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indole-based hole transport materials for green PhOLEDs
Accepted Manuscript Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs Panpan Wu, Wenxuan Song, Zhenyuan Xia, Yi Chen, Guojian Tian, Jinhai Huang, Jianhua Su PII:
S0143-7208(17)31914-9
DOI:
10.1016/j.dyepig.2017.11.017
Reference:
DYPI 6363
To appear in:
Dyes and Pigments
Received Date: 8 September 2017 Revised Date:
26 October 2017
Accepted Date: 7 November 2017
Please cite this article as: Wu P, Song W, Xia Z, Chen Y, Tian G, Huang J, Su J, Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.11.017. 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 Graphical Abstract
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Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs
ACCEPTED MANUSCRIPT Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs Panpan Wu,[a] Wenxuan Song,[a] Zhenyuan Xia,[b] Yi Chen,[a] Guojian Tian,[a] Jinhai Huang,*[c] and Jianhua Su*[a] a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
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of Science & Technology, Shanghai 200237, P. R. China. Fax: (86)21-64252288；Tel: (86)2164252288；E-mail: [email protected] b
Istituto per la Sintesi Organica e la Fotoreattività - Consiglio Nazionale delle Ricerche, via
Gobetti 101, 40129 Bologna, Italy
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Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, P. R. China. E-mail: [email protected]
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c
ACCEPTED MANUSCRIPT Abstract Three hole transport materials, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, incorporating fluorene/indole core with carbazole or triphenylamine unit were synthesized and fully characterized. The photophysical properties, thermal properties and electrochemical properties of
high thermal stability (Td > 420
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these three compounds were fully investigated. The FIPN-based hole transport materials show ) and appropriate highest occupied molecular orbital (HOMO)
level (~-5.2 eV). Green phosphorescent organic light-emitting diodes (PhOLEDs) using these FIPN-based derivatives were fabricated to investigate the device performance, compared with
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NPB as the reference hole transport material. It turned out that the devices using these three compounds exhibited superior performance than that of the NPB-based PhOLED device.
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Especially, the FIPN-p-PCz based device showed outstanding electroluminescence performance with the maximum current efficiency and external quantum efficiency of 53.7 cd/A and 17.3 %, respectively, which was almost twice that of the NPB based device.
Keywords: hole transport material; fluorene; indole; green phosphorescent organic light-emitting
1. Introduction
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diodes
Organic light-emitting devices (OLEDs) have been attracting great attention for their unique properties of fast response, high luminous efficiency, large view angle in flat panel displays and
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solid-state lighting.[1-10] Generally, small-molecule OLEDs are fabricated by sequential thermal evaporation of hole transport layer (HTL), emitting layer (EML), electron transport layer (ETL)
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and metallic layer of cathode on a indium tin oxide anode (ITO). Each of the organic functional layers should own their favorable thermal and electrochemical stabilities with balanced charge transport to guarantee the whole device performance. Among these functional materials, rational designed hole transport materials (HTM) are one of the key factors in achieving high efficiency OLEDs. In the past decades, many works have been done to address the outstanding hole transport materials with the following properties: 1. good hole mobility that could provide a charge hopping pathway for positive charge carriers to migrate from the anode into the EML; 2. high thermal stability and electrochemical stability for high temperature operations;[11-13] 3. a proper HOMO
ACCEPTED MANUSCRIPT energy level to ensure the hole can be injected easily from anode to emitting layer; 4. an appropriate LUMO energy level to block the electron transfer from emitting layer to hole transport layer.[14-18] Fluorene-based materials generally possess high thermal and morphological stability, bipolar carrier transporting ability and high fluorescence quantum yield, which are ideal materials
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for organic optoelectronic devices including organic light-emitting diodes, field-effect transistors and photovoltaic cells.[19-24] Meanwhile, indole-based materials are widely used in OLEDs, which can be attributed to their remarkable hole transporting property and high triplet energy.[25,26] Fluorene and indole are nice electron donors, compounds that are introduced with
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fluorene and indole not only are thermally stable, but also possess a relatively high HOMO energy level, which can decrease the energy barrier for hole transportation, thus enhancing the hole
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mobility.
Taking advantages of the fluorene and indole units, three hole transport materials, N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren2-amine
(FIPN-p-PCz),
N4-(4-(1H-indol-1-yl)phenyl)-N4-(9,9-dimethyl-9H-fluoren-2-yl)-N4',N4'-diphenyl-[1,1'-biphenyl] (FIPN-p-TPA)
and
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-4,4'-diamine
N-(4-(1H-indol-1-yl)phenyl)-N-(4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9,9-dimethyl-9H-fluo ren-2-amine (FIPN-DPCz) were designed and synthesized. The fluorene/indole core was
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connected with carbazole or triphenylamine end capping groups via an aryl bridge. Due to the high charge carrier mobility and low ionization of the aromatic amines, the introduction of the
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carbazole or triphenylamine units are supposed to adjust the energy level and improve the carrier mobility of these compounds.[27,28] To study the hole transport capacity of these compounds, green PhPLEDs using the FIPN-based compounds as hole transport materials were fabricated and evaluated. Meanwhile, NPB, which is the most commonly used hole transport material, was used for comparison with the same device structure.[29]
2. Experimental section 2.1. General information All the reagents and solvents used for the synthesis or measurements were purchased from Shanghai Taoe chemical technology Co., Ltd without further purification. The 1H and
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C NMR
ACCEPTED MANUSCRIPT spectra were recorded on a Bruker AM 400 spectrometer at room temperature. High resolution mass spectra were determined by a Waters LCT premier XE spectrometer. Elemental analyses were calculated by a Vario EL-III microanalyzer. UV-vis absorption spectra were recorded on a Varin Cary 500 recording spectrophotometer with baseline correction. Photoluminescence (PL)
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spectra were recorded on a Varian-Cary fluorescence spectrophotometer. Thermogravimetric analysis (TGA) was performed using a PerkinElmer Pyris Diamond instrument under a nitrogen atmosphere, and the thermal stability was determined by measuring their weight loss, at a heating rate of 10
/min. The differential scanning calorimetry (DSC) analysis were measured on a DSC
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Q2000 instrument under a nitrogen atmosphere. The cyclic voltammograms of the compounds were measured by a Versastat II electrochemical workstation 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, 0.1M TBAPF6 in dichloromethane solution as a supporting electrolyte.
2.2. Material synthesis
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2.2.1. 1-(4-bromophenyl)-1H-indole (1)[33]
A mixture of 1H-indole (5.00 g, 42.73 mmol), 1-bromo-4-iodobenzene (11.00 g, 38.87 mmol), potassium hydroxide (4.40 g, 78.57 mmol), cuprous iodide (28.5 mg, 0.15 mmol) and
150
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1,10-Phenanthroline monohydrate (27.0 mg, 0.15 mmol) in ortho-xylene (50 mL) was stirred at for 7 h under nitrogen atmosphere. After cooling to room temperature, the solvent was
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removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (6:1, v/v) afforded 1 (9.65 g, 91.3 %). 1H NMR (400 MHz, CDCl3) δ 7.61 (d, J = 7.8 Hz, 1H), 7.58 – 7.53 (m, 2H), 7.45 (d, J = 8.0 Hz, 1H), 7.34 – 7.28 (m, 2H), 7.22 (d, J = 3.4 Hz, 1H), 7.19 – 7.10 (m, 2H), 6.62 (d, J = 3.2 Hz, 1H). 2.2.2. N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-9H-fluoren-2-amine (2) A mixture of 1 (5.00 g, 18.38 mmol), 9,9-dimethyl-9H-fluoren-2-amine (4.60 g, 22.22 mmol), sodium tert-butoxylate (4.00 g, 41.67 mmol), palladium acetate (0.11 g, 0.50 mmol) and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (0.44 g, 1.00 mmol) was dissolved in
ACCEPTED MANUSCRIPT methylbenzene (50 mL) and heated at 120
for 7 h under nitrogen. After cooling to room
temperature, the solvent was removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column
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chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded 2 (6.50 g, 88.8 %). 1H NMR (400 MHz, DMSO) δ 8.60 (s, 1H), 7.75 (t, J = 7.0 Hz, 2H), 7.71 (d, J = 7.8 Hz, 1H), 7.63 (d, J = 3.2 Hz, 1H), 7.56 (dd, J = 7.2, 5.6 Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H), 7.34 (d, J = 8.6 Hz, 4H), 7.23 (m, 4H), 6.72 (s, 1H), 1.50 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 154.39, 152.10, 141.18,
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140.97, 138.09, 135.18, 132.16, 131.61, 127.93, 127.17, 125.96, 125.24, 124.77, 121.44, 121.08, 119.98, 119.88, 119.03, 118.16, 116.88, 116.38, 111.87, 109.44, 101.84, 45.79, 26.23.
