Pyridine/isoquinoline-carbazole containing bipolar host materials for green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1166/jnn.2011.3371

We demonstrated the electroluminescent properties of bipolar host materials such as 9-(3-(6-methylpyridin-2-yl)phenyl)-9H-carbazole (Czpmpy), 9-(6-phenylpyridin-2-yl)-9H-carbazole (Czppy), and 9-(3-(isoquinolin-1-yl) phenyl)-9H-carbazole (Czpiq). Particul

Full text of DOI:10.1166/jnn.2011.3371

Copyright © 2011 American Scientific Publishers
All rights reserved
Printed in the United States of America
Journal of
Nanoscience and Nanotechnology
Vol. 11, 1499–1502, 2011
Pyridine/Isoquinoline-Carbazole Containing Bipolar
Host Materials for Green Phosphorescent
Organic Light-Emitting Diodes
Kum Hee Lee1, Hyun Ju Kang1, Heo Min Kim2, Ji Hyun Seo2,
Young Kwan Kim2ꢀ∗, and Seung Soo Yoon1ꢀ∗
1Department of Chemistry, Sungkyunkwan University, Suwon, 440-746, Korea
2Department of Information Display, Hongik University, Seoul 121-791, Korea
We demonstrated the electroluminescent properties of bipolar host materials such as 9-(3-(6-
methylpyridin-2-yl)phenyl)-9H-carbazole (Czpmpy), 9-(6-phenylpyridin-2-yl)-9H-carbazole (Czppy),
and 9-(3-(isoquinolin-1-yl)phenyl)-9H-carbazole (Czpiq). Particularly, by using host (Czpiq) and
dopant (bis[2-(1,1ꢀ,2ꢀ,1ꢀꢀ-terphen-3-yl)pyridinato-C,N]iridium(III) (tphpy)2Ir(acac)) as the emitting
layer, a green phosphorescent OLED was fabricated, showing a maximum luminance of 12780
cd/m2 at 10 V, maximum luminous efficiency of 45.0 cd/A, power efficiency of 47.1 lm/W, maximum
external quantum efficiency of 12.3%, and CIE xꢀy coordinates of (0.34, 0.58) at 8 V.
Keywords: OLEDs, Green Phosphorescence, Bipolar Host, Pyridine/Isoquioline-Carbazole
Containing Materials.
Delivered by Publishing Technology to: Adelaide Theological Library
IP: 117.253.137.136 On: Mon, 28 Mar 2016 15:15:10
Copyright: American Scientific Publishers
1. INTRODUCTION
2. EXPERIMENTAL DETAILS
Organic light-emitting diodes (OLEDs) are being investi-
gated widely for their potential applications in full-color
flat-panel displays.1ꢀ2 Forrest and Thompson’s groups have
developed transition metal complexes, which emit phos-
phorescence from their triplet states, realizing nearly 100%
internal quantum efficiencies of electroluminescence.3–6
For efficient phosphorescent OLEDs (PHOLEDs), the
heavy-metal guest materials are doped into charge-
transporting host materials. Recently, bipolar molecules
have attracted much interest in this field, since a balanced
density of charge could be achieved by simultaneously
supplying the electron and hole to electroluminescent (EL)
materials, resulting in high efficiency and excellent device
stablility.7–11
In this paper, we report a molecular design strategy of
combining the carbazole with the good hole-transporting
ability and pyridine or isoqunoline with high electron
affinity and high triplet energy to yield the bipolar host
material 9-(3-(6-methylpyridin-2-yl) phenyl)-9H-carba-
zole (Czpmpy), 9-(6-phenylpyridin-2-yl) -9H- carbazole
(Czppy), and 9-(3-(isoquinolin-1-yl)phenyl)-9H- carbazole
(Czpiq). As will be seen below, by using these mate-
rials as host materials, efficient green PHOLEDs were
demonstrated.
2.1. Synthesis and Characterization
9-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
9H-carbazole9 and 2-bromo-6-phenylpyridine12 were
prepared by a previously-reported method. Low resolu-
tion mass spectra were measured using a Jeol JMS-600
spectrometer in EI mode. Elemental Analysis was done
using an EA1108 Elemental analyzer (Fisons Instrument).
