A star-shaped bipolar host material based on carbazole and dimesitylboron moieties for fabrication of highly efficient red, green and blue electrophosphorescent devices

Article abstract of DOI:10.1039/c3tc32236j

A new bipolar host material based on carbazole and dimesitylboron moieties, 3,6-bis(dimesitylboryl)-9-(4-(dimesitylboryl)phenyl) carbazole (BDDPC), has been successfully synthesised and characterised by elemental analysis, nuclear magnetic resonance spectroscopy, mass spectrometry and thermogravimetric analysis. The electrochemical and photophysical properties of BDDPC are studied by both experimental and theoretical methods. BDDPC exhibits excellent thermal stability (T_d = 234 °C), electrochemical stability, high fluorescence quantum yield (0.95) and high triplet energy (2.83 eV). A red phosphorescent organic light-emitting diode (PhOLED) device comprising BDDPC as the host material and Os(bpftz)₂(PPh₂Me)₂ as the dopant is fabricated and displays promising electrophosphorescence properties with a turn-on voltage of 3.0 V, a maximum brightness of 12337 cd m^-2 and a maximum current efficiency of 11.04 cd A^-1. Similarly, BDDPC is used to fabricate a green PhOLED device with Ir(ppy) ₂(acac) as the dopant, possessing a turn-on voltage of 2.5 V, a maximum brightness of 26473 cd m^-2 and a maximum current efficiency of 38.60 cd A^-1. Furthermore, a blue PhOLED device with BDDPC as the host material and FIrpic as the dopant is fabricated with a turn-on voltage of 3.0 V, a maximum brightness of 7622 cd m^-2 and a maximum current efficiency of 7.39 cd A^-1. It is anticipated that BDDPC has great potential in manufacturing PhOLED devices for display or lighting applications.

Full text of DOI:10.1039/c3tc32236j

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Graphical Abstract
Bipolar host material for PhOLEDs
Highlights
PhOLEDs fabricated from a new bipolar host material with high triplet energy exhibit
reasonable good performance

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A star-shape bipolar host material based on carbazole and dimesitylboron
moieties for fabrication of highly efficient red, green and blue
electrophosphoresent devices
Heping Shi,*a Dehua Xin,a Xiuqing Dong,a Jian-xin Dai,a Xiaohuan Wu,a Yanqin Miao,b Li
Fang,a Hua Wang*b and Martin M. F. Choi*c
Abstract
A
new bipolar host material based on carbazole and dimesitylboron moieties,
3,6ꢀbis(dimesitylboryl)ꢀ9ꢀ(4ꢀ(dimesitylboryl)phenyl) carbazole (BDDPC), has been successfully
synthesised and characterised by elemental analysis, nuclear magnetic resonance spectroscopy,
mass spectrometry and thermogravimetric analysis. The electrochemical and photophysical
properties of BDDPC are studied by both experimental and theoretical methods. BDDPC exhibits
excellent thermal stability (Td = 234 oC), electrochemical stability, high fluorescence quantum yield
(0.95) and high triplet energy (2.83 eV). A red phosphorescent organic lightꢀemitting diodes
(PhOLEDs) device comprising BDDPC as the host material and Os(bpftz)2(PPh2Me)2 as the dopant
is fabricated which displays promising electrophosphoresence properties with a turnꢀon voltage of
3.0 V, a maximum brightness of 12,337 cd/m2 and a maximum current efficiency of 11.04 cd/A.
Similarly, BDDPC is used to fabricate a green PhOLEDs device with Ir(ppy)2(acac) as the dopant,
possessing a turnꢀon voltage of 2.5 V, a maximum brightness of 26,473 cd/m2 and a maximum
current efficiency of 38.60 cd/A. Furthermore, a blue PhOLEDs device with BDDPC as the host
material and FIrpic as the dopant was fabricated with a turnꢀon voltage of 3.0 V, a maximum
brightness of 7622 cd/m2 and a maximum current efficiency of 7.39 cd/A. It is anticipated that
BDDPC has great potential in manufacturing PhOLEDs devices for displaying or lighting
applications.
Keywords:
Carbazole,
Dimesitylboron,
Photophysical
properties,
Host
material,
Electrophosphoresence
aSchool of Chemistry and Chemical Engineering, Shanxi University, 92 Wucheng Road, Taiyuan 030006, Shanxi Province, China. Eꢀmail:
hepingshi@sxu.edu.cn; Fax: +86ꢀ351ꢀ7011688; Tel.: +86ꢀ351ꢀ7010588
bKey Laboratory of Interface Science and Engineering in Advanced Materials, Taiyuan University of Technology, Taiyuan 030024, Shanxi Province,
China. Eꢀmail: wanghua001@tyut.edu.cn
cDepartment of Chemistry, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong SAR, China. Eꢀmail: mfchoi@hkbu.edu.hk;
Fax: +852ꢀ34117348; Tel.: +852ꢀ34117839
1

