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Dimethyl modified terphenyl core based compounds as hosts of blue phosphorescent emitters
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Dimethyl modified terphenyl core based compounds as hosts of blue phosphorescent emitters Ju Hui Yun, Si Hyun Han, Jun Yeob Lee * School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Terphenyl Dimethyl substitution Triplet energy Host Blue device
High triplet energy materials were derived from dimethyl modified terphenyl core structure in order to apply them as the host materials of blue-emitting phosphors. The dimethyl modification was effective to increase the triplet energy of the terphenyl and carbazole based host materials and enabled application of the host materials in blue phosphorescent organic light-emitting diodes. Four hosts based on the dimethyl modified terphenyl and carbazole derived charge transport units were synthesized and the hosts with the carbazole functionalized backbone structure performed effectively as the host materials of phenylpyridine type Ir emitter. An external quantum efficiency of 18.2% was achieved in the blue phosphorescent organic light-emitting diodes using the new host developed in this work.
1. Introduction
based host materials were synthesized to adjust the triplet energy for blue PhOLEDs. The methyl group was substituted in the central phenyl unit of the terphenyl backbone through ortho-position to have large dihedral angle for high triplet energy. The DMT backbone structure was functionalized with carbazole and cyanocarbazole moieties for good hole transport properties and high triplet energy. Four host materials with different charge transport units and substitution positions were synthesized. It was demonstrated that the carbazole functionalized DMT hosts show high external quantum efficiency (EQE) in the blue PhO LEDs, confirming the effectiveness of the host the design approach adopting the DMT backbone structure.
High triplet energy host materials are gathering great interest in recent years because of strong needs of host materials for high efficiency blue phosphorescent organic light-emitting diodes (PhOLEDs) and blue thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) [1–11]. Both devices utilize triplet excitons for light emission even though emission modes of PhOLEDs and TADF OLEDs are different. Therefore, high triplet energy host materials which fully take advantage of the triplet excitons for radiative electronic transition modes are required. There exist a lot of high triplet energy host materials for phospho rescent emitters and TADF emitters and they were built based on mo lecular design methods widening the gap between triplet excited state and ground state. Commonly, the energy gap increasing approach was to reduce degree of conjugation of the molecular structure. Distortion of backbone structure by ortho-linkage [4,12–18] or bulky substituent [19–22], and use of conjugation breaking linking groups like ether, Si, C, or P were the most popular approaches to increase the triplet energy [3, 11,13,14,17,19,22–31]. However, the conjugation breaking units are weak to chemical degradation due to small bond dissociation energy and are not suitable for long lifetime in the PhOLED or TADF OLEDs [31–36]. Therefore, molecular designs with only aromatic units are preferred to develop the high triplet energy hosts. In this research, 20 ,50 -dimethyl-1,1’:40 ,100 -terphenyl (DMT) backbone
2. Results and discussion para-Terphenyl has a para connected molecular structure of three phenyl units in series. It has an aromatic based molecular structure, but it is difficult to obtain high triplet energy because of extended conju gation length. Therefore, isomers of terphenyl, ortho-terphenyl or metaterphenyl, were commonly used to increase the triplet energy of the terphenyl derived compounds by distorting the aromatic unit from the central phenyl ring [16,37–40]. Likewise, we developed host materials with high triplet energy by introducing methyl group in the central phenyl unit to largely distort the backbone structure. Four terphenyl modified host materials, 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3, 300 -diyl)bis(9H-carbazole) (m-CzDMT), 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,
* Corresponding author. E-mail address: [email protected] (J.Y. Lee). https://doi.org/10.1016/j.dyepig.2019.107947 Received 4 September 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 2 October 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ju Hui Yun, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.107947
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100 -terphenyl]-4,400 -diyl)bis(9H-carbazole) (p-CzDMT), 9,9’-(20 , 50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carbazole-3-carbo nitrile) (m-CzCNDMT) and 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-4, 400 -diyl)bis(9H-carbazole-3-carbonitrile) (p-CzCNDMT) were synthe sized to prove the design concept. Carbazole and CN modified carbazole units were merged with the DMT backbone through either para or meta position to prepare the four hosts. Carbazole was used as the hole transport type functional unit, while the CN modified carbazole was used to enhance electron transport property. The synthesis of the four host materials was carried out by the same process mediated by Suzuki coupling reaction. Halogenated phenyl carbazole reagents were synthesized by nucleophilic substitution reac tion and then coupled with borylated dimethyl substituted phenyl. Two
carbazole intermediates and two CN modified carbazole intermediates were used to prepare the four hosts. Four host materials were synthe sized with good yield over 50% and purified by column chromatography for high purity above 99%. The detailed synthetic route is described in Scheme 1 and experimental section. Molecular orbital simulation of host materials was implemented to identify geometrical structure and photophysical properties. The calculated results are presented in Fig. 1. The highest occupied molec ular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distributions of m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT were quite similar in that the HOMOs were localized on carbazole units because of its strong electron donating character and the LUMOs were spread widely over the terphenyl backbone. The HOMOs of
Scheme 1. The synthetic procedure of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. 2
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Fig. 1. Calculated results of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT.
p-CzDMT and p-CzCNDMT were distributed slightly to the central phenyl unit due to the para- orientation of the building blocks. Dihedral angles of the m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT hosts between central phenyl unit and adjacent phenylene units were 57.6, 54.4, 57.0 and 55.3� , respectively, and they were larger than 30.6� of para-terphenyl without the methyl groups [41]. The dihedral angles between carbazole and phenylene unit were similarly about 60� . The HOMO and LUMO levels of the hosts were measured by ana lysing oxidation and reduction potentials using cyclic voltammetry (CV) in Fig. 2. The HOMO/LUMO levels of m-CzDMT, p-CzDMT, mCzCNDMT and p-CzCNDMT were 6.13/-2.26, 6.11/-2.30, 6.33/2.63 and 6.31/-2.67 eV, respectively. The HOMO and LUMO levels of m-CzCNDMT and p-CzCNDMT were relatively deep compared to those of the m-CzDMT and p-CzDMT because of electron deficiency of the CN modified carbazole. The orientation of the carbazole and CN modified carbazole had little effect on the HOMO and LUMO energy levels although the HOMO-LUMO gap was slightly increased in the meta ori ented compounds. Photophysical properties of the hosts were estimated by ultra violet–visible (UV–vis) absorption and photoluminescence (PL) emis sion in Fig. 3. Main absorption below 300 nm was shared in the four hosts by the methyl substituted terphenyl backbone and carbazole functional groups. The m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT showed comparable UV–vis gap values of 3.56, 3.55, 3.53 and 3.52 eV, respectively. The singlet energy (ES) and triplet energy (ET) were obtained from fluorescence and phosphorescence spectra of low temperature PL at 77K in tetrahydrofuran (THF) solution. The ES/ET of m-CzDMT, p-CzDMT m-CzCNDMT and p-CzCNDMT were 3.60/2.84, 3.58/2.62, 3.56/2.85 and 3.53/2.61 eV. The ES values of four materials were similar, but the ET values were found to be different. m-CzDMT and m-CzCNDMT showed higher ET than p-CzDMT and p-CzCNDMT because the conjugation length was reduced by coupling DMT backbone and carbazole unit through meta-position. Extended backbone structure of pCzDMT and p-CzCNDMT caused reduction of the ET by 0.2 eV. The
Fig. 2. Cyclic voltammetry (CV) curves of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT.
