Simple bipolar hosts with high glass transition temperatures based on 1,8-disubstituted carbazole for efficient blue and green electrophosphorescent devices with ideal turn-on voltage

Article abstract of DOI:10.1002/chem.201202329

Two hybrids based on 1,8-disubstituted carbazole, 1,8-OXDCz and 1,8-mBICz, have been designed and synthesized through a facile process. The incorporation of oxadiazole or N-phenylbenzimidazole moieties at the 1,8-positions of carbazole greatly improves its morphological stability, giving glass transition temperatures (T_g) as high as 138 and 154 °C, respectively. Blue phosphorescent organic light-emitting devices (PhOLEDs) with 1,8-mBICz exhibit almost the same performance as a similarly structured device based on the mCP host, and green PhOLEDs employing the new host material 1,8-OXDCz exhibit an ideal turn-on voltage (2.5 V at 1.58 cd m^-2), a maximum current efficiency (η_c,max) of 73.9 cd A^-1, and a power efficiency (η_p,max) of 89.7 lm W^-1. These results are among the best performances of [Ir(ppy)₃]-based devices with simple device configurations. Copyright

Full text of DOI:10.1002/chem.201202329

DOI: 10.1002/chem.201202329
Simple Bipolar Hosts with High Glass Transition Temperatures Based on 1,8-
Disubstituted Carbazole for Efficient Blue and Green Electrophosphorescent
Devices with “Ideal” Turn-on Voltage
Hong Huang,[a] Yixin Wang,[a] Biao Pan,[a] Xiao Yang,[a] Lei Wang,*[a]
Jiangshan Chen,*[b] Dongge Ma,[b] and Chuluo Yang*[a, c]
Abstract: Two hybrids based on 1,8-di-
ACHTUNGTRENNUNGsubstituted carbazole, 1,8-OXDCz and
light-emitting devices (PhOLEDs) with
1,8-mBICz exhibit almost the same
performance as a similarly structured
device based on the mCP host, and
green PhOLEDs employing the new
host material 1,8-OXDCz exhibit an
ideal turn-on voltage (2.5 V at
1.58 cdmÀ2), a maximum current effi-
ciency (hc,max) of 73.9 cdAÀ1, and a
power efficiency (hp,max) of 89.7 lmWÀ1.
These results are among the best per-
1,8-mBICz, have been designed and
synthesized through a facile process.
The incorporation of oxadiazole or N-
phenylbenzimidazole moieties at the
1,8-positions of carbazole greatly im-
proves its morphological stability,
giving glass transition temperatures
(Tg) as high as 138 and 1548C, respec-
tively. Blue phosphorescent organic
formances of [Ir
ACHTUNGRTEN(NUGN ppy)3]-based devices
Keywords: carbazoles
· heterocy-
with simple device configurations.
cles · host–guest systems · phos-
phorescence · organic light-emitting
devices
Introduction
rials satisfying the requirements for practical applications
are still rare, and further study is needed.
The host–guest system has been used widely in phosphores-
cent organic light-emitting devices (PhOLEDs) since the
triplet emitter of heavy-metal complexes (PtOEP) was first
doped in a host matrix by Baldo and Forrest to reduce con-
As we know, carbazole has a high triplet energy of
2.95 eV[6] and an excellent hole-transporting ability.[7] In ad-
dition, the simple structure of the carbazole unit offers
many options for the introduction of building blocks into
the carbazole core through a facile chemical approach. Of
the reported carbazole-based host materials, most focused
on 2,7-, 3,6-, or N-position functionalization. Recently, some
1,8-disubstituted carbazole derivatives were reported as flu-
orescent probes[8] or to demonstrate the relationship be-
tween the rigidity of the structure and the linking posi-
tions.[9,10] However, 1,8-disubstituted carbazole derivatives
used as host materials for PhOLEDs have not yet been re-
ported. According to the electron distribution rules of car-
bazole, the 1,8-positions of carbazole are electron-deficient;
this will reduce the conjugation between carbazole and the
other conjugated moieties, leading to a relatively high triplet
energy level. Note that the relatively high spatial hindrance
of 1,8-position derivatives will enhance their glass transition
temperatures. Generally, the introduction of one or more
1,3,4-oxadiazole and N-phenylbenzimidazole groups to the
corresponding carbazole/oxadiazole or carbazole/benz-
centration quenching and triplet–triplet annihilation.[1]
A
suitable host material for separation of the triplet emitters is
vital for the achievement of efficient electrophosphores-
cence. Recently, large numbers of carbazole-based host ma-
terials have been developed, such as the widely utilized
hole-transporting host 4,4’-bis (9-carbazolyl)-2,2’-biphenyl
(CBP),[2] 2,7-bis(N-carbazolyl)-N-phenyl carbazole (BCC-
27),[3] and the bipolar host material bis-4-(N-carbazolyl)phe-
nylphosphine oxide (BCPO),[4] 2,5-bis[2-(9H-carbazol-9-yl)-
phenyl]-1,3,4-oxadiazole (o-CzOXD).[5] However, host mate-
[a] Dr. H. Huang, Y. Wang, B. Pan, X. Yang, Prof. Dr. L. Wang,
Prof. Dr. C. Yang
Wuhan National Laboratory for Optoelectronics
Huazhong University of Science and Technology
Wuhan 430074 (P. R. China)
E-mail: wanglei@mail.hust.edu.cn
[b] Dr. J. Chen, Prof. Dr. D. Ma
AHCTUNGTREGiNNUN midazole hybrids will impart a greater electron-transporting
State Key Laboratory of Polymer Physics and Chemistry
Changchun Institute of Applied Chemistry
Graduate School of Chinese Academy of Sciences
Chinese Academy of Sciences, Changchun 130022 (P. R. China)
E-mail: jschen@ciac.jl.cn
capability to the carbazole derivatives, coupled with in-
creased conductivity.[5,11] Herein, we present the synthesis
and photophysical properties of a series of novel carbazole
derivatives: 1,8-disubstituted carbazole/oxadiazole (1,8-
OXDCz) and carbazole/benzimidazole (1,8-mBICz) hybrids.
