High thermal-stability benzocarbazole derivatives as bipolar host materials for phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1016/j.dyepig.2015.07.040

Two novel host materials based on 5- and 9-substituted benzocarbazole have been developed for application in phosphorescent organic light-emitting diodes. Both host materials exhibit excellent thermal stability with high decomposition temperatures of ca.515 °C, while the 9-substituted benzocarbazole derivative exhibits a high glass transition temperature of 181 °C. Additionally, the electroluminescence characteristics of green phosphorescent organic light-emitting diodes made with two new host materials were also evaluated. The devices exhibited an electroluminescent emission peak at 528 nm with CIE coordinates of (0.31, 0.64) and possessed low turn-on voltages of <2.84 V.

Full text of DOI:10.1016/j.dyepig.2015.07.040

Dyes and Pigments 123 (2015) 196e203
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Review
High thermal-stability benzocarbazole derivatives as bipolar host
materials for phosphorescent organic light-emitting diodes
,
,
, **
Yi Chen a 1, Wenqing Liang b, Wing Hong Choi c, Jinhai Huang a *, Qingchen Dong b
,
Furong Zhu c ***, Jianhua Su a
,
, 1
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China
b MOE Key Laboratory for Interface Science and Engineering in Advanced Materials and Research Center of Advanced Materials Science and Technology,
Taiyuan University of Technology, 79 Yingze West Street, Taiyuan 030024, PR China
c Department of Physics and Institute of Advanced Materials, Hong Kong Baptist University, 224 Waterloo Road, Kowloon Tong, Hong Kong
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel host materials based on 5- and 9-substituted benzocarbazole have been developed for
application in phosphorescent organic light-emitting diodes. Both host materials exhibit excellent
thermal stability with high decomposition temperatures of ca.515 ꢀC, while the 9-substituted benzo-
carbazole derivative exhibits a high glass transition temperature of 181 ꢀC. Additionally, the electrolu-
minescence characteristics of green phosphorescent organic light-emitting diodes made with two new
host materials were also evaluated. The devices exhibited an electroluminescent emission peak at
528 nm with CIE coordinates of (0.31, 0.64) and possessed low turn-on voltages of <2.84 V.
© 2015 Elsevier Ltd. All rights reserved.
Received 12 May 2015
Received in revised form
28 July 2015
Accepted 29 July 2015
Available online 10 August 2015
Keywords:
Green phosphorescent OLEDs
Host materials
Bipolar
Benzocarbazole derivatives
High thermal stability
Low turn-on voltages
1. Introduction
the emitting layer [5,6]; iii) a high glass transition temperature (Tg)
to enhance thermal and morphological stability during device
Phosphorescent organic light-emitting diodes (PHOLEDs) have
attracted worldwide attention for application in flat panel displays
and area lighting. The heavy-metal complexes employed allows
harvesting of both singlet and triplet excitons for realizing 100%
internal quantum efficiency in the PHOLEDs [1]. To prevent effi-
ciency roll-off induced by concentration quenching and triplee-
triple annihilation, the heavy-metal phosphors are usually doped in
a host matrix [2,3]. Thus, the choice of host materials is crucial to
realize high efficiency PHOLEDs. In general, the following host
material properties are a prerequisite for achieving efficient PHO-
LEDs: i) the triplet energy level (ET) should be higher than the
dopant to prevent the energy transfer from guest to host [4]; ii) the
holeeelectron mobility is balanced to confine the excitons within
operation that could extend the operational lifetime of the device
[7,8].
In recent years, tremendous efforts have been devoted to
developing the host materials with desired characteristics [9e11].
Among them, the carbazole-based materials are widely used as a
host in the PHOLEDs to improve the device efficiency [12e14]. For
example, the carbazole-based material 4,40-bis(N-carbazolyl)-1,10-
biphenyl (CBP) with a high triplet energy level and excellent hole
transporting property is commonly used as the host material for
green and red PHOLEDs [15e18]. However, the glass transition
temperature of CBP is low (62 ꢀC), which would shorten the device
lifetime due to the crystallization at a low dopant concentration
[19].
To increase the glass transition temperature, two novel host
materials 9-(dibenzo[b,d]thiophen-4-yl)-7-(4-(1-phenyl-1H-benzo
[d]imidazole-2-yl)phenyl)-7H-benzo[c]carbazole (DYNS) and 5-
* Corresponding author. Tel./fax: þ86 21 64252288.
** Corresponding author.
*** Corresponding author.
(dibenzo[b,d]thiophen-4-yl)-7-(4-(1-phenyl-1H-
benzo[d]imid-
azole-2-yl)phenyl)-7H-benzo[c]carbazole (DYSN) which contain a
benzocarbazole core were designed and synthesized. In addition,
the introduction of dibenzothiophene on the 5- or 9- position of
E-mail addresses: dele12@163.com (J. Huang), dongqingchen@tyut.edu.cn
(Q. Dong), frzhu@hkbu.edu.hk (F. Zhu).
1
Tel./fax: þ86 21 64252288.
http://dx.doi.org/10.1016/j.dyepig.2015.07.040
0143-7208/© 2015 Elsevier Ltd. All rights reserved.

Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
197
benzocarbazole core was expected to enhance the molecular
2.2.2. Synthesis of 9-bromo-7H-benzo[c]carbazole (1e2)
distortion and reduce the intramolecular charge transfer for
increasing their triplet energy levels and thermal stability.
