Carbazolyl benzo[1,2-b:4,5-b']difuran: An ambipolar host material for full-color organic light-emitting diodes

Article abstract of DOI:10.1002/asia.201200062

We have designed an ambipolar material, 3,7-bis[4-(N-carbazolyl)-phenyl]-2, 6-diphenylbenzo[1,2-b:4,5-b']difuran (CZBDF), and synthesized it by zinc-mediated double cyclization. Its physical properties clarified that CZBDF possesses a wide-gap character,

Full text of DOI:10.1002/asia.201200062

DOI: 10.1002/asia.201200062
Carbazolyl Benzo[1,2-b:4,5-b’]difuran: An Ambipolar Host Material for Full-
Color Organic Light-Emitting Diodes
[
a]
[a]
[b]
[a, b]
Chikahiko Mitsui, Hayato Tsuji,* Yoshiharu Sato,* and Eiichi Nakamura*
À3
2
À1 À1
Abstract: We have designed an ambi-
polar material, 3,7-bis[4-(N-carbazol-
yl)-phenyl]-2,6-diphenylbenzo[1,2-
b:4,5-b’]difuran (CZBDF), and synthe-
sized it by zinc-mediated double cycli-
zation. Its physical properties clarified
10 cm V s , and a high thermal sta-
bility. Using CZBDF as a host material
for heterojunction OLED devices,
a full range of visible emission was ob-
tained. Notably, CZBDF also enabled
us to fabricate RGB-emitting homo-
junction OLEDs, with performances
comparable or superior to the hetero-
junction devices composed of several
materials.
Keywords: ambipolar material
·
benzodifuran · host–guest system ·
organic light-emitting diodes · semi-
conductors
that CZBDF possesses
a wide-gap
character, well-balanced and high hole
and electron mobilities of larger than
Introduction
result in either intra- or intermolecular annihilation of the
hole and electron pair rather than intermolecular carrier
transport between two ends of a macroscopic solid because
the latter is obviously intrinsically more difficult to achieve.
Thus, an ideal ambipolar molecule must possess both high-
HOMO donor and low-LUMO acceptor moieties that have
no direct intramolecular p-conjugation with each other. We
conjecture that any substituent that can transport an elec-
tron can be installed on a recently designed hole-transport-
ing 2,6-phenylbenzo[1,2-b:4,5-b’]difuran (hereafter called
Organic synthesis is playing a pivotal role in the develop-
ment of functional p-conjugated materials for use in or-
ganic semiconducting devices, such as organic light-emitting
for portable and flexible displays. Am-
bipolar organic semiconductors that transport both holes
and electrons serve either as an emissive material or as
a conductive host matrix in the emission layer. Considera-
tions of both inter- and intramolecular interactions are man-
datory for the design of efficient semiconductors. For a com-
pound to act as a good semiconductor, the intermolecular
orbital interaction should be maximized. For the compound
to show ambipolarity, the installation of both p- and n-type
moieties in one molecule is one possible strategy, in which
each molecule can transfer both holes and electrons in op-
posite directions. In addition, for the ambipolar compound
to be a versatile host in an emissive layer, it needs to have
a wide HOMO/LUMO gap so that a single host material
[1]
[2–4]
diodes (OLEDs)
[5]
[7]
BDF), without direct p-conjugation, by exploiting the
steric congestion that will twist the added electron-trans-
porting moiety AHCUTNGRNENUG( ies) away from the BDF core (Figure 1).
Herein, we report the design and synthesis of a new ambi-
polar material, 3,7-bis[4-(N-carbazolyl)phenyl]-2,6-diphenyl-
[7]
benzo[1,2-b:4,5-b’]difuran (CZBDF) possessing the follow-
ing features: 1) well-balanced hole and electron mobilities in
À3
2
À1 À1
the order of 10 cm V
s
when amorphous; 2) a wide
HOMO/LUMO gap with a value higher than 3 eV; and
3) deep-blue light emission, which allows the material to
serve as a host for blue, green, and yellow fluorescent dop-
ants as well as a red-phosphorescent dopant. We first de-
[6]
can be used for light emission from blue to red dopants.
