Design of star-shaped molecular architectures based on carbazole and phosphine oxide moieties: Towards amorphous bipolar hosts with high triplet energy for efficient blue electrophosphorescent devices

Article abstract of DOI:10.1039/c0jm00846j

With a carbazole moiety as the electron donor and a phosphine-oxide moiety as the electron acceptor, two novel star-shaped bipolar hosts, 4,4′,4″-tri(N-carbazolyl)triphenylphosphine oxide (TCTP) and 3,6-bis(diphenylphosphoryl)-9-(4′-(diphenylphosphoryl)phenyl)carbazole (TPCz), have been designed and synthesized. Their topology structure differences are that the phosphine-oxide moiety is located in the molecular centre and the periphery for TCTP and TPCz, respectively. The star-shaped architecture imparts them with high decomposition temperatures (T_d: 497°C for TCTP and 506°C for TPCz) and results in the formation of a stable amorphous glassy state (T_g: 163°C for TCTP and 143°C for TPCz), while the phosphine oxide linkage ensures the disrupted conjugation and the high triplet energy (>3.0 eV). In addition, both TCTP and TPCz possess a bipolar transporting capability. However, TCTP mostly transports holes and TPCz primarily conducts electrons. On the basis of appropriate device configurations, high performance blue electrophosphorescent devices with comparable efficiency (35.0-36.4 cd A^-1, 15.9-16.7%) have been realized using TCTP and TPCz as the host for the blue phosphor, respectively. Compared with the unipolar host, 4,4′,4″-tri(N-carbazolyl)triphenylamine (TCTA, 15.9 cd A ^-1, 7.8%), the efficiency is improved by more than two-fold. As far as the obtained state-of-the-art performance is concerned, we think that these novel materials should provide an avenue for the design of amorphous bipolar hosts with high triplet energy used for blue PhOLEDs on a star-shaped scaffold.

Full text of DOI:10.1039/c0jm00846j

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Design of star-shaped molecular architectures based on carbazole and
phosphine oxide moieties: towards amorphous bipolar hosts with high triplet
energy for efficient blue electrophosphorescent devices
a
a
a
Junqiao Ding,a Qi Wang,ab Lei Zhao, Dongge Ma, Lixiang Wang, Xiabin Jinga and Fosong Wanga
*
Received 27th March 2010, Accepted 29th June 2010
DOI: 10.1039/c0jm00846j
With a carbazole moiety as the electron donor and a phosphine-oxide moiety as the electron acceptor, two
novel star-shaped bipolar hosts, 4,40,400-tri(N-carbazolyl)triphenylphosphine oxide (TCTP) and 3,6-
bis(diphenylphosphoryl)-9-(40-(diphenylphosphoryl)phenyl)carbazole (TPCz), have been designed and
synthesized. Their topology structure differences are that the phosphine-oxide moiety is located in the
molecular centre andthe peripheryfor TCTP andTPCz, respectively. Thestar-shapedarchitectureimparts
them with high decomposition temperatures (Td: 497 ꢀC for TCTP and 506 ꢀC for TPCz) and results in the
formation of a stable amorphous glassy state (Tg: 163 ꢀC for TCTP and 143 ꢀC for TPCz), while the
phosphine oxide linkage ensures the disrupted conjugation and the high triplet energy (>3.0 eV). In
addition, both TCTP and TPCz possess a bipolar transporting capability. However, TCTP mostly
transports holes andTPCzprimarily conductselectrons. Onthe basisofappropriatedeviceconfigurations,
high performance blue electrophosphorescent devices with comparable efficiency (35.0–36.4 cd Aꢁ1, 15.9–
16.7%)havebeenrealizedusing TCTPandTPCzas thehostfor the bluephosphor, respectively. Compared
withthe unipolar host, 4,40,400-tri(N-carbazolyl)triphenylamine(TCTA, 15.9 cd Aꢁ1, 7.8%), the efficiency is
improved by more than two-fold. As far as the obtained state-of-the-art performance is concerned, we
think that these novel materials should provide an avenue for the design of amorphous bipolar hosts with
high triplet energy used for blue PhOLEDs on a star-shaped scaffold.
electron-rich and -deficient moieties in one molecule has been
intensely studied by several groups.16–18 Considering all these
1. Introduction
For a potential application in solid state lighting,1 phosphores-
features, therefore, we aim to develop the novel amorphous
cent organic light-emitting diodes (PhOLEDs) have attracted
bipolar hosts with high triplet energy used for the blue PhO-
much attention due to their possibility to achieve comparable or
LEDs.
