Synthesis and properties of 3,8-bis[4-(9H-carbazol-9-yl)phenyl]-1,10- phenanthroline for phosphorescent OLEDs

Article abstract of DOI:10.1246/cl.2008.262

A novel bipolar molecule, 3,8-bis[4-(9H-carbazol-9-yl)-phenyl]-1,10- phenanthroline (CZPT) was synthesized and employed as the host in phosphorescent organic light-emitting devices (OLEDs). A maximum luminance of 7000 cd/m ² was achieved using fac-tris(2-phenylpyridine)iridium (Ir(ppy) ₃) as the emitting material. Copyright

Full text of DOI:10.1246/cl.2008.262

262
Chemistry Letters Vol.37, No.3 (2008)
Synthesis and Properties of 3,8-Bis[4-(9H-carbazol-9-yl)phenyl]-1,10-phenanthroline
for Phosphorescent OLEDs
Ziyi Ge,1 Teruaki Hayakawa,1 Shinji Ando,1 Mitsuru Ueda,1 Toshiyuki Akiike,2 Hidetoshi Miyamoto,2
Toru Kajita,2 and Masa-aki KakimotoÃ1
1Department of Organic & Polymeric Materials, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo 152-8552
2Display Research Laboratories JSR Corporation, 100, Kawajiri-cho, Yokkaichi 510-8552
(Received December 12, 2007; CL-071376; E-mail: mkakimot@o.cc.titech.ac.jp)
A novel bipolar molecule, 3,8-bis[4-(9H-carbazol-9-yl)-
phenyl]-1,10-phenanthroline (CZPT) was synthesized and
employed as the host in phosphorescent organic light-emitting
devices (OLEDs). A maximum luminance of 7000 cd/m2 was
achieved using fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) as
the emitting material.
(DBPT)6 and BRCZ7 were prepared according to the literatures.
BRCZ was then converted to 4-(9H-carbazol-9-yl)phenylboron-
ic acid (CZBA) treated with n-BuLi followed by trimethyl
borate.8 The Suzuki cross-coupling reaction of DBPT and
CZBA led to CZPT. The structure of CZBT was identified by
IR and NMR sprectra.9
Thermal gravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) were employed to investigate the
thermal properties of CZPT. As shown in Figure 1, CZPT was
thermally stable up to 500 ꢀC with Td of 530 ꢀC in nitrogen. It
has neither obvious Tg nor melting point when heated up to
400 ꢀC and cooled to room temperature by DSC investigation,
suggesting it possesses an amorphous state. The excellent ther-
mal stability is attributed to the rigid aromatic structure, which
enables preparation of homogeneous and stable morphological
thin films by vacuum deposition.
Considerable progress has been made on phosphorescent
organic light-emitting diodes (OLEDs) for their high external
quantum efficiency via the intersystem crossing from singlet to
triplet excited states followed by relaxing through phosphores-
cence.1,2 Maintaining the charge balance in OLEDs is an impor-
tant issue for achieving highly efficient devices.3 Bipolar mole-
cules combining both electron-rich and -deficient moieties in the
molecular structures, have attracted much interest in OLEDs
arising from the fact that a balanced density of charge and high
efficiency could be attained by simultaneously supplying the
electron and hole to electroluminescent (EL) materials sand-
wiched between two electrodes.4 Further, the bipolar feature
bearing both electron- and hole-transporting units enables the
simplification of the fabrication for achieving low-cost and
large-area devices.
In this study, we report a new bipolar molecule, 3,8-bis[4-
(9H-carbazol-9-yl)phenyl]-1,10-phenanthroline (CZPT), con-
taining both electron-transporting phenanthroline and hole-
transporting carbazole units. Phenanthroline and carbazole were
chosen as the building blocks in the molecule since the electron-
deficient phenanthroline derivatives, such as 2,9-dimethyl-4,7-
diphenyl-1,10-phenanthroline (BCP), are well known as elec-
tron-transporting and hole-blocking materials, while the elec-
tron-rich carbazole derivatives have been widely used as hole-
transporting materials due to their high hole-transport mobility.5
CZPT was prepared according to the synthetic procedure as
illustrated in Scheme 1. First, 3,8-dibromo-1,10-phenanthroline
(a)
0
-10
(b)
-20
-30
-40
-50
-60
-70
-80
50
100
150
200
250
300
Temperature/°C
-90
-100
100
200
300
400
500
600
700
800
Temperature/°C
Figure 1. (a) TGA curve, (b) DSC curve.
1.0
1.0
EL
0.8
0.6
0.4
0.2
0.8
0.6
0.4
0.2
UV
Br2
+
Br
Br
Br
HCl.H2
O
N
N
PhNO2
N
N
N
N
+
SBPT
DBPT
CuI / K2CO3
DMPU
n-BuLi
B(OCH3
Br
Br
NH
N
Br
N
B(OH)2
)
3
CZBA
BRCZ
Pd(PPh3
)
4
300
400
500
600
700
DBPT
+
CZBA
N
N
K2CO3 /THF
N
N
Wavelength/nm
CZPT
Figure 2. UV spectrum of CZPT (in THF) and EL spectrum of
the device B.
Scheme 1. Synthetic procedure of CZPT.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.3 (2008)
263
Table 1. EL performance of the devices
(a)
Al (100 nm)
