Unsymmetrically amorphous 9,10-disubstituted anthracene derivatives for high-efficiency blue organic electroluminescence devices

Article abstract of DOI:10.1016/j.dyepig.2010.10.007

Two unsymmetrically amorphous 9,10-disubstituted anthracene derivatives have been synthesized and characterized. They emit the light in the blue region and possess glass transition temperature of 132.5 °C and 133.8 °C. The scanning electron microscope (SE

Full text of DOI:10.1016/j.dyepig.2010.10.007

Dyes and Pigments 89 (2011) 155e161
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Unsymmetrically amorphous 9,10-disubstituted anthracene derivatives
for high-efficiency blue organic electroluminescence devices
,
,
*
Jinhai Huang a, Bo Xu a, Mei-Ki Lam b, Kok-Wai Cheah b, Chin H. Chen b c, Jian-Hua Su a
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science & Technology, Shanghai 200237, PR China
b Centre for Advanced Luminescence Materials, Department of Physics, Hong Kong Baptist University, Kowloon Tong, Hong Kong, PR China
c Displays & Lighting Centre, SEIEE, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two unsymmetrically amorphous 9,10-disubstituted anthracene derivatives have been synthesized and
characterized. They emit the light in the blue region and possess glass transition temperature of 132.5 ꢀC
and 133.8 ꢀC. The scanning electron microscope (SEM) was employed in studying the thin-film
morphological stability of the two unsymmetrical isomers. These two compounds were fabricated into
typical trilayers devices with pure blue lighting performance. In addition, a series of highly efficient and
stable sky blue devices doped with a sky blue dopant BUBD-1 have been fabricated by using these two
isomers as the host.
Received 3 September 2010
Received in revised form
12 October 2010
Accepted 16 October 2010
Available online 11 November 2010
Keywords:
OLEDs
Ó 2010 Elsevier Ltd. All rights reserved.
Anthracene derivatives
Amorphous materials
Morphological stability
Symmetry
High efficiency
1. Introduction
Anthracene has been intensively studied as an attractive
building block and starting material in OLEDs, due to its unusual
Organic light-emitting diodes (OLEDs) have drawn great scien-
tific and commercial attention, due to their potential applications in
full-color, flat-displays as well as solid-state lighting [1e3]. Despite
many efforts to improve the device performance, compared to the
efficient and stable red and green device, sufficiently wide band gap
of highly efficient and stable blue luminescent materials are still rare
[4,5]. The thermal stability is one very important factor for the device
operation efficiency and lifetime, because the morphological change
of the organic layers caused by Joule heat during device operation
limits the device durability [6,7]. Therefore, amorphous materials
with high glass transition temperature (Tg) are significantly required
to effectively suppress formation of grain boundaries [8e13].
Most of the recent studies have been devoted to optimize the
morphological stability and photophysical properties of the blue
emitters [10e16]. However, most synthesized stable blue-emitting
materials are still far from commercial application because of the
insufficient efficiency and the complicated synthesis routes, as well
as excessively expensive cost.
photoluminescence and electroluminescence properties and excel-
lent electrochemical properties, as well as easier modification
[15,17e23]. Moreover, the fluorescence spectral color of anthracene
can be easily tuned from blue to green by introducing the different
bulks at the C-9 and 10 positions [24]. Among these anthracene
derivatives, the 9,10-di(2-naphthyl)anthrance (ADN) was firstly
reported as blue host and 2,5,8,11-tetra (t-butyl)-perylene (TBP) as
dopant to achieve an EL efficiency of around 3.5 cd/A with a blue
emission CIE(x,y) of (0.154,0.232) and a half-time of 4000 h at an
initial light output of 700 cd/m2 [25]. However, the film morphology
of ADN was found to be unstable and easily tend to crystallize after
long operation [26]. Compounds with an unsymmetrical structure
have less tendency to crystallize and favor amorphous morphology,
thus yielding higher stability over that of the symmetric structures
[11, 27]. 2-Methyl-9,10-di-(2-naphthyl) anthracene (MADN), a well-
known and deeply investigated blue emitter, was engineered based
on ADN. The methyl substituent at C-2 position was introduced into
the anthracene unit for slightly disrupting the symmetry of ADN,
which can effectively suppress the problematic crystallization to
fabricate the stable and amorphous film [28,29]. But a moderate Tg
and a low luminance efficiency are the key problems for its wide-
spread commercial use [30]. Thus, blue light-emitting materials with
* Corresponding author. Tel./fax: þ86 21 64252288.
