Controllable molecular configuration for significant improvement of blue OLEDs based on novel twisted anthracene derivatives

Article abstract of DOI:10.1016/j.dyepig.2015.03.005

Two novel twisted anthracene derivatives, 2-(4-(10-(phenanthren-9-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]-imidazole (p-PABPI) and 2-(3-(10-(phenan-thren-9-yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro-[9,10-d]imidazole (m-PABPI), have been synthesized. Their photophysical and photochemical properties are also investigated systemically. The non-doped fluorescent organic light-emitting diodes are fabricated by using anthracene derivatives as the emitters. The maximum current efficiencies are achieved to be 3.98 and 1.32 cd A^-1 and the maximum power efficiencies are 2.80 and 1.14 lm W^-1, respectively. The external quantum efficiency maximum (EQE_max) is 3.61% and 1.33% for p-PABPI and m-PABPI. Intriguingly, the efficiencies of p-PABPI are almost three times larger than that of m-PABPI with only the different molecular configuration. The results revealed a new rule of molecular design based on anthracene derivatives for obtaining high performance blue emission materials.

Full text of DOI:10.1016/j.dyepig.2015.03.005

Dyes and Pigments 118 (2015) 137e144
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Controllable molecular configuration for significant improvement of
blue OLEDs based on novel twisted anthracene derivatives
a
,
b
c
b
b
a
c, *
Juan Wang , Xia Lou , Yaqing Liu , Guizhe Zhao , Amjad Islam , Suidong Wang
Ziyi Ge
a
,
a, *
Ningbo Key Laboratory of Silicon and Organic Thin Film Optoelectronic Technologies, Ningbo Institute of Materials Technology & Engineering, Chinese
Academy of Sciences, Ningbo 315201, PR China
b
Institute of Materials Science & Engineering, The North University of China, Taiyuan 030051, PR China
Institute of Functional Nano & Soft Materials, Soochow University, Suzhou 215123, PR China
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 15 December 2014
Received in revised form
Two novel twisted anthracene derivatives, 2-(4-(10-(phenanthren-9-yl)anthracen-9-yl)phenyl)-1-
phenyl-1H-phenanthro[9,10-d]-imidazole (p-PABPI) and 2-(3-(10-(phenan-thren-9-yl)anthracen-9-yl)
phenyl)-1-phenyl-1H-phenanthro-[9,10-d]imidazole (m-PABPI), have been synthesized. Their photo-
physical and photochemical properties are also investigated systemically. The non-doped fluorescent
organic light-emitting diodes are fabricated by using anthracene derivatives as the emitters. The
3
February 2015
Accepted 4 March 2015
Available online 12 March 2015
ꢀ
1
maximum current efficiencies are achieved to be 3.98 and 1.32 cd A and the maximum power effi-
ꢀ
1
ciencies are 2.80 and 1.14 lm W , respectively. The external quantum efficiency maximum (EQEmax) is
.61% and 1.33% for p-PABPI and m-PABPI. Intriguingly, the efficiencies of p-PABPI are almost three times
Keywords:
Blue emission
Twisted anthracene derivatives
Molecular configuration
OLEDs
3
larger than that of m-PABPI with only the different molecular configuration. The results revealed a new
rule of molecular design based on anthracene derivatives for obtaining high performance blue emission
materials.
Significant improvement
Fluorescent emitters
© 2015 Elsevier Ltd. All rights reserved.
1. Introduction
triplet excitons, and EQEmax of blue electrophosphorescence re-
ported over 20% [7e10]. However, blue phosphorescent emitters
Organic light-emitting diodes (OLEDs) have been attracting
great scientific and industrial attention due to their potential ap-
plications in high-quality flat-panel displays and solid-state light-
ing [1e3]. For full-color display, red (R), green (G), and blue (B)
emission of relatively equal stability, efficiency, and color purity is
immensely required. Among the color of RGB, the blue emitter is of
special significance because such emitters not only effectively
reduce the power consumption, but can also be utilized to generate
light of other colors by energy cascade to a lower-energy fluores-
cent or phosphorescent dopants [4]. However, owing to their
intrinsic band gap, the state of art performance of blue OLEDs is
inferior to that of green and red [4e6]. Therefore, blue OLEDs with
high efficiency have been urgently required to kick off the
commercialization of OLEDs.
are less preferable than their fluorescent counterparts because of
the difficulties encountered in the synthesis of large band-gap
dopants and poor color purity, stability, longevity and expensive
price [11]. Hence, design and synthesis of highly efficient blue
fluorescent emitters is still a key requirement to accelerate the
industrialization of OLEDs.
Since the first report about organic electroluminescence from
organic crystals of anthracene was observed in the 1960s [12], there
has been great interest in the study of anthracene derivatives as an
attractive building block for OLEDs. However, anthracene with its
intrinsic planarity and grid structure easily causes fluorescence
concentration quenching and emission wavelength bathochromic
shift in the solid state. In this regard, Adachi et al. reported one of
the anthracene derivatives (DPA) by introducing two phenyl units
at 9,10-positions of anthracene and achieved a luminescence of
Recently, great efforts have been made to devise efficient blue
phosphors due to their advantage of harvesting both singlet and
ꢀ
2
ꢀ2
0.09 cd m at 100 mA cm [13]. However, the reported DPA
showed easy crystallization in the solid-state thin film, which
would directly lead to low device efficiency. Then, Lee et al.
designed 2-methyl-9,10-di(2-naphthyl)anthracene based on the
*
Corresponding authors.
E-mail addresses: wangsd@suda.edu.cn (S. Wang), geziyi@nimte.ac.cn (Z. Ge).
http://dx.doi.org/10.1016/j.dyepig.2015.03.005
143-7208/© 2015 Elsevier Ltd. All rights reserved.
