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Received 00th January 2012, Accepted 00th January 2012
A Versatile Efficient One-step Approach for Carbazole/Pyridine Hybrid Molecules: Highly Efficient Host Materials for Blue Phosphorescent OLEDs a
b
*a
a
a
a
Chao Tang, Ran Bi, Youtian Tao, FangfangWang, XudongCao, ShifanWang, Tao a c *b *ab Jiang, Cheng Zhong, Hongmei Zhang and Wei Huang
DOI: 10.1039/x0xx00000x www.rsc.org/
By using various fluoro-substituted pyridines and carbazole as starting materials, we have synthesized a series of carbazole/pyridine hybrid compounds through an efficient catalyst free C-N coupling reaction with high yields of 8595%. The bi-, tri- and tetra-carbazole substituted pyridine derivatives exhibit increased thermal stabilities and comparatively high triplet energy levels of ~3.0 eV. By using them as host materials for blue phosphorescent OLEDs, maximum current efficiencies are achieved ranging from 3242 cd A-1. Phosphorescent organic light-emitting diodes (PhOLEDs) continue to attract considerable interests because they can utilize both singlet and triplet excitons to theoretically approach 100% internal quantum efficiency, which is four times higher as conventional fluorescence.1 To achieve high efficiency, phosphorescent emitters based on heavy metal complexes are usually doped into suitable organic host materials to reduce concentration quenching and triplet-triplet annihilation.2 Recently, organic host materials especially bipolar transport hosts simultaneously containing electron-donating and electronwithdrawing moieties have drawn intensive attention in OLED research.3-10 It is interesting that, the previously reported organic host materials, which exhibited state-of-the-art device external quantum efficiencies (EQE) over 20% all possess very simple molecular
structures (Scheme 1).7-10 And their design and synthesis have been focused mainly on a multi-step strategy. For example, a maximum EQE of 23.5, 21.6 and 17.0% for blue, green and red phosphorescent devices, respectively were obtained in the universal host bis[4-(Ncarbazolyl)-phenyl]phenylphosphine oxide (BCPO),7 which was synthesized in three steps starting from 4-bromophenylcarbazole (4BrPCz). After treating (4-BrPCz) with n-butyl lithium, followed by the reaction with dichlorophenylphosphine, and then the oxidation with H2O2 to afford the desired product. Besides, 4,5’- N,N’-dicarbazolyl-(2phenylpyrimidine) (CPHP),8 a green host with EQE of 26.8%, was prepared by two steps strategy including Pd-catalyzed Suzuki coupling and Cu-catalyzed Ullman reactions. In addition, 3,3’-bis(9H pyrido[2,3- b ]indol-9-yl)-1,1’-biphenyl (CbBPCb),9 a blue PhOLED host, with EQE greater than 30%, was designed in three steps from 3(9H-carbazol-9-yl)phenylboronic acid. Another universal host, 3’,5’di(carbazol-9-yl)-[1,1’-biphenyl]-3,5-dicarbonitrile (DCzDCN),10 which achieved high quantum efficiency close to 25% either for thermally activated delayed fluorescence (TADF) or phosphorescent OLEDs, allowed for four steps synthesis. Both CbBPCb and DCzDCN involved one step of Ullman as well as two other steps of Pd-catalyzed reactions.
a
Key Laboratory for Flexible Electronics, Institute of Advanced Materials (IAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, P.R. China E-mail: [email protected]; [email protected] b Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced Materials, Nanjing University of Posts and Telecommunications 9 Wenyuan Road, Nanjing 210046 (P.R. China) E-mail : [email protected] c Department of Chemistry, Wuhan University Wuhan, 430072 (P.R. China)
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Scheme 1.Representative host materials with simple chemical structures and the state-of-the-art efficiencies.