E-mail address: bbsjh@ecust.edu.cn (J.-H. Su).
0143-7208/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2010.10.007

156
J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
high efficiency, excellent morphological and thermal stability, as
well as simple synthetic routes should be persistently investigated
for the full-color displays and solid-lighting.
2.3. Synthesis
2.3.1. 2-Methyl-9-(2-naphthyl)anthracene (MNA)
In this study, we sought to pursue two new high performance
blue light-emitting amorphous materials from MADN based
on the relationship between the molecular structures and the
morphological stability. 2-methyl-9-(2-naphthyl)-10-(4-biphenyl)
anthracene (MNBPA) and 2-methyl-9-(4-biphenyl)-10-(2-naph-
thyl) anthracene (MBPNA) were easily synthesized with good yield
by using biphenyl moiety instead of one of the naphthyl moiety in
the MADN structure. The slight change with the unsymmetrical
substituent at C-9 and -10 positions of anthracene, not only
efficiently improved the efficiency, but also strengthened amor-
phous morphological stability.
2-Methyl-9-bromoanthracene (2 g, 7.38 mmol) and 2-nathphyl
boronic acid (1.53 g, 8.89 mmol) were mixed in 10 ml of THF. K2CO3
(2.0 M,10 ml) was added, and the mixture was stirred with magnetic
stirring. Then tetrakis(triphenylphsosphine) palladium (100 mg,
0.075 mmol) was added to the mixture. The reaction solution was
heated to reflux for 12 h under the atmosphere of nitrogen. After the
mixture cooled, the solvent was evaporated and the product was
extracted with dichloromethane. The organic was washed with
brine and water, and then dried by anhydrous MgSO4. The solvent
was evaporated, and the residue was purified by using column
chromatography with DCM: n-Hexane ¼ 1:3 eluent to afford a white
solid (2.1 g, 89.7%). 1H NMR: (400 MHz, CDCl3,
d): 8.45 (s, 1H),
7.94e8.04 (m, 4H), 7.87e7.89 (m, 2H), 7.51e7.61 (m, 4H), 7.38e7.41
2. Experimental
(m, 2H), 7.24e7.29 (m, 2H), 2.34 (s, 3H). 13C NMR (400 MHz, CDCl3,
d): 22.23, 124.73, 124.93, 125.33, 126.15, 126.34, 126.48, 126.77,
2.1. General information
127.91, 127.93, 128.04, 128.15, 128.28, 128.40, 129.64, 130.05, 130.11,
130.58, 130.90, 132.74, 133.45, 135.17, 135.74, 136.59. HRMS (m/z)
calculated for C25H18: 318.1409, Found [Mþ]: 318.1404.
Unless otherwise specified, all reactions and manipulations
were performed under nitrogen atmosphere using standard Schlenk
techniques. All chemical reagents were used as received from
commercial sources without further purification. And the solvents
were dried using standard procedures. 1H and 13C NMR spectra were
recorded on Brüker AV-400, spectrometer tetramethylsilane (TMS)
as the internal standard. The film morphologies were recorded
using an LEO1530 FE-SEM. High-resolution mass spectrometric
measurements were carried out using a Brüker autoflex MALDI-TOF
mass spectrometer. UVevis spectra were obtained on a Varian Cary
200 spectrophotometer. Fluorescence spectra were obtained on
a Perkin Elmer LS55 luminescence spectrometer with the excitation
at 380 nm. The concentration was adjusted so that the absorbance of
the solution would be lower than 0.1.The differential scanning
calorimetry (DSC) analysis was performed under a nitrogen atmo-
sphere on a TA Instruments DSC 2920 with a heating and cooling
scan rate of 20 ꢀC/min. Thermogravimetric analysis (TGA) was
undertaken using a TGA instrument under a nitrogen atmosphere
with a heating scan rate of 20 ꢀC/min. The quantum yield of
2-methyl-9,10-di(2-naphthyl)anthracene was set as 100% in CH2Cl2
as standard [27]. Cyclic voltammetric (CV) measurements were
carried out in a conventional three electrode cell using a Pt button
working electrode of 2 mm in diameter, a platinum wire counter
electrode, and an SCE reference electrode on a computer-controlled
EG&G Potentiostat/Galvanostat model 283 at room temperature.
