Indolo acridine-based hole-transport materials for phosphorescent OLEDs with over 20% external quantum efficiency in deep blue and green

Article abstract of DOI:10.1021/cm201634w

Thermally stable high-triplet-energy hole transport materials based on 8H-indolo[3,2,1-de]acridine core (FPC) were synthesized and the device performances of FPC-based phosphorescent organic light-emitting diodes (OLEDs) were investigated. The FPC-based hole transport materials showed a high triplet energy (above 2.90 eV) and high glass-transition temperature (above 140 °C). The FPC-based hole-transport materials (HTMs) were effective as the HTMs for green and deep-blue phosphorescent OLEDs, and a high quantum efficiency (over 20%) was achieved in both devices.

Full text of DOI:10.1021/cm201634w

ARTICLE
pubs.acs.org/cm
Indolo Acridine-Based Hole-Transport Materials for Phosphorescent
OLEDs with Over 20% External Quantum Efficiency in Deep Blue and
Green
Min Su Park and Jun Yeob Lee*
Department of Polymer Science and Engineering, Dankook University 126, Jukjeon-dong, Suji-gu, Yongin, Gyeonggi, 448-701, Korea
ABSTRACT: Thermally stable high-triplet-energy hole trans-
port materials based on 8H-indolo[3,2,1-de]acridine core (FPC)
were synthesized and the device performances of FPC-based
phosphorescent organic light-emitting diodes (OLEDs) were
investigated. The FPC-based hole transport materials showed a
high triplet energy (above 2.90 eV) and high glass-transition
temperature (above 140 °C). The FPC-based hole-transport
materials (HTMs) were effective as the HTMs for green and
deep-blue phosphorescent OLEDs, and a high quantum efficiency (over 20%) was achieved in both devices.
KEYWORDS: high quantum efficiency, high triplet energy, hole transport material, thermal stability
’ INTRODUCTION
have been used as high-triplet-energy HTMs for blue PHOLEDs,
because of the high triplet energy of 2.9 eV. However, mCP has
a problem of poor thermal stability and TAPC has a problem
of improper highest occupied molecular orbital (HOMO)
level for hole injection. The HOMO level of TAPC is app-
roximately ꢀ5.5 eV and hole injection from TAPC to deep-blue
host materials with a HOMO level below ꢀ6.0 eV is difficult,
because of the high energy barrier for hole injection. Other
compounds could not be applied in deep-blue PHOLEDs,
because of low triplet energy or poor hole injection. Therefore,
it is strongly required to develop thermally stable high-triplet-
energy HTMs for efficient charge injection and triplet exciton
blocking in deep-blue PHOLEDs.
In this work, thermally stable high-triplet-energy HTMs based
on an 8H-indolo[3,2,1-de]acridine (FPC) core will be synthe-
sized, and the device performances of the deep blue and green
PHOLEDs are investigated. Over 20% external quantum effi-
ciency in deep-blue and green PHOLEDs were demonstrated
using the FPC-based HTMs.
Phosphorescent organic light-emitting diodes (PHOLEDs) have
been developed for more than 10 years, because of their merit of
high quantum efficiency.1 Over 20% external quantum efficiency
has already been reported in red/green/blue PHOLEDs, although
the high efficiency was hardly reported in deep-blue PHOLEDs.2ꢀ5
There are several factors affecting the quantum efficiency of
PHOLEDs, which include charge balance in the emitting layer,
triplet exciton blocking, and triplet energy transfer. In particular,
the triplet exciton blocking was critical to the quantum efficiency
of PHOLEDs.6ꢀ12 The lifetime of triplet excitons (on the scale of
microseconds) is much longer than that of the singlet excitons
(on the scale of nanoseconds). Therefore, the triplet excitons can
diffuse out of the emitting layer and be quenched by the charge
transport layer.10 The triplet exciton quenching can be sup-
pressed by using a high-triplet-energy exciton blocking layer,
which has higher triplet energy than the emitting material.