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2.2.3. N-(4-(1H-indol-1-yl)phenyl)-N-(4-bromophenyl)-9,9-dimethyl-9H-fluoren-2-amine (3) A mixture of 2 (5.00 g, 12.56 mmol), 1-bromo-4-iodobenzene (4.30 g, 15.19 mmol), potassium hydroxide (1.41 g, 25.18 mmol), cuprous iodide (15.2 mg, 0.08 mmol) and 1,10-Phenanthroline monohydrate (14.4 mg, 0.08 mmol) in ortho-xylene (50 mL) was stirred at 150
for 8 h under nitrogen atmosphere. After cooling to room temperature, the solvent was
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removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using
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petroleum ether/dichloromethane (4:1, v/v) afforded 3 (5.83 g, 83.88 %). 1H NMR (400 MHz, DMSO) δ 7.85 (dd, J = 16.0, 7.6 Hz, 2H), 7.71 (t, J = 5.4 Hz, 2H), 7.65 – 7.53 (m, 6H), 7.38 (m, 13
C NMR (101 MHz,
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3H), 7.27 (m, 3H), 7.14 (m, 4H), 6.75 (d, J = 3.2 Hz, 1H), 1.46 (s, 6H).
CDCl3) δ 154.36, 152.51, 145.88, 145.42, 144.86, 137.64, 134.89, 134.05, 133.54, 131.32, 128.14, 126.93, 126.02, 125.76, 124.29, 124.22, 123.55, 122.74, 121.50, 121.23, 120.09, 119.85, 119.23, 118.55, 118.04, 114.17, 109.44, 102.30, 45.87, 26.06. 2.2.4.
N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluor en-2-amine (FIPN-p-PCz) A mixture of compounds 3 (0.90 g, 0.16 mmol), (9-phenyl-9H-carbazol-3-yl)boronic acid (0.50 g, 0.17 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (23.0 mg, 0.02 mmol) was
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ether/dichloromethane (4:1, v/v) afforded FIPN-p-PCz (0.65 g, 59.0 %). 1H NMR (400 MHz, DMSO) δ 8.67 (d, J = 1.2 Hz, 1H), 8.43 (d, J = 7.6 Hz, 1H), 7.87 (m, 5H), 7.76 (m, 6H), 7.63 (m, 5H), 7.50 (m, 4H), 7.40 (m, 2H), 7.31 (m, 6H), 7.20 (m, 2H), 6.77 (d, J = 3.2 Hz, 1H), 1.50 (s, 6H).
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C NMR (101 MHz, CDCl3) δ 155.31, 153.62, 146.97, 146.46, 146.39, 141.36, 140.23,
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138.92, 137.69, 136.67, 136.04, 134.66, 134.15, 132.89, 129.96, 129.18, 128.14, 128.10, 127.54, 127.09, 127.06, 126.65, 126.17, 125.26, 125.18, 124.59, 124.40, 123.95, 123.66, 123.50, 122.56,
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122.25, 121.13, 120.83, 120.39, 120.23, 120.09, 119.55, 118.94, 118.41, 110.59, 110.08, 109.97, 103.22, 46.95, 27.18. HRMS (ESI, m/z): [M+H]+ calculated for C53H39N3, 718.3222, found 718.3221. Anal. calcd for C53H39N3: C 88.67, H 5.48, N 5.85. found: C 88.35, H 5.41, N 5.58. 2.2.5.
N4-(4-(1H-indol-1-yl)phenyl)-N4-(9,9-dimethyl-9H-fluoren-2-yl)-N4',N4'-diphenyl-[1,1'-biphe
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nyl]-4,4'-diamine (FIPN-p-TPA)
A mixture of compounds 3 (1.60 g, 0.29 mmol), (4-(diphenylamino)phenyl)boronic acid (0.90 g, 0.31 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under
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nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (46.0 mg, 0.04 mmol) was added to the mixture, and the resulting mixture was refluxed for 6 h under nitrogen atmosphere. After
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cooling to room temperature, the mixture was poured into H2O and then extracted with dichloromethane (3×20 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded FIPN-p-TPA (1.40 g, 67.3 %). 1H NMR (400 MHz, DMSO) δ 7.81 (d, J = 8.2 Hz, 1H), 7.77 (d, J = 7.2 Hz, 1H), 7.59 (m, 10H), 7.20 (m, 22H), 6.69 (d, J = 3.2 Hz, 1H), 1.41 (s, 6H).
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C NMR (101 MHz, CDCl3) δ 155.31, 153.60, 147.73, 146.89,
146.87, 146.61, 146.33, 138.87, 136.01, 135.15, 134.74, 134.57, 134.22, 129.31, 129.18, 128.07, 127.48, 127.35, 127.06, 126.68, 125.25, 124.46, 124.37, 124.31, 124.11, 123.73, 122.90, 122.55, 122.25, 121.13, 120.83, 120.24, 119.56, 119.03, 110.57, 103.24, 46.93, 27.16. HRMS (ESI, m/z): [M+H]+ calculated for C53H41N3, 720.3379, found 720.3381. Anal. calcd for C53H41N3: C 88.42, H
ACCEPTED MANUSCRIPT 5.74, N 5.84. found: C 88.04, H 5.46, N 5.57. 2.2.6. N-(4-(1H-indol-1-yl)phenyl)-N-(4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9,9-dimethyl-9H-f luoren-2-amine (FIPN-DPCz)
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A mixture of compounds 3 (1.50 g, 0.27 mmol), (4-(9H-carbazol-9-yl)phenyl)boronic acid (0.90 g, 0.31 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (46.0 mg, 0.04 mmol) was added to the mixture, and the resulting mixture was refluxed for 6 h under nitrogen atmosphere.
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After cooling to room temperature, the mixture was poured into H2O and then extracted with dichloromethane (3×20 mL). The combined organic phase was collected, filtered and dried over
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MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded FIPN-p-TPA (1.35 g, 69.6 %). 1H NMR (400 MHz, DMSO) δ 8.28 (d, J = 7.8 Hz, 2H), 7.98 (d, J = 8.6 Hz, 2H), 7.82 (m, 4H), 7.69 (m, 4H), 7.57 (m, 4H), 7.45 (dd, J = 12.4, 2.8 Hz, 5H), 7.37 (m, 6H), 7.23 (dd, J = 13.0, 7.9 Hz, 3H), 7.14 (m, 2H), 6.71 (d, J = 3.0 Hz, 1H), 1.44 (s, 6H).
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C NMR (101 MHz, CDCl3) δ 155.40, 153.62, 147.46,
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146.74, 146.17, 140.90, 139.67, 138.82, 136.49, 136.01, 135.03, 134.52, 134.37, 129.22, 128.06, 128.00, 127.97, 127.40, 127.10, 126.79, 126.00, 125.30, 124.79, 124.00, 123.44, 122.59, 122.30, 121.17, 120.92, 120.38, 120.30, 120.00, 119.63, 119.30, 110.57, 109.87, 103.35, 46.98, 27.18.
