A simple systematic design of phenylcarbazole derivatives for host materials to high-efficiency phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1039/c3tc30410h

A series of novel blue phosphorescent host materials, namely, CTP-1, CTP-2 and CTP-3 have been designed and synthesized through the Suzuki-Miyaura cross-coupling reaction between N-phenylcarbazole and biphenyl. Their thermal, photophysical and electrochemical properties were systematically investigated. These novel hosts show excellent thermal stability and high glass transition temperatures (T_g) ranging from 113 to 127°C. The triplet energies of these three materials are significantly higher than that of (4,4′-bis(N-carbazolyl)-2,2′-biphenyl) (CBP, 2.56 eV) and that of the most popular blue phosphorescent material iridium(iii) bis[(4,6- difluorophenyl)pyridinato-N,C²′] picolinate (FIrpic, 2.65 eV). These three novel materials have superior thermal and electronic properties for blue and white phosphorescent OLEDs. Blue emitting devices with an Ir-complex FIrpic as the phosphorescent dopant have been fabricated and show high efficiency with low roll-off. In particular, 40.4 cd A^-1 at 100 cd m^-2 and 38.2 cd A^-1 at 1000 cd m^-2 were achieved when CTP-1 was used as the host material. On the basis of this work, an all-phosphor white device with a current efficiency of 64.5 cd A^-1 at 100 cd m^-2 and 61.9 cd A^-1 at 1000 cd m^-2 has also been fabricated.

Full text of DOI:10.1039/c3tc30410h

Journal of
Materials Chemistry C
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PAPER
A simple systematic design of phenylcarbazole
derivatives for host materials to high-efficiency
phosphorescent organic light-emitting diodes†
Cite this: J. Mater. Chem. C, 2013, 1,
3967
*
Lin-Song Cui, Shou-Cheng Dong, Yuan Liu, Qian Li, Zuo-Quan Jiang
and Liang-Sheng Liao
*
A series of novel blue phosphorescent host materials, namely, CTP-1, CTP-2 and CTP-3 have been designed
and synthesized through the Suzuki–Miyaura cross-coupling reaction between N-phenylcarbazole and
biphenyl. Their thermal, photophysical and electrochemical properties were systematically investigated.
These novel hosts show excellent thermal stability and high glass transition temperatures (Tg) ranging
from 113 to 127 ꢀC. The triplet energies of these three materials are significantly higher than that of
(4,40-bis(N-carbazolyl)-2,20-biphenyl) (CBP, 2.56 eV) and that of the most popular blue phosphorescent
2
0
material iridium(III) bis[(4,6-difluorophenyl)pyridinato-N,C ] picolinate (FIrpic, 2.65 eV). These three novel
materials have superior thermal and electronic properties for blue and white phosphorescent OLEDs.
Blue emitting devices with an Ir-complex FIrpic as the phosphorescent dopant have been fabricated and
show high efficiency with low roll-off. In particular, 40.4 cd Aꢁ1 at 100 cd mꢁ2 and 38.2 cd Aꢁ1 at 1000
cd mꢁ2 were achieved when CTP-1 was used as the host material. On the basis of this work, an all-
phosphor white device with a current efficiency of 64.5 cd Aꢁ1 at 100 cd mꢁ2 and 61.9 cd Aꢁ1 at 1000
cd mꢁ2 has also been fabricated.
Received 5th March 2013
Accepted 17th April 2013
DOI: 10.1039/c3tc30410h
www.rsc.org/MaterialsC
the CBP (4,40-bis(N-carbazolyl)-2,20-biphenyl) molecule which
consists of two carbazole moieties and a biphenyl group and its
Introduction
OLEDs, an abbreviation of Organic Light-Emitting Diodes, have
broken through in the past decades, and they have been put into
a wide range of applications in MP3 players, smartphones and
cameras since 2003 because of their slimness and power-saving
features. At the same time, OLED-based lighting is considered
as another attractive application in future markets, although
these products consist almost entirely of luxury lighting pres-
ently.1–5 All of the progress can be traced back to the invention of
phosphorescent OLEDs (PHOLEDs) in 1998,6,7 which shed a new
light on OLED technology and now it is widely accepted that
only the use of PHOLEDs will enable the devices to achieve
theoretical 100% internal quantum efficiencies.
