Controllably tunable phenanthroimidazole-carbazole hybrid bipolar host materials for efficient green electrophosphorescent devices

Article abstract of DOI:10.1039/c3tc30832d

A series of phenanthroimidazole-carbazole (1:2) hybrids as bipolar host materials have been designed and synthesized through facile typical Ullmann reactions. These compounds with rigid configurations exhibit excellent thermal and morphological stabilities with high glass transition temperatures (T _g) (143-282 °C). Their photoelectronic properties, energy levels, charge transport mobility, and film morphologies can be controllably tuned through judicious engineering of the linkage modes between the two carbazole groups and the 2,5-diphenyl-1,3,4-phenanthroimidazole (para and meta). The promising physical properties of these new compounds make them suitable for use as hosts doped with Ir-based phosphor for realizing highly efficient phosphorescent organic light emitting diodes (PhOLEDs). A green device hosted by compound PhBIDmpCP shows a maximum current efficiency of 74.3 cd A^-1 and a maximum power efficiency of 74.4 lm W^-1 (corresponding EQE_max = 20.2%).

Full text of DOI:10.1039/c3tc30832d

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Page 1 of 22
Journal of Materials Chemistry C
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DOI: 10.1039/C3TC30832D
Controllably Tunable Phenanthroimidazole/Carbazole Hybrid
Bipolar Host Materials for Efficient Green
Electrophosphorescent Devices
Hong Huang,a Yixing Wang,a Bo Wang,a Shaoqing Zhuang,a Biao Pan,a Xiao
Yang,a Lei Wang,*a Chuluo Yang.*ab
aWuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology,
Wuhan 430074, P. R. China.
bDepartment of Chemistry, Wuhan University, Wuhan 430072, P. R. China
Email：wanglei@mail.hust.edu.cn; clyang@whu.edu.cn.
A series of phenanthroimidazole/carbazole (1 : 2) hybrids as bipolar host materials
have been designed and synthesized through facile typical Ullmann reactions. These
compounds with regid configurations exhibit excellent thermal and morphological
stability with high glass transition temperatures (Tg) (143 ~ 282 ℃). Their
photoelectronic properties, energy levels, charge transport mobility, film
morphologies can be controllably tuned through judicious engineering of the linkage
modes
between
the
two
carbazole
groups
and
the
2,5ꢀdiphenylꢀ1,3,4ꢀphenanthroimidazole (para and meta).. The promising physical
properties of these new compounds made them suitable for use as hosts doped with
Irꢀbased phosphor for realizing highly efficient phosphorescent organic light emitting
diodes (PhOLEDs). Green device hosted by compound PhBIDmpCP shows a
maximum current efficiency of 74.3 cd/A, and a maximum power efficiency of 74.4
lm/W (corresponding EQEmax = 20.2%).
1. Introduction
Organic lightꢀemitting diodes (OLEDs) have attracted much interest due to their
potential applications in organic flatꢀpanel displays and flexible lighting sources1. And
phosphorescent emitters have been recognized as the most effective approach to
achieve high efficiency devices since both the singlet and triplet excitons can be
harvested in this way2ꢀ5. Generally, phosphorescent organic lightꢀemitting diodes
(PhOLEDs) adopt a hostꢀguest system to reduce concentration quenching and T1ꢀT1

Journal of Materials Chemistry C
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DOI: 10.1039/C3TC30832D
annihilation6. As a result, a diverse of host materials have been developed based on
carbozole and their derivatives.. However, those host materials such as
N,N′ꢀdicarbazolylꢀ3,5ꢀbenzene
(mCP)7,
4,4'ꢀdi(9Hꢀcarbazolꢀ9ꢀyl)ꢀ1,1'ꢀbiphenyl
(CBP)8 and tris(4ꢀ(9Hꢀcarbazolꢀ9ꢀyl)phenyl) amine (TCTA)9 with holeꢀtransporting
properties, show an unbalanced carrierꢀtransporting property, which is harmful to the
stability and the efficiency of PhOLEDs. With the aim to solve the problem, some
bipolar host materials10ꢀ13, such as bisꢀ4ꢀ(Nꢀ carbazolyl)phenyl)phenylphosphine
oxide (BCPO)14, 4,7ꢀdicarbazolꢀ9ꢀylꢀ[1,10] phenanthroline (BUPH1)15, and
2,5ꢀbis(2ꢀ(9Hꢀcarbazolꢀ9ꢀyl)phenyl)ꢀ1,3,4ꢀoxadiazole (oꢀCzOXD)16, have been
developed. When applying BCPO, BUPH1 and oꢀCzOXD as host material for green
PhOLEDs, the maximum power efficiencies of 87.5 lm/W, 33.0 lm/W and 59.3 lm/W
were achieved ,respectively.14ꢀ16. As indicated by literatures, an excellent host material
must possess: (i) a suitable ET for phosphors, transferring energy to the guest
effectively, (ii) bipolar carrier transporting property, rendering
a
broad
excitonꢀformation zone and consequently reducing the efficiency rollꢀoff, (iii) the
appropriate HOMO/LUMO levels, well matching with that of the neighboring layers,
and allowing the effective charge injection and charge confinement, (iv) high thermal
and morphological stability, which is required for a long operational lifetime. As
mentioned before, carbazole and its derivatives have been widely used in designing
host materials due to their sufficiently high triplet energy levels and good
holeꢀtransporting properties17. On the other hand, the phenanthroimidazole moiety has
been demonstrated to be effective for the electron injection and hole blocking18,
accompanying with strong blue fluorescence19ꢀ21. In addition, the rigid structure of
phenanthroimidazole made its derivatives possess high thermal stability. Herein, the
combination of carbazole and phenanthroimidazole moieties provides a reasonable
strategy for designing bipolar host materials in PhOLEDs.
In our previous work,22 we have also developed
phenanthroimidazole/carbazole hybrids (1:1) as bipolar host materials, which exhibit
a maximum current efficiencies ranging from 60.2 ~ 77.6 cd
Aꢀ1 and 49.6 ~ 57.3
cd
Aꢀ1 for green and yellow PhOLEDs, respectively. It was found that the hosts based
a
series of
•
•