UV-vis absorption spectra were measured on a Shimadzu
UV-1650PC spectrometer. Photoluminescence (PL) spectra
were measured on an Aminco-Bowman series 2 lumi-
nescence spectrometer. The UV-vis and PL spectra of
phosphorescent host materials were measured in a 10−5
M
dilute CH2Cl2 solution. The energy levels were measured
with a low-energy photo-electron spectrometer (Riken-
Keiki, AC-2). Thermal properties were measured using
thermogravimetric analysis (TGA) (DTA-TGA, TA-4000)
and differential scanning calorimeter (DSC) (Mettler
Toledo; DSC 822) under N2 at a heating rate of 10 ꢁC/min
ꢁ
and cooling rate of 10 C/min.
Synthesis of 9-(3-(6-methylpyridin-2-yl)phenyl)-9H-
carbazole (Czpmpy). 9-(3-(4,4,5,5-Tetramethyl-1,3,2-di-
oxaborolan-2-yl)phenyl)-9H-carbazole (1.29 g, 2.90 mol),
2-bromo-6-methylpyridine (0.5 g, 3.49 mol), Pd(PPh3ꢁ4
(0.04 mol), aqueous K2CO3 (2.0 M, 29.1 mol), Aliquat
336, and toluene were mixed in a flask. The mixture was
∗Authors to whom correspondence should be addressed.
J. Nanosci. Nanotechnol. 2011, Vol. 11, No. 2
1533-4880/2011/11/1499/004
doi:10.1166/jnn.2011.3371
1499

Pyridine/Isoquinoline-Carbazole Containing Bipolar Host Materials for Green Phosphorescent OLEDs
Lee et al.
refluxed for 4 h. After the reaction finished, the reaction
mixture was extracted with ethyl acetate and washed with
water. The organic layer was dried with anhydrous MgSO4
and filtered with silica gel. The solution was then evap-
orated. Czpmpy was obtained as a white solid (0.76 g,
(50 nm)/4,4ꢀ,4ꢀꢀ-tris(N-carbazole)triphenylamine (TCTA)
(10 nm)/Phosphorescent host materials: Green dopant
material (tphpy)2Ir(acac)13 (30 nm, 8%)/4,7-diphenyl-1,10-
phenanthroline (Bphen) (30 nm)/lithium quinolate (Liq)
(2 nm)/Al (100 nm). The current density (J), luminance
(L), luminous efficiency (LE), and CIE chromaticity coor-
dinates of the OLEDs were measured with a Keithly 2400,
Chroma meter CS-1000A. Electroluminance was measured
using a Roper Scientific Pro 300i.
1
78%) after recrystallization from CH2Cl2/EtOH. H-NMR
(300 MHz, CDCl3ꢁ: ꢂ 8.16 (t, J = 7ꢃ7 Hz, 2H), 8.08 (d,
J = 7ꢃ8 Hz, 1H), 7.70–7.51 (m, 5H), 7.46–7.37 (m, 4H),
7.31–7.26 (m, 2H), 7.09 (d, J = 7ꢃ5 Hz, 1H). 13C-NMR
(75 MHz, CDCl3ꢁ: ꢂ 158.9, 156.1, 142.1, 141.2, 138.4,
137.3, 130.5, 127.6, 126.3, 126.2, 126.1, 123.6, 122.4,
120.6, 120.2, 118.0, 110.1. FT-IR [ATR]: ꢄ = 3015ꢀ 2970,
1366, 1092, 1033 cm−1. MS(EI+ꢁ m/z 334 (M+ꢁ. Anal.
calcd for C24H18N2: C 86.20, H 5.43, N 8.38 found: C
85.56, H 5.44, N 8.32.
3. RESULTS AND DISCUSSION
The syntheses of the phosphorescent host materials
are outlined in Scheme 1. Suzuki cross-coupling of
9-(3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-
9H-carbazole with the corresponding halide afforded
Czpmpy and Czpiq in moderated yields. Czppy was
prepared by the Buchwald-Hartwig cross-coupling with
2-bromo-6-phenylpyridine and cabazole in 52% yield.