However, installation of donor and acceptor moieties in one
molecule and its stacking in a macroscopic solid tends to
[
a] Dr. C. Mitsui, Dr. H. Tsuji, Prof. Dr. E. Nakamura
Department of Chemistry
The University of Tokyo
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax : (+81)3-5800-6889
E-mail: nakamura@chem.s.u-tokyo.ac.jp
[
b] Dr. Y. Sato, Prof. Dr. E. Nakamura
Nakamura Functional Carbon Cluster Project, ERATO
Japan Science and Technology Agency
Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
E-mail: ysato@chem.s.u-tokyo.ac.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/asia.201200062.
Figure 1. Structures and properties of BDF derivatives.
Chem. Asian J. 2012, 00, 0 – 0
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FULL PAPERS
scribe its utility as a host matrix in the emission layer of
[7]
a multilayer heterojunction OLED device, and then as
a material for a homojunction OLED device that utilizes
this compound as a single matrix of the whole organic layer.
The latter device with a very simple architecture showed
performances comparable with or superior to the state-of-
the-art multilayer heterojunction devices, suggesting the fea-
sibility of homojunction OLED devices for practical applica-
tions.
Results and Discussion
Molecular Design and Synthesis of CZBDF
The benzo[1,2-b:4,5-b’]difuran framework was recorded in
[8]
the literature in the 19th century but has received little at-
[9]
tention until we reported in 2007 that its triphenylamine-
substituted derivative (DPABDF) shows a high hole mobili-
À3
2
À1 À1
ty (m =2.8ꢁ10 cm V s ; yet undetectable electron mo-
h
bility, m ), most probably because of the synergetic effect of
Scheme 1. Synthesis of CZBDF.
e
the BDF core and the substituents, and serves as a useful
[10–13]
hole-transporting material in OLEDs.
The material has
Properties of CZBDF
a wide band gap (optical gap: DE=3.06 eV) due to the
twisted orientation of the 3- and the 7-aromatic substituents
against the BDF framework. This molecular structure in-
spired us to consider that installation of a proper group at 3-
and 7-positions would create an ambipolar material having
a wide HOMO/LUMO gap suitable for blue emission. We
have chosen to install a carbazolyl group, which is a widely
used motif to render an ambipolar property for a host mate-
The wide-gap and ambipolar character of CZBDF was indi-
cated by the electrochemical and photophysical measure-
ments in solution. Cyclic voltammograms in CH Cl2 and
2
THF showed irreversible oxidation and reduction waves, re-
spectively (Figure 2). Oxidation and reduction potentials de-
termined by differential pulse voltammetry were 0.72 V (in
CH Cl ) and À2.60 V (in THF), respectively (vs Fc/Fc+).
2
2
[14]
rial in OLEDs despite its inherent hole-transporting prop-
erty. We thus synthesized CZBDF (Figure 1), expecting am-
bipolarity based on the synergetic effect between the BDF
The HOMO and LUMO energy levels (E
and E
)
LUMO
HOMO
were estimated to be À5.52 eV and À2.20 eV, respectively,
using the equation, E
the HOMO/LUMO gap is 3.32 eV. This gap is wide enough
or E
=À(4.80+E ). Thus,
HOMO
LUMO
DPV
[15]
and carbazole moieties.
Synthesis of CZBDF has been performed by using
[16]
a tandem zinc-mediated cyclization /cross-coupling reac-
tion (Scheme 1), which makes a variety of BDF derivatives
[17]
in a modular way using a 3,7-dizincio-BDF intermediate
3). Thus, 2,5-di(phenylethynyl)hydroquinone (2), which was
(
obtained from commercially available 2,5-dibromo-1,4-hy-
droquinone (1) in three steps, was subjected to intramolecu-
lar double cyclization using zinc chloride at 1208C to afford
3
, which was then coupled with N-(p-bromophenyl)carba-
[18]
zole (4) in the presence of a palladium catalyst using tri-
tert-butylphosphine
[19]
to obtain CZBDF in 76% isolated
yield. The product was purified by repeated train sublima-
tion for use in the following measurements and device appli-
cations.
Abstract in Japanese:
Figure 2. DPV (top) and CV (bottom) traces of CZBDF. Measurements
2 2
were performed in CH Cl (a) and THF (b) to determine oxidation and
reduction potentials, respectively.