Reduction CV of all compounds was performed in dichloromethane
containing Bu4NPF6 (0.1 M) as the supporting electrolyte. The E1/2
values were determined by (Epa þ Epc)/2, where Epa and Epc are the
anodic and cathodic peak potentials, respectively. Ferrocene was
used as an external standard. Electrochemistry was done at a scan
rate of 100 mV/s.
2.3.2. 2-Methyl-9-(2-naphthyl)-10-bromoanthracene (MNBA)
2-Methy-9-(2-naphthyl)anthracence (1.45 g, 4.68 mmol) was
mixed in 10 ml of dehydrated dimethylformamide (DMF). After
the material was dissolved, N-bromosuccinimide (NBS) (0.92 g,
5.15 mmol) was added at 50 ꢀC, and the resultant mixture was stirred
with magnetic stirring for 10 h. After the reaction was completed, the
reaction solution was poured into purified water, and formed crystals
were separated by filtration. The separated crystals were recrystallized
from THF and ethanol to afford a yellow solid (1.58 g, 85.4%). 1H NMR:
(400 MHz, CDCl3,
d
): 8.58e8.60 (d, 1H, J ¼ 8.8 Hz), 8.51e8.53 (d, 1H,
J ¼ 9.2 Hz), 8.00e8.05 (m, 2H), 7.88e7.90 (d, 1H, J ¼ 6.8 Hz), 7.49e7.62
(m, 5H), 7.39e7.43 (m, 2H), 7.28e732 (m, 1H), 2.36 (s, 3H). 13C NMR
(400 MHz, CDCl3, d): 21.86, 122.75, 125.48, 125.54, 126.34, 126.52,
126.55, 127.32, 127.80, 127.87, 127.94, 128.06, 128.15, 128.98, 129.36,
129.72, 129.85, 130.10, 131.38, 132.81, 133.35, 135.48, 136.14, 136.51.
HRMS (m/z) calculated for C25H17Br: 396.0514, Found [Mþ]: 396.0502.
2.3.3. 2-Methyl-9-(2-naphthyl)-10-(4-biphenyl)anthracene
(MNBPA)
MNBPA was prepared from MNBA and 4-biphenyl boronic acid
as described for MNBA. To afford a yellow solid (90.3%). 1H NMR:
(400 MHz, CDCl3,
d
): 8.07e8.09 (d, 1H, J ¼ 8.4 Hz), 8.02e8.04 (m,
1H), 7.98 (s, 1H), 7.92e7.94 (m, 1H), 7.83e7.85 (d, 2H, J ¼ 6.8 Hz),
7.71e7.73 (d, 1H, J ¼ 8.4 Hz), 7.66e7.68 (m, 2 H), 7.53e7.63 (m, 7 H),
7.51 (s, 1H), 7.40e7.46 (m, 1H), 7.27e7.34 (m, 2H), 7.19e7.21 (d, 1H,
J ¼ 9.2 Hz), 2.35 (s, 3H). 13C NMR (400 MHz, CDCl3,
d): 21.99, 124.69,
125.03, 125.12, 126.15, 126.35, 126.92, 127.01, 127.08, 127.18, 127.36,
127.43, 127.54, 127.91, 127.90, 128.15, 128.59, 128.82, 128.90, 129.45,
129.69, 130.21, 130.28, 131.79, 132.75, 133.47, 134.84, 135.91, 136.73,
136.84, 138.18, 140.22, 140.88. HRMS (m/z) calculated for C37H26
:
470.2035, Found [Mþ]: 470.2032.