A
mixture of 1-(4-bromo-2-nitrophenyl) naphthalene 1e1
(3.27 g, 10 mmol) and triphenylphosphine (5.25 g, 20 mmol) in 1,2-
dichlorobenzene 20 mL was added into a round bottom flask, then
heated to reflux for 5 h. After cooling to room temperature, the
mixture was filtered under vacuum and washed with dichloro-
methane three times. The combined organic layer was dried over
anhydrous sodium sulfate. The solvent was removed in vacuum and
the crude product was purified by SiO2 column chromatography,
Furthermore, we also imported
a commonly used electron-
deficient moiety benzimidazole to modify the bipolar charge
transport property of the new compounds.
2. Experimental
affording the yellow solid (1.53 g, 52%). IR (KBr, disk)
3061.90, 1584.84, 1529.22, 1467.48, 1433.99 cmꢁ1
(400 MHz, CDCl3,
J ¼ 8.4 Hz, 1H), 8.01 (d, J ¼ 8.4 Hz, 1H), 7.89 (d, J ¼ 8.8 Hz, 1H),
7.76e7.68 (m, 2H), 7.63 (d, J ¼ 8.8 Hz, 1H), 7.50 (t, J ¼ 7.6 Hz, 2H).
HRMS (ESI, m/z): [MꢁH]þ calcd for: C16H9NS, 293.9918, found,
293.9920.
n
3404.88,
2.1. Materials and measurement
.
1H NMR
d
): 8.69 (d, J ¼ 8.4 Hz, 1H), 8.47 (s, 1H), 8.41 (d,
Chemicals and solvents used in the process were reagent grades
and purchased from J & K Chemical Co. and Aladdin Chemical Co.
without further purification. Tetrahydrofuran (THF) was disposed
by primary procedures. All reactions and manipulations were car-
ried out under N2 atmosphere. Silica gel (300e400 mesh) column
chromatography was used as the stationary phase in the column. 2-
(4-bromophenyl)-1-phenyl-1H-benzo [d] imidazole was synthe-
sized following the method reported in the literature [20,21].
The 1H and 13C NMR spectra were measured using a Bruker AM
400 spectrometer. Mass spectra were obtained on a Waters LCT
Premier XE spectrometer. The IR spectra were recorded in the range
4000e600 cmꢁ1 using the potassium bromide disk for solid sam-
ples by the FTIR instrument. The ultravioletevisible (UVeVis) ab-
sorption spectra of the samples were characterized using a Varian
Cary 500 spectrophotometer. Photoluminescence (PL) measure-
ments were conducted by a Varian-Cary fluorescence spectropho-
tometer at room temperature. The cyclic voltammetry experiments
were performed by a Versastat II electrochemical workstation
(Princeton applied research) using a conventional three-electrode
configuration with a glassy carbon working electrode, a Pt wire
counter electrode, and a regular calomel reference electrode in
2.2.3. Synthsis of 9-(dibenzo[b,d]thiophen-4-yl)-7H-benzo[c]
carbazole (1e3)
A
mixture of 9-bromo-7H-benzo[c]carbazole 1e2 (1.16 g,
4
mmol), dibenzo [b,d] thiophen-4-ylboronic acid (1.00 g,
4.4 mmol) in tetrahydrofuran (20 mL) and 2 M K2CO3 (20 mL) was
added into a round bottom flask and bubbled with argon stirring for
15 min. Pd(PPh3)4 (0.092 g, 0.08 mmol) was added to the mixture,
and the reaction was refluxed for 5 h under argon atmosphere. The
reaction mixture was cooled down to room temperature, poured
into H2O and then extracted with dichloromethane three times. The
combined organic layer was dried over anhydrous sodium sulfate.
The solvent was removed in vacuum and the crude product was
purified by SiO2 column chromatography, affording an orange solid
(1.20 g, 75%). IR (KBr, disk)
n
3402.79, 3061.27, 1440.63,
): 8.86 (d, J ¼ 8.4 Hz, 1H),
1382.53 cmꢁ1. 1H NMR (400 MHz, CDCl3,
d
saturated KCl solution, 0.1
M
tetrabutylammonium hexa-
8.71 (d, J ¼ 8.4 Hz,1H), 8.57 (s,1H), 8.24 (dd, J ¼ 8.8, 6.4 Hz, 2H), 8.06
(d, J ¼ 8.0 Hz, 1H), 7.98 (s, 1H), 7.92 (t, J ¼ 7.2 Hz, 1H), 7.88 (dd,
J ¼ 6.0, 2.4 Hz, 1H), 7.84e7.75 (m, 2H), 7.71e7.62 (m, 3H), 7.58e7.48
(m, 3H). HRMS (ESI, m/z): [MꢁH]þ calcd for: C28H16NS, 398.1003,
found, 398.1007.
fluorophosphate (TBAPF6) in dichloromethane solution as the
supporting electrolyte with a scan rate of 100 mV/s. The E1/2 values
were determined by (Epa þ Epc)/2 using Ferrocene as an external
standard, where Epa and Epc were the anodic and catholic peak
potentials, respectively. The differential scanning calorimetry (DSC)
analysis was performed under a nitrogen atmosphere using a
NETZSCH STA 409 PC/PG instrument with a heating scan rate of
10 ꢀC/min. Thermogravimetric analysis (TGA) was carried out using
a TGA instrument under a nitrogen atmosphere with a heating scan
rate of 10 ꢀC/min.
2.2.4. Synthesis of 9-(dibenzo[b,d]thiophen-4-yl)-7-(4-(1-phenyl-
1H-benzo[d] imidazole-2-yl)phenyl)-7H-benzo[c]carbazole (DYNS)
A mixture of 9-(dibenzo[b,d]thiophen-4-yl)-7H-benzo[c]carba-
zole 1e3 (0.099 g, 0.25 mmol), 2-(4-bromophenyl)-1-phenyl-1H-
benzo[d]imidazole (0.13 g, 0.375 mmol) in m-xylene (20 mL) and
potassium tert-butoxide (0.034 g, 0.3 mmol) were added into a
round bottom flask and bubbled with argon stirring for 15 min.