0

138
J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
structure of DPA, and the electroluminescent devices with styryl-
2.2.1. Preparation of 4-(10-bromoanthracen-9-yl)benzaldehyde (1)
In a 500 mL round-bottomed flask charged with nitrogen, 9,10-
dibromoanthracene (9.21 g, 18 mmol), (4-formylphenyl)boronic
acid (1.35 g, 9 mmol) and bis(triphenyl phosphine)palladium(II)
chloride (0.31 g, 0.9 mmol) were dissolved in THF (100 mL) and
stirred for 10 min. A solution of 10 mL potassium carbonate (2 M)
ꢀ1
amine as dopant were fabricated, and the efficiencies of 9.7 cd A
ꢀ
1
ꢀ2
and 5.5 lm W at 20 mA cm were achieved. It means the crys-
tallization in the film can be efficaciously suppressed [14]. Prior to
the symmetrical structure, the unsymmetrical structure can be a
promising candidate for the formation of efficient, stable and
amorphous film. For example, Tian et al. reported an unsymmet-
rical anthracene derivative with triphenylamine and imidazole, the
non-doped device showed a maximum current efficiency of
ꢁ
was added and the reaction mixture was heated to 70 C for 12 h.
The reaction mixture was cooled to room temperature and
extracted with dichloromethane and water. The organic layer was
evaporated with a rotary evaporator. The crude product was puri-
ꢀ1
3
.33 cd A [15]. Subsequently, Zhuang et al. designed and syn-
thesized three novel blue-emitters with unsymmetrical structure
by introducing 9-(naphthalen-2-yl)anthracene or pyrene, and
fied by column chromatography to give a yellow powder. Yield:
1
1.65 g, 51%. H NMR (400 MHz, CDCl
3
)
d
10.20 (s, 1H), 8.56e8.65 (m,
¼ 8.39 Hz), 7.54e7.64 (m, 4H), 7.48
(t, 2H, J ¼ 7.45 Hz), 7.38 (dd, 2H, J ¼ 10.92 Hz, J ¼ 7.76 Hz), which
was consistent with the reported literature [20].
ꢀ
1
gained a maximum current efficiency of 4.99 cd A with CIEx,y
0.15, 0.17) for the non-doped OLED devices [16]. Recently, Liu et al.
1H), 8.09 (dd, 3H, J
1
¼7.92 Hz, J
2
(
1
2
reported two twisted anthracene derivatives and showed high ef-
ficiency deep-blue emission [17]. Notably, the singlet generation
0
fraction is more than 25% by using 10,10 -(Bis-9-pentylcarbazol-3-
2.2.2. Preparation of 3-(10-bromoanthracen-9-yl)benzaldehyde (2)
0
yl)-[9,9 ]-bianthryl (CzBACz) as emitter. In view of this, it is highly
Using the same procedure described for (1), 3-(10-
desirable to develop novel materials with twisted D-A structure to
achieve high efficiency. Some progress has been made to obtain
high efficiency by synthesizing some materials with twisted donor-
acceptor (D-A) structures [18,19]. Therefore, the research about
twisted anthracene derivatives is still of significant importance to
the development of the blue OLEDs.
bromoanthracen-9-yl)benzald-ehyde was synthesized. Yield:
1
1.77 g, 54.6%. H NMR (400 MHz, CDCl
3
)
d
10.10 (s, 1H), 8.61 (d, 2H,
J ¼ 8.94 Hz), 8.07 (d, 1H, J ¼ 7.57 Hz), 7.91 (s, 1H), 7.75 (t, 1H,
J ¼ 7.70 Hz), 7.66 (d, 1H, J ¼ 7.55 Hz), 7.58 (t, 2H, J ¼ 7.70 Hz), 7.52 (d,
2H, J ¼ 8.83 Hz), 7.37 (t, 2H, J ¼ 7.49 Hz), which was consistent with
the reported literature [21].
Herein, we report synthesis and characterization of two novel
twisted anthracene derivatives isomers, 2-(4-(10-(phenanthren-9-
yl)anthracen-9-yl)phenyl)-1-phenyl-1H-phenanthro[9,10-d]imid-
azole (p-PABPI) and 2-(3-(10-(phenanthren-9-yl)anthracen-9-yl)
phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (m-PABPI). It
is particularly interesting to compare the electroluminescent
properties of p-PABPI and m-PABPI, both have the same con-
structed unit. When the molecular configuration of these de-
rivatives is changed, their EL devices are exhibited similar blue
emission but electroluminescent efficiency is varied almost three
times.
2.2.3. Preparation of 4-(10-(phenanthren-9-yl)anthracen-9-yl)
benzaldehyde (3)
In
a
three necked flask filled with nitrogen, 4-(10-
mmol), 9-
bromoanthracen-9-yl)benzald-ehyde (1.44 g,
4
phenanthreneboronic acid (1.11 g, 5 mmol) and bis(triphenyl
phosphine)palladium(II)chloride (0.14 g, 0.2 mmol) were dissolved
in THF (30 mL), forming yellow solution. A solution of 10 mL po-
tassium carbonate (2 M) was added and the reaction mixture was
ꢁ
heated to 70 C for 10 h. After that, the reaction mixture was cooled
to room temperature and extracted with dichloromethane and
water for three times. The organic layer was evaporated with a
rotary evaporator. The crude product was purified by column
2
. Experimental
1
chromatography to give a yellow powder. Yield: 1.10 g, 60%. H NMR
(
400 MHz, CDCl
3
)
d
10.25 (s, 1H), 8.89 (dd, 2H, J
1
¼ 4.23 Hz,
2
.1. Materials and measurements
J
2
¼ 3.83 Hz), 8.19 (t, 2H, J ¼ 6.16 Hz), 7.94 (d,1H, J ¼ 7.58 Hz), 7.88 (s,
1
7
7
H), 7.80 (d, 2H, J ¼ 7.58 Hz), 7.76 (dd, 2H, J
1
¼ 6.45 Hz, J
2
¼ 8.27 Hz),
Fourier transform-infrared (FTIR) spectra were performed using
.68 (t, 4H, J ¼ 7.90 Hz), 7.62 (d, 2H, J ¼ 8.85 Hz), 7.38e7.33 (m, 3H),
Thermo Nicolet 6700 spectrophotometer. Nuclear magnetic reso-
.24 (t, 2H, J ¼ 7.58 Hz); ESI-MS (m/z): 459.2 (Mþ þ H).