The ultraviolet-visible (UV-Vis) absorption and photolu-
minescence (PL) spectra of Czpmpy, Czppy, and Czpiq
in dichloromethane are shown in Figure 1 (see Table I).
The maximum absorption peaks were located at 282, 243,
and 241 nm for Czpmpy, Czppy, and Czpiq, respectively.
The PL spectra of Czpmpy, Czppy, Czpiq were 394,
392, and 444 nm in dichloromethane, respectively. The
band gap of Czpmpy (3.70 eV), Czppy (3.67 eV), Czpiq
Synthesis of 9-(6-phenylpyridin-2-yl)-9H-carbazole
(Czppy). 2-Bromo-6-phenylpyridine (0.80 g, 3.42 mmol),
cabazole (0.69 g, 4.10 mmol), Pd2(dba3ꢁ (0.16 g,
0.17 mmol), P(t-Bu)3 (0.01 g, 0.0342 mmol), and NaOt-
Bu (0.62 g, 6.40 mmol) were dissolved in anhydrous
toluene (20 mLꢁ). The mixture was heated to reflux with
stirring at 120 C for 18 h. After cooling to room tem-
perature, the solvent was removed and the residue was
purified by column chromatography eluting with 5–20%
ethyl acetate/hexane. Czppy was obtained as a white solid
1
(0.57 g, 52%). H-NMR (300 MHz, CDCl3ꢁ: ꢂ 8.15–8.11
(m,4H), 7.94 (d, J = 8ꢃ4 Hz, 3H), 7.73–7.70 (m, 1H),
Delivered by Publishing Technology to: Adelaide Theological Library
13
(3.59 eV) were broader than that (3.5 eV) of the CBP. The
phosphorescence of Czpmpy, Czppy, and Czpiq were
IP: 117.253.137.136 On: Mon, 28 Mar 2016 15:15:10
7.54–7.41 (m, 6H), 7.35–7.30 (m, 2H). C-NMR (125
Copyright: American Scientific Publishers
MHz, CDCl3ꢁ: ꢂ 157.3, 151.8, 139.9, 139.4, 138.7, 129.7,
129.1, 127.1, 126.4, 124.6, 121.2, 120.4, 117.5, 117.3,
111.7. FT-IR [ATR]: ꢄ = 3015, 1738, 1366, 1092, 1055,
1032 cm−1. MS(EI+ꢁ m/z 320 (M+ꢁ. Anal. calcd for
C23H16N2: C 86.22, H 5.03, N 8.74 found: C 85.86, H
5.03, N 8.76.
also measured at 77 K (in 2-methyltetrahydrofuran). From
the highest-energy 0–0 phosphorescence emissions, we
estimate that their values of ET are 2.64, 2.62, and 2.89 eV,
respectively. These values are higher than that of CBP
(ET = 2ꢃ56 eV) and (tphpy)2Ir(acac) (ET = 2ꢃ23 eV), which
indicates that Czpiq may be an appropriate host materials
for green, red, and even blue phosphorescent emitters.
Figure 2 displays the device structure and relative
HOMO/LUMO energy level of the materials used in study.
The HOMO energy levels of Czpmpy, Czppy, and Czpiq,
measured using a photoelectron spectrometer (Riken-Keiki
Synthesis of 9-(3-(isoquinolin-1-yl)phenyl)-9 H-
carbazole (Czpiq). White solid with a yield of 0.88 g
(76%). 1H NMR (300 MHz, CDCl3ꢁ: ꢂ 8.62 (d, J =
5ꢃ72 Hz, 1H), 8.19–8.12 (m, 3H), 7.89 (d, J = 11ꢃ7 Hz,
1H), 7.85–7.78 (m, 2H), 7.74 (d, J = 7ꢃ48 Hz, 1H), 7.71–
7.64 (m, 3H), 7.57–7.51 (m, 3H), 7.43–7.38 (m, 2H),
7.29–7.25 (m, 2H). 13C NMR (75 MHz, CDCl3ꢁ: ꢂ 159.8,
158.0, 142.6, 141.8, 141.1, 138.0, 137.2, 130.5, 130.3,
129.2, 128.8, 127.8, 127.4, 127.3, 126.9, 126.3, 123.7,
120.7, 120.6, 120.3, 110.1. FT-IR [ATR]: ꢄ = 3015, 1366,
1092, 1055, 1032 cm−1; MS (EI, m/z) 370 (M+ꢁ. Anal.