Pd(OAc)2 (2.8 mg, 0.0125 mmol) and 2-dicyclohexylphosphino-
20,40,60-triisopropylbiphenyl (XPhos, 8.8 mg, 0.025 mmol) were
added to the mixture, and the reaction was refluxed overnight
under argon atmosphere. After cooling down to room temperature,
the reaction mixture was extracted with dichloromethane three
times, and the organic layer was dried over anhydrous sodium
sulfate. The solvent was removed in vacuum and the crude product
was purified by SiO2 column chromatography, affording a white
2.2. Synthesis
2.2.1. Synthesis of 1-(4-bromo-2-nitrophenyl) naphthalene (1e1)
A mixture of naphthalen-1-ylboronic acid (3.44 g, 20 mmol),1,4-
dibromo-2-nitrobenzene (6.13 g, 22 mmol) in tetrahydrofuran
(100 mL) and 2 M K2CO3 (100 mL) was added into a round bottom
flask and bubbled with argon whilst stirring for 15 min. Pd(PPh3)4
(0.46 g, 0.4 mmol) was added to the mixture, and the resulting
mixture was refluxed for 5 h under argon atmosphere. The reaction
mixture was cooled down to room temperature, poured into H2O
and then extracted with dichloromethane three times. The com-
bined organic layer was dried over anhydrous sodium sulfate. The
solvent was removed in vacuum and the crude product was puri-
fied by SiO2 column chromatography, affording a white solid
solid (0.10 g, 60%). IR (KBr, disk)
1450.61, 1434.95, 744.64, 699.88 cmꢁ1
1H NMR (400 MHz, CDCl3,
n 3051.22, 1602.70, 1586.50,
.
): 8.94 (dd, J ¼ 16.0, 8.4 Hz, 2H),
d
8.50e8.38 (m, 2H), 8.15 (d, J ¼ 8.0 Hz, 1H), 8.05 (d, J ¼ 8.8 Hz, 2H),
7.92e7.77 (m, 8H), 7.73e7.62 (m, 3H), 7.62e7.50 (m, 7H), 7.43 (d,
J ¼ 4.0 Hz, 1H), 7.38e7.27 (m, 2H), 7.23 (d, J ¼ 8.0 Hz, 1H). 13C NMR
(3.33 g, 51%). IR (KBr, disk)
n
3058.26, 1529.69, 1348.88, 800.66,
): 8.23 (d, J ¼ 1.7 Hz, 1H),
776.71 cmꢁ1. 1H NMR (400 MHz, CDCl3,
d
(101 MHz, DMSO, d): 150.92, 142.58, 139.36, 138.46, 138.33, 137.53,
7.92 (d, J ¼ 8.2 Hz, 2H), 7.84 (dd, J ¼ 8.2, 1.9 Hz, 1H), 7.51 (td, J ¼ 7.9,
3.4 Hz, 2H), 7.42 (d, J ¼ 3.7 Hz, 2H), 7.38 (d, J ¼ 8.2 Hz, 1H), 7.32 (d,
J ¼ 7.0 Hz, 1H). HRMS (ESI, m/z): [MþK]þ calcd for: C16H10BrNO2,
365.9526, found, 365.9521.
137.16, 137.07, 136.62, 136.40, 136.31, 136.00, 135.30, 130.91, 130.03,
129.39, 129.34, 129.27, 128.99, 128.89, 128.25, 127.69, 127.54, 127.42,
127.28, 127.12, 125.79, 124.96, 123.76, 123.62, 123.26, 123.07, 122.99,
122.92, 122.74, 122.31, 121.28, 121.12, 119.50, 114.81, 111.57, 110.54,

198
Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
109.47. HRMS (ESI, m/z): [MþH]þ calcd for: C47H29N3S, 668.2160,
2.2.6. Synthesis of 1-bromo-4-(2-nitrophenyl)naphthalene (2-2), 5-
bromo- 7H- benzo[c]carbazole (2e3) and 5-(dibenzo[b,d]thiophen-
4-yl)-7H-benzo[c]carbazole (2e4)
found, 668.2163.
To a round bottom flask were added 1-(2-nitrophenyl) naph-
thalene 2e1 (2.49 g, 10 mmol) and carbon tetrachloride (5 mL). The
Br2 (0.5 mL, 10 mmol) dissolved in 10 mL carbon tetrachloride was
added to the mixture and the reaction was refluxed for 24 h. After
cooling down to room temperature, the solvent was removed in
vacuum and the crude product 2e2 was obtained. A mixture of
crude product 2e2, triphenylphosphine (5.25 g, 20 mmol) in 1,2-
dichlorobenzene 20 mL was added into a round bottom flask,
then heated to reflux for 5 h. After cooling to room temperature, the
mixture was filtered under vacuum and washed with dichloro-
methane three times. The solvent was removed in vacuum and the
crude product 2e3 was obtained and used in the reaction without
further purification. The crude product 2e3 and dibenzo[b,d] thi-
ophen-4-ylboronic acid (2.51 g, 11 mmol) were dissolved in tetra-
hydrofuran (50 mL) and 2 M K2CO3 (50 mL) and the reaction
2.2.5. Synthesis of 1-(2-nitrophenyl) naphthalene (2e1)
A mixture of naphthalen-1-ylboronic acid (3.44 g, 20 mmol), 1-
bromo-2-nitrobenzene (4.42 g, 22 mmol) in tetrahydrofuran
(100 mL) and 2 M K2CO3 (100 mL) was added into a round bottom
flask and bubbled with argon stirring for 15 min. Pd(PPh3)4 (0.46 g,
0.4 mmol) was added to the mixture, and the resulting mixture was
refluxed for 5 h under argon atmosphere. After cooling down to
room temperature, the reaction mixture was extracted with
dichloromethane three times. The combined organic layer was
dried over anhydrous sodium sulfate. The solvent was removed in
vacuum and the crude product was purified by SiO2 column chro-
matography, affording a white solid (3.44 g, 69%). 1H NMR
(400 MHz, CDCl3,
7.71 (t, J ¼ 7.6 Hz, 1H), 7.61 (t, J ¼ 7.6 Hz, 1H), 7.55e7.38 (m, 5H), 7.35
(d, J ¼ 7.2 Hz, 1H).