even higher power efficiencies than fluorescent tubes.2 However,
To date, a star-shaped molecule, 4,40,400-tri(N-carbazolyl)-
as one of the components necessary for white light, the perfor-
triphenylamine (TCTA), has been widely used as the host of the
mance of the blue PhOLEDs is not as good as the corresponding
phosphors.2,5,19,20 Nevertheless, it only behaves as a unipolar
red and green ones,3–9 where the host material remains a chal-
hole-transporter. On the other hand, some reported molecules
lenge. To be an effective host of the blue phosphor suitable for
containing phosphine oxide have been demonstrated to
vacuum deposition technology, three key factors must be
predominantly transport electrons.21–23 From the molecular
considered together. Firstly, the host should possess an amor-
design aspect, it is reasonable to replace the nitrogen atom
phous film morphology to ensure the device reproducibility and
in TCTA with phosphine oxide to afford a bipolar host,
stability.10 Hence, a nonplanar star-shaped structure, which has
been previously demonstrated to be a successful strategy to
obtain organic glasses,11 could be employed in the design of the
amorphous host. Secondly, the triplet energy of the host is
required to be higher than 2.75 eV12 in order to prevent the back
energy transfer from the phosphor to the host.13 Finally,
a significant point to emphasize is the capability of the host to
transport both holes and electrons, which is of paramount
importance to maintain the charge balance in the emissive layer
(EML) so as to improve the device efficiency accompanied by
a small roll-off.14,15 A desirabe, bipolar host containing suitable
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun, 130022, P. R. China. E-mail: lixiang@ciac.jl.cn; Fax:
(+86)0431-8568-5653
bGraduate School of the Chinese Academy of Sciences, Beijing, 100039,
Fig. 1 Schematic representations of the hosts and their corresponding
P. R. China
molecular structures.
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4,40,400-tri(N-carbazolyl)triphenylphosphine oxide (TCTP). As
shown in Fig. 1, TCTP is associated with topology structure I, in
which the electron acceptor (A), the phosphine oxide moiety, is
used as the core and the electron donor (D), the carbazole
moiety, acts as the branch. In view of the star-shaped scaffold,
there exists another inverted architecture, topology structure II,
where A locates at the molecular periphery and D locates in the
molecular center. Consequently, 3,6-bis(diphenylphosphoryl)-9-
(40-(diphenylphosphoryl)phenyl)carbazole (TPCz), with a phos-
phine oxide moiety as the branch and a carbazole moiety as the
core, is simultaneously designed. It is worth noting that, for both
TCTP and TPCz, the disrupted conjugation via phosphine-oxide
linkage can be well-preserved to ensure their high triplet energies.
Although the bipolar hosts containing phosphine oxide have
been reported recently,23,24 few endeavors have been paid to the
development of the amorphous star-shaped bipolar hosts, which
are believed to show better thermal and morphology stability. In
this article, based on carbazole and phosphine-oxide moieties,
TCTP and TPCz were conveniently synthesized, and the effect of
the topology structure on their thermal, photophysical, electro-
chemical, bipolar transporting and device properties was inves-
tigated in detail.
41% sequentially through a lithium-halogen exchange reaction
with n-butyllithium, a coupling reaction with trichlorophosphine
and an oxidation reaction with hydrogen peroxide. Similarly,
TPCz was obtained from 2 and chlorodiphenylphosphine with
a yield of 47%. Both TCTP and TPCz were purified by column
1
chromatography and their structures were confirmed with H
NMR, 13C NMR, 31P NMR, elemental analysis and MALDI-
TOF mass spectroscopy. Before device fabrication, both TCTP
and TPCz were further purified by sublimation. For comparison,
TCTA was also synthesized as the model compound.
From the 31P NMR spectrum of TCTP, only one signal peak
(d ¼ 27.5 ppm) was observed. However, the 31P NMR spectrum
of TPCz revealed two distinct signals, d ¼ 28.4 and 30.1 ppm.
According to the signal intensity, the former peak could be
ascribed to the 40-positioned P atom of the N-phenyl ring, and
the latter was related to the 3 and 6-position P atoms of the
carbazole unit. The upfield shift shows the higher electron
density of the 40-positioned P atom compared with the 3 and 6-
positions.