2.2. Device fabrication
2.3.4. 2-Methyl-9-(4-biphenyl)anthracene (MBPA)
Prior to the deposition of organic materials, indiumetin-oxide
(ITO)/glass was cleaned with a routine cleaning procedure and
pretreated with oxygen plasma, and then coated with a polymer-
ized fluorocarbon (CFx) film. Devices were fabricated under
about 10ꢁ6 Torr base vacuum in a thin-film evaporation coater.
The currentevoltageeluminance characteristics were measured
with a diode array rapid scan system using a Photo Research PR650
spectrophotometer and a computer-controlled, programmable,
direct-current (DC) source. All measurements were carried out in
ambient atmosphere at room temperature.
MBPA was prepared from 2-methyl-9-bromoanthracene and
4-biphenyl boronic acid as described for MNA. To afford a white
solid (88.6%). 1H NMR: (400 MHz, CDCl3,
d): 8.44 (s, 1 H), 8.00e8.02
(d, 1H, J ¼ 8.0 Hz), 7.37e7.60 (d, 1H, J ¼ 8.8 Hz), 7.75e7.81 (m, 4H),
7.67e7.69 (d, 1H, J ¼ 8.8 Hz), 7.46e7.52 (m, 5H), 7.37e7.45 (m, 2H),
7.28e7.37 (m, 2H), 2.41 (s, 3H). 13C NMR (400 MHz, CDCl3,
d): 22.29,
124.73, 124.88, 125.30, 126.41, 126.74, 127.06, 127.17, 127.43, 128.02,
128.26, 128.38, 128.92, 130.05, 130.44, 130.89, 131.74, 135.11, 135.59,
137.98, 140.07, 140.88. HRMS (m/z) calculated for C27H20: 344.1565,
Found [Mþ]: 344.1565.

J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
157
2.3.5. 2-Methyl-9-(4-biphenyl)-10-bromoanthracene (MBPBA)
126.39, 126.90, 126.94, 127.03, 127.09, 127.15, 127.42, 127.89, 127.91,
127.98, 128.08, 128.72, 128.90, 129.58, 130.14, 130.15, 130.20, 131.82,
132.73, 133.41, 134.78, 135.83, 136.71, 136.79, 138.25, 140.10, 140.86.
HRMS (m/z) calculated for C37H26: 470.2035, Found [Mþ]: 470.2052.
MBPBA was prepared from MBPA as described for MNBA.
To afford a yellow solid (85.4%). 1H NMR: (400 MHz, CDCl3,
d):
8.57e8.60 (d, 1H, J ¼ 9.2 Hz), 8.50e8.53 (m, 3H), 7.76e7.82 (m, 4H),
7.67e7.70 (d, 1H, J ¼ 8.8 Hz), 7.50e7.58 (m, 3H), 7.34e7.47 (m, 6H),
2.43 (s, 3H). 13C NMR (400 MHz, CDCl3,
d): 21.93, 122.70, 125.42,
3. Results and discussion
125.51, 126.55, 127.12, 127.17, 127.28, 127.39, 127.54, 127.79, 127.86,
128.95, 129.71, 129.84, 131.25, 131.63, 135.43, 136.36, 137.55, 140.37,
140.72. HRMS (m/z) calculated for C27H19Br: 424.0650, Found [Mþ]:
424.0635.
3.1. Synthesis
The synthetic routes to the two new compounds are shown in
Scheme 1. 2-Methyl-9-bromoanthracene, naphthyl boronic acid,
4-biphenly boronic acid were provided by e-Ray Optoelectronics
Technology Co., Ltd. Suzuki arylearyl coupling reactions and
bromination reactions were employed in all the synthesis with
a good yield. All compounds were availably purified with the silica
column chromatographic method or recrystallization because of
good solubility in the common solvents. Further purification was
readily accomplished by train sublimation in vacuum (10ꢁ6 Torr).