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1. Introduction
Organic lightꢀemitting diodes (OLEDs) have attracted a great deal of scientific and commercial
interests owing to their promising applications in flatꢀpanel display and solidꢀstate illumination
source. Besides the improvement in production process and structure of OLEDs, designing and
synthesizing organic electroluminescent materials with excellent performance is of great importance
for their commercialization. Organic electroluminescent materials are commonly classified as
organic electroluminescent fluorescent and phosphorescent materials based on their luminescent
mechanisms.1,2 The organic electroluminescent fluorescent materials emit light by using the singlet
state exciton. Unfortunately, the luminescence efficiency of devices made by fluorescent materials
is low in general. As such, the improvement of the luminous efficiency of device has become an
essential research topic in the organic electroluminescent field.
In 1998, Forrest and his coꢀworkers successfully prepared a luminous layer of OLED using
platinumꢀporphyrin complexes as the phosphor object materials and tris(8ꢀhydroxyquinoline)
aluminium with electron transport properties as the host material by coꢀevaporation. The luminous
efficiencies of their electroluminescent devices were greatly improved with the external quantum
efficiency of 4 and 8%.1 Since then, OLEDs based on phosphorescent emitters of transitional metal
complexes have attracted intensive attention owing to their ability to achieve an internal quantum
efficiency of 100% by harvesting both singlet and triplet excitons.1,3 To minimize selfꢀquenching
and tripletꢀtriplet annihilation for phosphorescent OLEDs, the emitters are normally dispersed in a
host matrix as emitting guests. Thus, suitable host materials are indispensable to the high
performance phosphorescent devices.4ꢀ6
In order to prevent reverse energy transfer from the guest back to the host and to confine the
excitons on guest molecules, the excellent host materials should have higher triplet energy than that
2

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of the guest.7ꢀ9 Moreover, it is desirable that the host material should have good carrier transport
properties for a balanced carrier recombination in the emitting layer.10,11 So far carbazoles are
considered as a prominent class of materials that can fulfil the aboveꢀmentioned requirements. Kang
et al. synthesized a carbazoleꢀtype host material containing tetraphenylsilane moiety for green
phosphorescent OLEDs. They found that Ir(ppy)3ꢀbased OLEDs using the carbazoleꢀtype host
material exhibited the maximum external quantum efficiency of 19.8%, the power efficiency of
59.4 lm/W and an initial luminance of 100 cd/m2.12 Very recently, Pu et al. synthesized two new
carbazoleꢀbased host materials, 1,3ꢀbis(3ꢀ(3,6ꢀdiꢀnꢀbutylcarbazolꢀ9ꢀyl)phenyl)benzene (BCzPPh)
and 4,6ꢀbis(3ꢀ(3,6ꢀdiꢀnꢀbutylcarbazolꢀ9ꢀyl)phenyl)pyrimidine. They observed that BCzPPh host
doped with a green phosphorescent emitter displayed power efficiencies and external quantum
efficiencies above 30 lm/W and 13%, respectively.13 In principle, a favourable host material should
give rise to a balanced carrier recombination in the emitting layer; thus, they ought to exhibit equal
hole and electron mobility. Generally, the hole mobility of many hosts is much higher than the
electron mobility since they mainly consist of strong electron donors such as carbazoles and
aromatic amines.14 An effective approach to optimize the hole and electron mobilities is to use
bipolar host materials. Wong and coꢀworkers developed a series of novel bipolar host materials
incorporating the electronꢀdonating carbazole core and electronꢀaccepting moieties such as
benzimidazole, phenylbenzimidazole, 1,3,5ꢀtriazine diarylborane, which generally exhibited high
morphological and thermal stability, suitable energy levels, and balanced electron/hole transporting
characteristics. Devices based on those materials demonstrated better external quantum efficiencies,
indicating the great potential of bipolar materials in OLEDs.15ꢀ19 Moreover, Zhuang et al. developed
a series of bipolar host materials containing carbazole and 1,2,4ꢀtriazole building blocks. Devices
based on the bipolar host materials exhibit better performance than those based on
3