To approach the target of high efficiency, phosphorescent
emitters, which are usually composed of heavy metal
complexes, should be doped into host materials by which the
exciton's triplet–triplet annihilation (TTA) and triplet polaron
quenching (TPQ) could thus be suppressed at a low concen-
tration.8 As we know, the host material reported in 1998 was
concise synthetic route makes it widely used in today's research
(Scheme 1).7 Although numerous host materials have been
developed to date, the carbazole derivatives are still the most
favorable materials for chemists, because the derivatives can be
readily tailored to full the requirements for designing a good
host, e.g. high triplet energy level, good thermal stability,
appropriate transport mobility and matched HOMO and LUMO
levels.9–19 Despite the success of the CBP molecule in green or
red PHOLEDs, for instance, the CBP itself possesses a low triplet
energy of 2.56 eV, which severely limits its application in the
most important blue PHOLEDs. In addition, its low glass-tran-
sition temperature of 78 ꢀC may also undermine the device's
operational stability. Hence, a series of modications have been
conducted to handle these disadvantages of CBP. In 2003,
another classic host material mCP was reported with a high
triplet energy (2.90 eV) by removing one phenyl group and
modifying two carbazole moieties at the meta-linkage instead of
the para-linkage, the power efficiency of the device hosted by
mCP increased to 8.9 lm Wꢁ1
.
In a similar way, Gong et al.
20
designed and synthesized CBP isomers as blue host materials
with high ET and achieved good power efficiency.21 Another
approach to promote the triplet energy of CBP-based materials
is by incorporating the steric effect to the molecular design. For
example, CDBP and CPD use methyl groups as steric effect
groups, and device power efficiencies arrived at 10.5 and 8.3 lm
Jiangsu Key Laboratory for Carbon-Based Functional Materials & Devices, Institute of
Functional Nano & So Materials (FUNSOM), Soochow University, Suzhou, Jiangsu
215123, China. E-mail: zqjiang@suda.edu.cn; lsliao@suda.edu.cn; Fax: +86-
65882846; Tel: +86-512-65880945
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c3tc30410h
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Scheme 1 CBP analogues host materials.
Wꢁ1 respectively.22,23 Interrupting the conjugation is also an attributed to its best hole mobility and most sufficient energy
effective strategy to raise the triplet energy. He et al. developed a transfer, as compared with the other two analogues. Meanwhile,
series of non-conjugated carbazole hosts based on CBP, but the the all-phosphor white devices hosted by these three materials
power efficiencies were moderate.13 On the other hand, to were also fabricated and current efficiencies of 64.5 cd Aꢁ1 at
overcome the thermal disadvantages of CBP, the molecular size 100 cd mꢁ2, 61.9 cd Aꢁ1 at 1000 cd mꢁ2 were achieved by using
should be enlarged. Noticeably, this molecular design must be CTP-1 as the host material.
carefully modulated, for the extent of conjugation is highly
correlated with maintaining the triplet energy. A feasible means
Results and discussion
Synthesis and characterization
to solve this tradeoff is the introduction of a bulky group in the
sp3-hybridized linkage. For example, Jiang et al. used this
The synthetic routes and chemical structures of the novel hosts
are shown in Scheme 2. Three phenylcarbazole derivatives were
facilely prepared through a classic Suzuki–Miyaura cross-
coupling reaction from 3,30-dibromodiphenyl with the corre-
sponding N-phenylcarbazole-3-boronic acid in high yields.
Detailed synthetic procedures are presented in the Experi-
mental section. Aerwards, all the target materials were further
puried by repeated temperature gradient vacuum sublimation.