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HRMS (ESI, m/z): [M+H]+ calculated for C53H39N3, 717.3144, found 717.3346. Anal. calcd for C53H39N3: C 88.67, H 5.48, N 5.85. found: C 88.27, H 5.49, N 5.62.
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3. Results and discussion
3.1. Synthesis and characterization The synthetic routes of the intermediates and three target compounds are shown in Scheme 1.
The intermediates 1-3 were synthesized by Ullmann and Buchwald-Hartwig reaction. The target compounds were prepared by Suzuki coupling reaction, then purified by flash column chromatography and recrystallized with yields of 59.0 %, 67.3 % and 69.6 %, respectively. The molecular structures of target compounds were confirmed by 1H NMR, 13C NMR, high-resolution mass spectrometry (HRMS) and elemental analysis.
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Scheme 1 Synthetic routes of the compounds
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3.2. Photophysical properties
The UV-vis absorption and fluorescence emission spectra of these three compounds were
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showed in Fig. 1, which were measured in tetrahydrofuran solution. It can be seen that, the UV-vis spectra of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were quite similar with maximum peaks and shoulders, the intense UV-vis absorption peaks (~278 nm) of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz could be attributed to the π-π* transitions of the conjugated aromatic segments, the weak absorption peaks of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz are respectively located at around 343 nm, 352 nm and 345 nm, which should be assigned to the n-π* transition for the extended conjugation of the aromatic amines.[29] The optical energy bandgaps (Eg) of the three compounds calculated from the onset of the absorption spectra were determined to be 2.88 eV, 2.82 eV and 2.86 eV, respectively. In terms of emission, the maximum PL emission peaks were located at 406 nm, 415 nm and 431 nm for FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz,
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respectively. The detailed data are summarized in Table 1.
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Fig. 1 Normalized UV-vis absorption and fluorescence emission spectra of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz in tetrahydrofuran solution at room temperature
3.3. Thermal properties
The thermal properties of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) under
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nitrogen atmosphere with a heating rate of 10
/min. As shown in Fig. 2 and Table 1, the
decomposition temperatures (Td, correspond to 5% weight loss) of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were measured to be 443
, 420
and 475
, respectively. Meanwhile, the , 122
and 136
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corresponding glass transition temperature (Tg) were determined to be 127
for FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, respectively. These values are higher than that of ),[30] which suggesting better morphological stability of FIPN-based compounds.
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NPB (Tg, 98
Therefore, these three compounds are expected to be potential alternatives of NPB from their excellent thermal stability behavior.
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3.4. Electrochemical properities
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Fig. 2 TGA and DSC curves of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
The electrochemical behavior of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were investigated by cyclic voltammograms (CV) using a standard three-electrode electrochemical cell in an electrolyte solution (0.1M TBAPF6/DCM), with ferrocene as an external reference. As
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shown in Fig. 3 and Table 1, the highest occupied molecular orbital (HOMO) energy level of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were calculated to be -5.26 eV, -5.19 eV and -5.27 eV, respectively, which is slightly higher than that of NPB (-5.5 eV). The corresponding LUMO values were calculated to be -2.38 eV for FIPN-p-PCz, -2.37 eV for FIPN-p-TPA and -2.41 eV for
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FIPN-DPCz respectively, according to the equation ELUMO = EHOMO + Eg. The energetically favorable HOMO energy levels of these three compounds are supposed to be further facilitate the
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hole injection and transport properties from anode side.
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Fig. 3 Cyclic voltammograms of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz in dichloromethane containing 0.1 M TBAPF6 electrolytes, scanning rate: 100 mV/s
Table 1 The physical properties of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz. Absorption λmaxa [nm]
Emission λmaxa [nm]
Egb [eV]
HOMOc [eV]
LUMOd [eV]
Tge [ ]
T df [ ]
FIPN-p-PCz
343
406
2.88
-5.26
-2.38
127
443
FIPN-p-TPA
352
415
2.82
-5.19
-2.37
122
420
FIPN-DPCz
345
431
2.86
-5.27
-2.41
136
475
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Compounds
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Measured in tetrahydrofuran at a concentration of 1.0×10 M.
b
Estimated from onset of the absorption spectra (Eg = 1241/λonset).
c
The HOMO energy level was determined from cyclic voltammetry (EHOMO = -4.4 - Eox).[31]
d
The LUMO energy level was calculated by the equation: ELUMO = EHOMO + Eg.[31]
e
Measured by DSC.
f
Measured by TGA.
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a
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3.5. Electroluminescence of PhOLEDS To evaluate the hole-transporting properties of these three compounds, OLEDs with the
structure of [ITO/PEDOT (45nm)/HTL (40 nm)/CBP:Ir(ppy)3 (20 nm, 8wt%)/TmPyPB (40 nm)/LiF (0.6 nm)/Al (80 nm)] were fabricated using FIPN-p-PCz, FIPN-p-TPA or FIPN-DPCz as hole transporting layer (HTL), ITO and LiF/Al were used as the anode and composite cathode, respectively. PEDOT and TmPyPB were employed as hole injection layer (HIL) and electron transporting layer (ETL), respectively. CBP served as host, and Ir(ppy)3 was used as the emitting material. For comparison, the control device with structure of [ITO/PEDOT (45 nm)/NPB (40 nm)/CBP:Ir(ppy)3 (20 nm, 8wt %)/TmPyPB (40 nm)/LiF (0.6 nm)/Al (80 nm)] was also fabricated.
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shown in Fig. 5, Fig. 6 and Fig.7, and the relevant parameters are summarized in Table 2.
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Fig. 4 The energy level diagram of the materials in devices
Fig. 5 Current density-voltage-luminance (J-V-L) curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 6 External quantum efficiency-current density curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 7 Current efficiency-luminance-power efficiency curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 8 EL spectra at 5 V of green PhOLEDs
As revealed in Fig. 5 and Table 2, the turn-on voltages were 3.3 V, 3.4 V, 3.2 V and 3.5 V for NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, respectively. The device using FIPN-p-TPA as hole transporting layer exhibited the lowest turn-on voltage, which might be attributed to the suitable HOMO energy level of FIPN-p-TPA and efficient hole transport properties from triphenylamine group. The maximum luminance of devices based on FIPN-p-PCz and
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FIPN-DPCz were 59784 cd/m2 and 72052 cd/m2, which was comparable to the value of NPB based device (63458 cd/m2). Meanwhile, the maximum luminance of FIPN-p-TPA based device achieved 82680 cd/m2, which was nearly 130 % higher than that of NPB based ones. The high EL
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improvements from FIPN-p-TPA could be attributed to its proper HOMO energy level, which is more favorable for hole injection/transport from anode side to EML interface. 6
and
Fig.7
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Fig.
show
EQE-current
density
characteristic
and
current
efficiency-luminance-power efficiency characteristic, respectively. From Fig. 6, Fig. 7 and Table 2, it can be seen that, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz based devices exhibited much better performance than NPB based device. Especially, the FIPN-p-PCz based device showed the best performance with maximum EQE of 17.3 %, maximum current efficiency of 53.7 cd/A, and maximum power efficiency of 30.7 lm/W, respectively. The better performance of these three materials based devices might be ascribed to their high HOMO energy level and more balanced charge transfer than that of NPB. In addition, all these three devices showed small efficiencies roll-off over a wide range of luminance. For example, under the representative luminance of
ACCEPTED MANUSCRIPT 10000 cd/m2, the current efficiency of device based on FIPN-p-PCz still reached up to 45.9 cd/A. Furthermore, EL spectra of Fig. 8 suggested that the green emission peaks of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz are similarly located at 511 nm, with the corresponding CIE coordinates of (0.26, 0.61), (0.29, 0.63), (0.28, 0.62) and (0.26, 0.60),
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respectively.
