Highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio deep-blue emitter with improved concentration of phosphorescent dopant

Article abstract of DOI:10.1039/c5ra05432j

An improved concentration of phosphorescent dopant for highly-efficient hybrid white organic light-emitting diodes based on a high radiative exciton ratio (80%) deep-blue emitter has been developed. The high radiative exciton ratio for the deep-blue emitter was found to be the transfer from the higher triplet (T₅) to the lowest singlet state (S₁) by a "hot-exciton" process. Notably, when the concentration of Ir(2-phq)₃ is up to 0.9 wt%, the OLED still exhibited white emission with a maximum total EQE, CE and PE of 22.3%, 53.7 cd A^-1 and 60.2 lm W^-1, respectively. The exciton transfer mechanism in a high concentration of phosphorescent dopant was also discussed. The studies provide a way to obtain high performance F/P hybrid WOLEDs with a simple architecture and improved doping concentration. This journal is

Full text of DOI:10.1039/c5ra05432j

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Highly-efficient hybrid white organic light-emitting
diodes based on a high radiative exciton ratio deep-
blue emitter with improved concentration of
phosphorescent dopant†
Cite this: RSC Adv., 2015, 5, 32298
a
b
a
a
a
*
Xinhua Ouyang, Xiang-Long Li, Yongqi Bai, Dongbo Mi, Ziyi Ge
and Shi-Jian Su
b
*
An improved concentration of phosphorescent dopant for highly-efficient hybrid white organic light-
emitting diodes based on a high radiative exciton ratio (80%) deep-blue emitter has been developed. The
high radiative exciton ratio for the deep-blue emitter was found to be the transfer from the higher triplet
(T5) to the lowest singlet state (S1) by a “hot-exciton” process. Notably, when the concentration of Ir(2-
phq)3 is up to 0.9 wt%, the OLED still exhibited white emission with a maximum total EQE, CE and PE of
Received 27th November 2014
Accepted 30th March 2015
22.3%, 53.7 cd Aꢀ1 and 60.2 lm Wꢀ1
, respectively. The exciton transfer mechanism in a high
DOI: 10.1039/c5ra05432j
concentration of phosphorescent dopant was also discussed. The studies provide a way to obtain high
performance F/P hybrid WOLEDs with a simple architecture and improved doping concentration.
www.rsc.org/advances
WOLEDs is considered to be an ideal solution to approach high-
efficiency and long life-time WOLEDs.
1. Introduction
Commonly, the hybrid WOLEDs can be constructed using
two architectures, including multi-emissive-layers (multi-
EML), or a single-emissive-layer (single-EML) with different
color emitting dopants. The multi-EML structure usually
requires complexity of device fabrication with an interlayer
between the uorophore and phosphor to prevent mutual
exciton transfer and quenching processes, which decrease the
reproducibility and raise the fabrication cost. Furthermore,
the complicated multilayer structure is also difficult to be
achieved by solution processing. Compared to the multi-EML
counterpart, the single-EML can be avoided effectively the
accumulation of charges and the generation of a higher elec-
tric eld at the interfaces, especially for the single-dopant
single-EML WOLEDs. Therefore, the single-dopant single-
EML structure is thought to be a potential strategy to fabri-
cate hybrid WOLEDs with high efficiencies.
In the early work, Leo et al. demonstrated one kind of single-
dopant single-EML F/P hybrid WOLEDs by doping a yellow
phosphor (Ir(MDQ)2(acac)) into a blue uorescent host (4P-NPD).10
Their exhibited external quantum efficiencies (EQEs) at a practical
brightness of 1000 cd mꢀ2 are much lower than the theoretically
maximum EQE (EQEmax) of 20%, which can be attributed to the
unipolar of 4P-NPD. And the corresponding power efficiencies are
not yet high enough for practical applications. In the meantime,
Ma et al. reported one of bipolar derivatives containing triphe-
nylamine and oxadiazole units for single-dopant single-EML
WOLEDs.11 A maximum current efficiency (CE) of 7.9 cd Aꢀ1 (a
corresponding EQE of 5.2%) was achieved. Most recently, higher
White organic light-emitting diodes (WOLEDs) have been
attracting great attention due to their favorable properties of
good color rendering, exibility, and homogenous large-area
emission for lighting sources and full color OLEDs.1–4 In the
past two decades, some effective approaches have been used to
achieve WOLEDs including full uorescent,5 phosphorescent6
and hybrid WOLEDs.7 Generally, for the full uorescent
WOLEDs, they have been demonstrated with excellent stability.