[c] Prof. Dr. C. Yang
Department of Chemistry, Wuhan University
Wuhan 430072 (P. R. China)
E-mail: clyang@whu.edu.cn
The PhOLEDs with [IrACHTNUGTRNEUNG(ppy)3] doped in 1,8-OXDCz exhibit
a maximum power efficiency of 89.7 lmWÀ1 and an external
1828
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1828 – 1834

FULL PAPER
Scheme 1. Molecular structures and synthesis of 1,8-OXDCz and 1,8-mBICz. Reagents and conditions: a) Br2, AcOH, 908C, 3.5 h, 85%; b) NaH, CH3I,
DMF, 18 h, 96%; c) [Pd
N
3ACHTUNGERTN(NUNG 2m), toluene, ethanol, 908C,
24 h, 78%; d) [Pd
24 h, 74%.
ACHTUNGTRENNUNG
quantum efficiency (EQE) of 20.3%; the turn-on voltage is
even lower than 2.5 V. These are among the best reported
results for simple carbazole-based devices.[11,12] The results
demonstrate that 1,8-disubstituted-carbazole-based bipolar
host materials could be a promising class of phosphorescent
host materials for PhOLEDs.
Results and Discussion
Synthesis: The new hosts were synthesized according to the
synthetic route shown in Scheme 1. Starting from commer-
cially available 3,6-di-tert-butyl-9H-carbazole, 1,8-dibromo-
3,6-di-tert-butyl-9H-carbazole 1 was prepared by the tradi-
tional bromination reaction; the key precursor 1,8-dibromo-
3,6-di-tert-butyl-9-methyl-9H-carbazole 2 was obtained by
treatment of compound 1 with sodium hydrogen for 0.5 h
and subsequent methylation with iodomethane. The Suzuki
coupling reaction between compound 2 and 2-phenyl-5-[3-
(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl]-1,3,4-
oxadiazole or 1-phenyl-2-[3-(4,4,5,5-tetramethyl-1,3,2-dioxa-
borolan-2-yl)phenyl]-1H-benzo[d]imidazole gave rise to the
final products 1,8-OXDCz and 1,8-mBICz, respectively. The
detailed procedures and characterization of the new com-
pounds are described in the Experimental Section. The pure
compounds 1,8-OXDCz and 1,8-mBICz are white powders,
and exhibit excellent solubility in common solvents such as
THF, CH2Cl2, and toluene for the incorporation of two tert-
butyl groups. The structures of 1,8-OXDCz and 1,8-mBICz
were determined by atmospheric-pressure chemical ioniza-
Figure 1. TGA thermograms of 1,8-OXDCz and 1,8-mBICz recorded at a
heating rate of 108CminÀ1. Inset: DSC thermograms of 1,8-OXDCz and
1,8-mBICz recorded at a heating rate of 108CminÀ1
.
sition temperatures (Tg) were observed at 138 and 1548C,
respectively, through differential scanning calorimetry
(DSC); these values are higher than those of the carbazole
analogues such as CBP (628C) and mCP (608C).[6] The high
Tg values of these host materials may be attributed mainly
to the two tert-butyl groups on the 3,6-positions of the car-
bazole, which can suppress intermolecular aggregation and
improve the film morphology. The relatively high Tg and Td
values mean that these compounds avoid phase separation
upon heating and have the potential for use in high-per-
formance devices fabricated by vacuum thermal evaporation
technology.
1
tion mass spectrometry (APCI-MS), H NMR and 13C NMR
spectroscopy, and elemental analyses.