The compounds were verified by 1H NMR, 13C NMR, high-resolution
mass spectrometry.
2.3.6. 2-Methyl-9-(4-biphenyl)-10-(2-naphthyl)anthracene
(MBPNA)
MBPNA was prepared from MBPBA and 2-naphtyl boronic acid as
described for MNA. To afford a yellow solid (92.2%). 1H NMR:
(400 MHz, CDCl3,
d
): 8.05e8.06 (d,1H, J ¼ 8.0 Hz), 7.97e8.02 (m, 2H),
7.89e7.91 (m, 1H), 7.84e7.86 (d, 1H, J ¼ 8.4 Hz), 7.75e7.78 (m, 1H),
7.69e7.71 (d, 1H, J ¼ 8.4 Hz), 7.50e7.65 (m, 9H), 7.39e7.43 (t, 1H,
J ¼ 7.6 Hz), 7.25e7.34 (m, 2H), 7.14e7.16 (d, 1H, J ¼ 8.8 Hz), 2.39 (s, 3
H). 13C NMR (400 MHz, CDCl3,
d): 22.03,124.72,125.00,125.07,126.17,
B(OH)2
i)
ii)
Br
+
Br
MBA
MNA
MNBA
B(OH)2
i)
Br
+
MNBA
MNBPA
B(OH)2
ii)
i)
Br
Br
+
MBPA
MNA
MBPBA
B(OH)2
i)
Br
+
MBPBA
MBPNA
i) Pd(PPh3)4, K2CO3, THF; ii) NBS, DMF
Scheme 1. Synthetic routes to MNBPA and MBPNA.

158
J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
Table 1
Physical data for the compounds.
Quantum yieldb Vf
HOMO/LUMO (eV)
Eg (eV)
Td (ꢀC)
Tm (ꢀC)
Tg (ꢀC)
a
UVmax (nm)
in CH2Cl2
PLmax (nm)
in CH2Cl2
PLmax (nm)
in film
MADN
MNBPA
MBPNA
400, 262
400, 263
400, 263
431
432
431
448
452
452
1
1.13
1.10
5.5c/2.5c
5.58/2.60
5.58/2.60
3.0c
2.98
2.98
397c
401.8
412.1
255c
236.5
265.2
120c
132.5
133.8
a
Film on quartz plate (40 nm).
In CH2Cl2 solution, Ref. [30].
Ref. [26].
b
c
3.2. Thermal properties
This suggests the color of the anthracene derivatives are kept in
pure blue region in term of using the diphenyl moiety instead of
naphthyl moiety. The absorption peaks at 262 nm and 263 nm can
The thermal properties of the new compounds were determined
by differential scanning calorimetry (DSC) and thermogravimetric
analysis (TGA) measurements (Table 1). The two isomers, MNBPA and
MBPNA, possess thermal decomposition temperatures (Td; 5% weight
loss) as high as 402 ꢀC and 412 ꢀC, and exhibit the melting temper-
atures (Tm) at 236.5 ꢀC and 265.2 ꢀC. Moreover, MNBPA and MBPNA
have a similar Tg values, 132.5 ꢀC and 133.8 ꢀC, respectively, both of
which are higher than that of MADN (120 ꢀC) [26]. This is because the
unsymmetrical nature of the two new hosts with biphenyl group
instead of one naphthyl group may dramatically hampers the facile
packing of the molecules and enhances the Tg value [12].
be assigned as the
pe
p* transition of biphenyl and naphthyl
moiety. Also, the PL spectra of the synthesized compounds in thin
films show a little red-shift compared with those in solution due to
the solid-effect [32], but the phenomena are also similar to MADN
and do not show significant excimer emission. For convenient
comparison with MADN, we set the quantum yield (V) as 1 and
calculated that of MNBPA and MBPNA to be 1.13 and 1.10. Both of
them improve the quantum yield to around 10% higher than MADN.
The improved quantum yield should be expected to increase the EL
efficiency.