However, these devices show low efficiencies owing to the limit
of exciton statistics.8 So, in order to obtain high efficiencies, the
researchers begin to focus on the full phosphorescent WOLEDs,
which have been reported with high efficiencies up to 100%
internal quantum efficiency. Unfortunately, these devices are
not stable enough to ensure a long device life-time as the limit
of the availability of high performance blue hosts/emitters.9
Moreover, an intrinsic exchange energy loss arising from energy
transfer from the host's singlets to the phosphorescent guest's
triplets cannot be prevented fully. In view of this, the develop-
ment of uorescence (F) and phosphorescence (P) hybrid
aNingbo Institute of Materials Technology & Engineering (NIMTE), Chinese Academy of
Sciences (CAS), Ningbo, 315201, P. R. China. E-mail: geziyi@nimte.ac.cn
bState Key Laboratory of Luminescent Materials and Devices (South China University of
Technology), Institute of Polymer Optoelectronic Materials and Devices, South China
University of Technology, Guangzhou, 510641, P. R. China. E-mail: mssjsu@scut.
edu.cn
† Electronic supplementary information (ESI) available. CCDC 1035328. For ESI
and crystallographic data in CIF or other electronic format see DOI:
10.1039/c5ra05432j
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performance with maximum total EQE of 26.6%, total effi- emission. The single-dopant single-EML F/P hybrid WOLED
ciencies of 53.5 cd Aꢀ1 and 67.2 lm Wꢀ1 has been obtained by has been fabricated with improved dopant concentration to
Zhang's group based on a blue uorophor (DADBT) and 0.9%, which dose not follow the conventional principle of the
orange phosphors (Ir(2-phq)3) hybrid single-dopant single- doping concentration regulation. The device still shows white
EML.12 Nevertheless, the dopant concentration of phosphors emission and excellent electroluminescence (EL) performance
in these WOLEDs with single-dopant single-EML structure was with a low turn-on voltage of 2.6 V, maximum total EQE, CE and
very low (0.1%), which was difficult to be controlled by vacuum PE of 22.3%, 53.7 cd Aꢀ1 and 60.2 lm Wꢀ1, respectively.
deposition. Moreover, the color was very sensitive to the
dopant concentration of phosphors. A small variation in the
dopant concentration would result in a pronounced change in
the energy transfer between the emitting dopants. Therefore,
2. Results and discussion
2.1 Synthesis and crystal structure
developments of improving dopant concentration design
For the synthesis of DPTPA (Fig. 1), the important intermediate
strategies for exible manipulation have been considered as
was 400-(diphenyl-amino)-[1,10:40,100-terphenyl]-4-carbaldehyde
the current key challenges for progressing single-dopant
(3), which was synthesized by Suzuki couplings of 40-bromo-
N,N-diphenyl-[1,10-biphenyl]-4-amine (2) and (4-formyl-phenyl)
single-EML F/P hybrid WOLEDs.
In this work, we demonstrated the rst time with improved
boronic acid using Pd(PPh3)2Cl2 as catalysis. While the
dopant concentration by design and synthesis of an ideal
compound 2 can be prepared by mixing (4-(diphenylamino)
saturated deep-blue uorophor, N,N-diphenyl-400-(1,4,5-tri-
phenyl)boronic acid (1) and 1-bromo-4-iodobenzene in the
phenyl-1H-imidazol-2-yl)-[1,10:40,100-terphenyl]-4-amine (DPTPA),
presence of Pd(PPh3)2Cl2, K2CO3 and toluene. Finally, the
which exhibits high radiative exciton ratio and deep-blue
condensation of compound 3 and benzil resulted in the target
Fig. 1 The synthetic route and crystal structure of DPTPA.
