Synthesis, characterization, physical properties, and blue electroluminescent device applications of phenanthroimidazole derivatives containing anthracene or pyrene moiety

Article abstract of DOI:10.1016/j.dyepig.2013.08.027

By introducing a highly fluorescent anthracene, 9-(naphthalen-2-yl) anthracene or pyrene unit at the N1-phenyl of 1,2-diphenyl-1H-phenanthro[9,10-d] imidazole, three novel blue-emissive materials (ANPI, 2-NaNPI and PNPI) were synthesized and characterized, of which HOMO and LUMO orbitals are specifically segregative. Their high luminescence efficiency and excellent stability render them promising materials in electroluminescent device. The undoped three layers device using PNPI exhibits a maximum current efficiency of 4.99 cd A ^-1 with Commission Internationale d'Eclairage (CIE_x,y) color coordinates of (x = 0.15, y = 0.17). Meanwhile, the property differences between N1-substituted and C2-substituted of diphenyl-1H-phenanthro[9,10-d] imidazole derivatives were studied. The results indicate that the change of conjugated position and moiety for those blue hosts can affect the electrical field polarization of device. Specifically, all the blue host materials containing anthracene moiety show an impressive luminous current efficiency around 12 cd A^-1 when doped with sky blue fluorescent material BUBD-1.

Full text of DOI:10.1016/j.dyepig.2013.08.027

Dyes and Pigments 101 (2014) 93e102
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Synthesis, characterization, physical properties, and blue
electroluminescent device applications of phenanthroimidazole
derivatives containing anthracene or pyrene moiety
,
,
*
Shaoqing Zhuang a b, Ronggang Shangguan a, Hong Huang a, Guoli Tu a, Lei Wang a
**
,
, c,
Xunjin Zhu b
a Wuhan National Laboratory for Optoelectronics, School of Optical and Electronic Information, Huazhong University of Science and Technology, Wuhan
430074, PR China
b Department of Chemistry and Institute of Advanced Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China
c HKBU Institute of Research and Continuing Education, Shenzhen Virtual University Park, Shenzhen 518057, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
By introducing a highly fluorescent anthracene, 9-(naphthalen-2-yl)anthracene or pyrene unit at the N1-
phenyl of 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole, three novel blue-emissive materials (ANPI, 2-
NaNPI and PNPI) were synthesized and characterized, of which HOMO and LUMO orbitals are specif-
ically segregative. Their high luminescence efficiency and excellent stability render them promising
materials in electroluminescent device. The undoped three layers device using PNPI exhibits a maximum
current efficiency of 4.99 cd Aꢀ1 with Commission Internationale d’Eclairage (CIEx,y) color coordinates of
(x ¼ 0.15, y ¼ 0.17). Meanwhile, the property differences between N1-substituted and C2-substituted of
diphenyl-1H-phenanthro[9,10-d]imidazole derivatives were studied. The results indicate that the change
of conjugated position and moiety for those blue hosts can affect the electrical field polarization of
device. Specifically, all the blue host materials containing anthracene moiety show an impressive lu-
minous current efficiency around 12 cd Aꢀ1 when doped with sky blue fluorescent material BUBD-1.
Ó 2013 Elsevier Ltd. All rights reserved.
Received 3 July 2013
Received in revised form
30 August 2013
Accepted 30 August 2013
Available online 13 September 2013
Keywords:
Blue host material
Phenanthro[9,10-d]imidazole
Anthracene
Pyrene
Organic light-emitting device
High Tg
1. Introduction
versatile blue fluorescent materials aiming at further improving
device efficiency, chromaticity and lifetime. Intrigued by the good
Organic light emitting diodes (OLED) have been expected to find
applications as next generation display devices [1,2]. For realizing
active matrix full-color OLED displays, it is essential to have the
three primary colors, red, green and blue. Red and green phos-
phorescent electroluminescent devices with high efficiencies, long
lifetimes, and proper CIE coordinates have been well developed.
However, efficient blue phosphorescent electroluminescent de-
vices remain a challenge due to their high CIE coordinates (y-co-
ordinate value >0.30) and short device lifetime [3e5]. In this
context, highly efficient blue fluorescent materials have been
considered as a promising alternative to promote the commer-
cialization of OLEDs. Significant efforts have been made to design
PL properties of anthracene in solution, much effort has been
devoted to develop anthracene derivatives as deep-blue emitters
with reduced close packings and consequent high fluorescent
quantum yields in films [6,4a]. Besides anthracene, fluorene de-
rivatives have also been widely investigated as blue emitters due to
their high fluorescent quantum yields and good thermal stabilities
[7]. In addition to small molecules, oligofluorene have been of much
interest for use in OLEDs because of the associated high quantum
yield, good optical stability, and film forming ability along with the
easy tuning of the structures [8].
Phenanthroimidazole derivatives have also attracted great at-
tentions as blue electroluminescent materials because of their high
thermal stability and efficient electron transporting ability. For
example,
4,40-Bis(1-phenyl-1H-phenanthro[9,10-d]phenanthro-
* Corresponding author. Tel./fax: þ86 27 87793032.
** Corresponding author. Department of Chemistry and Institute of Advanced
Materials, Hong Kong Baptist University, Waterloo Road, Hong Kong, China.
Tel.: þ852 34115157; fax: þ852 34117348.
imidazolyl-2-yl)biphenyl (PPIP) was reported recently as an excel-
lent non-doped blue emitting material, corresponding to a high
external quantum efficiency of 6.31% and power efficiency of
7.30 lm Wꢀ1 [9]. Very recently, pure blue device was fabricated with
E-mail addresses: wanglei@mail.hust.edu.cn (L. Wang), xjzhu@hkbu.edu.hk
(X. Zhu).