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Carbazolyl Benzo[1,2-b:4,5-b’]difuran
to confine carriers and excitons formed from common dop-
ants for blue, green, yellow, and red colors (Figure 3), and
suggests that CZBDF serves as a viable ambipolar material
for OLEDs.
at 300 nm in CH Cl , CZBDF emits deep-blue light with
2 2
a maximum wavelength of 414 nm. The emission quantum
yield was 0.97, as determined by the absolute method. The
emission spectrum in the solid state was slightly red-shifted
to 422 nm, thereby suggesting the presence of an intermolec-
ular interaction. This agrees with the decrease of the emis-
sion quantum yield in the solid state to 0.61, a value still
viable for an emissive material.
The thermal stability of CZBDF was unambiguously con-
firmed. Thermogravimetric analysis (TGA) showed that
weight loss started at 4508C and is less than 5% even at
5
008C, thus indicating that CZBDF will survive the vacuum
deposition process used for device fabrication. The thermal
stability of the amorphous state was evaluated by differen-
tial scanning calorimetry (DSC). An amorphous solid
sample was obtained by the rapid cooling of a molten
sample with liquid N , and this was then subjected to DSC
2
À1
analysis at a heating rate of 10 Kmin . The glass-transition
temperature (T ) of CZBDF (1628C) is much higher than
g
that of the most commonly used ambipolar material CBP
[
20]
(
628C),
which is beneficial for the longevity of the
[
21]
device.
Time-of-flight (TOF) carrier mobility was measured for
an amorphous film of CZBDF (thickness: 3.3 mm) deposited
under vacuum on a glass substrate coated with ITO. The
transient photocurrent signals of both the hole and electron
were dispersive. The carrier mobilities of CZBDF were plot-
Figure 3. Orbital energy levels of host and dopants used in this study, to-
gether with the structures of dopants.
ted against the square root of the electric field (Figure 5).
À3
Hole and electron mobilities were 3.7ꢁ10
and 4.4ꢁ
À3
2
À1 À1
1
1
0
cm V s , respectively, in an electric field of 2.5ꢁ
0 Vcm at room temperature. This level of well-balanced,
5
À1
The UV/Vis absorption measurement in CH Cl showed
an absorption edge at 400 nm (Figure 4), and the optical gap
DE) was estimated to be 3.10 eV, which is close to that of
high carrier mobilities rivals those reported for amorphous
materials, and we consider that it originated from the coop-
erative effect of the BDF core and the carbazolyl groups.
2
2
(
DPABDF (DE=3.06 eV), suggesting that the expected
twisting inhibited the electronic interaction between the
BDF core and the carbazole moieties. Upon photoexcitation
Heterojunction OLED Devices
To investigate the viability of the new ambipolar material,
we first examined the use of CZBDF as the host of the light
Figure 4. Absorption (left) and emission (right) spectra of CZBDF. Solid-
Figure 5. Hole and electron mobilities of CZBDF plotted against the
square root of the electric field measured using the TOF method at room
temperature employing a vacuum deposition film.
line spectra were obtained in CH
2 2
Cl solution at room temperature.
Dotted-line spectrum was obtained in powder form.
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Hayato Tsuji, Yoshiharu Sato, Eiichi Nakamura et al.
FULL PAPERS
emission layer (EML) in heterojunction OLEDs. CZBDF
was found to function by itself as a blue-emitting dye and to
serve as a host material for various dopants over a full-color
range. The device configurations of blue, green, and yellow
[22]
[1]
OLEDs were ITO/MCC-PC1020 /a-NPD (45 nm)/EML
emitting layer(s)) (30 nm)/Alq (30 nm)/Liq (1 nm)/Al (as
(
3
shown in Figure 6), where CZBDF was used in the EML
Figure 6. a) Heterojunction OLED structure of dye-doped devices.
b) Structures of representative materials used in this study.
either as an emitting material by itself (undoped) or as
a host material for various dopants (1–3 wt%). The struc-
tures of the dopants are shown in Figure 3. The electrolumi-
nescence (EL) spectra of the devices are shown in Figure 7.