Both hole-transport-type and electron-transport-type exciton
blocking materials have been developed to improve the quantum
efficiency of PHOLEDs. Various carbazole- or aromatic amine-
based hole transport materials (HTMs) have been successful to
block triplet excitons,9,13 and imidazole-, triazole-, or oxadiazole-
type electron transport materials could suppress the triplet exciton
diffusion.4ꢀ7,11 The quantum efficiency could be greatly enhanced
by using the high-triplet-energy exciton blocking layers.
’ EXPERIMENTAL SECTION
General Information. The following chemicals (from Aldrich
Chemical Co.) were used without further purification: 9H-carbazole,
n-butyllithium, dichlorobenzene, copper iodide(I), and 18-crown-6. Hy-
drogen chloride (Duksan Science Co.), potassium carbonate (Duksan
Science Co.), 1-bromo-2-iodebenzene (TCI Chemical Co.), and 4,40-
dibromobenzophenone (Alfa Aesar. Co.) also were used as received.
Tetrahydrofuran (THF) was distilled over sodium and calcium hydride.
However, it was difficult to develop an exciton blocking layerfor
deep-blue PHOLEDs. Deep-blue-emitting phosphorescent dopants
have a triplet energy over 2.7 eV and the exciton blocking
material should have a triplet energy over 2.8 eV for efficient
triplet exciton blocking. N,N0-dicarbazolyl-3,5-benzene (mCP),14,15
1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC),5 2,20-bis(4-
ditolylaminophenyl)-1,10-biphenyl,16 2,20-bis(3-ditolylaminophenyl)-
1,10-biphenyl,16 and bis[4-(p,p0-ditolylamino)-phenyl]diphenylsilane17
Received: June 9, 2011
Revised:
August 22, 2011
Published: September 06, 2011
r
2011 American Chemical Society
4338
dx.doi.org/10.1021/cm201634w Chem. Mater. 2011, 23, 4338–4343
|

Chemistry of Materials
ARTICLE
Scheme 1. Synthetic Scheme of FPCA and FPCC
Nuclear magnetic resonance (1H NMR and 13C NMR) spectra were
recorded on a Varian 200 (200 MHz) spectrometer. A fluorescence
spectrophotometer (Hitachi, Model F-7000) and ultravioletꢀvisible light
(UVꢀvis) spectrophotometer (Shimadzu, Model UV-2501PC) were
used to measure the photoluminescence (PL) spectra and UVꢀvis
spectra. Low-temperature PL measurement of the synthesized materials
was carried out at 77 K, using a dilute solution of the materials. The
differential scanning calorimeter (DSC) measurements were performed
using a Mettler Model DSC 822e system under nitrogen at a heating rate
of 10 °C/min. The mass spectra were recorded using a JEOL Model JMS-
AX505WA spectrometer in fast-atom bombardment (FAB) mode. The
HOMO energy levels were measured using cyclic voltammetry (CV). CV
measurement of the organic materials was carried out in acetonitrile
solution with tetrabutylammonium perchlorate at 0.1 M concentration.
Silver was used as the reference electrode, and platinum was used as the
counter electrode. Organic materials were coated on an indium tin oxide
(ITO) substrate and were immersed in electrolyte for analysis.
Synthesis. Synthetic scheme of the FPC compounds is described in
Scheme 1. The synthesis of the intermediate compound was reported in
other work.18
4,40-(8H-indolo[3,2,1-de]acridine-8,8-diyl)bis(N,N-dipheny-
laniline) (FPCA). Diphenylamine (0.748 g, 4.422 mmol), 8,8-bis-
(4-bromophenyl)-8H-indolo[3,2,1-de]acridine (1 g, 1.76 mmol), potassium
carbonate (0.51 g, 3.70 mmol), copper(I) iodide (0.085 g, 0.442 mmol),
and dibenzo 18-crown-6 (0.046 g, 0.176 mmol) were dissolved in
o-dichlorobenzene under nitrogen atmosphere. The reaction mixture was
stirred for 12 h at 100 °C. The mixture was diluted with dichloromethane
and washed with distilled water (100 mL) three times. The organic layer
was dried over anhydrous MgSO4 and evaporated in vacuo to give the
crude product. The extract was evaporated to dryness, affording a white
solid (0.852 g), which was further purified by column chromatography,
using dichloromethane/n-hexane.