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Journal of Materials Chemistry C
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DOI: 10.1039/C3TC30832D
on carbazole/phenanthroimidazole hybrid conjugated through the Nꢀphenyl exhibit
better EL performances than those of C2ꢀphenyl conjugated analogues under virtually
identical conditions. However, the glass transition temperatures (Tg) of those hybrids
conjugated by Nꢀphenyl (pPhBINCP and mPhBINCP) is only around 110 ℃. The
primary results motivate us to develop new host materials with high glass transition
temperatures and balanced charge transport properties for high efficiency PhOLEDs.
In this contribution, aiming to achieve high glass transition temperatures and
balance charge transport host materials for PHOLEDs, we designed and synthesized a
series of bipolar host materials by the conjugation of two carbazole units to one
phenanthroimidazole moiety through different linkage modes. As expected, these new
compounds exhibit high thermal stabilities, with the decomposition temperatures (Td,
5 wt % weight loss) and the glass transition temperatures (Tg) in the range of 468 ~
514 ℃and 143 ~ 282 ℃, respectively. The device doping the phosphorescent emitter
Ir(ppy)3 into the PhBIDmpCP shows a low onset voltage of 2.8 V, high brightness of
42160 cd/m2 at 10.0 V, corresponding to a maximum current efficiency of 74.3 cd/A
and a maximum power efficiency of 74.4 lm/W, as well as maximum external
quantum efficieny (EQEmax) of 20.2%.
2. Experimental Section
2.1. General Information
All the reagents and solvents used for the synthesis were purchased from Aldrich
and used without further purification. All reactions were performed under a dry
nitrogen atmosphere. 1H NMR and 13C NMR spectra were measured on a
BrukerꢀAF301 AT 400MHz spectrometer. Elemental analyses of carbon, hydrogen,
and nitrogen were performed on an Elementar (Vario Micro cube) analyzer. Mass
spectra were carried out on an Agilent (1100 LC/MSD Trap) using ACPI ionization.
UVꢀVis absorption spectra were recorded on
a
Shimadzu UVꢀVISꢀNIR
Spectrophotometer (UVꢀ3600). PL spectra were recorded on Edinburgh instruments
(FLSP920 spectrometers). Differential scanning calorimetry (DSC) was performed on

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a PE Instruments DSC 2920 unit at a heating rate of 10 ℃/min from 30 to 300 ℃
under nitrogen. The glass transition temperature (Tg) was determined from the second
heating scan. Thermogravimetric analysis (TGA) was undertaken with a PerkinElmer
Instruments (Pyris1 TGA). The thermal stability of the samples under a nitrogen
atmosphere was determined by measuring their weight loss when heating at a rate of
10 ℃/min from 30 to 700 ℃. Cyclic voltammetry (CV) measurements were carried
out in a conventional three electrode cell using a glassy carbon working electrode of 2
mm in diameter, a platinum wire counter electrode, and an Ag/AgNO3 (0.1 M)
reference electrode on a computerꢀcontrolled EG&G Potentiostat/Galvanostat model
283 at room temperature. Reductions of all compounds were performed in
dichloromethane containing 0.1
M
tetrabutylammoniumhexafluorophosphate
(Bu4NPF6) as the supporting electrolyte. And all solutions were purged with a
nitrogen stream for 10 min before measurement. In the timeꢀofꢀflight (TOF)
transientꢀphotocurrent technique, those samples were illuminated at 355 nm through
the ITO electrode from an UVꢀFQ qꢀswitched DPSS laser (pulse: 4 ns). The transient
currents were measured using a digital phosphor oscilloscope (DPO 3034;
bandwidth:300MHz).
2.2. Computational Details
The theoretical investigation of geometrical and electronic properties was
performed with the Amsterdam Density Functional (ADF) 2009.01 program package.
And the calculation was optimized by means of the B3LYP (Becke three parameters
hybrid functional with LeeꢀYangꢀ Perdew correlation functionals)23 with the 6ꢀ31G(d)
atomic basis set. Then the electronic structures were calculated at
τꢀHCTHhyb/6–311++G(d,p)level24. Molecular orbital was visualized using ADFview.
2.3. Device Fabrication and Measurement
The holeꢀinjection material MoO3, holeꢀtransporting material 1,4ꢀbis[(1ꢀnaphthylꢀ
phenyl)amino]biphenyl (NPB), electron/excitonꢀblocking material TCTA, and
electronꢀtransporting material 3,3'ꢀ(5'ꢀ(3ꢀ(pyridineꢀ3ꢀyl)phenyl)ꢀ[1,1':3',1''ꢀterphenyl]
ꢀ3,3''ꢀdiyl)dipyridine (TmPyPB) were commercially available. Commercial ITO