d
): 8.08 (d, J ¼ 8.0 Hz, 1H), 7.91 (d, J ¼ 8.0 Hz, 2H),
Scheme 1. Synthetic routes of DYNS and DYSN.

Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
199
solid (0.10 g, 60%). IR (KBr, disk)
n
3048.78, 1604.53, 1560.18,
1485.62, 1452.81, 763.42, 694.96 cmꢁ1. 1H NMR (400 MHz, DMSO,
DYNS
DYSN
d
): 9.06 (d, J ¼ 8.0 Hz, 1H), 8.84 (d, J ¼ 7.6 Hz, 1H), 8.53 (d, J ¼ 7.6 Hz,
1H), 8.48 (d, J ¼ 7.6 Hz, 1H), 7.92 (d, J ¼ 8.0 Hz, 1H), 7.82 (d,
J ¼ 8.0 Hz, 4H), 7.74 (dd, J ¼ 12.4, 8.4 Hz, 4H), 7.63 (d, J ¼ 6.4 Hz, 2H),
7.59e7.43 (m, 10H), 7.33 (dd, J ¼ 15.6, 5.6 Hz, 3H), 7.20 (d, J ¼ 8.0 Hz,
1H). 13C NMR (100 MHz, CDCl3,
d): 151.46, 142.96, 140.95, 140.02,
139.75, 138.13, 137.59, 137.52, 137.34, 136.78, 135.96, 135.77, 130.92,
130.35,130.02, 129.35, 128.85, 128.55, 127.44, 127.29, 126.84, 124.87,
124.65, 124.45, 124.04, 123.78, 123.62, 123.46, 123.19, 122.82,
122.30, 121.81, 121.20, 120.75, 119.95, 116.16, 112.73, 110.55, 110.43.
HRMS (ESI, m/z): [MþH]þ calcd for: C47H29N3S, 668.2160, found,
668.2155.
2.3. OLED fabrication
Organic layers were deposited on the indium-tin-oxide (ITO)/
glass substrate by thermal evaporation. The ITO/glass substrate was
cleaned sequentially by detergent, de-ionized water and ethanol.
Then the ITO/glass was treated by ozone and polymerized fluoro-
carbon (CFx) plasma before loading into a 10-source evaporator,
with a base pressure of 5.0 ꢂ 10ꢁ4 Pa, for device fabrication. All
devices were encapsulated in a N2 purged glove box connected to
the evaporator after the fabricated for device characterization.
The currentevoltageeluminance characteristics of the PHOLEDs
were measured with a diode array rapid scan system using a Photo
Research PR650 spectrophotometer and a computer-controlled,
programmable, direct-current (DC) source. All measurements
were carried out in ambient atmosphere at room temperature.
oC)
Fig. 1. The thermal properties of DYNS and DYSN host materials.
bubbled with argon stirring for 15 min. Pd(PPh3)4 (0.23 g,
0.2 mmol) was added to the mixture, and the reaction was refluxed
for 5 h under argon atmosphere. After cooling down to room
temperature, the reaction mixture was extracted with dichloro-
methane three times, and the organic layer was dried over anhy-
drous sodium sulfate. The solvent was removed in vacuum and the
crude product was purified by SiO2 column chromatography,
affording a white solid (2.8 g, yield 65%). IR (KBr, disk)
n
3048.78,
): 8.93
3061.12, 1445.79, 1354.83 cmꢁ1. 1H NMR (400 MHz, CDCl3,
d
3. Results and discussion
(d, J ¼ 8.4 Hz, 1H), 8.67 (d, J ¼ 8.0 Hz, 1H), 8.56 (s, 1H), 8.29 (dd,
J ¼ 13.2, 7.8 Hz, 2H), 7.86e7.71 (m, 4H), 7.65 (dd, J ¼ 16.4, 7.6 Hz,
3H), 7.56e7.42 (m, 4H), 7.38 (t, J ¼ 7.6 Hz, 1H). HRMS (ESI, m/z):
[MþH]þ calcd for: C28H18NS, 400.1160, found, 400.1160.
3.1. Synthesis and characterization
The structures of target molecules and their synthetic routes
were outlined in Scheme 1. Here, two key intermediate compounds
9-(dibenzo [b,d]thiophen-4-yl)- 7H-benzo[c] carbazole (1e3) and
5-(dibenzo[b,d]thiophen-4-yl)-7H-benzo[c]carbazole (2e4) were
prepared through multiple steps according to the methods re-
ported in the litertures [22,23]. Subsequently, the desired products
(DYNS and DYSN) were obtained by the palladium catalyzed
BuchwaldeHartwig cross coupling reaction of benzocarbazole
species (1e3 and 2e4) and bromide intermediate 2-(4-
bromophenyl)-1-phenyl-1H-benzo[d]imidazole in 60% yield. The
two target compounds were identified and characterized by 1H and
13C NMR as well as the high-resolution mass spectrometry (HRMS).