Photophysical properties: The absorption and fluorescence
properties of 1,8-OXDCz and 1,8-mBICz were tested in
both solution and thin film form (Figure 2a). The absorption
peak at 287 nm is attributed to the n–p* transitions of the
carbazole, and the absorption maximum peak at about
Thermal properties: The good thermal stabilities of 1,8-
OXDCz and 1,8-mBICz are indicated by their high decom-
position temperatures (Td, corresponding to 5% weight loss,
see Figure 1) of 416 and 4338C, respectively. The glass tran-
Chem. Eur. J. 2013, 19, 1828 – 1834
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1829

L. Wang, J. Chen, C. Yang et al.
(2.65 eV) and [IrACTHNUTRGNEUNG
(ppy)3] (2.4 eV),[14] and that 1,8-OXDCz
could be applied as a host for green phosphorescent emit-
ters. Note that when phenylbenzimidazole was attached to
the 3,6-position of carbazole, ET was only 2.48 eV.[15] The rel-
atively high ET of 1,8-mBICz also verified our design princi-
ple that 1,8-disubstitued carbazole derivatives possess higher
ET values than the corresponding 3,6-disubstitued carbazole
derivatives.
Electrochemical properties: For evaluation of the HOMO/
LUMO energy levels, the electrochemical properties of 1,8-
mBICz and 1,8-OXDCz were probed by cyclic voltammetry
(CV). Both compounds exhibited reversible oxidation peaks
(Figure 3); the respective electrochemical data are summa-
Figure 2. a) Absorption and emission spectra of 1,8-OXDCz and 1,8-
mBICz in toluene solution or as thin films at room temperature. b) Phos-
phorescence spectra of 1,8-OXDCz and 1,8-mBICz in 2-methyl tetrahy-
drofuran at 77 K.
Figure 3. Cyclic voltammograms of 1,8-mBICz and 1,8-OXDCz in CH2Cl2
solution.
362 nm may be due to the intermolecular or intramolecular
interactions resulting from the higher degree of conjugation,
as the benzene units are linked at the 1,8-positions. The
emission maxima in toluene solution are centered at about
394 and 385 nm, respectively, and the emission maximum in
the film are redshifted by just 16 and 22 nm, respectively,
AHCTUNGTREGrNNNU ized in Table 1. The oxidation potentials of 1,8-mBICz and
1,8-OXDCz were found at 0.88 V and 0.90 V versus Ag/Ag+
, respectively. The HOMO levels of 1,8-mBICz and 1,8-
OXDCz were estimated to be approximately À5.58 and
À5.60 eV, respectively, according to the equation EHOMO
=
suggesting that the two tert-butyl groups suppress in
N
À(Eoxonset+4.7) eV.[16] The corresponding LUMO levels were
calculated from the HOMO values, and the energy gaps
(3.27 and 3.26 eV), and were found to be approximately
À2.31 and À2.34 eV, respectively. Moreover, the HOMO-
level values were all significantly higher than that of the
CBP host (À6.0 eV), implying lower hole-injection barriers
from the hole-transporting layer to the emissive layer com-
pared with CBP.[17]
ACHTUNGTRENNUNG
sorption edges of 1,8-OXDCz and 1,8-mBICz in toluene, the
corresponding optical energy gaps (Eg) were calculated to
be 3.27 and 3.26 eV, respectively. Figure 2b depicts the phos-
phorescence spectra of 1,8-OXDCz and 1,8-mBICz meas-
ured in a frozen 2-methyltetrahydrofuran matrix at 77 K.
The triplet energies (ET) were about 2.6 and 2.7 eV, respec-
tively, by the highest-energy vibronic sub-band of the phos-
phorescence spectrum, which suggests that ET of 1,8-mBICz
is high enough for blue or green emitters such as FIrpic
Theoretical calculations: Quantum chemical calculations
performed using the DFT/B3LYP/6-31G (d,p) method re-
Table 1. Optical, photophysical, and thermal properties of 1,8-mBICz and 1,8-OXDCz.
labs,max [nm]
lem,max [nm]
Eg [eV][b]
HOMO/LUMO [eV][c]
HOMO/LUMO [eV][d]
ET [eV][e]
Tg/Td [8C][f]
Sol[a]
Film
Sol[a]
Film
1,8-mBICz
1,8-OXDCz
362/299
365/281
363/302
362/288
385
394
407
410
3.27
3.26
À5.58
À5.60
À2.31
À2.34
À5.34
À5.30
À2.46
À2.45
2.7
2.6
154/433
138/416
[a] Measured in toluene solution. [b] Calculated from the edge of the absorbance of the solution. [c] Measured from the CV. [d] Calculated from DFT.
[e] Measured in 2-MeTHF at 77 K. [f] Measured from DSC and TGA.
1830
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1828 – 1834

Bipolar Hosts Based on 1,8-Disubstituted Carbazole
FULL PAPER
Figure 5. Current density versus voltage characteristics of the hole-only
and electron-only devices.