2.2 Thermal and photophysical properties
The thermal properties of TCTP and TPCz with respect to
TCTA were estimated by differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA). Fig. 2 displays
their DSC curves recorded over the temperature range 50–
2. Results and discussion
2.1 Synthesis and chemical characterization
ꢀ
350 C. For all the sublimated samples during the first heating
Scheme 1 outlines the synthesis of the bipolar hosts, TCTP and
TPCz. The key intermediate 9-(40-bromophenyl)carbazole (1)
was synthesized by a modified Ullmann reaction.6 Further
bromination with N-bromosuccinimide (NBS) gave another
important intermediate 9-(40-bromophenyl)-3,6-dibromocarba-
zole (2). Then, TCTP was prepared from 1 with a total yield of
process, only an endothermic peak due to melting appeared. On
the second heating, the DSC of TCTA revealed a glass transition
(Tg ¼ 152 ꢀC), followed by an exotherm at 237 ꢀC caused by
crystallization and an endotherm at 268 ꢀC associated with
melting. As for TCTP and TPCz, however, an amorphous glassy
state was formed upon cooling from the melt. After heating
Scheme 1 Synthesis of the bipolar hosts. Reagents and conditions: (i) carbazole, CuI, K2CO3, 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone
(DMPU), 18-crown-6, 190 ꢀC; (ii) n-BuLi, THF, PCl3, ꢁ78 ꢀC; (iii) 30% H2O2, CH2Cl2; (iv) N-bromosuccinimide (NBS), dimethylformamide (DMF),
0 ꢀC; (v) n-BuLi, THF, chlorodiphenylphosphine, ꢁ78 ꢀC; (vi) 30% H2O2, CH2Cl2.
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emission maximum of TCTP is blue-shifted by about 23 nm.
Moreover, the triplet energy estimated from the highest energy
peak of the phosphorescence spectra at 77 K enhances by about
0.18 eV from TCTA to TCTP. As a result, TCTP gives a triplet
energy as high as 3.03 eV, which is sufficient to host the blue
phosphor, iridium(III)[bis(4,6-difluorophenyl)-pyridinato-N,C2]-
picolinate (FIrpic, 2.65 eV). This enhanced triplet energy, which
suggests potentially effective prevention of the back energy
transfer from the phosphor to the host,13 is a very desirable feature
for PhOLEDs. In contrast, the shifts of the absorption and
emission spectra are almost negligible when going from TCTP to
TPCz. This demonstrates that the photophysical properties are
hardly influenced by their topology structure, no matter if the
phosphine oxide moiety locates in the center or at the periphery of
the architecture.
Fig. 2 DSC curves of the hosts (heating rate: 20 ꢀC minꢁ1). Tg: glass-
transition temperature; Tc: crystallization temperature; Tm: melting-point
temperature.
2.3 Electrochemical properties, theoretical calculations and
bipolar transporting characteristics
The cyclic voltammograms (CV) were measured in dichloro-
methane to study the energy levels of the hosts. The highest
occupied and lowest unoccupied molecular orbital (HOMO/
LUMO) energy levels can be calculated from the electrochemical
data together with the absorption spectra, and the data are
summarized in Table 1. Interestingly, TCTP and TPCz display
different redox behavior. During the anodic and cathodic scans,
there are two quasi-reversible oxidation waves and no reduction
waves for TCTP in Fig. 4. As for TPCz, however, only irre-
versible reduction waves are detected. In comparison to TPCz,
TCTP has a much higher HOMO energy level of ꢁ5.25 eV,
indicative of its effective hole injection capability. In contrast,
a much lower LUMO energy level of ꢁ2.69 eV for TPCz is
obtained, which suggests that electrons are more easily injected
from the cathode.
again, a well-defined glass transition occurred at 163 and 143 ꢀC
for TCTP and TPCz, respectively, and no exothermic peak
relatꢀed to crystallization was observed at temperatures up to
350 C. These observations indicate that the enhanced thermal
stability of the amorphous glass states of TCTP and TPCz may
arise from the repulsive effect of the additional oxygen atom,
which causes the more crowded arrangement of the aryl groups
around the P atom, and prevents the easy intermolecular
packing and hence ready crystallization. In addition, TCTP
and ꢀTPCz exhibited a decomposꢀition temperature of 497 and
506 C, respectively, about 8–17 C higher than that of TCTA
ꢀ
(Td ¼ 489 C).
Upon going from TCTA to TCTP, this repulsive effect from the
oxygen atom can also lead to the decrease of the molecular
conjugation length after the replacement of the N atom in TCTA
with phosphine oxide. For example, an obvious blue shift for the
lowest-energy absorption bands in the range 300–360 nm is clearly
seen in Fig. 3, and thus the optical band gap (Eg) estimated from
the onset of the absorption spectra increases from 3.46 eV of
TCTA to 3.58 eV of TCTP (Table 1). Correspondingly, the
To further understand the effect of the topology structure on
the electronic properties, density functional theory (DFT)
calculations were carried out using B3LYP hybrid functional
theory with 6-31G* basis sets (see Experimental). The optimized
structures and the HOMO/LUMO distribution for TCTP and
TPCz are given in Fig. 5. As can be seen from Fig. 5, the HOMO
and LUMO of TCTP are separately distributed over the
periphery carbazole units and the central triphenyl phosphine
oxide, respectively. However, the situation is entirely opposite
for TPCz, for its HOMO mainly consists of the highest p-orbitals
of the carbazole moiety located in the center, while its LUMO
mostly corresponds to the lowest p-orbitals of the periphery
triphenyl phosphine oxide at the N-position of the carbazole
moiety. This result is consistent with the higher electron density
of the 40-positioned P atom observed in the 31P NMR spectrum of
TPCz.
