Novel electron-transport material containing boron atom with a high triplet excited energy level

Article abstract of DOI:10.1246/cl.2007.262

Tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) with high triplet excited energy level was synthesized and investigated as an electron-transport material for organic light-emitting devices (OLEDs). Larger current density and higher luminance were observed for

Full text of DOI:10.1246/cl.2007.262

262
Chemistry Letters Vol.36, No.2 (2007)
Novel Electron-transport Material Containing Boron Atom
with a High Triplet Excited Energy Level
Daisaku Tanaka,1 Takashi Takeda,1 Takayuki Chiba,2 Soichi Watanabe,2 and Junji Kidoꢀ1;2
1Optoelectronic Industry and Technology Development Association (OITDA), Bunkyo, Tokyo 112-0014
2Department of Polymer Science and Engineering, Yamagata University, Yonezawa 992-8510
(Received November 14, 2006; CL-061338; E-mail: kid@yz.yamagata-u.ac.jp)
Br
B
Tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) with high
triplet excited energy level was synthesized and investigated
as an electron-transport material for organic light-emitting de-
vices (OLEDs). Larger current density and higher luminance
were observed for the OLED using 3TPYMB, compared with
the OLED with a conventional electron-transport material,
tris(8-quinolinolato)aluminum (Alq3).
Br
Br
a), b)
Br
Br
1
Three-coordinate boron atom, with its vacant p-orbital, is a
strong ꢀ-electron acceptor in conjugated organic molecules.
Therefore, organoboron compounds have been investigated as
electron-transporting materials in organic light-emitting devices
(OLEDs).1–3 Recently, OLEDs having phosphorescent emitters
have received considerable attention, since such emitting mate-
rials can provide higher quantum efficiencies.4,5
N
O
O
B
N
B
N
c)
To achieve high efficiencies in OLEDs having phosphores-
cent emitters, it is important to suppress the transfer of the triplet
excited energy from the phosphorescent emitter not only to the
host material but also to the carrier transport materials adjacent
to the emitting layer (EML). It is, therefore, necessary to use
carrier transport materials with triplet energy levels higher than
that of the phosphorescent emitter.
N
3TPYMB
←
a) n-BuLi 1 equiv., -78 °C rt, 3 h
b) Boron trifluoride diethyletherate 1/3 equiv.,
←
-78 °C rt, 12 h
c) Pd(PPh3)4, Toluene/EtOH, 2M Na2CO3 aq, 72 °C, 12 h
Concerning electron-transport materials (ETMs) for
OLEDs, a variety of materials have been reported.6–8 However,
there are few ETMs with sufficiently high triplet energy levels.
From such a point of view, we synthesized a novel electron-
transport material containing trimesitylborane unit, tris[3-(3-
pyridyl)mesityl]borane (3TPYMB), with a wide HOMO–
LUMO energy gap and a high triplet excited energy level. In
designing the molecular structure, the following points were
considered. First, because of boron atom with vacant p-orbital,
we can expect a high electron affinity, resulting in reducing
the barrier height for the electron injection from the cathode.
Second, by introduction of electron-withdrawing units such as
pyridine units, a high electron affinity is expected. Third, the
twisted molecular structure, by the introduction of mesityl
groups, leads to wide HOMO–LUMO energy gaps and high
triplet excited energy levels.
Scheme 1. Synthesis of 3TPYMB.
observed at 106 ꢁC from differential scanning calorimetry analy-
sis, which is high enough for OLED application. Crystallization
of 3TPYMB was found to be suppressed in the solid state be-
cause of its twisted structure. Optoelectronic properties of Alq3
and 3TPYMB are summarized in Table 1. Ionization potential
(Ip) of 3TPYMB was determined to be 6.77 eV by photoelectron
spectroscopy under ambient atmosphere, which is larger as com-
pared to Alq3, indicating high hole blocking ability of 3TPYMB.
The HOMO–LUMO energy gap (Eg) of 3TPYMB was deter-
mined to be 3.45 eV from UV absorption edge, which can lead
to high triplet energy level. Electron affinity (Ea) of 3TPYMB
was estimated to be 3.32 eV, which is greater than that of Alq3.
Low barrier height for the electron injection from the cathode is
expected.
Scheme 1 shows the chemical structure of 3TPYMB and the
synthetic scheme. Compound 1 was prepared via monolithiation
of 2,4-dibromomesitylene using 1.0 equiv. of n-butyllithium,
followed by the reaction with boron trifluoride diethyl etherate
in 70% yield. 3TPYMB was synthesized by palladium-catalyzed
Suzuki coupling reaction of 1 with 3-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)pyridine.9 The crude material was purified by
silica-gel column chromatography to give colorless glassy mate-
rial (yield 87%). 3TPYMB was further purified by train sublima-
tion method for the OLED application.
Electron drift mobility of 3TPYMB and Alq3 were deter-
mined by the time-of-flight method (Figure 1). The electron mo-
bility of 3TPYMB was about 10ꢂ5 cm2 Vꢂ1 sꢂ1 order, which is
a
Table 1. Optoelectronic properties of 3TPYMB and Alq3
Ip/eV
Eg/eV
Ea/eV
3TPYMB
Alq3
6.77
5.93
3.45
2.71
3.32
3.22
aIp: measured by Riken Keiki AC-3, Eg: determined from
UV absorption edge, Ea: calculated from Ip and Eg values.
The glass transition temperature (Tg) of 3TPYMB was
Copyright Ó 2007 The Chemical Society of Japan