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4,4'ꢀN,N'ꢀdicarbazoleꢀbiphenyl and 1,3ꢀbis(9ꢀcarbazolyl)benzene. The best host material could
achieve efficiencies at 32.7 and 21.1 cd/A for green and blue devices, respectively.20 Recently,
Huang et al. synthesized a series of bipolar hosts based on benzimidazoleꢀcarbazole cores and
fabricated highly efficient blue and singleꢀemitting layer white devices using the bipolar compounds
as
host
materials.
The
devices
hosted
by
9ꢀ(3ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenyl)ꢀ9Hꢀcarbazole
(mBICP),
5ꢀ(3ꢀ(1ꢀPhenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenyl)ꢀ5Hꢀpyrido[4,3ꢀb]indole
(mBINCP)
and
9ꢀPhenylꢀ3ꢀ(3ꢀ(1ꢀphenylꢀ1Hꢀbenzo[d]imidazolꢀ2ꢀyl)phenyl)ꢀ9Hꢀcarbazole (mBIPhCP) obtain
maximum external quantum efficiencies of 18.7, 13.8 and 17.0% for blue phosphorescent organic
lightꢀemitting diodes (PhOLEDs), respectively. The white devices hosted by mBICP, mBINCP and
mBIPhCP acquire a maximum external quantum efficiency of 20.5, 12.7 and 16.6%, corresponding
to a maximum power efficiency of 53.3, 31.7 and 39.4 lm Wꢀ1, respectively.21 Besides, some other
groups have also done impressive work on the exploration of bipolar host materials based on
carbazole.22ꢀ26
Triarylboron compounds have attracted a considerable attention attributing to their intriguing
electronic and photophysical properties.27,28 These excellent properties derived from the pꢀπ
conjugation between the vacant pꢀorbital on the boron centre and the πꢀorbital of the πꢀconjugated
molecular system.29ꢀ33 Recently, many triarylboron compounds have been synthesized as nonlinear
optical materials, chargeꢀtransport and emissive materials in OLEDs.34ꢀ41 As such, triarylboron
compounds possessing high effective photoelectric and transmission performance should be ideal
materials for organic electroluminescent devices. Lin et al. synthesized a new bipolar host material
(CMesB) containing the dimesityl borane/carbazole hybrid. They reported that red/green/blue/white
OLEDs based on CMesB all show high external quantum efficiencies. The results indicated that
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CMesB is a promising universal host material for phosphorescent OLEDs of various colours.42
In this work, we propose to combine triarylboron compounds and carbazoles into a single
molecular unit in order to further improve the luminescent, holeꢀtransporting and
electronꢀtransporting performance of PhOLEDs device. A novel starꢀshape compound based on
carbazole
and
dimesitylboron
moieties,
3,
6ꢀbis(dimesitylboryl)ꢀ9ꢀ(4’ꢀ(dimesitylboryl)phenyl)carbazole (BDDPC), was synthesized by
introducing triarylboron units to the 3,6ꢀpositions and 9ꢀ(4ꢀphenyl) position of a carbazole. The
chemical structure of BDDPC was fully characterized by proton nuclear magnetic resonance (1H
NMR), 13carbon (13C NMR) spectroscopy, mass spectrometry (MS) and elemental analysis. The
thermal, electrochemical, photoluminescent and electrophosphoresent properties of BDDPC were
studied in detail. Finally, BDDPC was employed as a host material for fabricating a red, a green and
a blue PhOLEDs device with excellent electroluminescent properties. It is anticipated that BDDPC
will be a new and promising host material in phosphorescent organic electroluminescent devices for
displaying or lighting applications
2.1. Experimental
2.1. Synthesis of 9-(4’-bromophenyl)carbazole (1)
The synthesis routes are summarized in Scheme 1. A mixture of carbazole (5.02 g, 30 mmol),
1ꢀbromoꢀ4ꢀiodobenzene (8.49 g, 30 mmol), 18ꢀcrownꢀ6 (0.40 g, 1.5 mmol), K2CO3 (8.29 g, 60
mmol) and CuI (0.57 g, 3.0 mmol) and 1,3ꢀdimethylꢀ3,4,5,6ꢀtetrahydroꢀ2(1H)ꢀpyrimidinone (10 mL)
was heated at 170 °C under nitrogen (N2) for 18 h. After cooling to room temperature, the reaction
was quenched with ice water. The mixture was extracted with CH2Cl2, washed with water, and
dried over Na2SO4. After the solvent had been completely removed, the residue was purified by
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column chromatography on silica gel (CH2Cl2/hexane) to afford the desired product as a white solid
1
(1) (7.65 g, 79% yield). H NMR (600 MHz, CDCl3) δ 8.13 (dd, J = 7.8, 0.5 Hz, 2H), 7.92 (d, J =
8.3 Hz, 2H), 7.39 (m, 4H), 7.32 (d, J = 8.3 Hz, 2H), and 7.29 (t, J = 7.3 Hz, 2H).
Scheme 1
2.2. Synthesis of 3,6-dibromo-9-(4’-bromophenyl)carbazole (2)
NꢀBromosuccinimide (2.0 g NBS, 11.0 mmol) in dimethylformamide (10 mL DMF) was added
dropwise to a solution of 1 (1.6 g, 5.0 mmol) in DMF (20 mL) at 0 °C. The mixture was stirred for
3 h at room temperature. Then, ice water was added to the mixture to give a white precipitate. The
obtained white solid was purified by flash silicaꢀgel column chromatography using petroleum ether
1
as eluent to afford 3,6ꢀdibromoꢀ9ꢀ(4’ꢀbromophenyl)carbazole (2). H NMR (600 MHz, CDCl3) δ
8.19 (d, J = 2.0 Hz, 2H), 7.74 (d, J = 8.7 Hz, 2H), 7.51 (dd, J = 8.7, 1.7 Hz, 2H), 7.38 (d, J = 8.7 Hz,
2H), and 7.22 (d, J = 8.7 Hz, 2H).
2.3. Synthesis of 3, 6-bis(dimesitylboryl)-9-(4’-(dimesitylboryl)phenyl)carbazole (BDDPC)
To a solution of 2 (0.48 g, 1.0 mmol) in anhydrous tetrahydrofuran (40 mL THF), tertꢀbutyllithium
(1.6 M tꢀBuLi solution) in nꢀhexane (3.75 mL, 6.0 mmol) was injected at ꢀ78 °C under N2. The
reaction was kept at this temperature for 3 h, and dimesitylboron fluoride (1.69 g Mes2BF, 6.3
mmol) in THF (20 mL) was added dropwise. The temperature was allowed to rise to room
temperature and the mixture was stirred overnight. Then, water was added and the mixture was
extracted with CH2Cl2, washed with water, dried over Na2SO4. After the solvent had been
completely removed, the residue was purified by column chromatography (CH2Cl2/nꢀhexane) to
obtain BDDPC. 1H NMR (600 MHz, CDCl3) δ 8.35 (s, 2H), 7.74 (d, J = 8.1 Hz, 3H), 7.57 (m, 4H),
7.38 (d, J = 8.3 Hz, 2H), 6.85 (s, 4H), 6.82 (s, 8H), 2.26 (s, 6H), 2.25 (s, 12H), 2.02 (s, 12H), 2.01
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(s, 24H). 13C NMR (CDCl3): d 142.8, 139.7, 134.2, 133.7, 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, and 110.2. Anal. calcd for C54H40NO3B3: C, 76.86;
H, 4.78; N, 1.66. Found: C, 75.90; H, 4.77; N, 1.58. LCꢀMS (m/z): 844.2 [M + H]+.
2.4. Measurement and characterization
Melting points were determined on an Xꢀ5 melting point detector. All NMR spectra were measured
on a Bruker 600 MHz spectrometer. Thermogravimetric analysis (TGA) was performed on a TGA
2050 thermogravimetric analyser under N2 atmosphere with a heating rate of 10 °C/ min from room
temperature to 750 °C. Elemental analyses were performed on an Element Analysis System. Mass
spectra were recorded with a LCꢀMS system consisted of a Waters 1525 pump and a Micromass
ZQ4000 single quadrupole mass spectrometer detector. Cyclic voltammetry (CV) was performed on
a CHIꢀ600C electrochemical analyser. The CV measurements were carried out with a conventional
threeꢀelectrode configuration consisting of a glassy carbon working electrode, a platinumꢀdisk
auxiliary electrode and a Ag/AgCl reference electrode, and the scan speed was 50 mV/s. UVꢀvis
absorption spectrum was acquired on a Shimadzu UVꢀ2450 spectrophotometer. Fluorescence
spectra were obtained on a Hitachi Fꢀ4500 spectrofluorometer. Fluorescence quantum yield was
determined by using quinine sulfate as the reference. All measurements were performed at room
temperature.
2.5. Device fabrication and testing
The multilayer OLEDs were fabricated by the vacuumꢀdeposition method. Organic layers were
deposited by highꢀvacuum (5×10−4 Pa) thermal evaporation onto a glass (3 cm×4 cm) substrate
preꢀcoated with an indium tin oxide (ITO) layer. N,Nꢀbis(naphthyl)ꢀN,Nꢀbis(phenyl)benzidine,
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4,4'ꢀbis(Nꢀcarbazolyl)ꢀ1,1'ꢀbiphenyl, BDDPC, 4,7ꢀdiphenylꢀ1,10ꢀphenanthroline (BPhen), and
LiF/Al were used as the holeꢀtransport layer, electronꢀblocking layer, emitting layer,
electronꢀtransport layer and cathode, respectively. All organic layers were sequentially deposited.
Thermal deposition rates for organic materials, LiF and Al were 0.5, 0.5 and 1 Å/s, respectively.
The active area of the devices was 12 mm2. The electroluminescent spectra were measured on a
Hitachi MPFꢀ4 spectrofluorometer. The voltage–current density (V–J) characteristics of OLEDs
were recorded on a Keithley 2400 Source Meter. The characterization of brightness–current–voltage
(B–I–V) were measured with a 3645 DC power supply combined with a 1980A spot photometer
and were recorded simultaneously. All measurements were done at room temperature in ambient
conditions.
2.6. Theoretical calculations
The groundꢀstate geometry of BDDPC was optimized at B3LYP level with 6ꢀ31G (d, p) basis
set.43ꢀ45 The vibration frequencies and the frontier molecular orbital characteristics were analysed on