Yield, 65%; Tg, 142 °C. 1H NMR (200 MHz, CDCl3): δ 8.13 (d, 1H,
J = 8.0 Hz), 8.09 (d, 1H, J = 8.0 Hz), 7.95 (d, 1H, J = 8.0 Hz), 7.74ꢀ7.62
(m, 3H), 7.56ꢀ7.46 (m, 5H), 7.40ꢀ6.80 (m, 28H). 13C NMR (50
MHz, CDCl3): δ 148.1, 146.4, 140.4, 139.0, 138.0, 137.3, 132.7, 131.7,
130.7, 130.1, 129.0, 128.4, 127.2, 126.1, 125.3, 124.4, 123.7, 123.1, 122.7,
122.2, 121.0, 119.0, 117.9, 115.5, 114.5, 113.5, 56.2. MS (FAB) m/z 741
[(M + H)+]. Anal. Calcd for C55H39N3: C, 89.02; H, 5.30; N, 5.66.
Found: C, 89.12; H, 5.51; N, 5.41.
Device Fabrication and Measurements. The device structure of
blue PHOLEDs was indium tin oxide (ITO, 150 nm)/N,N0-diphenyl-N,N0-
bis-[4-(phenyl-m-tolyl-amino)-phenyl]-biphenyl-4,40-diamine (DNTPD,
60 nm)/N,N0-di(1-naphthyl)-N,N0-diphenylbenzidine (NPB, 5 nm)/
FPCC or FPCA (10 nm)/9-(3-(9H-carbazole-9-yl)phenyl)-3-(dibro-
mophenylphosphoryl)-9H-carbazole (mCPPO1):bis((3,5-difluoro-4-
cyanophenyl)pyridine) iridium picolinate (FCNIrpic) (30 nm, 3%)/
diphenylphosphine oxide-4-(triphenylsilyl)phenyl (TSPO1, 25 nm)/
LiF(1 nm)/Al(200 nm). Green PHOLED had the device structure
of ITO (150 nm)/DNTPD (60 nm)/NPB (20 nm)/FPCA or
FPCC (10 nm)/bis-9,90-spirobi[fluoren-2-yl]-methanone (BSFM):tris-
(2-phenylpyridine) iridium (Ir(ppy)3)/TSPO1 (25 nm)/LiF (1 nm)/
Al (200 nm). The device performances of the blue and green PHOLEDs
were measured with Keithley Instruments Model 2400 source measure-
ment unit and Model CS1000 spectroradiometer.
Synthesis of 8,8-bis(4-(9H-carbazol-9-yl)phenyl)-8H-
indolo[3,2,1-de]acridine (FPCC). 9H-Carbazole (0.736 g, 4.422 mmol),
8,8-bis(4-bromophenyl)-8H-indolo[3,2,1-de]acridine (1 g, 1.76 mmol),
potassium carbonate (0.51 g, 3.70 mmol), copper(I) iodide (0.085 g,
0.442 mmol), and dibenzo 18-crown-6 (0.046 g, 0.176 mmol) were dissolved
in dimethylformamide (DMF) under a nitrogen atmosphere. The reaction
mixture was stirred for 12 h at 100 °C. The mixture was diluted with
dichloromethane and washed with distilled water (100 mL) three times. The
organic layer was dried over anhydrous MgSO4 and evaporated in vacuo to
give the crude product. The extract was evaporated to dryness, affording a
white solid (0.849 g), which was further purified by column chromatography,
using dichloromethane/n-hexane.