1
13
nance ( H NMR & C NMR) spectra were recorded on a Bruker
DRX-400 spectrometer with chemical shifts. ESI-Ms spectra were
measured with an FINNIGAN Trace DSQ mass spectrometer at
7
2
recorded on a PerkineElmer Lambda 950 spectrophotometer.
Photoluminescent (PL) measurements were with a FLSP920 spec-
trophotometer in a solution of 10
respectively. Thermogravimetric analyses (TGA) were carried out
using a PerkineElmer Pyris thermogravimeter at 10 C min under
nitrogen flushing. Cyclic voltammetry (CV) measurements were
performed on a CHI 660D electrochemical workstation with a Pt
disk as the working electrode, a Pt wire as the counter electrode,
and Ag/AgCl as the reference electrode, in a dichloromethane so-
2
.2.4. Preparation of 3-(10-(phenanthren-9-yl)anthracen-9-yl)
benzaldehyde (4)
-(10-(phenanthren-9-yl)anthracen-9-yl)benzaldehyde
obtained with the same procedure described for (3). Yield: 1.12 g,
0 eV. Elemental analyses were performed with a PerkineElmer
400 II elemental analyzer. UVevis absorption spectra were
3
was
1
6
2
1%. H NMR (400 MHz, CDCl
H, J ¼ 3.69 Hz), 8.16e8.14 (m, 2H), 7.95 (dd, 1H,
¼ 3.57 Hz, J
¼ 4.04 Hz, J ¼ 3.95 Hz), 7.91e7.84 (m, 3H), 7.79 (t, 1H,
3
)
d
10.20 (d, 1H, J ¼ 4.99 Hz), 8.90 (dd,
ꢀ
6
1
2
mol/L and a solid state,
J
1
2
J ¼ 7.62 Hz), 7.69 (dd, 4H J
1
¼ 7.90 Hz, J ¼ 8.64 Hz),7.63 (d, 2H,
2
ꢁ
ꢀ1
J ¼ 8.87 Hz), 7.36 (t, 3H, J ¼ 7.75 Hz), 7.26 (t, 3H, J ¼ 4.25 Hz); ESI-MS
þ
(m/z): 458.1 (M ).
2
.2.5. Synthesis of 2-(4-(10-(phenanthren-9-yl)anthracen-9-yl)
phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (p-PABPI)
-(10-(phenanthren-9-yl)anthracen-9-yl)benzaldehyde (0.92 g,
mmol), 9,10-phenanthraquinone (0.62 g, 3 mmol) and ammo-
lution containing 0.1
fluorophosphate as the supporting electrolyte.
M
of tetrabutylammonium hexa-
4
2
nium acetate (0.3 g, 5 mmol) were dissolved in acetic acid (30 mL).
The reaction mixture was stirred in a three-necked flask for 30 min.
Then, aniline (0.46 g, 5 mmol) was added to the mixture and
2
.2. Synthesis of the compounds
ꢁ
9
,10-dibromoanthracene, (4-formylphenyl)boronic acid, (3-
refluxed overnight at 120 C. After cooled to room temperature, the
formylphenyl)boroni-c acid, 9-phenanthreneboronic acid, 9,10-
phenanthraquinone, aniline were purchased from TCI co.
reaction mixture was poured into much water, forming a large
quantity of precipitates. The solid was collected by filtration and

J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
139
dissolved in dichloromethane. The mixed organic layer was evap-
orated with a rotary evaporator. The crude product was purified by
column chromatography to give a yellow target product. Yield:
for fluorescent OLEDs that exhibits a good blue emission [18,19]. To
expand the family of twisted anthracene-type derivative with
excellent performance of OLEDs, we adopted anthracene and its
derivatives as the plane to form twisted structure. In addition,
recently we have reported one kind of the constitution isomerism
to manage their excited-state and intramolecular charge-transfer
[22,23]. Therefore, based on our group's work, two novel twisted
anthracene derivatives were designed with the same building
blocks but only difference of phenyl substitution position. As shown
in Scheme 1, the compounds 1, 2, 3 and 4 were prepared by the
Suzuki coupling with high yields. Subsequently, the target com-
ꢀ
1
1
.20 g, 83.2%. FTIR (KBr, cm ) 3060, 756, 725 (aromatic CeH), 1596,
1
1
496, 1450, 1439 (aromatic C]C), 1474 (C]N), 1345 (CeN). H NMR
400 MHz, CDCl
9.01 (d, 1H, J ¼ 7.99 Hz), 8.90 (d, 2H, J ¼ 8.16 Hz),
.84 (d, 1H, J ¼ 8.50 Hz), 8.78 (d, 1H, J ¼ 8.67 Hz), 7.94 (t, 2H,
J ¼ 7.99 Hz), 7.89 (s, 2H), 7.81 (d, 2H, J ¼ 7.65 Hz), 7.76e7.67 (m,
0H), 7.59 (dd, 4H, J ¼ 7.90 Hz), 7.51 (d, 1H,
¼ 8.69 Hz, J
J ¼ 8.10 Hz), 7.38e7.31 (m, 5H), 7.24 (t, 3H, J ¼ 7.11 Hz). C NMR
100 MHz, CDCl 135.27, 132.68, 131.73, 131.56, 130.66, 130.51,
30.44, 129.99, 129.76, 129.68, 129.18, 128.79, 128.56, 127.69, 127.49,
27.14,126.99,126.95,126.92,126.84,126.73,126.55,125.34,124.26,
(
8
3
) d
1
1
2
13
(
1
1
3
) d
pounds were obtained by condensing
3 and 4 with 9,10-
phenanthraquinone, and aniline (more than 80% yield). All the
þ
1
13
123.17, 122.88, 122.75, 121.00; ESI-MS (m/z): 723.3 (M þ H); Anal.
