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A zig-zag type bidibenzofuran based host material for green phosphorescent organic light-emitting diodes
Dyes and Pigments 114 (2015) 278e282
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Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A zig-zag type bidibenzofuran based host material for green phosphorescent organic light-emitting diodes Yu Jin Kang, Sang Kyu Jeon, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2014 Received in revised form 27 November 2014 Accepted 2 December 2014 Available online 9 December 2014
A zig-zag type carbazole modified bidibenzofuran compound, 2,2'-di(carbazol-9-yl)-4,4'-bidibenzo[b,d] furan, was synthesized as the host material for green phosphorescent organic light-emitting diodes by coupling two 9-(dibenzo[b,d]furan-2-yl)carbazole moieties via 4- position of dibenzofuran. The carbazole modified bidibenzofuran host showed a triplet energy of 2.75 eV and the highest occupied molecular orbital/the lowest unoccupied molecular orbital of 6.10 eV/2.87 eV for hole and electron injection. The bidibenzofuran host was doped with green emitting tris(2-phenylpyridine) iridium and high quantum efficiency of 20.6% was achieved in the green phosphorescent organic light-emitting diodes. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Bidibenzofuran Carbazole modification Green device High efficiency Phosphorescent device Triplet energy
1. Introduction Dibenzofuran is an aromatic compound with two benzene rings fused to a central furan ring and has been used as a building block of triplet host materials because of high triplet energy, good charge transport properties and facile modification by electrophilic substitution reaction [1e11]. Dibenzofuran was modified with Br at 2- or 8positions by direct halogenation [1e4], [12] and was modified at 4- or 6- position by lithiation [5e8], [13]. Therefore, various dibenzofuran derivatives could be designed and synthesized as the host materials for phosphorescent organic light-emitting diodes (OLEDs). Most dibenzofuran derivatives have been developed for use as the host materials for blue phosphorescent OLEDs due to high triplet energy of the dibenzofuran moiety [1e11]. Typically, the dibenzofuran core was substituted with diphenylphosphine oxide to keep the high triplet energy of the dibenzofuran core and to produce electron transport type host materials [1], [5e8]. Monosubstitution of diphenylphosphine oxide at 4- position or disubstitution of diphenylphosphine oxide at 2- and 8-positions of dibenzofuran was effective to develop electron transport type host materials with a high triplet energy above 3.0 eV. The dibenzofuran was also substituted with carbazole [2e4], [10] or triphenylsilyl [8]
* Corresponding author. Fax: þ82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). http://dx.doi.org/10.1016/j.dyepig.2014.12.001 0143-7208/© 2014 Elsevier Ltd. All rights reserved.
moiety for better charge transport properties and could improve the external quantum efficiency of blue phosphorescent OLEDs. Additionally, hybrid host materials of dibenzofuran and spirobifluorene were also applied as the host materials for green and blue phosphorescent OLEDs [9]. Although several classes of dibenzofuran compounds were reported as the host materials for phosphorescent OLEDs, there has been no study about bidibenzofuran compounds as the host materials. Herein, we describe the synthesis and device performances of a zig-zag type bidibenzofuran derivative, 2,2'-di(carbazol-9-yl)-4,4'bidibenzo[b,d]furan (BDBFCz), as the host material for green phosphorescent OLEDs. Photophysical characterization of BDBFCz was carried out and the device performances of the BDBFCz based green phosphorescent OLEDs were studied. It was demonstrated that the BDBFCz host showed high external quantum efficiency of 20.6% in the green phosphorescent OLEDs doped with iridium (III) tris(2-phenylpyridine) (Ir(ppy)3). 2. Experimental 2.1. General information Dibenzofuran and diiodoethane were purchased from TCI. Co. Tetrahydrofuran (THF), n-hexane, magnesium sulfate anhydrous, sulfuric acid, acetic acid, 1,4-dioxane, K3PO4, K2CO3 and Na2S2O3 were purchased from Duksan Sci. Co. 9H-Carbazole, periodic acid,
Y.J. Kang et al. / Dyes and Pigments 114 (2015) 278e282
cupper iodide, Triethylborate and n-BuLi were purchased from Aldrich. Chem. Co. Tetrakis(triphenylphosphine)palladium, were purchased from P&H Co. These chemical were used without further purification. THF was distilled over sodium and calcium hydride. Chemical characterization of the synthesized material was carried out according to the method reported in the literature [14]. 2.2. Synthesis 2-Iododibenzofuran Dibenzofuran (10.0 g, 59.4 mmol) and periodic acid (16.2 g, 71.3 mmol) were dissolved in acetic acid (600 ml) and iodine (9.0 g, 71.3 mmol) was added to the solution. The solution was stirred at 60 C for 30 min followed by addition of distilled water (120 ml) and sulfuric acid (1.20 ml). The solution was refluxed for 12 h, cooled to room temperature and poured into distilled water. White powder was filtered off and then the filtrate was extracted using ethyl acetate. The extracted solution was washed with 5% NaOH solution and aqueous sodium sulfate. Ethyl acetate was removed by evaporation and yellowish white powder was obtained (12.0 g, yield: 69%) after drying in vacuum. 2Iododibenzofuran was included 85% of all powder and it was used in amination without purification. 9-(Dibenzofuran-2-yl)-9H-carbazole 2-Iododibenzofuran (11.0 g, 37.4 mmol) and carbazole (7.2 g, 43.0 mmol) were dissolved in 1,4-dioxane (250 ml) at room temperature under nitrogen. After 30 min, trans-1,2-diaminocyclohexane (2.0 ml,18.7 mmol) was added to the solution and the solution was refluxed overnight. After cooling to room temperature, the solution was extracted using methylene chloride. Methylene chloride was removed by evaporation and obtained product was purified by column chromatography on silica gel using toluene/n-hexane (1: 2) as an eluent. A white powder was obtained as a product (7.9 g, yield: 63%). 