Yield, 65%; Tg, 149 °C. 1H NMR (200 MHz, CDCl3): δ 8.25 (d, 1H,
J = 8.0 Hz), 8.21 (d, 1H, J = 8.0 Hz), 8.13 (d, 2H, J = 8.0 Hz), 8.05 (d, 1H,
J = 8.0 Hz), 7.64ꢀ7.20 (m, 30H). 13C NMR (50 MHz, CDCl3): δ 145.4,
141.1, 139.1, 138.0, 137.5, 136.6, 132.9, 132.4, 131.9, 131.5, 128.9, 127.8,
127.2, 126.8, 126.0, 125.7, 123.8, 122.7, 122.3, 121.3, 120.9, 120.1, 119.8,
119.5, 118.5, 115.4, 114.8, 114.2, 113.8, 110.8, 109.7, 57.1. MS (FAB)
m/z 737 [(M + H)+]. Anal. Calcd for C55H35N3: C, 89.52; H, 4.78; N,
5.69. Found: C, 89.17; H, 4.73; N, 5.71.
’ RESULTS AND DISCUSSION
Two FPC-based HTMs were synthesized; the synthetic
scheme of the materials has already been shown in Scheme 1.
4339
dx.doi.org/10.1021/cm201634w |Chem. Mater. 2011, 23, 4338–4343

Chemistry of Materials
ARTICLE
Table 1. Photophysical properties of FPCA and FPCC
UVꢀvis (nm)a
PL (nm)b
Tg (°C)
HOMO (eV)
LUMO (eV)
bandgap (eV)
ET (eV)
FPCA
FPCC
259
368
366
142
149
ꢀ5.78
ꢀ6.03
ꢀ2.42
ꢀ2.65
3.36
3.38
2.90
2.96
295
a Ultraviolet-visible light spectroscopy. Tetrahydrofuran (THF) was used as the solvent. b Photoluminescence. Tetrahydrofuran (THF) was used as the
solvent.
Figure 2. Molecular orbital simulation results of FPCA and FPCC.
orbital (LUMO) of the FPC-based HTMs were measured from
the absorption edge of the UVꢀvis absorption and oxidation
potential from CV. The HOMO level of the FPCA was ꢀ5.78 eV,
while that of the FPCC was ꢀ6.03 eV. The HOMO level of
the FPCA was shifted upward by 0.25 eV, compared to that of
the FPCC. As can be seen in the molecular orbital distribution of
the FPCA and FPCC (Figure 2), the HOMO is localized on the
aromatic amine and carbazole units of each molecule. Therefore,
the HOMO level was upward shifted in the FPCA, because of the
Figure 1. UVꢀvis absorption and PL spectra of (a) FPCC and
strong electron-donating character of the aromatic amine unit.
(b) FPCA.
The bandgaps were calculated from the UVꢀvis absorption edge
of the HTMs; they were determined to be 3.36 and 3.39 eV.
The FPC was designed as the high-triplet-energy core with high-
temperature stability. The FPC core can have the high triplet
energy of phenylcarbazole, because the conjugation of the
phenylcarbazole is not extended through the sp3 carbon. In
addition, it can show high-temperature stability as the rotation of
the phenyl group is limited by the additional bond between the
phenyl and carbazole groups. Based on the rigid FPC core, two
different hole transport units—FPCA and FPCC—were intro-
duced in the molecular structure, to improve the hole transport
properties.