Table 2 EL performance of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz. Vona [V]
Lmaxb [cd/m2]
η cc [cd/A]
η pc [lm/W]
ηextd [%]
NPB
3.3
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28.4, 26.8, 23.1
19.9, 19.4, 12.3
8.5
(0.26, 0.61)
FIPN-p-PCz
3.4
59784
53.7, 47.3, 45.9
30.7, 29.9, 23.0
17.3
(0.29, 0.63)
FIPN-p-TPA
3.2
82680
41.9, 40.5, 38.5
28.3, 27.8, 21.5
12.4
(0.28, 0.62)
FIPN-DPCz
3.5
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50.8, 50.1, 46.3
31.6, 31.0, 22.3
15.4
(0.26, 0.60)
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Compounds
CIE(x,y)e
a
Turn-on voltage to give a luminance of 1 cd/m2.
b
Lmax: maximum luminance.
c
Order of measured values: maximum, then at 1000 cd/m2 and 10000 cd/m2. ηc: current efficiency. ηp: power
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efficiency. d
maximum values. ηext: external quantum efficiency.
e
Measured from the EL spectra at 5 V by inverting chromaticity coordinates on the CIE 1931 diagram.
4. Conclusion
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In conclusion, three hole transport materials, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were synthesized and characterized, all these three compounds exhibited high HOMO energy level
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and excellent thermal stability. The electroluminescent properties of green PhOLEDs using these three compounds as hole transport layer were investigated, and NPB was introduced as comparison. The results indicated that three synthesized compounds exhibited superior properties than NPB in green PhOLEDs, the device using FIPN-p-PCz as hole transport material showed the best performance with current efficiency and external quantum efficiency of 53.7 cd/A and 17.3 %, respectively, which are almost twice that of NPB. The above results indicated that FIPN-based derivatives are promising candidates for HTMs in OLEDs and its related optoelectronic devices.
ACCEPTED MANUSCRIPT Notes and references [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987; 51: 913-5. [2] 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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[8] Zheng ZW, Dong QC, Gou L, Su JH, Huang JH. Novel hole transport materials based on N,N’-disubstituted-dihydrophenazine derivatives for electroluminescent diodes. J Mater Chem C
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[9] Xing ZH, Zhuang JY, Xu XP, Ji SJ, Su WM, Cui Z. Novel oxazole-based emitters for high
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efficiency fluorescent OLEDs: Synthesis, characterization, and optoelectronic properties. Tetrahedron 2017; 73: 2036-42. [10] Chen HW, Liang WQ, Chen Y, Tian GJ, Dong QC, Huang JH, Su JH. Efficient blue fluorescent organic light-emitting diodes based on novel 9,10-diphenyl-anthracene derivatives. RSC Adv 2015; 5: 70211-19. [11] Yu MX, Duan JP, Lin CH, Cheng CH, Tao YT. Diaminoanthracene derivative as high-performance green host electroluminescent materials. Chem Mater 2002; 14: 3958-63 [12] Shen JY, Lee CY, Huang TH, Lin JT, Tao YT, Chien CH, Tsai C. High Tg blue emitting materials for electroluminescent devices. J Mater Chem 2005; 15: 2455-63
ACCEPTED MANUSCRIPT [13] Wong WY, Ho CL. Functional metallophosphors for effective charge carrier injection/transport: new robust OLED materials with emerging applications. J Mater Chem 2009; 19: 4457-82 [14] Lu J, Tao Y, D’iorio M, Li Y, Ding J, Day M. Pure deep blue light-emitting diodes from fluorene/carbazole
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[17] Xiang N, Gao ZX, Tian GJ, Chen Y, Liang WQ, Huang JH, Dong QC, Wong WY, Su JH. Novel fluorene/indole-based hole transport materials with high thermal stability for efficient OLEDs. Dyes and Pigments 2017; 137: 36-42.
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[18] Tian GJ, Wei X, Xiang N, Huang JH, Cao J, Wang ZX, Zhang JH, Su JH. Small organic molecules based on oxazole/thiazole with excellent performances in green and red phosphorescent organic light-emitting diodes. RSC Adv 2016; 6: 51575-82.
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[19] Zhen CG, Chen ZK, Liu QD, Dai YF, Shin RYC, Chang SY, et al. Fluorene-based oligomers for highly efficient and stable organic blue-light-emitting diodes. Adv Mater 2009; 21: 2425-29.
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[20] Romain M, Quinton C, Tondelier D, Geffroy B, Jeannin O, Berthelot JR, et al. Thioxanthene and dioxothioxanthene dihydroindeno[2,1-b]fluorenes: synthesis, properties and applications in green and sky blue phosphorescent OLEDs. J Mater Chem C 2016; 4: 1692-703. [21] Romain M, Chevrier M, Bebiche S, Brahim TM, Berthelot JR, Jacques E, et al. The structure-property
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dihydroindeno[2,1-b]fluorene
derivatives for n-type organic field effect transistors. J Mater Chem C 2015; 3: 5742-53. [22] Lin YW, Lin CJ, Chou YH, Liu CL, Chang HC, Chen WC. Nonvolatile organic field effect transistor memory devices using one-dimensional aligned electrospun nanofiber channels of semiconducting polymers. J Mater Chem C 2013; 1: 5336-43. [23] Tian GJ, Liang WQ, Chen Y, Xiang N, Dong QC, Huang JH, Su JH. A novel spiro-annulated
ACCEPTED MANUSCRIPT host based on carbazole with good thermal stability and high triplet energy for efficient blue and green phosphorescent organic light-emitting diodes. Dyes and Pigments 2016; 126: 296-302. [24] Liu WQ, Wu YZ, Li X, Xie YS, Zhu WH. Absorption and photovoltaic properties of organic solar cell sensitizers containing fluorene unit as conjunction bridge. Energy Environ Sci 2011; 4:
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1830-37. [25] Park MS, Choi DH, Lee BS, Lee JY. Fused indole derivatives as high triplet energy hole transport materials for deep blue phosphorescent organic light-emitting diodes. J Mater Chem 2012; 22: 3099-104.
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[26] Chen Y, Xie JW, Wang ZX, Cao J, Chen HW, Huang JH, et al. Highly efficient bipolar host materials based-on indole and triazine moiety for red phosphorescent light-emitting diodes. Dyes
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and Pigments 2016; 124: 188-95.
[27] Chang CH, Kuo MC, Lin WC, Chen YT, Wong KT, Chou SH, et al. A dicarbazole-triazine hybrid bipolar host material for highly efficient green phosphorescent OLEDs. J Mater Chem 2012; 22: 3832-38.
[28] Thangthong A, Prachumrak N, Tarsang R, Keawin T, Jungsuttiwong S, Sudyoadsuk T, et al. light-emitting
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9,9-bis(4-diphenylaminophenyl)fluorenes for efficient electroluminescent devices. J Mater Chem 2012; 22: 6869-77.
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[29] Li JY, Zhang T, Liang YJ, Yang RX. Solution-processible carbazole dendrimers as host materials for highly efficient phosphorescent organic light-emitting diodes. Adv Funct Mater 2013;
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23: 619-28.
[30] Aziz H, Popovic ZD, Hu NX, Hor AM, Xu G. Degradation mechanism of small molecule-based organic light-emitting devices. Science 1999; 283: 1900-02. [31] Hu MM, Liu Y, Chen Y, Song WX, Gao L, Mu HC, Huang JH, Su JH. Highly efficient triazine/carbazole-based host material for green phosphorescent organic light-emitting diodes with low efficiency roll-off. RSC Adv 2017; 7: 7287-92. [32] Tian GJ, Jiang YX, Wu PP, Huang JH, Zou Q, Wang QC, Mu HC, Su JH. Pure hydrocarbon host materials based on spirofluorene with excellent performances for green phosphorescent light-emitting devices. J Name 2012; 00: 1-3. [33] Ma HC, Jiang XZ. N-Hydroxyimides as Efficient Ligands for the Copper-Catalyzed
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N-Arylation of Pyrrole, Imidazole, and Indole. J Org Chem 2007; 72: 8943-46.
ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs Panpan Wu,[a] Wenxuan Song,[a] Zhenyuan Xia,[b] Yi Chen,[a] Guojian Tian,[a] Jinhai Huang,*[c] and Jianhua Su*[a] a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
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of Science & Technology, Shanghai 200237, P. R. China. Fax: (86)21-64252288；Tel: (86)2164252288；E-mail: [email protected] b
Istituto per la Sintesi Organica e la Fotoreattività - Consiglio Nazionale delle Ricerche, via
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Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, P. R. China. E-mail: [email protected]
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Gobetti 101, 40129 Bologna, Italy
ACCEPTED MANUSCRIPT Contents OLED fabrication procedure, 1H NMR,
13
C NMR, HRMS of intermediates and three target
compounds.
OLED fabrication
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In general, organic layers were deposited on the indium-tin-oxide (ITO)/glass substrate by thermal evaporation. Before use, the ITO/glass substrate was cleaned sequentially by detergent, de-ionized water and ethanol. Then the ITO/glass was treated by oxygen (O2) plasma before
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loading into a 10-source evaporator, with a base pressure of 5.0 × 10-4 Pa, for device fabrication.
Fig. S1 1H NMR of intermediate 1
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Fig. S2 1H NMR of intermediate 2
Fig. S3 13C NMR of intermediate 2
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Fig. S4 1H NMR of intermediate 3
Fig. S5 13C NMR of intermediate 3
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Fig. S6 1H NMR of FIPN-p-PCz
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Fig. S7 13C NMR of FIPN-p-PCz
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Fig. S8 Mass spectrometry of FIPN-p-PCz
Fig. S9 1H NMR of FIPN-p-TPA
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Fig. S10 13C NMR of FIPN-p-TPA
Fig. S11 Mass spectrometry of FIPN-p-TPA
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Fig. S12 1H NMR of FIPN-DPCz
Fig. S13 13C NMR of FIPN-DPCz
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Fig. S14 Mass spectrometry of FIPN-DPCz
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1. The research was based on indole and fluorene, which is excellent hole transport moieties. 2. The three synthesized compounds possess high thermal stabilities, which can be used in OLED stably. 3. The OLEDs based on the three synthesized materials indicated much higher efficiency than NPB in the same device structure.
S0143-7208(17)31914-9
DOI:
10.1016/j.dyepig.2017.11.017
Reference:
DYPI 6363
To appear in:
Dyes and Pigments
Received Date: 8 September 2017 Revised Date:
26 October 2017
Accepted Date: 7 November 2017
Please cite this article as: Wu P, Song W, Xia Z, Chen Y, Tian G, Huang J, Su J, Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs, Dyes and Pigments (2017), doi: 10.1016/j.dyepig.2017.11.017. 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 Graphical Abstract
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Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs
ACCEPTED MANUSCRIPT Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs Panpan Wu,[a] Wenxuan Song,[a] Zhenyuan Xia,[b] Yi Chen,[a] Guojian Tian,[a] Jinhai Huang,*[c] and Jianhua Su*[a] a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
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of Science & Technology, Shanghai 200237, P. R. China. Fax: (86)21-64252288；Tel: (86)2164252288；E-mail: [email protected] b
Istituto per la Sintesi Organica e la Fotoreattività - Consiglio Nazionale delle Ricerche, via
Gobetti 101, 40129 Bologna, Italy
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Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, P. R. China. E-mail: [email protected]
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ACCEPTED MANUSCRIPT Abstract Three hole transport materials, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, incorporating fluorene/indole core with carbazole or triphenylamine unit were synthesized and fully characterized. The photophysical properties, thermal properties and electrochemical properties of
high thermal stability (Td > 420
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these three compounds were fully investigated. The FIPN-based hole transport materials show ) and appropriate highest occupied molecular orbital (HOMO)
level (~-5.2 eV). Green phosphorescent organic light-emitting diodes (PhOLEDs) using these FIPN-based derivatives were fabricated to investigate the device performance, compared with
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NPB as the reference hole transport material. It turned out that the devices using these three compounds exhibited superior performance than that of the NPB-based PhOLED device.
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Especially, the FIPN-p-PCz based device showed outstanding electroluminescence performance with the maximum current efficiency and external quantum efficiency of 53.7 cd/A and 17.3 %, respectively, which was almost twice that of the NPB based device.
Keywords: hole transport material; fluorene; indole; green phosphorescent organic light-emitting
1. Introduction
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diodes
Organic light-emitting devices (OLEDs) have been attracting great attention for their unique properties of fast response, high luminous efficiency, large view angle in flat panel displays and
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solid-state lighting.[1-10] Generally, small-molecule OLEDs are fabricated by sequential thermal evaporation of hole transport layer (HTL), emitting layer (EML), electron transport layer (ETL)
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and metallic layer of cathode on a indium tin oxide anode (ITO). Each of the organic functional layers should own their favorable thermal and electrochemical stabilities with balanced charge transport to guarantee the whole device performance. Among these functional materials, rational designed hole transport materials (HTM) are one of the key factors in achieving high efficiency OLEDs. In the past decades, many works have been done to address the outstanding hole transport materials with the following properties: 1. good hole mobility that could provide a charge hopping pathway for positive charge carriers to migrate from the anode into the EML; 2. high thermal stability and electrochemical stability for high temperature operations;[11-13] 3. a proper HOMO
ACCEPTED MANUSCRIPT energy level to ensure the hole can be injected easily from anode to emitting layer; 4. an appropriate LUMO energy level to block the electron transfer from emitting layer to hole transport layer.[14-18] Fluorene-based materials generally possess high thermal and morphological stability, bipolar carrier transporting ability and high fluorescence quantum yield, which are ideal materials
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for organic optoelectronic devices including organic light-emitting diodes, field-effect transistors and photovoltaic cells.[19-24] Meanwhile, indole-based materials are widely used in OLEDs, which can be attributed to their remarkable hole transporting property and high triplet energy.[25,26] Fluorene and indole are nice electron donors, compounds that are introduced with
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fluorene and indole not only are thermally stable, but also possess a relatively high HOMO energy level, which can decrease the energy barrier for hole transportation, thus enhancing the hole
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mobility.
Taking advantages of the fluorene and indole units, three hole transport materials, N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluoren2-amine
(FIPN-p-PCz),
N4-(4-(1H-indol-1-yl)phenyl)-N4-(9,9-dimethyl-9H-fluoren-2-yl)-N4',N4'-diphenyl-[1,1'-biphenyl] (FIPN-p-TPA)
and
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-4,4'-diamine
N-(4-(1H-indol-1-yl)phenyl)-N-(4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9,9-dimethyl-9H-fluo ren-2-amine (FIPN-DPCz) were designed and synthesized. The fluorene/indole core was
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connected with carbazole or triphenylamine end capping groups via an aryl bridge. Due to the high charge carrier mobility and low ionization of the aromatic amines, the introduction of the
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carbazole or triphenylamine units are supposed to adjust the energy level and improve the carrier mobility of these compounds.[27,28] To study the hole transport capacity of these compounds, green PhPLEDs using the FIPN-based compounds as hole transport materials were fabricated and evaluated. Meanwhile, NPB, which is the most commonly used hole transport material, was used for comparison with the same device structure.[29]
2. Experimental section 2.1. General information All the reagents and solvents used for the synthesis or measurements were purchased from Shanghai Taoe chemical technology Co., Ltd without further purification. The 1H and
13
C NMR
ACCEPTED MANUSCRIPT spectra were recorded on a Bruker AM 400 spectrometer at room temperature. High resolution mass spectra were determined by a Waters LCT premier XE spectrometer. Elemental analyses were calculated by a Vario EL-III microanalyzer. UV-vis absorption spectra were recorded on a Varin Cary 500 recording spectrophotometer with baseline correction. Photoluminescence (PL)
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spectra were recorded on a Varian-Cary fluorescence spectrophotometer. Thermogravimetric analysis (TGA) was performed using a PerkinElmer Pyris Diamond instrument under a nitrogen atmosphere, and the thermal stability was determined by measuring their weight loss, at a heating rate of 10
/min. The differential scanning calorimetry (DSC) analysis were measured on a DSC
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Q2000 instrument under a nitrogen atmosphere. The cyclic voltammograms of the compounds were measured by a Versastat II electrochemical workstation 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, 0.1M TBAPF6 in dichloromethane solution as a supporting electrolyte.