Combining their electrochemical behavior with the HOMO
and LUMO distribution, we can postulate that TCTP is favor-
able for the hole injection and transporting, while TPCz is suit-
able for the electron injection and transporting. To verify our
hypothesis, the same device structure (Fig. 6) with a 10 nm thick
film of TCTA, TCTP, or TPCz inserted between N,N0-bis((1-
naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine (NPB) and
tris(8-hydroquinolinato)aluminium (Alq3) was adopted accord-
ing to the literature.25 Fig. 6 shows their electroluminescence
Fig. 3 The absorption spectra (—) in dichloromethane solutions and PL
spectra (/) in toluene solutions at 298 K, and the phosphorescence
spectra (---) at 77 K of the hosts.
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Table 1 The thermal, photophysical and electrochemical properties of the hosts
labs (log 3) c/nm
lem d/nm
T1 e/eV
Eg f/eV
HOMO/eV
LUMO/eV
a
b
Tg/Tc/Tm /ꢀC
Td /ꢀC
TCTA
TCTP
TPCz
152; 237; 266
163; —; —
143; —; —
489
497
506
238 (5.1), 293 (4.8), 326 (4.6)
233 (5.2), 292 (4.7), 326 (4.6), 338 (4.5)
229 (4.8), 258 (4.8), 276 (4.6)
386
363
363
2.85
3.03
3.07
3.46
3.58
3.58
ꢁ5.09
ꢁ5.25
ꢁ6.27
ꢁ1.63
ꢁ1.67
ꢁ2.69
a
b
c
Data obtained from the second DSC scans. The temperature at which 5% weight loss was detected. Measured in CH2Cl2 at 298 K with
d
e
a concentration of 10ꢁ5 M. Measured in toluene at 298 K with a concentration of 10ꢁ5 M and an excitation wavelength of 330 nm. Estimated
f
from the highest energy peak of the phosphorescence spectra at 77 K. Estimated from the onset of the absorption spectrum. HOMO ¼ ꢁe (4.8 V
+ Eox), LUMO ¼ ꢁe (4.8 V + Ered), and LUMO ¼ Eg + HOMO, where Eox and Ered were taken from the onset of the oxidation and reduction
potential, respectively.
Fig. 6 Electroluminescent spectra of devices with a configuration of
ITO/NPB(50nm)/TCTA, TCTP, or TPCz (10nm)/Alq3(50nm)/LiF(1nm)/
Al(100nm) measured at a driving voltage of 10 V. The dotted lines are the
fitted emissions corresponding to NPB and Alq3.
Fig. 4 The cyclic voltammograms of TCTP and TPCz. Scan rate ¼
50 mV sꢁ1; solvent ¼ dichloromethane; concentration ¼ 1 mM.
(EL) spectra at a driving voltage of 10 V. It can be clearly seen
that the EL spectrum of the TCTA-based device was completely
dominated by the emission of Alq3 with the Commission Inter-
nationale de L’Eclairage (CIE) coordinates of (0.33, 0.57).
However, for the TCTP-based device, the emissions from both
Alq3 and NPB with a ratio of 0.74 : 0.26 were observed, and the
corresponding CIE coordinates blue-shifted to (0.23, 0.36).
Furthermore, for TPCz-based device, the ration of the emission
coming from NPB further increased from 26% to 58% accom-
panied by the CIE coordinates shifting to (0.18, 0.26). These
observations indicate that the TCTA interlayer only transports
holes, whereas the TCTP and TPCz layers permit both electrons
and holes to pass. That is to say, both TCTP and TPCz exhibit
a bipolar transporting nature. Nevertheless, as discussed above,
TCTP mostly transports holes, and TPCz primarily transports
electrons.
2.4 Electrophosphorescent OLED characterization
2.4.1 TCTP-based blue electrophosphorescent devices. The
bipolar transporting characteristic as well as the excellent
morphology stability and high triplet energy suggests that TCTP
and TPCz can be used as the ideal hosts for the blue electro-
phosphorescent devices. Therefore, their electroluminescent
properties were investigated using FIrpic as the corresponding
blue dopant.
As indicated in Fig. 7, device I was fabricated using TCTP as
the host with a configuration of ITO/NPB (70 nm)/TCTA (5 nm)/
8 wt% FIrpic: TCTP (20 nm)/TAZ (40 nm)/LiF (1 nm)/Al
(100 nm). Here, NPB and TCTA were used as the hole
Fig. 5 Molecular simulation results of TCTP and TPCz, showing the
HOMO and LUMO distribution.