For further confirming the intrinsic amorphous characteristics,
thin-film morphology of these unsymmetrical isomers and MADN
were investigated to unveil the morphological stability by scanning
electron microscope (SEM). There is no morphological change on
the MNBPA film after annealing at 80 ꢀC for 1 h under air atmo-
sphere shown in Fig.1(a). Moreover, we also cannot find the MBPNA
film surface crystalline as shown in Fig. 1(b), when we improve the
annealing temperature to 110 ꢀC for 1 h under a vacuum atmo-
sphere. In contrast, the film of MADN induced considerable crys-
tallization after annealing at 80 ꢀC for 1 h under air atmosphere as
shown in Fig. 1(c). As we know, crystallization is a serious problem
The HOMO and LUMO energy levels of the isomers were also
listed in Table 1. The HOMO energies of the anthracene derivatives
are ꢁ5.58 eV, which were determined by cyclic voltammetry (CV)
analyses. The wide optical energy band gaps (Eg) were found about
2.98 eV, determined from the threshold of UVevis absorption
spectra. The LUMO values are ꢁ2.60 eV, calculated based on the
HOMO energy levels and energy band gaps.
3.4. Electroluminescent properties
To study the electroluminescent performance of the two
asymmetrical isomers, we fabricated six devices (1e6), with
the following device configurations: ITO/CFx/NPB (60 nm)/Host
(40 nm)/Alq3 (15 nm)/LiF (1 nm)/Al (device 1: Host ¼ MBPNA;
device 2: Host ¼ MNBPA); ITO/CFx/2-TNATA (60 nm)/NPB (10 nm)/
Host: 3% BUBD-1(40 nm)/TPBI (20 nm)/LiF (1 nm)/Al (device 3:
Host ¼ MBPNA; device 4: Host ¼ MNBPA); ITO/CFx/EHI (60 nm)/
NPB (10 nm)/Host: 3% BUBD-1 (40 nm)/Alq3 (20 nm)/LiF (1 nm)/Al
(device 5: Host ¼ MBPNA; device 6: Host ¼ MNBPA), where indium
tin oxide (ITO) was used as the anode, polymerized fluorocarbon
(CFx) and 4,40,400-tris[N- (1-naphthyl)-N-phenylamino] triphenyl-
amine (2-TNATA) were the hole-injection layer, 4,40-bis[N-(1-
naphthyl)-N-phenyl-l-amino]biphenyl (NPB) was used as the hole
transporting layer, Alq3 or TPBI was used as the electron trans-
porting layer, and LiF was used as the electron injecting layer, EHI is
a new hole-injection material. The detailed device structures are
shown in Fig. 3. Devices 1 and 2 were the typical trilayer non-doped
for molecular
p-conjugated materials. The boundary of crystalline
regions in OLEDs layers usually act as traps for mobile electrons or
holes as they move past [31]. Hence, the two amorphous unsym-
metrical isomers with a high Tg can efficiently hinder crystallization
to promote the device lifetime.
3.3. Photophysical and electrochemical properties
Fig. 2 shows the absorption and PL spectra of the unsymmetrical
isomers and MADN in dilute dichloromethane solution as well as in
solid thin films deposited on quartz plates. The similar absorption
and PL spectra in solution of the two isomers exhibit the charac-
teristic vibrational structure of the anthracene moiety, which are
identical to them of MADN. They have similar absorption peaks in
the range from 350 to 400 nm, and these characteristic vibronic
bands are assigned to the pe
p* transitions of the anthracene core.
Table 2
Detailed data of the EL performance for the compounds.
Max EL spectrab
(nm)
CIE(x,y)
c
Device
Turn-on
Max brightness
(cd/m2)
Max quantum
efficiency (%)
Max power
efficiency (lm/W)
Max current
efficiency (cd/A)
voltage (V)a
1
2
3
4
5
6
3.6
3.9
2.9
2.8
3.5
3.5
3157
3740
12090
8057
14060
12890
0.72
0.82
6.34
4.07
4.75
3.15
0.59
0.70
6.46
3.82
4.38
3.01
0.92
1.07
10.22
6.7
7.85
5.32
452
456
464
460
464
464
(0.164,0.156)
(0.162,0.160)
(0.152,0.222)
(0.153,0.231)
(0.148,0.236)
(0.154,0.244)
a
At 1 cd/m2.
b
c
The maximum EL spectra for devices 1e4 at 20 mA/m2 and devices 5e6 at 6 V.