2.2. Material synthesis
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2.2.1. 1-(4-bromophenyl)-1H-indole (1)[33]
A mixture of 1H-indole (5.00 g, 42.73 mmol), 1-bromo-4-iodobenzene (11.00 g, 38.87 mmol), potassium hydroxide (4.40 g, 78.57 mmol), cuprous iodide (28.5 mg, 0.15 mmol) and
150
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1,10-Phenanthroline monohydrate (27.0 mg, 0.15 mmol) in ortho-xylene (50 mL) was stirred at for 7 h under nitrogen atmosphere. After cooling to room temperature, the solvent was
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removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (6:1, v/v) afforded 1 (9.65 g, 91.3 %). 1H NMR (400 MHz, CDCl3) δ 7.61 (d, J = 7.8 Hz, 1H), 7.58 – 7.53 (m, 2H), 7.45 (d, J = 8.0 Hz, 1H), 7.34 – 7.28 (m, 2H), 7.22 (d, J = 3.4 Hz, 1H), 7.19 – 7.10 (m, 2H), 6.62 (d, J = 3.2 Hz, 1H). 2.2.2. N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-9H-fluoren-2-amine (2) A mixture of 1 (5.00 g, 18.38 mmol), 9,9-dimethyl-9H-fluoren-2-amine (4.60 g, 22.22 mmol), sodium tert-butoxylate (4.00 g, 41.67 mmol), palladium acetate (0.11 g, 0.50 mmol) and 2-dicyclohexylphosphino-2',4',6'-triisopropylbiphenyl (0.44 g, 1.00 mmol) was dissolved in
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for 7 h under nitrogen. After cooling to room
temperature, the solvent was removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column
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chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded 2 (6.50 g, 88.8 %). 1H NMR (400 MHz, DMSO) δ 8.60 (s, 1H), 7.75 (t, J = 7.0 Hz, 2H), 7.71 (d, J = 7.8 Hz, 1H), 7.63 (d, J = 3.2 Hz, 1H), 7.56 (dd, J = 7.2, 5.6 Hz, 2H), 7.51 (d, J = 8.6 Hz, 2H), 7.34 (d, J = 8.6 Hz, 4H), 7.23 (m, 4H), 6.72 (s, 1H), 1.50 (s, 6H). 13C NMR (101 MHz, CDCl3) δ 154.39, 152.10, 141.18,
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140.97, 138.09, 135.18, 132.16, 131.61, 127.93, 127.17, 125.96, 125.24, 124.77, 121.44, 121.08, 119.98, 119.88, 119.03, 118.16, 116.88, 116.38, 111.87, 109.44, 101.84, 45.79, 26.23.
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2.2.3. N-(4-(1H-indol-1-yl)phenyl)-N-(4-bromophenyl)-9,9-dimethyl-9H-fluoren-2-amine (3) A mixture of 2 (5.00 g, 12.56 mmol), 1-bromo-4-iodobenzene (4.30 g, 15.19 mmol), potassium hydroxide (1.41 g, 25.18 mmol), cuprous iodide (15.2 mg, 0.08 mmol) and 1,10-Phenanthroline monohydrate (14.4 mg, 0.08 mmol) in ortho-xylene (50 mL) was stirred at 150
for 8 h under nitrogen atmosphere. After cooling to room temperature, the solvent was
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removed by rotary evaporation under vacuum. Then, cold water was added to the mixture and finally extracted with DCM (3×30 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using
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petroleum ether/dichloromethane (4:1, v/v) afforded 3 (5.83 g, 83.88 %). 1H NMR (400 MHz, DMSO) δ 7.85 (dd, J = 16.0, 7.6 Hz, 2H), 7.71 (t, J = 5.4 Hz, 2H), 7.65 – 7.53 (m, 6H), 7.38 (m, 13
C NMR (101 MHz,
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3H), 7.27 (m, 3H), 7.14 (m, 4H), 6.75 (d, J = 3.2 Hz, 1H), 1.46 (s, 6H).
CDCl3) δ 154.36, 152.51, 145.88, 145.42, 144.86, 137.64, 134.89, 134.05, 133.54, 131.32, 128.14, 126.93, 126.02, 125.76, 124.29, 124.22, 123.55, 122.74, 121.50, 121.23, 120.09, 119.85, 119.23, 118.55, 118.04, 114.17, 109.44, 102.30, 45.87, 26.06. 2.2.4.
N-(4-(1H-indol-1-yl)phenyl)-9,9-dimethyl-N-(4-(9-phenyl-9H-carbazol-3-yl)phenyl)-9H-fluor en-2-amine (FIPN-p-PCz) A mixture of compounds 3 (0.90 g, 0.16 mmol), (9-phenyl-9H-carbazol-3-yl)boronic acid (0.50 g, 0.17 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (23.0 mg, 0.02 mmol) was
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ether/dichloromethane (4:1, v/v) afforded FIPN-p-PCz (0.65 g, 59.0 %). 1H NMR (400 MHz, DMSO) δ 8.67 (d, J = 1.2 Hz, 1H), 8.43 (d, J = 7.6 Hz, 1H), 7.87 (m, 5H), 7.76 (m, 6H), 7.63 (m, 5H), 7.50 (m, 4H), 7.40 (m, 2H), 7.31 (m, 6H), 7.20 (m, 2H), 6.77 (d, J = 3.2 Hz, 1H), 1.50 (s, 6H).
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C NMR (101 MHz, CDCl3) δ 155.31, 153.62, 146.97, 146.46, 146.39, 141.36, 140.23,
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138.92, 137.69, 136.67, 136.04, 134.66, 134.15, 132.89, 129.96, 129.18, 128.14, 128.10, 127.54, 127.09, 127.06, 126.65, 126.17, 125.26, 125.18, 124.59, 124.40, 123.95, 123.66, 123.50, 122.56,
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122.25, 121.13, 120.83, 120.39, 120.23, 120.09, 119.55, 118.94, 118.41, 110.59, 110.08, 109.97, 103.22, 46.95, 27.18. HRMS (ESI, m/z): [M+H]+ calculated for C53H39N3, 718.3222, found 718.3221. Anal. calcd for C53H39N3: C 88.67, H 5.48, N 5.85. found: C 88.35, H 5.41, N 5.58. 2.2.5.
N4-(4-(1H-indol-1-yl)phenyl)-N4-(9,9-dimethyl-9H-fluoren-2-yl)-N4',N4'-diphenyl-[1,1'-biphe
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nyl]-4,4'-diamine (FIPN-p-TPA)
A mixture of compounds 3 (1.60 g, 0.29 mmol), (4-(diphenylamino)phenyl)boronic acid (0.90 g, 0.31 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under
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nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (46.0 mg, 0.04 mmol) was added to the mixture, and the resulting mixture was refluxed for 6 h under nitrogen atmosphere. After
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cooling to room temperature, the mixture was poured into H2O and then extracted with dichloromethane (3×20 mL). The combined organic phase was collected, filtered and dried over MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded FIPN-p-TPA (1.40 g, 67.3 %). 1H NMR (400 MHz, DMSO) δ 7.81 (d, J = 8.2 Hz, 1H), 7.77 (d, J = 7.2 Hz, 1H), 7.59 (m, 10H), 7.20 (m, 22H), 6.69 (d, J = 3.2 Hz, 1H), 1.41 (s, 6H).