2.2.7. Synthesis of 5-(dibenzo[b,d]thiophen-4-yl)-7-(4-(1-phenyl-
1H-benzo[d] imidazole-2-yl)phenyl)-7H-benzo[c]carbazole (DYSN)
A mixture of 5-(dibenzo[b,d]thiophen-4-yl)-7H-benzo[c]carba-
zole 2e4 (0.099 g, 0.25 mmol), 2-(4-bromophenyl)-1-phenyl-1H-
benzo[d]imidazole (0.13 g, 0.375 mmol) in m-xylene (20 mL) and
potassium tert-butoxide (0.034 g, 0.3 mmol) were added into a
round bottom flask and bubbled with argon stirring for 15 min.
Pd(OAc)2 (2.8 mg, 0.0125 mmol) and 2-dicyclohexylphosphino-
20,40,60-triisopropylbiphenyl (XPhos, 8.8 mg, 0.025 mmol) were
added to the mixture, and the reaction was refluxed overnight
under argon atmosphere. After cooling down to room temperature,
the reaction mixture was extracted with dichloromethane three
times, and the organic layer was dried over anhydrous sodium
sulfate. The solvent was removed in vacuum and the crude product
was purified by SiO2 column chromatography, affording a white
3.2. Thermal properties
The thermal gravimetric analyses (TGA) and differential scan-
ning calorimetry (DSC) measurements were carried out under
Table 1
Properties of DYNS and DYSN host materials.
Compound
lmax abs (nm)a
lmax em (nm)a
lmax em (nm)b
Eg (eV)c
HOMO (eV)d
LUMO (eV)e
ET (eV)f
Tg (ꢀC)g
Td (ꢀC)g
DYNS
DYSN
372
372
388
387
398
397
3.21
3.22
ꢁ5.83
ꢁ5.78
ꢁ2.62
ꢁ2.56
2.45
2.45
NA
181
513
517
a
Measured in dichloromethane.
b
c
d
e
f
Measured in film.
Estimated from onset of the absorption spectra (EOg pt ¼ 1241/lonset).
Calculated from cyclic voltammetry.
Calculated by the equation EHOMO ¼ ELUMO ꢁ Eg.
Calculated by the first peak of phosphorescence spectra measured at 77 k.
Measured by DSC and TGA.
g

200
Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
nitrogen atmosphere to investigate the thermal properties of DYNS
and DYSN; the related data for DYNS and DYSN were displayed in
Fig. 1 and summarized in Table 1. The thermal decomposition
temperatures (Td), corresponding to 5% weight loss, were 513 and
517 ꢀC for DYNS and DYSN, respectively, while the glass transition
temperature (Tg) of DYSN was 181 ꢀC. However, the glass transition
temperature of DYNS was not observed even in the second DSC
scan. From the above data new host materials exhibit high thermal
ability, which can be mainly attributed to the high molecule weight
of them [24]. Besides, the Td value of DYSN was obviously higher
than that of DYNS, which could be ascribed to the more steric
molecular structure of DYSN [25e27].
1.2
1.0
0.8
0.6
0.4
0.2
0.0
a)
UV-Vis solution
FL solution
FL solid film
LT PL
E
T
3.3. Optical properties
The photophysical properties of DYNS and DYSN were investi-
gated by ultravioletevisible (UVevis) and photoluminescence (PL)
spectroscopy, and the detailed data were shown in Fig. 2 and
Table 1. As shown in the spectra, DYNS and DYSN exhibit similar
UVevis absorption profile with three characteristic absorption
peaks in the range of 270e380 nm. The low energy band around
300
400
500
Wavelength (nm)
600
700
1.2
1.0
0.8
0.6
0.4
0.2
0.0
UV-Vis solution
FL solution
FL solid flim
LT PL
b)
370 nm can be assigned to the intramolecular charge transfer pep*
transitions from the electron-donating group benzocarbazole to the
electron-accepting moiety benzimidazole, whereas the two higher
energy bands at around 330 nm and 270 nm could be attributed to
the nep* transitions of carbazole and pep* transitions of N-phe-
nylcarbazole [28], respectively. The corresponding optical band
gaps of DYNS and DYSN estimated from the absorption edge of
UVevis spectra were 3.21 eV and 3.22 eV, respectively.
It can be seen from the PL spectra that, the maximum emission
peaks of DYNS and DYSN were 388 nm and 387 nm in solution,
whereas the maximum emission peaks of DYNS and DYSN in the
solid state were red shifted to 398 nm and 397 nm, respectively,
owing to the intermolecular p-stacking in the solid state [29]. The
maximum PL emission peaks of DYNS and DYSN at 390 nm are
completely overlapped with the maximum absorption peak of
ET
300
400
500
wavelength (nm)
600
700
€
Ir(ppy)2acac [30], implying that the Forster energy could be effi-
ciently transferred from host to dopant. Additionally, the phos-
phorescence spectra measured in 2-methyltetrahydrofuran at 77 k
were also shown in Fig. 2. The triplet energy level of DYNS evalu-
ated from the highest-energy vibronic sub-bands [31] was 2.45 eV,
which was identical to that of DYSN.
Fig. 2. UVeVis, PL and low temperature PL spectra of a) DYNS and b) DYSN.
3.4. Electrochemical properties and energy level
The electrochemical behaviors of DYNS and DYSN were inves-
tigated by cyclic voltammetry (CV) with the tetra (n-butyl)
ammonium hexauorophosphate (TBAPF6) in CH2Cl2 as supporting
electrolytes and the results were exemplified in Fig. 3 and Table 1.