Calcd for C27H18N2: C 87.54, H 4.90, N 7.56; found: C
87.02, H 4.84, N 7.57.
Pd(PPh3)4,
2MK2CO3
Aliquat336
O
B
N
Br
N
N
O
N
Toluene
Czpmpy
Pd2(dba)3,
P(t-Bu)3,
NaOtBu
H
N
Br
N
N
N
Toluene
Czppy
2.2. Device Fabrication and Characterization
Pd(PPh3)4,
2MK2CO3
Aliquat336
Cl
OLEDs using green-light-emitting molecules were
fabricated by vacuum (5×10−7 torr) thermal evapo-
ration onto precleaned ITO coated glass substrates.
The structure was as follows: ITO/N,Nꢀ-diphenyl-
O
B
N
N
N
O
N
Toluene
Czpiq
N,Nꢀ-(1-napthyl)-(1,1ꢀ-phenyl)-4,4ꢀ-diamine
(NPB)
Scheme 1. Synthesis of Czpmpy, Czppy, and Czpiq.
J. Nanosci. Nanotechnol. 11, 1499–1502, 2011
1500

Lee et al.
Pyridine/Isoquinoline-Carbazole Containing Bipolar Host Materials for Green Phosphorescent OLEDs
Table I. Optical properties of host materials (Czpmpy, Czppy, and
Czpiq).
Czpmpy
Czppy
Czpiq
Czpmpy 0.8
Czppy
Czpiq
Czpmpy
Czppy
Czpiq
1.0
0.8
0.6
0.4
0.2
0.0
1.0
Compound
Czpmpy
Czppy
Czpiq
UV ꢅmax [nm]a
PL ꢅmax [nm]a
fwhm [nm]a
HOMO [eV]b
LUMO [eV]b
Egb
282
394
93
−5.89
−2.19
3.70 eV
2.64 eV
271/90
243
398
59
−5.90
−2.23
3.67 eV
2.62 eV
292/98
241
444
80
−5.85
−2.26
3.59 eV
2.89 eV
287/84
0.6
0.4
0.2
0.0
ETc
Td [ꢁC]/ Tg[ꢁC]
aMeasured in CH2Cl2 solution. bObtained from AC-2 and UV-vis absorption mea-
surements. cMeasured in 2-MeTHF solution at 77 K.
300
400
500
600
Wavelenght (nm)
Fig. 1. UV-Vis absorption (CH2Cl2, room temperature, solid line), fluo-
rescence spectra (CH2Cl2, room temperature, opened symbols), and phos-
phorescence spectra of (2-MeTHF, 77 K, closed symbols) for Czpmpy,
Czppy, and Czpiq.
(30 nm)/Liq (2 nm)/Al (100 nm). NPB, Bphen and Liq
were used as hole-transporting, electron-transporting and
electron-injecting materials, respectively. TCTA was used
as exciton-blocking materials to prevent exciton leakages
from the emitting layer to NPB layer to improve the EL
performances.
The current density–voltage–luminance (I–V–L) charac-
teristics (a) and efficiency versus current density curves
of devices examined in this work are shown in Figure 3.
Table II summarizes the electroluminescence characteris-
tics of the resulting devices.
AC-2), were −5.89, −5.90, and −5.85 eV, respectively.
Compared to the HOMO energy level of Czpiq, Czpmpy
and Czppy showed lower HOMO energy levels of 0.04
and 0.05 eV, respectively. The LUMO energy levels
were calculated at −2.19, −2.23, and −2.26 eV for
Czpmpy, Czppy, and Czpiq. The LUMO energy level
was decreased by directly connecting carbazole with the
pyridine moieties. In addition, compared to LUMO energy
Delivered by Publishing Technology to: Adelaide Theological Library
level of Czpmpy, Czpiq showed lower LUMO energy
(a)
160
IP: 117.253.137.136 On: Mon, 28 Mar 2016 15:15:10
level of 0.07 eV due to difference in electron affinity.
