Syntheses and properties of novel quarterphenylene-based materials for blue organic light-emitting devices

Article abstract of DOI:10.1246/cl.2007.316

A series of quarterphenylene-based compounds were prepared and investigated as the hole-transport layer and the host materials in organic light-emitting devices (OLEDs). These compounds have wide HOMO-LUMO energy gaps (ca. 3.57 eV) due to the twisted back

Full text of DOI:10.1246/cl.2007.316

316
Chemistry Letters Vol.36, No.2 (2007)
Syntheses and Properties of Novel Quarterphenylene-based Materials
for Blue Organic Light-emitting Devices
Yuya Agata,1 Hitoshi Shimizu,1 and Junji Kidoꢀ1;2
1Optoelectronic Industry and Technology Development Association, 1-20-10 Sekiguchi, Bunkyo-ku, Tokyo 112-0014
2Department of Polymer Science and Engineering, Yamagata University, 4-3-16 Jonan, Yonezawa 992-8510
(Received November 24, 2006; CL-061392; E-mail: kid@yz.yamagata-u.ac.jp)
H3C
2
CH3
A
series of quarterphenylene-based compounds were
prepared and investigated as the hole-transport layer and the
host materials in organic light-emitting devices (OLEDs). These
compounds have wide HOMO–LUMO energy gaps (ca. 3.57
eV) due to the twisted backbone. A maximum external efficiency
of 17% was achieved for blue organic light-emitting devi0ce
using iridium(III)–bis[2-(4,6-difluorophenyl)pyridinate-N,C2 ]-
picolinate (FIrpic) as an emitting material.
B(OH)2
N
+
(HO)2B
Br
CH3
N
CH3
Pd(PPh3)4, TBAOH
Toluene / methanol
H3C
N
Recently, OLEDs using phosphorescent emitters have been
intensively investigated because these devices provide high
quantum efficiencies. For example, the devices using green-
emitting fac-tris(2-phenylpyridine)iridium [Ir(ppy)3] exhibited
the maximum external quantum efficiency (EQE) of 8.0% in
combination with hole-transporting 4,40-bis[N-(1-naphthyl)-
N-phenylamino]biphenyl (ꢀ-NPD), electron-transporting 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), and N,N0-
dicarbazolylbiphenyl (CBP) as a host material.1 Furthermore,
the incorporation of an exciton-blocking layer improved the
EQE up to 19.2%.2 On the other hand, blue OLEDs based on
phosphorescent materials such as iridium(III) bis[4,6-di(fluoro-
phenyl)pyridinato-N,C20] picolinate (Firpic) exhibited lower
efficiencies compared with the green phosphorescent devices.
The OLED, using N,N0-dicarbazolyl-3,5-benzene (mCP) as a
host, exhibited an EQE of 8.0%.3 Tokito et al. reported that
the OLED consisting of 4,40-bis(9-carbazolyl)-2,20-dimethylbi-
phenyl (CDBP) exhibited an EQE of 10.4%.4 It is considered
that the triplet energy transfer from the emitting Firpic to the host
or the hole-transport layer prevents efficient phosphorescence
from the triplet excited level of Firpic. Therefore, the high triplet
energy, or the wide HOMO–LUMO energy gap, of the host and
hole-transporting materials are required to achieve high efficien-
cy in blue phosphorescent devices.4 In this study, we prepared a
new series of quarterphenylene-based hole-transporting materi-
als and host materials for blue electrophosphorescent devices.5
As shown in Scheme 1, 2,20-bis(4-ditolylaminophenyl)-1,10-
biphenyl (1a) was synthesized by the Suzuki coupling reaction
of 2,20-biphenyldiboronic acid6 and 4-(N,N0-ditolylamino)-1-
bromobenzene obtained from p,p0-ditolylamine and 1-bromo-
4-iodobenzene.7 2,20-Bis(3-ditolylaminophenyl)-1,10-biphenyl
(1b), 2,20-bis(4-carbazolylphenyl)-1,10-biphenyl (2a), and 1,10-
bis(3-carbazolylphenyl)-2,20-biphenyl (2b) were prepared in
the similar manner. All products purified by column chromato-
graphy were identified by 1H NMR and elemental analyses.
The thermal properties of these materials were measured
by thermogravimetric analysis (TGA) and differential scanning
calorimetry (DSC). The electrochemical properties were studied
using UV photoelectron spectrometry (AC-1, Riken Keiki Co.,
Japan) and absorption spectroscopy. The glass transition temper-
1a
CH3
CH3
H3C
N
N
CH3
1b
CH3
N
N
N
N
2a
2b
Scheme 1. Synthesis and chemical structures of 1a–2b.
ature (Tg), melting temperature (Tm), decomposition temperature
(Td), ionization potential (Ip), electron affinity (Ea), and HOMO–
LUMO energy gap (Eg) are summarized in Table 1. The Tg
values of the compounds were between 87 and 120 ꢁC. As
can be seen from Table 1, the Tg values of 1a and 1b having
triphenylamine moieties are lower than those of the carbazole-
containing compounds (2a and 2b). This is due to the fact that
triphenylamine moieties possess nonplaner chemical structures.
Since 1b and 2b did not show melting points, they form stable
amorphous states. From the absorption edges, the HOMO–
LUMO energy gaps of the compounds were determined to be