Physical properties of the HTMs were analyzed and are
summarized in Table 1. Two materials showed different UVꢀvis
absorption and PL spectra (Figure 1). The UVꢀvis absorption of
the FPCC is from πꢀπ* transition of the carbazole and 8H-
indolo[3,2,1-de]acridine moieties, while that of FPCA is from
πꢀπ* transition of the 8H-indolo[3,2,1-de]acridine and the
diphenylamine unit. In addition to the main absorption peak at
295 nm, which is attributed to the 8H-indolo[3,2,1-de]acridine
group, the FPCA showed a shoulder peak that originated from
the aromatic amine group at long wavelength. The PL spectra
were slightly red-shifted in the FPCA, because of the aromatic
amine unit. The HOMO and the lowest unoccupied molecular
Considering the bandgap of the FPCA and FPCC, the LUMO
levels were ꢀ2.42 eV and ꢀ2.65 eV for FPCA and FPCC,
respectively. The triplet energies of the HTMs were measured
from low-temperature PL spectra and they were 2.90 and 2.96 eV
for FPCA and FPCC, respectively. The triplet energy of the two
HTMs was high enough for triplet exciton blocking in deep-blue
PHOLEDs. Therefore, two HTMs were adopted as the hole-
transport-type exciton blocking layer in deep-blue and green
PHOLEDs. The glass-transition temperatures (Tg) of the FPCC
and FPCA are also presented in Table 1. Both compounds
showed high Tg values (over 140 °C), because a rigid core
structure was introduced in the FPCC and FPCA. Therefore, the
use temperature of the FPCA and FPCC HTMs can be as high as
140 °C. In particular, the FPCC showed a high Tg value of
149 °C, because of the rigid aromatic carbazole unit. The thermal
stability of the two materials were tested at 100 °C for 10 min and
two materials did not show any change of the film morphology
even after thermal treatment, demonstrating the thermal stability
of the FPCC and FPCA.
Deep-blue PHOLEDs with a device configuration of ITO/
DNTPD/NPB/FPCC or FPCA/mCPPO1:FCNIrpic/TSPO1/
LiF/Al were fabricated and device performances were investigated.
4340
dx.doi.org/10.1021/cm201634w |Chem. Mater. 2011, 23, 4338–4343

Chemistry of Materials
ARTICLE
Figure 3. Current densityꢀvoltageꢀluminance data of blue PHOLEDs
Figure 6. Quantum efficiencyꢀluminance and power efficiencyꢀ
with FPCC and FPCA.
luminance curves of blue PHOLEDs with FPCC and FPCA.
Figure 4. Current densityꢀvoltage curves of hole-only devices of
FPCC and FPCA.
Figure 7. Electroluminescence spectra of blue PHOLEDs with FPCC
and FPCA.
The FPCA and FPCC have the HOMO levels of ꢀ5.78 eV and
ꢀ6.03 eV for hole injection from NPB (ꢀ5.5 eV) to mCPPO1
(ꢀ6.13 eV) and high triplet energy over 2.90 eV for triplet
exciton blocking. Therefore, high quantum efficiency can be
obtained from the deep-blue PHOLED, using the FPC-based
HTMs. Figure 3 shows the current densityꢀvoltageꢀluminance
curves of the deep-blue PHOLEDs. The FPCA device showed
higher current density than FPCC device. The difference of the
current density is due to the different hole density in the emitting
layer as the same electron transport layer was used in two devices.
Therefore, this result indicates that the FPCA is better than
FPCC in terms of hole transport.
To confirm the hole-transport properties, hole-only devices of
two HTMs were fabricated and the hole current density was
compared. Figure 4 shows the current densityꢀvoltage curves
of the hole-only device of two HTMs. The device structure
was ITO/DNTPD (60 nm)/NPB (5 nm)/FPCC or FPCA
(30 nm)/Au. The hole current density was increased in the
order of FPCA > FPCC. As expected, the hole current density
showed the same tendency as the current density of PHOLEDs.
In general, the hole current density depends on the energy level
and hole-transport properties of the materials. Considering the
HOMO level, FPCA is better than FPCC, because the energy
barriers for hole injection from NPB to HTMs were 0.28 and
0.53 eV for FPCA and FPCC, respectively. FPCA is better than
Figure 5. Energy-level diagram of blue and green phosphorescent
organic light-emitting diodes (PHOLEDs).