the optimal structure at the same level. The excitedꢀstate geometry of compound was optimised
based on the ground state structures using timeꢀdependent density functional theory (DTꢀDFT)
method at B3LYP level with 6ꢀ31G (d, p) basis set. The absorption and emission spectra of
compound were carried out using TDꢀDFT method based on the optimal ground state structure and
the lowest singlet excited state structure, respectively. Solvent effect was also taken into account by
using the polarised continuum model.46,47 All calculations were carried out with the Gaussian 09
program package48 and performed in the Supercomputing Centre of Computer Network Information
Centre of the Chinese Academy of Sciences.
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3. Results and discussion
3.1. Synthesis and characterization
Scheme 1 displays the synthetic route of BDDPC. First, Compound 1 was synthesized via the
modified Ullmann reaction. Then, Compound 2 was prepared by bromination of 1 with NBS. The
final product BDDPC was obtained by reacting 2 with tꢀBuLi, followed with the addition of
Mes2BF. The chemical structure of BDDPC was fully characterized by 1H NMR, 13C NMR and MS.
1
The H NMR spectrum of BDDPC displays downfield peaks at 7.00–8.48 ppm which were
assigned to the protons of carbazole moieties. The downfield peaks at 6.59–6.89 ppm and the
upfield peaks at 2.09 and 2.37 ppm were corresponding to the dimesitylboron moieties protons. The
13C NMR, MS and elemental analysis are consistent with the desirable structures. Our synthesis
routes successfully synthesized BDDPC which is soluble in many common organic solvents
including CH2Cl2, CHCl3, DMF, THF, toluene, and dimethylsulfoxide.
3.2. Thermal properties
The thermal property of BDDPC was investigated by TGA and differential scanning calorimetry
(DSC) under N2 atmosphere as depicted in Fig. S1 and Fig. S2 (ESI), respectively. The
decomposition temperature (Td) corresponding to 5% weight loss of BDDPC is at 234 °C and a
glass transition temperature (Tg) is at 110 °C. These results are summarized in Table 1. The high Td
might be attributed to its large molecular mass, the rigid carbazole core and the nonꢀplanar
dimesitylboron peripheries. The result indicates that BDDPC has good thermal stability and high Tg
which should be adequate for most optoelectronic device applications.
3.3. Theoretical calculation
Theoretical calculations can provide a reasonable qualitative indication of the excitation and
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emission properties of a compound.49 BDDPC comprise a 9ꢀphenylcarbazole unit as the core and
three dimesitylboron units as the terminals. Its geometry at ground state was optimised at the
DFT/B3LYP/6ꢀ31G (d, p) level by using Gaussian 09 software. Fig. 1 depicts the carbazole moiety
is of planar structure and the three mesityl groups form propellerꢀlike conformations originated
from the trigonal boron centre. The highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) of BDDPC (Fig. 1) were also calculated at
DFT/B3LYP/6ꢀ31G (d, p) level using the polarised continuum model for geometry optimisations.
The HOMO is localized mainly in the carbazole moiety while the LUMO is in the
dimesitylborylphenyl moiety. Thus, the electronic transition from the ground state to the first
excited state mainly involves intramolecular charge transfer (ICT) from the carbazole
(electronꢀdonor) to the dimesitylborylphenyl (electronꢀacceptor) moieties.
Generally, holes and electrons in OLEDs are transferred through the individual HOMO and
LUMO levels.50 BDDPC exhibits complete separation and localisation of HOMO and LUMO,
inferring that the HOMO to LUMO transition follows a typical charge transfer process. In addition,
the complete localisation of HOMO and LUMO is essential to the efficient hole and electron
transport and to the prevention of reverse energy transfer as well.51 Table 1 displays the calculated
HOMO and LUMO energy levels of BDDPC at ꢀ5.55 and ꢀ1.90 eV, respectively. The
HOMOꢀLUMO energy gap (Eg) is 3.65 eV. The theoretical Eg is ca. 0.39 eV larger than the optical
energy band gap obtained from UVꢀvis absorption measurement.
Fig. 1
Table 1
3.4. Photophysical properties
Fig. 2 displays the UVꢀvis absorption, fluorescence and phosphorescence emission spectra of
BDDPC in THF (10 ꢁM). The spectroscopic data are summarized in Table 1. BDDPC exhibits two
major absorption bands at 290–330 and 330–380 nm. The absorption peak maximum (λabs) at 348
nm is corresponding to the πꢀπ* electronic transition and the λabs of 390 nm is assigned to the ICT
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from the carbazole core to dimesitylboron terminal. The optical energy band gap calculated from
the absorption band edge of the absorption spectrum is approximately 3.26 eV. BDDPC exhibits a
strong fluorescence emission band at 369–550 nm with an emission peak maximum (λem) of 420
nm. The emission band is assigned to ICT from the carbazole core to dimesitylboron terminal. The
fluorescent quantum yield (Φ) of BDDICZ in THF solvent was measured by using quinine sulfate
in 0.10 M sulfuric acid as the reference.52 BDDPC possesses a high Φ of 0.95, revealing its
potential application as an excellent optoelectronic material in optical field.
The phosphorescence emission spectrum of BDDPC was performed in a frozen THF at 77 K in
order to determine its triplet energy (ET). The highestꢀenergy phosphorescence peak is 438 nm and
the triplet energy is 2.83 eV. Such the triplet energy implies that BDDPC could serve as an
appropriate host material for red and green phosphors. This result clearly suggests the effective
interruption of πꢀconjugation between carbazole core and three dimesitylboron terminals due to the
highly twisted conformation.
Fig. 2
3.5. Electrochemical properties
The electrochemical properties of BDDPC were studied by CV with Ag/AgCl as the reference
electrode. The measurement was conducted in 1.0 mM BDDPC in acetonitrile containing 0.10 M
tetrabutylammonium perchlorate as the supporting electrolyte under N2 atmosphere. The CV of
BDDPC is displayed in Fig. S3 (ESI). A reversible oxidation peak and three reversible reduction
peaks are observed within the entire electrochemical window of acetonitrile. The reversible
oxidation peak around +1.34 V is assigned to the oxidation of carbazole, whereas three reversible
reduction peaks around ꢀ0.99, ꢀ2.06 and ꢀ2.28V are attributed to the electron injection into the
vacant pꢀorbital of dimesitylboron.42 Obviously, the higher reduction potentials (ꢀ0.99V) of
BDDPC suggest the electron distributions on its LUMO are solely localised on the
dimesitylborylphenyl moiety.15 The HOMO energy level can be calculated with the empirical
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equation: HOMO = – (Eox + 4.40) eV, where Eox is the onset oxidation potential.53 The Eox is 1.21
eV and the HOMO energy level of BDDPC is ꢀ5.61 eV. The Eg is estimated to be 3.26 eV by the
absorption edges of the absorption spectrum. The LUMO energy level of BDDPC is thus ꢀ2.35 eV
which is calculated from the HOMO energy level and Eg. The HOMO, LUMO and Eg are very
close to the theoretical values. All the data are summarized in Table 1.
The HOMO level of BDDPC is well matched with those of two common holeꢀtransporting
materials,
4,4ꢀbis[Nꢀ(1ꢀnaphthyl)ꢀNꢀphenylꢀaminobiphenyl
(5.4
eV)54
and
4,4′ꢀcyclohexylidenebis[N,Nꢀbis(4ꢀmethylphenyl)benzenamine] (TAPC) (5.5 eV),55 indicating that
BDDPC can possibly facilitate hole injection and transportation. Similarly, the LUMO level of
BDDPC is also well matched with those of two common electronꢀtransporting materials,
1,3,5ꢀtris(1ꢀphenylꢀ1Hꢀbenzimidazolꢀ2ꢀyl)benzene
(2.39
eV)
and
2,9ꢀdimethylꢀ4,7ꢀdiphenylꢀ1,10ꢀphenanthroline () (2.44 eV). The barrier of electron transport from
the electronꢀtransporting materials to the host materials is quite small and thus is beneficial to be
used as electronꢀtransporting materials for OLEDs.20 The electrochemical properties of BDDPC
indicate that it can be utilized both as a hole and an electron transporter. Furthermore, the low
HOMO energy level of BDDPC suggests that the compound has high oxidative stability.56 Its
electrochemical stability will improve the lifeꢀtime of electroluminescent devices. It is anticipated
that BDDPC should be a desirable host material in lightꢀemitting applications.
3.6. Charge transporting properties
Single carrier devices were fabricated to investigate the charge transporting properties of BDDPC.
First, a holeꢀonly device comprising ITO/TAPC (8 nm)/BDDPC (30 nm)/TAPC (8 nm)/Al was
prepared to examine the hole transporting capability of BDDPC. Since TAPC has a high LUMO
12