In 2008, we designed a simple carbazole/oxadiazole hybrid host, 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole(o-CzOXD) with a suitable triplet energy of 2.68 eV.3 The highest efficiency of 20.2%
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for green electrophosphorescence was achieved. o-CzOXD was synthesized via a catalyst free aromatic nucleophilic substitution reaction by using carbazole as nitrogen nucleophiles and 2,5-bis(2fluorophenyl)-1,3,4-oxadiazole as the electrophiles. The reaction showed superior of competitive advantages over widely reported Cuand Pd- catalyzed C-N coupling reactions. However, the intermediate product, 2,5-bis(2-fluorophenyl)-1,3,4-oxadiazole was prepared by a continuous five steps reactions which involves rigorous conditions and/or use of harsh reagents such as thionyl chloride, sulfuric acid and phosphorus oxychloride. Therefore, from the materials point of view for the commercialization of OLED, it is highly desirable to develop organic host materials possessing simple chemical structures and high device efficiencies together with the simplest synthetic chemistry by directly using commercially available inexpensive starting materials. Based on this concept, we maintain carbazole as the nitrogen nucleophiles in the catalyst free C-N coupling reaction, because carbazole is the most popular building block for organic host materials due to its high triplet energy and good hole transporting properties.11 On the other hand, we expand the electrophiles to various multifluoro-substituted pyridines other than previously reported fluorobenzene derivatives with an electron-withdrawing activation group like oxadiazole,3 CN,12 NO2,13 CHO14 et.al directly linked to the phenyl ring. All the seven as designed di- (2,6-/3,5-/2,4-/2,3-CzPy), tri(2,4,6-CzPy) and tetra- (2,3,5,6-/2,3,4,6-CzPy)carbazole hybrid pyridine compounds are allowed to be synthesized conveniently by a one-step reaction with high yields of more than 85%. Devices with CzPy as hosts show maximum current efficiencies as high as 32.5-42.1 cd A-1 for blue FIrpic based electrophosphorescence. Scheme 2 illustrates the chemical structure and synthetic routes for carbazole/pyridine hybrid compounds 1-7 studied in this work. It should be mentioned that all the starting materials for the one-step catalyst free C-N couplings are easily accessible from commercial resources. As shown, the C-N coupling reaction is suitable for various multi-fluoro-substituted pyridine at all ortho-(2,6-), meta-(3,5-) and para-(4-) positions of N-heterocyclic ring. The reaction yields are as high as 85-95%, which is comparable to our previously reported oCzOXD and p-CzOXD, but significantly higher than that of 30% of mCzOXD,3 indicating there is little difference on the reaction activity at each position in fluoro-pyridines compared to other electrophiles such as electron-deficient fluorobenzene derivatives. Moreover, The synthetic methodology in this work is superior to general procedures for 2,6-CzPy and 3,5-CzPy under Cu-catalyzed Ullman conditions with relatively low reaction yields of 53 and 46%, respectively (Scheme S1).15 The thermal properties were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure S1 and S2). The decomposition temperatures (Td, corresponding to 5% weight loss) determined from TGA for compounds 1-7 were in the range of 286-434oC. The glass transition temperatures (Tg) from DSC for dicarbazole structured compounds 1-4 were observed at 70-78oC. Under similar molecular skeleton, the Tg for hetercyclic pyridine containing 2,6-CzPy is 15oC higher than that of phenyl based mCP (60oC).1d Moreover, Tg for the bulky tri-and tetra-carbazole structured 5 and 6 increased to 124 and 150oC, respectively. As shown in Table 1,
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C4CC08335K the thermal properties are greatly improved by increasing the number of carbazole units.
Scheme 2. Synthetic routes for various carbazole/pyridine hybrid compounds.