compounds were characterized by H and C NMR spectrometry,
mass spectrometry, and elemental analysis.
calcd for C55 : C, 91.38; H, 4.74; N, 3.88; Found: C, 91.41; H,
34 2
H N
4
.71; N, 3.85.
3.2. Thermal properties
2.2.6. Synthesis of 2-(3-(10-(phenanthren-9-yl)anthracen-9-yl)
phenyl)-1-phenyl-1H-phenanthro[9,10-d]imidazole (m-PABPI)
The thermal properties of p-PABPI and m-PABPI were measured
Using the same procedure described for p-PABPI, 2-(3-(10-
phenanthren-9-yl)a-nthracen-9-yl)phenyl)-1-phenyl-1H-phe-
using thermogravimetric analysis (TGA) under a nitrogen atmo-
sphere, and their related data are illustrated in Table 1 and Fig. 1. As
(
nanthro[9,10-d]imidazole (m-PABPI) was obtained. Yield: 1.17 g,
shown in Fig. 1, p-PABPI and m-PABPI exhibit high decomposition
ꢀ1
ꢁ
8
1%. FTIR (KBr, cm ) 3057, 754, 725, (aromatic CeH), 1597, 1496,
temperature (T
d
, 5% weight loss), which are 511 and 438 C,
1
1
(
8
451, 1438 (aromatic C]C), 1476 (C]N), 1346 (CeN). H NMR
400 MHz, CDCl
8.94 (d,1H, J ¼ 6.32 Hz), 9.90 (d, 2H, J ¼ 8.10 Hz),
.80 (d, 1H, J ¼ 8.49 Hz), 8.74 (d, 1H, J ¼ 8.49 Hz), 7.98 (dd, 2H,
¼7.11 Hz, J ¼ 7.30 Hz), 7.89 (d,1H, J ¼ 8.10 Hz), 7.82e7.75 (m, 3H),
.73e7.63 (m, 5H), 7.59e7.48 (m, 11H), 7.37 (dd, 1H, J
¼ 7.74 Hz,
¼ 7.26 Hz), 7.30 (m, 2H), 7.24 (q, 4H, J ¼ 7.50 Hz). C NMR
100 MHz, CDCl 139.28, 135.38, 135.23, 135.19, 132.65, 132.62,
32.22, 131.72, 131.68, 130.64, 130.55, 130.51, 130.45, 130.41, 130.05,
29.92, 129.84, 129.82, 129.40, 128.79, 128.73, 127.45, 127.01, 126.80,
25.52, 125.31, 124.35, 123.12, 122.87, 122.77, 121.01; ESI-MS (m/z):
respectively. The high T
d
values allow to use these derivatives in the
3
)
d
fabrication of high performance OLEDs through vacuum thermal
evaporation technology.
J
1
2
7
1
3.3. Theoretical calculations
13
J
2
(
1
1
1
3
)
d
The ground-state geometries and the frontier molecular orbital
energy levels were calculated using density functional theory (DFT)
in Gaussian 03 program. From Fig. 2, the calculated highest occu-
pied molecular orbital (HOMO) and lowest unoccupied molecular
orbital (LUMO) of p-PABPI and m-PABPI are found to be mainly
localized on the anthracene moiety. The 9,10-substituents are
þ
722.1 (M ); Anal. calcd for C55
34 2
H N : C, 91.38; H, 4.74; N, 3.88;
Found: C, 91.37; H, 4.70; N, 3.93.
ꢁ
leaned almost 90 to the anthracene core due to large torsional
2
.3. Device fabrication and characterization
stress. Both compounds with twisted structure can efficiently
prevent molecular recrystallization and excimer or exciplex for-
mation [24]. The HOMO and LUMO energy levels of p-PABPI are
calculated to be ꢀ5.11 and ꢀ2.66 eV, and the HOMO and LUMO
energy levels of m-PABPI are ꢀ5.07 eV and ꢀ2.60 eV, respectively.
Before the fabrication of OLEDs, the compounds p-PABPI and m-
PABPI were purified by vacuum sublimation with the first pipe
heating up to 360 C. The ITO-coated glass substrates were
routinely cleaned and then treated with UV-ozone for 15 min. Two
ꢁ
kinds of devices were constructed with the structure of ITO/MoO
5 nm)/NPB (40 nm)/TCTA (5 nm)/p-PABPI or m-PABPI (20 nm)/
PyPB (40 nm)/LiF (1 nm)/Al (100 nm) and ITO/MoO (5 nm)/NPB
40 nm)/TCTA (5 nm)/p-PABPI or m-PABPI: CBP (20 nm, 3 wt% or
wt%)/B PyPB (40 nm)/LiF (1 nm)/Al (100 nm). All the organic and
inorganic layers were manufactured in sequence at 10 Torr. The
deposition rate of organic compounds was 0.9e1.1 Å s . The
electroluminescence spectra and the Commission Internationale de
l'Eclairage coordination of the device were tested on a PR655
spectra scan spectrometer. The luminanceecurrent and densi-
tyevoltage characteristics were measured simultaneously from the
measurement of the EL spectra by combining the spectrometer
with a Keithley 2400 programmable voltageecurrent source. All
measurements were conducted at room ambient conditions.