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scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The Tgs of m-CzDMT and m-CzCNDMT were 105.2 and 140.9 � C. Both ma terials showed high Tg values above 100 � C and the Tg of m-CzCNDMT was increased because the polar CN groups induced strong intermolec ular interactions. p-CzDMT and p-CzCNDMT did not show the Tg due to rapid crystallization during cooling. The Td values of m-CzDMT, pCzDMT, m-CzCNDMT and p-CzCNDMT were 440.5, 451.1, 492.0 and 500.8 � C, respectively. The results of DSC and TGA measurements are provided in Fig. S1 and all analysed data are summarized in Table 1. Prior to figure out the device performances, single charge devices were fabricated to confirm carrier transport of host materials. The car rier density data of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT are presented in Fig. 4. The hole current densities of p-CzDMT and pCzCNDMT were higher than those of m-CzDMT and m-CzCNDMT in the hole only devices. The result was correlated with geometrical structure of molecules and HOMO distributions. The wide distribution of the HOMOs of p-CzDMT and p-CzCNDMT increases orbital overlap integral for hole transport and the extended molecular structure also assists the hole transport by large orbital overlap integral. In the electron only devices, m-CzCNDMT and p-CzCNDMT showed higher electron current density than m-CzDMT and p-CzDMT because the CN modified carba zole unit promoted the electron transport character. The blue PhOLEDs were fabricated to figure out device performances of four materials as host. Bis[2-(4,6-difluorophenyl)pyridinato-C2,N] (picolinato)iridium(III) (FIrpic) was used as the triplet emitter at a doping concentration of 15%. The blue PhOLED with 3,30 -di(9H-car bazol-9-yl)-1,10 -biphenyl (mCBP) host was used as a reference device. Current density (J) and luminance (L) according to voltage are presented in Fig. 5. In the PhOLEDs doped with FIrpic, the J was increased in the order of p-CzCNDMT > m-CzCNDMT > p-CzDMT � m-CzDMT. The LUMO energy level gap between hosts and FIrpic was large in the pCzDMT and m-CzDMT devices, which caused strong electron trapping in the FIrpic dopant, decreasing the J of the p-CzDMT and m-CzDMT de vices. Whereas, the good electron transport character of m-CzCNDMT and p-CzCNDMT, and small LUMO level gap between the hosts and FIrpic increased the J of the m-CzCNDMT and p-CzCNDMT devices. The energy diagram of devices is described in Fig. 6 and the J and L values according to doping concentration are shown in Fig. S2. The EQEs of blue PhOLEDs using FIrpic are presented in Fig. 7. Maximum EQEs of the m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT devices were 18.2%, 16.9%, 17.3% and 17.7%, respectively. The maximum EQE of the four devices was not quite different, indicating that all host equivalently harvested the triplet excitons of FIrpic. How ever, the efficiency roll-off of the devices was dissimilar according to the molecular structure of the host. The two hosts with the CN modified carbazole showed relatively small EQE drop at high L. The CN assisted electron transport character of the m-CzCNDMT and p-CzCNDMT could maintain the carrier balance and suppress degradation pathways by the good electron transport properties widening the recombination zone in the emitting layer. Electroluminescence (EL) spectra are provided in Fig. 8. The EL spectra showed only FIrpic emission without host emission. It indicates that energy transfer from host to FIrpic were occurred effectively. The color coordinates of all phosphorescent devices represented blue emis sion. All device results are in Table 2. 3. Conclusions In conclusion, the four host materials of m-CzDMT, p-CzDMT, mCzCNDMT and p-CzCNDMT were synthesized for blue PhOLEDs. In order to achieve high triplet energy, methyl groups were introduced in terphenyl backbone. The host materials worked effectively in blue PhOLEDs with Firpic emitter. The PhOLED devices showed good EQEs close to 20%. Therefore, the design strategy can be helpful to synthesize host materials and the four host materials can be used for blue triplet emitters.
Fig. 3. Photoluminescence (PL) spectra of (a)m-CzDMT, (b)p-CzDMT, (c)mCzCNDMT and (d)p-CzCNDMT.
phosphorescence emission peak at 408 nm in m-CzDMT and mCzCNDMT was originated from local emission of carbazole moiety. Thermal properties represented by glass transition temperature (Tg) and decomposition temperature (Td) were obtained by differential 4
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Table 1 Analysed properties of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. HOMO/LUMO (eV) m-CzDMT p-CzDMT m-CzCNDMT p-CzCNDMT
6.13/-2.26 6.11/-2.30 6.33/-2.63 6.31/-2.67
Band gap (eV)
ES/ET (eV)
Tg (� C)
Td (� C)
3.87 3.81 3.70 3.64
3.60/2.84 3.58/2.62 3.56/2.85 3.53/2.61
105.16 – 140.94 –
440.5 451.1 492.0 500.8
Fig. 6. Energy diagram of blue PhOLED devices. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Current density (J) plots of (a)hole only and (b)electron only devices of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. Fig. 7. External quantum efficiency plots of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Experimental 4.1. General information 2,5-Dibromo-p-xylene and 1,4-dibromobenzene were purchased from Thermo Fisher Scientific Inc. 9H-carbazole-3-carbonitrile, 9-(3bromophenyl)-9H-carbazole and 9-(4-bromophenyl)-9H-carbazole were supplied from P&H Tech Co. and 1,10-phenanthroline and copper(I) iodide were received from Sigma Aldrich Co. Bis(pinacolato) diboron, (1,10 -bis(diphenylphosphino)ferrocene)palladium(II) dichloride (Pd (dppf)Cl2) and tetrakis(triphenylphosphine) palladium (0) were pur chased from GOM Technology Co. Ltd. Potassium acetate, potassium carbonate, magnesium sulfate, celite, N,N-dimethylformamide (DMF), 1,2-dibromobenzene (DCB) were from Daejung Chemicals & Metals Co. Ltd. and methylene chloride (MC), tetrahydrofuran (THF), toluene, ethanol, and acetone were products of Samchun Pure Chemical Co.