The chemical structures of the target compounds were fully
characterized by 1H NMR and 13C NMR spectroscopy, mass
spectrometry and elemental analysis.
linkage to bridge one N-phenylcarbazole moiety of CBP and Li
et al. used this linkage to connect two N-phenylcarbazole
moieties of CBP, and subsequently, high glass-transition
temperatures (Tg) were obtained for these materials.24,25
However, it is reported that the increased distance resulted
from these bulky groups may separate the conducting units
from each other and lead to a lower carrier mobility.26 Thus, in
general, a rational molecular design is needed to face the
dilemma among the sufficient triplet energy, the high
morphological stability and the suitable carrier mobility of a
host material.
In this work, we report the design and syntheses of three new
carbazole-based materials CTP-1, CTP-2 and CTP-3. All of them
are comprised of biphenyl and N-phenylcarbazole units and
could be quite simply prepared in one-step. By altering the
linking positions, these materials exhibited obvious differences
in device performance. Their electrical, thermal and photo-
physical properties were investigated. With regards to the
aspect of molecular size, these new materials just add one more
biphenyl group, as compared with the CBP molecule. Due to the
meta-linkage, the triplet energies do not reduce but increase.
The enlarged molecular size could also lead to the increased Tg
of these materials. Blue PHOLEDs containing CTP-1, CTP-2, and
CTP-3 as hosts and FIrpic as a dopant exhibited excellent effi-
ciency performances with low roll-off. In particular, the device
with CTP-1 as the host material showed the highest current
efficiency of 40.4 cd Aꢁ1 at 100 cd mꢁ2 and 38.2 cd Aꢁ1 at 1000
cd mꢁ2 among the tested devices, which could possibly be
Thermal properties
To conrm their thermal stabilities, these three compounds
were investigated with thermogravimetric analysis (TGA)
and differential scanning calorimetry (DSC)ꢁ1in a nitrogen
atmosphere at a scanning rate of 10 ꢀC min (Fig. 1). The
three materials exhibited high thermal decomposition
temperatures (Td, corresponding to 5% weight loss, see
Fig. S1†) 448 ꢀC for CTP-1, 429 ꢀC for CTP-2 and 452 ꢀC for
CTP-3. Their glass-transition temperatures were observed at
123 ꢀC for CTP-1, 127 ꢀC for CTP-2 and 113 ꢀC for CTP-3,
which are substantially higher than those of CBP-based
derivative host materials, such as CBP (62 ꢀC), m-CBP (60
ꢀC), o-CBP (82 ꢀC) and CDBP (94 ꢀC). Therefore these three
novel hosts show excellent thermal and morphological
stability. The high Tg may be attributed to the extended
molecular structure.
3968 | J. Mater. Chem. C, 2013, 1, 3967–3975
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Scheme 2 Synthetic routes and chemical structures of the host materials.
Electrochemical properties
The electrochemical properties of CTP-1, CTP-2 and CTP-3
probed by cyclic voltammetry (CV) are shown in Fig. 3. The
measurements were performed in CH2Cl2 with ferrocene as the
internal reference. All compounds exhibited irreversible oxida-
tion behaviors which could unambiguously be assigned to the
oxidation of the carbazole units.
The HOMO energies were deduced from the onset of the
oxidation potentials with regard to the energy level of ferro-
cene (4.8 eV below the vacuum). The HOMO levels of CTP-1,
CTP-2 and CTP-3 were estimated as 5.96, 5.99 and 6.00 eV,
respectively, which are similar to that of CBP (6.01 eV). The
LUMO energy levels were calculated from the HOMO values
Fig. 1 DSC traces recorded at a heating rate of 10 ꢀC minꢁ1
.
Photophysical properties
Fig. 2 depicts the electronic absorption and emission photo-
luminescence (PL) of CTP-1, CTP-2 and CTP-3 in CH2Cl2 solu-
tion at room temperature. The absorption peaks at around 300
nm of the three materials are associated with the carbazole
unit’s p–p* transitions. The absorption in the range of 315 nm
to 352 nm can be assigned to the p–p* transition between the
carbazole moiety to its adjacent phenyl unit on the molecule.