Figure 4. Spatial distributions of the HOMO and LUMO levels for 1,8-
mBICz and 1,8-OXDCz.
of the hole-only device of 1,8-mBICz is higher than that of
the 1,8-OXDCz device, which may be ascribed to the stron-
ger electron-withdrawing ability of oxadiazole. The results
also demonstrate that electron injection from the cathode
into the host 1,8-mBICz is more difficult than into 1,8-
OXDCz, which means the device based on 1,8-OXDCz may
exhibit a better performance.
vealed that the molecules exhibit non-coplanar conforma-
tions (Figure 4). In particular, the 1,8-disubstituted carbazole
twists nearly perpendicularly to the adjacent two oxadiazole
or benzimidazole moieties because of the steric repulsion
between them. Such special structural characteristics can in-
fluence their electronic and physical properties. The HOMO
and LUMO orbitals of 1,8-mBICz and 1,8-OXDCz are de-
Electrophosphorescent OLED characterization: Compounds
1,8-mBICz and 1,8-OXDCz were also examined in terms of
their use as host materials for phosphorescent devices. Be-
cause the triplet energy level of 1,8-mBICz is higher than
that of FIrpic (ET =2.65 eV), Device A was fabricated using
FIrpic as the dopant, with the device structure ITO/MoO3
(10 nm)/NPB (40 nm)/mCP (5 nm)/1,8-mBICz: 6 wt%
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
ly. The separation of the HOMO and LUMO orbitals is
beneficial for efficient charge-carrier transport and the pre-
vention of reverse energy transfer.[18] The calculated
HOMO/LUMO values are À5.35/À2.46 eV and À5.30/
À2.45 eV, respectively, which also correlate well with the ex-
perimental data.
FIrpic (20 nm)/TmPyPB (40 nm)/LiF
ACHTUNGTREN(NUNG 1 nm)/Al (100 nm). In
this device, 1,4-bis[(1-naphthyl-phenyl)amino]biphenyl
(NPB) acts as a hole-transporting material, and mCP acts as
both a hole-transporting material and an exciton blocker
(ET, 2.9 eV) to prevent diffusion of the excitons to the NPB
layer. 3,3’-{5’-[3-(pyridine-3-yl)phenyl]-(1,1’:3’,1’’-terphenyl)-
3,3’’-diyl}dipyridine (TmPyPB) acts as both the electron-
transporting and hole-blocking layer. The device perform-
ance is summarized in Table 2. As shown in Figure 6a, De-
Bipolar transporting characteristics: For a better under-
standing of the effect of the structural change on the energy
levels and the carrier-injection and transport abilities, hole-
only and electron-only devices of these bipolar host materi-
als were fabricated.[4] The configuration of the hole-only
device was ITO/NPB (10 nm)/Host (30 nm)/NPB (10 nm)/Al
(100 nm), and that of the electron-only device was ITO/BCP
(10 nm)/Host (30 nm)/BCP (10 nm)/LiF (1 nm)/Al (100 nm).
AHCTUNGTREGvNNNU ice A exhibits a maximum external quantum efficiency and
power efficiency of 11.6% and 27.0 lmWÀ1, respectively.
The EL spectra are the same as the emission of FIrpic (see
Figure 6b), suggesting that the triplet energy of 1,8-mBICz
is high enough to transfer energy to the dopant FIrpic. The
relatively poor performance of Device A may be induced by
reverse energy transfer; nevertheless, the performance is
comparable with that of the blue device based on mCP.[20]
NPB
(N,N’-bis-(1-naphthyl)-N,N’-diphenyl-1,10-biphenyl-
4,4’-diamine) and BCP (2,9-dimethyl-4,7-diphenyl-1,10-phe-
nanthroline) layers were used to prevent electron and hole
injection from the cathode and anode, respectively.[19] As
shown in Figure 5, the hole-only device gives a slightly
lower current density than the
electron-only device for the
Table 2. Electroluminescence characteristics of the devices.
compound 1,8-OXDCz, but for
1,8-mBICz, the current density
of the hole-only device is
higher than that of the elec-
tron-only device. However,
both devices exhibit bipolar
[b]
[d]
Device
Host
Von
Lmax [cdmÀ2
]
hc[c]
[cdAÀ1
hp
[lmWÀ1
EQE[e]
[%]
CIE
[V][a]
(V at Lmax, [V])
G
]
G
]
x, y[f]
A
B
C
1,8-mBICz
1,8-mBICz
1,8-OXDCz
3.0
2.7
2.5
9565 (12.3)
38440 (9.5)
37920 (10.3)
27.6
64.0
73.9
27.0
70.0
89.7
11.6
17.5
20.3
0.15, 0.31
0.30, 0.63
0.30, 0.63
[a] Turn-on voltage (V) at a luminance of 1.58 cdmÀ2. [b] Maximum brightness of the device. [c] Maximum cur-
properties. The current density rent efficiency. [d] Maximum power efficiency. [e] Maximum external quantum efficiency. [f] CIE coordinates.