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compound DPTPA with high yield. Single-crystal of DPTPA† was sublimation temperatures and therefore proves the high
obtained from a layering of ethanol onto a mixed solution of thermal stability. The DSC curve shows the glass-transition
ꢁ
dichloromethane and n-hexane. As shown in the crystal struc- temperatures (Tg) is found to be 151 C.
ture, the highly twisted geometry is found in the DPTPA to
contain N-substituted benzene ring with imidazole plane and
triphenylamine (TPA) moiety in DPTPA with the imidazole
plane, and the dihedral angles are 97.6ꢁ and 23.3ꢁ, respectively.
2.3 Theoretical calculation
The geometry and electronic structures of DPTPA were obtained
using density functional theory (DFT) at the B3LYP/6-31G(d,p)
level in the Gaussian 03 program.13 The calculated dihedral
angles between imidazole and N-substituted benzene ring and
triphenylamine moieties are 87.8ꢁ and 41.5ꢁ, respectively. The
large twisted geometry conguration might prevent intra-
molecular extending of p-electron and suppress molecular
packing in the solid state effectively, which is agreement with
the results of the crystal structure. The electron dentistry of
DPTPA was shown in Fig. 3. The HOMO orbit of DPTPA mainly
populate on the whole molecule, while the corresponding
LUMO orbit mainly locate on the linker and imidazole moieties.
A large overlapping was found in the molecule, which would be
benet to balance the carriers. The values of HOMO and LUMO
levels are 4.9 eV and 2.24 eV, respectively. In contrast, the
absolute values of the frontier orbital energies have a 0.36 eV
discrepancy between the experimental and theoretical data. As
DFT-calculations were performed in gas-phase only, solvent
effects occurring during experimental characterization were not
taken into account. Additionally, time-dependent (TD) DFT
calculations were used to predict the lowest and higher exited
states (triplets and singlets), two degenerate triplet levels (T3
and T4) of 3.007 eV, which is lower than the value of the S1 level
(3.267 eV). And the lower triplet states (T1 and T2) are 2.614 eV
and 2.848 eV, respectively. All of these low triplet states are
ascribed to the transition from HOMOꢀ1 to LUMO and from
HOMOꢀ2 to LUMO+1, both located on the same imidazole unit.
The transitions between spatially non-overlapped HOMOꢀ2
and LUMO orbitals are ascribed to S1 and T5, and their energy
difference is zero. HOMOꢀ2 and LUMO+2 also do not overlap,
and the transitions between them are ascribed to S2 and T9;
their energy difference is also zero. These zero gaps between the
exited singlet (Sn $ 1) and triplet (Tm $ 3) state may offer some
advantage with respect to increasing the radiative singlet-state
population from the nonradiative high triplet state, generated
by the “hot-exciton” process from the T5 to S1 states.
2.2 Electrochemical and thermal and properties
The electrochemical behavior of DPTPA was studied by cyclic
voltammetry (CV). The HOMO level of DPTPA was determined
from the onset potential of the rst oxidation relative to ferro-
cene (Fig. 2a), and the LUMO level was calculated by adding the
optical band gap to the HOMO levels. The results are summa-
rized in Table 1. The rst oxidation event is observed at 0.86 (vs.
Fc/Fc+). The potentials can be translated to HOMO level of
5.26 eV. The thermal properties of DPTPA were investigated
with TGA and DSC under a nitrogen atmosphere as shown in
Fig. 2b, and their thermal data are summarized in Table 1. The
TGA measurement shows that DPTPA possesses high thermal
stabilities of more than 400 ꢁC. A weight loss of 5% (Td) is
observed at 469 ꢁC. The value is more than 100 ꢁC above the
2.4 Absorption and photoluminescent properties
The absorption and photoluminescent (PL) spectra in lm and
solution are shown in Fig. 4. The absorption spectra in solution
and lm are nearly identical, implying small dipolar changes at
the ground state in different surroundings. The absorptive
spectra is characterized by an absorption maximum at lmax,abs
¼ 351 nm, while the onset is determined at l ¼ 397 nm. It can
be attributed to be the transition of p / p*.14 Weaker
absorption band is observed at l ¼ 305 nm, which is assigned to
be the transition of d / p.15 A small bathochromic shi occurs
in the solid state. The PL spectra in solution features the similar
proles with that of in lm. The peak is located at ꢂ435 nm and
the FWHM of 58 nm in solution and 46 nm in lm, respectively.