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Fig. 7 A schematic diagram of EL device configurations and the molecular structures of the relevant compounds used in these devices.
transporting materials, and 3-(4-biphenyl)-4-phenyl-5-(4-tert-
butylphenyl)-1,2,4-triazole (TAZ) was used as the electron
transporting material. For comparison, the control device II:
ITO/NPB (70 nm)/TCTA (5 nm)/8 wt% FIrpic: TCTA (20 nm)/
TAZ (40 nm)/LiF (1 nm)/Al (100 nm) was also prepared.
Fig. 8 depicts the current density-voltage-luminance charac-
teristics of devices I–II, and the EL spectrum of device I. The EL
spectrum of device I shows the CIE coordinates of (0.15, 0.34),
corresponding to the emission of FIrpic. Moreover, no additional
emission from TCTP is observed, which demonstrates that the
exciton energy can be efficiently transferred from TCTP to FIrpic.
As presented in Fig. 8, device I and II have a similar turn-on
voltage (voltage at 1 cdmꢁ2) of about3.2 V, but the current and the
luminance of device I are higher than those of device II at the same
driving voltage. For instance, the maximum luminance of device I
and II are 35 100 and 5500 cd mꢁ2, respectively. At the same time,
the peak luminous efficiency increases from 15.9 cd Aꢁ1 of TCTA
to 35.0 cd Aꢁ1 of TCTP, and a more than two-fold improvement is
achieved (Fig. 9a). Correspondingly, the external quantum
Fig. 9 Comparison of the current density dependence of the luminous
efficiency (a) and EQE (b) between device I and II. In (a), the arrows
indicate the maximum luminous efficiency, as well as the efficiency at 1
and 100 mA cmꢁ2, respectively.
Fig. 8 Comparison of the current density-voltage-luminescence char-
acteristics between device I and II. Inset: the EL spectrum at a voltage of
8 V of device I.
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efficiency (EQE) of TCTP (15.9%) is about double that of TCTA
(7.8%) (Fig. 9b). Furthermore, it is found that at high current
density, the efficiency of TCTP decays more slowly than that of
TCTA. For example, on going from 1 to 100 mA cmꢁ2, the effi-
ciency decreases by about 58 and 40% for TCTA and TCTP,
respectively. The obtained small roll-off can be mainly attributed
to the bipolar nature of TCTP. Owing to the conductivity of holes
and electrons simultaneously through the whole EML, the exci-
tons are believed to be formed in the bulk EML rather than
located at the narrow interface. Consequently, the density of the
triplet excitons can be controlled at a relatively low level to
minimize the triplet–triplet (T–T) annihilation, and thus prohibit
the steep efficiency roll-off.26,27
additional thin hole-transporting and electron-blocking layer
should be inserted between the NPB and EML. Because of its
high LUMO energy level (ꢁ1.5 eV),28 N,N0-dicarbazolyl-3,5-
benzene (mCP) was selected to block electrons, and device IV
(ITO/NPB (65 nm)/mCP (10 nm)/8 wt% FIrpic: TPCz (20 nm)/
TAZ (40 nm)/LiF (1 nm)/Al (100 nm)) was prepared (Fig. 7). As
expected, device IV gave an efficiency of 36.4 cd Aꢁ1 (16.7%),
which is comparable with that of device I. This suggests that,
although TCTP and TPCz have different bipolar transporting
ability, they both can be used as an effective host for the fabri-
cation of high-performance blue PhOLEDs with appropriate
device configurations. In addition, compared with device III, the
efficiency of device IV is enhanced by about 47 times, indicating
the above-mentioned excellent electron-transporting ability of
TPCz. Therefore, we used TPCz as the electron transporting
material instead of TAZ to fabricate device V: ITO/NPB (65 nm)/
mCP (10 nm)/8 wt% FIrpic: TPCz (20 nm)/TPCz (40 nm)/LiF
(1 nm)/Al (100 nm). As shown in Fig. 10, the efficiency of device
V still remains as high as 27.6 cd Aꢁ1 (12.7%), close to that of
device IV. This promising result has prompted us to further study
the potential application of TPCz as the electron transporting
material, and the detailed work will be reported elsewhere.
2.4.2 TPCz-based blue electrophosphorescent devices. To
investigate the host capability of TPCz, device III was fabricated
with an OLED structure: ITO/NPB (65 nm)/8 wt% FIrpic: TPCz
(20 nm)/TAZ (40 nm)/LiF (1 nm)/Al (100 nm). Unfortunately,
a very low efficiency of 0.75 cd Aꢁ1 (0.3%) was obtained (Fig. 10).
