Diarylmethylene-bridged 4,4′-(bis(9-carbazolyl))biphenyl: Morphological stable host material for highly efficient electrophosphorescence

Article abstract of DOI:10.1039/b910247g

A novel compound (BCBP) based on the modification of a well-known host material 4,4′-(bis(9-carbazolyl))biphenyl (CBP) through arylmethylene bridge linkage was synthesized, and fully characterized. Its thermal, electrochemical, electronic absorption and p

Full text of DOI:10.1039/b910247g

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Diarylmethylene-bridged 4,40-(bis(9-carbazolyl))biphenyl: morphological
stable host material for highly efficient electrophosphorescence†
a
a
b
a
Zuoquan Jiang, Xichen Xu, Zhiqiang Zhang, Chuluo Yang, Zhongyin Liu,a Youtian Tao,a Jingui Qina
*
b
*
and Dongge Ma
Received 26th May 2009, Accepted 4th August 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b910247g
A novel compound (BCBP) based on the modification of a well-known host material
4,40-(bis(9-carbazolyl))biphenyl (CBP) through arylmethylene bridge linkage was synthesized, and fully
characterized. Its thermal, electrochemical, electronic absorption and photoluminescent properties
ꢀ
were studied. A high glass transition temperature (Tg) of 173 C is observed for BCBP due to the
introduction of the bridged structure, remarkably contrasting with a low Tg of 62 ꢀC for CBP.
Furthermore, the bridged structure enhances the conjugation and raises the HOMO energy, thus
facilitating hole-injection and leading to a low turn-on voltage in an electroluminescent device. With the
device structure of ITO/MoO3/NPB/Ir complex: BCBP/BCP/Alq3/LiF/Al, maximum power efficiencies
of 41.3 lm/W and 6.3 lm/W for green- and blue-emitting OLED were achieved, respectively.
morphological stabililty with a low Tg of 62 ꢀC,9 which could be
Introduction
the result of its low molecular weight, non-rigid and symmetric
molecular architecture. It is of important significance to develop
morphologically stable CBP derivatives as host materials. Up to
now, to improve morphological stability of carbazole-based host
materials, molecular modifications mainly involved the intro-
duction of nonconjugated moieties (such as sp3-hybridized C-9 of
fluorene or a Si atom of triphenylsilyl group) to the para position
of the nitrogen atom,10,4c In this paper, we report a new solution
to achieve a thermally and morphologically stable CBP deriva-
tive by diarylmethylene-bridged 9-phenyl-9H-carbazole. The
bridged group could serve as not only the nonconjugated group
to the CBP, but also as a rigid and steric group, therefore ful-
filling the trade off between the increasing molecular size and
constraint of molecular conjugation.
During the past decade, phosphorescent organic light-emitting
diodes (PHOLEDs) have attracted much attention from both
academic and industrial communities.1 PHOLEDs are able to
fully utilize both singlet and triplet exciton emission, and there-
fore realize nearly 100% internal quantum efficiencies in theory.2
For PHOLEDs, the appropriate host material is of equal
importance to the triplet guest to achieve high device efficiency.3
The development of an effective host material for a triplet emitter
imposes some challenges to molecular design. The triplet energy
of the host must be higher than that of the guest to prevent
energy transfer from the guest back to the host and confine triplet
excitons on guest molecules.4 To this goal, the extent of conju-
gation and molecular size of the host must be constrained. This
requirement makes it difficult for the molecules to acquire the