The photophysical properties of compounds 1-7 are summarized in Table 1. As shown in Figure 1, the strong absorption band at ~ 250 nm could be ascribed to pyridine unit, and the absorption bands at ~ 290 and 340 nm to the carbazole moiety. From the onset of absorption curves, a clear progression to lower energy (Eg) can be observed as the number of carbazole increases from bicarbazole substituted 1-4 to triand tetra-carbazole 5-7. All the compounds exhibit ultra-violet to deep blue emission either in CHCl3 solution or film state (Figure S3). Among the seven CzPy hybrids, 2,3,5,6-CzPy containing the most electron-donating carbazole substituent and the symmetrical structure, displayed exceptional photophysical properties, e.g. narrowest Eg, most red shifted emission, and lowest triplet energy. The triplet energies (ET) determined by the highest-energy vibronic sub-band of the 77 K phosphorescence spectra for 2,3,5,6-CzPy is 2.81 eV, significantly lower than those of ~3.0 eV for other CzPy counterparts. From the theoretical calculation on the spin density distribution (Figure S5), the T1 state for all the carbazole/pyridine hybrids 1-7 are well-controlled and located only at carbazole moiety.16 Thus, their triplet energies are almost equivalent to ~3.0 eV of carbazole. Therefore, the lower triplet energy for 2,3,5,6-CzPy from the experimental value may be attributed to its relatively stronger donor-acceptor interactions between carbazole and pyridine, which is in accordance with its obvious red-shifted intramolecular charge transfer transition absorption (350-400 nm). The highest occupied molecular orbitals (HOMO) energy levels are mainly located on the electron-donating carbazole moieties, and lowest occupied molecular orbitals (LUMO) on electron-deficient pyridine ring (Figure S6). The HOMO levels determined from the onset of oxidation for 1-7 probed by cyclic voltammetry (CV) are similar as 5.63-5.72 eV. All compounds exhibited reversible or quasireversible oxidation behavior whereas irreversible reduction. Their LUMO levels calculated from the difference of HOMO and optical bandgap varied from 2.07-2.48 eV. And the HOMO/LUMO energy levels trends are in good agreement with the theoretical results. Due to the high triplet energy levels of the as conveniently synthesized carbazole/pyridine hybrid compounds, blue
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Table 1.Physical properties of CzPy derivatives. Compounds (1) 2,6-CzPy (2) 3,5-CzPy (3) 2,4-CzPy (4) 2,3-CzPy (5) 2,4,6-CzPy (6) 2,3,5,6-CzPy (7)2,3,4,6-CzPy
Tg/ Tm/ Td [oC] 75/222/329 74/209/310 70/218/337 78/256/286 124/312/412 150/330/434 - /384/419
λabs[a] [nm] 289, 337 290, 336 290, 333 290, 331 288, 334 287, 334 287, 333
λmax[a] [nm] 377 392 387 401 387 429 417
λmax[b] [nm] 379 377 364 391 376 424 413
Eg[c] [eV] 3.54 3.56 3.61 3.59 3.46 3.19 3.24
HOMO/LUMO [eV] [d] -5.63/-2.08 -5.72/-2.16 -5.69/-2.08 -5.66/-2.07 -5.67/-2.21 -5.67/-2.48 -5.67/-2.43
HOMO/LUMO [eV] [e] -5.43/-1.07 -5.57/-1.23 -5.50/-1.15 -5.40/-1.18 -5.51/-1.27 -5.43/-1.50 -5.41/-1.50
ET[f] [eV] 3.00 3.00 2.95 2.95 2.99 2.81 2.99
ET[e] [eV] 3.00 3.00 3.16 3.16 2.99 3.00 2.99
[a] Measured in CHCl3 solution at room temperature.[b] Measured in film state at room temperature. [c] Bandgap was calculated from the absorption edge of UV-Vis spectra. [d] HOMO was measured by CV, LUMO was calculated from the difference of HOMO and bandgap. [e] Values from DFT calculations. [f] Triplet energy was calculated from low temperature (77 K) phosphorescence spectra. optimizing the device structures. To the best of our knowledge, reports based on a series of organic donor/acceptor based host materials that involve in only one-step of simple synthetic chemistry as well as favourable device efficiencies have been very rare.