Taking into account the fact that TPCz can conduct electrons
well, this is understandable. The injected electrons from the
cathode can easily pass through the EML and enter into NPB
layer, this inevitably results in the loss of the excited excitons and
thus low efficiency. In order to suppress such undesirable loss, an
3. Conclusion
In summary, with a carbazole moiety as the electron donor and
a phosphine oxide as the electron acceptor, we have demon-
strated the design and synthesis of two novel star-shaped bipolar
hosts, TCTP and TPCz. In terms of their amorphous
morphology, high triplet energy and bipolar transporting char-
acteristic, high performance blue electrophosphorescent devices
with comparable efficiency have been realized using TCTP and
TPCz as the hosts of the blue phosphor. Most importantly, these
novel materials should provide an avenue for the design of the
amorphous bipolar hosts with high triplet energy used for blue
PhOLEDs based on the star-shaped scaffold.
4. Experimental
Synthesis
All chemicals and reagents were used as received from commercial
sources without further purification. Solvents for chemical
synthesis were purified according to the standard procedures. All
chemical reactions were carried out under an inert atmosphere.
4,40,400-tri(N-carbazolyl)triphenylamine (TCTA) was prepared
according to the literature procedures.29
Synthesis of 9-(40-bromophenyl)carbazole (1). A mixture of 1,4-
dibromobenzene (14.2 g, 60.0 mmol), carbazole (5.0 g, 30.0
mmol), CuI (0.3 g, 1.5 mmol), 18-crown-6 (0.4 g, 1.5 mmol),
K2CO3 (6.24 g, 45.0 mmol) and 1,3-dimethyl-3,4,5,6-tetrahydro-
ꢀ
2(1H)-pyrimidinone (DMPU) (6 mL) was heated at 190 C for
24 h under argon. After cooling to room temperature, the reac-
tion was quenched with 1 N hydrochloric acid. The mixture was
extracted with CH2Cl2, washed with NH3$H2O and water, and
dried over Na2SO4. After the solvent had been completely
removed, the residue was purified by column chromatography on
silica gel using petroleum as eluent to give 9-(40-bromophe-
nyl)carbazole (1) with a yield of 59% (5.7 g). 1H NMR (CDCl3):
Fig. 10 The current density dependence of the luminous efficiency (a)
and EQE (b) of the TPCz-based device III–V.
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d 8.13 (d, J ¼ 7.5 Hz, 2H), 7.72 (d, J ¼ 8.4 Hz, 2H), 7.45 (d, J ¼
8.4 Hz, 2H), 7.35–7.41 (m, 4H), 7.29 (t, J ¼ 7.8 Hz, 2H).
132.6, 132.3, 132.2, 132.0, 131.2, 130.3, 128.8, 128.5,
126.8, 125.7, 125.3, 123.9, 123.3, 110.2. 31P NMR
(CDCl3): d 28.4, 30.1. Anal. calcd for C54H40NO3P3: C,
76.86; H, 4.78; N, 1.66. Found: C, 75.90; H, 4.77; N, 1.58.
MALDI-TOF (m/z): 844.2 [M+ + H].
Synthesis of 4,40,400-tri(N-carbazolyl)triphenylphosphine oxide
(TCTP). n-butyllithium (2.5 M ꢂ 7.5 mL, 18.8 mmol) was slowly
ꢀ
added at ꢁ78 C to a solution of 1 (5.5 g, 17.1 mmol) in THF
(80 mL). The reaction was kept at this temperature for 3 h, and
then 0.48 mL (5.5 mmol) of trichlorophosphine was added. The
resulting mixture was further stirred for 3 h more at ꢁ78 ꢀC before
quenching with 4 mL of methanol. Water was added, and the
mixture was extracted with CH2Cl2, washed with water, and dried
over Na2SO4. After the solvent had been completely removed,
30% hydrogen peroxide (6 mL) and CH2Cl2 (20 mL) were added
to the obtained residue and they were stirred overnight at room
temperature. The organic layer was separated and washed with
water and then brine. The extract was evaporated to dryness, and
the residue was purified by column chromatography on silica gel
using dichloromethane–methanol (50 : 1 to 20 : 1) as eluent to
give 4,40,400-tri(N-carbazolyl)triphenylphosphine oxide (TCTP)
with a yield of 41% (1.7 g). 1H NMR (CDCl3): d 8.09 (d, J ¼ 7.5 Hz,
6H), 8.03 (dd, J ¼ 10.0, 3.2 Hz, 6H), 7.79 (dd, J ¼ 2.2, 8.5 Hz, 6H),
7.49 (d, J ¼ 8.2 Hz, 6H), 7.38 (t, J ¼ 8.2 Hz, 6H), 7.26 (t, J ¼ 7.5 Hz,
6H). 13C NMR (CDCl3): d 141.8, 140.2, 133.9, 131.3, 129.9, 126.8,
126.3, 123.9, 120.6, 109.7. 31P NMR (CDCl3): d 27.5. Anal. calcd
for C54H36N3OP: C, 83.81; H, 4.69; N, 5.43. Found: C, 84.03; H,
4.59; N, 5.08. MALDI-TOF (m/z): 774.3 [M+ + H].