Moreover, the compound of DPTPA exhibits a pronounced
Fig. 2 (a) The oxidation curve of DPTPA from cyclic voltammetry; (b)
the TGA and DSC curves of DPTPA.
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Table 1 Summaries of physical properties of DPTPA
Absa (nm)
PLa (nm)
Tg (ꢁC)
Td (ꢁC)
Fc (%)
Absd (nm)
355
PLd (nm)
438
HOMO (eVe, eVf)
LUMO (eVe, eVf)
Egg (eV)
3.15
Egh (eV)
b
349
434
151
469
23.8
ꢀ5.26, ꢀ4.90
ꢀ2.11, ꢀ2.24
2.66
a
UV-vis absorption and PL spectra in THF. b Loss of 5 wt%. c Measured by the integrating sphere system with neat lm. d UV-vis absorption and PL
spectra in neat lm. e HOMO was measured from the onset of oxidation potential, LUMO was deduced from HOMO and Eg. f Obtained from DFT
calculations using B3LYP/6-31G*. g Obtained from the onset of UV-vis absorption spectra in lm. h Calculated from the HOMO and LUMO based on
the calculations of B3LYP/6-31G*.
Fig. 3 Calculated energy levels, molecular orbitals, and excited state of DPTPA.
Stokes-shi by almost 80 nm, which is characteristic for a large
reorganization in the excited state.16 Additionally, the PL
spectra in various polarity solvents were recorded and are
shown in Fig. 5. With increase of the solvent polarity gradually
from petroleum ether to acetonitrile, the PL spectra exhibit a
larger red-shied, broader shape. Obviously, a large difference
in the dipole moment between the excited state and the
ground state, the CT state with a large polarity is usually
stabilized in polar solvents, and the change is consistent with
a variety of the excited state from the LE-state to an excited
state with the strong CT character.17 The dipole moments were
calculated according to the Lippert–Mataga theory to be
21.7 and 5.4 D, respectively.18 The present results are very
similar with the pioneer work by Ma and his colleagues.19
Considering that the two compounds in this work have similar
Fig. 4 The absorption and fluorescence spectra of DPTPA in THF
molecular structures, we can conrm these emitters also show
the excited states of hybridized local and charge-transfer
(HLCT). Furthermore, the phosphorescent spectra and PL
decay characteristics were also investigated and the lifetimes
of DPTPA was observed to be 1.49 ns, respectively, which could
be assigned to the ordinary uorescence.
solution and in a thin film.
of ITO/HAT-CN (5 nm)/NPB (40 nm)/TCTA (5 nm)/DPTPA
(20 m)/TPBI (40 nm)/LiF (1 nm)/Al was fabricated. Here, ITO
(indium tin oxide) and LiF/Al are the anode and the cathode,
respectively; 4,40-bis[N-(1-naphthyl)-N-phenylamino]biphenyl
(NPB) and 1,3,5-tris(N-phenylbenz-imidazol-2-yl)benzene (TPBI)
2.5 Electroluminescent properties
To investigate DPTPA's electroluminescence (EL) characteristics are employed respectively as the hole-transporting layer (HTL)
as a saturated deep-blue emitter, a device with a conguration and the electron-transporting layer (ETL); whereas, 4,40,400-
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uorescent OLEDs.20 The corresponding theoretical value of
the radiative exciton ratio was calculated by the following
equation: hext ¼ ghrhPLhout, in which hr is the radiative exciton
ratio, hext is the external quantum efficiency, hout is the light
out-coupling efficiency (ca. 20%), hPL is the intrinsic photo-
luminescence efficiency (ca. 23.8%), and g is the recombina-
tion efficiency of injected holes and electrons, which is ideally
100% only if holes and electrons are fully balanced and
completely recombined to form excitons. Thus, the hr value of
the DPTPA device was calculated to be 80%, which breaks
through the limit of the radiative exciton ratio of 25% for
conventional FOLEDs. Since no delayed uorescence was
observed from transient PL, and the luminance of EL dis-
played a linear increase with increasing current density, the
high radiative exciton ratio does not seem to be in accordance
with some mainstream views, such as thermally activated
delayed uorescence (TADF) or triplet–triplet annihilation
(TTA).21,22
The hybrid WOLED with the architecture of single-EML was