Figure 1. Normalized absorption spectra of compounds 1-7 in CHCl3 solution at room temperature (left) and solid state phosphorescence spectra at 77 K (right) .
phosphorescent devices A-G with the configuration of ITO/MoO3 (5 nm)/NPB (60 nm)/TCTA (5 nm)/Host (1-7):FIrpic (13%, 10 nm)/TmPyPb(35 nm)/CsCO3 (2 nm)/Al were fabricated to evaluate their host performances. MoO3 and LiF was used as the hole- and electron-injection materials, bis[2-(4,6-difluorophenyl)-pyridinatoN,C2]picolinate iridium(III) (FIrpic) was used as blue-emitting dopant, 1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) and 1,3,5-tri(mpyrid-3-ylphenyl)benzene (TmPyPB) acted as the hole and electron transporting materials, respectively. (tris(4-(9H-carbazol-9yl)phenyl)amine) (TCTA) as hole transporting and triplet exciton block layer. The luminance-Voltage-Current density (L-V-J) characteristics and Efficiency versus Luminance curves of the devices examined in this work were shown in Figure 2, 3 and S7. The key device data are summarized in Table 2. As shown, all devices using CzPy hybrid compounds as host materials exhibited low turn-on voltages in the range of 2.8-3.4 eV and FIrpic featured blue emission. No obvious efficiency disparity was observed between these CzPy devices, due to their similar molecular structures and energy levels. Maximum current and external quantum efficiency in the range of 32.5-42.1 cd A-1 and 15.4-18.0% were obtained respectively. It is noted that both the bicarbazole containing 2,6-CzPy and tri-carbazole based 2,4,6-CzPy hosted devices showed the highest EQE of 18% among the seven hosts. We believe the device performance can be further improved by
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Figure 2.Luminance-Voltage-Current characteristics of the blue phosphorescent devices A-G.
Figure 3. External quantum Efficiency versus Luminance (L) curves of devices AG.
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Table 2. EL properties of the blue phosphorescent devices A-G.
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A 2,6-CzPy B 3,5-CzPy C 2,4-CzPy D 2,3-CzPy E 2,4,6-CzPy F 2,3,5,6-CzPy G 2,3,4,6-CzPy
Von[a] [V] 2.9 2.8 3.0 3.4 2.9 3.0 3.0
ηc[b] [cd A-1] 39.7/39.0 39.1/33.8 37.0/32.0 33.0/24.4 42.1/39.2 32.5/29.0 35.6/32.2
ηp[c] [lm W-1] 33.9/32.2 36.1/26.5 34.1/26.4 23.6/19.2 38.9/32.4 27.4/23.9 28.2/25.3
ηext[d] [%] 18.0/17.7 16.7/14.7 15.4/14.2 15.5/14.5 18.0/16.9 15.8/14.6 15.8/14.5
CIE (x, y) at 8V 0.16, 0.33 0.16, 0.35 0.16, 0.33 0.15, 0.32 0.16, 0.35 0.15, 0.33 0.16, 0.34
[a] Turn-on voltage. [b] Maximum current efficiency and at 100 cd m-2. [c] Maximum power efficiency and at 100 cd m-2. [d] The maximum EQE and at 100 cd m-2. In conclusion, we have developed a convenient and effective onestep synthetic methodology for various multi-carbazole substituted pyridine host materials through catalyst free C-N coupling reaction. The starting materials involved for CzPy compounds are commercially accessible and inexpensive. From experimental and theoretical studies, we have found that the triplet energy levels are nearly irrelevant to the number of electron-donating carbazole unit, but slightly affected by D-A interactions. Blue electrophosphorescent devices based on these high triplet hosts exhibit maximum current efficiency efficiencies up to 42.1cd/A, corresponding to external quantum efficiencies of 18.0%. The encouraging cost-effective simple synthetic chemistry as well as inspiring high efficiencies provides an attractive approach for the development of OLED and other organic semiconducting materials. Further researches based on the one-step simple aromatic nucleophilic substitution reactions have been expanded to other nitrogen nucleophiles (e.g. indoles, imidazoles and benzimidazoles) and various electrophiles in the application of diverse OLED materials. We thank the financial support from NSFC (61474062) and Youth Project 973 (No. 2014CB648300). We also thank Prof. Chuluo Yang and Prof. Jingui Qin for the measurement of physical properties.