Copyright: American Scient1if4ic0 Publishers
12000
9000
6000
3000
0
The thermal stability of Czpmpy, Czppy, and Czpiq
Czpmpy
Czppy
Czpiq
120
100
80
60
40
20
0
are shown by the decomposition temperature (Td, corre-
ꢁ
sponding to 5% weight loss) of 271, 292, and 287 C by
TGA. The glass transition temperature (Tgꢁ were observed
Czpmpy
Czppy
Czpiq
ꢁ
at 90, 98, and 84 C for Czpmpy, Czppy, and Czpiq,
ꢁ
which is much higher than that of CBP (62 C). We found
that Czppy, with its direct connecting carbazole with pyri-
dine moieties, exhibited higher value of Td and Tg than
Czpmpy andCzpiq with benzene ring.
To evaluate Czpmpy, Czppy, and Czpiq as host materi-
als in PHOLEDs, we have chosen the triplet green emitter
(tphpy)2Ir(acac)13 for the device fabrications. The device
structure of ITO/NPB (50 nm)/TCTA (10 nm)/ Phospho-
rescent host materials:(tphpy)2Ir(acac) (30 nm, 8%)/Bphen
0
2
4
6
8
10
12
14
Voltage (V)
(b)
100
80
60
40
20
Czpmpy
Czppy
Czpiq
Czpmpy
Czppy
Czpiq
14
12
10
8
6
4
2
0
0.01
0.1
1
10
100
Current density (mA/cm2)
Fig. 3. (a) I-V-L characteristics of devices and (b) luminous efficiency
and external quantum efficiency versus current density relationship for
PHOLEDs.
Fig. 2. Energy-level diagram of the materials used in devices.
J. Nanosci. Nanotechnol. 11, 1499–1502, 2011
1501

Pyridine/Isoquinoline-Carbazole Containing Bipolar Host Materials for Green Phosphorescent OLEDs
Lee et al.
Table II. EL performance characteristic of the devices Czpmpy,
Czppy, and Czpiq.
energy transfer from Czpiq host to the dopant compared
to devices of Czpmpy and Czppy.
This study clearly demonstrated the simple molecu-
lar design strategy of combining carbazole and isoquino-
line groups for bipolar host materials for the efficient
PHOLEDs.
Devices
Czpmpy
Czppy
Czpiq
ELmax [nm]
Vtaurn-on[V]
531
7.0 V
526
3.5 V
529
3.0 V
Lb [cd/m2]
LEb [cd/A]
PEb [lm/W]
EQEb [%]
CIEc (xꢀyꢁ
438 (10 V)
6.44 (5.0 V)
4.05 (5.0 V)
4.41 (5.0 V)
(0.34, 0.62)
4362 (10 V)
17.9 (5.0 V)
11.6 (4.5 V)
5.08 (5.0 V)
(0.35, 0.61)
12780 (10 V)
45.0 (3.0 V)
47.1 (3.0 V)
12.3 (3.0 V)
(0.34, 0.58)
4. CONCLUSION
We demonstrated a molecular design strategy of combin-
ing a carbazole as an electron donor and pyridine or iso-
quilonine as an electron acceptor to give the bipolar host
materials for the efficient PHOLEDs. Particularly, a green
PHOLED using Czpiq a s a bipolar host material and
(tphpy)2Ir(acac)) as a dopant in the emitting layer showed
the maximum luminous efficiency of 45.0 cd/A, power
efficiency of 47.1 lm/W, the maximum external quantum
efficiency of 12.3% and CIE xꢀy coordinates of (0.34,
058) at 8 V.
aTurn-on voltage at 1 cd/m2. bValues in parentheses are the voltages at which
the maximum value were obtained. cCommission Internationale d’Énclairage (CIE)
coordinates at a 8 V.