1 H NMR (400 MHZ, CDCl3): d 8.17 (d, 2H, J ¼ 3.8 Hz), 8.11 (d, 1H, J ¼ 1 Hz), 7.94 (d, 1H, J ¼ 4 Hz), 7.78 (d, 1H, J ¼ 4 Hz), 7.66e7.59 (m, 2H), 7.54e7.50 (m, 1H), 7.41e7.35 (m, 5H), 7.31e7.25 (m, 2H). 9-(4-Iododibenzofuran-2-yl)-9H-carbazole 9-(Dibenzofuran2-yl)-9H-carbazole (3.9 g, 12.0 mmol) was dissolved in dry tetrahydrofuran (50 ml) under a nitrogen atmosphere at room temperature. The solution was cooled to 78 C and 2.5 M nbutyllithium solution (5.4 ml, 14.0 mmol) was slowly added to the solution. After 3 h, the mixture was warmed to room temperature for 30 min and then cooled to 78 C. Diiodoethane (3.8 g, 14.0 mmol) in dry tetrahydrofuran (30 ml) was slowly added to the solution and was stirred for overnight. The reaction was quenched with distilled water and the solution was extracted using methylene chloride and aqueous sodium sulfate solution. The methylene chloride layer was separated and dried in vacuum oven after removing methylene chloride by evaporation. A white powder was obtained as a crude product and it was used without further purification. (2-(9H-carbazol-9-yl)dibenzofuran-4-yl)boronic acid 9(Dibenzofuran-2-yl)-9H-carbazole (4.0 g, 12.0 mmol) was dissolved in dry tetrahydrofuran (50 ml) under a nitrogen atmosphere at room temperature. The solution was cooled to 78 C and 2.5 M nbutyllithium solution (5.5 ml, 14.0 mmol) was slowly added. After 3 h, the mixture was warmed to room temperature and cooled to 78 C. Triethylborate (2.4 ml, 14.0 mmol) was slowly added to the solution and the solution was stirred at 78 C for 3 h. The reaction was quenched with a 5% HCl solution and the solution was extracted with methylene chloride. After removing methylene chloride, crude product was obtained as a white powder. The crude product was used for the coupling reaction without further purification. 2,2′-Di(9H-carbazol-9-yl)-4,4'-bidibenzofuran 9-(4Iododibenzofuran-2-yl)-9H-carbazole (1.4 g, 5.0 mmol) and (2-
279
(9H-carbazol-9-yl)dibenzofuran-4-yl)boronic acid (1.9 g, 5.0 mmol) were dissolved in dry tetrahydrofuran under nitrogen. Tetrakis(triphenylphosphine)palladium(0) (37.0 mg, 0.2 mmol) and 2 M aqueous potassium carbonate (8.3 g, 60.0 mmol) were added to the reaction mixture successively and the solution was refluxed for overnight. After cooling the solution to room temperature, a white powder was filtered off and a filtrate was extracted using ethyl acetate. Ethyl acetate was evaporated and a solid product was purified by column chromatography on silica gel using chloroform/nhexane (1: 2) as an eluent. A white product was obtained after purification by vacuum train sublimation (2.2 g, yield: 66%). 1 H NMR (400 MHZ, CDCl3): d 8.46 (d, 2H, J ¼ 1.0 Hz), 8.226 (m, 6H), 8.00 (d, 2H, J ¼ 3.8 Hz), 7.72 (d, 4H, J ¼ 4 Hz), 7.63 (d, 2H, J ¼ 4 Hz), 7.54e7.50 (m, 6H), 7.42e7.34 (m, 6H) 13C NMR (100 MHZ, CDCl3): d 156.92, 152.31, 141.42, 133.01, 128.19, 127.54, 126.64, 126.14, 123.82, 123.56, 123.40, 121.22, 121.07, 120.49, 120.17, 119.17, 112.25, 110.03, MS (FAB) miz 664 [(M)þ]. Elemental Analysis (calculated for C48H28N2O2): C, 86.73; H, 4.25; N, 4.21; O, 4.81. Found: C, 86.31; H, 4.24; N, 4.12; O, 5.05. 2.3. Device fabrication and measurements Green phosphorescent OLEDs were fabricated using vacuum evaporation process by stacking 4,4'-cyclohexylidenebis[N,N-bis(4methylphenyl)aniline] (TAPC, 20 nm), N,N'-dicarbazolyl-3,5benzene (mCP, 10 nm), BDBFCz:Ir(ppy)3 (25 nm), diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 30 nm), LiF (1 nm) and Al (200 nm) on poly(3,4-ethylenedioxythiophene); poly(styrenesulfonate) (PEDOT:PSS, 60 nm) coated indium tin oxide (ITO, 120 nm) substrate. The BDBFCz:Ir(ppy)3 emitting layer was deposited by co-evaporation of BDBFCz and Ir(ppy)3 at a relative weight ratio of 97:3 (3%), 95:5 (5%) and 90:10 (10%). A control device with 4,40 -bis(N-carbazolyl)-1,10 -biphenyl (CBP) as a host was fabricated for comparison. Hole only device was fabricated by depositing TAPC (30 nm), mCP (10 nm), BDBFCz (25 nm) and Al (200 nm) on ITO (120 nm)/PEDOT:PSS (60 nm) substrate and electron only device was prepared by evaporating Ca (5 nm), BDBFCz (25 nm), TSPO1 (30 nm), LiF (1 nm) and Al (200 nm), consecutively. Green phosphorescent OLEDs, hole only and electron only devices were protected from moisture and oxygen by encapsulating the device with a glass cover with a desiccant inside. Electrical characterization of the green phosphorescent OLEDs and single carrier devices was carried out in ambient condition using Keithley 2400 source measurement unit and CS 2000 spectroradiometer. 3. Results and discussion Recently, there was a report that carbazole modified dibenzofuran compounds could improve the quantum efficiency of the mixed host based phosphorescent OLEDs due to high triplet energy and good hole transport properties [4]. Triplet excitons and carriers were confined in the emitting layer and efficient light emission was observed in the mixed host device of the carbazole modified dibenzofuran host. That work motivated us to develop a bidibenzofuran derivative as the host material for phosphorescent OLEDs because better thermal stability and high quantum efficiency can be obtained by proper design of the bidibenzofuran based host material. The design concept of the BDBFCz was to develop 4,4’-bidibenzofuran core based carbazole compound for high triplet energy, high thermal stability and good charge transport properties. In particular, the bidibenzofuran core can improve the thermal stability significantly compared with a common dibenzofuran core structure because of a rigid zig-zag type design.