morphological stability that is necessary for forming stable and
uniform amorphous thin films.5 Furthermore, decreasing the
p-aromatic conjugation system may adversely affect the charge
transport properties.6
Experimental
General information
1H NMR and 13C NMR spectra were measured on a Varian
Mercury 300 spectrometer in CDCl3. Elemental analyses of
carbon, hydrogen, and nitrogen were performed on a PE-2400C
micro-analyzer. Mass spectra were measured on a VJ-ZAB-3F-
Mass spectrometer. UV-Vis absorption spectra were recorded on
a Shimadzu UV-2500 recording spectrophotometer. PL spectra
were recorded on a Hitachi F-4500 fluorescence spectropho-
tometer. Differential scanning calorimetry (DSC) was performed
on a NETZSCH DSC 200 PC unit. The samples were heated at
a heating rate of 10 ꢀC minꢁ1 from 30 to 220 ꢀC under argon, then
cooled quickly to room temperature, and then heated again from
As we know, carbazole derivatives can usually be used as host
materials due to their high triplet energy and good hole-trans-
porting ability.7 For example, 4,40-(bis(9-carbazolyl))biphenyl
(CBP) is a prominent host for triplet emitters. PHOLEDs using
CBP as a host material with a peak efficiency as high as 28 cd/A
for green Ir(ppy)3 have been reported.1b Unfortunately, the CBP
host is prone to crystallize, especially when the dopant concen-
tration is too low.8 Moreover, the CBP host shows poor
aDepartment of Chemistry, Hubei Key Lab on Organic and Polymeric
Optoelectronic Materials, Wuhan University, Wuhan 430072, People’s
Republic of China. E-mail: clyang@whu.edu.cn
ꢀ
30 to 220 C. The glass transition temperature (Tg) was deter-
bState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People’s Republic of China. E-mail: mdg1014@ciac.
jl.cn
mined from the second heating scan. Thermogravimetric analysis
(TGA) was undertaken with a NETZSCH STA 449C instru-
ment. The thermal stability of the samples under a nitrogen
atmosphere was determined by measuring their weight loss while
heating at a rate of 10 ꢀC minꢁ1 from 25 to 600 ꢀC. Cyclic
† CCDC reference number 732327. For crystallographic data in CIF or
other electronic format see DOI: 10.1039/b910247g
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 7661–7665 | 7661

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voltammetry (CV) was carried out in nitrogen-purged dichloro-
methane at room temperature with a CHI voltammetric
analyzer. Tetrabutylammonium hexafluorophosphate (TBAPF6)
(0.1 M) was used as the supporting electrolyte. The conventional
three-electrode configuration consists of a platinum working
electrode, a platinum wire auxiliary electrode, and a Ag wire
pseudo-reference electrode with ferrocenium–ferrocene (Fc+/Fc)
as the internal standard. Cyclic voltammograms were obtained at
a scan rate of 100 mV sꢁ1. Formal potentials are calculated as the
average of cyclic voltammetric anodic and cathodic peaks.
distilled under reduced pressure. The crude product was purified
by column chromatography on silica gel using 10:1 petroleum/
ethyl acetate as the eluent. Yield: 1.71 g, 45%. 1H NMR
(300 MHz, CDCl3, d): 8.22 (s, 1H), 8.11 (d, J ¼ 7.8 Hz, 2H), 7.85
(d, J ¼ 8.7 Hz, 1H), 7.45 (d, J ¼ 7.8 Hz, 1H), 7.35 (t, J ¼ 7.8 Hz,
2H), 7.25 (t, J ¼ 7.2 Hz, 2H), 7.09 (d, J ¼ 7.8 Hz, 2H), 3.19
(s, 3H); 13C NMR (75 MHz, CDCl3, d): 164.95, 141.30, 136.43,
135.96, 134.93, 131.69, 131.49, 126.12, 123.40, 121.80, 120.41,
120.15, 109.16, 52.38. Anal. Calcd. for C20H14BrNO2 (%): C,
63.18; H, 3.71; N, 3.68. Found: C, 63.33; H, 3.28; N, 3.54. GC-
MS m/z 379 [M+].