0143-7208/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.dyepig.2013.08.027

94
S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
an attractive luminance efficiency of 6.87 cd Aꢀ1, CIE coordinates of
(0.15, 0.21) and a turn on voltage of 2.8 V, in which PPIP was used as a
bifunctional luminescent material [10]. In addition, a series of phe-
nanthroimidazole derivatives bridged by thiophene or benzene ring
havealso been synthesizedandappliedasemittingor function layers
in organic light-emitting devices [11]. Among them, the best device
performance using deep-blue emitting phenanthroimidazole de-
rivative with a structure of donor-linker-acceptor was obtained by
Tong and Ma’s group [12]. Specifically, an impressive device perfor-
mance with an external quantum efficiency (EQE) of 7.8% and a
currentefficiencyof10.4cdAꢀ1 hasbeenreportedbased onexcellent
blue dopant PPIE containing n-type imidazole moiety [13]. However,
device performances of blue materials often suffer from the retarded
charge injection and transportation in the emission layer [14]. The
preparation of blue materials by a facial synthetic process with
multiple functions, such as high emissive efficiency, balanced charge
injection and transport properties and good morphological and
thermal stability still remains one of the most active and challenging
areasin this field. In ourprevious work, wedesignedandsynthesized
some asymmetric phenanthroimidazole C2-phenyl position
substituted anthracene derivatives [15]. The introduction of
anthracene moieties can effectively increase the electron injection
and transport abilityand finelyadjust the ionizationpotentials (Ip) of
the compounds, resulting in reduced hole injection barrier and
balanced recombination ability. In the subsequent research, some
hybrid bipolar phosphorescent hosts for green and orange OLEDs
have also been synthesized by conjugating carbazole moiety to the
rigid skeleton 1, 2-diphenyl-1H-phenanthro[9,10-d]imidazole
(Scheme 1), and we found that the host with carbazole unit conju-
gated to N-1 position exhibited better performance than that of C-2
positionsubstitutedinelectroluminescentdevices[16]. Notethatthe
structural engineering of the phenanthroimidazole by changing
the conjugated position and moiety can affect the EL device perfor-
mance of the materials obtained. Is there any principle for designing
versatile materials and fabricating highly efficient devices based on
phenanthroimidazole?
In this contribution, by introducing a highly fluorescent
anthracene, 9-(naphthalen-2-yl)anthracene or pyrene unit at the
N1-phenyl of 1,2-diphenyl-1H-phenanthro[9,10-d]imidazole,
three new blue-emissive materials (ANPI, 2-NaNPI and PNPI)
were synthesized and characterized by thermal, photophysical
and electrochemical studies. The undoped three layers device
PNPI exhibits a maximum current efficiency of 4.99 cd Aꢀ1 with
Commission Internationale d’Eclairage (CIEx,y) color coordinates
of (x ¼ 0.15, y ¼ 0.17). Meanwhile, the property differences be-
tween N1-substituted and C2-substituted of diphenyl-1H-phe-
nanthro[9,10-d]imidazole derivatives were studied carefully. It
was found that all the blue host material containing anthracene
moiety showed an impressive luminous current efficiency
around 12 cd Aꢀ1 when doped with sky blue fluorescent material
BUBD-1.
2. Experimental section
2.1. Material and methods
All reagents and solvents were purchased from Aldrich and used
without further purification. 1H NMR spectra were recorded using a
Bruker-AF301 400 MHz. Mass spectra were carried out on an Agi-
lent (1100 LC/MSD Trap) using APCI ionization. Elemental analyses
of carbon, hydrogen, and nitrogen were performed on an Elementar
(Vario Micro cube) analyzer. Fluorescence spectra were obtained on
Edinburgh instruments (FLSP920 spectrometers) and UVeVis
spectra were measured using a Shimadzu UV-VIS-NIR Spectro-
photometer (UV-3600). The differential scanning calorimetry (DSC)
analyses were performed on PE Instruments DSC 2920 under
a nitrogen atmosphere at a heating rate of 10 ꢁC/min. Thermogra-
vimetric analyses (TGA) were undertaken on PerkinElmer
Scheme 1. Synthetic routes of ANPI, 2-NaNPI and PNPI and structure of ACPI, 1-NaCPI,2-NaCPI Reagents and conditions: i) NH4Ac, HAc, reflux; ii) toluene, K2CO3, ethanol, aryl boric
acid, Pd(PPh3)4, reflux.

S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
95
Instruments (Pyris1 TGA) under nitrogen atmosphere at a heating
rate of 10 ꢁC/min. Atomic force microscopies (AFMs) were
measured on Veeco (DIMENSION 3100). To measure the PL quan-
tum yields (Ff), degassed solutions of the compounds in CH2Cl2
were prepared. The concentration was adjusted with the absor-
bance between 0.05 and 0.1, and 9,10-diphenylanthracene (DPA) in
with dichloromethane. The organic layer was separated and
dried by anhydrous MgSO4. Then the solvent was removed under
a reduced pressure and the residue was purified by column
chromatography to yield a white solid. Yield: 90%. 1H NMR:
(DMSO-d6, 400 MHz):
d
(ppm) 9.01e8.98 (d, J ¼ 8.0 Hz, 1 H),
8.94e8.91 (d, J ¼ 8.0 Hz, 1 H), 8.77e8.74 (d, J ¼ 5.6 Hz, 2 H), 8.24e
8.20 (t, J ¼ 7.2 Hz, 2 H), 7.95e7.92 (d, J ¼ 8.0 Hz, 2 H), 7.84e7.79
(d, J ¼ 8.0 Hz, 2 H), 7.70e7.48 (m, 16 H). MS (APCI) (m/z):
[M þ Hþ] calcd for C41H27N2, 547.7; found, 547.2. 13C NMR
cyclohexane (
F
¼ 0.9 in cyclohexane) was used as a standard [17].
Cyclic voltammetry (CV) measurements were carried out in a
conventional three electrode cell using a Pt button working elec-
trode of 2 mm in diameter, a platinum wire counter electrode, and a
Ag/AgNO3 (0.1 M) reference electrode on a computer-controlled
EG&G Potentiostat/Galvanostat model 283 at room temperature.
The supporting electrolyte was 0.1 M tetra-n-butylammonium
hexafluorophosphate (Bu4NPF6) in CH2Cl2 solution. The onset po-
tentials were determined from the intersection of two tangents
drawn at the rising and background current of the cyclic voltam-
mogram and calibrated to the ferrocene/ferrocenium (Fc/Fcþ) redox
couple. DFT calculations were performed to study the 3D geome-
tries and the frontier molecular orbital energy levels of ANPI, 2-
NaNPI and PNPI at the B3LYP/6-31G* level based on ADF2009.01
program.