Turn on
voltage/V luminance/cd mÀ2
Maximum
Luminance efficiency at
100 cd mÀ2/cd AÀ1
Device
Cs:BCP (1:1, 20 nm)
TPBI (30 nm)
Al (100 nm)
Cs:BCP (1:1, 20 nm)
CZPT+6%Ir(ppy)3 (70 nm)
PEDOT (20 nm)
ITO
A
B
8.1
4100
7000
2.5
3.0
10.5
CZPT+6%Ir(ppy)3 (70 nm)
PEDOT (20 nm)
ITO
and exciton blocker, the performance of device B increased to
7000 cd/m2 and 3.0 cd/A, respectively. The HOMO level of
TPBI (6.20 eV as determined) is higher than that of CZBP
(5.76 eV) by 0.44 eV, which increased the HOMO energy level
barrier between the cathode and the emitting layer. Thus, the
holes were blocked and the charge recombination was efficiently
confined in the emitting layer consequently, suggesting that
TPBI is an effective hole-blocking layer to improve the device
performance.
Device B
Device A
(b)
7000
Device A
Device B
6000
5000
4000
3000
2000
1000
0
In summary, a novel bipolar host, CZPT was prepared
and employed as the host in phosphorescent OLEDs. CZPT
exhibited excellent thermal stability and desirable amorphous
state. Two Ir(ppy)3-based devices, with CZPT as the hosts
possessed high performance. The device exhibited improved
efficiency when a hole-blocking layer of TPBI was used.
0
100
200
300
400
500
600
References and Notes
-2
Current Density/mA cm
1
J. Ding, J. Gao, Y. Cheng, Z. Xie, L. Wang, D. Ma, X. Jing,
F. Wang, Adv. Funct. Mater. 2006, 16, 575.
M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395,
151.
Figure 3. Characteristics of OLEDs: (a) EL device structure,
(b) current–luminance curves.
2
Figure 2 outlines the UV spectrum, in which the absorption
at 375 nm originates from the ꢀ–ꢀÃ transitions from the
electron-donating carbazole to the electron-accepting phenan-
throline moiety. The experimental values of HOMO levels were
determined with a Riken AC-2 photoemission spectrometer
(PES), and those of LUMO (S1) and the triplet energy levels
(T1) were estimated from the UV and phosphorescent spectra,
respectively. The HOMO–LUMO energy gap of CZPT was
determined to be 3.18 eV. The triplet energy level of CZPT
(3.24 eV) is higher than that of Ir(ppy)3 (2.44 eV), indicating
that CZPT is suitable to be used as the host material for Ir(ppy)3
due to the prevention of back energy transfer from the guest
to the host.10
Phosphorescent OLED devices using CZPT as the host were
fabricated by vacuum deposition in two structures as outlined
in Figure 3a. As shown in Figure 2, the EL spectra exhibit
the typical green luminescence around 520 nm originated from
Ir(ppy)3, implying the energy of triplet exitons are well trans-
ferred from the host of CZPT to the guest of Ir(ppy)3.
Figure 3b outlines the representative current–luminance
(I–L) characteristics of CZPT devices and the results are summa-
rized in Table 1. In device A, poly(3,4-ethylenedioxythiophene
(PEDOT) is employed to promote the hole injection, and CZPT
doped with 6% mol of Ir(ppy)3 served as the emitting layer.
Device A exhibits good performance with the maximum lumi-
nance of 4100 cd/m2 and current efficiency at 100 cd/m2 of
2.5 cd/A, individually. When a layer of 1,3,5-tris(N-phenyl-
benzimidazol-2-yl)benzene (TPBI) was employed as the hole
3
4
Y. Shirota, M. Kinoshita, T. Noda, K. Okumoto, T. Ohara,
J. Am. Chem. Soc. 2000, 122, 11021.
T. H. Huang, J. T. Liu, L. Y. Chen, Y. T. Lin, C. C. Wu,
Adv. Mater. 2006, 18, 602.
5
6
Y. Agata, H. Shimizu, J. Kido, Chem. Lett. 2007, 36, 316.
D. Tzalis, Y. Tor, S. Failla, J. S. Siegel, Tetrahedron Lett.
1995, 36, 3489.
7
8
Q. Zhang, J. Chen, Y. Cheng, L. Wang, D. Ma, X. Jing,
F. Wang, J. Mater. Chem. 2004, 14, 895.
CZBA: H NMR (DMSO-d6, ppm) 8.26 (s, 2H), 8.24–8.22
1
(d, 2H), 8.10–8.07 (d, 2H), 7.60–7.58 (d, 2H), 7.42–7.38
(m, 4H), 7.28–7.24 (t, 2H). 13C NMR (DMSO-d6, ppm)
139.9, 138.4, 135.9, 126.3, 125.3, 122.8, 120.5, 120.1, 109.7.
The mixture of CZBA (1.03 g, 3.6 mmol), DBPT (1.01 g,
3 mmol), Pd(PPh3)4 (0.10 g, 0.09 mmol), K2CO3 (2 M,
15 mL), and THF (60 mL) was degassed and refluxed for
48 h. The precipitation was filtered and washed with THF,
then recrystallized in DMF to give a yellow solid of CZPT.
Yield: 0.83 g (42%). 1H NMR (DMSO-d6, ppm) 9.70 (s, 2H),
9.07–9.06 (d, 2H), 8.41–8.37 (m, 8H), 8.28 (s, 2H), 7.99–
7.96 (d, 4H), 7.64–7.56 (m, 8H), 7.46–7.41 (t, 4H). 13C NMR
(DMSO-d6, ppm) 151.4, 149.6, 140.6, 137.5, 136.2, 135.3,
133.4, 129.0, 128.4, 128.0, 126.1, 126.0, 123.5, 120.4,
120.2, 109.7. IR (KBr, cmÀ1): 3438, 1602, 1525, 1449,
1335, 1229.
9
10 K.-T. Wong, Y.-M. Chen, Y.-T. Lin, H.-C. Su, C.-C. Wu,
Org. Lett. 2005, 7, 5361.
Published on the web (Advance View) February 2, 2008; doi:10.1246/cl.2008.262