The highest occupied molecular orbital (HOMO) energy levels of
DYNS and DYSN were calculated from the onset oxidation poten-
tials by the equation: EHOMO ¼ ꢁEox ꢁ 4.4 eV [32], while the lowest
unoccupied molecular orbital (LUMO) energy levels could be esti-
mated from the EHOMO and optical band gaps. The onset oxidation
potentials of DYNS and DYSN were measured as 1.43 V and 1.38 V,
respectively. Hence, the HOMO/LUMO energy levels of DYNS and
DYSN were calculated to be ꢁ5.83 eV/ꢁ2.60 eV and ꢁ5.78 eV/
ꢁ2.57 eV, respectively.
DYNS
DYSN
The energy levels of those related materials used in green
PHOLEDs in this work are shown in Fig. 4. As seen from the energy
level diagram, two host materials possess reasonable energy levels
with relatively higher HOMO and lower LUMO than that of Ir(p-
py)2acac [30], indicating that the holes and electrons in the two
host materials can be trapped by the dopant to facilitate the effi-
cient charge recombination in the emitting layer. Moreover, the
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
2.5
Potential (V)
Fig. 3. Cyclic voltammograms of compounds DYNS and DYSN in CH2Cl2 solution
containing 0.1 M TBAPF6 electrolytes, scanning rate: 100 mV/s.

Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
201
Fig. 4. The energy lever diagram of device and the molecular structure of materials used in PHOLEDs.
HOMO level of TmPyPB was 0.85 eV and 0.90 eV higher than that of
DYNS and DYSN, respectively, providing efficient hole-confinement
and improve the device efficiency [33].
structure and the related HOMO and LUMO energy levels of those
materials are presented in Fig. 4. As shown in Fig. 4, HAT-CN and LiF
act as hole injection layer (HIL) and electron injection layer (EIL),
respectively. TAPC and TmPyPB are employed as hole transport
layers (HTL) and electron transport layers (ETL), respectively. The
green phosphor Ir(ppy)2acac used as the emitting material, is co-
evaporated with the host materials to fabricate green PHOLED
device 1 (DYNS) and device 2 (DYSN).
The normalized EL spectra of device 1 and device 2 are shown in
Fig. 5. Each device exhibits the same green emission at 528 nm,
consisting with the triplet emission of Ir(ppy)2acac (520 nm) [30],
which indicates that the green emission peak solely originated
from the triplet metal ligand charge transfer (3MLCT) transition of
Ir(ppy)2acac. The CIE coordinates of the devices were measured as
(0.31, 0.64) by converting the EL spectra into chromaticity co-
ordinates on the CIE 1931 diagram.
3.5. Electroluminescence
In order to investigate the electroluminescence performance of
green PHOLEDs, both DYNS and DYSN were used as the host ma-
terials, the multilayer diodes with the structure of ITO/1,4,5,8,9,12-
hexaazatriphenylenehexacarbonitrile (HAT-CN, 5 nm)/4,40- (cyclo-
hexane-1,1-diyl)bis(N,N-di-p-tolylaniline) (TAPC, 65 nm)/Host:
bis(2-phenylpyridine)(acetylacetonato)iridium(III) (Ir(ppy)2acac,
7 wt%, 10 nm)/1,3,5-tri[(3-pyridyl)-phen-3-yl]benzene (TmPyPB,
50 nm)/LiF (1 nm)/Al (100 nm) were fabricated. The device
The current density-voltage (JeV) and luminance-voltage (LeV)
characteristics of device 1 and device 2 are plotted in Fig. 6, and the
1.0
Device 1
Device 2
0.8
600
Device 1
Device 2
Device 1
Device 2
10000
1000
100
10
0.6
0.4
0.2
0.0
400
200
0
400
500
600
700
800
1
0
2
4
6
8
10
Wavelength (nm)
Voltage (V)
Fig. 5. The normalized EL spectra of green PHOLEDs with DYNS (Device 1) and DYSN
(Device 2) host materials.
Fig. 6. JeVeL characteristics of PHOLEDs with DYNS and DYSN host materials.

202
Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
Table 2
Performance parameters of PHOLEDs with DYNS and DYSN host materials.
Turn-on voltage (V)a
Max brightness (cd/m2)
EQE (%)b
Power efficiency (lm/W)b
Current efficiency (cd/A)b
CIE(x,y)
c
Device
1
2
2.72
2.84
13096
8196
2.12
1.62
8.07
6.26
8.35
6.35
(0.31, 0.64)
(0.31, 0.64)
a
At 1 cd/m2.
b
c
Estimated at the luminance of 100 cd/m2.
Measured from the EL spectra by inverting into chromaticity coordinates on the CIE 1931 diagram.
corresponding data are summarized in Table 2. The turn-on volt-
ages of device 1 and device 2 at 1 mA/cm2 are 2.72 V and 2.84 V,
respectively. The maximum brightness of device 1 is 13,096 cd/m2
at the current density of 424 mA/cm2, and 8196 cd/m2 for device
2 at the current density of 393 mA/cm2. Both the current density
and luminance of the device1 (DYNS) are higher than that of device
2 (DYSN), suggesting that the DYNS can be a favorable host material
for application in green PHOLEDs. Moreover, it is worth noting that
both devices exhibited low turn-on voltage, benefiting from the
lower injection barrier and good carrier mobility.