The device performances were evaluated with regard to
three standard criteria: driving voltage (V1000), maximum ex-
À2
Figure 7. Electroluminescence spectra at 1000 cdm of the OLED devi-
ces comprising CZBDF as an emissive material and a host material.
a) Blue, green, yellow, and red emissive devices A–D and F. b) White
emissive OLED device E.
ternal quantum efficiency (EQEmax), and current efficiency
À1
(
LJ1000 ), as summarized in Table 1.
The undoped device (device A) showed a deep-blue emis-
sion at 444 nm with Commission Internationale de l’ꢂclair-
age (CIE) coordinates of (0.177; 0.139). This emission can
be assigned to that from CZBDF, but the maximum wave-
length was slightly red-shifted when compared with the pho-
toluminescence. No emission from any other material, such
nescence only from each dye, and not from the CZBDF
host, indicating that the emissive dopants functioned as ef-
fectively as the recombination sites. The TBP (tetra-tert-bu-
[23]
tylperylene)-doped
device (device B), for example,
as Alq , was observed, indicating that efficient charge re-
showed EL exclusively from the TBP dye with CIE coordi-
nates of (0.160; 0.219), and the performance was higher than
that of device A. Thus, device B showed a lower driving
voltage (V1000 =8.3 V for device A vs. 6.7 V for device B)
and higher external quantum efficiencies (EQEmax =1.2%
for device A vs. 1.7% for device B) than device A. Similarly,
3
combination takes place within the CZBDF layer. We next
employed commercially available emissive dyes, the
HOMO/LUMO levels of which were located between those
of CZBDF, for an efficient carrier confinement within the
emissive dopant molecules, leading to color tuning as well as
enhanced EL performances. It was found that OLED devi-
ces using the dopants shown in Figure 3 show electrolumi-
[24]
[25]
C545T- (a coumarin-based dye) and rubrene-doped de-
vices C and D showed green and yellow luminescence from
[
a]
Table 1. Performances of OLEDs comprising either CZBDF or CBP in an emission layer.
[
b]
[c]
1000À1 [d]
À1
[e]
Device
Host
Dopant
V
1000 [V]
EQEmax [%]
LJ
[cdA
]
CIE coordinates
x
y
A
B
C
D
E
F
CZBDF
CZBDF
CZBDF
CZBDF
CZBDF
CZBDF
CBP
None
TBP
C545T
rubrene
TBP and rubrene
8.3
6.7
6.2
6.8
7.4
6.6
9.1
1.2
1.7
2.7
2.5
1.8
6.8
6.4
1.2
1.9
5.3
5.7
3.2
4.1
5.4
0.177
0.160
0.269
0.446
0.304
0.659
0.653
0.139
0.219
0.570
0.477
0.325
0.312
0.317
Ir
A
H
U
G
R
N
U
G
3
G
Ir ACHTUNGTR(EUNNNG piq)
3
À2
[
a] OLED performance data collected at a luminance of 1000 cdm . [b] Driving voltage. [c] Maximum external quantum efficiency. [d] Current efficien-
cy. [e] Commission Internationale de l’ꢂclairage coordinates.
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Carbazolyl Benzo[1,2-b:4,5-b’]difuran
each dopant with driving voltages of 6.2 V and 6.8 V and
good external quantum efficiencies of 2.7% and 2.5%, re-
spectively.
We next fabricated a white-emitting OLED by stacking
a TBP-doped blue-emitting layer over the rubrene-doped
yellow-emitting layer (device E). The device structure of
this white OLED is as follows: ITO/MCC-PC1020/a-NPD
Homojunction OLED Devices
With promising results from the heterojunction devices in
hand, we next examined the use of CZBDF in a homojunc-
tion setting, as it offers a much simpler architecture than the
popular heterojunction device (see, for example, Figure 6a).
While such a homojunction architecture was previously
[
28–30]
(
45 nm)/1.7 wt% TBP-doped CZBDF (20 nm)/2.7% ru-
studied for organic photovoltaic devices by Leo et al.,
it
brene-doped CZBDF (10 nm)/Alq (30 nm)/Liq (1 nm)/Al.
has seldom been reported in OLED research except for
3
[31]
[32]
The CIE coordinates of this device were (0.304; 0.325) at
a report in a conference proceeding, and some studies
À2
[7]
a luminance of 1,000 cdm , which is close to the ideal white
that were published following our original communication.