(100 MHz, DMSO-d6)
d (ppm) 151.18, 140.23, 138.26, 137.10,
135.37, 133.29, 131.38, 131.37, 130.80, 130.08, 129.87, 129.79,
129.75, 129.67, 129.11, 128.73, 128.30, 128.23, 128.01, 127.61,
127.28, 127.26, 126.93, 126.74, 126.27, 126.09, 125.97, 125.91,
125.84, 125.12, 124.20, 123.04, 122.55, 120.87. Anal. Calcd for
C
41H26N2: C, 90.08; H, 4.79; N, 5.12. Found: C, 89.91; H, 4.88; N,
5.21.
2.2.3. Synthesis of compound 2-NaNPI
The compound was synthesized similarly as to ANPI. Yield: 86%.
1H NMR: (DMSO-d6, 400 MHz):
d
(ppm) 9.02e9.00 (d, J ¼ 8.0 Hz,
1 H), 8.95e8.92 (d, J ¼ 8.0 Hz, 1 H), 8.78e8.75 (d, J ¼ 8.0 Hz, 1 H),
8.23e8.20 (d, J ¼ 8.0 Hz, 1 H), 8.15e8.12 (d, J ¼ 8.0 Hz, 1 H), 8.09e
7.98 (m, 4 H), 7.90e7.87 (d, J ¼ 8.8 Hz,1 H), 7.84e7.75 (m, 5 H), 7.73e
7.64 (m, 8 H), 7.64e7.44 (m, 9 H). MS (APCI) (m/z): [M þ Hþ] calcd
for C51H33N2, 673.8; found, 673.5. 13C NMR (100 MHz, DMSO-d6)
2.2. Preparation of compounds
2.2.1. Synthesis of intermediate 1
d
(ppm) 151.23, 140.49, 138.39, 137.53, 137.10, 136.03, 135.80, 133.52,
A 250 ml round-bottomed flask was charged with 9,10-
phenanthrenequinone (2.08 g, 10 mmol), 4-iodoaniline (2.19 g,
10 mmol), benzaldehyde (1.06 g, 10 mmol) and ammonium acetate
(9.24 g, 120 mmol) under nitrogen atmosphere. Subsequently,
120 mL acetic acid was added and the reaction mixture was heated
to reflux for 36 h. After cooled to room temperature, the mixture
was poured into water. And the solid was collected by filtration,
washed with methanol and dried to give a white solid for next step
reaction without further purification. Yield 4.22 g (85%). 1H NMR:
133.41, 132.90, 130.83, 130.22, 129.90, 129.83, 129.71, 129.63, 129.51,
129.13, 128.76, 128.65, 128.49, 128.33, 128.24, 128.03, 127.29, 127.16,
126.99,126.46,126.40,126.30,125.88,125.16,124.23,126.07,122.54,
120.90. Anal. Calcd for C51H32N2: C, 91.04; H, 4.79; N, 4.16. Found: C,
90.81; H, 4.87; N, 4.32.
2.2.4. Synthesis of compound PNPI
The compound was synthesized similarly as to ANPI. Yield: 90%.
1H NMR: (DMSO-d6, 400 MHz):
d
(ppm) 8.95e8.92 (d, J ¼ 8.0 Hz,
(DMSO-d6, 400 MHz):
d
(ppm) 8.94e8.91 (d, J ¼ 8.4 Hz, 1 H), 8.88e
1 H), 8.83e8.80 (d, J ¼ 8.0 Hz, 1 H), 8.75e8.72 (d, J ¼ 8.0 Hz, 1 H),
8.30e8.27 (d, J ¼ 8.0 Hz, 1 H), 8.25e8.20 (t, J ¼ 8.0 Hz, 2 H), 8.18e
8.09 (m, 5 H), 8.07e8.02 (t, J ¼ 8.0 Hz, 2 H), 7.83e7.81 (d, J ¼ 8.0 Hz,
2 H), 7.79e7.65 (m, 6 H), 7.60e7.55 (m,1 H), 7.50e7.48 (d, J ¼ 8.0 Hz,
1 H), 7.43e7.39 (m, 1 H). MS (APCI) (m/z): [M þ Hþ] calcd for
8.85 (d, J ¼ 8.4 Hz, 1 H), 8.71e8.68 (dd, J ¼ 1.2, 8.0 Hz, 1 H), 8.02e
8.00 (d, J ¼ 8.4 Hz, 2 H), 7.79e7.75 (t, J ¼ 7.2 Hz, 1 H), 7.71e7.66 (m,
1 H), 7.59e7.55 (m, 3 H), 7.51e7.49 (d, J ¼ 8.4 Hz, 2 H), 7.42e7.36 (m,
4 H), 8.16e8.14 (d, J ¼ 8.0 Hz, 1 H). MS (APCI) (m/z): [M þ Hþ] calcd
for C27H18IN2, 497.3; found, 497.0. 13C NMR: (100 MHz,DMSO-d6):
C
d
43H27N2, 570.7; found, 570.6. 13C NMR (100 MHz, DMSO-d6)
(ppm) 151.12, 142.80, 137.77, 135.91, 132.23, 131.50, 131.14, 130.90,
d
(ppm) 151.16, 139.58, 138.37, 137.04, 131.77, 130.58, 129.76, 129.58,
128.97, 128.74, 128.16, 128.04, 127.96, 127.27, 127.13, 126.27, 125.75,
125.01, 124.13, 122.84, 122.51, 120.59, 97.72. Anal. Calcd for
129.65,129.42,129.13,129.04,128.48,128.36,128.33,128.20,128.13,
127.89, 127.48, 127.42, 127.38, 127.25, 126.384, 126.27, 125.72,
125.56, 125.18, 125.06, 125.02, 124.86, 124.77, 124.48, 124.26, 123.17,
123.13, 122.88, 121.00. Anal. Calcd for C43H26N2: C, 90.50; H, 4.59; N,
4.91. Found: C, 90.35; H, 4.67; N, 4.98.
C
27H17IN2: C, 65.34; H, 3.45; N, 5.64. Found: C, 65.19; H, 3.58; N,
5.66.