X-Ray structural analysis
Synthesis of 10-bromo-8,8-di-p-tolyl-8H-indolo[3,2,1-de]acri-
dine (2). A solution of 1 (3.81 g, 10 mmol) in dry THF (100 ml)
was added slowly a 0.6 M 4-methylphenylmagnesium bromide
(36.7 ml, 22 mmol), then the reaction mixture was stirred over-
night at 80 ꢀC. After that, the reaction mixture was quenched
with a dilute solution of ammonium chloride, extracted with
ether, and the organic layers were washed with brine, dried over
anhydrous sodium sulfate, and evaporated to get a yellow solid
(used for the next step without further purification). The yellow
solid was dissolved in the boiling acetic acid (38 ml), and
concentrated HCl(aq.) (4 ml) was added dropwise. After reflux
for 3 h, the mixture was poured into ice water (300 ml). The
precipitate was collected and washed twice with ethanol, then
purified by column chromatography on silica gel using 3:1
petroleum/chloroform as the eluent to afford a white solid. Yield:
2.4 g, 47%. 1H NMR (300 MHz, CDCl3, d): 8.12 (d, J ¼ 7.5 Hz,
1H), 8.00 (t, J ¼ 6.6 Hz, 2H), 7.93 (d, J ¼ 8.1 Hz, 1H), 7.55–7.45
(m, 2H), 7.36–7.29 (m, 2H), 7.22 (d, J ¼ 1.5 Hz, 1H), 7.05–6.9 (m,
J ¼ 8.1 Hz, 5H), 6.91 (d, J ¼ 8.1 Hz, 4H), 2.36 (s, 6H); 13C NMR
(75 MHz, CDCl3, d): 143.16, 138.95, 137.80, 136.86, 136.57,
135.28, 134.77, 130.84, 130.50, 129.19, 128.31, 127.41, 126.93,
126.85, 123.23, 122.53, 122.03, 121.79, 118.77, 115.93, 115.34,
114.87, 57.08, 21.53. Anal. Calcd. for C33H24BrN (%): C, 77.04;
H, 4.70; N, 2.72. Found: C, 76.71; H, 4.88; N, 2.99. MS (EI) m/z
513.0 [M+].
Single crystal X-ray diffraction data were obtained from
a Bruker AXS Smart CCD diffractometer using a graphite-
˚
monochromated Mo Ka (l ¼ 0.71073A) radiation. The data
were collected using the u/2q scan mode and corrected for Lor-
entz and polarization effects as well as the absorption during
data reduction suing Shelxtl 97 software. Crystal data for BCBP:
(C51H36N2)2.EtOH.CH2Cl2, triclinic, P-1, cell parameters a ¼
˚
˚
˚
10.1508(11) A, b ¼ 12.2571(13) A, c ¼ 16.6606(18) A, a ¼
3
˚
79.161(2), b ¼ 77.551(2), g ¼ 88.507(2), V ¼ 1987.8(4) A , z ¼ 1,
Mr ¼ 1484.69, Dc ¼ 1.240 g/cm3, F(000) ¼ 780, m ¼ 0.137 mmꢁ1
,
10 775 reflections were collected yielding 6918 independent data
(Rint ¼ 0.0156).
Device fabrication and performance measurements
The hole-injection material MoO3, hole-transporting material
NPB (1,4-bis(1-naphthylphenylamino)-biphenyl), hole-blocking
material BCP (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)
and electron-transporting materials Alq3 (tris(8-hydroxy-
quinoline)aluminium) were commercially available. Commercial
indium tin oxide (ITO) coated glass with a sheet resistance of
10U/, was used as the starting substrates. Before device fabri-
cation, the ITO glass substrates were pre-cleaned carefully and
treated by UV/O3 for 2 min. Then the sample was transferred to
the deposition system. 10 nm of MoO3 was firstly deposited to
ITO substrate, followed by 40 nm NPB, 30 nm emissive layer,
10 nm BCP, and 30 nm Alq3. Finally, a cathode composed of
1 nm of lithium fluoride and 100 nm of aluminium were
sequentially deposited onto the substrate in the vacuum of 10ꢁ6
Torr to construct the device. The I–V–B of EL devices was
measured with a Keithey 2400 Source meter and a Keithey 2000
Source multimeter equipped with a calibrated silicon photo-
diode. The EL spectra were measured by a JY SPEX CCD3000
spectrometer. All measurements were carried out at room
temperature under ambient conditions.