This is clearly because of the paucity of suitable ambipolar
materials. First, the diode properties of two types of
CZBDF-based homojunction devices were examined:
a single-layer device using only CZBDF (Figure 9a, or
device H in Table 2) and a p/n-doped homojunction device,
(
0.333; 0.333). The CIE coordinates barely changed (less
À2
than 0.02) at a brightness in the range 200–47,000 cdm ,
suggesting the steady and balanced charge recombination
within both TBP- and rubrene-doped layers.
CZBDF was also applicable as the host material for
[26]
3
a red-phosphorescent dye, Ir
ture of device F is ITO/MCC-PC1020/a-NPD (45 nm)/
CZBDF: 4.0 wt% Ir(piq) (30 nm)/BCP (10 nm)/Alq3
20 nm)/Liq (1 nm)/Al, in which BCP (2,9-dimethyl-4,7-di-
A
T
N
T
E
N
N
(piq)
(device F). The struc-
AHCTUNGTRENNUNG
3
(
phenyl-9,10-phenanthroline) was used as hole- and exciton-
blocking material to confine carrier and triplet excitons
within the emissive layer. Device F showed an external
quantum efficiency of 6.8%, which is higher than the theo-
[27]
retical limit for a fluorescent OLED device (ca. 5%). The
performance of this device was superior to that of an OLED
device using CBP (N,N’-dicarbazolyl-4,4’-biphenyl), a com-
monly used host material (device G). The external quantum
efficiency of device F was higher than that of device G
Figure 9. Schematic device structures of the OLEDs. a) Single-layer
device (device H) and b) p-i-n homojunction device (devices I–L).
(
6.4%), with a significant improvement in the driving volt-
age (6.6 V for device F and 9.1 V for device G). The lifetime
of device F was also much longer (4,250 h) than device G
(
1
260 h), as measured using a constant current mode (at
2.5 mAcm , Figure 8). The long lifetime of CZBDF-based
in which vanadium oxide (V O , p-dopant) and cesium (Cs,
2
5
À2
n-dopant) were respectively doped on the bottom and top
parts in the device by co-deposition under vacuum (Fig-
ure 9b or device I). As shown in Figure 10a, the current
density-voltage (J–V) characteristics indicate that only the
p/n-doped device I showed a good diode character (on/off
devices can be attributed to the stable amorphicity of
CZBDF and its lower driving voltage due to a suitable
HOMO/LUMO energy level for neighboring layers and
a high CZBDF carrier mobility. Further optimization of the
CZBDF-based structure will prolong the lifetime of such
a device for practical use.
3
ratio: ca. 10 and turn-on voltage: ca. 1 V), while device H
did not. Thus, the inorganic p/n-doping endows the homoge-
neous CZBDF layer with diode characteristics by facilitating
carrier generation and injection from the electrodes to the
[33,34]
organic layer.
The p/n-doping device I also increases the
efficiency of charge recombination in the CZBDF layer, and
thus device I had a deep-blue emission that peaked at
4
28 nm (Figure 10b) with an EQEmax of 0.5% at 4 V. The
emission maximum wavelength is close to that obtained by
photoluminescence (422 nm), while blue shifted when com-
pared with the corresponding heterojunction device A
(
444 nm). We conjecture that this shift in the heterojunction
device was a result of the filtering effect caused by layers
other than the emission layer.
We next examined the performance of various emissive
dyes in a p-i-n homojunction device using CZBDF, where
we doped the i-layer by co-deposition of a dye dopant and
CZBDF (matrix), and achieved full-color emission in the
homojunction architecture. We chose blue-fluorescent TBP,
Figure 8. Operation-lifetime measurement for devices F and G.
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Table 2. Summary of the OLED structures studied and their characteristics.