Chemistry Letters Vol.36, No.2 (2007)
263
10-3
10-4
10-5
10-6
350
400
450
500
550
600
650
650
700
750
1/2 [(V/cm)1/2
800
850
900
Wavelength / nm
]
E
Figure 3. Phosphorescence spectrum of a vacuum deposited
3TPYMB film at 4.2 K, measured by streak camera with a N2
gas laser (337 nm) as an excitation source.
Figure 1. Electric field dependence of electron mobility for
3TPYMB (squares) and Alq3 (circles).
densities and higher luminance at the same driving voltages
compared with the device II. Such low drive voltages of the
device I are due to the low LUMO level of 3TPYMB, resulting
in the reduction of the barrier height for the electron injection
from the cathode and to the high electron mobility of 3TPYMB.
Figure 3 shows phosphorescence spectrum of 3TPYMB.
The onset of 3TPYMB phosphorescence was observed at about
420 nm, which is equivalent to the excited energy level of
2.95 eV. Therefore, it is considered that 3TPYMB is useful for
OLEDs having phosphorescent emitter.
105
104
103
102
101
100
10-1
10-2
10-3
103
102
101
100
10-1
10-2
In conclusion,
a novel trimesitylborane compound,
3TPYMB, was synthesized. 3TPYMB has high electron mobili-
ty of 10ꢂ5 cm2 Vꢂ1 sꢂ1 and high triplet excited energy level of
2.95 eV. It was also demonstrated to be useful as the electron-
transport layer in OLEDs.
0
2
4
6
8
10
12
14
Voltage / V
This study was supported, in part, by NEDO, through High
Efficiency Organic Device Project.
Figure 2. Luminance–voltage and current density–voltage
characteristics for the NPD/Alq3 (open) and NPD/Alq3/
3TPYMB (close) devices.
References and Notes
about 10 times higher than that of Alq3. These results suggest
that 3TPYMB can provide low drive voltages when used as
the electron-transport layer in OLEDs.
1
2
3
T. Noda, Y. Shirota, J. Am. Chem. Soc. 1998, 120, 9714.
T. Noda, H. Ogawa, Y. Shirota, Adv. Mater. 1999, 11, 283.
C. D. Entwistle, T. B. Mardaer, Chem. Mater. 2004, 16,
4574.
M. A. Baldo, D. F. O’Brien, Y. You, A. Shoustikov, S.
Sibley, M. E. Thompson, S. R. Forrest, Nature 1998, 395,
151.
D. F. O’Brien, M. A. Baldo, M. E. Thompson, S. R. Forrest,
Appl. Phys. Lett. 1999, 74, 442.
A. P. Kulkarni, C. J. Tonzola, A. Babel, S. A. Jenekhe,
Chem. Mater. 2004, 16, 4556.
3TPYMB was examined as an electron-transport layer of
typical OLEDs consisting of [N-(1-naphthyl)-N-phenylamino]-
biphenyl (NPD) and tris(8-hydroxyquinolinolato)aluminium
(Alq3) as a hole-transport layer and an electron-transporting
emitter layer, respectively. OLEDs with a structure of ITO/
NPD (50 nm)/Alq3 (40 nm)/3TPYMB (30 nm)/LiF (0.5 nm)/
Al (100 nm) (device I) was fabricated. As a reference, the
device with a structure of ITO/NPD (50 nm)/Alq3 (70 nm)/
LiF (0.5 nm)/Al (100 nm) (device II) was also fabricated. All
organic materials were vacuum deposited under 5:0 ꢃ 10ꢂ6
Torr. Lithium fluoride and aluminum were also deposited under
1:0 ꢃ 10ꢂ5 Torr.
4
5
6
7
8
9
G. Hughes, M. R. Bryce, J. Mater. Chem. 2005, 15, 94.
Y. Shirota, J. Mater. Chem. 2005, 15, 75.
3TPYMB: 1H NMR (270 MHz, CDCl3) ꢁ 1.70–1.85 (m,
9H), 1.93–2.14 (m, 18H), 6.89 (dd, 3H, J ¼ 5:94, 16.2 Hz),
7.31–7.50 (m, 6H), 8.29–8.39 (m, 3H), 8.54–8.58 (m, 3H);
EI–MS: Calcd MW, 599.6; m=z ¼ 600 (Mþ), 585, 402,
387, 372, 300, 197, 182, 167. Anal. Calcd for C42H42BN3:
C, 84.13; H, 7.06; B, 1.80; N, 7.01%. Found: C, 83.84; H,
7.06; N, 6.93%.
Green emission from the Alq3 layer was observed from the
device I, using 3TPYMB as the electron-transport layer, which
indicates that 3TPYMB has hole-blocking property. Figure 2
shows current density–voltage and luminance–voltage charac-
teristics of both devices. The device I exhibits larger current
Published on the web (Advance View) January 18, 2007; doi:10.1246/cl.2007.262