Fig. 5. J and luminance (L) plots of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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4.2.4. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9Hcarbazole) (m-CzDMT) 2,2’-(2,5-Dimethyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2dioxaborolane) (0.7 g, 1.95 mmol) and 9-(3-bromophenyl)-9H-carba zole (1.39 g, 4.30 mmol) were dissolved in THF (20 ml). A 10 ml aqueous solution of potassium carbonate (0.81 g, 5.86 mmol) was added to the solution. Tetrakis(triphenylphosphine) palladium (0) (0.07 g, 0.06 mmol) was added and the solution was refluxed overnight. The mixture was extracted with MC and purified by column chromatog raphy. A solid was recrystallized in a toluene/ethanol mixed solvent and then vacuum sublimed. A white powder was obtained (0.88 g, yield 76.5%). 1H NMR (500 MHz, CDCl3):δ 8.149 (d, 4H, J ¼ 8.0 Hz), 7.655 (t, 2H, J ¼ 7.75 Hz), 7.597–7.551 (m, 4H), 7.493–7.463 (m, 6H), 7.416 (t, 4H, J ¼ 8.0 Hz), 7.304–7.265 (m, 6H), 2.392 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 143.61, 141.05, 140.40, 137.72, 132.98, 132.20, 129.83, 128.48, 127.97, 126.16, 125.67, 123.60, 120.53, 120.17, 110.01, 20.31. GC/MS (m/z): found, 589.2715 ([FAB]þ); Calcd. for C44H32N2, 588.7383.
Fig. 8. Electroluminescence (EL) spectra of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4.2.5. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9Hcarbazole) (p-CzDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9H-carba zole) was synthesized according to the synthetic method of 9,9’-(20 ,50 dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carbazole) except for the reagent. 9-(4-Bromophenyl)-9H-carbazole (0.99 g, 3.07 mmol) was used instead of 9-(3-bromophenyl)-9H-carbazole (0.46 g, yield 56.1%). 1 H NMR (500 MHz, CDCl3):δ 8.180 (d, 4H, J ¼ 8.0 Hz), 7.672–7.632 (m, 8H), 7.536 (d, 4H, J ¼ 8.5 Hz), 7.457 (t, 4H, J ¼ 7.5 Hz), 7.344–7.305 (m, 6H), 2.456 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 141.09, 140.92, 140.51, 136.65, 133.10, 132.30, 130.88, 126.89, 126.17, 123.64, 120.56, 120.19, 110.08, 20.30. GC/MS (m/z): found, 589.2489 ([FAB]þ); Calcd. for C44H32N2, 588.7383.
4.2. Synthesis 4.2.1. 2,2’-(2,5-Dimethyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2dioxaborolane) 2,5-Dibromo-p-xylene (6.0 g, 22.7 mmol), bis(pinacolato) diboron (18.0 g, 68.2 mmol), potassium acetate (13.4 g, 136 mmol) and Pd(dppf) Cl2 (1.0 g, 0.68 mmol) were dissolved in DMF (100 ml) and the mixture was stirred at 85 � C for 48 h. After the reaction, the mixture was extracted with MC and dehydrated with magnesium sulfate. A crude product was purified by column chromatography and white powder was obtained (6.37 g, yield 78.3%). 1H NMR (500 MHz, DMSO‑d6):δ 7.535 (s, 2H), 2.480 (s, 6H), 1.338 (s, 24H). LC/MS (m/z): found, 358.99 ([M þ H]þ); Calcd. for C20H32B2O4, 358.09.
4.2.6. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9Hcarbazole-3-carbonitrile) (m-CzCNDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole-3-carbonitrile) was synthesized according to the synthetic method of 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole). 9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile (1.49 g, 4.30 mmol) was used as reagent instead of 9-(3-bromophenyl)-9Hcarbazole. The product was recrystallized with MC/ethanol and purified by train vacuum sublimation (0.83 g, yield 66.4%). 1H NMR (500 MHz, CDCl3):δ 8.462 (s, 2H), 8.169 (d, 2H, J ¼ 8.0 Hz), 7.706 (t, 2H, J ¼ 8.0 Hz), 7.661 (d, 2H, J ¼ 8.5 Hz), 7.550–7.465 (m, 12H), 7.386 (t, 2H, J ¼ 7.25 Hz), 7.266 (s, 2H), 2.389 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 143.90, 142.77, 141.83, 140.21, 136.55, 133.02, 132.21, 130.22, 129.45, 127.95, 127.66, 125.76, 125.54, 123.79, 122.47, 121.55, 120.94, 120.57, 103.00, 20.28. GC/MS (m/z): found, 639.3158 ([FAB]þ); Calcd. for C46H30N4, 638.7572.
4.2.2. 9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile The synthetic method of 9-(3-bromophenyl)-9H-carbazole-3-car bonitrile was provided in our previous work [42]. 4.2.3. 9-(4-Bromophenyl)-9H-carbazole-3-carbonitrile 9H-carbazole-3-carbonitrile (1.0 g, 5.20 mmol), 1,4-dibromoben zene (3.68 g, 15.6 mmol), potassium carbonate (2.16 g, 15.6 mmol), 1,10-phenanthroline (0.47 g, 2.60 mmol) and copper(I) iodide (0.50 g, 2.60 mmol) were dissolved in DMF (30 ml) and refluxed in a pressure tube. After 2 h, the reaction mixture was filtered through celite and the organic solvent was removed by vacuum evaporation. The product was obtained as a powder after purification using column chromatography (1.14 g, yield 63.0%). 1H NMR (500 MHz, DMSO‑d6):δ 8.864 (s, 1H), 8.377 (d, 1H, J ¼ 8.0 Hz), 7.890 (d, 2H, J ¼ 9.0 Hz), 7.810 (d, 1H, J ¼ 8.5 Hz), 7.631 (d, 2H, J ¼ 8.5 Hz), 7.540 (t, 1H, J ¼ 7.75 Hz), 7.494 (d, 1H, J ¼ 8.0 Hz), 7.405 (t, 2H, J ¼ 9.0 Hz). LC/MS (m/z): found, 348.22 ([M þ H]þ); Calcd. for C19H11BrN2, 347.21.
Table 2 Device performances of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. Host
Dopant
Von (V)a
ηmax (%)b
η1000 (%)c
ηp (lm/W)d
ηp,1000 (lm/W)e
Color coordinate
m-CzDMT p-CzDMT m-CzCNDMT p-CzCNDMT
Firpic
7.0 6.7 5.7 5.3
18.2 16.9 17.3 17.7
16.3 15.8 16.0 15.8
20.6 19.6 29.6 29.4
15.0 15.4 18.5 19.5
(0.16, 0.34) (0.15, 0.35) (0.16, 0.35) (0.15, 0.35)
a b c d e
Von is turn on voltage at 1cdm-2 ηmax is maximum external quantum efficiency. η1000 is quantum efficiency measured at 1000cdm-2 ηp is maximum power efficiency. ηp,1000 is power efficiency measured at 1000cdm-2 6
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4.2.7. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9Hcarbazole-3-carbonitrile) (p-CzCNDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9H-carba zole-3-carbonitrile) was synthesized according to the synthetic method of 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole). Instead of 9-(3-bromophenyl)-9H-carbazole, 9-(4-bromophenyl)9H-carbazole-3-carbonitrile (1.49 g, 4.30 mmol) was used. After purifi cation using column chromatography, a white powder was further pu rified by recrystallization with DCB/acetone and train vacuum sublimation (0.65 g, 52.0%). 1H NMR (500 MHz, CDCl3):δ 8.491 (s, 2H), 8.197 (d, 2H, J ¼ 8.0 Hz), 7.688 (t, 6H, J ¼ 8.0 Hz), 7.627 (d, 4H, J ¼ 8.0 Hz), 7.565–7.517 (m, 6H), 7.414 (t, 2H, J ¼ 8.0 Hz), 7.342 (s, 2H), 2.455 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 142.83, 141.94, 141.89, 140.37, 135.46, 133.14, 132.31, 131.17, 129.47, 127.67, 127.00, 125.57, 123.83, 122.52, 121.58, 120.96, 120.63, 110.85, 110.69, 103.02, 20.28. GC/MS (m/z): found, 639.3655 ([FAB]þ); Calcd. for C46H30N4, 638.7572.