[
a]
[b]
[c]
À2
[d]
Device
Device structure
p/n-doping
Emissive
dopant
Emission color
l
max [nm]
L
max [cdm
]
EQEmax [%]
[
e]
[e]
[e]
[e]
H
I
J
K
L
7a
7b
7b
7b
7b
À
+
+
+
+
None
None
TBP
NA
NA
428
464
506
628
NA
NA
0.5
1.0
Blue
Blue
Green
Red
1,170
10,200
60,590
4,690
[f]
[g]
C545T
4.2
5.6
[
h]
Ir
A
H
U
G
R
N
U
G
3
À2
[
a] p-dopant=V
2
O
5
, n-dopant=Cs. [b] Emission maximum wavelength in nanometers at a luminance of 1000 cdm . [c] Maximum luminance. [d] Maxi-
À2
mum external quantum efficiency. [e] Not available because the maximum luminance did not reach 1000 cdm . [f] At 14 V. [g] At 13 V. [h] At 10 V.
Figure 10. a) Diode characteristics of the single-layer device H (without
doping) and the p/n-doped device I. b) EL spectrum of device I.
Figure 11. EL characteristics for p-i-n homojunction devices (devices J–
L) using CZBDF as a host matrix. a) L–V characteristics. b) Plot of EQE
green-fluorescent C545T, and red-phosphorescent material,
Ir
The configuration of the homojunction devices (devices J–
L) was ITO/CZBDF:V O (30 nm)/CZBDF(:emissive
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(piq) , the same dopants as for the heterojunction devices.
against applied voltage.
3
2
5
dopant)/CZBDF:Cs (20 nm)/Al, where emissive dopants
were introduced by co-deposition with CZBDF under
vacuum. The thickness of the emissive layer was 50 nm for
device J and 100 nm for devices K and L. The dye-doped de-
vices showed electroluminescence from the dopants, and no
light emission was observed from the CZBDF host, indicat-
ing that the emissive dopants functioned very well as the re-
combination sites. The L–V characteristics and EQE values
of devices J–L are plotted against the applied voltage in
Figure 11. Devices J–L showed much better efficiencies than
device I (Table 2). Remarkably, the homojunction devices J–
L displayed a similar, and sometimes even better perfor-
mance than the corresponding heterojunction devices (devi-
ces B, C, and F). With regard to the blue-fluorescent TBP-
doped devices, the homojunction device J drove at a lower
voltage than the heterojunction device B (V1000 =4.9 V for
device J vs. 6.7 V for device B); however, the EQE were
somewhat low (EQEmax =1.0% for device J vs. 1.7% for
device B). The green-fluorescent C545T-doped homojunc-
tion device K achieved a high EQE value of 4.2% at a lu-
max
À2
minance higher than 60000 cdm , while the corresponding
heterojunction device C reached only 2.7%. The high EQE
value is close to the theoretical limit of a fluorescent OLED
(ca. 5%).
The p-i-n homojunction architecture was also effective in
a red-phosphorescent OLED. The Ir CAHTUNGTERNN(UGN piq) -doped device L
3
gave an EQEmax value of 5.6%, which is higher than the the-
oretical limit for a fluorescent OLED device. However, this
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Carbazolyl Benzo[1,2-b:4,5-b’]difuran
EQEmax value was somewhat lower than that of the six-layer
heterojunction device F with a hole- and exciton-blocking
BCP layer (6.8%).
combined organic layers were passed through a silica gel column employ-
ing CHCl as the eluent. After removal of the solvent in vacuo, the crude
3
material was purified by gel permeation chromatography to afford the
1
title compound (48 mg, 15%). Hence, the total yield was 76%. H NMR
(
500 MHz, 1,2-dideutero-1,1,2,2-tetrachloroethane): d=7.29–7.34 (m, 6H,
carbazole and p-Ph), 7.37 (dd, J=7.4 Hz, 7.4 Hz, 4H, m-Ph), 7.46 (dd,
J=8.1 Hz, 8.1 Hz, 4H, carbazole), 7.57 (d, J=8.1 Hz, 4H, carbazole),
Conclusions
7
8
.70–7.76 (m, 10H, benzodifuran, phenylene, and o-Ph), 7.79 (d, J=
.0 Hz, 4H, phenylene), 8.14 ppm (d, J=8.1 Hz, 4H, carbazole);
C NMR (125 MHz, 1,2-dideutero-1,1,2,2-tetrachloroethane at 1008C):
We have developed a new ambipolar material, CZBDF,
composed of a BDF skeleton and two carbazole moieties
connected to each other through twisted CÀC bonds.