2.2.2. Synthesis of compound ANPI
To a 100 ml two-neck round-bottomed flask were added
compound
1
(0.99 g,
2
mmol), anthracen-9-ylboronic acid
2.3. Device fabrication and measurement
(0.49 g, 2.2 mmol), toluene (30 mL), ethanol (15 mL), 2 M K2CO3
(30 mL, 60 mmol) aqueous solution and tetrakis-(triphenyl-
phosphine) palladium(0) (0.12 g, 0.1 mmol) in turn, then the
reaction mixture was refluxed under nitrogen for 24 h in the
absence of light. After cooled down the solution was extracted
The EL devices were fabricated by vacuum deposition of those
materials at a base pressure of 5 ꢂ 10ꢀ6 Torr onto glass precoated
with a layer of indium tin oxide (ITO) with a sheet resistance of
25 U/square. Before the deposition of an organic layer, the clear ITO
Table 1
The optical, electrochemical and thermal properties of ANPI, 2-NaNPI and PNPI.
Compound
HOMO/LUMO (eV)a
HOMO/LUMO (eV)b
FF
Eg(eV)
PL (nm)
Solution
Abs (nm)
Tg/Tm/Td (ꢁC)
Film
ANPI
2-NaNPI
PNPI
ꢀ5.76/ꢀ2.66
ꢀ5.75/ꢀ2.64
ꢀ5.77/ꢀ2.45
ꢀ5.65/ꢀ2.31
ꢀ5.74/ꢀ2.27
ꢀ5.45/ꢀ2.50
0.62
0.61
0.48
3.11
3.01
3.32
400, 420
420, 435
391, 401
448
448
457
347, 365, 386
358, 375, 396
331, 344
115/288/396
168/365/469
128/230/446
a
Determined from the onset of oxidation potentials and the Eg ¼ HOMO-LUMO.
b
Values from DFT calculation.

96
S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
100
ANPI
2-NaNPI
PNPI
80
60
40
20
ANPI
2-NaNPI
PNPI
230 C
288 C
168 C
365 C
115 C
128 C
100
200
300
400
500
600
700
50 100 150 200 250 300 350 400 450
Temperature(oC)
Temperature (oC)
Fig. 1. (a) DSC curves of ANPI, 2-NaNPI and PNPI; (b) TGA curves of ANPI, 2-NaNPI and PNPI.
substrates were treated with oxgen plasma for 5 min. The deposi-
characterized fully by 1H and 13C NMR, mass spectrometry, and
elemental analysis.
tion rate of organic compounds was 0.9e1.1 A sꢀ1. Finally, a cathode
ꢀ
composed of cesium pivalate (2 nm) and aluminium (100 nm) was
sequentially deposited onto the substrate under a vacuum of
10ꢀ5 Torr. The LeVeJ curves of the devices were measured with a
Keithley 2400 Source meter and PR655. All measurements were
carried out at room temperature under ambient conditions. By the
way, in counting the layers of a device, we only consider the hole
transport layer, emissive layer and electron transport layer.
3.2. Thermal properties
The thermal properties of the three compounds have been
investigated by thermal gravimetric analysis (TGA) and differential
scanning calorimetry (DSC) under a nitrogen atmosphere, and the
related data is listed in Table 1. As shown in Fig. 1, compounds ANPI,
2-NaNPI and PNPI exhibit good thermal stability with decompo-
sition temperatures (Td, 5% weight loss) of 396, 469 and 446 ꢁC,
respectively. All the compounds exhibit high glass transition tem-
peratures (Tg) in the range of 115e168 ꢁC in differential scanning
calorimetry, while endothermic melting transition temperatures
(Tm) appear differentially at 288, 365 ꢁC and 230 for ANPI, 2-NaNPI
and PNPI, respectively. The high Tg and Tm values demonstrate that
the introduction of functional rigid group at N1 position of phenyl
phenanthroimidazole can greatly improve their thermal stability.
In contrast to our previously reported ACPI, 1-NaCPI, 1-NaCPI,
discernible glass transition temperatures peak was observed, which
can be ascribed to the sp3 hybridization of N1. No doubt that those
materials are stable enough to be fabricated into devices by vacuum
thermal evaporation technology, which can improve the film
morphology and reduce the possibility of phase separation upon
heating, and prolong the lifetime of the devices.
3. Results and discussion
3.1. Synthesis
The synthetic protocol of the three target compounds, ANPI, 2-
NaNPI and PNPI is depicted in Scheme 1. The synthesis of the
anthracen-9-ylboronic acid and (10-(naphthalen-2-yl)anthracen-
9-yl) boronic acid follows the previously reported strategy [8a,18].
Pyren-1-ylboronic acid and the intermediate 1 were prepared ac-
cording to the literature methods [19,20,12b]. Then Pd-catalyzed
Suzuki cross-coupling of aryl boric acid and compound 1 gave the
target compounds in excellent yields, which were purified by a
simple column chromatography. Repeated temperature-gradient
vacuum sublimation is required for further purification of these
materials when used in OLEDs. The three new compounds were
Fig. 2. AFM topographic images of the three compounds in unannealed and annealed films.

S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
97
Fig. 3. Three-dimensional structures and calculated HOMO and LUMO density maps of ANPI, 2-NaNPI and PNPI.
3.3. Morphology properties
3.4. Theoritical calculations
Since excellent film-forming property of emitting materials in
OLEDs is crucial to the performance of the EL devices, the surface
morphologies of vacuum-deposited thin films for the three com-
pounds (ANPI, 2-NaNPI and PNPI) have been examined by atomic
force microscopy (AFM). For comparison purposes, we prepared
both unannealed films and thermally annealed samples films at
120 ꢁC for 1 h under a N2 atmosphere. As showed in Fig. 2, the
unannealed films exhibit a root-mean-square (RMS) roughness of
0.467, 0.285, and 0.296 nm for ANPI, 2-NaNPI and PNPI, respec-
tively, while the annealed films exhibit a fairly smooth surface
morphology with a root-mean-square (RMS) roughness of 0.285,
0.178, and 0.200 nm for ANPI, 2-NaNPI and PNPI, respectively. The
smaller RMS difference between unannealed and annealed film for
both 2-NaNPI and PNPI indicates their more excellent thermal and
amorphous stability, which demonstrate in turn that the intro-
duction of different rigid moieties at the N1 position of phenyl
phenanthroimidazole can effectively influence the assembling
modes of the molecules in the thin films.