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(indium tin oxide) coated glass with sheet resistance of 20 ꢁ per square was used as
the substrates. Before device fabrication, the ITO glass substrates were preꢀcleaned
carefully and treated by oxygen plasma for 2 min. MoO3 was firstly deposited to ITO
substrate, followed by NPB, TCTA, emissive layer, and TmPyPB. Finally, a cathode
composed of lithium fluoride (LiF) and aluminum (Al) was sequentially deposited
onto the substrate in the vacuum of 10ꢀ6 Torr. The JꢀVꢀL of the devices was measured
with a Keithley 2400 source meter equipped with a calibrated silicon photodiode. The
EL spectra were measured by PR655 spectrometer. The EQE values were calculated
according to previously reported methods25.
2.4. Synthesis
The phenanthroimidazole bromide intermediates were synthesized according to
the reported20 procedure. A mixture of phenanthrenequinone (3.0 mmol),
bromoaniline (15.0 mmol), bromide aromatic aldehyde (3.0 mmol) ammonium acetate
(12.0 mmol) and acetic acid (200 ml) was refluxed for about 18 h under nitrogen
atmosphere26. Then the mixture was cooled to room temperature, and the solid
product was collected after filtration and washing with water and CH3OH. The
product was dried under vacuum and used directly for the next step.
1
1,2-bis(4-bromophenyl)-1H-phenanthro[9,10-d]imidazole (1). Yield: 78%. H
NMR (CDCl3, 400 MHz) δ (ppm): 8.86 ~ 8.84 (d, J = 8.0 Hz, 1H)， 8.79 ~ 8.77 (d, J
= 8.4 Hz, 1H), 8.71 ~ 8.69 (d, J = 8.0 Hz, 1H), 7.96 ~ 7.94 (d, J = 7.6 Hz, 2H), 7.75 ~
7.67 (m, 2H), 7.57 ~ 7.53 (m, 1H), 7.48 ~ 7.41 (m, 4H), 7.35 ~ 7.26 (m, 1H), 7.26 ~
7.20 (m, 3H). 13C NMR (100 MHz, CDCl3) δ (ppm): 149.68, 139.60, 138.09, 131.67,
130.92, 130.75, 129.48, 128.39, 127.94, 127.52, 126.55, 126.02, 125.34, 124.28,
123.15, 122.84, 122.60, 120.70. MS (APCI): calcd for C27H16Br2N2: 526.0, found,
527.0 (M+H)+.
1-(3-bromophenyl)-2-(4-bromophenyl)-1H-phenanthro[9,10-d]imidazole (2).
1
Yield: 80%. H NMR (CDCl3, 400 MHz) δ (ppm): 8.84 ~ 8.82 (d, J = 7.6 Hz, 1H),
8.79 ~ 8.77 (d, J = 8.0 Hz, 1H), 8.71 ~ 8.69 (d, J = 8.4 Hz, 1H), 7.81 ~ 7.73 (m, 2H),
7.69 ~ 7.65 (m, 2H), 7.56 ~ 7.43 (m, 7H), 7.34 ~ 7.30 (m, 1H), 7.18 ~ 7.16 (d, J = 8.0

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Hz, 1H). 13C NMR (100 MHz, CDCl3) δ (ppm): 149.64, 139.82, 133.33, 132.11,
131.64, 131.43, 130.82, 129.46, 128.38, 128.12, 127.86, 127.45, 126.96, 126.54,
125.93, 125.24, 124.28, 123.69, 123.59, 123.15, 122.77, 122.65, 120.67. MS (APCI):
calcd for C27H16Br2N2: 526.0, found, 527.2 (M+H)+.
2-(3-bromophenyl)-1-(4-bromophenyl)-1H-phenanthro[9,10-d]imidazole (3).
Yield: 80%. 1H NMR (CDCl3, 400 MHz) δ (ppm): 8.84 ~8.82 (m, 1H), 8.77 ~ 8.75 (d,
J = 8.4 Hz, 1H)，8.70 ~ 8.68 (d, J = 8.4 Hz, 1H)，7.95 ~ 7.93 (d, J = 8.4 Hz, 2H)，7.86
~ 7.85 (m, 1H)，7.74 ~ 7.72 (m, 1H), 7.68 ~ 7.63 (m, 1H), 7.55 ~ 7.51 (m, 1H), 7.48 ~
7.45 (m, 1H), 7.34 ~ 7.29 (m, 2H), 7.25 ~ 7.20 (m, 3H), 7.16 ~ 7.12 (m, 1H). 13C
NMR (100 MHz, CDCl3) δ (ppm): 149.14, 139.53, 138.14, 137.58, 132.65, 132.16,
132.02, 130.76, 129.72, 129.48, 128.37, 128.05, 127.54, 127.47, 127.00, 126.51,
125.94, 125.29, 124.26, 123.15, 122.76, 122.68, 122.58, 120.73, 95.58. MS (APCI):
calcd for C27H16Br2N2: 526.0, found, 527.1 (M+H)+.
1
1,2-bis(3-bromophenyl)-1H-phenanthro[9,10-d]imidazole (4). Yield: 75%. H
NMR (CDCl3, 400 MHz) δ (ppm): 8.85 ~ 8.83 (m, 1H), 8.79 ~ 8.77 (d, J = 8.4 Hz,
1H), 8.72 ~ 8.70 (d, J = 8.0 Hz, 1H), 7.87 ~ 7.86 (m, 1H), 7.82 ~ 7.79 (m, 1H), 7.77 ~
7.73 (m, 1H), 7.70 ~ 7.65 (m, 2H), 7.57 ~ 7.47 (m, 4H), 7.36 ~ 7.31 (m, 2H), 7.20 ~
7.14 (m, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 149.12, 139.69, 137.53, 133.34,
132.60, 132.09, 131.39, 129.72, 129.52, 128.39, 128.15, 127.84, 127.54, 127.48,
126.98, 126.55, 125.96, 125.31, 124.28, 123.57, 123.16, 122.78, 122.63, 122.56,
120.73. MS (APCI): calcd for C27H16Br2N2: 526.0, found, 527.0 (M+H)+.
These target compounds were prepared according to a general procedure
described below27. A mixture of carbazole (5.0 mmol), the intermediates 1 (2, 3 or 4)
(2.0 mmol), CuI (0.2 mmol), 18ꢀcrownꢀ6 (0.2 mmol), and K2CO3 (20.0 mmol) in
1,3ꢀdimethyltetrahydropyrimidinꢀ2(1H)ꢀone (DMPU) (5.0 ml) was heated at 170 ℃
for 48 h under nitrogen atmosphere. Then cooled to room temperature, the reaction
mixture was quenched with 1N HCl, and extracted with dichloromethane, washed
with NH3 H2O and water, and dried by anhydrous MgSO4. The solvent was removed
•
under vacuum, and the residue was purified by column chromatography on silica gel
with dichloromethane as eluent. All the target compounds are white powder.