3
3.4. Photophysical and electrochemical properties
(
B
(
6
3
3
The UVevis absorption and photoluminescent (PL) spectra of p-
PABPI and m-PABPI in dilute CH Cl solutions as well as in the solid
2 2
film are shown in Fig. 3. A summary of the precise photophysical
data of the compounds is also given in Table 1. The p-PABPI and m-
PABPI exhibited similar absorption spectra (360, 377 and 397 nm
for p-PABPI, 357, 375 and 396 nm for m-PABPI) in the dilute solu-
3
ꢀ
6
ꢀ
1
tion, which can be originated from the pep* transition of the iso-
lated anthracene core with archetypal vibronic characteristics [25].
In the film state, they exhibited a 3e5 nm bathochromic-shift.
Compared with UVevis absorption spectra, fluorescence spectra
2 2
of p-PABPI and m-PABPI in CH Cl showed maximum emission
wavelengths at 434 and 413 nm. Also, the shape of the spectrum in
film and solution was rather similar, but there was an about 14 nm
red-shifted in thin films. Furthermore, it is highly worth noting that
the PL spectrum of p-PABPI exhibits more red-shifted as compared
to m-PABPI. It can be assigned to the greater steric hindrance of m-
PABPI, which leads to an increase in non-planarity and prevents
close molecular stacking in the solid film [26]. The fluorescent
3
. Results and discussion
3.1. Synthesis
Scheme 1 displays the synthetic route of the molecules, which
contain an anthracene-phenanthrene and imidazole unit. Our
design of the anthracene-type compounds was inspired by the
twisted structure of anthracene derivatives, a widely used material
quantum yields (
respectively, indicating both are promising candidates for blue
F
emitters in OLEDs. Obviously, the fluorescent quantum yields (F )
F
F ) of them are 74% (p-PABPI) and 56% (m-PABPI),

140
J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
Scheme 1. The synthetic route to p-PABPI, m-PABPI.
of p-PABPI is higher than that of m-PABPI. It can be attributed to the
largely twisted structure of m-PABPI, which will reduce the
conjugation and suppress its fluorescence [27].
The HOMO energy levels of p-PABPI and m-PABPI were
measured by cyclic voltammetry (CV), and the results are shown in
Fig. 4 and Table 1. The onset oxidation peaks (Eox) of p-PABPI and m-
PABPI are 1.09 and 0.97 eV versus the ferrocenium/ferrocene redox
couple, respectively. HOMO values are calculated from the formula
p-PABPI and m-PABPI are calculated to be about 2.94 and 2.98 eV,
which are determined from the threshold of UVevis absorption
spectra. Finally, the LUMO levels of p-PABPI and m-PABPI are found
to be ꢀ2.65 and ꢀ2.39 eV.
3.5. Electroluminescent properties
Initially, to evaluate electroluminescent (EL) performances of p-
PABPI and m-PABPI, the non-doped OLEDs with the structure of
of EHOMO ¼ ꢀ([Eonset ox þ 4.4). The values of p-PABPI and m-PABPI
]
are ꢀ5.59 and ꢀ5.37 eV, respectively. Such deep HOMO energy
levels are lower than the work function of ITO, which can effectively
reduce the energy barrier for hole injection into the emission layer
g
and improve the device efficiency [28]. The energy band gaps (E ) of
Table 1
Thermal properties, optional properties and energy levels of p-PABPI and m-PABPI.
maxAbsa,b
C) (nm)
l
maxPLa,b HOMO/LUMO
exp
HOMO/LUMO
cal
Compounds
T
(
d
ꢁ
l
opt
g
c
opt
g
d
(nm)
ðE
Þ (ev)
ðE
Þ (ev)
p-PABPI
511 360, 377, 434.2
ꢀ5.59/ꢀ2.65(2.94) ꢀ5.11/ꢀ2.66(2.45)
3
3
4
97
64, 380, 448.4
01
m-PABPI
438 357, 375, 413.0
ꢀ5.37/ꢀ2.39(2.98) ꢀ5.07/ꢀ2.60(2.47)
3
3
4
96
62, 378, 426.6
01
a
2 2
Measured in CH Cl .
Measured in solid film on quartz plates.
b
c
Estimated
based
on
absorption
onset
.
and
cyclic-voltammetry.
opt
E
HOMO ¼ ꢀ(qEox þ 4.4) eV, ELUMO ¼ E
g
þ EHOMO
Fig. 1. TGA thermograms of p-PABPI and m-PABPI recorded at a heating rate of
10 C min .
d
ꢁ
ꢀ1
DFT calculation with B3LYP/6-31G.

J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
141
Fig. 2. Molecular orbital distribution and optimized geometries of p-PABPI and m-PABPI.