Measurement and characterization
1H NMR, 13C NMR and 31P NMR spectra were recorded with
Bruker Avance 300 NMR spectrometer. Elemental analysis was
performed using a Bio-Rad elemental analysis system. MALDI/
TOF (matrix assisted laser desorption ionization/time-of-flight)
mass spectra were performed on AXIMA CFR MS apparatus
(COMPACT). Thermal gravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC) were performed under a flow of
nitrogen with a Perkin-Elmer-TGA 7 and Perkin-Elmer-DSC 7
system, respectively. UV-vis absorption and photoluminescence
spectra were measured with a Perkin-Elmer Lambda 35 UV/vis
spectrometer and a Perkin-Elmer LS 50B spectrofluorometer,
respectively. Phosphorescence spectra at 77 K were measured in
a toluene–ethanol–methanol (5 : 4 : 1) solvent misxture. The
highest energy peaks in the phosphorescence spectra at 77 K were
referred to as the S0n ¼ 0 ) T1n ¼ 0 transitions. Cyclic voltammetry
(CV) experiments were performed on an EG&G 283 (Princeton
Applied Research) potentiostat/galvanostat system. With regard
to the energy level of the ferrocene reference (4.8 eV relative to the
vacuum level), the HOMO and LUMO energy levels were calcu-
lated according to the following three equations: HOMO ¼ ꢁe
(4.8 V + Eox), LUMO ¼ ꢁe (4.8 V + Ered), and LUMO ¼ Eg +
HOMO. Here, Eox and Ered were taken from the onset of the
oxidation and reduction potential, respectively, and Eg was the
optical band gap estimated from the onset of the absorption
spectrum.9 Theoretical calculations were performed using the
Gaussian 03 program, and the chemical structures of TCTP and
TPCz were fully optimized by density functional theory (DFT)
using Beck’s three-parameterized Lee–Yang–Parr exchange
functional (B3LYP) with 6-31G* basis sets.
Synthesis of 9-(40-bromophenyl)-3,6-dibromocarbazole (2). To
a solution of 1 (1.6 g, 5.0 mmol) in DMF (20 mL) at 0 ꢀC, NBS
(2.0 g, 11.0 mmol) in DMF (10 mL) was added drop-wise. The
mixture was stirred for 3 h at room temperature. Then, water was
added to the mixture to give a white precipitate. After filtration
and drying, the obtained white solid was recrystallized from
petroleum to afford 9-(40-bromophenyl)-3,6-dibromocarbazole
(2) with a yield of 86% (2.1 g). 1H NMR (CDCl3): d 8.19 (d, J ¼
1.8 Hz, 2H), 7.74 (d, J ¼ 8.7 Hz, 2H), 7.51 (dd, J ¼ 8.7, 1.8 Hz,
2H), 7.38 (d, J ¼ 8.4 Hz, 2H), 7.22 (d, J ¼ 8.7 Hz, 2H).
Synthesis of 3,6-bis(diphenylphosphoryl)-9-(40-(diphenylphos-
phoryl)phenyl)carbazole (TPCz). n-butyllithium (2.5 M, 5.3 mL,
13.2 mmol) was slowly added at ꢁ78 ꢀC to a solution of 2 (1.9 g,
4.0 mmol) in THF (100 mL). The reaction was kept at this
temperature for 3 h, and then 2.4 mL (13.2 mmol) of chlor-
odiphenylphosphine was added. The resulting mixture was stir-
red for 3 h more at ꢁ78 ꢀC before quenching with 5 mL of
methanol. Water was added, and the mixture was extracted with
CH2Cl2, washed with water, dried over Na2SO4. After the
solvent had been completely removed, 30% hydrogen peroxide
(15 mL) and CH2Cl2 (20 mL) were added to the obtained residue
and they were stirred overnight at room temperature. The
organic layer was separated and washed with water and then
brine. The extract was evaporated to dryness, and the residue was
purified by column chromatography on silica gel using ethyl
acetate–methanol (20 : 1) as eluent to give 3,6-bis(diphenyl-
phosphoryl)-9-(40-(diphenylphosphoryl)phenyl)carbazole
(TPCz) with a yield of 47% (1.6 g). 1H NMR (CDCl3):
d 8.45 (d, J ¼ 12.3 Hz, 2H), 7.95 (d, J ¼ 8.4 Hz, 1H), 7.91
(d, J ¼ 8.1 Hz, 1H), 7.65–7.79 (m, 16H), 7.44–7.62
(m, 20H). 13C NMR (CDCl3): d 142.8, 139.7, 134.2, 133.7,
Device fabrication and testing
N,N0-bis((1-naphthyl)-N,N0-diphenyl-1,10-biphenyl-4,40-diamine
(NPB) was purchased from Acros, while TCTA, N,N0-dicarb-
azolyl-3,5-benzene (mCP), 3-(4-biphenyl)-4-phenyl-5-(4-tert-
butylphenyl)-1,2,4-triazole (TAZ) and iridium(III)[bis(4,6-
difluorophenyl)-pyridinato-N,C2]-picolinate
(FIrpic)
were
prepared in our lab. OLEDs were fabricated by means of vacuum
deposition with ITO glass as the substrate (10 U/,). The ITO
surface was degreased in an ultrasonic solvent bath and then
dried at 120 ꢀC prior to use. For device I, 70 nm thick NPB film
followed by 5 nm thick TCTA film was first deposited on the ITO
glass substrates. 8 wt% FIrpic and TCTP were co-evaporated to
form a 20 nm emitting layer (EML). Successively, TAZ (40 nm),
LiF (1nm) and Al (100 nm) were evaporated at a base pressure
less than 10ꢁ6 Torr (1 Torr ¼ 133.32 Pa). For comparison, device
II was also fabricated with TCTA as the host instead of TCTP.