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1,2-bis(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazole
(PhBIDpCP). Yield: 80 %. 1H NMR (400 MHz, CDCl3) δ (ppm): 9.0 (s, 1H), 8.87 ~
8.84 (d, J = 8.4Hz, 1H), 8.78 ~ 8.76 (d, J = 8.0Hz, 1H), 8.21 ~ 8.19 (d, J = 7.6Hz, 2H),
8.15 ~ 8.13 (d, J = 7.6Hz, 2H), 7.97 ~ 7.86 (m, 6H), 7.83 ~ 7.80 (m, 1H), 7.74 ~ 7.72
(m, 1H), 7.65 ~ 7.60 (m, 3H), 7.57 ~ 7.55 (d, J = 8.4Hz, 2H), 7.51 ~ 7.35 (m, 10H),
7.32 ~ 7.28 (m, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 140.48, 140.43, 131.10,
130.82, 128.64, 128.54, 126.72, 126.64, 126.44, 126.16, 124.46, 123.92, 123.68,
123.24, 120.79, 120.69, 120.41, 120.36, 109.73, 109.38. MS (APCI): calcd for
C51H32N4: 700.3, found, 701.2 (M+H)+. Anal. calcd for C51H32N4 (%): C, 87.40; H,
4.60; N, 8.00; Found: C, 87.52; H, 4.74; N, 7.74.
1-(3-(9H-carbazol-9-yl)phenyl)-2-(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthr
1
o[9,10-d]imidazole (PhBIDpmCP). Yield: 70%. H NMR (400 MHz, CDCl3) δ
(ppm): 8.87 ~ 8.85 (d, J = 8.4Hz, 1H), 8.77 ~ 8.75 (d, J = 8.4Hz, 1H), 8.15 ~ 8.10 (m,
4H), 7.98 (s, 4H), 7.84 ~ 7.81 (m, 3H), 7.75 ~ 7.62 (m, 4H), 7.57 ~ 7.55 (d, J = 8.0Hz,
1H), 7.52 ~ 7.30 (m, 11H), 7.28 ~ 7.23 (m, 3H). 13C NMR (100 MHz, CDCl3) δ
(ppm): 40.47, 140.36, 139.93, 131.70, 131.13, 129.67, 128.49, 127.84, 127.62, 126.83,
126.61, 126.43, 126.12, 124.47, 123.77, 123.66, 123.21, 120.81, 120.69, 120.59,
120.36, 120.33. MS (APCI): calcd for C51H32N4: 700.3, found, 701.4 (M+H)+. Anal.
calcd for C51H32N4 (%): C, 87.40; H, 4.60; N, 8.00; Found: C, 87.53; H, 4.48; N, 7.99.
2-(3-(9H-carbazol-9-yl)phenyl)-1-(4-(9H-carbazol-9-yl)phenyl)-1H-phenanthr
1
o[9,10-d]imidazole (PhBIDmpCP). Yield: 72%. H NMR (400 MHz, CDCl3) δ
(ppm): 8.91 ~ 8.89 (d, J = 8.4Hz, 1H), 8.78 ~ 8.76 (d, J = 8.4Hz, 1H), 8.71 ~ 8.69 (d,
J = 8.4Hz, 1H), 8.18 ~ 8.16 (d, J = 6.8Hz, 2H), 8.08 ~ 8.06 (d, J = 7.6Hz, 2H), 8.01 ~
7.98 (d, J = 8.4Hz, 1H), 7.81 ~ 7.75 (m, 6H), 7.68 ~ 7.64 (m, 2 H), 7.57 ~ 7.52 (m,
2H), 7.40 ~ 7.26 (m, 8H), 7.23 ~ 7.22 (m, 4H), 7.16 ~ 7.14 (m, 2H). 13C NMR (100
MHz, CDCl3) δ (ppm): MS (APCI): 149.75, 140.69, 140.22, 139.38, 137.75, 136.92,
130.62, 130.23, 129.59, 128.86, 128.53, 128.45, 127.95, 127.52, 127.06, 126.52,
126.37, 126.02, 125.97, 125.28, 124.36, 123.87, 123.38, 123.19, 122.83, 120.69,
120.66, 120.51, 120.31, 120.14, 109.51. MS (APCI): calcd for C51H32N4(%): 700.3,
found, 701.3 (M+H)+. Anal. calcd for C51H32N4 (%): C, 87.40; H, 4.60; N, 8.00;