To get an insight into the electronic structures of the three new
compounds, density functional theory (DFT) calculations were
performed at B3LYP/6-31G(d) level. The three-dimensional geom-
etries and the frontier molecular orbital energy levels of the three
compounds are shown in Fig. 3 and the calculated highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) energy levels are depicted in Table 1. The spatial
distributions of electron density of HOMO for the three compounds
ANPI, 2-NaNPI and PNPI, are mainly localized on the phenan-
throimidazole units and the LUMO orbitals are dispersed in the
anthracene or pyrene moieties. The separation of spatial distribu-
tions of HOMO and LUMO can be rationalized by the broken
conjugation at N1 position of phenyl phenanthroimidazole. It
should be noted that the HOMOs/LUMOs of those previously re-
ported ACPI, 1-NaCPI and 2-NaCPI are all distributed along
anthracene moiety [15]. Moreover, the calculated geometries of
ANPI, 2-NaNPI and PNPI indicate their non-planar molecular
structures. And non-planarity arising from the attached
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
ANPI
(a)
Film FP
(b)
2-NaNPI
0.8
ANPI
2-NaNPI
PNPI
PNPI
0.6
0.4
0.2
Solution FP
0.0
300 320 340 360 380 400 420 440
250 300 350 400 450 500 550 600 650
Wavelength (nm)
Wavelength (nm)
Fig. 4. (a) Normalized absorption spectra of compound ANPI, 2-NaNPI and PNPI in dichloromethane; (b) Normalized photoluminescence spectra in dichloromethane and film state.

98
S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
anthracence, 9-(naphthalen-2-yl)anthracene or pyrene moiety at
N1 position provides steric hindrance and prevents intermolecular
interactions. In addition, the energy gaps of ANPI, 2-NaNPI and
PNPI locate at 3,34, 3.47 and 2.95 eV, respectively. ANPI and 2-
NaNPI are in good agreement with the experimental results
(Table 1). For the compounds PNPI, the calculated data deviate
from the experimental result, which may be induced by the solvent
effect.
3.5. Photophysical properties
The UVevisible absorption and emission spectra of ANPI, 2-
NaNPI and PNPI in both CH2Cl2 solution and solid state are pre-
sented in Fig. 4. ANPI and 2-NaNPI have similarly structured ab-
sorption spectra in solution in the range of 350e410 nm, which
can be assigned to the VeV* transition of the isolated anthracene
moiety with vibrational characteristics. In comparison to 2-NaNPI,
ANPI shows a 10 nm blue shift due to its shorter length of
conjugation. The absorption spectrum of PNPI shows similar vi-
bration characteristics to pyrene molecule [20], but the peak po-
sition is red-shifted due to the extension of conjugation. Upon
excitation at 330 nm, the maximum emission wavelengths of
ANPI, 2-NaNPI and PNPI in CH2Cl2 solution were observed at 400,
420 and 391 nm, and a shoulder at 420, 435, 401 nm, respectively.
Compared to ANPI, 2-NaNPI shows bathochromic shift of about
20 nm, due to the lengthened conjugation by the naphthalene
moiety. Moreover, the steric hindrance of 2-naphthyl leads to an
increasing non-planarity property of naphthalene ring to anthra-
cene group, which also annotates the reason that 2-NaNPI shows
smaller bathochromic shift than ANPI [18]. The larger shoulder of
ANPI can be attributed to a more efficient intramolecular charge
transfer from the donor phenanthroimidazole group to the
acceptor anthracene moiety, due to smaller steric hindrance. On
the other hand, the maximum emission peaks of ANPI, 2-NaNPI
and PNPI in solid state are red-shifted by 48, 28 and 66 nm,
respectively, compared to the corresponding ones in CH2Cl2 so-
lution. The feature is reasonable for those organic and polymer
materials with intermolecular interactions in the solid state.
Moreover, a gradual decreasing red-shift effect was observed with
an increasing elongation of conjugation and planarity, PNPI
(66 nm) > ANPI (48 nm) > 2-NaNPI (28 nm), implying that the
intermolecular interactions in PNPI occured seriously in the
ground state. In addition, the full widths at half maximum
(FWHM) of the solid state of 2-NaNPI is smaller than those of
ANPI and PNPI. It indicates that aryl substituents at the 9- and 10-
positions of the anthracene disable the molecular packing in the
solid state and effectively inhibit excimer formation as well as
fluorescence quenching. The quantitative enhancement of emis-
sion was evaluated by the photoluminescence (PL) quantum yields
Fig. 5. The CV measurement of ANPI, 2-NaNPI and PNPI.
of oxidation and their absorption spectra. The HOMO levels of the
three compounds vary in a range from ꢀ5.75 eV to ꢀ5.77 eV and
LUMO levels vary in a range of ꢀ2.45 w ꢀ2.66 eV (Table 1). The
nearly same HOMO levels could be rationalized that the oxidation
centers mainly focus on the phenanthroimidazole group, which is
in good agreement with the calculated values. The spatial distri-
butions of electron densities of HOMOs for the three compounds
are mainly localized on the phenanthroimidazole unit.
3.7. Electroluminescence
To evaluate EL device performances of ANPI, 2-NaCPI and PNPI
as blue emitters, we fabricated devices IeIII with typical three-
layers configuration as follows: ITO/2-TNATA(60 nm)/NPB
(10 nm)/emitting layer (40 nm)/Alq3 (15 nm)/(CH3)3CCOOCs
(2 nm)/Al (100 nm). Indium tin oxide (ITO) was used as the anode,
40,4"-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine
(2-
TNATA) or MoO3 was used as the hole-injection layer, 4,4 -bis[N-
(1-naphthyl)-N-phenyl-l-amino]biphenyl (NPB) or 1,1- bis[4-[N,N0-
di(p-tolyl)amino]phenyl]cyclohexane (TAPC) was chosen as the
hole transporting layer (HTL), tris(8-hydroxy-quinolinato)
aluminium (Alq3) or 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)
benzene (TPBI) was selected as the electron transporting layer
(ETL), and (CH3)3CCOOCs was used as the electron injecting layer
[21]. The related HOMO/LUMO energy levels of the materials are
illustrated in Scheme 2, and key performance parameters of the
devices are summarized in Table 2.
The current density-voltage-luminance (IeVeL) and current
density-current efficiency characteristics are shown in Fig. 6.