Synthesis of 10-(4-(9H-carbazol-9-yl)phenyl)-8,8-di-p-tolyl-8H-
indolo[3,2,1-de]acridine (BCBP, 3). A mixture of 9-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole (0.37
g, 1 mmol), 2 (0.51 g, 1 mmol), Pd(PPh3)4 (60 mg, 4.5% mmol)
and potassium carbonate (1.38 g, 11.4 mmol) in 18 ml of THF
and 5.8 ml of distilled water in a 100 ml round bottom flask was
stirred at 90 ꢀC for 36 h under argon. The mixture was extracted
with chloroform, and the organic layers were washed with brine
and dried over anhydrous sodium sulfate. The crude product was
purified by column chromatography on silica gel using 3:1
petroleum/chloroform as the eluent to afford a white solid. Yield:
0.45 g, 67%. 1H NMR (300 MHz, CDCl3, d): 7.09–7.01 (m, 6H),
6.92–6.89 (m, 12H), 6.91 (s, 6H), 6.72–6.68 (m, 6H), 2.37 (s, 6H),
2.22 (s, 6H), 1.09 (s, 9H); 13C NMR (75 MHz, CDCl3, d): 149.50,
141.03, 139.80, 138.81, 137.63, 136.79, 136.67, 136.36, 134.43,
133.35, 130.55, 130.32, 128.79, 128.40, 128.16, 127.54, 127.12,
126.59, 126.24, 126.17, 123.60, 122.60, 122.21, 121.57, 121.46,
120.55, 120.17, 118.35, 114.68, 114.18, 110.07, 56.89, 21.22. Anal.
Calcd. for C51H36N2 (%): C, 90.50; H, 5.36; N, 4.14. Found: C,
90.05; H, 5.76; N, 4.39. MS (EI) m/z 676.4 [M+].
Materials
Synthesis of methyl 5-bromo-2-(9H-carbazol-9-yl)benzoate (1).
A 100 ml of round bottom flask equipped with a magnetic stirrer
bar and reflux condenser was charged with carbazole (1.67 g,
10 mmol), methyl 4-bromo-2-iodobenzoate (3.41 g, 10 mmol),
K2CO3 (5 g, 36.2 mmol), activated copper bronze (0.1 g,
1.6 mmol), and 18-crown-6 (0.10 g, 0.4 mmol), and then 20 ml of
o-dichlorobenzene was added. The reaction mixture was refluxed
under argon for 24 h. After it was cooled to room temperature,
the mixture was filtered through celite and the solvent was
7662 | J. Mater. Chem., 2009, 19, 7661–7665
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Results and discussion
Synthesis and characterization
The synthesis of BCBP is outlined in Scheme 1. The intermediate
methyl 5-bromo-2-(9H-carbazol-9-yl)benzoate was prepared by
a classic Ullmann reaction, and treated with p-tolylmagnesium
bromide followed by subsequent ring closure through a Friedel–
Crafts reaction to construct the bridged structure 2. BCBP was
afforded by the Suzuki-coupling reaction of 2 with 9-(4-(4,4,5,5-
tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)-9H-carbazole. These
compounds were fully characterized by 1H NMR, 13C NMR, mass
spectrometry, and elemental analysis.
Colorless crystals of BCBP suitable for X-ray crystallographic
analysis were grown by carefully layering a dichloromethane
solution of BCBP with ethanol. The molecular structure by
ORTEP drawing is shown in Fig. 1. The geometry of BCBP
reveal a torsion angle of 20.8ꢀ between the carbazole and phenyl
ring on the briged moiety, which is much smaller than 56.3ꢀ
between the carbazole and phenyl ring on the non-bridged
moiety. The value of 56.3ꢀ is close to the torsion angle 49.5ꢀ in
CBP.11
Fig. 2 TGA thermogram of BCBP at a heating rate of 10 ꢀC minꢁ1
Inset: DSC traces of BCBP recorded at a heating rate of 10 ꢀC minꢁ1
.