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Scheme 1. Synthetic scheme of BDBFCz.
Fig. 1. UVevis, solution PL and low temperature PL of BDBFCz.
Synthetic scheme of BDBFCz is shown in Scheme 1. The BDBFCz host was synthesized by Suzuki coupling reaction between boronic acid of 2-(9-carbazolyl)dibenzofuran and iodinated 2-(9carbazolyl)dibenzofuran. Both boronic acid and iodine functional groups were introduced by activation of 4- position of 2-(9carbazolyl)dibenzofuran using butyllithium as reported in other works [4]. The coupling reaction of the two intermediates produced the BDBFCz as a white powder at a synthetic yield of 63% after purification by column chromatography and vacuum train
sublimation. Chemical structure of the final compound was confirmed by 1H and 13C nuclear magnetic resonance, mass and elemental analysis. Purity of the sublimed product was above 99%. Photophysical analysis of the host materials was performed by ultravioletevisible (UVevis) and photoluminescence (PL) measurements. Normalized UVevis absorption, solution PL and low temperature PL spectra in liquid nitrogen are presented in Fig. 1. Intense pp* absorption by the carbazole modified bidibenzofuran core was observed at short wavelength below 300 nm and weak np* absorption by lone pair electrons of dibenzofuran and carbazole was detected above 300 nm, which was extended to 370 nm. Fluorescent emission peak of BDBFCz in tetrahydrofuran solvent was positioned at 405 nm and phosphorescent emission peak position in liquid nitrogen was 450 nm, which corresponded to a triplet energy of 2.75 eV. The coupling of two dibenzofuran moieties reduced the triplet energy of the BDBFCz host compared to that of carbazole modified dibenzofuran host by 0.15 eV [4]. Although the two dibenzofuran moieties were linked through 4position of dibenzofuran, extension of conjugation resulted in the reduction of the triplet energy. The dihedral angle between two dibenzofuran moieties was 31.4 . The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of BDBFCz was calculated from the optimized geometrical structure using B3LYP 631G* basis set of Gaussian 09 program. Orbital distribution of the HOMO and LUMO of BDBFCz is shown in Fig. 2. Carbazole is an electron rich moiety because lone pair electrons of nitrogen are shared with aromatic units of carbazole. Dibenzofuran is also an electron rich moiety because of lone pair electron sharing between oxygen and aromatic units of dibenzofuran. However, high electronegativity of oxygen compared to that of nitrogen makes
Fig. 2. Molecular orbital simulation results of bidibenzofuran.
Y.J. Kang et al. / Dyes and Pigments 114 (2015) 278e282
Fig. 3. Oxidation and reduction scan curves of BDBFCz.
281
Fig. 5. Current densityevoltage-luminance curves of green phosphorescent organic light-emitting diodes with BDBFCz host.
dibenzofuran relatively more electron deficient than carbazole. Therefore, the HOMO and LUMO were dispersed over carbazole and bidibenzofuran, respectively. It can be inferred from the molecular orbital distribution that electron transport properties of BDBFCz would be dominated by the bidibenzofuran core. Cyclic voltammetry scan of BDBFCz was performed to characterize the oxidation and reduction behaviour of BDBFCz. Oxidation and reduction curves of BDBFCz are presented in Fig. 3. Oxidation and reduction potentials of BDBFCz were 1.22 V and 2.01 V, respectively, which corresponded to an ionization potential (IP) and electron affinity (EA) of 6.02 eV and 2.79 eV. The IP of BDBFCz followed the oxidation potential of carbazole and the EA matched
Fig. 6. Quantum efficiencyeluminance curves of green phosphorescent organic lightemitting diodes with BDBFCz host.
with the reduction of bidibenzofuran as the HOMO and LUMO were localized on the carbazole and bidibenzofuran moieties, respectively. The HOMO and LUMO can be estimated to be 6.02 eV and 2.79 eV from the IP and EA. Thermal properties of BDBFCz were analysed using differential scanning calorimeter (DSC) and thermogravimetric analyser (TGA). Melting temperature of BDBFCz was 335 C and glass transition temperature was 181 C in the DSC scans. Thermal decomposition temperature at 5% weight loss was 419 C in the TGA thermogram.
Fig. 4. Current densityevoltage curves of hole only and electron only devices of BDBFCz (a) and CBP (b).
Fig. 7. EL spectra of green phosphorescent organic light-emitting diodes with BDBFCz host.
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Y.J. Kang et al. / Dyes and Pigments 114 (2015) 278e282
Table 1 Device performances of BDBFCz green phosphorescent OLEDs. Doping concentration
3% 5% 10% a
Voltage (V)
Quantum efficiency (%)
Current efficiency (cd/A)
Power efficiency (lm/W)
Turn-ona1000 cd/m2
Max1,000 cd/m2
Max1,000 cd/m2
Max1,000 cd/m2
3.7 6.6 3.7 6.6 3.7 6.5
20.6 16.0 20.6 18.3 20.0 17.7
80.2 76.3 71.1 63.8 69.7 60.3
47.9 25.3 43.4 30.5 47.0 29.7
Color index
0.28,0.63 0.29,0.63 0.30,0.63
Voltage at 0.1 cd/m2.