fabricated with the same construction as the previous DPTPA-
based device with the only difference that the DPTPA layer
contains different concentration of Ir(2-phq)3 from 0.1 to
0.9 wt%. As shown in Fig. 7a, all of devices with different
concentration of Ir(2-phq)3 are showing extremely low turn-on
voltage of 2.6 V, and low driving voltages of 4.2 eV (0.1 wt%),
3.8 eV (0.3 wt%), and 3.5 eV (0.65 and 0.9 wt%) at a practical
brightness of 1000 cd mꢀ2. The EL efficiencies are illustrated
in Fig. 7b and c and listed in Table 2. When the concentration
of Ir(2-phq)3 is ꢂ0.9%, the hybrid WOLED exhibits maximum
Fig. 5 (a) The fluorescence spectra of DPTPA in various polarity
solvents; (b) linear correlation of the orientation polarization (Df) of
solvent media with the Stokes shift (va ꢀ vf; a: absorbed light; f: fluo-
rescence) for DPTPA. In low-polarity and high-polarity solvents, the
S1 state is the HLCT state and the CT state, respectively.
total EQE, CE and PE of 22.3%, 53.7 cd Aꢀ1 and 60.2 lm Wꢀ1
,
respectively. As decrease of the concentration of Ir(2-phq)3, the
efficiencies reduce gradually. When the concentration of
Ir(2-phq)3 is down to 0.1%, the efficiencies are declined to be
13.2%, 25.8 cd Aꢀ1 and 29.1 lm Wꢀ1, respectively, indicating
that about 50% IQE could be harvested in this concentration.
In Fig. 7d, a warm white light emission can be realized when
the concentration of Ir(2-phq)3 is up to 0.9 wt%. The corre-
sponding CIE coordinates show a moderate blue shi from
(0.51, 0.41) to (0.43, 0.34) when the brightness increases from
10 to 10 000 cd mꢀ2. While the doped concentration was
decreased to 0.6 wt%, the CIE coordinates changed from (0.49,
0.40) to (0.39, 0.31), the present white emission is near to the
standard white light at high brightness (ESI, S1†). When the
concentration was down to 0.35 wt%, the CIE coordinates
changed from (0.45, 0.36) to (0.29, 0.22), Thus, a cold white
light can be obtained at high luminance (ESI, S2†). As the
concentration is changed to be 0.1%, which is following the
conventional principle of the doping concentration regula-
tion, the present spectra cannot be found enough the emission
from the phosphorescence even at the high luminance
(ESI, S2†).
tris(N-carbazolyl)triphenylamine (TCTA) serves as the electron-
blocking layer (EBL).
The electroluminescent (EL) spectra, current density–
voltage–luminance (J–V–L) characteristics, and CE–current
density–PE plots of the device based on DPTPA are shown in
Fig. 6, and the key performance parameters of the device are
summarized in Table 2. The device shows a saturated deep-
blue emission solely from DPTPA (peaked at 438 nm) and
Commission Internationale de L'Eclairage (CIE) coordinates
of (0.15, 0.09). The EL spectra show little changes as the
brightness increases from 10 to 10 000 cd mꢀ2. Furthermore, a
low turn-on voltages (voltage required to give 1 cd mꢀ2) of 2.8 V
was observed. The low turn-on voltages are mainly attributed
to charge injection and balanced charge recombination in the
emitter layer. For the EL efficiencies, the device exhibits
impressive maximum EQE, CE and PE of 3.81%, 3.45 cd Aꢀ1
and 3.52 lm Wꢀ1, respectively, and these efficiencies remain to
be 3.78%, 3.43 cd Aꢀ1 and 3.12 lm Wꢀ1 at a brightness of
100 cd mꢀ2, even at the brightness of 1000 cd mꢀ2, these
efficiencies still remain to be 3.40%, 3.07 cd Aꢀ1 and
1.88 lm Wꢀ1. Considering the low uorescent quantum yield
(23.8%) and high device external quantum efficiency (3.81%),
the performances are among the best for saturated deep-blue
2.6 Discussion about exciton transfer mechanism in the
high concentration of phosphorescent dopant
To clearly demonstrate the exciton transfer mechanism in the
high concentration of phosphorescent dopant WOLEDs, we
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Fig. 6 The performance of device based on pure DPTPA, (a) the curves of voltage–brightness–current density; (b) the curves of brightness–
luminescent efficiency–power efficiency; (c) the curve of brightness–EQE; (d) the electroluminescent spectra.