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Found: C, 87.50; H, 4.46; N, 8.04.
1,2-bis(3-(9H-carbazol-9-yl)phenyl)-1H-phenanthro[9,10-d]imidazole
(PhBIDmCP). Yield: 50%. 1H NMR (400 MHz, CDCl3) δ (ppm): 8.88 ~ 8.86 (d, J =
8.0Hz, 1H), 8.82 ~ 8.80 (d, J = 8.0Hz, 1H), 8.72 ~ 8.70 ((d, J = 8.4Hz, 1H), 8.14 ~
8.12 (d, J = 7.6Hz, 2H), 8.06 ~ 7.99 (m, 3H), 7.92 ~ 7.88(m, 2H), 7.76 ~ 7.58 (m, 8H),
7.48 ~ 7.39 (m, 2H), 7.32 ~ 7.14 (m, 10H), 7.00 (s, 2H). 13C NMR (100 MHz, CDCl3)
δ (ppm): 149.66, 140.77, 140.28, 139.99, 137.84, 131.84, 130.40, 129.63, 129.14,
128.45, 128.41, 128.17, 128.00, 127.69, 127.60, 127.51, 126.50, 126.29, 126.02,
125.37, 124.42, 123.67, 123.41, 123.16, 122.83, 122.78, 120.78, 120.53, 120.41,
120.32, 120.14, 109.69, 109.17. MS (APCI): calcd for C51H32N4: 700.3, found, 701.3
(M+H)+. Anal. calcd for C51H32N4 (%): C, 87.40; H, 4.60; N, 8.00; Found: C, 87.34;
H, 4.52; N, 8.14.
3. Results and Discussion
3.1. Synthesis
As shown in Scheme 1, the PhBIꢀBr2 intermediates (the compounds 1, 2, 3 and 4)
were synthesized in oneꢀpot reactions in high yields (> 75 %) (See the Experimental
Section). The target products were prepared through the typical Ullmann coupling
reaction of carbazole and PhBIꢀBr2 intermediates, and the crude products were
purified by column chromatography on silica gel using dichloromethane as the eluent.
The simple synthetic method provides a common and facile way to construct
phenanthroimidazole derivatives with tunable photoelectronic properties through
judicious engineering of the linkage modes. Repeated temperatureꢀgradient vacuum
sublimation is required for further purification of these materials used in OLEDs. All
the target products were fully characterized by 1H NMR, 13C NMR, mass
spectrometry and elemental analysis.

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C2-Phenyl
H
1
4
5
N
N
2
N
3
imidazole
N
N
N
Br
PhBIDpCP
N
Br
N
N
1
Br
N
N
N
N
N
Br
PhBIDpmCP
2
Br
a
NH
N
N
N
N
N
N
3
Br
PhBIDmpCP
Br
N
N
4 N
Br
N
N
N
PhBIDmCP
Scheme 1 The synthetic routes of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP.
Reagents and conditions: (a) CuI, K2CO3, 18ꢀCrownꢀ6, 180 ℃, 48 h.
3.2. Thermal properties
All the phenanthroimidazole and carbazole hybrids show high thermal stabilities
(Fig. 1). The thermalꢀdecomposition temperatures (Td) for 5 % weight loss are in
range of 468 ~ 514 ℃. In particular, their glassꢀtransition temperatures (Tg) are
ranging from 147 to 282 ℃, which are distinctly higher than that of carbazole based

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analogous, such as CBP (62 ℃) and mCP (60 ℃)17, pPhBINCP(113℃) and
mPhBINCP(113℃)22. The significant enhancement of Tg can be attributed to the
introduction of two carbazole units to phenanthroimidazole moiety, which would
reduce the possibility of phase separation upon heating, and prolong the lifetime of
the devices. Specifically, the compound PhBIDpCP with carbazole at paraꢀ positions
of the C2ꢀphenyl and N1ꢀphenyl of the phenanthroimidazole shows the highest Tg
reached to 282 ℃, which further verified the principle that the small energetic
disorder the paraꢀ positions of the C2ꢀphenyl of the phenanthroimidazole can
efficiently enhance the morphological stability28. It is regret that no obvious glass
transition temperatures (Tg) are observed for the compound PhBIDpmCP.
a)
b)
100
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
80
60
40
20
0
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
o
150
C
o
282
C
o
147
150
C
200
250
100
200
300
400
500
600
700
Temperature (oC)
Temperature (oC)
Fig. 1 DSC (a) and TGA (b) Curves of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP.
3.3. Morphology properties
Since the excellent filmꢀforming properties of light emitting materials are crucial
for the high performances of the devices, the surface morphologies of
vacuumꢀdeposited thin films of four new compounds, PhBIDmpCP, PhBIDmCP,
PhBIDpCP and PhBIDpmCP were studied by atomic force microscopy (AFM). As
shown in Fig. 2, the organic films exhibit a rootꢀmeanꢀsquare (RMS) roughness of
0.358, 0.445, 0.443 and 0.457 nm for PhBIDmpCP, PhBIDmCP, PhBIDpCP and
PhBIDpmCP, respectively. The relatively small RMS of the thin films for these hosts
make them desirable for the highꢀvacuum deposition process. Specifically, the thin
film of PhBIDmpCP shows the lowest RMS, which meets the necessity for high
efficiency device.

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DOI: 10.1039/C3TC30832D
Fig. 2 The AFM topographic images of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP.
3.4. Photophysical properties
The absorption and fluorescence spectra of these compounds in CH2Cl2 solution
and film state were studied to investigate the relationship between linkage modes and
the photophysical properties (Fig. 3). For the four target compounds, their absorption
spectra exhibit no distinct differences and all exhibit two peaks at around 290 nm and
336 nm in CH2Cl2 solution. The absorption peaks at about 290 nm for the four
compounds could be assigned to the carbazole centered nꢀπ* transition, while the
absorption bands between 330 to 340 nm arise from the πꢀπ* transition from the
carbazole and the phenanthroimidazole moiety. The photoluminescence spectra for
PhBIDpCP and PhBIDpmCP show a sharp emission peak at 383 nm and a shoulder
peak at 403 nm, while for PhBIDmpCP and PhBIDmCP, two peaks blue shift to 368
nm and 488 nm due to their limited conjugation of the meta linkage to C2ꢀphenyl
compared with para linked former two compounds. Above all, the absorption and
emission spectra of these compounds varied with the linkage positions of the
carbazole to phenanthroimidazole, the compounds with the same substituted position
( para or meta) to C2ꢀphenyl almost display the similar spectrum. The absorbance and