.
asymmetric molecular design may decrease the crystallization
tendency. As a consequence, the novel compound can form
morphologically stable and uniform amorphous films, an
essential property for OLEDs upon thermal evaporation.
Thermal analysis
The bridge linkage of 9-phenyl-9H-carbazole is strongly benefi-
cial to the thermal stability of the new compound,12 as indicated
by the high decomposition temperature (Td, corresponding to 5%
weight loss) of 412 ꢀC in the thermogravimetric analysis (Fig. 2).
Noticeably, the BCBP shows a glass-transition temperature (Tg)
of 173 ꢀC, which is much higher than those of carbazole ana-
logues,7b such as CBP (62 ꢀC) and mCP (60 ꢀC).9 In addition, the
Electrochemical analysis
The electrochemical property of BCBP was studied by cyclic
voltammetry (CV), and the compound shows two irreversible
oxidation potentials in CH2Cl2 solution at 0.81 and 1.02 V,
respectively (Fig. 3, CV of CBP was also measured under iden-
tical conditions for comparison). The HOMO energy level
determined from the onset of the oxidation is ꢁ5.45 eV (relative
to vacuum energy level). This value is significantly higher than
that of CBP (ꢁ5.7 eV), indicating that the bridge linkage could
raise the HOMO energy level, and thus lead to a lower barrier of
hole-injection. The LUMO energy level is estimated to be
ꢁ2.13 eV from the difference between the HOMO level and the
optical band gap.
Photophysical properties
Fig. 4 shows the absorption (in CH2Cl2), fluorescence (in
toluene), and phosphorescence (in a frozen toluene matrix
Scheme 1 Synthesis of BCBP.
Fig. 1 ORTEP diagram of BCBP. (For clarity, atoms are shown with
30% thermal ellipsoids probability; hydrogen atoms and disordered
solvent molecules of EtOH and CH2Cl2 in BCBP are omitted).
Fig. 3 Cyclic voltammograms of BCBP and CBP.
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Fig. 6 Power efficiency and current efficiency versus current density
Fig. 4 Absorption and emission spectra of BCBP at room temperature,
curves of device I to device III.
and phosphorescence spectrum of BCBP in toluene at 77 K.
at 77 K) spectra of BCBP. The absorptions at 293 and around
329 nm can be attributed to p–p* transitions of carbazole and
bridged-carbazole subunits, respectively. BCBP exhibits ultra-
violet fluorescence emission at 374 nm at room temperature. The
triplet energy (ET) is determined to be ca. 2.60 eV by the highest-
energy vibronic sub-band of the phosphorescence spectrum. This
value is close to that of CBP (2.56 eV).9 The photophysical results
indicate evidently that there is little effect on the triplet energy
from the partial planarization.
Table 1 EL data of the devices with BCBP as host
a
b
c
d
Von Lmax
[V]
hc.max
hp.max
hext.max
Device Host/guest
[cd/m2] [cd/A)] [lm/W] [%]
I e
II
BCBP/Ir(ppy)3 2.9
48 420 52.7
4613 9.0
44 733 52.8
41.1
6.3
35.5
13.7
4.0
13.6
BCBP/FIrpic
CBP/Ir(ppy)3
3.5
3.3
III
a
b
c
Maximum luminance. Maximum current efficiency. Maximum
power efficiency. d Maximum external quantum efficiency.
Electroluminescence properties
curves of the devices are shown in Fig. 5 and Fig. 6, respectively.
The device data are summarized in Table 1. The devices show
typical emissions from the Ir complexs with CIE coordinates of
(0.26, 0.65) for device I and (0.11, 0.34) for device II.