The intensity of the p–p* transitions is signicantly reduced in
comparison with that of CBP, which could be ascribed to the
meta-conjugation for CTP-1, CTP-2 and CTP-3 instead of the
para-linkage.27
Compounds CTP-1, CTP-2 and CTP-3 exhibit deep blue
emissions with vibronic peaks located in the range 350–382
nm in CH2Cl2 solution. The optical energy bandgaps (Eg) of
CTP-1, CTP-2 and CTP-3 are 3.49, 3.56, and 3.55 eV, respec-
tively, which were calculated from the threshold of the
absorption spectra in CH2Cl2 solution. The phosphorescence
spectra were measured in frozen 2-methyltetrahydrofuran
matrix at 77 K, and the ET values calculated from the spectra
followed the sequence CTP-2 (2.73 eV) < CTP-1 (2.75 eV) < CTP-
3 (2.81 eV). All the novel hosts exhibited higher ET levels than
those of CBP (2.64 eV) and the blue phosphor FIrpic (2.65 eV).
Thus they can probably serve as good host materials for blue
triplet emitters while CBP cannot due to its close ET to that of
the guest.
Fig. 2 (a) UV-Vis absorption and PL spectra of CBP, CTP-1, CTP-2 and CTP-3 in
dichloromethane solution at 10ꢁ5 M. (b) Phosphorescence spectra of these
compounds measured in a frozen 2-methyltetrahydrofuran matrix at 77 K. The
arrows indicate the inferred triplet level positions.
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Fig. 4 The current density–voltage (J–V) curves for the hole-only devices (ITO/
Fig. 3 Cyclic voltammograms of CBP, CTP-1, CTP-2 and CTP-3 in dichloro-
MoO3 (10 nm)/CBP, CTP-1, CTP-2 or CTP-3 (100 nm)/MoO3 (10 nm)/Al (100 nm)).
methane solution for oxidation.
and thus only the phenyl rings on the carbazole moiety occupy
the LUMO while the ring at the p-bridge does not.
and optical band gaps, and which were about 2.47, 2.43 and
2.45 eV, respectively. All the physical property data is
summarized in Table 1.
Phosphorescent OLEDs
In order to evaluate the functioning of the novel materials for
blue phosphorescent emitters, we fabricated PHOLEDs with
Hole transport properties
CBP possesses excellent hole transport mobility, which is much typical sandwiched structures by sequential vapor deposition of
higher than that of the widely used electron transport material the materials onto a glass substrate coated with ITO, the blue Ir-
tris(8-hydroxyquinoline)aluminium (Alq3).28–30 To evaluate the complex FIrpic doped into the three hosts as the emitting layer.
hole transport characters of these hosts, we fabricated hole-only We utilized strongly electron-decient 1,4,5,8,9,11-hexaaza-
devices with a device structure of ITO/MoO3 (10 nm)/host (100 triphenylene–hexacarbonitrile (HAT–CN) as the hole-injection
nm)/MoO3 (10 nm)/Al (100 nm). The MoO3 layer was utilized to layer (HIL), 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]cyclohexane
prevent electron-injection from the cathode. Fig. 4 shows the (TAPC) was used as the hole-transporting layer (HTL) and the
current density versus voltage characteristics of the hole-only electron-blocking layer. 1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene
devices. Evidently, the hole mobility of the compound CTP-1 is (TmPyPB) was utilized as the electron-transporting as well as
slightly higher than that of CBP, but much higher than those of the hole-blocking layer. Liq. served as the electron-injecting
CTP-2 and CTP-3. These results could be supported by the layer and Al served as the cathode. The blue PHOLED’s struc-
molecular simulation. In the view of the structure from tures were ITO/HAT-CN (10 nm)/TAPC (45 nm)/Host: FIrpic (8%,
computational calculation, the HOMO of CTP-1 is distributed 20 nm)/TmPyPB (40 nm)/Liq. (2 nm)/Al (120 nm) (Host ¼ CTP-1:
on the main chain composed of six phenyl rings, while the device C; Host ¼ CTP-2: device D; Host ¼ CTP-3: device E).