Chem. Eur. J. 2013, 19, 1828 – 1834
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1831

L. Wang, J. Chen, C. Yang et al.
Figure 6. Blue electrophosphorescence properties of Device A with 1,8-
mBICz as the host. a) EQE–J–PE curves of Device A; b) EL spectra of
Device A.
For further evaluation of the performances of 1,8-mBICz
and 1,8-OXDCz as host materials, green PhOLEDs were
fabricated with the following configuration: ITO/MoO3
(10 nm)/NPB (80 nm)/TCTA (5 nm)/host: 9 wt% [Ir
ACHTUNGERTN(NUNG ppy)3]
(20 nm)/TmPyPB (40 nm)/LiF(1 nm)/Al (100 nm) (1,8-
AHCTUNGTRENNUNG
mBICz for Device B; 1,8-OXDCz for Device C). 1,4-bis[(1-
naphthylphenyl)amino]biphenyl (NPB) was used as the
hole-transporting material, TmPyPB was used as both the
electron-transporting and hole-blocking layer, and MoO3
and LiF served as the hole- and electron-injecting layers, re-
spectively.[21] In the two devices, TCTA was chosen as the
exciton blocker to prevent the diffusion of excitons to the
hole-transporting layer. TCTA has been proven to be an ef-
ficient exciton blocker for green and red phosphorescence
Figure 7. Green electrophosphorescence properties of Devices B and C:
a) J–V–L curves; b) EQE–J–PE curves; c) EL spectra.
properties of the host and the suitable HOMO and LUMO
energy levels of 1,8-OXDCz with the adjacent layers. Sche-
matic energy level diagrams of the devices are shown in
Figure 8. The small energy barriers show that carrier injec-
tion from TCTA into 1,8-mBICz or 1,8-OXDCz is easy. Fur-
devices.[22] In addition, the phosphor [Ir
OXDCz was used as the emitting layer, with the doping
level of [Ir(ppy)3] optimized at 9%. The current density–
ACHTUNGRTEN(NNUG ppy)3] doped in 1,8-
ACHTUNGTRENNUNG
voltage–luminance (J–V–L) characteristics, efficiency–cur-
rent density (J–h) characteristics, and EL spectra of the de-
vices are shown in Figure 7, and the key parameters are
summarized in Table 2.
Both the 1,8-mBICz- and 1,8-OXDCz-based devices have
low turn-on voltages of 2.7 and 2.5 V, respectively, at a
brightness of 1.58 cdmÀ2. Considering that the triplet energy
of [Ir
Device C has already reached the limit of [Ir
green PhOLEDs; this could be attributed to the bipolar
ACHTUNGTRENNUNG
(ppy)3] is about 2.42 eV,[17] the turn-on voltage value of
AHCTUNGRTEN(NGUN ppy)3]-based
Figure 8. Energy levels of the materials used in Devices B and C.
1832
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1828 – 1834

Bipolar Hosts Based on 1,8-Disubstituted Carbazole
FULL PAPER
loss while heating at a rate of 108CminÀ1 from 30 to 7008C. Cyclic vol-
tammetry measurements were recorded for a conventional three-elec-
trode cell using a Pt carbon working electrode of 2 mm in diameter, a
platinum wire counter electrode, and an Ag/AgNO3 (0.1m) reference
thermore, there are energy barriers of approximately 1.1 eV
between the EML and TmPyPB interfaces. This means that
the carriers can be confined efficiently in the EML, which
will result in high exciton formation efficiencies even at high
current densities. Both devices show very high and stable ef-
ficiencies. Device C has a maximum current efficiency of
73.9 cdAÀ1, a maximum power efficiency of 89.7 lmWÀ1,
and a maximum luminance of 37920 cdmÀ2 at a low driving
voltage of 10.3 V. On the other hand, the efficiency of De-
electrode on
a computer-controlled EG&G Potentiostat/Galvanostat
model 283 at room temperature. Reduction CVs of all compounds were
obtained in dichloromethane containing 0.1m tetrabutylammoniumhexa-
fluorophosphate (Bu4NPF6) as the supporting electrolyte. All solutions
were purged with a nitrogen stream for 10 min before measurement.
Computational details: The geometrical and electronic properties were
analyzed with the Amsterdam Density Functional (ADF) 2009.01 pro-
gram package. The calculation was optimized by means of the B3LYP
(Becke three-parameters hybrid functional with Lee–Yang–Perdew corre-
lation functionals)[23] with the 6–31G(d) atomic basis set. Then, the elec-
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
a maximum power efficiency of 70.0 lmWÀ1. These results
can be understood by considering the hole- and electron-
transporting properties of these bipolar compounds and
their HOMO/LUMO spatial distributions. More balanced
carriers in the 1,8-OXDCz-based device could lead to a
higher utilization ratio of the charge carriers. The low turn-
on voltage and high power efficiency are attractive in com-
tronic structures were calculated at the t-HCTHhyb/6–311+ +GACHTUNGTRENNUNG(d,p)
level.[24] Molecular orbitals were visualized using ADFview.