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A Versatile Efficient One-step Approach for Carbazole/Pyridine Hybrid Molecules: Highly Efficient Host Materials for Blue Phosphorescent OLEDs a
b
*a
a
a
a
Chao Tang, Ran Bi, Youtian Tao, FangfangWang, XudongCao, ShifanWang, Tao a c *b *ab Jiang, Cheng Zhong, Hongmei Zhang and Wei Huang
DOI: 10.1039/x0xx00000x www.rsc.org/
By using various fluoro-substituted pyridines and carbazole as starting materials, we have synthesized a series of carbazole/pyridine hybrid compounds through an efficient catalyst free C-N coupling reaction with high yields of 8595%. The bi-, tri- and tetra-carbazole substituted pyridine derivatives exhibit increased thermal stabilities and comparatively high triplet energy levels of ~3.0 eV. By using them as host materials for blue phosphorescent OLEDs, maximum current efficiencies are achieved ranging from 3242 cd A-1. Phosphorescent organic light-emitting diodes (PhOLEDs) continue to attract considerable interests because they can utilize both singlet and triplet excitons to theoretically approach 100% internal quantum efficiency, which is four times higher as conventional fluorescence.1 To achieve high efficiency, phosphorescent emitters based on heavy metal complexes are usually doped into suitable organic host materials to reduce concentration quenching and triplet-triplet annihilation.2 Recently, organic host materials especially bipolar transport hosts simultaneously containing electron-donating and electronwithdrawing moieties have drawn intensive attention in OLED research.3-10 It is interesting that, the previously reported organic host materials, which exhibited state-of-the-art device external quantum efficiencies (EQE) over 20% all possess very simple molecular
structures (Scheme 1).7-10 And their design and synthesis have been focused mainly on a multi-step strategy. For example, a maximum EQE of 23.5, 21.6 and 17.0% for blue, green and red phosphorescent devices, respectively were obtained in the universal host bis[4-(Ncarbazolyl)-phenyl]phenylphosphine oxide (BCPO),7 which was synthesized in three steps starting from 4-bromophenylcarbazole (4BrPCz). After treating (4-BrPCz) with n-butyl lithium, followed by the reaction with dichlorophenylphosphine, and then the oxidation with H2O2 to afford the desired product. Besides, 4,5’- N,N’-dicarbazolyl-(2phenylpyrimidine) (CPHP),8 a green host with EQE of 26.8%, was prepared by two steps strategy including Pd-catalyzed Suzuki coupling and Cu-catalyzed Ullman reactions. In addition, 3,3’-bis(9H pyrido[2,3- b ]indol-9-yl)-1,1’-biphenyl (CbBPCb),9 a blue PhOLED host, with EQE greater than 30%, was designed in three steps from 3(9H-carbazol-9-yl)phenylboronic acid. Another universal host, 3’,5’di(carbazol-9-yl)-[1,1’-biphenyl]-3,5-dicarbonitrile (DCzDCN),10 which achieved high quantum efficiency close to 25% either for thermally activated delayed fluorescence (TADF) or phosphorescent OLEDs, allowed for four steps synthesis. Both CbBPCb and DCzDCN involved one step of Ullman as well as two other steps of Pd-catalyzed reactions.
a
Key Laboratory for Flexible Electronics, Institute of Advanced Materials (IAM), Nanjing Tech University, 30 South Puzhu Road, Nanjing, 211816, P.R. China E-mail: [email protected]; [email protected] b Key Laboratory for Organic Electronics and Information Displays and Institute of Advanced Materials, Nanjing University of Posts and Telecommunications 9 Wenyuan Road, Nanjing 210046 (P.R. China) E-mail : [email protected] c Department of Chemistry, Wuhan University Wuhan, 430072 (P.R. China)
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Scheme 1.Representative host materials with simple chemical structures and the state-of-the-art efficiencies.