Device III was similarly assembled with the sequence: ITO on
glass substrate, 65 nm of NPB, 20 nm of the EML made of TPCz
doped with 8 wt% FIrpic, 40 nm of TAZ, 1 nm of LiF and
100 nm Al. To improve the device performance, device IV was
8132 | J. Mater. Chem., 2010, 20, 8126–8133
This journal is ª The Royal Society of Chemistry 2010

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€
9 J. Ding, B. Zhang, J. Lu, Z. Xie, L. Wang, X. Jing and F. Wang, Adv.
Mater., 2009, 21, 4983.
prepared through inserting an additional mCP electron-blocking
layer (10 nm) between NPB and the EML. For device V, TPCz
was used as the electron-transporting layer instead of TAZ
compared with device IV. The active area of all the devices is
16 mm2. The electroluminescence (EL) spectra and Commission
Internationale de L’Eclairage (CIE) coordinates were measured
using a PR650 spectra colorimeter. The current–voltage and
brightness–voltage curves of devices were measured using
a Keithley 2400/2000 source meter and a calibrated silicon
photodiode. All the experiments and measurements were carried
out at room temperature under ambient conditions.
10 Z. Ge, T. Hayakawa, S. Ando, M. Ueda, T. Akiike,
H. Miyamoto, T. Kajita and M. –a. Kakimoto, Adv. Funct.
Mater., 2008, 18, 584.
11 Y. Shirota, J. Mater. Chem., 2000, 10, 1.
12 R. J. Holmes, S. R. Forrest, Y. –J. Tung, R. C. Kwong,
J. J. Brown, S. Garon and M. E. Thompson, Appl. Phys. Lett.,
2003, 82, 2422.
13 M. Sudhakar, P. I. Djurovich, T. E. Hogen-Esch and
M. E. Thompson, J. Am. Chem. Soc., 2003, 125, 7796.
14 S. H. Kim, J. Jang, K. S. Yook and J. Y. Lee, Appl. Phys. Lett., 2008,
92, 023513.
15 N. C. Giebink and S. R. Forrest, Phys. Rev. B: Condens. Matter
Mater. Phys., 2008, 77, 235215.
16 T. H. Huang, J. T. Liu, L. Y. Chen, Y. T. Lin and C. C. Wu, Adv.
Mater., 2006, 18, 602.
Acknowledgements
17 Z. H. Li, M. S. Wong, H. Fukutani and Y. Tao, Org. Lett., 2006, 8,
4271.
18 Y. Tao, Q. Wang, Y. Shang, C. Yang, L. Ao, J. Qin, D. Ma and
Z. Shuai, Chem. Commun., 2009, 77.
19 Q. Wang, J. Ding, D. Ma, Y. Cheng and L. Wang, Appl. Phys. Lett.,
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20 G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstick, R. Pudzich and
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21 P. E. Burrows, A. Padmaperuma, L. S. Sapochak,
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22 A. B. Padmaperuma, L. S. Sapochak and P. E. Burrows, Chem.
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23 L. S. Sapochak, A. B. Padmaperuma, X. Cai, J. L. Male and
P. E. Burrows, J. Phys. Chem. C, 2008, 112, 7989.
24 S. O. Jeon, K. S. Yook, C. W. Joo and J. Y. Lee, Adv. Funct. Mater.,
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The authors are grateful to the National Natural Science
Foundation of China (No. 50803062), Science Fund for Creative
Research Groups (No. 20621401), and 973 Project
(2009CB623601) for financial support of this research. We also
thank Prof. Jingping Zhang in Northeast Normal University for
her help on the theoretical calculations.
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