Table 2 Summary of device performance in correlation with the concentration of Ir(2-phq)3
At 100 cd mꢀ2
At 1000 cd mꢀ2
a
Doping
concentration (V)
Von CEmax
PEmax
EQEmax
(%)
V
CE
PE
CIE
(x, y)
EQE
(%)
V
CE
PE
CIE
(x, y)
EQE
(%)
(cd Aꢀ1
)
(lm Wꢀ1
)
(V) (cd Aꢀ1
)
(lm Wꢀ1
)
(V) (cd Aꢀ1
)
(lm Wꢀ1
)
0
2.8
3.45
3.52
29.1
49.3
54.4
60.2
3.81
13.2
19.2
21.3
22.3
3.5 3.43
3.1 17.3
3.0 35.5
2.9 45.9
2.9 51.8
3.12
17.3
37.1
50.2
55.1
(0.15, 0.09) 3.78 5.1 3.07
(0.28, 0.18) 10.9 4.2 10.7
(0.41, 0.33) 16.5 3.8 21.8
(0.48, 0.39) 20.0 3.5 32.6
(0.50, 0.49) 21.9 3.5 38.3
1.88
7.99
17.8
28.9
33.2
(0.15, 0.09) 3.40
(0.23, 0.16) 8.1
(0.35, 0.28) 11.6
(0.45, 0.36) 14.9
(0.48, 0.38) 16.8
0.1
0.35
0.6
0.9
2.6 25.8
2.6 44.1
2.6 48.5
2.6 53.7
a
At the luminance of 1 cd mꢀ2
.
combined the results of theoretical calculations, mono- from singlets of the uorescent to phosphorescent emitters
chromatic light devices and WOLEDs. Considering the can be calculated the ratio of their uorescent lifetimes
improved dopant concentration in the single-EML-SD F/P (LF/LP ¼ (sF/sP)0.5
)
in the mixed lm, and the value from
23
hybrid WOLEDs, it is suspected that the current relatively singlets of the uorescent to phosphorescent emitters is
high doping concentration should be resulted from the blue 19.2%. The energy transfer from the triplet excited states of
emitters with HLCT characters and thus “hot exciton” energy DPTPA to those of Ir(2-phq)3 is found to be only 20%.
transfer from T5 to S1. Due to the part transfer from the triplet Accordingly, a more than 55% of triplet excitons cannot
excited states to the singlet excited states of the blue emitter, transfer from DPTPA to Ir(2-phq)3. It means that the insuffi-
the phosphorescent dopant cannot obtain sufficient energy to cient energy can be transferred to the orange-red dopant, and
emit orange-red light of the traditional devices with the same the doping concentration should be improved to a high level.
doping concentration as shown in Fig. 8. At the 0.9 wt% Furthermore, the present results can not only make the future
doping concentration, the uorescence emission utilize about mass production much easier to control accurately, but also a
ꢂ60.8% excitons, and about 55% excitons are from the triplet great breakthrough in the device mechanism, and then the
state T5 by hot-excitons process, while the excitons transferred design strategy.
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Fig. 7 (a) The current density–voltage–luminance curves; (b) the brightness–LE–power efficiency curves; (c) the luminance–EQE curve; and (d)
the electroluminescent spectra of the concentration of 0.9 wt%.
Fig. 8 (a) The illustration of exciton forming process of emitter from hole and electron recombination in electric excitation condition of
WOLEDs; (b) the transient singlet PL of DPTPA films doped with 0.9 wt% of Ir(2-phq)3, the corresponding transient triplet PL is shown in inserted
figure.
EML-simplied F/P-WOLEDs. The studies provide a way to obtain
high performance F/P hybrid WOLEDs with a simple architecture
3. Conclusions
and high doping concentration for easily manipulation.