In 2008, we designed a simple carbazole/oxadiazole hybrid host, 2,5-bis(2-(9H-carbazol-9-yl)phenyl)-1,3,4-oxadiazole(o-CzOXD) with a suitable triplet energy of 2.68 eV.3 The highest efficiency of 20.2%
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for green electrophosphorescence was achieved. o-CzOXD was synthesized via a catalyst free aromatic nucleophilic substitution reaction by using carbazole as nitrogen nucleophiles and 2,5-bis(2fluorophenyl)-1,3,4-oxadiazole as the electrophiles. The reaction showed superior of competitive advantages over widely reported Cuand Pd- catalyzed C-N coupling reactions. However, the intermediate product, 2,5-bis(2-fluorophenyl)-1,3,4-oxadiazole was prepared by a continuous five steps reactions which involves rigorous conditions and/or use of harsh reagents such as thionyl chloride, sulfuric acid and phosphorus oxychloride. Therefore, from the materials point of view for the commercialization of OLED, it is highly desirable to develop organic host materials possessing simple chemical structures and high device efficiencies together with the simplest synthetic chemistry by directly using commercially available inexpensive starting materials. Based on this concept, we maintain carbazole as the nitrogen nucleophiles in the catalyst free C-N coupling reaction, because carbazole is the most popular building block for organic host materials due to its high triplet energy and good hole transporting properties.11 On the other hand, we expand the electrophiles to various multifluoro-substituted pyridines other than previously reported fluorobenzene derivatives with an electron-withdrawing activation group like oxadiazole,3 CN,12 NO2,13 CHO14 et.al directly linked to the phenyl ring. All the seven as designed di- (2,6-/3,5-/2,4-/2,3-CzPy), tri(2,4,6-CzPy) and tetra- (2,3,5,6-/2,3,4,6-CzPy)carbazole hybrid pyridine compounds are allowed to be synthesized conveniently by a one-step reaction with high yields of more than 85%. Devices with CzPy as hosts show maximum current efficiencies as high as 32.5-42.1 cd A-1 for blue FIrpic based electrophosphorescence. Scheme 2 illustrates the chemical structure and synthetic routes for carbazole/pyridine hybrid compounds 1-7 studied in this work. It should be mentioned that all the starting materials for the one-step catalyst free C-N couplings are easily accessible from commercial resources. As shown, the C-N coupling reaction is suitable for various multi-fluoro-substituted pyridine at all ortho-(2,6-), meta-(3,5-) and para-(4-) positions of N-heterocyclic ring. The reaction yields are as high as 85-95%, which is comparable to our previously reported oCzOXD and p-CzOXD, but significantly higher than that of 30% of mCzOXD,3 indicating there is little difference on the reaction activity at each position in fluoro-pyridines compared to other electrophiles such as electron-deficient fluorobenzene derivatives. Moreover, The synthetic methodology in this work is superior to general procedures for 2,6-CzPy and 3,5-CzPy under Cu-catalyzed Ullman conditions with relatively low reaction yields of 53 and 46%, respectively (Scheme S1).15 The thermal properties were measured by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Figure S1 and S2). The decomposition temperatures (Td, corresponding to 5% weight loss) determined from TGA for compounds 1-7 were in the range of 286-434oC. The glass transition temperatures (Tg) from DSC for dicarbazole structured compounds 1-4 were observed at 70-78oC. Under similar molecular skeleton, the Tg for hetercyclic pyridine containing 2,6-CzPy is 15oC higher than that of phenyl based mCP (60oC).1d Moreover, Tg for the bulky tri-and tetra-carbazole structured 5 and 6 increased to 124 and 150oC, respectively. As shown in Table 1,
2 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C4CC08335K the thermal properties are greatly improved by increasing the number of carbazole units.
Scheme 2. Synthetic routes for various carbazole/pyridine hybrid compounds.