ITO/MoO
PABPI (20 nm)/B
fabricated by vacuum deposition. Indium tin oxide (ITO) and Al
3
(5 nm)/NPB (40 nm)/TCTA (5 nm)/EML: p-PABPI and m-
1.33% for m-PABPI from Fig. 5d. Notably, when substituted position
of phenyl is changed between anthracene and imidazole for p-
PABPI and m-PABPI, the efficiency is changed by approximately
three times, which results from the higher PL quantum yields of p-
PABPI and the significantly improved carrier injection/transport
ability of p-PABPI (Fig. S9) [30]. Interestingly, when luminance is up
to 1000 cd m , the current efficiency for p-PABPI-based devices
still maintain an upward trend. Likewise, the results illustrate that
p-PABPI has much lower roll-off. The CIE coordinates (x, y) of p-
PABPI- and m-PABPI-based devices are discovered to be (0.15, 0.13)
and (0.16, 0.12) from Table 2, which is classified as strong blue
emission. Overall, the non-doped devices based on p-PABPI as an
emitter are superior than m-PABPI-based devices.
3
PyPB (40 nm)/LiF (1 nm)/Al (100 nm) were
were utilized as anode and cathode, MoO
3
was used as a hole
0
0
injecting
layer,
N,N -bis(naphthalene-1-yl)-N,N -bis(phenyl)-
0
00
benzidine (NPB) was as a hole transporting layer, 4,4 ,4 -tri(N-
carbazolyl)benzene (TCTA) was as an electron blocking layer,
ꢀ2
00 00 0 0 00
3
,3 ,5,5 -tetra-(pyridin-3-yl)-1,1 :3 ,1 -terphenyl (B PyPB) as an
3
electron transporting and hole blocking layer, and LiF was used as
an electron injecting layer.
A and B represent the non-doped devices based on p-PABPI and
m-PABPI as emitters. Performance of the devices is shown in Table 2
and Fig. 5. From Fig. 5a, the maximum emission (lmax) of p-PABPI
and m-PABPI are located at 456 and 448 nm, and the full width at
half-maximum (FWHM) values are located at 59 and 60 nm,
respectively. There is a little red-shift for both, relative to photo-
luminescent (PL) spectra, which can be attributed to face-to-face
Then, to further discuss the potential application of the com-
pounds, the doped OLED devices with the configuration of ITO/
MoO
3
(5 nm)/NPB (40 nm)/TCTA (5 nm)/p-PABPI or m-PABPI: CBP
PyPB (40 nm)/LiF (1 nm)/Al (100 nm)
(20 nm, 3 wt% or 6 wt%)/B
3
p
e
p
stacking for the vacuum-deposited film [29]. The current
were fabricated by vacuum deposition, where CBP was used as a
host. In Table 2, The devices p1 and p2 represent 3% and 6% dopant
concentration for p-PABPI as the emitting dopant, and the devices
m1 and m2 stand for 3% and 6% dopant concentration for m-PABPI
as the emitting dopant [31].
density-voltage-luminance characteristics of the devices are given
in Fig. 5b. Maximum luminance of p-PABPI-based OLEDs are as high
as 18,980 cd m , but the devices based on m-PABPI are only
ꢀ2
ꢀ2
6140 cd m . As shown in Fig. 5c, the maximum current efficiency
ꢀ
1
for p-PABPI and m-PABPI is 3.98 and 1.32 cd A , and the maximum
power efficiency is 2.80 and 1.14 lm W , respectively. The EQEmax
of the non-doped fluorescent OLEDs are 3.61% for p-PABPI and
ꢀ1
Fig. 3. UV absorption and PL spectra of p-PABPI and m-PABPI.
2 2
Fig. 4. Cyclic voltammograms of compounds p-PABPI and m-PABPI in CH Cl .

142
J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
Table 2
Electroluminescent data of the devices.
CIEa
x,y)
L
l
maxELa
FWHMa
(nm)
CEmax
b,c
PEmaxb,c
ꢀ1
)
EQEmaxb,c
(%)
Device
max
(
(cd mꢀ2)
(nm)
ꢀ1
(cd A )
(lm W
A
B
p1
p2
m1
m2
0.15,0.13
0.16,0.12
0.15,0.08
0.15,0.09
0.15,0.04
0.15,0.04
18,980
6140
9902
9539
3748
4091
456
448
444
444
436
436
59
60
52
52
51
51
3.98/3.50/3.95
1.32/1.32/1.20
3.5/3.15/2.36
3.2/3.08/2.41
1.2/0.92/e
2.80/2.64/1.96
1.14/0.89/0.53
2.59/2.04/1.03
2.51/2.10/1.11
0.95/0.53/e
3.61/3.10/3.57
1.33/1.30/1.27
4.68/4.40/3.35
3.92/3.92/3.09
2.81/2.26/e
1.2/0.90/e
0.93/0.52/e
2.84/2.24/e
a
ꢀ2
.
At 100 cd m
b
c
ꢀ2
ꢀ2
Maximum at 100 cd m
Maximum at 1000 cd m
.
.
Electroluminescent characteristics of the doped devices are also
shown in Table 2 and Fig. 6. From Fig. 6a, the maximum emission
max) of devices of p1, p2, m1 and m2 are located at 444, 444, 436
the host to trigger short wavelength emission [32]. More impor-
tantly, prior to p-PABPI, the CIEx,y (0.15, 0.04) of m-PABPI-doped
devices is independent to dopant concentration. The independence
may be related to the more twisted molecular configuration that
prevents close molecular stacking.