Journal of Materials Chemistry C
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DOI: 10.1039/C3TC30832D
photoluminescence of the four compounds in film state exhibit a similar red shift
phenomenon compared with that in the solution. The shoulder emission peaks has
been weakened because the intramolecular energy transfer could be effectively
blocked in the solid state.
a)
b)1.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
PhBIDpCP
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
0.8
0.6
0.4
0.2
0.0
250
300
350
400
450
500
550
250
300
350
400
450
500
550
600
Wavelength (nm)
Wavelength (nm)
Fig. 3 (a) UVꢀvisible absorption and PL spectra of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and
PhBIDmCP in dilute CH2Cl2 solution; (b) PL spectra of PhBIDpCP, PhBIDpmCP, PhBIDmpCP
and PhBIDmCP in film state.
4
PhBIDmCP
3
PhBIDmpCP
2
PhBIDpmCP
1
PhBIDpCP
600 650
0
400
450
500
550
Wavelength (nm)
Fig. 4 Phosphorescence spectra of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP in
2ꢀmethyltetrahydrofuran at 77 K.
The triplet energies (ET) of these new compounds have also been determined by
the highest energy vibronic subꢀband of the phosphorescence spectra at 77 K (Fig. 4).
They all show low ET (2.54 for PhBIDpCP, 2.56 eV for PhBIDpmCP, 2.59 eV for
PhBIDmpCP and 2.58 eV for PhBIDmCP). The relative lower ET of PhBIDpCP can
be ascribed to the two paraꢀconjugations between the carbazole and
phenanthroimidazole moieties. The ET values of the four compounds are all higher
than that of Ir(ppy)3 (2.40 eV), implying that they may act as appropriate host
materials for green phosphorescent emitters.
3.5. Electrochemical properties

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The electrochemical properties of the four compounds were studied in solution
through cyclic voltammetry (CV) measurements using Ag/AgNO3 as the reference
electrode. The cyclic voltammograms are shown in Fig. 5 and the electrochemical
data are summarized in Table 1. All the compounds exhibit similar reversible
oxidation waves as most carbazole derivatives29. The highest occupied molecular
orbital (HOMO) energy levels of the compounds were determined from the onset of
the oxidation potentials with regard to energy level of Ag/Ag+ (4.6 eV below vacuum).
The lowest unoccupied molecular orbital (LUMO) energy levels of the compounds
were calculated from the HOMO energy levels and energy gap deduced from their
absorption spectra. The HOMO levels of the four compounds vary in range of ꢀ5.82 ~
ꢀ5.87 eV and the LUMO levels range from ꢀ2.49 to ꢀ2.62 eV (Table 1), depending on
the linkage modes of the electronic donor and acceptor moieties.
Table 1 The photophysical, electrochemistry and thermal data of phenanthroimidazole derivatives
λmax,abs (nm)
λem (nm)
Solutiona
Eg
HOMO/LU
MO (eV)c
HOMO/LU
MO (eV)d
ET
Tg/Td
Compound
(eV)
(eV)e
(℃)
Solutiona
Filmb
Filmb
414
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
338/289
338/289
336/290
335/289
342/296
344/297
340/295
340/295
403/383
404/384
388/368
388/368
3.25
3.27
3.32
3.34
ꢀ5.87/ꢀ2.62
ꢀ5.84/ꢀ2.57
ꢀ5.82/ꢀ2.50
ꢀ5.33/ꢀ2.33
ꢀ5.35/ꢀ2.30
ꢀ5.36/ꢀ2.29
ꢀ5.38/ꢀ2.27
2.54
2.56
2.59
2.58
282/514
ꢀꢀꢀ/490
419
422/403
147/503
421/403
ꢀ5.83/ꢀ2.49
150/468
b
c
a Measured in CH2Cl2 solvent at room temperature. Measured in solid state. Determined from the onset of the
oxidation. d Values from DFT calculation. e Measured in 2ꢀmethyltetrahydrofuran at 77 K.
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
0.0
0.5
1.0
1.5
2.0
Potential (V vs Ag/Ag+)
Fig. 5 Cyclic voltammgrams of PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP.
3.6. Theoretical calculations
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP

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DOI: 10.1039/C3TC30832D
HOMO
LUMO
Fig. 6 Calculated spatial distributions of the HOMO and LUMO energy densities of the
phenanthroimidazole derivatives.
A quantum chemical calculations was performed on the phenanthroimidazole
derivatives at the B3LYP/6ꢀ31G(d) theoretical level. As depicted in Fig. 6. The
electron density distributions of the HOMO and LUMO of the phenanthroimidazole
derivatives are localized predominantly on the electronꢀrich carbazole and
electronꢀdeficient phenanthroimidazole fragments, respectively. The difference
among the four compounds could be attributed to the linkage modes of carbazole to
the C2ꢀphenyl of the phenanthroimidazole. With the carbazole moiety conjugated to
the meta of the C2ꢀphenyl, the distributions of the HOMO and LUMO are completely
separated, which are consistent with the aim of structure engineering that the p type
unit is responsible for hole transporting and the n type group plays a key role in
electron transporting. The separation between the HOMO and LUMO orbitals is
beneficial to the efficient chargeꢀcarrier transport and the prevention of reverse energy
transfer. The values of the calculation HOMO and LUMO levels are listed in Table 1.
3.7. Bipolar transporting characteristics
To investigate the bipolar transporting properties of these compounds, the hole
only device: ITO/NPB(10 nm)/host(30 nm)/NPB(10 nm)/Al(100 nm) and the
electroꢀonly device: ITO/BCP(10 nm)/host(30 nm)/BCP(30 nm)/LiF(1 nm)/Al(100
nm) were fabricated. 1,4ꢀbis[(1ꢀnaphthylphenyl)amino]biphenyl (NPB) and
2,9ꢀdimethylꢀ4,7ꢀdiphenylꢀ1,10ꢀphenanthroline (BCP) layers were used to prevent