The BDBFCz host material exhibited good thermal stability due to the rigid zig-zag type bidibenzofuran core. The glass transition temperature and decomposition temperature of the BDBFCz host were much higher than those of the dibenzofuran compound known as a thermally stable host material [9]. The increase of glass transition temperature by about 60 C and thermal decomposition temperature by more than 100 C was observed. Hole and electron current densities of BDBFCz were compared using hole and electron only devices and the current densityvoltage data of the single carrier devices are shown in Fig. 4. The hole current density of the BDBFCz device was higher than the electron current density, which suggests that BDBFCz shows better hole transport properties than electron transport properties. However, the difference of hole and electron current densities was less than an order, implying that BDBFCz has a bipolar charge transport character. The hole and electron current densities were similar at high voltage. In the case of CBP, the hole current density was much higher than that of electron current density. Device performances of BDBFCz host material were investigated by doping green emitting Ir(ppy)3 dopant at doping concentrations of 3, 5 and 10%. Current density and luminance of BDBFCz devices at different doping concentrations were plotted against driving voltage in Fig. 5. Although the current density was slightly reduced at high doping concentration by charge trapping effect of Ir(ppy)3, the luminance was maintained constant irrespective of the doping concentration because of high recombination efficiency at high doping concentration as shown in the quantum efficiencyeluminance curves in Fig. 6. Quantum efficiency of the BDBFCz devices was presented against luminance in Fig. 6. Maximum quantum efficiency at 3% and 5% doping concentrations was 20.6%, but it was 19.9% at 10% doping concentration owing to concentration quenching effect. The BDBFCz device was much better than the CBP device in terms of quantum efficiency. Although the molecular structures of BDBFCz and CBP were derived from hole transport carbazole moieties, the bidibenzofuran core facilitated electron injection from the TSPO1 electron transport layer to the emitting layer compared to biphenyl core of CBP as can be inferred from the deeper LUMO level of BDBFCz (2.79 eV) than that of CBP (2.40 eV). The better electron injection of BDBFCz enhanced the quantum efficiency of the BDBFCz device through charge balance in the emitting layer. The bipolar charge transport properties of BDBFCz as shown in the single carrier device data support this explanation. The reduction of the high maximum quantum efficiency at high luminance is mostly due to tripletetriplet and triplet-polaron quenching effect. The BDBFCz device exhibited typical electroluminescence (EL) spectra of the Ir(ppy)3 doped BDBFCz green devices as presented in Fig. 7. Peak position of the green emission was 509 nm at 3% doping concentration and it was shifted to 513 nm at 10% doping concentration by intermolecular interaction of Ir(ppy)3 at high doping concentration. At all doping concentrations, no host or hole transport layer emission was observed owing to charge confinement and
complete energy transfer. Color coordinate of the BDBFCz device was (0.28,0.63) at 3% doping concentration and it was red-shifted to (0.30,0.63) at 10% doping concentration. All device performances of the BDBFCz devices are summarized in Table 1. 4. Conclusions BDBFCz compound derived from carbazole and bidibenzofuran was synthesized by Suzuki coupling reaction of carbazolyldibenzofuran moieties. The coupling of two carbazolyldibenzofuran moieties induced bipolar charge transport properties and high triplet energy of 2.75 eV. The quantum efficiency of the BDBFCz device was better than that of the CBP device and reached 20.6% by doping Ir(ppy)3. Therefore, the bidibenzofuran core may be useful to develop bipolar host materials for phosphorescent OLEDs. References [1] Vecchi PA, Padmaperuma AB, Qiao H, Sapochak LS, Burrows PE. A dibenzofuran-based host material for blue electrophosphorescence. Org Lett 2006;8:4211e4. [2] Jeong SH, Lee JY. Carbazolyldibenzofuran-type high-triplet-energy bipolar host material for blue phosphorescent organic light-emitting diodes. J Lumin 2014;146:333e6. [3] Jeong SH, Lee JY. High power efficiency in blue phosphorescent organic lightemitting diodes using 2,4-substituted dibenzofuran with a carbazole and a diphenylphosphine oxide. Org Electron 2012;13:2589e93. [4] Jeong SH, Seo CW, Lee JY, Cho NS, Kim JK, Yang JH. Comparison of bipolar hosts and mixed-hosts as host structures for deep-blue phosphorescent organic light emitting diodes. Chem Asia J 2011;6:2895e8. [5] Han C, Xie G, Li J, Zhang Z, Xu H, Deng Z, et al. A new phosphine oxide host based on ortho-disubstituted dibenzofuran for efficient electrophosphorescence: towards high triplet state excited levels and excellent thermal, morphological and efficiency stability. Chem Eur J 2011;17:8947e56. [6] Han C, Xie G, Xu H, Zhang Z, Yu D, Zhao Y, et al. Towards highly efficient bluephosphorescent organic light-emitting diodes with low operating voltage and excellent efficiency stability. Chem Eur J 2011;17:445e9. [7] Han C, Xie G, Xu H, Zhang Z, Xie L, Zhao Y, et al. A single phosphine oxide host for high-efficiency White organic light-emitting diodes with extremely low operating voltages and reduced efficiency roll-off. Adv Mater 2011;23:2491. [8] May F, Al-Helwi M, Baumeier B, Kowalsky W, Fuchs E, Lennartz C, et al. Design rules for charge-transport efficient host materials for phosphorescent organic light-emitting diodes. J Am Chem Soc 2012;134:13818e22. [9] Dong SC, Gao CH, Zhang ZH, Jiang ZQ, Lee ST, Liao LS. New dibenzofuran/ spirobifluorene hybrids as thermally stable host materials for efficient phosphorescent organic light-emitting diodes with low efficiency roll-off. Phys Chem Chem Phys 2012;14:14224e8. [10] Lee CW, Seo J, Gong M, Lee JY. The effect of the substitution position of dibenzofuran on the photophysical and charge-transport properties of host materials for phosphorescent organic light-emitting diodes. Chem Eur J 2013;19:1194e8. [11] Jeong SH, Lee JY. Dibenzofuran derivative as high triplet energy host material for high efficiency in deep blue phosphorescent organic light-emitting diodes. Org Electron 2012;7:1141e5. [12] Sovocool GW, Munslow WD, Donnelly JR, Mitchum RK. Electrophilic bromination of dibenzofuran. Chemosphere 1987;16:221e4. [13] Haenel MW, Jakubik D, Rothenberger E, Schroth G. Bidentate phosphines of heteroarenes: 4,6-Bis(diphenylphosphino)dibenzofuran and 4,6-Bis(diphenylphosphino)dibenzothiophene. Chem Ber 1991;124:1705e10. [14] Lee CW, Lee JY. Structureproperty relationship of pyridoindole-type host materials for high-efficiency blue phosphorescent organic light-emitting diodes. Chem Mater 2014;26:1616e21.
Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig
A zig-zag type bidibenzofuran based host material for green phosphorescent organic light-emitting diodes Yu Jin Kang, Sang Kyu Jeon, Jun Yeob Lee* School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 October 2014 Received in revised form 27 November 2014 Accepted 2 December 2014 Available online 9 December 2014
A zig-zag type carbazole modified bidibenzofuran compound, 2,2'-di(carbazol-9-yl)-4,4'-bidibenzo[b,d] furan, was synthesized as the host material for green phosphorescent organic light-emitting diodes by coupling two 9-(dibenzo[b,d]furan-2-yl)carbazole moieties via 4- position of dibenzofuran. The carbazole modified bidibenzofuran host showed a triplet energy of 2.75 eV and the highest occupied molecular orbital/the lowest unoccupied molecular orbital of 6.10 eV/2.87 eV for hole and electron injection. The bidibenzofuran host was doped with green emitting tris(2-phenylpyridine) iridium and high quantum efficiency of 20.6% was achieved in the green phosphorescent organic light-emitting diodes. © 2014 Elsevier Ltd. All rights reserved.
Keywords: Bidibenzofuran Carbazole modification Green device High efficiency Phosphorescent device Triplet energy
1. Introduction Dibenzofuran is an aromatic compound with two benzene rings fused to a central furan ring and has been used as a building block of triplet host materials because of high triplet energy, good charge transport properties and facile modification by electrophilic substitution reaction [1e11]. Dibenzofuran was modified with Br at 2- or 8positions by direct halogenation [1e4], [12] and was modified at 4- or 6- position by lithiation [5e8], [13]. Therefore, various dibenzofuran derivatives could be designed and synthesized as the host materials for phosphorescent organic light-emitting diodes (OLEDs). Most dibenzofuran derivatives have been developed for use as the host materials for blue phosphorescent OLEDs due to high triplet energy of the dibenzofuran moiety [1e11]. Typically, the dibenzofuran core was substituted with diphenylphosphine oxide to keep the high triplet energy of the dibenzofuran core and to produce electron transport type host materials [1], [5e8]. Monosubstitution of diphenylphosphine oxide at 4- position or disubstitution of diphenylphosphine oxide at 2- and 8-positions of dibenzofuran was effective to develop electron transport type host materials with a high triplet energy above 3.0 eV. The dibenzofuran was also substituted with carbazole [2e4], [10] or triphenylsilyl [8]
* Corresponding author. Fax: þ82 31 8005 3585. E-mail address: [email protected] (J.Y. Lee). http://dx.doi.org/10.1016/j.dyepig.2014.12.001 0143-7208/© 2014 Elsevier Ltd. All rights reserved.
moiety for better charge transport properties and could improve the external quantum efficiency of blue phosphorescent OLEDs. Additionally, hybrid host materials of dibenzofuran and spirobifluorene were also applied as the host materials for green and blue phosphorescent OLEDs [9]. Although several classes of dibenzofuran compounds were reported as the host materials for phosphorescent OLEDs, there has been no study about bidibenzofuran compounds as the host materials. Herein, we describe the synthesis and device performances of a zig-zag type bidibenzofuran derivative, 2,2'-di(carbazol-9-yl)-4,4'bidibenzo[b,d]furan (BDBFCz), as the host material for green phosphorescent OLEDs. Photophysical characterization of BDBFCz was carried out and the device performances of the BDBFCz based green phosphorescent OLEDs were studied. It was demonstrated that the BDBFCz host showed high external quantum efficiency of 20.6% in the green phosphorescent OLEDs doped with iridium (III) tris(2-phenylpyridine) (Ir(ppy)3). 2. Experimental 2.1. General information Dibenzofuran and diiodoethane were purchased from TCI. Co. Tetrahydrofuran (THF), n-hexane, magnesium sulfate anhydrous, sulfuric acid, acetic acid, 1,4-dioxane, K3PO4, K2CO3 and Na2S2O3 were purchased from Duksan Sci. Co. 9H-Carbazole, periodic acid,
Y.J. Kang et al. / Dyes and Pigments 114 (2015) 278e282
cupper iodide, Triethylborate and n-BuLi were purchased from Aldrich. Chem. Co. Tetrakis(triphenylphosphine)palladium, were purchased from P&H Co. These chemical were used without further purification. THF was distilled over sodium and calcium hydride. Chemical characterization of the synthesized material was carried out according to the method reported in the literature [14]. 2.2. Synthesis 2-Iododibenzofuran Dibenzofuran (10.0 g, 59.4 mmol) and periodic acid (16.2 g, 71.3 mmol) were dissolved in acetic acid (600 ml) and iodine (9.0 g, 71.3 mmol) was added to the solution. The solution was stirred at 60 C for 30 min followed by addition of distilled water (120 ml) and sulfuric acid (1.20 ml). The solution was refluxed for 12 h, cooled to room temperature and poured into distilled water. White powder was filtered off and then the filtrate was extracted using ethyl acetate. The extracted solution was washed with 5% NaOH solution and aqueous sodium sulfate. Ethyl acetate was removed by evaporation and yellowish white powder was obtained (12.0 g, yield: 69%) after drying in vacuum. 2Iododibenzofuran was included 85% of all powder and it was used in amination without purification. 9-(Dibenzofuran-2-yl)-9H-carbazole 2-Iododibenzofuran (11.0 g, 37.4 mmol) and carbazole (7.2 g, 43.0 mmol) were dissolved in 1,4-dioxane (250 ml) at room temperature under nitrogen. After 30 min, trans-1,2-diaminocyclohexane (2.0 ml,18.7 mmol) was added to the solution and the solution was refluxed overnight. After cooling to room temperature, the solution was extracted using methylene chloride. Methylene chloride was removed by evaporation and obtained product was purified by column chromatography on silica gel using toluene/n-hexane (1: 2) as an eluent. A white powder was obtained as a product (7.9 g, yield: 63%). 