We investigated the utility of BCBP as the host material for blue
and green phosphorescent devices with the following configura-
tions: ITO/MoO3 (8 nm)/1,4-bis[(1-naphthylphenyl)- amino]-
biphenyl (NPB, 40 nm)/9% Ir(ppy)3:BCBP (25 nm) (device I) or
7% FIrpic:BCBP (25 nm) (device II) or 9% Ir(ppy)3: CBP (25 nm)
Device I with Ir(ppy)3 as the dopant exhibits a maximum
current efficiency of 52.7 cd/A at 0.40 mA/cm2, equivalent to an
external quantum efficiency (EQE) of 13.7%, and a maximum
luminance of 48 420 cd/m2. The device turns on at a rather low
voltage of 2.9 V, which may be attributed to the low hole-injec-
tion barrier of BCBP (HOMO ¼ ꢁ5.45 eV) as indicated by the
energy level diagram (Fig. 7).13 High quantum efficiency along
with low voltage gives a maximum power efficiency of 41.1 lm/W.
At the practical brightness of 100 cd/m2 and 1000 cd/m2, the
power efficiencies are still as high as 39.6 lm/W and 23.7 lm/W,
(device
III)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline
(BCP, 10 nm)/tris(8-hydroxyquinoline)aluminium (Alq3, 30 nm)/
LiF (1 nm)/Al (100 nm). Device III is constructed for compar-
ison. NPB and Alq3 are used as the hole- and electron-trans-
porting materials, respectively; Ir(ppy)3 (ET ¼ 2.42 eV) or FIrpic
(ET ¼ 2.62 eV) doped in host BCBP is used as the emitting layer,
with optimized doping levels of Ir(ppy)3 at 9 wt% and FIrpic at
7 wt%; BCP is used as hole and exciton blocking layer; MoO3
and LiF serve as hole- and electron-injecting layer, respectively.
The luminance–voltage–current density (L–V–J) characteristics,
and current efficiency and power efficiency versus current density
Fig. 5 Luminance–voltage–current density (L–V–J) curves of device I to
Fig. 7 Proposed energy level diagram showing HOMO and LUMO
device III.
levels for the materials used in devices.
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respectively. We note that the power efficiency of the BCBP-
hosted device I is enhanced as compared with CBP-hosted device
III. Device II with FIrpic as the dopant shows a maximum
current efficiency of 9.0 cd/A at 0.52 mA/cm2 (EQE of 4.0%),
a maximum power efficiency of 6.3 lm/W, and a maximum
luminance of 4613 cd/m2. The device data are comparable with
those using CBP/Firpic as the host/guest system under the
configuration of ITO/CuPc/NPB/CBP:6%FIrpic/BAlq/LiF/Al
reported by Forrest and coworkers.4a
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Conclusions
In summary, we have designed and synthesized a novel CBP
derivative (BCBP) through diarylmethylene-bridged 9-phenyl-
9H-carbazole. The asymmetric BCBP with confined carbazole
shows a remarkably enhanced glass transition temperature
(173 ꢀC) as compared to a low Tg of 62 ꢀC for CBP. Moreover the
increased molecular size does not decrease its triplet energy in
comparison with CBP. Utilizing the new compound as host
material, green phosphorescent OLEDs with Ir(ppy)3 as emitter
exhibit a low turn-on voltage of 2.9 V and a high power efficiency
of 41.1 lm/W; blue phosphorescent OLEDs with FIrpic as
a dopant show a maximum current efficiency of 9.0 cd/A, and
a maximum power efficiency of 6.3 lm/W. This work reveals that
aryl bridge linkages could be a good approach to fulfil the trade
off between the increasing molecular size and constraint of
molecular conjugation.
P.-I. Shih, C.-H. Chien, C.-Y. Chuang, C.-F. Shu, C.-H. Yang,
J.-H. Chen and Y. Chi, J. Mater. Chem., 2007, 17, 1692.
Acknowledgements
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M. E. Thompson, Chem. Mater., 2004, 16, 4743; (b) J.-J. Lin,
W.-S. Liao, H.-J. Huang, F.-I. Wu and C.-H. Cheng, Adv. Funct.
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We thank the National Natural Science Foundation of China
(Project Nos. 50773057 and 20474047) and the Ministry of
Science and Technology of China through the 973 program
(grant no. 2009CB623602, 2009CB930603, 2009CB623604) for
financial support.
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J. Mater. Chem., 2009, 19, 7661–7665 | 7665