Simple devices I, II and III utilizing compounds ANPI, 2-NaNPI
and PNPI as hosts exhibit threshold voltage of 3.74, 3.32,
3.59 V, respectively (see Table 2). In addition, at the same current
density, the operating voltage of device III is lower than those of
(Ff), using 9,10-diphenylanthracene as a standard. In toluene, the
three blue emitters ANPI, 2-NaNPI and PNPI exhibit quantum
yields of 0.62, 0.61 and 0.48, respectively. The relative lower
quantum yields for the three compounds compared to C-
substituted phenanthroimidazole derivatives [15] might be caused
by the sp3 hybrid of N1.
3.6. Electrochemical properties
To evaluate material’s usefulness in optoelectronic applications,
the electrochemical properties of ANPI, 2-NaNPI and PNPI were
further studied in solution through cyclic voltammetry (CV) mea-
surements. CV analyses indicate the three compounds display
similarly reversible oxidation waves with an oxidative onset po-
tential of about 0.96 V (Fig. 5). The HOMO and LUMO energy levels
of the three compounds were calculated from the onset potentials
Scheme 2. Energy-level diagram of the materials used in the devices.

S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
99
Table 2
EL performances of devices IeVI.
c,d
e,f
Device
Von (V)a
Lmax(Voltage) (cd mꢀ2) (V)b
hP (lm Wꢀ1
)
hC (cd Aꢀ1
)
hext (%)g
L(Voltage) (cd mꢀ2) (V)h
lem (FWHM) (nm)i
CIE (x, y)j
I
II
3.74
3.32
3.59
2.93
2.96
3.07
3.21
2.91
3.20
3.17
3.18
2.55
2.53
2.52
7554 (10.75)
9224 (11.00)
12250 (10.30)
8914 (10.50)
10960 (11.50)
13680 (11.75)
11300 (13.00)
4193 (10.00)
22290 (11.25)
14680 (10.50)
11840 (9.25)
23800 (9.25)
21860 (9.00)
22000 (10.00)
1.01 (0.73)
1.51 (0.73)
1.76 (1.03)
2.42 (1.17)
2.51 (1.20)
2.68 (1.34)
2.33 (1.16)
0.99 (0.48)
4.64
5.21
3.38
6.42
5.55
2.41 (2.50)
3.24 (2.36)
3.64 (3.06)
4.99 (3.39)
5.70 (3.98)
6.75 (4.77)
6.84 (4.68)
1.75 (1.33)
11.94
12.85
7.81
12.97
10.61
1.56
2.71
2.82
3.63
3.69
3.82
3.45
1.16
5.92
6.40
3.67
5.97
5.40
5.87
479 (7.50)
583 (6.75)
644 (6.50)
998 (6.28)
1140 (7.21)
1350 (7.90)
1368 (9.01)
350 (5.68)
2223 (8.0)
2586 (7.75)
1516 (7.25)
2580 (6.34)
2193 (5.99)
2465 (6.49)
472 (83)
460 (76)
462 (74)
468 (70)
472 (67)
484 (75)
492 (89)
468 (79)
472 (64)
472 (63)
476 (61)
472 (63)
468 (63)
472 (59)
(0.166, 0.213)
(0.154, 0.147)
(0.153, 0.163)
(0.153, 0.174)
(0.149, 0.199)
(0.150, 0.265)
(0.162, 0.301)
(0.160, 0.205)
(0.153, 0.323)
(0.155, 0.319)
(0.163, 0.351)
(0.153, 0.329)
(0.160, 0.305)
(0.157, 0.319)
III
IV
V
VI
VII
VIII
IX
X
XI
XII
XIII
XIV
5.91
12.20
a
Von: turn-on voltage.
b
c
d
e
f
Lmax: maximum luminance. Voltage: voltage at the maximum luminance.
hP: power efficiency measured at 200 mA cmꢀ2
hP: power efficiency measured at 20 mA cmꢀ2
hC: current efficiency measured at 200 mA cmꢀ2
hC: current efficiency measured at 20 mA cmꢀ2
hext: external quantum efficiency measured at 20 mA cmꢀ2
.
.
.
.
g
h
i
.
L: luminance measured at 20 mA cmꢀ2, V: voltage at 20 mA cmꢀ2
lem: emission wavelength at 20 mA cmꢀ2. FWHM: full width at half maximum at 20 mA cmꢀ2
Values at 20 mA cmꢀ2
.
.
j
.
devices I and II, which may be ascribed to the better charge
carrier transporting ability of PNPI. Devices I, II and III show a
maximum brightness of 7554, 9224, 12250 cd mꢀ2, a luminous
efficiency of 2.41, 3.24, 3.64 cd Aꢀ1, and a power efficiency of 1.01,
1.51, and 1.76 lm Wꢀ1, respectively, at a current density of
20 mA cmꢀ2. The EL spectra of I, II and III show blue light
emission with peaks centered at 472, 460 and 462 nm, respec-
tively (Fig. 7(a)). Compared with the fluorescence spectra in
films, the emission peaks of ANPI and 2-NaNPI shift to red
clearly, which may be caused by the intermolecular interactions
at the excited states. Since the compound fabricated in device
is in a thicker solid state, inevitably, the intermolecular interac-
tion and the electrical field polarization on the excited states
must be considered. Interestingly, the EL of the compound PNPI
only exhibits 5 nm red shift compared to the PL in solid state.
Taking account of the properties of previously reported ACPI,
1-NaCPI and 2-NaCPI [15], it can be concluded that the electrical
field polarization induced red shift has more impact on those
compounds possessing separated HOMO/LUMO distribution.
Meanwhile, the spatial steric configuration of the compound also
affects the electrical field polarization. The electroluminescence
(EL) spectra of the devices show similar change trends as the PL
in solid state. Narrow EL peaks in the devices IeIII with FWHMs
of 83, 76 and 74 nm, respectively, are observed, which are almost
correlated with the PLs in solid state. At a current density of
20 mA cmꢀ2, the CIE coordinates of devices I, II and III are (0.166,
0.213), (0.154, 0.147) and (0.153, 0.163), respectively (see Table 2).