13
d=101.0, 110.0, 117.2, 120.3, 120.4, 123.8, 126.2, 127.2, 127.6, 128.6, 128.7,
129.2, 130.7, 131.4, 132.3, 137.5, 141.1, 151.7, 152.2 ppm; IR (solid): n˜ =
3
046 (w), 1594 (m), 1521 (m), 1490 (m), 1451 (s), 1227 (s), 1146 (m), 1061
CZBDF possesses well-balanced hole and electron mobili-
À1
+
(
m), 957 (m), 749 cm (s); MS (APCI+): 793 [M ]; elemental analysis
calcd. (%) for C58 : C 87.86; H 4.58, N 3.53; found: C 87.97, H
.71, N 3.45.
À3
2
À1 À1
ties of greater than 10 cm V s . The carrier mobilities
are higher than BDF (hole mobility) or CBP (electron mo-
bility) alone, which we consider is due to the twisted molec-
ular structure that may have resulted in intertwining and
larger interactions between the molecular orbitals than for
BDF and CBP alone; however, we have not been able to
obtain crystal structures to confirm this assumption.
CZBDF has a wide HOMO/LUMO gap of >3.3 eV, and
serves as a deep-blue emissive material as well as an effec-
tive emission host material for various dopants emitting
over the full range of visible light. A CZBDF-based device
shows better performance and a much longer lifetime than
the widely examined CBP-based device. These useful prop-
erties of CZBDF allowed us to develop full-color emissive
p-i-n homojunction OLEDs, which performed at a level sim-
ilar to, or higher than, the structurally more complex hetero-
junction devices made from several material combinations.
Although homojunction devices using a single ambipolar
material are attractive because of their structural simplicity,
they require much more stringent materials design than het-
erojunction devices in which the properties of the unipolar
material for each layer can be optimized pertaining to the
specific function of each layer. The BDF-based homojunc-
tion devices reported here illustrate the feasibility of a high-
performance homojunction OLED enabled by the develop-
ment of suitable ambipolar materials.
36 2 2
H N O
4
General
UV/Vis absorption spectra were recorded with a JASCO V-570 spectrom-
eter at a resolution of 0.5 nm. Spectroscopy-grade dichloromethane was
used as a solvent. Sample solutions (ca. 10 mm) in a 1 cm square quartz
cell were used for the measurements. Photoluminescence spectra (PL)
and absolute quantum yields were recorded with a Hamamatsu Photonics
C9920–02 Absolute PL Quantum Yield Measurement System. Degassed
spectroscopy-grade dichloromethane was used as a solvent. All measure-
ments were performed at room temperature.
Cyclic voltammetry (CV) was performed using a HOKUTO DENKO
HZ-5000 voltammetric analyzer. A glassy carbon electrode was used as
+
the working electrode, a platinum coil as the counter electrode, and Ag
À1
/Ag as the reference electrode, at a scan rate of 100 mVs . The com-
pound was dissolved in degassed, dry CH
of ca. 5 mm, and tetrabutylammonium perchlorate at a concentration of
.1 m was added as an electrolyte. All potentials were determined by dif-
2 2
Cl and THF at a concentration
0
ferential pulse voltammetry (DPV), and were corrected from a ferrocene
standard.
Thermogravimetric analysis (TGA) was performed on a Rigaku Thermo
Plus 2 instrument. The samples (~5 mg) were placed in aluminum pans
À1
À1
2
and heated at 10 Kmin under N gas at a flow rate of 100 mLmin .
Differential scanning calorimetry (DSC) was performed on a NETZSCH
thermal analyzer (DSC 204/F1). An amorphous sample was obtained by
fast cooling the melt of CZBDF, and the obtained sample was heated at
À1
À1
2
a rate of 10 Kmin under N gas, at a flow rate of 18 mLmin .