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level, it was used to block the electron from the cathode. Second, an electronꢀonly device consisting
of ITO/BCP (8 nm)/BDDPC (30 nm)/BCP (8 nm)/LiF (1 nm)/Al was prepared to examine the
electron transporting capability of BDDPC. BCP was used to block the hole from the anode
because of its low HOMO level. Fig. 3 depicts the currentꢀcharacteristics of these two devices as a
function of the average electric field. Both of these devices could conduct significant currents,
indicating that BDDPC possesses the bipolar transporting property. The electronꢀonly device
conducts higher current than that of the holeꢀonly device under the same average electric field.
Although BDDPC has the bipolar transporting capability, its electronꢀtransport is better than its
holeꢀtransport ability.
Fig. 3
3.7. Electroluminescent property
To evaluate BDDPC as the potential host material for fabrication of OLEDs, phosphorescent
OLEDs were made with BDDPC as the host and 8.0 weight (wt) % Ir(ppy)2(acac) or
Os(bpftz)2(PPh2Me)2 as the dopant. The first device was a red phosphorescent OLED (Device R)
with the configuration of ITO/TAPC (50 nm)/BDDPC doped with 8.0 wt % Os(bpftz)2(PPh2Me)2
(30 nm)/tris[3ꢀ(3ꢀpyridyl)ꢀmesityl]borane (3TPYMB) (60 nm)/LiF (0.5 nm)/Al(150 nm). TAPC,
3TPYMB and BDDPC doped with 8.0 wt % Os(bpftz)2(PPh2Me)2 were used as the holeꢀinjection
layer, electronꢀtransporting layer and emitting layer, respectively. Fig. S4 (ESI) displays the
normalized electroluminescence spectra of Device R under various applied voltages. All the spectra
show the same spectral characteristics with an emission peak of 624 nm which are almost identical
with the International Commission on Illumination (CIE) coordinates of (0.62,0.31), corresponding
to the emission of Os(bpftz)2(PPh2Me)2. As BDDPC has higher ET than that of
Os(bpftz)2(PPh2Me)2,57 no additional emission from the host material was observed, indicating that
13