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High triplet energy host materials for blue phosphorescent organic light-emitting diodes derived from carbazole modified orthophenylene. J Mater Chem C 2014;2(35):7256–63. [17] Son HS, Lee JY. Structure–property relationship in high triplet energy host materials with a phenylcarbazole core and diphenylphosphine oxide substituent. Org Electron 2011;12(6):1025–32. [18] Liu T, Sun H, Fan C, Ma D, Zhong C, Yang C. High efficiency blue PhOLEDs using spiro-annulated triphenylamine/fluorene hybrids as host materials with high triplet energy, high HOMO level and high Tg. Org Electron 2014;15(12):3568–76. [19] Hu D, Lu P, Wang C, Liu H, Wang H, Wang Z, et al. Silane coupling di-carbazoles with high triplet energy as host materials for highly efficient blue phosphorescent devices. J Mater Chem 2009;19(34):6143–8. [20] Jiang Z, Xu X, Zhang Z, Yang C, Liu Z, Tao Y, et al. Diarylmethylene-bridged 4,40 (bis(9-carbazolyl))biphenyl: morphological stable host material for highly efficient electrophosphorescence. J Mater Chem 2009;19(41):7661–5. [21] Li W, Qiao J, Duan L, Wang L, Qiu Y. Novel fluorene/carbazole hybrids with steric bulk as host materials for blue organic electrophosphorescent devices. Tetrahedron 2007;63(41):10161–8. [22] Zhao X-H, Zhang Z-S, Qian Y, Yi M-D, Xie L-H, Hu C-P, et al. A bulky pyridinylfluorene-functionalizing approach to synthesize diarylfluorene-based bipolar host materials for efficient red, green, blue and white electrophosphorescent devices. J Mater Chem C 2013;1(21):3482–90. [23] Yun JH, Han SH, Lee JY. CN-carbazole modified diphenylsilane-type high triplet energy hosts for blue phosphorescent organic light-emitting diodes. Org Electron 2018;62:342–50. [24] Gong S, Fu Q, Wang Q, Yang C, Zhong C, Qin J, et al. Highly efficient deep-blue electrophosphorescence enabled by solution-processed bipolar tetraarylsilane host with both a high triplet energy and a high-lying HOMO level. Adv Mater 2011;23 (42):4956–9. [25] Cho YJ, Lee JY. Tetraphenylsilane-based high triplet energy host materials for blue phosphorescent organic light-emitting diodes. J Phys Chem C 2011;115(20): 10272–6. [26] Tsai M-H, Lin H-W, Su H-C, Ke T-H, Wu C-c, Fang F-C, et al. Highly efficient organic blue electrophosphorescent devices based on 3,6-Bis(triphenylsilyl)carba zole as the host Material. Adv. Mater. 2006;18(9):1216–20. [27] Wu M-F, Yeh S-J, Chen C-T, Murayama H, Tsuboi T, Li W-S, et al. The quest for high-performance host materials for electrophosphorescent blue dopants. Adv Funct Mater 2007;17(12):1887–95. [28] Xiao L, Su S-J, Agata Y, Lan H, Kido J. Nearly 100% internal quantum efficiency in an organic blue-light electrophosphorescent device using a weak electron transporting material with a wide energy gap. Adv Mater 2009;21(12):1271–4. [29] Kim SH, Jang J, Lee SJ, Lee JY. Deep blue phosphorescent organic light-emitting diodes using a Si based wide bandgap host and an Ir dopant with electron withdrawing substituents. Thin Solid Films 2008;517(2):722–6. [30] Jeon SO, Jang SE, Son HS, Lee JY. External quantum efficiency above 20% in deep blue phosphorescent organic light-emitting diodes. Adv Mater 2011;23(12): 1436–41.
4.3. Device fabrication and measurements Device structure of PHOLEDs was ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/EML (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al (200 nm). ITO was indium tin oxide and thickness was 150 nm. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and 4,40 -cyclohexylidenebis[N,N-bis(4-methyphenyl) benzenamine] (TAPC) were used as a hole injection layer and hole transport layer. 1,3-Bis(N-carbazolyl)benzene (mCP) and diphenyl(4(triphenylsilyl)phenyl)phosphine oxide (TSPO1) were used as exciton blocking layers. 1,3,5-Tris(1-phenyl-1H-benzo[d]imidazole-2-yl)ben zene (TPBi) was used as an electron transporting layer. The emitting layers were composed of host and triplet emitters. m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT were used as host materials. Firpic was used as a triplet emitter doped in host materials at doping concentration of 3%–15%. The hole only and electron only devices were fabricated using the structure of ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/host materials (25 nm)/TAPC (5 nm)/Al (200 nm) and ITO/ PEDOT:PSS (60 nm)/TSPO1 (10 nm)/host materials (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al (200). Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Nano Materials Technology Develop ment Program (2016M3A7B4909243) through the National Research Foundation of Korea funded by Ministry of Science, ICT and Future Planning. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.107947. References [1] Holmes RJ, Forrest SR, Tung Y-J, Kwong RC, Brown JJ, Garon S, et al. Blue organic electrophosphorescence using exothermic host–guest energy transfer. Appl Phys Lett 2003;82(15):2422–4. [2] Lin M-S, Yang S-J, Chang H-W, Huang Y-H, Tsai Y-T, Wu C-C, et al. Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices. J Mater Chem 2012;22(31):16114–20. [3] Yeh S-J, Wu M-F, Chen C-T, Song Y-H, Chi Y, Ho M-H, et al. New dopant and host materials for blue-light-emitting phosphorescent organic electroluminescent devices. Adv Mater 2005;17(3):285–9.