Device fabrication: The hole-injection material MoO3, hole-transporting
material 1,4-bis[(1-naphthyl- phenyl)amino]biphenyl (NPB), electron/ex-
citon-blocking material TCTA, and electron-transporting material 3,3’-
{5’-[3-(pyridine-3-yl)phenyl]-(1,1’:3’,1’’-terphenyl)-3,3’’-diyl}dipyridine
(TmPyPB) were commercially available. Commercial indium tin oxide
(ITO)-coated glass with a sheet resistance of 20 W per square was used as
the substrates. Before device fabrication, the ITO glass substrates were
precleaned carefully and treated with oxygen plasma for 2 min. MoO3
was deposited first on the ITO substrate, followed by NPB, mCP or
TCTA, the emissive layer, and TmPyPB. Finally, a cathode composed of
lithium fluoride and aluminum was deposited sequentially onto the sub-
strate under a vacuum of 10À6 Torr. The current density—voltage–bright-
ness (J–V–L) curves of the devices were measured with a Keithley 2400
Source meter equipped with a calibrated silicon photodiode. The EL
spectra were measured with a PR655 spectrometer. The EQE values
were calculated according to previously reported methods.[25]
parison with that of the [IrACHTNUGTRENUNG(ppy)3]-based PhOLEDs.
Conclusion
In summary, we have synthesized two novel bipolar 1,8-dis-
ubstituted carbazole derivatives, 1,8-mBICz and 1,8-
OXDCz, containing two benzimidazole or oxadiazole units
as the electron-transporting groups. Excellent photophysical
and thermal properties mean that these compounds are
good candidates for high-performance blue and green PhO-
LEDs. Blue PhOLEDs with 1,8-mBICz exhibit similar per-
formances to similarly structured devices based on the tradi-
tional mCP host. The PhOLEDs with the 1,8-OXDCz as the
Synthesis: 1,3,4-oxidazole and N-phenylbenzimidazole boronic esters
were prepared by the procedure reported in the literature.[26] N-methyl-
1,8-dibromo-3,6-ditetrabutyl-carbazole was synthesized according to the
literature method.[8] The Suzuki coupling reaction was conducted under a
nitrogen atmosphere, avoiding light exposure.
1,8-Dibromo-3,6-di-tert-butyl-9H-carbazole
(1):
Bromine
(0.92 g,
5.8 mmol) was added to a warm (908C) solution of 3,6-di-tert-butyl-9H-
carbazole (0.81 g, 2.9 mmol) in glacial AcOH (100 mL). The mixture was
stirred at 908C under N2 for 3.5 h. After being cooled to room tempera-
ture, the reaction mixture was concentrated and dried. The solid obtained
was washed with hexane, filtered, and dried to provide 1 as a white solid
(1.07 g, 85%). 1H NMR: (CDCl3, 400 MHz): d=8.13 (s, 1H), 7.97 (s,
2H), 7.63 (s, 2H), 1.44 ppm (s, 18H); MS (APCI): m/z: 436.1 [M+1]+.
host and [IrACHTUNGTRENNUNG(ppy)3] as the dopant exhibit a maximum bright-
ness of 37920 cdmÀ2 and a maximum power efficiency of
89.7 lmWÀ1, with a turn-on voltage as low as 2.5 eV. The
good performance of the 1,8-OXDCz-based PhOLEDs is
among the best reported results for [IrACTHNUTRGENUG(N ppy)3]-based green
PhOLEDs, indicating their great potential for further com-
mercial application.
1,8-Dibromo-3,6-di-tert-butyl-9-methyl-9H-carbazole (2): NaH (1.1 g,
4.5 mmol) was added slowly to a mixture of compound 1,8-dibromo-3,6-
di-tert-butyl-9H-carbazole (1) (1.30 g, 3.0 mmol) in anhydrous DMF
(100 mL) solution. After 30 min, CH3I (0.64 g, 4.5 mmol) was added. The
mixture was stirred at room temperature for 18 h under nitrogen. The re-
action was quenched with H2O and extracted with CH2Cl2. The organic
fractions were dried under sodium sulfate and the solvent was removed
under reduced pressure. The product was purified by silica gel column
chromatography (10% EtOAc in hexane as eluent) to give the title
powder (1.30 g, 96%). 1H NMR (400 MHz, CDCl3, 258C, TMS): d=7.94
Experimental Section
Materials and measurements: All the reagents and solvents used for the
synthesis were purchased from Aldrich and were used without further
purification. All reactions were performed under a dry nitrogen atmo-
ACHTUNGTRENNUNG
sphere. 1H NMR and 13C NMR spectra were measured on a Bruker-
(s, 1H), 7.64 (s, 1H), 4.42 (s, 3H), 1.45 ppm (s, 18H); MSACTHNUTRGENUG(N APCI): m/z:
AF301 AT 400 MHz spectrometer. Elemental analyses of carbon, hydro-
gen, and nitrogen were performed on an Elementar (Vario Micro cube)
analyzer. Mass spectra were obtained on an Agilent (1100 LC/MSD
Trap) instrument using ACPI ionization. UV/Vis absorption spectra were
recorded on a Shimadzu UV-VIS-NIR Spectrophotometer (UV-3600).