Under a constant voltage of 6 V, the current density of
Czpmpy, Czppy, and Czpiq were found to be 0.02, 7.65,
and 12.5 mA/cm2, respectively. As expected, the current
flow with Czpiq was significantly improved because of
the lower electron injection barrier and reduction of car-
rier trapping by the dopant materials. The turn-on volt-
age, defined as the bias required attaining a luminance of
1 cd/m2, is 7.0, 3.5, and 3.0 V for the Czpmpy, Czppy,
and Czpiq, respectively. Interestingly, the device using
Czpiq as a host shows the much superior current density
flow, and the lower operation driving voltage and turn-on
voltage than those using Czpmpy and Czppy as hosts.
The luminous and external quantum efficiencies (EQE)
achieved with bipolar host Czpiq (45.0 cd/A; 12.3% EQE)
References and Notes
1. J. Kido, M. Kimura, and K. Nagai, Science 267, 1332 (1995).
2. R. F. Service, Science 273, 878 (1996).
3. M. A. Baldo, D. F. O’Brine, Y. You, A. Shustikov, S. Sibley, M. E.
Thompson, and S. R. Forrest, Nature 398, 151 (1998).
4. C. Adachi, R. C. Kwong, P. Djurovich, V. Adamovich, M. A. Baldo,
Delivered by Publishing Technology to: Adelaide Theological Library
M. E. Thompson, and S. R. Forrest, Appl. Phys. Lett. 79, 2082
IP: 117.253.137.136 On: Mon, 2(820M01a).r 2016 15:15:10
are higher than those obtain with Czppy (17.9 cd/A;
5.08% EQE) and Czpmpy (6.44 cd/A; 4.41% EQE) as the
host.
Copyright: American Scientific Publishers
5. X. Ren, J. Li, R. J. Holmes, P. I. Djuorvich, S. R. Forrest, and
M. E. Thompson, Chem. Mater. 16, 4743 (2004).
6. C. Yang, X. Zhang, H. You, L.-Y. Zhu, L. Chen, L.-N. Zhu, Y. Tao,
D. Ma, Z. Shuai, and J. Qin, Adv. Funct. Mater. 17, 651 (2007).
7. C. T. Chen, Y. Wei, J. S. Lin, M. V. R. K. Moturu, W. S. Chao,
Y. T. Tao, and C. H. Chien, J. Am. Chem. Soc. 128, 10992
(2006).
8. Y.-L. Liao, C.-Y. Lin, K.-T. Wong, T.-H. Hou, and W. -Y. Hung,
Org. Lett. 9, 4511 (2007).
9. S.-J. Su, H. Sasabe, T. Takeda, and J. Kido, Chem. Mater. 20, 1691
(2008).
10. M.-H. Tasi, H.-W. Lin, H.-C. Su, T.-H. Ke, C. Wu, F.-C. Fang, Y.-Li,
Liao, K.-T. Wong, and C.-I. Wu, Adv. Mater. 18, 1216 (2006).
11. H. Wang, J. Ryu, C. Cao, Y. Shin, and Y. Kwon, J. Nanosci.
Nanotechnol. 9, 7240 (2009).
12. A. Bouillon, J.-C. Lancelot, J. S. O. Santos, V. Collot, P. R. Bovy,
and S. Rault, Tetrahedron 59, 10043 (2003).
A green OLED using Czpiq as a host exhibited
the improved EL performances, compared to that using
Czpmpy and Czppy as a host because Czpiq has the
lower LUMO level and thus the improved electron injec-
tion property. This may be attributed to the more suit-
able energy-level matching for the host and the dopant
in device using Czpiq. The green dopant (tphpy)2Ir(acac)
has HOMO and LUMO levels of −5.25 and −2.68 eV,
respectively.13 Therefore, the HOMO and LUMO levels
of (tphpy)2Ir(acac) fall within those of the Czpiq host,
with shallow hole and electron trap depth of 0.60 and
0.42 eV, respectively, which leads to a more balanced
charge trapping and recombination, and more efficient
13. K. H. Lee, J. H. Seo, Y. K. Kim, and S. S. Yoon, J. Nanosci.
Nanotechnol. 9, 7099 (2009).
Received: 13 November 2009. Accepted: 24 March 2010.
1502
J. Nanosci. Nanotechnol. 11, 1499–1502, 2011