1 H NMR (400 MHZ, CDCl3): d 8.17 (d, 2H, J ¼ 3.8 Hz), 8.11 (d, 1H, J ¼ 1 Hz), 7.94 (d, 1H, J ¼ 4 Hz), 7.78 (d, 1H, J ¼ 4 Hz), 7.66e7.59 (m, 2H), 7.54e7.50 (m, 1H), 7.41e7.35 (m, 5H), 7.31e7.25 (m, 2H). 9-(4-Iododibenzofuran-2-yl)-9H-carbazole 9-(Dibenzofuran2-yl)-9H-carbazole (3.9 g, 12.0 mmol) was dissolved in dry tetrahydrofuran (50 ml) under a nitrogen atmosphere at room temperature. The solution was cooled to 78 C and 2.5 M nbutyllithium solution (5.4 ml, 14.0 mmol) was slowly added to the solution. After 3 h, the mixture was warmed to room temperature for 30 min and then cooled to 78 C. Diiodoethane (3.8 g, 14.0 mmol) in dry tetrahydrofuran (30 ml) was slowly added to the solution and was stirred for overnight. The reaction was quenched with distilled water and the solution was extracted using methylene chloride and aqueous sodium sulfate solution. The methylene chloride layer was separated and dried in vacuum oven after removing methylene chloride by evaporation. A white powder was obtained as a crude product and it was used without further purification. (2-(9H-carbazol-9-yl)dibenzofuran-4-yl)boronic acid 9(Dibenzofuran-2-yl)-9H-carbazole (4.0 g, 12.0 mmol) was dissolved in dry tetrahydrofuran (50 ml) under a nitrogen atmosphere at room temperature. The solution was cooled to 78 C and 2.5 M nbutyllithium solution (5.5 ml, 14.0 mmol) was slowly added. After 3 h, the mixture was warmed to room temperature and cooled to 78 C. Triethylborate (2.4 ml, 14.0 mmol) was slowly added to the solution and the solution was stirred at 78 C for 3 h. The reaction was quenched with a 5% HCl solution and the solution was extracted with methylene chloride. After removing methylene chloride, crude product was obtained as a white powder. The crude product was used for the coupling reaction without further purification. 2,2′-Di(9H-carbazol-9-yl)-4,4'-bidibenzofuran 9-(4Iododibenzofuran-2-yl)-9H-carbazole (1.4 g, 5.0 mmol) and (2-
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(9H-carbazol-9-yl)dibenzofuran-4-yl)boronic acid (1.9 g, 5.0 mmol) were dissolved in dry tetrahydrofuran under nitrogen. Tetrakis(triphenylphosphine)palladium(0) (37.0 mg, 0.2 mmol) and 2 M aqueous potassium carbonate (8.3 g, 60.0 mmol) were added to the reaction mixture successively and the solution was refluxed for overnight. After cooling the solution to room temperature, a white powder was filtered off and a filtrate was extracted using ethyl acetate. Ethyl acetate was evaporated and a solid product was purified by column chromatography on silica gel using chloroform/nhexane (1: 2) as an eluent. A white product was obtained after purification by vacuum train sublimation (2.2 g, yield: 66%). 1 H NMR (400 MHZ, CDCl3): d 8.46 (d, 2H, J ¼ 1.0 Hz), 8.226 (m, 6H), 8.00 (d, 2H, J ¼ 3.8 Hz), 7.72 (d, 4H, J ¼ 4 Hz), 7.63 (d, 2H, J ¼ 4 Hz), 7.54e7.50 (m, 6H), 7.42e7.34 (m, 6H) 13C NMR (100 MHZ, CDCl3): d 156.92, 152.31, 141.42, 133.01, 128.19, 127.54, 126.64, 126.14, 123.82, 123.56, 123.40, 121.22, 121.07, 120.49, 120.17, 119.17, 112.25, 110.03, MS (FAB) miz 664 [(M)þ]. Elemental Analysis (calculated for C48H28N2O2): C, 86.73; H, 4.25; N, 4.21; O, 4.81. Found: C, 86.31; H, 4.24; N, 4.12; O, 5.05. 2.3. Device fabrication and measurements Green phosphorescent OLEDs were fabricated using vacuum evaporation process by stacking 4,4'-cyclohexylidenebis[N,N-bis(4methylphenyl)aniline] (TAPC, 20 nm), N,N'-dicarbazolyl-3,5benzene (mCP, 10 nm), BDBFCz:Ir(ppy)3 (25 nm), diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 30 nm), LiF (1 nm) and Al (200 nm) on poly(3,4-ethylenedioxythiophene); poly(styrenesulfonate) (PEDOT:PSS, 60 nm) coated indium tin oxide (ITO, 120 nm) substrate. The BDBFCz:Ir(ppy)3 emitting layer was deposited by co-evaporation of BDBFCz and Ir(ppy)3 at a relative weight ratio of 97:3 (3%), 95:5 (5%) and 90:10 (10%). A control device with 4,40 -bis(N-carbazolyl)-1,10 -biphenyl (CBP) as a host was fabricated for comparison. Hole only device was fabricated by depositing TAPC (30 nm), mCP (10 nm), BDBFCz (25 nm) and Al (200 nm) on ITO (120 nm)/PEDOT:PSS (60 nm) substrate and electron only device was prepared by evaporating Ca (5 nm), BDBFCz (25 nm), TSPO1 (30 nm), LiF (1 nm) and Al (200 nm), consecutively. Green phosphorescent OLEDs, hole only and electron only devices were protected from moisture and oxygen by encapsulating the device with a glass cover with a desiccant inside. Electrical characterization of the green phosphorescent OLEDs and single carrier devices was carried out in ambient condition using Keithley 2400 source measurement unit and CS 2000 spectroradiometer. 3. Results and discussion Recently, there was a report that carbazole modified dibenzofuran compounds could improve the quantum efficiency of the mixed host based phosphorescent OLEDs due to high triplet energy and good hole transport properties [4]. Triplet excitons and carriers were confined in the emitting layer and efficient light emission was observed in the mixed host device of the carbazole modified dibenzofuran host. That work motivated us to develop a bidibenzofuran derivative as the host material for phosphorescent OLEDs because better thermal stability and high quantum efficiency can be obtained by proper design of the bidibenzofuran based host material. The design concept of the BDBFCz was to develop 4,4’-bidibenzofuran core based carbazole compound for high triplet energy, high thermal stability and good charge transport properties. In particular, the bidibenzofuran core can improve the thermal stability significantly compared with a common dibenzofuran core structure because of a rigid zig-zag type design.