4341
dx.doi.org/10.1021/cm201634w |Chem. Mater. 2011, 23, 4338–4343

Chemistry of Materials
ARTICLE
Table 2. Device Performances of FPCA and FPCC Blue and Green Devices
driving voltage
(V)a
maximum quantum
quantum
efficiency (%)a
maximum power
power efficiency
maximum current
color
coordinatea
efficiency (%)
efficiency (lm/W)
(lm/W)a
efficiency (cd/A)
Blue-FPCA
Blue-FPCC
Green-FPCA
Green-FPCC
6.3
7.1
5.4
6.2
22.0
18.8
24.5
24.3
19.7
15.6
24.2
21.1
25.2
19.0
58.5
54.4
13.0
9.1
26.0
22.3
79.0
78.3
(0.14, 0.17)
(0.14, 0.17)
(0.33, 0.62)
(0.33, 0.62)
50.1
37.0
a Measured at 1000 cd/m2.
Figure 8. (a) Current densityꢀvoltageꢀluminance, (b) quantum efficiencyꢀluminance, (c) power efficiencyꢀluminance, and (d) electrolumines-
cence data of green PHOLEDs with FPCA and FPCC HTMs.
FPCC, in terms of hole transport, because it is well-known that
the order of hole-transport character is amine > carbazole. The
hole mobility of FPCA (1.4 ꢁ 10ꢀ3 cm2/(V s)) was higher than
that of the FPCC (5.2 ꢁ 10ꢀ4 cm2/(V s)). The order of hole-
transport character coincided with the hole current density of the
hole-only devices. Therefore, the different current density of the
deep-blue PHOLEDs is due to different hole-transport character of
the high-triplet-energy HTMs and energy barrier for hole injection.
The energy level diagram of the devices is shown in Figure 5.
The quantum efficiencyꢀluminance and power efficiencyꢀ
luminance data are shown in Figure 6. The quantum efficiency of
the deep-blue PHOLEDs was on the order of FPCA > FPCC.
The mCPPO1 host has rather strong electron transport proper-
ties, because of the diphenylphosphine oxide group. Therefore,
more hole injection can improve the quantum efficiency of the
deep-blue PHOLEDs. Because the FPCA showed better hole-
injection properties than FPCC, the quantum efficiency of the
FPCA device was higher than that of FPCC. The maximum
external quantum efficiency of the FPCA device was 22.0%, and
the quantum efficiency at 1000 cd/m2 was 19.7%. The maximum
power efficiency was 25.2 lm/W, and the power efficiency at
1000 cd/m2 was 13.0 lm/W. The color coordinate of the device
was (0.14, 0.17), as shown in the electroluminescence spectra
(see Figure 7). Detailed device data are summarized in Table 2.
The high quantum and power efficiencies of the deep-blue
PHOLEDs with the FPCA can be explained by the high
recombination efficiency and triplet exciton blocking. The re-
combination efficiency was improved because of efficient hole
injection from FPCA to the mCPPO1 emitting layer. In addition,
the triplet exciton quenching by the HTMs was suppressed due
to the high triplet energy of 2.90 eV. The triplet energy of the
FPCA was high enough for exciton blocking, although it was
slightly lower than that of the FPCC. The FPCC was also
effective for triplet exciton blocking, but rather poor hole
injection properties degraded the efficiency of the deep-blue
PHOLEDs.
4342
dx.doi.org/10.1021/cm201634w |Chem. Mater. 2011, 23, 4338–4343

Chemistry of Materials
ARTICLE
Similarly, the FPCC and FPCA were also applied as the HTMs
in green PHOLEDs. Device data are presented in Figure 8. The
current density and luminance of the green PHOLEDs showed
the same tendency as that of deep-blue PHOLEDs. The quantum
efficiency was also high in the FPCA device. The maximum
quantum efficiency was 24.5%, and the quantum efficiency at
1000 cd/m2 was 24.2%. There was little efficiency rolloff in the
green PHOLED with the FPCA hole-transport layer. The power
efficiency was also high; the maximum power efficiency and
power efficiency at 1000 cd/m2 were 58.5 lm/W and 50.1 lm/W,
respectively. The FPCA hole-transport layer was also very
effective in green PHOLED, because of good hole injection
and suppressed triplet exciton quenching. Detailed device data
are summarized in Table 2.