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Device R has very efficient energy transfer from the host to the dopant. Fig. 4 and 5 display the
current density–voltage–luminance curve and current density–luminance efficiency curve of Device
R, respectively. Table 2 summarizes the electroluminescent characteristics of Device R. Device R
exhibits a turnꢀon voltage of 3.0 V and a maximum luminance efficiency (Lmax) of 11.04 cd/A and a
maximum luminance (η
L,max) of 12,337 cd/m2.
Fig. 4
Fig. 5
Table 2
Similarly, a green phosphorescent OLED (Device G) with the configuration of ITO/TAPC (50
nm)/BDDPC doped with 8.0 wt % Ir(ppy)2(acac)(25 nm)/BPhen (20 nm)/BPhen doped with 5.0 wt
% of Cs2CO3 (30 nm)/Ag (150 nm) was prepared. TAPC, BPhen, BPhen doped with 5.0 wt % of
Cs2CO3, and BDDPC doped with 8.0 wt % Ir(ppy)2(acac) function as the holeꢀinjection layer,
electronꢀtransporting layer, conductivity enhancement and electronꢀinjection, and emitting layer of
Device G, respectively. The electroluminescence spectra of Device G at various applied voltages
are depicted in Fig. S5. All spectra show similar spectral characteristics with an λEL of 508 nm and
almost identical with the CIE coordinates of (0.30, 0.61), corresponding to the emission of
Ir(ppy)2(acac). Furthermore, no additional emission coming from the host materials was observed
owing to the higher ET of BDDPC than that of Ir(ppy)2(acac).57 The result indicates that Device G
has efficient energy transfer from the host to the dopant. Fig. 6 and 7 display the current
density–voltage–luminance curves and current density–luminance efficiency curve of Device G,
respectively. Table 2 summarizes the electroluminescent characteristics of Device G. Device G
exhibits a turnꢀon voltage of 2.5 V and a Lmax of 38.60 cd/A and a η
L,max of 26,473 cd/m2.
Fig. 6
14