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Dyes and Pigments xxx (xxxx) xxx [37] Li W, Li J, Liu D, Wang F, Zhang S. Bipolar host materials for high-efficiency blue phosphorescent and delayed-fluorescence OLEDs. J Mater Chem C 2015;3(48): 12529–38. [38] Lee CW, Lee JY. Carbazole modified terphenyl based high triplet energy host materials for blue phosphorescent organic light-emitting diodes. Dyes Pigments 2014;101:150–5. [39] Wu C-A, Chou H-H, Shih C-H, Wu F-I, Cheng C-H, Huang H-L, et al. Synthesis and physical properties of meta-terphenyloxadiazole derivatives and their application as electron transporting materials for blue phosphorescent and fluorescent devices. J Mater Chem 2012;22(34):17792–9. [40] Lee CW, Lee JY. High quantum efficiency in blue phosphorescent organic light emitting diodes using ortho-substituted high triplet energy host materials. Org Electron 2013;14(6):1602–7. [41] Wee K-R, Cho Y-J, Jeong S, Kwon S, Lee J-D, Suh I-H, et al. Carborane-based optoelectronically active organic molecules: wide band gap host materials for blue phosphorescence. J Am Chem Soc 2012;134(43):17982–90. [42] Byeon SY, Kim JH, Lee JY. CN-modified host materials for improved efficiency and lifetime in blue phosphorescent and thermally activated delayed fluorescent organic light-emitting diodes. ACS Appl Mater Interfaces 2017;9(15):13339–46.
[31] Jeon SO, Yook KS, Joo CW, Lee JY. High-efficiency deep-blue-phosphorescent organic light-emitting diodes using a phosphine oxide and a phosphine sulfide high-triplet-energy host material with bipolar charge-transport properties. Adv Mater 2010;22(16):1872–6. [32] Lin N, Qiao J, Duan L, Wang L, Qiu Y. Molecular understanding of the chemical stability of organic materials for OLEDs: a comparative study on sulfonyl, phosphine-oxide, and carbonyl-containing host materials. J Phys Chem C 2014;118 (14):7569–78. [33] Schmidbauer S, Hohenleutner A, K€ onig B. Chemical degradation in organic lightemitting devices: mechanisms and implications for the design of new materials. Adv Mater 2013;25(15):2114–29. [34] Tsuboi T, Liu S-W, Wu M-F, Chen C-T. Spectroscopic and electrical characteristics of highly efficient tetraphenylsilane-carbazole organic compound as host material for blue organic light emitting diodes. Org Electron 2009;10(7):1372–7. [35] Ren X, Li J, Holmes RJ, Djurovich PI, Forrest SR, Thompson ME. Ultrahigh energy gap hosts in deep blue organic electrophosphorescent devices. Chem Mater 2004; 16(23):4743–7. [36] Song W, Lee JY. Degradation mechanism and lifetime improvement strategy for blue phosphorescent organic light-emitting diodes. Adv Opt Mater 2017;5(9): 1600901.
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Contents lists available at ScienceDirect
Dyes and Pigments journal homepage: http://www.elsevier.com/locate/dyepig
Dimethyl modified terphenyl core based compounds as hosts of blue phosphorescent emitters Ju Hui Yun, Si Hyun Han, Jun Yeob Lee * School of Chemical Engineering, Sungkyunkwan University, 2066, Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, South Korea
A R T I C L E I N F O
A B S T R A C T
Keywords: Terphenyl Dimethyl substitution Triplet energy Host Blue device
High triplet energy materials were derived from dimethyl modified terphenyl core structure in order to apply them as the host materials of blue-emitting phosphors. The dimethyl modification was effective to increase the triplet energy of the terphenyl and carbazole based host materials and enabled application of the host materials in blue phosphorescent organic light-emitting diodes. Four hosts based on the dimethyl modified terphenyl and carbazole derived charge transport units were synthesized and the hosts with the carbazole functionalized backbone structure performed effectively as the host materials of phenylpyridine type Ir emitter. An external quantum efficiency of 18.2% was achieved in the blue phosphorescent organic light-emitting diodes using the new host developed in this work.
1. Introduction
based host materials were synthesized to adjust the triplet energy for blue PhOLEDs. The methyl group was substituted in the central phenyl unit of the terphenyl backbone through ortho-position to have large dihedral angle for high triplet energy. The DMT backbone structure was functionalized with carbazole and cyanocarbazole moieties for good hole transport properties and high triplet energy. Four host materials with different charge transport units and substitution positions were synthesized. It was demonstrated that the carbazole functionalized DMT hosts show high external quantum efficiency (EQE) in the blue PhO LEDs, confirming the effectiveness of the host the design approach adopting the DMT backbone structure.
High triplet energy host materials are gathering great interest in recent years because of strong needs of host materials for high efficiency blue phosphorescent organic light-emitting diodes (PhOLEDs) and blue thermally activated delayed fluorescent (TADF) organic light-emitting diodes (OLEDs) [1–11]. Both devices utilize triplet excitons for light emission even though emission modes of PhOLEDs and TADF OLEDs are different. Therefore, high triplet energy host materials which fully take advantage of the triplet excitons for radiative electronic transition modes are required. There exist a lot of high triplet energy host materials for phospho rescent emitters and TADF emitters and they were built based on mo lecular design methods widening the gap between triplet excited state and ground state. Commonly, the energy gap increasing approach was to reduce degree of conjugation of the molecular structure. Distortion of backbone structure by ortho-linkage [4,12–18] or bulky substituent [19–22], and use of conjugation breaking linking groups like ether, Si, C, or P were the most popular approaches to increase the triplet energy [3, 11,13,14,17,19,22–31]. However, the conjugation breaking units are weak to chemical degradation due to small bond dissociation energy and are not suitable for long lifetime in the PhOLED or TADF OLEDs [31–36]. Therefore, molecular designs with only aromatic units are preferred to develop the high triplet energy hosts. In this research, 20 ,50 -dimethyl-1,1’:40 ,100 -terphenyl (DMT) backbone
2. Results and discussion para-Terphenyl has a para connected molecular structure of three phenyl units in series. It has an aromatic based molecular structure, but it is difficult to obtain high triplet energy because of extended conju gation length. Therefore, isomers of terphenyl, ortho-terphenyl or metaterphenyl, were commonly used to increase the triplet energy of the terphenyl derived compounds by distorting the aromatic unit from the central phenyl ring [16,37–40]. Likewise, we developed host materials with high triplet energy by introducing methyl group in the central phenyl unit to largely distort the backbone structure. Four terphenyl modified host materials, 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3, 300 -diyl)bis(9H-carbazole) (m-CzDMT), 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,
* Corresponding author. E-mail address: [email protected] (J.Y. Lee). https://doi.org/10.1016/j.dyepig.2019.107947 Received 4 September 2019; Received in revised form 1 October 2019; Accepted 1 October 2019 Available online 2 October 2019 0143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Ju Hui Yun, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.107947
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100 -terphenyl]-4,400 -diyl)bis(9H-carbazole) (p-CzDMT), 9,9’-(20 , 50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carbazole-3-carbo nitrile) (m-CzCNDMT) and 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-4, 400 -diyl)bis(9H-carbazole-3-carbonitrile) (p-CzCNDMT) were synthe sized to prove the design concept. Carbazole and CN modified carbazole units were merged with the DMT backbone through either para or meta position to prepare the four hosts. Carbazole was used as the hole transport type functional unit, while the CN modified carbazole was used to enhance electron transport property. The synthesis of the four host materials was carried out by the same process mediated by Suzuki coupling reaction. Halogenated phenyl carbazole reagents were synthesized by nucleophilic substitution reac tion and then coupled with borylated dimethyl substituted phenyl. Two
carbazole intermediates and two CN modified carbazole intermediates were used to prepare the four hosts. Four host materials were synthe sized with good yield over 50% and purified by column chromatography for high purity above 99%. The detailed synthetic route is described in Scheme 1 and experimental section. Molecular orbital simulation of host materials was implemented to identify geometrical structure and photophysical properties. The calculated results are presented in Fig. 1. The highest occupied molec ular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) distributions of m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT were quite similar in that the HOMOs were localized on carbazole units because of its strong electron donating character and the LUMOs were spread widely over the terphenyl backbone. The HOMOs of
Scheme 1. The synthetic procedure of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. 2
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Fig. 1. Calculated results of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT.
