Highly efficient electron-transporting/injecting and thermally stable naphthyridines for organic electrophosphorescent devices

Article abstract of DOI:10.1002/adfm.201202194

A series of 1,8-naphthyridine derivatives is synthesized and their electron-transporting/injecting (ET/EI) properties are investigated via a multilayered electrophosphorescent organic light-emitting device (OLED) using fac-tris(2-phenylpyridine)iridium [Ir(ppy)₃] as a green phosphorescent emitter doped into a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host with 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (a-NPD) as the hole-transporting layer, and poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped with tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH) as the hole-injecting layer. The turn-on voltage of the device is 2.5 V using 2,7-bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9- dimethylfluorene (DNPF), lower than that of 3.0 V for the device using a conventional ET material. The maximum current efficiency (CE) and power efficiency (PE) of the DNPF device are much higher than those of a conventional device. With the aid of a hole-blocking (HB) and exciton-blocking layer of bathocuproine (BCP), 13.2-13.7% of the maximum external quantum efficiency (EQE) and a maximum PE of 50.2-54.5 lm W^-1 are obtained using the naphthyridine derivatives; these values are comparable with or even higher than the 13.6% for conventional ET material. The naphthyridine derivatives show high thermal stabilities, glass-transition temperatures much higher than that of aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate (BAlq), and decomposition temperatures of 510-518 °C, comparable to or even higher than those of Alq₃. A series of 1,8-naphthyridine derivatives with high thermal stabilities are synthesized and their electron-transporting properties for green electrophosphorescence are investigated via a multilayered organic light-emitting device using fac-tris(2-phenylpyridine)iridium [Ir(ppy) ₃] as the phosphorescent emitter. A maximum external quantum efficiency of 13.2-13.7% and a maximum power efficiency of 50.2-54.5 lm W ^-1 are obtained using the naphthyridine derivatives. Copyright