13
C NMR (101 MHz, CDCl3) δ 155.31, 153.60, 147.73, 146.89,
146.87, 146.61, 146.33, 138.87, 136.01, 135.15, 134.74, 134.57, 134.22, 129.31, 129.18, 128.07, 127.48, 127.35, 127.06, 126.68, 125.25, 124.46, 124.37, 124.31, 124.11, 123.73, 122.90, 122.55, 122.25, 121.13, 120.83, 120.24, 119.56, 119.03, 110.57, 103.24, 46.93, 27.16. HRMS (ESI, m/z): [M+H]+ calculated for C53H41N3, 720.3379, found 720.3381. Anal. calcd for C53H41N3: C 88.42, H
ACCEPTED MANUSCRIPT 5.74, N 5.84. found: C 88.04, H 5.46, N 5.57. 2.2.6. N-(4-(1H-indol-1-yl)phenyl)-N-(4'-(9H-carbazol-9-yl)-[1,1'-biphenyl]-4-yl)-9,9-dimethyl-9H-f luoren-2-amine (FIPN-DPCz)
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A mixture of compounds 3 (1.50 g, 0.27 mmol), (4-(9H-carbazol-9-yl)phenyl)boronic acid (0.90 g, 0.31 mmol), 2M aq.K2CO3 (10 mL) and tetrahydrofuran (20 mL) was stirred for 30 min under nitrogen atmosphere. Tetrakis(triphenylphosphine)palladium (46.0 mg, 0.04 mmol) was added to the mixture, and the resulting mixture was refluxed for 6 h under nitrogen atmosphere.
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After cooling to room temperature, the mixture was poured into H2O and then extracted with dichloromethane (3×20 mL). The combined organic phase was collected, filtered and dried over
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MgSO4. The crude product was purified by SiO2 column chromatography using petroleum ether/dichloromethane (4:1, v/v) afforded FIPN-p-TPA (1.35 g, 69.6 %). 1H NMR (400 MHz, DMSO) δ 8.28 (d, J = 7.8 Hz, 2H), 7.98 (d, J = 8.6 Hz, 2H), 7.82 (m, 4H), 7.69 (m, 4H), 7.57 (m, 4H), 7.45 (dd, J = 12.4, 2.8 Hz, 5H), 7.37 (m, 6H), 7.23 (dd, J = 13.0, 7.9 Hz, 3H), 7.14 (m, 2H), 6.71 (d, J = 3.0 Hz, 1H), 1.44 (s, 6H).
13
C NMR (101 MHz, CDCl3) δ 155.40, 153.62, 147.46,
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146.74, 146.17, 140.90, 139.67, 138.82, 136.49, 136.01, 135.03, 134.52, 134.37, 129.22, 128.06, 128.00, 127.97, 127.40, 127.10, 126.79, 126.00, 125.30, 124.79, 124.00, 123.44, 122.59, 122.30, 121.17, 120.92, 120.38, 120.30, 120.00, 119.63, 119.30, 110.57, 109.87, 103.35, 46.98, 27.18.
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HRMS (ESI, m/z): [M+H]+ calculated for C53H39N3, 717.3144, found 717.3346. Anal. calcd for C53H39N3: C 88.67, H 5.48, N 5.85. found: C 88.27, H 5.49, N 5.62.
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3. Results and discussion
3.1. Synthesis and characterization The synthetic routes of the intermediates and three target compounds are shown in Scheme 1.
The intermediates 1-3 were synthesized by Ullmann and Buchwald-Hartwig reaction. The target compounds were prepared by Suzuki coupling reaction, then purified by flash column chromatography and recrystallized with yields of 59.0 %, 67.3 % and 69.6 %, respectively. The molecular structures of target compounds were confirmed by 1H NMR, 13C NMR, high-resolution mass spectrometry (HRMS) and elemental analysis.
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Scheme 1 Synthetic routes of the compounds
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3.2. Photophysical properties
The UV-vis absorption and fluorescence emission spectra of these three compounds were
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showed in Fig. 1, which were measured in tetrahydrofuran solution. It can be seen that, the UV-vis spectra of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were quite similar with maximum peaks and shoulders, the intense UV-vis absorption peaks (~278 nm) of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz could be attributed to the π-π* transitions of the conjugated aromatic segments, the weak absorption peaks of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz are respectively located at around 343 nm, 352 nm and 345 nm, which should be assigned to the n-π* transition for the extended conjugation of the aromatic amines.[29] The optical energy bandgaps (Eg) of the three compounds calculated from the onset of the absorption spectra were determined to be 2.88 eV, 2.82 eV and 2.86 eV, respectively. In terms of emission, the maximum PL emission peaks were located at 406 nm, 415 nm and 431 nm for FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz,
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respectively. The detailed data are summarized in Table 1.
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Fig. 1 Normalized UV-vis absorption and fluorescence emission spectra of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz in tetrahydrofuran solution at room temperature
3.3. Thermal properties
The thermal properties of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were investigated by thermogravimetric analysis (TGA) and differential scanning calorimetry analysis (DSC) under
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nitrogen atmosphere with a heating rate of 10
/min. As shown in Fig. 2 and Table 1, the
decomposition temperatures (Td, correspond to 5% weight loss) of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were measured to be 443
, 420
and 475
, respectively. Meanwhile, the , 122
and 136
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corresponding glass transition temperature (Tg) were determined to be 127
for FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, respectively. These values are higher than that of ),[30] which suggesting better morphological stability of FIPN-based compounds.
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NPB (Tg, 98
Therefore, these three compounds are expected to be potential alternatives of NPB from their excellent thermal stability behavior.
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3.4. Electrochemical properities
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Fig. 2 TGA and DSC curves of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
The electrochemical behavior of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were investigated by cyclic voltammograms (CV) using a standard three-electrode electrochemical cell in an electrolyte solution (0.1M TBAPF6/DCM), with ferrocene as an external reference. As
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shown in Fig. 3 and Table 1, the highest occupied molecular orbital (HOMO) energy level of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were calculated to be -5.26 eV, -5.19 eV and -5.27 eV, respectively, which is slightly higher than that of NPB (-5.5 eV). The corresponding LUMO values were calculated to be -2.38 eV for FIPN-p-PCz, -2.37 eV for FIPN-p-TPA and -2.41 eV for
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FIPN-DPCz respectively, according to the equation ELUMO = EHOMO + Eg. The energetically favorable HOMO energy levels of these three compounds are supposed to be further facilitate the
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hole injection and transport properties from anode side.
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Fig. 3 Cyclic voltammograms of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz in dichloromethane containing 0.1 M TBAPF6 electrolytes, scanning rate: 100 mV/s
Table 1 The physical properties of FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz. Absorption λmaxa [nm]
Emission λmaxa [nm]
Egb [eV]
HOMOc [eV]
LUMOd [eV]
Tge [ ]
T df [ ]
FIPN-p-PCz
343
406
2.88
-5.26
-2.38
127
443
FIPN-p-TPA
352
415
2.82
-5.19
-2.37
122
420
FIPN-DPCz
345
431
2.86
-5.27
-2.41
136
475
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Compounds
-5
Measured in tetrahydrofuran at a concentration of 1.0×10 M.
b
Estimated from onset of the absorption spectra (Eg = 1241/λonset).
c
The HOMO energy level was determined from cyclic voltammetry (EHOMO = -4.4 - Eox).[31]
d
The LUMO energy level was calculated by the equation: ELUMO = EHOMO + Eg.[31]
e
Measured by DSC.
f
Measured by TGA.
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a
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3.5. Electroluminescence of PhOLEDS To evaluate the hole-transporting properties of these three compounds, OLEDs with the
structure of [ITO/PEDOT (45nm)/HTL (40 nm)/CBP:Ir(ppy)3 (20 nm, 8wt%)/TmPyPB (40 nm)/LiF (0.6 nm)/Al (80 nm)] were fabricated using FIPN-p-PCz, FIPN-p-TPA or FIPN-DPCz as hole transporting layer (HTL), ITO and LiF/Al were used as the anode and composite cathode, respectively. PEDOT and TmPyPB were employed as hole injection layer (HIL) and electron transporting layer (ETL), respectively. CBP served as host, and Ir(ppy)3 was used as the emitting material. For comparison, the control device with structure of [ITO/PEDOT (45 nm)/NPB (40 nm)/CBP:Ir(ppy)3 (20 nm, 8wt %)/TmPyPB (40 nm)/LiF (0.6 nm)/Al (80 nm)] was also fabricated.