HOMOs of CTP-2 and CTP-3 are barely distributed on the Meanwhile, the control devices using CBP (device A) and mCP
peripheral carbazole rings. This is mainly attributed to the fact (device B) as hosts were also fabricated. Fig. 5 depicts the rela-
that the p-orbital on the nitrogen atom with two lone pairs of tive energy levels of the materials employed in the devices.
electrons could also participate in the conjugation and the
The current density–voltage–luminance (J–V–L) and effi-
phenyl rings at the biphenyl p-bridge locate at the para-position ciency versus current density curves for the devices are depicted
to the nitrogen (e.g. at the 3-postion of carbazole), and which in Fig. 6. All the OLED’s performances are summarized in Table
could give rise to longer effective conjugation on the HOMO (see 2. As expected, the operating voltage of the devices at 100 cd
Fig. S1†) and indicates better hole-mobility. In contrast, in the mꢁ2 based on CTP-1 (3.8 eV) is lower than that of the devices
excited state, there is no empty orbital on the nitrogen atom and based on CTP-2 (4.5 eV) and CTP-3 (4.6 eV), which is consistent
it cannot participate in the formation of the LUMO anymore, with the hole-only device results observed in Fig. 4. This may be
Table 1 Physical properties of CBP, CTP-1, CTP-2 and CTP-3
Compound
labs (nm)
lemb (nm)
lph (nm)
Egd (eV)
ET (eV)
HOMO/LUMOf (eV)
a
Tg (ꢀC)
a
Td (ꢀC)
b
c
e
CBP
62
123
127
113
283
448
429
452
293, 319
288, 336
293, 305
292,328
378
470
448
454
441
3.40
3.49
3.56
3.55
2.64
2.75
2.73
2.81
6.01/2.61
5.96/2.47
5.99/2.43
6.00/2.45
CTP-1
CTP-2
CTP-3
365,380
352, 368
349, 365
a
b
c
Tg: glass transition temperatures. Measured in dichloromethane solution at room temperature. Measured in 2-MeTHF glass matrix at 77 K.
d
e
Eg: the band gap energies were estimated from the optical absorption edges of the UV-Vis absorption spectra. ET: the triplet energy were
f
estimated from the onset peak of the phosphorescence spectra. HOMO and LUMO estimated from the onset of oxidation potentials and the
optical band gap from the absorption spectra.
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In addition, we were motivated to test the applicability of
CTP-1, CTP-2 and CTP-3 for all the phosphor white
OLEDs with the conguration of ITO/HAT-CN (10 nm)/TAPC
(45 nm)/Host: FIrpic (8%, 19 nm)/Host: PO-01 (5%, 1 nm)/
TmPyPB (40 nm)/Liq. (2 nm)/Al (120 nm) (Host ¼ CTP-1:
device F; Host ¼ CTP-2: device G; Host ¼ CTP-3: device H).
Fig. 7 shows the J–V–L characteristics and curves of current
efficiency, power efficiency and external quantum efficiency
versus current density. The devices F–H obtained the
maximum current efficiencies of 64.5, 56.0, 60.0 cd Aꢁ1 and
the maximum power efficiencies of 49.5, 37.1, 37.9 lm Wꢁ1
,
Fig. 5 Energy level diagrams for the devices.
respectively. Moreover, the emission color of these devices
exhibited stability and varied slightly from (0.34, 0.47) at 4 V
to (0.35, 0.48) at 8 V. The detailed device performances are
due to the higher hole mobility of CTP-1 resulted from its summarized in Table 2.
relatively higher conjugation. As illustrated in Fig. 6, when
For the WOLEDs, the current/power efficiencies of the
operated at the luminance of 100 cd mꢁ2, device C hosted by devices hosted by these materials are in the order device F >
CTP-1 exhibited the best performance with a current efficiency device J z device H. This data agrees well with the tendencies of
of 40.4 cd Aꢁ1 and a power efficiency of 33.8 lm Wꢁ1; while the blue phosphorescence performance, which demonstrates
device D hosted by CTP-2 and device E hosted by CTP-3 the superiority of CTP-1.