p-CzDMT and p-CzCNDMT were distributed slightly to the central phenyl unit due to the para- orientation of the building blocks. Dihedral angles of the m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT hosts between central phenyl unit and adjacent phenylene units were 57.6, 54.4, 57.0 and 55.3� , respectively, and they were larger than 30.6� of para-terphenyl without the methyl groups [41]. The dihedral angles between carbazole and phenylene unit were similarly about 60� . The HOMO and LUMO levels of the hosts were measured by ana lysing oxidation and reduction potentials using cyclic voltammetry (CV) in Fig. 2. The HOMO/LUMO levels of m-CzDMT, p-CzDMT, mCzCNDMT and p-CzCNDMT were 6.13/-2.26, 6.11/-2.30, 6.33/2.63 and 6.31/-2.67 eV, respectively. The HOMO and LUMO levels of m-CzCNDMT and p-CzCNDMT were relatively deep compared to those of the m-CzDMT and p-CzDMT because of electron deficiency of the CN modified carbazole. The orientation of the carbazole and CN modified carbazole had little effect on the HOMO and LUMO energy levels although the HOMO-LUMO gap was slightly increased in the meta ori ented compounds. Photophysical properties of the hosts were estimated by ultra violet–visible (UV–vis) absorption and photoluminescence (PL) emis sion in Fig. 3. Main absorption below 300 nm was shared in the four hosts by the methyl substituted terphenyl backbone and carbazole functional groups. The m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT showed comparable UV–vis gap values of 3.56, 3.55, 3.53 and 3.52 eV, respectively. The singlet energy (ES) and triplet energy (ET) were obtained from fluorescence and phosphorescence spectra of low temperature PL at 77K in tetrahydrofuran (THF) solution. The ES/ET of m-CzDMT, p-CzDMT m-CzCNDMT and p-CzCNDMT were 3.60/2.84, 3.58/2.62, 3.56/2.85 and 3.53/2.61 eV. The ES values of four materials were similar, but the ET values were found to be different. m-CzDMT and m-CzCNDMT showed higher ET than p-CzDMT and p-CzCNDMT because the conjugation length was reduced by coupling DMT backbone and carbazole unit through meta-position. Extended backbone structure of pCzDMT and p-CzCNDMT caused reduction of the ET by 0.2 eV. The
Fig. 2. Cyclic voltammetry (CV) curves of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT.
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scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The Tgs of m-CzDMT and m-CzCNDMT were 105.2 and 140.9 � C. Both ma terials showed high Tg values above 100 � C and the Tg of m-CzCNDMT was increased because the polar CN groups induced strong intermolec ular interactions. p-CzDMT and p-CzCNDMT did not show the Tg due to rapid crystallization during cooling. The Td values of m-CzDMT, pCzDMT, m-CzCNDMT and p-CzCNDMT were 440.5, 451.1, 492.0 and 500.8 � C, respectively. The results of DSC and TGA measurements are provided in Fig. S1 and all analysed data are summarized in Table 1. Prior to figure out the device performances, single charge devices were fabricated to confirm carrier transport of host materials. The car rier density data of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT are presented in Fig. 4. The hole current densities of p-CzDMT and pCzCNDMT were higher than those of m-CzDMT and m-CzCNDMT in the hole only devices. The result was correlated with geometrical structure of molecules and HOMO distributions. The wide distribution of the HOMOs of p-CzDMT and p-CzCNDMT increases orbital overlap integral for hole transport and the extended molecular structure also assists the hole transport by large orbital overlap integral. In the electron only devices, m-CzCNDMT and p-CzCNDMT showed higher electron current density than m-CzDMT and p-CzDMT because the CN modified carba zole unit promoted the electron transport character. The blue PhOLEDs were fabricated to figure out device performances of four materials as host. Bis[2-(4,6-difluorophenyl)pyridinato-C2,N] (picolinato)iridium(III) (FIrpic) was used as the triplet emitter at a doping concentration of 15%. The blue PhOLED with 3,30 -di(9H-car bazol-9-yl)-1,10 -biphenyl (mCBP) host was used as a reference device. Current density (J) and luminance (L) according to voltage are presented in Fig. 5. In the PhOLEDs doped with FIrpic, the J was increased in the order of p-CzCNDMT > m-CzCNDMT > p-CzDMT � m-CzDMT. The LUMO energy level gap between hosts and FIrpic was large in the pCzDMT and m-CzDMT devices, which caused strong electron trapping in the FIrpic dopant, decreasing the J of the p-CzDMT and m-CzDMT de vices. Whereas, the good electron transport character of m-CzCNDMT and p-CzCNDMT, and small LUMO level gap between the hosts and FIrpic increased the J of the m-CzCNDMT and p-CzCNDMT devices. The energy diagram of devices is described in Fig. 6 and the J and L values according to doping concentration are shown in Fig. S2. The EQEs of blue PhOLEDs using FIrpic are presented in Fig. 7. Maximum EQEs of the m-CzDMT, p-CzDMT, m-CzCNDMT and pCzCNDMT devices were 18.2%, 16.9%, 17.3% and 17.7%, respectively. The maximum EQE of the four devices was not quite different, indicating that all host equivalently harvested the triplet excitons of FIrpic. How ever, the efficiency roll-off of the devices was dissimilar according to the molecular structure of the host. The two hosts with the CN modified carbazole showed relatively small EQE drop at high L. The CN assisted electron transport character of the m-CzCNDMT and p-CzCNDMT could maintain the carrier balance and suppress degradation pathways by the good electron transport properties widening the recombination zone in the emitting layer. Electroluminescence (EL) spectra are provided in Fig. 8. The EL spectra showed only FIrpic emission without host emission. It indicates that energy transfer from host to FIrpic were occurred effectively. The color coordinates of all phosphorescent devices represented blue emis sion. All device results are in Table 2. 3. Conclusions In conclusion, the four host materials of m-CzDMT, p-CzDMT, mCzCNDMT and p-CzCNDMT were synthesized for blue PhOLEDs. In order to achieve high triplet energy, methyl groups were introduced in terphenyl backbone. The host materials worked effectively in blue PhOLEDs with Firpic emitter. The PhOLED devices showed good EQEs close to 20%. Therefore, the design strategy can be helpful to synthesize host materials and the four host materials can be used for blue triplet emitters.