The time-of-flight (TOF) measurement was performed using a TOF-401
intrument (Sumitomo Heavy Industries Advanced Machinery). Films
were deposited on indium tin oxide (ITO, 145 nm)-coated glass sub-
strates. The vacuum deposition was performed using a VPC-260 system
(
ULVAC KIKO). The films were deposited by vacuum sublimation at 1ꢁ
À3
À1
Experimental Section
10 Pa, with an average deposition rate of 20–30 nms . The ITO-coated
glass substrate was spaced at 100 mm from the sample, and it was kept at
Synthesis of CZBDF
2
58C. The thickness of the films obtained was 3.3 mm.
For the device fabrication and evaluation, CZBDF with analytical purity
was purified further by train sublimation. All other materials were used
To
a suspension of 2,5-bis(phenylethynyl)-1,4-benzenediol (124 mg,
0
.400 mmol) in THF (0.8 mL) was added a solution of n-butyllithium in
À1
as obtained commercially. An ITO-coated glass substrate treated by O3-
hexane (0.48 mL, 1.66 molL , 0.80 mmol) at 08C. The resulting yellow
suspension was allowed to warm to ambient temperature and stirred for
plasma was used as the anode. For the fabrication of heterojunction
OLED devices, MCC-PC1020 was spin-coated onto this substrate, dried,
À1
3
0 min. A solution of zinc chloride in THF (0.80 mL, 1.0 molL
,
and annealed at 2308C under nitrogen atmosphere. All other layers were
0
.80 mmol) was then added. Subsequently, the volatiles were removed in
À4
vacuum deposited onto this substrate at a pressure of 2ꢁ10 Pa or less.
vacuo and toluene (0.8 mL) was added. The resulting yellow solution was
heated to 1208C and stirred for 2.5 h at this temperature. After cooling
to ambient temperature, N-methylpyrrolidone (NMP) (0.2 mL), Pd
Finally, the device was sealed by encapsulation with a fresh desiccant
under nitrogen atmosphere. The emissive area of the device was 2ꢁ
2
2
2
mm .
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(dba)
3
·CHCl
3
(41.4 mg, 0.04 mmol),
0.16 mmol), and 4-bromophenylcarbazole
P
A
H
U
G
R
N
U
G
3
in toluene (160 mL,
(309 mg,
À1
1
0
.0 molL
,
.960 mmol) were successively added. The resulting mixture was stirred
for 25 h at 808C. After cooling to ambient temperature, the precipitate
was collected by filtration and washed several times with MeOH and
EtOAc. The resulting crude solid product was purified by gradient
vacuum sublimation (pressure <20 mTorr) at 380–4008C to obtain the
title compound (193 mg, 61%) as a white powder. A second crop was ob-
tained from the mother liquor. After removal of the solvent in vacuo, the
Acknowledgements
We thank MEXT (KAKENHI for E.N., No. 22000008, H.T., No.
20685005) and the Global COE Program for Chemistry Innovation. C.M.
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
7
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&
&
These are not the final page numbers! ÞÞ

Hayato Tsuji, Yoshiharu Sato, Eiichi Nakamura et al.
FULL PAPERS
thanks the Research Fellowship of the Japan Society for the Promotion
of Science for Young Scientists (No. 219262).
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Received: January 18, 2012
Revised: February 27, 2012
Published online: && &&, 0000
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www.chemasianj.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!

FULL PAPERS
The (Ambi)Polar Express: A newly
designed ambipolar material, CZBDF,
was synthesized. CZBDF possesses
a wide-gap character, well-balanced
and high hole and electron mobilities
of larger than 10 cm V s , and
a high thermal stability. This material
functions as an emissive material and
a host material for OLEDs and ena-
bled us to produce heterojunction as
well as p-i-n homojunction OLED
devices that emit full-color visible light
with high efficiency.
Organic Light-Emitting Diodes
Chikahiko Mitsui, Hayato Tsuji,*
Yoshiharu Sato,*
Eiichi Nakamura*
&&&& — &&&&
À3
2
À1 À1
Carbazolyl Benzo[1,2-b:4,5-b’]difuran:
An Ambipolar Host Material for Full-
Color Organic Light-Emitting Diodes
Chem. Asian J. 2012, 00, 0 – 0
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemasianj.org
9
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These are not the final page numbers! ÞÞ