exhibited 36.9 cd Aꢁ1 (25.6 lm Wꢁ1) and 37.4 cd Aꢁ1 (25.5 lm
To further verify the exciton connement properties of the
Wꢁ1), respectively. Remarkably, all the devices showed a low hosts, transient photoluminescence decays of thin lms
efficiency roll-off. For example, when the luminance reached (formed on quartz substrates with a thickness of 40 nm) with
1000 cd mꢁ2, the current efficiency was still as high as 38.2 cd 10 wt% FIrpic dispersed in CBP, CTP-1, CTP-2 and CTP-3 were
Aꢁ1 for device C. In comparison, the control device A/B hosted measured. According to Fig. 8, the emission of FIrpic-doped
by CBP/mCP as the emitting layer showed a relatively poorer CTP-1, CTP-2 and CTP-3 lms display nearly mono-exponen-
performance.
tial decay curves, indicating that the triplet energy transfer
The EL spectra and the Commission International de from FIrpic to hosts is completely suppressed and the energy
I'Eclairage (CIE) coordinates vary very little for these devices. All is well conned in emission layer. However the CBP: FIrpic
the devices show identical spectra with a peak at 476 nm and a lm exhibited an unclear exponential curve with the lowest
shoulder at 500 nm, which is arising from the typical emission decay, indicating the insufficient energy transfer from CBP to
of the phosphor FIrpic.
FIrpic in the emission layer. This could be attributed to the
Fig. 6 (a) Current density–voltage–luminance characteristics, (b) current efficiency and external quantum efficiency versus current density curves, (c) power efficiency
and external quantum efficiency versus current density curves, (d) the EL spectrum of devices A–E at 10 mA cmꢁ2
.
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Table 2 Summary of OLED performances
At 100 cd mꢁ2
hc/hp/hext/Va
(cd/A/lm/W/%/V)
b
hc.max/hp.max/hext.max
(cd/A/lm/W/%/V)
Device
Host/guest
At 1000 cd mꢁ2
CIE (x,y)c
A
B
C
D
E
F
CBP/B
mCP/B
CTP-1/B
CTP-2/B
26.4/18.3/10.4/4.5
34.3/22.8/13.7/4.7
40.4/33.8/15.9/3.8
36.9/25.6/14.7/4.5
37.4/25.5/14.4/4.6
64.5/49.5/19.1/4.0
56.0/37.1/17.7/4.7
60.0/37.9/18.9/4.9
24.4/14.4/9.78/5.3
32.1/17.1/13.0/5.9
38.2/24.2/15.7/4.9
35.6/18.6/14.4/6.1
34.1/17.9/13.7/6.2
61.9/40.5/19.5/4.7
47.5/24.7/15.0/6.0
50.2/25.0/15.9/6.2
26.5/18.8/10.4/4.0
34.5/23.1/13.7/4.2
40.5/33.9/15.9/3.7
37.0/25.7/14.9/4.5
37.5/25.6/15.4/4.6
64.5/49.5/19.1/4.0
56.0/37.1/17.7/4.7
60.0/37.9/18.9/4.9
(0.17, 0.38)
(0.16, 0.38)
(0.16, 0.38)
(0.16, 0.39)
(0.17, 0.38)
(0.36, 0.48)
(0.35, 0.48)
(0.35, 0.48)
CTP-3/B
CTP-1/B + Y
CTP-2/B + Y
CTP-3/B + Y
G
H
a
b
Current efficiency (hc), power efficiency (hp), external quantum efficiency (hext), voltage (V) at 100 cd mꢁ2 and 1000 cd mꢁ2
.
Maximum current
efficiency (hc.max), maximum power efficiency (hp.max), maximum external quantum efficiency (hext.max). Measured at 10 mA cmꢁ2
.
c
close triplet energy level of CBP (2.64 eV) and FIrpic (2.65 eV),
which results in the diffusion of excitons out of the emissive
layer and thus lowers the device performance. On the
contrary, the CTP-1: FIrpic lm exhibited the quickest
energy transfer, and this tendency is consistent with the
device performance.
As outlined in the introduction, there is a dilemma in design
among the high triplet energy, stable morphology, appropriate
HOMO/LUMO levels for carrier injection and the high carrier
transport mobility. Satisfactorily, CTP-1 could fulll all the
aspects proposed in the dilemma and in fact, according to the
device performances, it could be utilized as a good host mate-
Fig.