Fig. 3. Photoluminescence (PL) spectra of (a)m-CzDMT, (b)p-CzDMT, (c)mCzCNDMT and (d)p-CzCNDMT.
phosphorescence emission peak at 408 nm in m-CzDMT and mCzCNDMT was originated from local emission of carbazole moiety. Thermal properties represented by glass transition temperature (Tg) and decomposition temperature (Td) were obtained by differential 4
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Table 1 Analysed properties of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. HOMO/LUMO (eV) m-CzDMT p-CzDMT m-CzCNDMT p-CzCNDMT
6.13/-2.26 6.11/-2.30 6.33/-2.63 6.31/-2.67
Band gap (eV)
ES/ET (eV)
Tg (� C)
Td (� C)
3.87 3.81 3.70 3.64
3.60/2.84 3.58/2.62 3.56/2.85 3.53/2.61
105.16 – 140.94 –
440.5 451.1 492.0 500.8
Fig. 6. Energy diagram of blue PhOLED devices. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
Fig. 4. Current density (J) plots of (a)hole only and (b)electron only devices of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. Fig. 7. External quantum efficiency plots of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4. Experimental 4.1. General information 2,5-Dibromo-p-xylene and 1,4-dibromobenzene were purchased from Thermo Fisher Scientific Inc. 9H-carbazole-3-carbonitrile, 9-(3bromophenyl)-9H-carbazole and 9-(4-bromophenyl)-9H-carbazole were supplied from P&H Tech Co. and 1,10-phenanthroline and copper(I) iodide were received from Sigma Aldrich Co. Bis(pinacolato) diboron, (1,10 -bis(diphenylphosphino)ferrocene)palladium(II) dichloride (Pd (dppf)Cl2) and tetrakis(triphenylphosphine) palladium (0) were pur chased from GOM Technology Co. Ltd. Potassium acetate, potassium carbonate, magnesium sulfate, celite, N,N-dimethylformamide (DMF), 1,2-dibromobenzene (DCB) were from Daejung Chemicals & Metals Co. Ltd. and methylene chloride (MC), tetrahydrofuran (THF), toluene, ethanol, and acetone were products of Samchun Pure Chemical Co.
Fig. 5. J and luminance (L) plots of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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4.2.4. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9Hcarbazole) (m-CzDMT) 2,2’-(2,5-Dimethyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2dioxaborolane) (0.7 g, 1.95 mmol) and 9-(3-bromophenyl)-9H-carba zole (1.39 g, 4.30 mmol) were dissolved in THF (20 ml). A 10 ml aqueous solution of potassium carbonate (0.81 g, 5.86 mmol) was added to the solution. Tetrakis(triphenylphosphine) palladium (0) (0.07 g, 0.06 mmol) was added and the solution was refluxed overnight. The mixture was extracted with MC and purified by column chromatog raphy. A solid was recrystallized in a toluene/ethanol mixed solvent and then vacuum sublimed. A white powder was obtained (0.88 g, yield 76.5%). 1H NMR (500 MHz, CDCl3):δ 8.149 (d, 4H, J ¼ 8.0 Hz), 7.655 (t, 2H, J ¼ 7.75 Hz), 7.597–7.551 (m, 4H), 7.493–7.463 (m, 6H), 7.416 (t, 4H, J ¼ 8.0 Hz), 7.304–7.265 (m, 6H), 2.392 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 143.61, 141.05, 140.40, 137.72, 132.98, 132.20, 129.83, 128.48, 127.97, 126.16, 125.67, 123.60, 120.53, 120.17, 110.01, 20.31. GC/MS (m/z): found, 589.2715 ([FAB]þ); Calcd. for C44H32N2, 588.7383.
Fig. 8. Electroluminescence (EL) spectra of blue PhOLEDs of FIrpic emitter at 15% doping concentration. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
4.2.5. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9Hcarbazole) (p-CzDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9H-carba zole) was synthesized according to the synthetic method of 9,9’-(20 ,50 dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carbazole) except for the reagent. 9-(4-Bromophenyl)-9H-carbazole (0.99 g, 3.07 mmol) was used instead of 9-(3-bromophenyl)-9H-carbazole (0.46 g, yield 56.1%). 1 H NMR (500 MHz, CDCl3):δ 8.180 (d, 4H, J ¼ 8.0 Hz), 7.672–7.632 (m, 8H), 7.536 (d, 4H, J ¼ 8.5 Hz), 7.457 (t, 4H, J ¼ 7.5 Hz), 7.344–7.305 (m, 6H), 2.456 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 141.09, 140.92, 140.51, 136.65, 133.10, 132.30, 130.88, 126.89, 126.17, 123.64, 120.56, 120.19, 110.08, 20.30. GC/MS (m/z): found, 589.2489 ([FAB]þ); Calcd. for C44H32N2, 588.7383.
4.2. Synthesis 4.2.1. 2,2’-(2,5-Dimethyl-1,4-phenylene)bis(4,4,5,5-tetramethyl-1,3,2dioxaborolane) 2,5-Dibromo-p-xylene (6.0 g, 22.7 mmol), bis(pinacolato) diboron (18.0 g, 68.2 mmol), potassium acetate (13.4 g, 136 mmol) and Pd(dppf) Cl2 (1.0 g, 0.68 mmol) were dissolved in DMF (100 ml) and the mixture was stirred at 85 � C for 48 h. After the reaction, the mixture was extracted with MC and dehydrated with magnesium sulfate. A crude product was purified by column chromatography and white powder was obtained (6.37 g, yield 78.3%). 1H NMR (500 MHz, DMSO‑d6):δ 7.535 (s, 2H), 2.480 (s, 6H), 1.338 (s, 24H). LC/MS (m/z): found, 358.99 ([M þ H]þ); Calcd. for C20H32B2O4, 358.09.
4.2.6. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9Hcarbazole-3-carbonitrile) (m-CzCNDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole-3-carbonitrile) was synthesized according to the synthetic method of 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole). 9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile (1.49 g, 4.30 mmol) was used as reagent instead of 9-(3-bromophenyl)-9Hcarbazole. The product was recrystallized with MC/ethanol and purified by train vacuum sublimation (0.83 g, yield 66.4%). 1H NMR (500 MHz, CDCl3):δ 8.462 (s, 2H), 8.169 (d, 2H, J ¼ 8.0 Hz), 7.706 (t, 2H, J ¼ 8.0 Hz), 7.661 (d, 2H, J ¼ 8.5 Hz), 7.550–7.465 (m, 12H), 7.386 (t, 2H, J ¼ 7.25 Hz), 7.266 (s, 2H), 2.389 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 143.90, 142.77, 141.83, 140.21, 136.55, 133.02, 132.21, 130.22, 129.45, 127.95, 127.66, 125.76, 125.54, 123.79, 122.47, 121.55, 120.94, 120.57, 103.00, 20.28. GC/MS (m/z): found, 639.3158 ([FAB]þ); Calcd. for C46H30N4, 638.7572.