Fig. 7 and Fig. 8, respectively. At 100 cd/m2, the current and power
efficiency are 8.07 cd/A and 8.35 lm/W for device 1, which were
higher than that of device 2 (6.26 cd/A and 6.35 lm/W). The external
quantum efficiencies of device 1 and device 2 were 2.12% and 1.62%,
respectively. The detailed electroluminescence data are summa-
rized in Table 2. The luminous efficiency of devices with DYNS and
DYSN as host materials are not high, this is because the triplet
energy level of Ir(ppy)2acac was 2.3 eV [30] which is only 0.15 eV
lower than that of DYNS and DYSN, due to the possible exciton
energy transfer from guest to host, leading to a low external
quantum efficiency. Another reason is that the energy-barrier for
electrons at the interface of the host and TAPC are only 0.08 eV and
0.02 eV, respectively, which could not efficiently confine the elec-
trons within the emissive-layer.
The dependence of current and power efficiency on luminance
measured for device 1 (DYNS) and device 2 (DYSN) are shown in
10
4. Conclusion
Device 1
Device 2
8
In summary, two benzocarbazole derived new host materials
were developed successfully via BuchwaldeHartwig cross-
coupling reaction for application in green PHOLEDs. The com-
pound DYSN that was synthesized by linking a dibenzothiophene
fragment to the 5- position of benzocarbazole presented non-
coplanar conformation, resulting in a steric bulk and relative high
Tg (181 ꢀC). PHOLEDs with DYNS and DYSN host materials possess a
low turn-on voltage of<2.84 V, with a maximum luminance of
13,096 cd/m2 and 8196 cd/m2, respectively. At the brightness of
100 cd/cm2, the power efficiency and the current efficiency were
8.35 lm/W and 8.07 cd/A for device 1 (DYNS), while 6.35 lm/W and
6.26 cd/A for device 2 (DYSN). Further optimization of devices with
high efficiency using DYNS and DYSN as host materials still de-
serves study.
6
4
2
0
0
2000 4000 6000 8000 10000 12000 14000
Luminance
cd/m2
(
)
Fig. 7. Current efficiency-Luminance characteristics of PHOLEDs.
Acknowledgments
Q. Dong thanks the financial support from the National Natural
Science Foundation of China (Grant No.: 61307030), Program for
the Outstanding Innovative Teams of Higher Learning Institutions
of Shanxi (OIT), Fund Program for the Scientific Activities of
Selected Returned Overseas Professionals in Shanxi Province, the
Natural Science Foundation for Young Scientists of Shanxi Province,
China (Grant No.: 2014021019-2), the Outstanding Young Scholars
Cultivating Program, Research Project Supported by Shanxi Schol-
arship Council of China (Grant No.: 2014-02) and the Qualified
Personal Foundation of Taiyuan University of Technology (Grant
No.: 2013Y003, tyut-rc201275a).
12
10
8
Device 1
Device 2
6
4
References
2
[1] Baldo MA, O'Brien DF, You Y, Shoustikov A, Sibley S, Thompson ME, et al.
Highly efficient phosphorescent emission from organic electroluminescent
devices. Nature 1998;395:151e4.
0
0
2000
4000
6000
8000 10000 12000 14000
[2] Suzukia H, Hoshino S. Effects of doping dyes on the electroluminescent
characteristics of multilayer organic light-emitting diodes.
1996;79:8816e22.
J Appl Phys
Luminance (cd/m2)
[3] Murawski C, Leo K, Gather MC. Efficiency roll-off in organic light-emitting
diodes. Adv Mater 2013;25:6801e27.
Fig. 8. Power efficiency-Luminance characteristics of PHOLEDs.

Y. Chen et al. / Dyes and Pigments 123 (2015) 196e203
203
[4] Inomata H, Goushi K, Masuko T, Konno T, Imai T, Sasabe H, et al. High-effi-
ciency organic electrophosphorescent diodes using 1,3,5-triazine electron
transport materials. Chem Mater 2004;16:1285e91.
[5] Chen CT, Wei Y, Lin JS, Moturu MVRK, Chao WS, Tao YT, et al. Doubly ortho-
linked quinoxaline/diphenylfluorene hybrids as bipolar, fluorescent chame-
leons for optoelectronic applications. J Am Chem Soc 2006;128:10992e3.
[6] Lundin NJ, Blackman AG, Gordon KC, Officer DL. Synthesis and characteriza-
tion of a multicomponent rhenium (I) complex for application as an OLED
dopant. Angew Chem Int Ed 2006;45:2582e4.
bipolar triphenylamine-benzimidazole derivatives. Adv Funct Mater 2008;18:
584e90.
[21] Ge Ziyi, Hayakawa Teruaki, Ando Shinji, Ueda Mitsuru, Akiike Toshiyuki,
Miyamoto Hidetoshi, et al. Solution-processible bipolar triphenylamine-
benzimidazole derivatives for highly efficient single-layer organic light-
emitting diodes. Chem Mater 2008;20:2532e7.
[22] Creencia EC, Kosaka M, Muramatsu T, Kobayashi M, Iizuka T, Horaguchi T.
Microwave-assisted Cadogan reaction for the synthesis of 2-aryl-2H-inda-
zoles, 2-aryl-1H-benzimidazoles, 2-carbonylindoles, carbazole, and phena-
zine. J Heterocycl Chem 2009;46:1309e17.
[23] Kim GH, Lampande R, Park MJ, Bae HW, Kong JH, Kwon JH, et al. Highly
efficient bipolar host materials with lndenocarbazole and pyrimidine moieties
for phosphorescent green light-emitting diodes. J Phys Chem C 2014;118:
28757e63.
[24] Jeon YM, Kim JW, Lee CW, Gong MS. Blue organic light-emitting diodes using
novel spiro[fluorene-benzofluorene]-type host materials. Dyes Pigments
2009;83:66e71.