The CIE(x,y) for devices 1e4 at 20 mA/m2 and devices 5e6 at 6 V.

J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
159
Fig. 1. SEM images of (a) MNBPA, (b) MBPNA, (c) MADN.
a
devices. Also, the highly efficient sky blue dopant BUBD-1 was
employed in the devices 3e6 with a common concentration of 3%.
The current densityevoltageeluminance (IeVeL) characteris-
tics for the devices 1e6 are shown in Fig. 4. The devices with
MBPNA as host emitting material present better performance than
the devices with MNBPA in both the current density and luminance
at the same applied voltage. They are attributed to the better film
spectra performance of the MBPNA. After doping 3% BUBD-1, the
luminance was significantly improved. For instance, the maximum
luminance for the devices 3e6 were 12090 cd/m2 (at 13.2 V),
8057 cd/m2 (at 13.9 V), 14060 cd/m2 (at 8.5 V), 12890 cd/m2
(at 9.5 V), respectively, while those for devices 1e2 were 3157 cd/
m2 (at 11.7 V) and 3740 cd/m2 (at 11.8 V). However, the devices 5e6
with a new hole injection material indicate the best current density
characteristics in the three device types. The different current
densities are attributed to the different hole injections, electron
transporting materials and the thickness. Moreover, as shown in
Fig. 4, the good hole injection material (EHI) and a doped system
(BUBD-1) will be helpful to improve the OLEDs performance.
Fig. 5(a) shows the current densityecurrent efficiency charac-
teristics for the six devices. The current efficiencies of doped devices
are several times higher than the non-doped system. In the pure
blue devices 1 and 2, these two devices indicate the mostly identical
current efficiency. In the doped devices, the MBPNA-based devices
display a higher efficiency than the MNBPA-based devices. More-
over, device 3 reveals a better efficiency than device 5, and the same
cases were happened in the MNBPA-based devices. The maximum
current efficiency of the devices 3e6 are 10.22 cd/A (at 6.53 V),
6.7 cd/A (at 9.07 V), 7.85 cd/A (at 7 V), and 5.32 cd/A (at 8 V),
respectively, which are several times higher than those of the
non-doped devices 1e2 (0.92 cd/A and 1.07 cd/A, respectively).
Meanwhile, for the devices 1, 2, 5 and 6, the EL efficiency rises
sharply to a maximumvalue and then displays a nearly flat response
from 10 mA/cm2 to 200 mA/cm2. Although the current efficiency
slightly decreases as the current density increases, the efficiencies
of devices 3 and 4 at 200 mA/cm2 are 6.04 cd/A and 4.02 cd/A,
1.0
0.8
0.6
0.4
0.2
0.0
UV-MADN
UV-MNBPA
UV-MBPNA
PL-MADN
PL-MNBPA
PL-MBPNA
300
400
500
600
Wavelength (nm)
b
1.0
0.8
0.6
0.4
0.2
0.0
MADN
MNBPA
MBPNA
450
500
550
600
650
Wavelength (nm)
Fig. 2. (a) The absorption and PL spectra of MADN, MNBPA, MBPNA in dichloro-
methane solution. (b) The film PL spectra of the compounds.