Photoluminescence (PL) spectra were recorded on Edinburgh instru-
ments (FLSP920 spectrometers). Differential scanning calorimetry
(DSC) was performed on a PE Instruments DSC 2920 unit at a heating
rate of 108CminÀ1 from 30 to 3008C under nitrogen. The glass transition
temperature (Tg) was determined from the second heating scan. Ther-
mogravimetric analysis (TGA) was undertaken using a PerkinElmer In-
struments (Pyris1 TGA) apparatus. The thermal stability of the samples
under a nitrogen atmosphere was determined by measuring their weight
450.3 [M+1]+.
5,5’-[(3,6-Di-tert-butyl-9-methyl-9H-carbazole-1,8-diyl)bis(3,1-phenyl-
ene)]bis(2-phenyl-1,3,4-oxadiazole) (1,8-OXDCz): A mixture of 2 (0.45 g,
1.0 mmol), 2-phenyl-5-[3-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-
phenyl]-1,3,4-oxadiazole (0.84 g, 2.4 mmol), [Pd(PPh3)4] (100 mg,
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
0.08 mmol), K2CO3 (2.0m aqueous solution, 5.0 mL, 10.0 mmol), toluene
(50 mL), and ethanol (25.0 mL) was stirred at 908C for 24 h. After being
cooled to room temperature, dichloromethane was added to the reaction
mixture. The organic phase was separated and washed with brine before
being dried over anhydrous MgSO4. The solvent was evaporated, and the
solid residues were purified by column chromatography on silica gel with
petroleum ether to afford the crude product as a white powder (0.57 g,
Chem. Eur. J. 2013, 19, 1828 – 1834
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1833

L. Wang, J. Chen, C. Yang et al.
78%). 1H NMR (400 MHz, CDCl3, 258C, TMS): d=8.36 (s, 2H), 8.21–
8.10 (m, 8H), 7.79–7.77 (d, J=7.6 Hz, 2H), 7.59–7.51 (m, 8H), 7.45 (s,
2H), 2.90 (s, 3H), 1.52 ppm (s, 18H); 13C NMR (100 MHz, CDCl3, 258C,
TMS): d=164.71, 164.53, 143.12, 141.95, 139.95, 132.70, 131.78, 129.11,
128.96, 127.61, 127.06, 126.96, 126.55, 125.45, 125.18, 124.47, 123.98,
123.95, 116.06, 38.17, 34.76, 32.04 ppm; MS (APCI): m/z: 734.5 [M+1]+;
elemental analysis calcd (%) for C49H43N5O2: C 80.19, H 5.91, N 9.54, O
4.36; found: C 80.28, H 5.54, N 9.68, O 4.50.
[6] M. H. Tsai, Y. H. Hong, C. H. Chang, H. C. Su, C. C. Wu, A. Mato-
liukstyte, J. Simokaitiene, S. Grigalevicius, J. V. Grazulevicius, C. P.
Hsu, Adv. Mater. 2007, 19, 862–866.
[7] F. Huang, Y. Zhang, M. S. Liu, Y. J. Cheng, A. K. Y. Jen, Adv. Funct.
Mater. 2007, 17, 3808–3815.
[8] H. C. Gee, C. H. Lee, Y. H. Jeong, W. D. Jang, Chem. Commun.
2011, 47, 11963–11965.
[9] H.-Y. Wang, F. Liu, L.-H. Xie, C. Tang, B. Peng, W. Huang, W. Wei,
J. Phys. Chem. C. 2011, 115, 6961–6967.
[10] a) T. Michinobu, H. Osako, K. Shigehara, Macromol. Rapid
Commun. 2008, 29, 111–116; b) T. Michinobu, H. Osako, K. Shige-
hara, Macromolecules 2009, 42, 8172–8180.
[11] Y. Tao, Q. Wang, C. Yang, C. Zhong, K. Zhang, J. Qin, D. Ma, Adv.
Funct. Mater. 2010, 20, 304–311.
[12] a) Y. Tao, S. Gong, Q. Wang, C. Zhong, C. Yang, J. Qin, D. Ma,
Phys. Chem. Chem. Phys. 2010, 12, 2438–2442; b) Y. Tao, Q. Wang,
C. Yang, C. Zhong, J. Qin, D. Ma, Adv. Funct. Mater. 2010, 20,
2923–2929.