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electron and hole injection from the cathode and anode, respectively14. The current
density versus voltage curves of the devices were shown in Fig. 7. Generally, the hole
mobility are much higher (about 1000 times) than the electron mobility in organic
semiconducting materials30. For the hosts PhBIDpCP, PhBIDpmCP and PhBIDmCP,
the hole only devices show higher current density than the electron only devices at the
same voltage. Interestingly, for compounds PhBIDmpCP, the electron only device
shows higher current density than the hole only device at the same voltage. It may be
attributed to the difference of molecular packing and nanostructures of PhBIDpCP,
PhBIDpmCP, PhBIDmpCP and PhBIDmCP in thin film systems. The charge densities
of hole only and electron only devices are often dependent on the interfacial
characteristics and mobility of host material. As for the interfacial effect, we can only
provide a little rational explanation from the AFM (RMS: PhBIDmpCP < PhBIDmCP
≈ PhBIDpCP < PhBIDpmCP, see Fig 2).
b)800
a)800
PhBIDpCP
Electron Only
PhBIDpmCP
Electron Only
600
600
Hole Only
Hole Only
400
200
0
400
200
0
0
5
10
15
20
0
5
10
15
20
Voltage (V)
Voltage (V)
c)800
800
d)
PhBIDmpCP
PhBIDmCP
Electron Only
Hole Only
Electron Only
Hole Only
600
600
400
200
0
400
200
0
0
5
10
15
20
0
5
10
15
20
Voltage (V)
Voltage (V)
Fig. 7 Current density versus voltage characteristics of the holeꢀonly and electronꢀonly devices for
PhBIDpCP, PhBIDpmCP, PhBIDmpCP and PhBIDmCP.
In order to further explain this interesting phenomenon, we selected PhBIDpCP,

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PhBIDmpCP as representative and tested their electron and hole mobility using the
conventional time of flight (TOF) technique based on the device configuration of
[ITO/ PhBIDpCP or PhBIDmpCP (um)/Al]. Fig. 8 shows typical TOF transients of
holes/electron for complex PhBIDpCP and PhBIDmpCP under an applied field. For
the compounds PhBIDmpCP, the electron mobility (3.92* 10ꢀ4 cm2/Vs) is larger than
the hole mobility (3.27* 10ꢀ4 cm2/Vs), while it is reversed for the compounds
PhBIDpCP, with the electron mobility (2.36* 10ꢀ4 cm2/Vs) vs the hole mobility (3.14*
10ꢀ4 cm2/Vs). The results further explain why the electron only device based on
PhBIDmpCP has the highest electron density among these new hosts. It is also
demonstrate that the charge transport of organic semiconductors is strongly molecular
packing and morphology dependent and organic molecules with similar molecular
structures can exhibit remarkably different mobility31. For the two representative
compounds, both the hole and electron mobility is at the level of 10ꢀ4 cm2/V s, which
further demonstrate their bipolar properties as host materials.
101
100
101
100
(b) PhBIDpCP hole
(a) PhBIDpCP electron
D=1
E=3*105V/cm
Tt=1.41
e=2.36*10-4cm2/Vs
µm
D=1
E=3*105V/cm
Tt=1.06
h=3.14*10-4cm2/Vs
µm
T
Tt
t
µs
µs
µ
µ
10-1
10-1
10-2
10-2
100
101
102
103
10-1
100
101
102
103
Time
(µs
)
Time
(µs
)
101
100
101
100
(c) PhBIDmpCP electron
(d) PhBIDmpCP hole
D=4
E=3*105V/cm
Tt=4.08
µm
D=4
E=3*105V/cm
Tt=3.4
e=3.92*10-4cm2/Vs
µm
Tt
Tt
µs
µ
s
µ
h=3.27*10-4cm2/Vs
µ
10-1
10-2
10-1
10-2
100
101
102
103
100
101
102
103
Time
(µs
)
Time
(µ
s)
Fig. 8 Log of the photocurrent vs. the log of time in a timeꢀofꢀflight measurement. The electron

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mobility (ꢂ) was calculated from the values of the transit time (Tt), the sample thickness and the
applied voltage (V) according to the equation ꢂ = D2/(VTt)
3.8. Electrophosphorescent OLED Characterization
To evaluate the suitability of these bipolar phenanthroimidazole derivatives as
host material in green phosphorescent OLEDs, devices with the configuration of
ITO/MoO3(10 nm)/NPB(80 nm)/TCTA(5 nm)/Host: 9wt% Ir(ppy)3(20 nm)/TmPyPB
(45 nm)/LiF(1 nm)/Al(100 nm) (Host: PhBIDpCP for Device A; PhBIDpmCP for
Device B; PhBIDmpCP for Device C; PhBIDmCP for Device D) were fabricated.
NPB acts as the holeꢀtransporting layer and TCTA is the exciton blocking layer.
Ir(ppy)3 doped into these hosts was used as the emitting layer, and the best
electroluminescence (EL) performance was achieved with 9 wt % Ir(ppy)3 for all the
hosts. TmPyPB serves as both the electronꢀtransporting layer and holeꢀblocking layer.
MoO3 and LiF (lithium fluoride) serves as the holeꢀ and electronꢀinjecting layers,
respectively. The energy level diagram of the devices based on the
phenanthroimidazole derivatives is shown in Fig. 9.
As shown in Fig. 10, all the fabricated devices emit brightly green light with the
CIE coordinates of (0.30, 0.63) when the bias is applied to the anode/cathode. The EL
spectra of the devices are almost the same as the emission spectrum of the Ir(ppy)3,
which indicates that the EL is indeed derived from the phosphorescent emitter and the
triplet energy transfer from the host to the dopant is completely. The results
demonstrate in turn that the triplet energies of the host materials are high enough to
the phosphorescent emitter Ir(ppy)3.
Fig. 9 Energy level diagram of the devices based on the phenanthroimidazole derivatives.