4.2.2. 9-(3-Bromophenyl)-9H-carbazole-3-carbonitrile The synthetic method of 9-(3-bromophenyl)-9H-carbazole-3-car bonitrile was provided in our previous work [42]. 4.2.3. 9-(4-Bromophenyl)-9H-carbazole-3-carbonitrile 9H-carbazole-3-carbonitrile (1.0 g, 5.20 mmol), 1,4-dibromoben zene (3.68 g, 15.6 mmol), potassium carbonate (2.16 g, 15.6 mmol), 1,10-phenanthroline (0.47 g, 2.60 mmol) and copper(I) iodide (0.50 g, 2.60 mmol) were dissolved in DMF (30 ml) and refluxed in a pressure tube. After 2 h, the reaction mixture was filtered through celite and the organic solvent was removed by vacuum evaporation. The product was obtained as a powder after purification using column chromatography (1.14 g, yield 63.0%). 1H NMR (500 MHz, DMSO‑d6):δ 8.864 (s, 1H), 8.377 (d, 1H, J ¼ 8.0 Hz), 7.890 (d, 2H, J ¼ 9.0 Hz), 7.810 (d, 1H, J ¼ 8.5 Hz), 7.631 (d, 2H, J ¼ 8.5 Hz), 7.540 (t, 1H, J ¼ 7.75 Hz), 7.494 (d, 1H, J ¼ 8.0 Hz), 7.405 (t, 2H, J ¼ 9.0 Hz). LC/MS (m/z): found, 348.22 ([M þ H]þ); Calcd. for C19H11BrN2, 347.21.
Table 2 Device performances of m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT. Host
Dopant
Von (V)a
ηmax (%)b
η1000 (%)c
ηp (lm/W)d
ηp,1000 (lm/W)e
Color coordinate
m-CzDMT p-CzDMT m-CzCNDMT p-CzCNDMT
Firpic
7.0 6.7 5.7 5.3
18.2 16.9 17.3 17.7
16.3 15.8 16.0 15.8
20.6 19.6 29.6 29.4
15.0 15.4 18.5 19.5
(0.16, 0.34) (0.15, 0.35) (0.16, 0.35) (0.15, 0.35)
a b c d e
Von is turn on voltage at 1cdm-2 ηmax is maximum external quantum efficiency. η1000 is quantum efficiency measured at 1000cdm-2 ηp is maximum power efficiency. ηp,1000 is power efficiency measured at 1000cdm-2 6
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4.2.7. 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9Hcarbazole-3-carbonitrile) (p-CzCNDMT) 9,9’-(20 ,50 -Dimethyl-[1,1’:40 ,100 -terphenyl]-4,400 -diyl)bis(9H-carba zole-3-carbonitrile) was synthesized according to the synthetic method of 9,9’-(20 ,50 -dimethyl-[1,1’:40 ,100 -terphenyl]-3,300 -diyl)bis(9H-carba zole). Instead of 9-(3-bromophenyl)-9H-carbazole, 9-(4-bromophenyl)9H-carbazole-3-carbonitrile (1.49 g, 4.30 mmol) was used. After purifi cation using column chromatography, a white powder was further pu rified by recrystallization with DCB/acetone and train vacuum sublimation (0.65 g, 52.0%). 1H NMR (500 MHz, CDCl3):δ 8.491 (s, 2H), 8.197 (d, 2H, J ¼ 8.0 Hz), 7.688 (t, 6H, J ¼ 8.0 Hz), 7.627 (d, 4H, J ¼ 8.0 Hz), 7.565–7.517 (m, 6H), 7.414 (t, 2H, J ¼ 8.0 Hz), 7.342 (s, 2H), 2.455 (s, 6H). 13C NMR (125 MHz, CDCl3):δ 142.83, 141.94, 141.89, 140.37, 135.46, 133.14, 132.31, 131.17, 129.47, 127.67, 127.00, 125.57, 123.83, 122.52, 121.58, 120.96, 120.63, 110.85, 110.69, 103.02, 20.28. GC/MS (m/z): found, 639.3655 ([FAB]þ); Calcd. for C46H30N4, 638.7572.
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4.3. Device fabrication and measurements Device structure of PHOLEDs was ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/EML (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al (200 nm). ITO was indium tin oxide and thickness was 150 nm. Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and 4,40 -cyclohexylidenebis[N,N-bis(4-methyphenyl) benzenamine] (TAPC) were used as a hole injection layer and hole transport layer. 1,3-Bis(N-carbazolyl)benzene (mCP) and diphenyl(4(triphenylsilyl)phenyl)phosphine oxide (TSPO1) were used as exciton blocking layers. 1,3,5-Tris(1-phenyl-1H-benzo[d]imidazole-2-yl)ben zene (TPBi) was used as an electron transporting layer. The emitting layers were composed of host and triplet emitters. m-CzDMT, p-CzDMT, m-CzCNDMT and p-CzCNDMT were used as host materials. Firpic was used as a triplet emitter doped in host materials at doping concentration of 3%–15%. The hole only and electron only devices were fabricated using the structure of ITO/PEDOT:PSS (60 nm)/TAPC (20 nm)/mCP (10 nm)/host materials (25 nm)/TAPC (5 nm)/Al (200 nm) and ITO/ PEDOT:PSS (60 nm)/TSPO1 (10 nm)/host materials (25 nm)/TSPO1 (5 nm)/TPBi (40 nm)/LiF (1.5 nm)/Al (200). Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by Nano Materials Technology Develop ment Program (2016M3A7B4909243) through the National Research Foundation of Korea funded by Ministry of Science, ICT and Future Planning. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.dyepig.2019.107947. References [1] Holmes RJ, Forrest SR, Tung Y-J, Kwong RC, Brown JJ, Garon S, et al. Blue organic electrophosphorescence using exothermic host–guest energy transfer. Appl Phys Lett 2003;82(15):2422–4. [2] Lin M-S, Yang S-J, Chang H-W, Huang Y-H, Tsai Y-T, Wu C-C, et al. Incorporation of a CN group into mCP: a new bipolar host material for highly efficient blue and white electrophosphorescent devices. J Mater Chem 2012;22(31):16114–20. [3] Yeh S-J, Wu M-F, Chen C-T, Song Y-H, Chi Y, Ho M-H, et al. New dopant and host materials for blue-light-emitting phosphorescent organic electroluminescent devices. Adv Mater 2005;17(3):285–9.
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