(l
and 436 nm, and the full width at half-maximum (FWHM) values
are 52, 52, 51 and 51 nm, respectively. The p-PABPI-doped devices
p1 and p2 exhibited the maximum current efficiencies of 3.5 and
ꢀ
1
3
2
.2 cd
A , the maximum power efficiencies of 2.59 and
4. Conclusions
ꢀ1
.51 lm W , and the EQE of 4.68% and 3.92%, respectively. Notably,
as dopant concentration of p-PABPI is increased from 3 wt% to 6 wt
, the integral performance of OLEDs declines to a certain extent,
Two novel twisted anthracene derivatives (p-PABPI and m-
PABPI) were successfully designed and synthesized as blue emitters
%
which is ascribed to the increase of dopant concentration for p-
PABPI that may strengthen the response of concentration-
quenching. The devices p1 and p2 showed pure blue emission
with CIE coordinates of (0.15, 0.08) and (0.15, 0.09), which are close
to the blue standards (0.14, 0.08) of NTSC. Curiously, a relatively rare
blue-violet emission was found for the m-PABPI-doped devices
with CIE coordinates (x, y) of (0.15, 0.04), which may owe to a low
doping concentration to prevent red-shift and an effective host and
guest energy level pairing that enables excitons to be generated on
in OLEDs. The non-doped blue fluorescent organic light-emitting
diodes were fabricated with the structure of ITO/MoO
3
(5 nm)/
NPB (40 nm)/TCTA (5 nm)/EML: p-PABPI and m-PABPI (20 nm)/
B
3
PyPB (40 nm)/LiF (1 nm)/Al (100 nm) by vacuum deposition. The
ꢀ2
devices with p-PABPI showed the efficiencies of 18,980 cd m
,
ꢀ1
ꢀ1
3.98 cd A , 2.80 lm W as well as a blue emission with CIE co-
ordinates of (0.15, 0.13) and little efficiency roll-off and color change
at high brightness. It is particularly intriguing that the efficiencies
of p-PABPI are almost three times larger than that of m-PABPI
Fig. 5. Electroluminescent result of the non-doped devices: (a) EL spectra; (b) Current density-voltage-luminance characteristics curves; (c) Current efficiency curves and power
efficiency curves versus current density; (d) The external quantum efficiency curves.

J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
143
Fig. 6. Electroluminescent result of the doped devices: (a) EL spectra; (b) Current density-voltage-luminance characteristics curves; (c) Current efficiency curves and power ef-
ficiency curves versus current density.
through changing the constitution of phenyl between anthracene
and imidazole. In addition, the doped OLED devices with the
applications: single-layer, blue, fluorescent OLEDs, hosts for single-layer,
phosphorescent OLEDs. Adv Funct Mater 2009;19:2661e70.
3] Zhang X, Lin J, Ouyang X, Liu Y, Liu X, Ge Z. Novel host materials based on
phenanthroimidazole derivatives for highly efficient green phosphorescent
OLEDs. J Photochem Photobiol A Chem 2013;268:37e43.
[
configuration of ITO/MoO
PABPI or m-PABPI: CBP (20 nm, 3 wt% or 6 wt%)/B
1 nm)/Al (100 nm) were fabricated, where CBP was used as a host.
3
(5 nm)/NPB (40 nm)/TCTA (5 nm)/p-
3
PyPB (40 nm)/LiF
[
4] Zhu M, Yang C. Blue fluorescent emitters: design tactics and applications in
organic light-emitting diodes. Chem Soc Rev 2013;42:4963e76.
(
The m-PABPI-doped devices showed a relatively rare blue-violet
emission with CIE coordinate of (0.15, 0.04), which is indepen-
dent to dopant concentration. The results provide a new approach
of molecular design for obtaining high performance blue emission
materials based on anthracene derivatives.
[
5] Kim M-J, Lee C-W, Gong M-S. New spirobenzoanthracene derivatives with
naphthylanthracene core: synthesis and application in sky-blue fluorescent
host materials. Dyes Pigments 2014;105:202e7.
[
6] Liang H, Wang X, Zhang X, Liu Z, Ge Z, Ouyang X, et al. Saturated deep-blue
emitter based on a spiro[benzoanthracene-fluorene]-linked phenanthrene
derivative for non-doped organic light-emitting diodes. New J Chem 2014;38:
4696e701.
[
7] Sasabe H, Gonmori E, Chiba T, Li Y-J, Tanaka D, Su S-J, et al. Wide-energy-gap
electron-transport materials containing 3,5-dipyridylphenyl moieties for an ultra
high efficiency blue organic light-emitting device. Chem Mater 2008;20:5951e3.
8] Su S-J, Gonmori E, Sasabe H, Kido J. Highly efficient organic blue-and white-
light-emitting devices having a carrier- and exciton-confining structure for
reduced efficiency roll-off. Adv Mater 2008;20:4189e94.
Acknowledgments
[
This work was financially supported from the National Natural
Science Foundation of China (51273209 and 514111004), the
External Cooperation Program of the Chinese Academy of Sciences
[9] Xiao L, Su S-J, Agata Y, Lan H, Kido J. Nearly 100% internal quantum efficiency
in an organic blue-light electrophosphorescent device using a weak electron
transporting material with a wide energy gap. Adv Mater 2009;21:1271e4.
10] Dias FB, Bourdakos KN, Jankus V, Moss KC, Kamtekar KT, Bhalla V, et al. Triplet
harvesting with 100% efficiency by way of thermally activated delayed fluo-
rescence in charge transfer OLED emitters. Adv Mater 2013;25:3707e14.
11] Yook KS, Lee JY. Organic materials for deep blue phosphorescent organic light-
emitting diodes. Adv Mater 2012;24:3169e90.
12] Pope M, Kallmann HP, Magnante P. Electroluminescence in organic crystals.
J Chem Phys 1963;38:2042e3.
13] Koguchi N, Takahashi S. Double-heterostructure Pb1ꢀxꢀyCd
Pb1ꢀxꢀyCd Sr S lasers grown by molecular beam epitaxy. Appl Phys Lett
991;58:799e800.
(
(
No.