The photophysical properties of compounds 1-7 are summarized in Table 1. As shown in Figure 1, the strong absorption band at ~ 250 nm could be ascribed to pyridine unit, and the absorption bands at ~ 290 and 340 nm to the carbazole moiety. From the onset of absorption curves, a clear progression to lower energy (Eg) can be observed as the number of carbazole increases from bicarbazole substituted 1-4 to triand tetra-carbazole 5-7. All the compounds exhibit ultra-violet to deep blue emission either in CHCl3 solution or film state (Figure S3). Among the seven CzPy hybrids, 2,3,5,6-CzPy containing the most electron-donating carbazole substituent and the symmetrical structure, displayed exceptional photophysical properties, e.g. narrowest Eg, most red shifted emission, and lowest triplet energy. The triplet energies (ET) determined by the highest-energy vibronic sub-band of the 77 K phosphorescence spectra for 2,3,5,6-CzPy is 2.81 eV, significantly lower than those of ~3.0 eV for other CzPy counterparts. From the theoretical calculation on the spin density distribution (Figure S5), the T1 state for all the carbazole/pyridine hybrids 1-7 are well-controlled and located only at carbazole moiety.16 Thus, their triplet energies are almost equivalent to ~3.0 eV of carbazole. Therefore, the lower triplet energy for 2,3,5,6-CzPy from the experimental value may be attributed to its relatively stronger donor-acceptor interactions between carbazole and pyridine, which is in accordance with its obvious red-shifted intramolecular charge transfer transition absorption (350-400 nm). The highest occupied molecular orbitals (HOMO) energy levels are mainly located on the electron-donating carbazole moieties, and lowest occupied molecular orbitals (LUMO) on electron-deficient pyridine ring (Figure S6). The HOMO levels determined from the onset of oxidation for 1-7 probed by cyclic voltammetry (CV) are similar as 5.63-5.72 eV. All compounds exhibited reversible or quasireversible oxidation behavior whereas irreversible reduction. Their LUMO levels calculated from the difference of HOMO and optical bandgap varied from 2.07-2.48 eV. And the HOMO/LUMO energy levels trends are in good agreement with the theoretical results. Due to the high triplet energy levels of the as conveniently synthesized carbazole/pyridine hybrid compounds, blue
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Table 1.Physical properties of CzPy derivatives. Compounds (1) 2,6-CzPy (2) 3,5-CzPy (3) 2,4-CzPy (4) 2,3-CzPy (5) 2,4,6-CzPy (6) 2,3,5,6-CzPy (7)2,3,4,6-CzPy
Tg/ Tm/ Td [oC] 75/222/329 74/209/310 70/218/337 78/256/286 124/312/412 150/330/434 - /384/419
λabs[a] [nm] 289, 337 290, 336 290, 333 290, 331 288, 334 287, 334 287, 333
λmax[a] [nm] 377 392 387 401 387 429 417
λmax[b] [nm] 379 377 364 391 376 424 413
Eg[c] [eV] 3.54 3.56 3.61 3.59 3.46 3.19 3.24
HOMO/LUMO [eV] [d] -5.63/-2.08 -5.72/-2.16 -5.69/-2.08 -5.66/-2.07 -5.67/-2.21 -5.67/-2.48 -5.67/-2.43
HOMO/LUMO [eV] [e] -5.43/-1.07 -5.57/-1.23 -5.50/-1.15 -5.40/-1.18 -5.51/-1.27 -5.43/-1.50 -5.41/-1.50
ET[f] [eV] 3.00 3.00 2.95 2.95 2.99 2.81 2.99
ET[e] [eV] 3.00 3.00 3.16 3.16 2.99 3.00 2.99
[a] Measured in CHCl3 solution at room temperature.[b] Measured in film state at room temperature. [c] Bandgap was calculated from the absorption edge of UV-Vis spectra. [d] HOMO was measured by CV, LUMO was calculated from the difference of HOMO and bandgap. [e] Values from DFT calculations. [f] Triplet energy was calculated from low temperature (77 K) phosphorescence spectra. optimizing the device structures. To the best of our knowledge, reports based on a series of organic donor/acceptor based host materials that involve in only one-step of simple synthetic chemistry as well as favourable device efficiencies have been very rare.