The EL spectrum of the device using ANPI as the emitting layer
has a little red shift compared to those based on 2-NaNPI and
PNPI, which can be ascribed to hypochromatic shift of EL spec-
trum and broader FWHMs of ANPI. In addition, the blue shift of
device II compared to III with the similar EL peak can also be
explained by the narrower FWHMs of 2-NaNPI. Inspired by the
relative good device performance of PNPI and its weak electrical
field polarization effect, the blue electroluminescent device
based on PNPI was further optimized with the device configu-
ration IVeVII as follows: ITO/MoO3(7 nm)/TAPC(30, 40, 50,
60 nm)/PNPI(40 nm)/TPBI(10 nm)/(CH3)3CCOOCs(2 nm)/Al and
single layer device VIII ITO/MoO3(7 nm)/PNPI(40 nm)/
(CH3)3CCOOCs(2 nm)/Al. Typically, MoO3 was used as the hole
injection layer, and TPBI functioned as the electron transporting
and hole blocking layer. The current density-voltage-luminance
(IeVeL), current density-current efficiency characteristics and
EL spectra are shown in Fig. 8. Both the turn on voltages and
driving voltages of the devices IVeVIII (see Table 2) are in the
following order: VIII < IV < V < VI < VII, which coincides well
with the sequence of thickness of devices. Device VI and VII
show almost the same efficiency curves (see Fig. 8(b)) and exhibit
4
I
II
III
3
2
1
0
(b)
10000
400 (a)
I
II
3
2
1
0
1000
100
10
III
300
200
100
0
1
0
50
100
150
200
250
2
4
6
8
10
12
Current density (mA/m2)
Voltage (V)
Fig. 6. (a) Voltage-current density-luminance (VeJeL) characteristics and (b) Efficiencies curves for blue devices I, II and III.

100
S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
1.0
1.0
0.8
0.6
0.4
0.2
0.0
(a)
I
II
III
(b)
IX
X
0.8
0.6
0.4
0.2
0.0
XI
350 400 450 500 550 600 650 700 750
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Wavelength (nm)
Fig. 7. (a) EL spectrum of devices I, II and III and (b) EL spectrum of devices IX, X and XI.
the current efficiency of 6.75 and 6.86 cd Aꢀ1 at 20 mA cmꢀ2
,
been investigated, indicating an optimal BUBD-1 concentration of
3% in these hosts. Herein, the devices were further fabricated with
the following structure: ITO/2-TNATA(60 nm)/NPB (10 nm)/emit-
ting layer (40 nm): 3% BUBD-1/Alq3 (15 nm)/(CH3)3CCOOCs (2 nm)/
Al (100 nm). The detailed device performances and EL emission
characteristics are summarized in Table 2. As shown in Fig. 9, the Ie
VeL curves indicate that the driving voltage of the device XI is
lower than those of device IX and X. Devices IX, X and XI show a
maximum brightness of 22290, 14680, 11840 cd mꢀ2, luminous
efficiency of 11.94, 12.85, 7.81 cd Aꢀ1, and power efficiency of 4.64,
5.21, and 3.38 lm Wꢀ1, respectively, at a current density of 20 mA
cmꢀ2 (see Table 2). The current efficiency of device IX and X based
on ANPI and 2-NaNPI are much higher than that of the device XI
based on PNPI, which is inconsistent with the nondoped devices. In
order to explain the phenomenon, the overlaps of the fluorescence
spectra of PNPI, ANPI and 2-NaNPI in film state with their corre-
sponding absorption spectra of the dopant (see Fig. 4(b)) were
tested, respectively, and no distinct difference was found. It is
assumed that the energy transfer between host and dopant is
respectively. In the device IV, a maximum current efficiency of
4.99 cd Aꢀ1 with CIEx,y color coordinates of (x ¼ 0.15, y ¼ 0.17)
was achieved. Specifically, the device IV utilizing thinner TAPC as
hole-transporting layer shows a higher color purity than those of
the devices VeVII. It can be induced by the movement of exciton
formation area. The simplest single layer device VIII using PNPI
as electron, hole transporting layer and emitting layer exhibit the
power efficiency of 1.75 lm Wꢀ1 at 20 mA cmꢀ2. Though the
efficiency can’t be competitive with the multi-layer devices, the
simple structure is a bold essay.
The good performance of the non-doped blue OLEDs prompted
us to explore the possibility of using these new blue emissive
materials as hosts in host/dopant hybrid devices. For this purpose,
we fabricated devices IX-XI utilizing ANPI, 2-NaNPI and PNPI as
the hosts and a sky blue styrylamine-type fluorescent material, 2-
ethyl-N-(4-((E)-4-((E)-4-((2-ethyl-6-methylphenyl)(phenyl)
amino) styryl)styryl)phenyl)-5-methyl-N-phenylaniline (BUBD-1)
as dopant [22]. Devices at different dopant concentrations have
8
500 (a)
6
IV
VI
VIII
V
VII
(b)
IV
VI
VIII
V
VII
6
4
10000
1000
100
10
5
4
3
2
1
400
300
200
100
0
2
0
-2
-4
0
1
0
100
200
300
400
3
6
9
12
Current density (mA/m2)
Voltage (V)
1.0
0.8
0.6
0.4
0.2
0.0
IV
V
VI
VII
VIII
(c)
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 8. (a) Voltage-current density-luminance (VeJeL) characteristics, (b) efficiencies curves and (c) EL spectrum for blue devices IV-VIII.

S. Zhuang et al. / Dyes and Pigments 101 (2014) 93e102
101
12
9
300
250
200
150
100
50
(a)
IX
X
XI
10000
12
(b)
IX
X
XI
1000
9
6
100
10
1
6
3
0
3
0
0
2
4
6
8
10
12
0
50
100
150
200
250
Current density (mA/m2)
Voltage (V)
Fig. 9. (a) Voltage-current density-luminance (VeJeL) characteristics and (b) efficiencies curves for blue devices IX, X and XI.
[4] [a] Shih PI, Chuang CY, Chien CH, Diau EW, Shu CFH. Highly efficient non-
doped blue-light-emitting diodes based on an anthrancene derivative end-
capped with tetraphenylethlyene groups. Adv Funct Mater 2007;17:3141e6;
[b] Chiechi RC, Tseng RJ, Marchioni F, Yang Y, Wudl F. Efficient blue-light-
mainly affected by the moieties of anthracene and pyrene. To verify
the hypothesis, we used our previously reported blue emissive
materials ACPI, 1-NaCPI and 2-NaCPI as hosts, and fabricated the
BUBD-1 doped device XIIeXIV, and found the devices base on the
three hosts containing anthracene moiety exhibited excellent effi-
ciencies (see Table 2).