[25] Hung WY, Chi LC, Chen WJ, Chen YM, Chou SH, Wong KT. A new benzimid-
azole/carbazole hybrid bipolar material for highly efficient deep-blue elec-
trofluorescence, yellowegreen electrophosphorescence, and two-color-based
white OLEDs. J Mater. Chem 2010;20:10113e9.
[7] Tao Y, Yang C, Qin J. Organic host materials for phosphorescent organic light-
emitting diodes. Chem Soc Rev 2011;40:2943e70.
[8] Loy DE, Koene BE, Thompson ME. Thermally stable hole-transporting mate-
rials based upon a fluorene core. Adv Funct Mater 2002;12:245e9.
[9] Joo Cho Yong, Soo Yook Kyoung, Yeob Lee Jun. Universal host material for high
external quantum efficiency close to 25% and long lifetime in green fluores-
cent and phosphorescent OLEDs. Adv Mater 2014;26:4050e5.
[10] Chou HH, Cheng CH. A highly efficient universal bipolar host for blue, green,
and red phosphorescent OLEDs. Adv Mater 2010;22:2468e71.
[11] Wang Q, Oswald IW, Yang X, Zhou G, Jia H, Qiao Q, et al. A non-doped
phosphorescent organic light-emitting device with above 31% external
quantum efficiency. Adv Mater 2014;26:8107e13.
[12] Lin SL, Chan LH, Lee RH, Yen MY, Kuo WJ, Chen CT, et al. Highly efficient
[26] Tao Y, Wang Q, Yang C, Wang Q, Zhang Z, Zou T, et al. A simple carbazole/
oxadiazole hybrid molecule: an excellent bipolar host for green and red
phosphorescent OLEDs. Angew Chem Int Ed Engl 2008;47:8104e7.
[27] Tsai MH, Hong YH, Chang CH, Su HC, Wu CC, Matoliukstyte A, et al. 3-(9-
carbazolyl)carbazoles and 3,6-Di(9-carbazolyl)carbazoles as effective host
materials for efficient blue organic electrophosphorescence. Adv Mater
2007;19:862e6.
carbazole-p-dimesitylborane bipolar fluorophores for nondoped blue organic
light-emitting diodes. Adv Mater 2008;20:3947e52.
[13] Mondal E, Hung WY, Chen YH, Cheng MH, Wong KT. Molecular topology
tuning of bipolar host materials composed of fluorene-bridged benzimidazole
and carbazole for highly efficient electrophosphorescence. Chemistry
2013;19:10563e72.
[14] Huang J, Su JH, Li X, Lam MK, Fung KM, Fan HH, et al. Bipolar anthracene
derivatives containing hole- and electron-transporting moieties for highly
efficient blue electroluminescence devices. J Mater Chem 2011;21:2957e64.
[15] Liu H, Bai Q, Yao L, Hu D, Tang X, Shen F, et al. Solution-processable hosts
constructed by carbazole/PO substituted tetraphenylsilanes for efficient blue
electro- phosphorescent devices. Adv Funct Mater 2014;24:5881e8.
[16] Baldo MA, Lamansky S, Burrows PE, Thompson ME, Forrest SR. Very high-
efficiency green organic light-emitting devices based on electro-
phosphorescence. Appl Phys Lett 1999;75:4e6.
[28] Cheng SH, Hung WY, Cheng MH, Chen HF, Chaskar A, Lee GH, et al. Highly
twisted biphenyl-linked carbazolee benzimidazole hybrid bipolar host ma-
terials for efficient PhOLEDs. J Mater. Chem C 2014;2:8554e63.
[29] Huang J, Su JH, Tian H. The development of anthracene derivatives for organic
light-emitting diodes. J Mater. Chem 2012;22:10977e89.
[30] Lee S, Kim KH, Limbach D, Park YS, Kim JJ. Low roll-off and high efficiency
orange organic light emitting diodes with controlled co-doping of green and
red phosphorescent dopants in an exciplex forming co-host. Adv Funct Mater
2013;23:4105e10.
[17] Holmes RJ, Forrest SR, Tung YJ, Kwong RC, Brown JJ, Garon S, et al. Blue
organic electrophosphorescence using exothermic hosteguest energy trans-
fer. Appl Phys Lett 2003;82:2422e4.
[18] Tokito S, Iijima T, Suzuri Y, Kita H, Tsuzuki T, Sato F. Confinement of triplet
energy on phosphorescent molecules for highly-efficient organic blue-light-
emitting devices. Appl Phys Lett 2003;83:569e71.
[31] Wang K, Wang SP, Wei JB, Chen SY, Liu D, Liu Y, et al. New multifunctional
phenanthroimidazole-phosphine oxide hybrids for high-performance red,
green and blue electroluminescent devices. J Mater. Chem C 2014;2:6817e26.
[32] Tonzola CJ, Alam MM, Kaminsky W, Jenekhe SA. New n-type organic semi-
conductors: synthesis, single crystal structures, cyclic voltammetry, photo-
physics, electron transport, and electroluminescence of
a series of
[19] Huang H, Wang Y, Wang B, Zhuang S, Pan B, Yang X, et al. Controllably tunable
phenanthroimidazoleecarbazole hybrid bipolar host materials for efficient
green electrophosphorescent devices. J Mater. Chem C 2013;1:5899e908.
[20] Ge Z, Hayakawa T, Ando S, Ueda M, Akiike T, Miyamoto H, et al. Spin-coated
highly efficient phosphorescent organic light-emitting diodes based on
diphenylanthrazolines. J Am Chem Soc 2003;125:13548e58.
[33] Jeon JH, Lee NJ, Lee JH, Suh MC. Thermally-stable 2,3-diphenylated benzo-
tiophene containing host materials for red phosphorescent organic light-
emitting diodes. Dyes Pigments 2014;111:116e23.