160
J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
a
12
Al
Al
Al
device 1
device 2
device 3
device 4
device 5
device 6
LiF (1 nm)
Alq3 (15 nm)
Host (40 nm)
NPB (70 nm)
ITO
LiF (1 nm)
Alq3 (20 nm)
Host:BUBD-1(40 nm)
NPB (10 nm)
EHI (60 nm)
ITO
10
8
LiF (1 nm)
TPBI (20 nm)
Host:BUBD-1(40nm)
NPB (10 nm)
2-TNATA (60 nm)
ITO
6
4
2
0
device 1, 2
device 3, 4
device 5, 6
0
50
100
150
200
250
Current (mA/cm2)
Fig. 3. Device structures.
b
7
6
respectively, which are still very efficient. These results suggest that
the two isomers are stable in the device and can be the promising
candidates for blue host materials in OLEDs. Fig. 5(b) reveals the
dependence of the power efficiency and current density charac-
teristics for the devices. The power efficiency drop down sharply as
the current density rises for devices 3 and 4 compared with those
for devices 5 and 6. That is attributed to the higher applied voltage
at the same current density in the devices 3 and 4. The detailed data
are summarized in Table 2.
device 1
device 2
device 3
device 4
device 5
device 6
5
4
3
EL spectra for the devices 1e4 at 20 mA/cm2 and for the devices
5e6 at 6 V are displayed in Fig. 6. The spectrum peaks of the non-
doped devices based on MBPNA and MNBPA at 20 mA/cm2 are
452 nm (0.164, 0.156) with a full with at half maximum of 76 nm and
456 nm (0.162, 0.160) with a full with at half maximum of 76 nm,
both of which are similar to the film PL spectra, indicating that all of
the emissions originate from the emitting layer. The BUBD-1 doped
devices exhibit the almost identical spectra with a dominating peak
at around 456 nm with a similar full with at half maximum of 60 nm
and a shoulder peak at around 490 nm. In comparison with the
non-doped devices, the emission from the host materials are totally
quenched, according to the doped spectra, suggesting the Förster
energy transfer form the host to the dopant is very efficient which is
beneficial to improve the EL performance. Moreover, there is almost
no EL color shift for all devices with varying driving current density
and voltage. For example, the CIE(x,y) color coordinates of device 3
only shifted from (0.152, 0.225) at 0.1 mA/cm2 to (0.161,0.226) at
250 mA/cm2 with ΔCIE(x,y) ¼ (0.01,0.001).
2
1
0
-1
0
50
100
150
200
250
Current (mA/cm2)
Fig. 5. (a) Current densityecurrent efficiency characteristics for the devices. (b) The
power efficiencyecurrent density characteristics for the devices.
1.0
device 1
400
device 2
10000
device 1
device 2
0.8
device 3
device 4
300
200
100
0
device 3
device 4
device 5
device 6
device 5
device 6
0.6
0.4
0.2
0.0
1000
100
400
450
500
550
600
650
700
0
2
4
6
8
10
12
14
Wavelength (nm)
Voltage (V)
Fig. 4. Current densityevoltageeluminance (IeVeL) characteristics for the devices.
Fig. 6. EL spectra for the devices 1e4 at 20 mA/cm2 and for the devices 5e6 at 6 V.

J. Huang et al. / Dyes and Pigments 89 (2011) 155e161
161
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In summary, we have synthesized two anthracene derivatives
isomers with a blue-emitting characteristic. The bipheny moiety
was introduced to anthracene core instead of the naphthyl moiety to
enhance the thermal and morphological stabilities. The pure blue
OLED devices were obtained in the non-doped system with a CIE(x,y) of
(0.164,0.156) for MBPNA and (0.162,0.160) for MNBPA at 20 mA/cm2 in
devices 1 and 2, respectively. Moreover, the Förster energy transfer
from the host to the dopant is very efficient which is beneficial to
improve the EL performance. A high luminance of 14060 cd/m2 (at
8.5 V) for device 5 and a high maximum current efficiency of 10.22 cd/
A (at 6.5 V) for device 3 in a sky blue dopant system are achieved.
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emitting diodes incorporating an anthracene derivative end-capped with
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Acknowledgments
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This work was financially supported by NSFC/China, National
Basic Research 973 Program and Hong Kong Baptist University.
J. Huang is deeply grateful for some suggestion from Dr. Z.Y. Xia, and
assistance from M.K. Lam. The author also thanks e-Ray Optoelec-
tronics Technology Co., Ltd. of Taiwan for generously supplying
some materials studied in this work.
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