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Scheme 1. Synthetic scheme of BDBFCz.
Fig. 1. UVevis, solution PL and low temperature PL of BDBFCz.
Synthetic scheme of BDBFCz is shown in Scheme 1. The BDBFCz host was synthesized by Suzuki coupling reaction between boronic acid of 2-(9-carbazolyl)dibenzofuran and iodinated 2-(9carbazolyl)dibenzofuran. Both boronic acid and iodine functional groups were introduced by activation of 4- position of 2-(9carbazolyl)dibenzofuran using butyllithium as reported in other works [4]. The coupling reaction of the two intermediates produced the BDBFCz as a white powder at a synthetic yield of 63% after purification by column chromatography and vacuum train
sublimation. Chemical structure of the final compound was confirmed by 1H and 13C nuclear magnetic resonance, mass and elemental analysis. Purity of the sublimed product was above 99%. Photophysical analysis of the host materials was performed by ultravioletevisible (UVevis) and photoluminescence (PL) measurements. Normalized UVevis absorption, solution PL and low temperature PL spectra in liquid nitrogen are presented in Fig. 1. Intense pp* absorption by the carbazole modified bidibenzofuran core was observed at short wavelength below 300 nm and weak np* absorption by lone pair electrons of dibenzofuran and carbazole was detected above 300 nm, which was extended to 370 nm. Fluorescent emission peak of BDBFCz in tetrahydrofuran solvent was positioned at 405 nm and phosphorescent emission peak position in liquid nitrogen was 450 nm, which corresponded to a triplet energy of 2.75 eV. The coupling of two dibenzofuran moieties reduced the triplet energy of the BDBFCz host compared to that of carbazole modified dibenzofuran host by 0.15 eV [4]. Although the two dibenzofuran moieties were linked through 4position of dibenzofuran, extension of conjugation resulted in the reduction of the triplet energy. The dihedral angle between two dibenzofuran moieties was 31.4 . The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distribution of BDBFCz was calculated from the optimized geometrical structure using B3LYP 631G* basis set of Gaussian 09 program. Orbital distribution of the HOMO and LUMO of BDBFCz is shown in Fig. 2. Carbazole is an electron rich moiety because lone pair electrons of nitrogen are shared with aromatic units of carbazole. Dibenzofuran is also an electron rich moiety because of lone pair electron sharing between oxygen and aromatic units of dibenzofuran. However, high electronegativity of oxygen compared to that of nitrogen makes
Fig. 2. Molecular orbital simulation results of bidibenzofuran.
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Fig. 3. Oxidation and reduction scan curves of BDBFCz.
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Fig. 5. Current densityevoltage-luminance curves of green phosphorescent organic light-emitting diodes with BDBFCz host.
dibenzofuran relatively more electron deficient than carbazole. Therefore, the HOMO and LUMO were dispersed over carbazole and bidibenzofuran, respectively. It can be inferred from the molecular orbital distribution that electron transport properties of BDBFCz would be dominated by the bidibenzofuran core. Cyclic voltammetry scan of BDBFCz was performed to characterize the oxidation and reduction behaviour of BDBFCz. Oxidation and reduction curves of BDBFCz are presented in Fig. 3. Oxidation and reduction potentials of BDBFCz were 1.22 V and 2.01 V, respectively, which corresponded to an ionization potential (IP) and electron affinity (EA) of 6.02 eV and 2.79 eV. The IP of BDBFCz followed the oxidation potential of carbazole and the EA matched
Fig. 6. Quantum efficiencyeluminance curves of green phosphorescent organic lightemitting diodes with BDBFCz host.
with the reduction of bidibenzofuran as the HOMO and LUMO were localized on the carbazole and bidibenzofuran moieties, respectively. The HOMO and LUMO can be estimated to be 6.02 eV and 2.79 eV from the IP and EA. Thermal properties of BDBFCz were analysed using differential scanning calorimeter (DSC) and thermogravimetric analyser (TGA). Melting temperature of BDBFCz was 335 C and glass transition temperature was 181 C in the DSC scans. Thermal decomposition temperature at 5% weight loss was 419 C in the TGA thermogram.
Fig. 4. Current densityevoltage curves of hole only and electron only devices of BDBFCz (a) and CBP (b).
Fig. 7. EL spectra of green phosphorescent organic light-emitting diodes with BDBFCz host.
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Table 1 Device performances of BDBFCz green phosphorescent OLEDs. Doping concentration
3% 5% 10% a
Voltage (V)
Quantum efficiency (%)
Current efficiency (cd/A)
Power efficiency (lm/W)
Turn-ona1000 cd/m2
Max1,000 cd/m2
Max1,000 cd/m2
Max1,000 cd/m2
3.7 6.6 3.7 6.6 3.7 6.5
20.6 16.0 20.6 18.3 20.0 17.7
80.2 76.3 71.1 63.8 69.7 60.3
47.9 25.3 43.4 30.5 47.0 29.7
Color index
0.28,0.63 0.29,0.63 0.30,0.63
Voltage at 0.1 cd/m2.