8
Transient photoluminescence decay (excited at 330 nm) curves at
rial, as compared to several representative CBP analogue host
materials (Table 3).
room temperature at 475 nm for the FIrpic co-deposited with CBP, CTP-1, CTP-2
and CTP-3.
Fig. 7 (a) Current density–voltage–luminance characteristics, (b) current efficiency and external quantum efficiency versus current density curves, (c) power efficiency
and external quantum efficiency versus current density curves, (d) the EL spectrum of devices A–E at 10 mA cmꢁ2
.
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Table 3 Physical properties and device performances of the CBP analogue host
by the PURE SOLV (Innovative Technology) purication system.
Chromatographic separations were carried out by using silica
gel (200–300 nm). All other reagents were used as received from
materials
a
Host
Tg (ꢀC)
ET (eV)
Dopant
hext (%)
Ref.
1
commercial sources unless otherwise stated. H NMR and 13C
NMR spectra were recorded on a Varian Unity Inova 400 spec-
trometer at room temperature. Mass spectra were recorded on a
Thermo ISQ mass spectrometer using a direct exposure probe.
UV-Vis absorption spectra were recorded on a Perkin Elmer
Lambda 750 spectrophotometer. PL spectra and phosphores-
cent spectra were recorded on a Hitachi F-4600 uorescence
spectrophotometer Transient PL decays were measured by a
single photon counting spectrometer from HORIBA JOBIN
YVON with a Nano LED pulse lamp as the excitation source.
Differential scanning calorimetry (DSC) was performed on a TA
DSC 2010 unit at a heating rate of 10 ꢀC minꢁ1 under nitrogen.
The glass transition temperatures (Tg) were determined from
the second heating scan. Thermogravimetric analysis (TGA) was
performed on a TA SDT 2960 instrument at a heating rate of
10 ꢀC minꢁ1 under nitrogen. The temperature at 5% weight loss
was used as the decomposition temperature (Td). Cyclic vol-
tammetry (CV) was carried out on a CHI600 voltammetric
analyzer at room temperature with a conventional three-elec-
trode conguration consisting of a platinum disk working
electrode, a platinum wire auxiliary electrode, and an Ag wire
pseudo-reference electrode with ferrocenium/ferrocene (Fc+/Fc)
as the internal standard. Nitrogen-purged dichloromethane was
used as the solvent for the oxidation scan and DMF for the
reduction scan with tetrabutylammonium hexauorophosphate
(TBAPF6) (0.1 M) as the supporting electrolyte. The cyclic vol-
CBP
mCP
CDBP
CPD
CPF
o-CBP
4CzPBP
BCBP
CTP-1
CTP-2
CTP-3
62
60
—
—
165
82
120
173
123
127
113
2.56
2.90
2.95
3.02
2.88
3.00
2.80
2.60
2.75
2.73
2.81
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
FIrpic
7.7
7.5
10.4
8.7
—
14.2
14.5
4.0
15.9
14.9
15.4
33
34
22
23
25
21
35
24
This work
This work
This work
a
Maximum external quantum efficiency.
Conclusion
In conclusion, three novel light-emitting host materials (CTP-1,
CTP-2 and CTP-3) comprised of biphenyl and N-phenylcarbazole
have been developed. All the compounds could be afforded by a
facile synthetic route. In view of the molecular design, to cope
with the major challenge of having both a large enough triplet
energy and acceptable morphological stability in a material, we
simply added a biphenyl group and tried to vary the link-position
of the N-phenylcarbazole moiety, which is unlike the other
modication methods, such as “torsion”, “steric effect”, “conju-
gation interruption” and “bulky group”. Our methods effectively
enlarged the molecular size and retained their triplet energies.