Table 1. Thermal and electrochemical data of 1a–2b
Compound Tg/ꢁCa Tm/ꢁCa Td/ꢁCb Ip/eVc Eg/eVd Ea/eVe
1a
1b
2a
2b
98
87
120
110
220
n.d.
277
n.d.
434
424
472
460
5.40
5.57
6.06
6.04
3.26
3.57
3.46
3.48
2.14
2.00
2.60
2.56
aDetermined by DSC measurement. bObtained from TGA analysis.
cMeasured by an AC-1 UV photoelectron spectrometer. dTaken as the point
of intersection of the normalized absorption spectra. eCalculated using Ip
and Eg values.
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
317
Table 2. The maximum power efficiencies, current efficiencies,
and external quantum efficiencies of the blue emitting device
a)
Al (100 nm)
N
N
O
Maximum external Maximum luminance Maximum power
quantum efficiency/% efficiency/cd Aꢂ1 efficiency/lm Wꢂ1
Ir
F
LiF (0.5 nm)
t-Bu TAZ (30 nm)
7%-FIrpic: HOST (30 nm)
HTL (40 nm)
Device
O
1
2
14.5
16.5
32.5
36.0
21.6
17.3
F
FIrpic
luminance reaches 18300 cd/m2 at an applied voltage of
15.0 V for Device 1.
N
N
N
ITO
t-Bu
The maximum power efficiencies, current efficiencies, and
external quantum efficiencies of the devices were summarized
in Table 2. The high external quantum efficiencies of 14.5–
16.5% and the high maximum power efficiencies of 17.3–
21.6 lm/W indicate that the triplet energy transfer from FIrpic
to the host or the HTL is effectively prevented.
In summary, novel quaterphenylene-based hole-transport
materials and host materials for OLED were prepared. The
FIrpic-based blue phosphorescent OLEDs with these materials
exhibited high performance.
Device 1 (
Device 2 (
,
,
): HTL = 1a, Host = 2a
): HTL = 1b, Host = 2b
t-Bu TAZ
105
104
103
102
101
100
10-1
10-2
103
102
101
100
10-1
b)
: Device 1
: Device 2
This study was financially supported, in part, by NEDO,
through ‘‘High Efficiency Organic Device’’ Project.
References and Notes
1
2
3
M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson,
S. R. Forrest, Appl. Phys. Lett. 1999, 75, 4.
M. Ikai, S. Tokito, Y. Sakamoto, T. Suzuki, Y. Taga, Appl.
Phys. Lett. 2001, 79, 156.
R. J. Holmes, S. R. Forrest, Y.-J. Tung, R. C. Kwong,
J. J. Brown, S. Garon, M. E. Thompson, Appl. Phys. Lett.
2003, 82, 2422.
: Device 1
: Device 2
10-2
0
5
10
Voltage/V
15
20
Figure 1. Characteristics of OLEDs: a) EL device structures,
b) current density–voltage–luminance curves.
4
5
6
7
S. Tokito, T. Iijima, Y. Suzuri, H. Kita, T. Tsuzuki, F. Sato,
Appl. Phys. Lett. 2003, 83, 569.
Y. Agata, H. Shimizu, J. Kido, Polym. Prepr. Jpn. 2004, 53,
1331.
E. G. Ijpeij, F. H. Beijer, H. J. Arts, C. Newton, J. G. de
Vries, G.-J. M. Gruter, J. Org. Chem. 2002, 67, 169.
3.26 to 3.57 eV. These values are greater than those of conven-
tional OLED materials such as ꢀ-NPD and CBP, which is due to
the twisted structures of the quarterphenylene backbone. The Ip
values of 1a and 1b were 5.40 and 5.57 eV, respectively. These
Ip values are almost the same as that of ꢀ-NPD, and hence, 1a
and 1b are expected to function as hole-transporting materials
in OLEDs.
We fabricated the blue phosphorescent OLEDs using these
materials. 1a and 1b were used as the hole-transport layer
(HTL), and 2a and 2b were used as the host materials in the
emitting layer. 3-(4-Biphenylyl)-4-phenyl-5-(4-tert-butylphen-
yl)-1,2,4-triazole (t-Bu TAZ) was used as the electron-transport
layer. All organic layers were successively formed by vacuum
deposition at 10ꢂ6 Torr. The device architecture was described
in Figure 1a. Figure 1b shows the current density–voltage and
luminance–voltage characteristics of the OLEDs using 1a and
2a (Device 1) and 1b and 2b (Device 2), respectively. The
turn-on voltage of Device 1 is lower than that of Device 2.
Because the Ip value of 1a is smaller than that of 2a, the barrier
height for the hole injection should be smaller for 1a. The
To
a solution of tetra(n-butyl)ammonium hydroxide
(TBAOH) (19.0 mL; 1.0 M; 19 mol) in methanol were added
toluene (30 mL), 4-(N,N0-ditolylamino)-1-bromobenzene
(4.00 g, 11.4 mmol), 2,20-biphenyldiboronic acid (1.50 g,
6.20 mmol), and tetrakis(triphenylphosphine)palladium(0)
(Pd(PPh3)4) (34 mg, 0.029 mmol). The mixture was refluxed
with stirring under N2 for 48 h. The solvent was removed on
a rotary evaporator and the residue was dissolved in chloro-
form. The solution was washed with 5% HCl and brine,
dried over anhydrous MgSO4, and filtered. The solvent
was evaporated and purified by column chromatography on
silica gel using a mixture of chloroform and hexanes (vol
ratio = 1/3) as eluent. The yield was 0.93 g (22%). 1H NMR
(CDCl3): ꢁ7.43–7.38 (m, 2H), 7.33–7.28 (m, 4H), 7.21–7.16
(m, 2H), 6.98–6.86 (m, 16H), 6.70–6.64 (d, 4H), 6.52–6.46
(d, 4H), 2.28–2.20 (s, 12H).
Published on the web (Advance View) January 27, 2007; doi:10.1246/cl.2007.316