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Fig. 7
Furthermore, a blue phosphorescent OLED (Device B) with the configuration of ITO/TAPC (50
nm)/BDDPC doped with 15 wt
%
bis[2ꢀ(4,6ꢀdifluorophenyl)pyridinatoꢀC2,N](picolinato)iridium(III) (FIrpic) (25 nm)/BPhen (40
nm)/LiF (0.5 nm)/Al (150 nm) was fabricated. TAPC, BPhen, and BDDPC doped with 15 wt %
FIrpic function as the holeꢀinjection layer, electronꢀtransporting layer, and emitting layer of Device
B, respectively. The electroluminescence spectra of Device B at various applied voltages are
depicted in Fig. S6. All spectra show similar spectral characteristics with an λEL of 472 nm and
almost identical with the CIE coordinates of (0.16, 0.33), corresponding to the emission of FIrpic.
Moreover, no additional emission coming from the host materials was observed owing to the higher
ET of BDDPC than that of FIrpic.57 The result indicates that Device B has efficient energy transfer
from the host to the dopant. Fig. 8 and 9 display the current density–voltage–luminance curves and
current density–luminance efficiency curve of Device B, respectively. Table 2 summarizes the
electroluminescent characteristics of Device B. Device B exhibits a turnꢀon voltage of 3.0 V and a
Lmax of 7.39 cd/A and a η
L,max of 7622 cd/m2.
Fig. 8
Fig. 9
The remarkable performance of Device R, G and B could be attributed to the excellent thermal
stability which significantly enhances the capability of forming stable amorphous thin films. The
brilliant bipolar charge transport capability of BDDPC significantly increases the carrier balance
between the emitting layer and the high triplet energy of BDDPC and transfers energy from the host
to the dopant. It should be pointed out that the electroluminescent performance was obtained in a
nonꢀoptimized test device under ordinary laboratory conditions. The device performance could be
15