The BDBFCz host material exhibited good thermal stability due to the rigid zig-zag type bidibenzofuran core. The glass transition temperature and decomposition temperature of the BDBFCz host were much higher than those of the dibenzofuran compound known as a thermally stable host material [9]. The increase of glass transition temperature by about 60 C and thermal decomposition temperature by more than 100 C was observed. Hole and electron current densities of BDBFCz were compared using hole and electron only devices and the current densityvoltage data of the single carrier devices are shown in Fig. 4. The hole current density of the BDBFCz device was higher than the electron current density, which suggests that BDBFCz shows better hole transport properties than electron transport properties. However, the difference of hole and electron current densities was less than an order, implying that BDBFCz has a bipolar charge transport character. The hole and electron current densities were similar at high voltage. In the case of CBP, the hole current density was much higher than that of electron current density. Device performances of BDBFCz host material were investigated by doping green emitting Ir(ppy)3 dopant at doping concentrations of 3, 5 and 10%. Current density and luminance of BDBFCz devices at different doping concentrations were plotted against driving voltage in Fig. 5. Although the current density was slightly reduced at high doping concentration by charge trapping effect of Ir(ppy)3, the luminance was maintained constant irrespective of the doping concentration because of high recombination efficiency at high doping concentration as shown in the quantum efficiencyeluminance curves in Fig. 6. Quantum efficiency of the BDBFCz devices was presented against luminance in Fig. 6. Maximum quantum efficiency at 3% and 5% doping concentrations was 20.6%, but it was 19.9% at 10% doping concentration owing to concentration quenching effect. The BDBFCz device was much better than the CBP device in terms of quantum efficiency. Although the molecular structures of BDBFCz and CBP were derived from hole transport carbazole moieties, the bidibenzofuran core facilitated electron injection from the TSPO1 electron transport layer to the emitting layer compared to biphenyl core of CBP as can be inferred from the deeper LUMO level of BDBFCz (2.79 eV) than that of CBP (2.40 eV). The better electron injection of BDBFCz enhanced the quantum efficiency of the BDBFCz device through charge balance in the emitting layer. The bipolar charge transport properties of BDBFCz as shown in the single carrier device data support this explanation. The reduction of the high maximum quantum efficiency at high luminance is mostly due to tripletetriplet and triplet-polaron quenching effect. The BDBFCz device exhibited typical electroluminescence (EL) spectra of the Ir(ppy)3 doped BDBFCz green devices as presented in Fig. 7. Peak position of the green emission was 509 nm at 3% doping concentration and it was shifted to 513 nm at 10% doping concentration by intermolecular interaction of Ir(ppy)3 at high doping concentration. At all doping concentrations, no host or hole transport layer emission was observed owing to charge confinement and
complete energy transfer. Color coordinate of the BDBFCz device was (0.28,0.63) at 3% doping concentration and it was red-shifted to (0.30,0.63) at 10% doping concentration. All device performances of the BDBFCz devices are summarized in Table 1. 4. Conclusions BDBFCz compound derived from carbazole and bidibenzofuran was synthesized by Suzuki coupling reaction of carbazolyldibenzofuran moieties. The coupling of two carbazolyldibenzofuran moieties induced bipolar charge transport properties and high triplet energy of 2.75 eV. The quantum efficiency of the BDBFCz device was better than that of the CBP device and reached 20.6% by doping Ir(ppy)3. Therefore, the bidibenzofuran core may be useful to develop bipolar host materials for phosphorescent OLEDs. References [1] Vecchi PA, Padmaperuma AB, Qiao H, Sapochak LS, Burrows PE. A dibenzofuran-based host material for blue electrophosphorescence. Org Lett 2006;8:4211e4. [2] Jeong SH, Lee JY. Carbazolyldibenzofuran-type high-triplet-energy bipolar host material for blue phosphorescent organic light-emitting diodes. J Lumin 2014;146:333e6. [3] Jeong SH, Lee JY. High power efficiency in blue phosphorescent organic lightemitting diodes using 2,4-substituted dibenzofuran with a carbazole and a diphenylphosphine oxide. Org Electron 2012;13:2589e93. [4] Jeong SH, Seo CW, Lee JY, Cho NS, Kim JK, Yang JH. Comparison of bipolar hosts and mixed-hosts as host structures for deep-blue phosphorescent organic light emitting diodes. Chem Asia J 2011;6:2895e8. [5] Han C, Xie G, Li J, Zhang Z, Xu H, Deng Z, et al. A new phosphine oxide host based on ortho-disubstituted dibenzofuran for efficient electrophosphorescence: towards high triplet state excited levels and excellent thermal, morphological and efficiency stability. Chem Eur J 2011;17:8947e56. [6] Han C, Xie G, Xu H, Zhang Z, Yu D, Zhao Y, et al. Towards highly efficient bluephosphorescent organic light-emitting diodes with low operating voltage and excellent efficiency stability. Chem Eur J 2011;17:445e9. [7] Han C, Xie G, Xu H, Zhang Z, Xie L, Zhao Y, et al. A single phosphine oxide host for high-efficiency White organic light-emitting diodes with extremely low operating voltages and reduced efficiency roll-off. Adv Mater 2011;23:2491. [8] May F, Al-Helwi M, Baumeier B, Kowalsky W, Fuchs E, Lennartz C, et al. Design rules for charge-transport efficient host materials for phosphorescent organic light-emitting diodes. J Am Chem Soc 2012;134:13818e22. [9] Dong SC, Gao CH, Zhang ZH, Jiang ZQ, Lee ST, Liao LS. New dibenzofuran/ spirobifluorene hybrids as thermally stable host materials for efficient phosphorescent organic light-emitting diodes with low efficiency roll-off. Phys Chem Chem Phys 2012;14:14224e8. [10] Lee CW, Seo J, Gong M, Lee JY. The effect of the substitution position of dibenzofuran on the photophysical and charge-transport properties of host materials for phosphorescent organic light-emitting diodes. Chem Eur J 2013;19:1194e8. [11] Jeong SH, Lee JY. Dibenzofuran derivative as high triplet energy host material for high efficiency in deep blue phosphorescent organic light-emitting diodes. Org Electron 2012;7:1141e5. [12] Sovocool GW, Munslow WD, Donnelly JR, Mitchum RK. Electrophilic bromination of dibenzofuran. Chemosphere 1987;16:221e4. [13] Haenel MW, Jakubik D, Rothenberger E, Schroth G. Bidentate phosphines of heteroarenes: 4,6-Bis(diphenylphosphino)dibenzofuran and 4,6-Bis(diphenylphosphino)dibenzothiophene. Chem Ber 1991;124:1705e10. [14] Lee CW, Lee JY. Structureproperty relationship of pyridoindole-type host materials for high-efficiency blue phosphorescent organic light-emitting diodes. Chem Mater 2014;26:1616e21.