Especially, the CTP-1 molecule possesses appropriate HOMO/
LUMO levels, without sacricing the carrier mobility by using
meta-linkage to the carbazole rings. The computational calcula-
tion also indicates this way of linking could lead to effectively
longer conjugation. These properties enable CTP-1 to be a good
material in blue and white phosphorescent devices, and the blue
light-emitting device exhibits 40.4 cd Aꢁ1 (33.8 lm Wꢁ1) at 100 cd
mꢁ2 and 38.2 cd Aꢁ1 (24.2 lm Wꢁ1) at 1000 cd mꢁ2 with very low
efficiency roll-off. Furthermore, the all-phosphor white device
hosted by CTP-1 achieved a current efficiency of 64.5 cd Aꢁ1
(49.5 lm Wꢁ1) at 100 cd mꢁ2 and 61.9 cd Aꢁ1 (40.5 lm Wꢁ1) at 1000
cd mꢁ2. These performances are believed to be among the best
results as comparedwith otherattemptsintailoringthe CBPto be
a better blue host material, indicating the simple modication
could evidently overcome its intrinsic disadvantages and main-
tain its advantages simultaneously to achieve good performance.
tammograms were obtained at a scan rate of 100 mV sꢁ1
.
Computational methodology
The geometrical and electronic properties of CTP-1, CTP-2 and
CTP-3 were performed with the Gaussian 09 program package.
The calculation was optimized by means of the wb97xd (Becke
three parameters hybrid functional with Lee–Yang–Perdew
correlation functionals) with the 6-31G(d) atomic basis set.
Molecular orbitals were visualized using Gaussview.
Fabrication of the OLEDs
The OLEDs were fabricated through vacuum deposition of the
materials at ca. 2 ꢂ 10ꢁ6 Torr onto ITO-coated glass substrates
having a sheet resistance of ca. 30 U per square. The ITO surface
was cleaned ultrasonically – sequentially with acetone, ethanol,
and deionized water, then dried in an oven, and nally exposed
to UV-ozone forꢁa1bout 30 min. Organic layers were depositedꢁa1t
˚
˚
a rate of 2–3 A s , subsequently, Liq. was deposited at 0.2 A s
Experimental section
Materials and methods
and then capped with Al (ca. 4 A sꢁ1) through a shadow mask
˚
without breaking the vacuum. For all the OLEDs, the emitting
All the chemicals i.e., (9-phenyl-9H-carbazol-3-yl) boronic acid, areas were determined by the overlap of two electrodes as
[4-(9H-carbazol-9-yl) phenyl] boronic acid, [3-(9H-carbazol-9-yl)- 0.09 cm2. The EL spectra, CIE coordinates and J–V–L curves of
phenyl] boronic acid, 1-bromo-3-iodobenzene, 3-bromobenze- the devices were measured with a PHOTO RESEARCH Spec-
neboronic acid were purchased from Bepharm limited and Alfa traScan PR 655 photometer and a KEITHLEY 2400 SourceMeter
Aesar. All of the materials were used without further purica- constant current source at room temperature. The EQE values
tion. The important intermediate 3,30-dibromodiphenyl was were values that were calculated according to the previously
prepared according to a literature method.31 THF was puried reported methods.32
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J. Mater. Chem. C, 2013, 1, 3967–3975 | 3973

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Journal of Materials Chemistry C
Paper
and the Natural Science Foundation of Jiangsu Province (no.
BK2010003). This was also a project funded by the Priority
Academic Program Development of Jiangsu Higher Education
Institutions (PAPD) and by the Research Supporting Program of
Suzhou Industrial Park.
General procedure for the Suzuki–Miyaura cross-coupling
reaction
3,30-Dibromodiphenyl (1 equiv.), carbazole-containing boronic
acid (2.2 equiv.) and tetrakis(triphenylphosphine)palladium(0)
(0.05 equiv.) were dissolved in THF/2 M K2CO3 (3/1, v/v). The
ꢀ
reaction mixture was heated to 60 C for 8 h under an argon
Notes and references
atmosphere. Aer cooling to room temperature, the organic
layer was separated and evaporated to remove solvent. The
residue was puried by column chromatography with 1 : 3 (v/v)
dichloromethane/petroleum ether as the eluent and recrystal-
lized from dichloromethane/petroleum and vacuum sublima-
tion to give the nal compound.
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1
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