GJHZ1219),
Ningbo
Natural
Science
Foundation
201401A6105063), and Open Research Fund of State Key Labora-
[
tory of Polymer Physics and Chemistry (201404), Changchun
Institute Applied Laboratory of Polymer Physics and Chemistry,
CAS. Ms J W is also thankful to Prof. Xinhua Ouyang (NIMTE) for
analysis and discussion.
[
[
[
x y
Sr S/PbS/
Appendix A. Supplementary data
x
y
1
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.dyepig.2015.03.005.
[14] Lee M-T, Chen H-H, Liao C-H, Tsai C-H, 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.
[
15] Huang JH, Su J-H, Li X, Lam M-K, Fung K-M, Fan H-H, et al. Bipolar anthracene
derivatives containing hole- and electron-transporting moieties for highly
efficient blue electroluminescence devices. J Mater Chem 2011;21:2957e64.
16] Zhuang S, Shangguan R, Huang H, Tu G, Wang L, Zhu X. Synthesis, charac-
terization, physical properties, and blue electroluminescent device applica-
tions of phenanthroimidazole derivatives containing anthracene or pyrene
moiety. Dyes Pigments 2014;101:93e102.
References
[
[
[
1] Sasabe H, Kido J. Development of high performance OLEDs for general light-
ing. J Mater Chem C 2013;1:1699e707.
2] Chen C-H, Huang W-S, Lai M-Y, Tsao W-C, Lin JT, Wu Y-H, et al. Versatile,
benzimidazole/amine-based ambipolar compounds for electroluminescent

144
J. Wang et al. / Dyes and Pigments 118 (2015) 137e144
0
[
17] Zhang P, Dou W, Ju ZH, Yang LZ, Tang XL, Liu WS, et al. A 9,9 -bianthracene-
[25] Zhuang S, Shangguan R, Jin J, Tu G, Wang L, Chen J, et al. Efficient nondoped
blue organic light-emitting diodes based on phenanthroimidazole-substituted
anthracene derivatives. Org Electron 2012;13:3050e9.
[26] Shin M-G, Kim SO, Park HT, Park SJ, Yu HS, Kim Y-H, et al. Synthesis and
characterization of ortho-twisted asymmetric anthracene derivatives for blue
organic light emitting diodes (OLEDs). Dyes Pigments 2012;92:1075e82.
[27] Gong M-S, Lee H-S, Jeon Y-M. Highly efficient blue OLED based on 9-
anthracene-spirobenzofluorene derivatives as host materials. J Mater Chem
2010;20:10735e46.
[28] Chang Y-C, Yeh S-C, Chen Y-H, Chen C-T, Lee R-H, Jeng R-J. New carbazole-
substituted anthracene derivatives based non-doped blue light-emitting de-
vices with high brightness and efficiency. Dyes Pigments 2013;99:577e87.
[29] Xing X, Zhong L, Zhang L, Chen Z, Qu B, Chen E, et al. Essential differences of
organic films at the molecular level via vacuum deposition and solution
cored molecule enjoying twisted intramolecular charge transfer to enhance
radiative-excitons generation for highly efficient deep-blue OLEDs. Org Elec-
tron 2013;14:915e25.
[
18] Li WJ, Liu DD, Shen FZ, Ma DG, Wang ZM, Feng T, et al. A twisting donor-
acceptor molecule with an intercrossed excited state for highly efficient,
deep-blue electroluminescence. Adv Funct Mater 2012;22:2797e803.
19] Ouyang X, Zhang X, Ge Z. Enhanced efficiency in nondoped, blue organic light-
emitting diodes utilizing simultaneously local excition and two charge-
transfer exciton from benzimidazole-based donoreacceptor molecules. Dyes
Pigments 2014;103:39e49.
20] Teng C, Yang X, Yang C, Li S, Cheng M, Hagfeldt A, et al. Molecular design of
anthracene-bridged metal-free organic dyes for efficient dye-sensitized solar
cells. J Phys Chem C 2010;114:9101e10.
[
[
[
[
[
21] Teki Y, Toichi T, Nakajima S.
p
topology and spin alignment in unique
photoexcited triplet and quintet states arising from four unpaired electrons of
an organic spin system. Chem A Eur J 2006;12:2329e36.
processes for organic light-emitting diodes. J Phys Chem C 2013;117:
25405e8.
[30] Wang K, Wang S, Wei J, Chen S, 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.
[31] Hu J-Y, Pu Y-J, Satoh F, Kawata S, Katagiri H, Sasabe H, et al. Bisanthracene-
based donoreacceptor-type light-emitting dopants: highly efficient deep-
blue emission in organic light-emitting devices. Adv Funct Mater 2014;24:
2064e71.
22] Zhang X, Xu Z, Ge Z, Ouyang X, Ji W. Tunable molecular configuration for
significant enhancement of two-photon absorption based on novel octupolar
benzoimidazole derivatives. J Photochem Photobiol A Chem 2014;290:22e30.
23] Liang HJ, Wang XX, Zhang XY, Ge ZY, Ouyang XH, Wang SD. Efficient tuning of
electroluminescence from sky-blue to deep-blue by changing the constitution
of spirobenzofluorene derivatives. Dyes Pigments 2014. http://dx.doi.org/
1
0.1016/j.dyepig.2014.04.019.
[32] Jou J-H, Chen Y-L, Tseng J-R, Wu R-Z, Shyue J-J, Justin Thomas KR, et al. The use
of a polarity matching and high-energy exciton generating host in fabricating
efficient purplish-blue OLEDs from a sky-blue emitter. J Mater Chem 2012;22:
15500e6.
[
24] Cho I, Kim SH, Kim JH, Parka S, Park SY. Highly efficient and stable deep-blue
emitting anthracene-derived molecular glass for versatile types of non-doped
OLED applications. J Mater Chem 2012;22:123e9.