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1.0
Device A (0.30, 0.63)
Device B (0.30, 0.63)
Device C (0.30, 0.64)
Device D (0.30, 0.63)
0.8
0.6
0.4
0.2
0.0
300
400
500
600
700
800
900 1000
Wavelength (nm)
Fig. 10 EL spectra of the devices based on the phenanthroimidazole derivatives.
Fig. 11 shows the JꢀVꢀL characteristic, efficiencyꢀversusꢀcurrent density curves of
the four devices, and the key device performance parameters are summarized in Table
2. The devices AꢀD display low turnꢀon voltages in a range of 2.8 ~ 3.0 V, which is
attributed to the bipolar transporting capabilities and the suitable HOMO and LUMO
energy levels of these hosts used. As shown in Fig. 9, there are injection barriers of
approximately 0.2ꢀ0.3 eV for electrons at TCTA/EML interface and about 0.9 eV for
the hole at EML/TmPyPB interfaces, which implies that the carriers can be confined
the EML layer and resulted in high exciton formation efficiencies even under high
current densities. Definitely, all the devices exhibit high and stable efficiencies.
b)
a)
102
Device A
Device B
Device C
Device D
150
104
103
102
101
100
101
100
100
50
0
Device A
Device B
Device C
Device D
10-1
2
4
6
8
10
12
1E-3
0.01
0.1
1
10
100
Current Intensity (mA/cm2)
Voltage (V)
2
d)10
c)102
101
101
100
Device A
Device B
Device C
Device D
Device A
Device B
Device C
Device D
100
10-1
10-1
1E-3
0.01
0.1
1
10
100
1E-3
0.01
0.1
1
10
100
Current Intensity (mA/cm2)
Current Intensity (mA/cm2)

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Fig. 11 (a) Current densityꢀvoltageꢀluminance characteristics (JꢀVꢀL) of device AꢀD; (b) Current
efficiency versus current density curves for device AꢀD; (c) Power efficiency versus current
density curves for device AꢀD; (d) External quantum efficiency versus current density for device
AꢀD.
Device C with the PhBIDmpCP as the host achieves the best EL performance, with
a maximum current and power efficiency of 74.3 cd/A and 74.4 lm/W, respectively,
corresponding maximum EQE of 20.2 %. Devices A, B and D exhibit a maximum
current efficiency of 59.6 cd/A, 45.7 cd/A, 60.3 cd/A and power efficiencies of 59.4
lm/W, 45.6 lm/W, 59.2 lm/W, respectively. Definitely, all of these values are higher
than those of the CBPꢀhosted device16. It is worth noting that both the current and
power efficiencies of the Device AꢀD are in the order of C > D ≈ A > B. The result can
be explained by the current density differences at the same voltage of the electronꢀ
and holeꢀ only devices of these hosts, which in turn demonstrates that the electron
injection from cathode into hosts are in the order of PhBIDmpCP > PhBIDmCP ≈
PhBIDpCP > PhBIDpmCP. Meanwhile, because of the excellent electron/hole
mobility of PhBIDmpCP, device C exhibits low efficiency rollꢀoff at high luminance,
and the external quantum efficiency (EQE) is still up to 20.0% and 19.3% at the
luminance of 100 cd/m2 and 1000 cd/m2, respectively. Notably, when the luminance
reaches as high as 10000 cd/m2, the EQE only slightly rolls down to 16.1%. On the
other side, the completely separation between the HOMO and LUMO levels in these
host materials, is also beneficial for efficient charge transport.
Table 2 EL performances of devices AꢀD based on the phenanthroimidazole derivatives
a
b
c
d
Von
Lmax
Voltage for
ηc,max
ηp,max
EQEmax
(%)
Device
host
CIE(x, y)e
(V)
(cd/m2)
Lmax(V)
(cd/A)
(lm/W)
A
B
C
D
PhBIDpCP
PhBIDpmCP
PhBIDmpCP
PhBIDmCP
2.8
3.0
2.8
3.0
33670
38500
40890
37520
10.0
11.2
10.0
11.0
59.6
45.7
74.3
60.3
59.4
45.6
74.4
59.2
16.1
12.6
20.2
16.3
(0.30, 0.63)
(0.30, 0.63)
(0.30, 0.64)
(0.30, 0.63)
a
b
c
d
The turnꢀon voltage (L = 1 cd/m2). Maximum luminance. Maximum current efficiency.
Maximum power efficiency. e The brightness is 100 cd/m2.
4. Conclusions
In summary, a series of simple phenanthroimidazole/carbazoles(1:2) hybrids as

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bipolar host materials have been developed by using different linkage modes. The
photophysical, charge transport abilities, film morphology and electrochemical
properties of these compounds can be controllably tuned by the different linkage
modes between the phenanthroimidazole and the two carbazoles, and further to affect
the device performances. The introduction of two carbazole group to
phenanthroimidazole moiety can significally improve the thermal stability of the host
materials. Device with 9 wt % Ir(ppy)3 hosted with PhBIDmpCP achieves a maximum
external quantum efficiency as high as 20.2% and exhibits an efficiency rollꢀoff at
high luminance. This work definitely demonstrates that tradeoffs among the bipolar
property, the triplet energy, charge transport abilities and the energy levels can be
controlled through judicious tuning the linkage modes between the electron donor and
acceptor.
Acknowledgements
This research work was supported by the NSFC/China (21161160442, 51203056) ,
the National Basic Research Program of China (973 Program 2013CB922104),
Wuhan Science and Technology Bureau (NO: 01010621227) and the Open Research
Fund of State Key Laboratory of Polymer Physics and Chemistry.
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