Peak wavelengths of the EL spectra of the devices IX-XI are very
similar to the emitter BUBD-1 [22,23]. This suggests that the
emission peaks are indeed derived from the dopant and the energy
transfer from the hosts to the dopant is complete. In addition, CIE
coordinates of devices IX, X and XI are (0.153, 0.323), (0.155, 0.319)
and (0.163, 0.351), respectively.
emitting electroluminescent devices with
a robust fluorophore: 7,8,10-
triphenylfluoranthene. Adv Mater 2006;18:325e8.
[5] [a] Tang S, Liu M, Lu P, Xia H, Li M, Xie Z, et al. A molecular glass for deep-blue
organic light-emitting diodes comprising a 9,9’-spirobifluorene core and pe-
ripheral carbazole groups. Adv Funct Mater 2007;17:2869e77;
[b] 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;
[c] Chen CH, Huang WS, Lai MY, Tsao WC, Lin JT, Wu YH, et al. Single-Layer,
Blue, fluorescent OLEDs, hosts for single-layer, phosphorescent OLEDs. Adv
Funct Mater 2009;19:2661e70;
[d] Fisher AL, Linton KE, Kamtekar KT, Pearson C, Bryce M, Petty MC. Efficient
deep-blue electroluminescence from an ambipolar fluorescent emitter in a
single-active-layer device. Chem Mater 2011;23:1640e2;
4. Conclusions
[e] Liu C, Li Y, Zhang Y, Yang C, Wu H, Qin J, et al. Solution-processed undoped
deep-blue organic light-emitting diodes based on starburst oligofluorenes
with a planar triphenylamine core. Chem Eur J 2012;18:6928e34;
[f] Lai MY, Chen CH, Huang WS, Lin JT, Ke TH, Chen LY, et al. Benzimidazole/
amine-based compounds capable of ambipolar transport for application in
single-layer blue-emitting OLEDs and as hosts for phosphorescent emitters.
Angew Chem Int Ed 2008;47:581e5.
In summary, three new robust blue-emissive materials have
been developed based on anthracene/pyrene and phenanthro[9,10-
d]imidazole functional core. The three compounds possess high
glass transition temperature and are stable enough to be fabricated
into devices by vacuum thermal evaporation technology. The
undoped device using PNPI in the typical three layers exhibits a
maximum current efficiency of 4.99 cd Aꢀ1 with CIEx,y color co-
ordinates of (x ¼ 0.15, y ¼ 0.17). Meanwhile, the property differ-
ences between N1-substituted and C2-substituted of diphenyl-1H-
phenanthro[9,10-d]imidazole derivatives were studied in detail.
When doped with sky blue fluorescent material BUBD-1, all the
blue host ANPI, 2-NaNPI, ACPI, 1-NaCPI and 2-NaCPI containing
anthracene moiety show an impressive luminous current efficiency
around 12 cd Aꢀ1. The primary results suggest that the efficient
energy transfer between fluorescence host and dopant are affected
not only by the Förster energy transfer mechanism, but also the
complicated interface effect among the hybrids.
[6] [a] Adachi C, Tsutsui T, Saito S. Blue light-emitting organic electroluminescent
devices. Appl Phys Lett 1990;56:799e801;
[b] Shi J, Tang CW. Anthracene derivatives for stable blue-emitting organic
electroluminescence devices. Appl Phys Lett 2002;80:3201e3;
[c] Lee MT, Chen HH, Liao CH, Tsai CH, Chen CH. Stable styrylamine-doped
blue organic electroluminescent device based on 2-methyl-9,10-di(2-
naphthyl)anthracene. Appl Phys Lett 2004;85:3301e3;
[d] Kim YH, Jeong HC, Kim SH, Yang K, Kwon SK. High-purity-blue and high-
efficiency electroluminescent devices based on anthracene. Adv Funct Mater
2005;15:1799e805.
[7] Chao TC, Lin YT, Yang CY, Hung TS, Chou HC, Wu CC, et al. Highly efficient UV
organic light-emitting devices based on bi(9,9-diarylfluorene)s. Adv Mater
2005;17:992e6.
[8] [a] Huang H, Fu Q, Zhuang S, Liu Y, Wang L, Chen J, et al. Novel deep blue OLED
emitters with 1,3,5-tri(anthracen-10-yl)-benzene-centered starburst oligo-
fluorenes. J Physic Chem C 2011;115:4872e8;
[b] Kanibolotsky AL, Berridge R, Skabara PJ, Perepichka IF, Bradley DDC,
Koeberg M. Synthesis and properties of monodisperse oligofluorene-
functionalized truxenes: highly fluorescent star-shaped architectures. J Am
Chem Soc 2004;126:13695e702;
Acknowledgments
[c] Luo J, Zhou Y, Niu ZQ, Zhou QF, Ma Y, Pei J. Three-dimensional architectures
for highly stable pure blue emission. J Am Chem Soc 2007;129:11314e5.
[9] Kuo CJ, Li TY, Lien CC, Liu CH, Wu FI, Huang MJ. Bis(phenanthroimidazolyl)
biphenyl derivatives as saturated blue emitters for electroluminescent de-
vices. J Mater Chem 2009;19:1865e71.
[10] Wang Z, Lu P, Chen S, Gao Z, Shen F, Zhang W, et al. Phenthro[9,10-d]imid-
azole as a new building block for blue light emitting materials. J Mater Chem
2011;21:5451e6.
This work was supported by Wuhan Science and Technology
Bureau (01010621227), the Doctoral Fund of the Ministry of Edu-
cation of China (20100142120049) and NSFC/China (21161160442).
X. Zhu thanks financial support from Hong Kong Baptist University
(FRG2/12-13/050).
[11] [a] Zhang Y, Lai SL, Tong QX, Chan MY, Ng TW, Wen ZC, et al. Synthesis and
characterization of phenanthroimidazole derivatives for applications in
organic electroluminescent devices. J Mater Chem 2011;21:8206e14;
[b] Yuan Y, Li D, Zhang X, Zhao X, Liu Y, Zhang J, et al. Phenanthroimidazole-
derivative semiconductors as funcitional layer in high performance OLED.
New J Chem 2011;35:1534e40.
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nondoped deep-blue organic light emitting devices based on imidazole-
V-triphenylamine derivatives. Chem Mater 2012;24:61e70;
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