[13] Y. Luo, H. Aziz, Adv. Funct. Mater. 2010, 20, 1285–1293.
[14] Z. Jiang, Y. Chen, C. Fan, C. Yang, Q. Wang, Y. Tao, Z. Zhang, J.
Qin, D. Ma, Chem. Commun. 2009, 3398–3400.
3,6-Di-tert-butyl-9-methyl-1,8-bis[3-(1-phenyl-1H-benzo[d]imidazole-2-
yl)phenyl]-9H-carbazole (1,8-mBICz): The synthesis procedure is similar
to that for compound 1,8-OXDCz. Yield: 74%. 1H NMR (400 MHz,
CDCl3, 258C, TMS): d=8.04 (s, 2H), 7.89–7.87 (d, J=8.0 Hz, 2H), 7.65
(s, 4H), 7.51–7.39 (s, 10H), 7.36–7.30 (m, 6H), 7.29–7.25 (m, 4H), 7.03 (s,
2H), 2.70 (s, 3H), 1.42 ppm (s, 18H); 13C NMR (100 MHz, CDCl3, 258C,
TMS): d=152.32, 142.92, 142.44, 141.06, 139.56, 137.15, 136.89, 130.95,
130.08, 129.90, 128.77, 128.19, 128.01, 127.40, 126.42, 124.89, 124.67,
123.48, 123.11, 119.86, 115.39, 110.51, 37.61, 34.63, 32.00 ppm; MS
(APCI): m/z: 830.5 [M+1]+; elemental analysis calcd (%) for C59H51N5:
C 85.37, H 6.19, N 8.44; found: C 85.44, H 6.04, N 8.52.
[15] W. Y. Hung, L. C. Chi, W. J. Chen, Y. M. Chen, S. H. Chou, K. T.
Wong, J. Mater. Chem. 2010, 20, 10113.
[16] Q. Sun, H. Wang, C. Yang, Y. Li, J. Mater. Chem. 2003, 13, 800–806.
[17] Z. Q. Gao, B. X. Mi, H. L. Tam, K. W. Cheah, C. H. Chen, M. S.
Wong, S. T. Lee, C. S. Lee, Adv. Mater. 2008, 20, 774–778.
[18] Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike, H. Miyamoto, T.
Kajita, M. Kakimoto, Adv. Funct. Mater. 2008, 18, 584–590.
[19] Z. Q. Gao, M. Luo, X. H. Sun, H. L. Tam, M. S. Wong, B. X. Mi,
P. F. Xia, K. W. Cheah, C. H. Chen, Adv. Mater. 2009, 21, 688–692.
[20] 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, 2422–2424.
[21] H. You, Y. Dai, Z. Zhang, D. Ma, J. Appl. Phys. 2007, 101, 026105.
[22] Y.-Y. Lyu, J. Kwak, W. S. Jeon, Y. Byun, H. S. Lee, D. Kim, C. Lee,
K. Char, Adv. Funct. Mater. 2009, 19, 420–427.
[23] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[24] A. D. Boese, N. C. Handy, J. Chem. Phys. 2002, 116, 9559–9569.
[25] S. R. Forrest, D. D. C. Bradley, M. E. Thompson, Adv. Mater. 2003,
15, 1043–1048.
[26] W. Kwon, B. Ahn, D. M. Kim, Y.-G. Ko, S. G. Hahm, Y. Kim, H.
Kim, M. Ree, J. Phys. Chem. C. 2011, 115, 19355–19363.
Acknowledgements
This research work was supported financially by the National Natural
Science Foundation of China (Project Nos.21161160442, 51203056), the
National Basic Research Program of China (973 Program
2013CB922100), and the Open Research Fund of the State Key Labora-
tory of Polymer Physics and Chemistry, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences.
[1] M. A. Baldo, D. F. OꢁBrien, Y. You, A. Shoustikov, S. Sibley, M. E.
Thompson, S. R. Forrest, Nature 1998, 395, 151–154.
[2] M. A. Baldo, S. Lamansky, P. E. Burrows, M. E. Thompson, S. R.
Forrest, Appl. Phys. Lett. 1999, 75, 4–6.
[3] W. Jiang, L. Duan, J. Qiao, G. Dong, D. Zhang, L. Wang, Y. Qiu, J.
Mater. Chem. 2011, 21, 4918–4926.
[4] H. H. Chou, C. H. Cheng, Adv. Mater. 2010, 22, 2468–2471.
[5] Y. Tao, Q. Wang, C. Yang, Q. Wang, Z. Zhang, T. Zou, J. Qin, D.
Ma, Angew. Chem. 2008, 120, 8224–8227; Angew. Chem. Int. Ed.
2008, 47, 8104–8107.
Received: June 30, 2012
Published online: November 23, 2012
1834
www.chemeurj.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1828 – 1834