In summary, we have presented a novel imidazole based emitter
material for high-efficient uorescent OLED and single-EML F/P
hybrid WOLED devices. It is notable that the WOLED still
exhibited white emission with a maximum total EQE, CE and PE
of 22.3%, 53.7 cd Aꢀ1 and 60.2 lm Wꢀ1, respectively, when the
concentration of Ir(2-phq)3 is up to 0.9 wt%, which is not following
the conventional principle of the doping concentration regulation.
The exciton transfer mechanism in the high concentration of
phosphorescent dopant is also discussed. Meanwhile, the devices
also showed low roll of with the efficiencies of 16.9%, 38.3 cd Aꢀ1
and 33.2 lm Wꢀ1 at 1000 cd mꢀ2, it is among the best results for
4. Experimental section
4.1 Synthesis
All reagents, unless otherwise specied, were obtained from
Alfa Aesar or Sigma-Aldrich and used without further purica-
tion. Some of the solvents used were further puried before use
(THF from sodium/benzophenone, methanol from CaH2). 1H
and 13C NMR spectra were measured using a Bruker AV-400.
Chemical shis were expressed in parts per million (ppm),
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and splitting patterns are designated as s (singlet), d (doublet), voltage characteristics were recorded simultaneously with the
m (multiplet) and br (broad). Coupling constants J are reported measurement of Commission Internationale De L'Eclairage
in Hertz (Hz). Elemental analyses were performed on a Vario EL (CIE) coordination of these devices by combining the spec-
elemental analysis instrument (Elementar Co.).
trometer CS200 with a Keithley model 2420 programmable
4.1.1 Synthesis of compound 2. (4-(Diphenylamino)phenyl) voltage–current source. All measurements were carried out at
boronic acid (2.89 g, 10.0 mmol), 1-bromo-4-iodobenzene room temperature under ambient conditions.
(1.415 g, 5 mmol) and Pd(PPh3)4 (50 mg, 0.1 mmol) were sus-
pended in toluene (30 mL) and K2CO3 (10 mL of 2 M aqueous
solution) and the reaction was stirred at 90 ꢁC for 24 h. The
Acknowledgements
orange solution was extracted with CH2Cl2 (40 mL ꢃ 3 times), This work was nancially supported from the National Natural
washed with water (20 mL ꢃ 3 times), dried over MgSO4 and Science Foundation of China (21102156, 51273209, 514111004
evaporated to dryness. Aer drying under vacuum, it was puri- and 91233116). ZYG and XO greatly appreciate the nancial
ed by silica gel column chromatography using CH2Cl2–petro- support from the External Cooperation Program of the Chinese
leum ether (60–90 ꢁC) (1 : 15) as an eluent to afford a white Academy of Sciences (no. GJHZ1219), Ningbo Natural Science
solid, yield: 1.68 g, 84%. MS (EI): m/z 399.1, 401.3 (M+), which Foundation (2014A610126) and Ningbo International Coopera-
was agreement with the literature.1
tion Foundation (2013D10013). SJS greatly appreciates the
4.1.2 Synthesis of compound 3. Compound 2 (2 g, 5 mmol), nancial support from the Ministry of Education (NCET-11-
1-bromo-4-iodobenzene (0.75 g, 5 mmol) and Pd(PPh3)4 (50 mg, 0159) and the Department of Education of Guangdong Prov-
0.1 mmol) were suspended in toluene (30 mL) and K2CO3 ince (2012KJCX0008). ZYG greatly appreciates the nancial
(10 mL of 2 M aqueous solution) and the reaction was stirred at support from by State Key Laboratory of Luminescence and
ꢁ
90 C for 24 h. The orange solution was extracted with CH2Cl2 Applications.
(40 mL ꢃ 3 times), washed with water (20 mL ꢃ 3 times), dried
over MgSO4 and evaporated to dryness. Aer drying under
References
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4.1.3 Synthesis of compound DPTPA. A mixture of aniline
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˚
˚
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(100 nm) through thermal evaporation at a rate of 4.0 A sꢀ1. The
˚
electroluminescence (EL) spectra were measured by a PR705
spectra scan spectrometer. The luminance–current density and
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