Figure 1. Normalized absorption spectra of compounds 1-7 in CHCl3 solution at room temperature (left) and solid state phosphorescence spectra at 77 K (right) .
phosphorescent devices A-G with the configuration of ITO/MoO3 (5 nm)/NPB (60 nm)/TCTA (5 nm)/Host (1-7):FIrpic (13%, 10 nm)/TmPyPb(35 nm)/CsCO3 (2 nm)/Al were fabricated to evaluate their host performances. MoO3 and LiF was used as the hole- and electron-injection materials, bis[2-(4,6-difluorophenyl)-pyridinatoN,C2]picolinate iridium(III) (FIrpic) was used as blue-emitting dopant, 1,4-bis[(1-naphthylphenyl)amino]biphenyl (NPB) and 1,3,5-tri(mpyrid-3-ylphenyl)benzene (TmPyPB) acted as the hole and electron transporting materials, respectively. (tris(4-(9H-carbazol-9yl)phenyl)amine) (TCTA) as hole transporting and triplet exciton block layer. The luminance-Voltage-Current density (L-V-J) characteristics and Efficiency versus Luminance curves of the devices examined in this work were shown in Figure 2, 3 and S7. The key device data are summarized in Table 2. As shown, all devices using CzPy hybrid compounds as host materials exhibited low turn-on voltages in the range of 2.8-3.4 eV and FIrpic featured blue emission. No obvious efficiency disparity was observed between these CzPy devices, due to their similar molecular structures and energy levels. Maximum current and external quantum efficiency in the range of 32.5-42.1 cd A-1 and 15.4-18.0% were obtained respectively. It is noted that both the bicarbazole containing 2,6-CzPy and tri-carbazole based 2,4,6-CzPy hosted devices showed the highest EQE of 18% among the seven hosts. We believe the device performance can be further improved by
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Figure 2.Luminance-Voltage-Current characteristics of the blue phosphorescent devices A-G.
Figure 3. External quantum Efficiency versus Luminance (L) curves of devices AG.
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Table 2. EL properties of the blue phosphorescent devices A-G.
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A 2,6-CzPy B 3,5-CzPy C 2,4-CzPy D 2,3-CzPy E 2,4,6-CzPy F 2,3,5,6-CzPy G 2,3,4,6-CzPy
Von[a] [V] 2.9 2.8 3.0 3.4 2.9 3.0 3.0
ηc[b] [cd A-1] 39.7/39.0 39.1/33.8 37.0/32.0 33.0/24.4 42.1/39.2 32.5/29.0 35.6/32.2
ηp[c] [lm W-1] 33.9/32.2 36.1/26.5 34.1/26.4 23.6/19.2 38.9/32.4 27.4/23.9 28.2/25.3
ηext[d] [%] 18.0/17.7 16.7/14.7 15.4/14.2 15.5/14.5 18.0/16.9 15.8/14.6 15.8/14.5
CIE (x, y) at 8V 0.16, 0.33 0.16, 0.35 0.16, 0.33 0.15, 0.32 0.16, 0.35 0.15, 0.33 0.16, 0.34
[a] Turn-on voltage. [b] Maximum current efficiency and at 100 cd m-2. [c] Maximum power efficiency and at 100 cd m-2. [d] The maximum EQE and at 100 cd m-2. In conclusion, we have developed a convenient and effective onestep synthetic methodology for various multi-carbazole substituted pyridine host materials through catalyst free C-N coupling reaction. The starting materials involved for CzPy compounds are commercially accessible and inexpensive. From experimental and theoretical studies, we have found that the triplet energy levels are nearly irrelevant to the number of electron-donating carbazole unit, but slightly affected by D-A interactions. Blue electrophosphorescent devices based on these high triplet hosts exhibit maximum current efficiency efficiencies up to 42.1cd/A, corresponding to external quantum efficiencies of 18.0%. The encouraging cost-effective simple synthetic chemistry as well as inspiring high efficiencies provides an attractive approach for the development of OLED and other organic semiconducting materials. Further researches based on the one-step simple aromatic nucleophilic substitution reactions have been expanded to other nitrogen nucleophiles (e.g. indoles, imidazoles and benzimidazoles) and various electrophiles in the application of diverse OLED materials. We thank the financial support from NSFC (61474062) and Youth Project 973 (No. 2014CB648300). We also thank Prof. Chuluo Yang and Prof. Jingui Qin for the measurement of physical properties.
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