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further improved by optimizing the layer thicknesses and processing conditions.58 This work is
currently conducted in our laboratories.
4. Conclusion
In summary, a new bipolar host material based on carbazole and dimesitylboron moieties, BDDPC,
has been successfully synthesised and characterised by elemental analysis, NMR, MS, and TGA.
The electrochemical and photophysical properties of BDDPC were studied by both experimental
and theoretical methods. Our results demonstrate that BDDPC has excellent thermal stability (Td =
o
234 C), electrochemical stability and high Φ (0.95) as well as high triplet energy (2.83 eV).
Moreover, utilizing BDDPC as the host material, red, green and blue PhOLEDs devices can be
fabricated with relatively good brightness and current efficiency. This work demonstrates that the
bipolar host BDDPC has potential in producing PhOLEDs devices for display or lighting
applications.
Acknowledgements
This work was supported by the Natural Science Foundation of Shanxi Province (2013011013ꢀ1);
Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi
Province (2012005); Program for Changjiang Scholar and Innovation Research Team in University
(IRT0972); International Science & Technology Cooperation Program of China (2012DFR50460);
National Natural Scientific Foundation of China (21101111, 61205179, 61307030, 61307029);
Shanxi Provincial Key Innovative Research Team in Science and Technology (2012041011); Shanxi
Scholarship Council of China (2012ꢀ006); Hundred Talent Programme of Shanxi Province, and
Fund of Key Laboratory of Optoelectronic Materials Chemistry and Physics, Chinese Academy of
Sciences (2008DP173016). The authors express their sincere thanks to the Advanced Computing
Facilities of the Supercomputing Centre of Computer Network Information Centre of Chinese
Academy of Sciences for all the theoretical calculations.
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Table 1. Physical properties of compound BDDPC
HOMO (eV) LUMO (eV) Eg (eV)
Theo Exp Theo Exp Theo Exp
ꢀ5.55 ꢀ5.61 ꢀ1.90 ꢀ2.35 3.65 3.26
Td (oC)
234
Tg (oC)
110
λ
abs (nm)
348
390
λ
em (nm)
420
Φ
0.95
λ
λ
abs : absorption peak maximum.
em : emission peak maximum.
Φ : fluorescent quantum yield.
HOMO: highest occupied molecular orbital.
LUMO: lowest unoccupied molecular orbital.
Eg : energy gap between HOMO and LUMO.
Td : decomposition temperature.
Tg: glass transition temperature
Table 2. Electroluminescent characteristics of Device R, G and B
Device
Von
3.0
2.5
3.0
Lmax (cd m−2)
12,337
λ
EL (nm)
624
508
η
L,max (cd A−1)
11.04
R
G
B
26,473
7622
38.60
7.39
472
λEL : electroluminescence emission peak.
Von : turnꢀon voltage at 1 cd m−2.
Lmax : maximum luminance.
η
L,max : maximum luminance efficiency.
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Br
Br
I
B
B
N
N
Mes2BF
t-BuLi
CuI
+
NBS
N
B
N
K2CO3
H
Br
Br
Br
1
2
BDDPC
Scheme 1 The synthetic route of BDDPC.

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HOMO
LUMO
Fig. 1. HOMO and LUMO diagrams of BDDPC.

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UV-vis
FL
Phos
0.9
0.6
0.3
0.0
300
350
Wavelength
400
450
nm
500
550
(
)
Fig. 2. UV-vis absorption, fluorescence (FL) and phosphorescence (Phos) spectra of
BDDPC.

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Hole-only Device
102
101
100
10-1
10-2
10-3
Electron-only Device
5.0x107 1.0x108 1.5x108
0.0
Average Electric Field (V/m)
Fig. 3. The current characteristics as a function of the average electric field for
hole-only and electron-only devices.

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103
102
101
100
10-1
10-2
10-3
Device R
105
104
103
102
101
100
2
4
6
Voltage (V)
Fig. 4. Current density–voltage–luminance curves of Device R.

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100
10
1
Device R
0.1
1
10
100
Current Density
(
mA/cm2
)
Fig. 5. Current density–luminance efficiency curve of Device R.