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shown in Fig. 5, Fig. 6 and Fig.7, and the relevant parameters are summarized in Table 2.
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Fig. 4 The energy level diagram of the materials in devices
Fig. 5 Current density-voltage-luminance (J-V-L) curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 6 External quantum efficiency-current density curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 7 Current efficiency-luminance-power efficiency curves of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz
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Fig. 8 EL spectra at 5 V of green PhOLEDs
As revealed in Fig. 5 and Table 2, the turn-on voltages were 3.3 V, 3.4 V, 3.2 V and 3.5 V for NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz, respectively. The device using FIPN-p-TPA as hole transporting layer exhibited the lowest turn-on voltage, which might be attributed to the suitable HOMO energy level of FIPN-p-TPA and efficient hole transport properties from triphenylamine group. The maximum luminance of devices based on FIPN-p-PCz and
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FIPN-DPCz were 59784 cd/m2 and 72052 cd/m2, which was comparable to the value of NPB based device (63458 cd/m2). Meanwhile, the maximum luminance of FIPN-p-TPA based device achieved 82680 cd/m2, which was nearly 130 % higher than that of NPB based ones. The high EL
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improvements from FIPN-p-TPA could be attributed to its proper HOMO energy level, which is more favorable for hole injection/transport from anode side to EML interface. 6
and
Fig.7
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Fig.
show
EQE-current
density
characteristic
and
current
efficiency-luminance-power efficiency characteristic, respectively. From Fig. 6, Fig. 7 and Table 2, it can be seen that, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz based devices exhibited much better performance than NPB based device. Especially, the FIPN-p-PCz based device showed the best performance with maximum EQE of 17.3 %, maximum current efficiency of 53.7 cd/A, and maximum power efficiency of 30.7 lm/W, respectively. The better performance of these three materials based devices might be ascribed to their high HOMO energy level and more balanced charge transfer than that of NPB. In addition, all these three devices showed small efficiencies roll-off over a wide range of luminance. For example, under the representative luminance of
ACCEPTED MANUSCRIPT 10000 cd/m2, the current efficiency of device based on FIPN-p-PCz still reached up to 45.9 cd/A. Furthermore, EL spectra of Fig. 8 suggested that the green emission peaks of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz are similarly located at 511 nm, with the corresponding CIE coordinates of (0.26, 0.61), (0.29, 0.63), (0.28, 0.62) and (0.26, 0.60),
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respectively.
Table 2 EL performance of devices based on NPB, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz. Vona [V]
Lmaxb [cd/m2]
η cc [cd/A]
η pc [lm/W]
ηextd [%]
NPB
3.3
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28.4, 26.8, 23.1
19.9, 19.4, 12.3
8.5
(0.26, 0.61)
FIPN-p-PCz
3.4
59784
53.7, 47.3, 45.9
30.7, 29.9, 23.0
17.3
(0.29, 0.63)
FIPN-p-TPA
3.2
82680
41.9, 40.5, 38.5
28.3, 27.8, 21.5
12.4
(0.28, 0.62)
FIPN-DPCz
3.5
72052
50.8, 50.1, 46.3
31.6, 31.0, 22.3
15.4
(0.26, 0.60)
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Compounds
CIE(x,y)e
a
Turn-on voltage to give a luminance of 1 cd/m2.
b
Lmax: maximum luminance.
c
Order of measured values: maximum, then at 1000 cd/m2 and 10000 cd/m2. ηc: current efficiency. ηp: power
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efficiency. d
maximum values. ηext: external quantum efficiency.
e
Measured from the EL spectra at 5 V by inverting chromaticity coordinates on the CIE 1931 diagram.
4. Conclusion
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In conclusion, three hole transport materials, FIPN-p-PCz, FIPN-p-TPA and FIPN-DPCz were synthesized and characterized, all these three compounds exhibited high HOMO energy level
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and excellent thermal stability. The electroluminescent properties of green PhOLEDs using these three compounds as hole transport layer were investigated, and NPB was introduced as comparison. The results indicated that three synthesized compounds exhibited superior properties than NPB in green PhOLEDs, the device using FIPN-p-PCz as hole transport material showed the best performance with current efficiency and external quantum efficiency of 53.7 cd/A and 17.3 %, respectively, which are almost twice that of NPB. The above results indicated that FIPN-based derivatives are promising candidates for HTMs in OLEDs and its related optoelectronic devices.
ACCEPTED MANUSCRIPT Notes and references [1] Tang CW, VanSlyke SA. Organic electroluminescent diodes. Appl Phys Lett 1987; 51: 913-5. [2] 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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[20] Romain M, Quinton C, Tondelier D, Geffroy B, Jeannin O, Berthelot JR, et al. Thioxanthene and dioxothioxanthene dihydroindeno[2,1-b]fluorenes: synthesis, properties and applications in green and sky blue phosphorescent OLEDs. J Mater Chem C 2016; 4: 1692-703. [21] Romain M, Chevrier M, Bebiche S, Brahim TM, Berthelot JR, Jacques E, et al. The structure-property
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ACCEPTED MANUSCRIPT Electronic Supplementary Information (ESI) Highly efficient fluorene/indole-based hole transport materials for green PhOLEDs Panpan Wu,[a] Wenxuan Song,[a] Zhenyuan Xia,[b] Yi Chen,[a] Guojian Tian,[a] Jinhai Huang,*[c] and Jianhua Su*[a] a
Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
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of Science & Technology, Shanghai 200237, P. R. China. Fax: (86)21-64252288；Tel: (86)2164252288；E-mail: [email protected] b
Istituto per la Sintesi Organica e la Fotoreattività - Consiglio Nazionale delle Ricerche, via
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Shanghai Taoe Chemical Technology Co., Ltd, Shanghai, P. R. China. E-mail: [email protected]
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c
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Gobetti 101, 40129 Bologna, Italy
ACCEPTED MANUSCRIPT Contents OLED fabrication procedure, 1H NMR,
13
C NMR, HRMS of intermediates and three target
compounds.
OLED fabrication
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In general, organic layers were deposited on the indium-tin-oxide (ITO)/glass substrate by thermal evaporation. Before use, the ITO/glass substrate was cleaned sequentially by detergent, de-ionized water and ethanol. Then the ITO/glass was treated by oxygen (O2) plasma before
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loading into a 10-source evaporator, with a base pressure of 5.0 × 10-4 Pa, for device fabrication.
Fig. S1 1H NMR of intermediate 1
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Fig. S2 1H NMR of intermediate 2
Fig. S3 13C NMR of intermediate 2
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Fig. S4 1H NMR of intermediate 3
Fig. S5 13C NMR of intermediate 3
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Fig. S6 1H NMR of FIPN-p-PCz
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Fig. S7 13C NMR of FIPN-p-PCz
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Fig. S8 Mass spectrometry of FIPN-p-PCz
Fig. S9 1H NMR of FIPN-p-TPA
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Fig. S10 13C NMR of FIPN-p-TPA
Fig. S11 Mass spectrometry of FIPN-p-TPA
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Fig. S12 1H NMR of FIPN-DPCz
Fig. S13 13C NMR of FIPN-DPCz
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Fig. S14 Mass spectrometry of FIPN-DPCz
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1. The research was based on indole and fluorene, which is excellent hole transport moieties. 2. The three synthesized compounds possess high thermal stabilities, which can be used in OLED stably. 3. The OLEDs based on the three synthesized materials indicated much higher efficiency than NPB in the same device structure.