(15) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Funct. Mater.
2009, 19, 3644.
(16) Agata, Y.; Shimizu, H.; Kido, J. Chem. Lett. 2007, 36, 316.
(17) Tanaka, D.; Agata, Y.; Takeda, T.; Watanabe, S.; Kido, J. Jpn. J.
Appl. Phys. 2007, 46, L117.
(18) Jeon, S. O.; Lee, J. Y. Tetrahedron 2010, 66, 7295.
’ CONCLUSIONS
In conclusion, thermally stable high-triplet-energy hole-trans-
port materials (HTMs), based on an 8H-indolo[3,2,1-de]-
acridine (FPC) core, were synthesized and they enhanced the
quantum efficiency of the deep-blue and green phosphorescent
organic light-emitting diodes (PHOLEDs) to over 20%. The
FPC-based HTMs showed high glass-transition temperatures
(over 140 °C). Therefore, stable HTMs for deep-blue PHOLED
application could be successfully developed, and they may
improve the device stability in practical applications.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: 82-31-8005-3585. Fax: 82-31-8005-3585. E-mail: leej17@
dankook.ac.kr.
’ REFERENCES
(1) Baldo, M. A.; O’Brien, D. F.; You, Y.; Shoustikov, A.; Sibley, S.;
Thompson, M. E.; Forrest, S. R. Nature 1998, 395, 151.
(2) Adachi, C.; Baldo, M. A.; Thompson, M. E.; Forrest, S. R. J. Appl.
Phys. 2001, 90, 5048.
(3) Su, S.-J.; Tanaka, D.; Li, Y.-J.; Sasabe, H.; Takeda, T.; Kido, J. Org.
Lett. 2008, 10, 941.
(4) Xiao, L.; Su, S.-J.; Agata, Y.; Lan, H.; Kido, J. Adv. Mater. 2009,
21, 1271.
(5) Chopra, N.; Lee, J.; Zheng, Y.; Eom, S.-H.; Xue, J.; So, F. Appl.
Phys. Lett. 2008, 93, 143307.
(6) Su, S. ꢀJ.; Chiba, T.; Takeda, T.; Kido, J. Adv. Mater. 2008, 20,
2125.
(7) Sasabe, H.; Gonmori, E.; Chiba, T.; Li, Y.-J.; Tanaka, D.; Su, S.-J.;
Takeda, T.; Pu, Y.-J.; Nakayama, K.; Kido, J. Chem. Mater. 2008, 20,
5951.
(8) Holmes, R. J.; Forrest, S. R.; Sajoto, T.; Tamayo, A.; Djurovich,
P. I.; Thompson, M. E.; Brooks, J.; Tung, Y.-J.; D’Andrade, B. W.;
Weaver, M. S.; Kwong, R. C.; Brown, J. J. Appl. Phys. Lett. 2005, 87,
243507.
(9) Kim, S. H.; Jang, J.; Lee, J. Y. Appl. Phys. Lett. 2007, 90, 223505.
(10) Baldo, M. A.; Thompson, M. E.; Forrest, S. R. Pure Appl. Chem.
1999, 71, 2095.
(11) Ikai, M.; Tokito, S.; Sakamoto, Y.; Suzuki, T.; Taga, Y. Appl.
Phys. Lett. 2001, 79, 156.
(12) Adamovich, V.; Cordero, S. R.; Djurovich, P. I.; Tamayo, A.;
Thompson, M. E.; Andrade, B.; Forrest, S. R. Org. Electron. 2003, 4, 77.
(13) Lee, J.; Chopra, N.; Eom, S.-H.; Zheng, Y.; Xue, J.; So, F.; Shi, J.
Appl. Phys. Lett. 2008, 93, 123306.
(14) Jeon, S. O.; Yook, K. S.; Joo, C. W.; Lee, J. Y. Adv. Mater. 2010,
22, 1872.
4343
dx.doi.org/10.1021/cm201634w |Chem. Mater. 2011, 23, 4338–4343