Full text of DOI:10.1002/adfm.201202194

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Highly Efficient Electron-Transporting/Injecting
and Thermally Stable Naphthyridines for Organic
Electrophosphorescent Devices
Lixin Xiao,* Xing Xing, Zhijian Chen, Bo Qu, Hsinglin Lan, Qihuang Gong,* and Junji Kido*
The efficiency of an OLED therefore
depends on the recombination efficiency
of holes with electrons and the photolumi-
nescence quantum yield of the emitter. An
internal quantum efficiency (IQE) of 25%
(corresponding to ≈5% external quantum
efficiency (EQE)) can be obtained in a fluo-
rescent emitter from the 1/4 singlet spin
states. If a phosphorescent emitting mate-
rial is used, the remaining 3/4 triplet spin
states can also emit light, therefore 100%
of the IQE (corresponding to ≈20% of the
EQE) can theoretically be obtained in an
electrophosphorescent device.[1,2] In the
case of green electrophosphorescence, fac-
tris(2-phenylpyridine)iridium [Ir(ppy)3] is
a highly efficient electrophosphorescence
emitter, and 100% photoluminescence
quantum yields have been reported by
Adachi and co-workers.[3]
Organic semiconductors are generally
better p-type conductors than they are
n-type conductors (i.e., the hole mobility
is much higher (around 1000 times) than
the electron mobility of an organic semi-
conducting material),[4] holes are much
easier to transport than electrons in an
OLED, therefore electron transport is
A series of 1,8-naphthyridine derivatives is synthesized and their electron-
transporting/injecting (ET/EI) properties are investigated via a multilayered
electrophosphorescent organic light-emitting device (OLED) using fac-tris(2-
phenylpyridine)iridium [Ir(ppy)3] as a green phosphorescent emitter doped
into a 4,4′-N,N′-dicarbazolebiphenyl (CBP) host with 4,4′-bis[N-(1-naphthyl)-
N-phenylamino]biphenyl (a-NPD) as the hole-transporting layer, and
poly(arylene ether sulfone) containing tetraphenylbenzidine (TPDPES) doped
with tris(4-bromophenyl)ammonium hexachloroantimonate (TBPAH) as the
hole-injecting layer. The turn-on voltage of the device is 2.5 V using 2,7-bis[3-
(2-phenyl)-1,8-naphthyridinyl]-9,9-dimethylfluorene (DNPF), lower than that
of 3.0 V for the device using a conventional ET material. The maximum cur-
rent efficiency (CE) and power efficiency (PE) of the DNPF device are much
higher than those of a conventional device. With the aid of a hole-blocking
(HB) and exciton-blocking layer of bathocuproine (BCP), 13.2–13.7% of the
maximum external quantum efficiency (EQE) and a maximum PE of 50.2–
54.5 lm W−1 are obtained using the naphthyridine derivatives; these values
are comparable with or even higher than the 13.6% for conventional ET
material. The naphthyridine derivatives show high thermal stabilities, glass-
transition temperatures much higher than that of aluminum(III) bis(2-methyl-
8-quinolinato)-4-phenylphenolate (BAlq), and decomposition temperatures of
510–518 °C, comparable to or even higher than those of Alq3.
1. Introduction
the limiting factor for the IQE of an OLED. To obtain a high-
efficiency electrophosphorescent device, it is crucial to develop
highly efficient electron-transporting (ET) materials. Although
tris(8-hydroxyquinoline)aluminum (Alq3)[5] and its analogs
The generation of light in an organic light-emitting device (OLED)
is the consequence of the recombination of holes and electrons
injected from the electrodes to the organic light-emitting layer.
Prof. L. Xiao
Prof. Q. Gong
State Key Laboratory for Mesoscopic Physics
and Department of Physics
Peking University
State Key Laboratory for Mesoscopic Physics
and Department of Physics
Peking University
Beijing 100871, P. R. China
E-mail: xiao66@pku.edu.cn
Beijing 100871, P. R. China
E-mail: qhgong@pku.edu.cn
X. Xing, Prof. Z. Chen, Dr. B. Qu
State Key Laboratory for Mesoscopic Physics
and Department of Physics
Peking University
Prof. J. Kido
Department of Organic Device Engineering
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
Optoelectronic Industry and Technology Development Association
(OITDA), 1-20-10 Sekiguchi, Bunkyo-ku, Tokyo 112-0014, Japan
E-mail: kid@yz.yamagata-u.ac.jp
Beijing 100871, P. R. China
Dr. H. Lan
Department of Organic Device Engineering
Yamagata University
4-3-16 Jonan, Yonezawa, Yamagata 992-8510, Japan
DOI: 10.1002/adfm.201202194
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aluminum(III) bis(2-methyl-8-quinolinato)-4-phenylphenolate
(BAlq)[6] are efficient ET materials, the triplet energies (ET)
of Alq3 and BAlq are 2.0 eV[7] and 2.2 eV,[6] respectively, both
lower than the value of 2.4 eV for Ir(ppy)3.[8] Thus, they cannot
effectively block the triplet exciton from the emitter layer of
an Ir(ppy)3-based electrophosphorescent device without com-
bining with hole-blocking (HB) and exciton-blocking materials
with wide energy gaps and high ET values. HB materials with
wide energy gaps (e.g., bathocuproine (BCP),[9] bathophenan-
throline,[10] oxadiazoles,[11,12] triazoles,[13] and 1,3,5-tris(N-phe-
nylbenzimidazol-2-yl-benzene),[14] have been used to improve
the efficiency of electrophosphorescent devices. Recently, Kido
and coworkers reported a series of phenylpyrimidines[15,16] and
phenylpyridines[17,18] as ET and HB materials with very high
electron mobilities and higher ET values than that of Ir(ppy)3.
An EQE of 29% and a power efficiency (PE) of 133 lm W−1 at
100 cd m−2 were obtained using bis-4,6-(3,5-di-3-pyridylphenyl)-
2-methylpyrimidine.[15] With the aid of the out-coupling method,
the PE can be further increased.[19] However, the thermal sta-
bility of ET materials needs to be improved.
In order to achieve this, we designed and synthesized a
series of naphthyridine derivatives, based on the electron-
withdrawing properties of the naphthyridine moiety and its
high thermal stability. Naphthyridines are widely used as anti-
bacterial substances.[20] Very recently, Liao et al.[21] reported a
series of 4-hydroxy-1,5-naphthyridine-based group-III metal
chelates as deep-blue emitters for OLEDs. In this paper, we
report a series of thermally stable 1,8-naphthyridine deriva-
tives with highly efficient ET properties, which also have HB
properties and energy gaps (2.94–3.33 eV) much wider than
that of Alq3. Efficient ET properties can therefore be expected
from the electron-withdrawing naphthyridine moiety, and holes
and/or excitons injected from the emitter layer can be expected
to be effectively blocked because of the wide energy gaps. Flu-
orene and anthracene moieties were introduced to improve the
thermal stabilities of the compounds. Their ET and HB proper-
ties were investigated via multilayered devices using Ir(ppy)3 as
the phosphorescent emitter.
2. Results and Discussion
2.1. Syntheses
The1,8-naphthyridinemoietywassynthesizedfromnicotinamide.
2-(3′-Pyridyl)pyrido[2,3-d]pyrimidineand2-aminonicotinaldehyde
were obtained, and then 2-(3-bromophenyl)-1,8-naphthyridine
was obtained via a Friedlander reaction.[22] Finally, naphthyri-
dine derivatives, 2,7-bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9-
dimethylfluorene (DNPF) and 9,10-bis[3-(1,8-naphthyridin-2-yl)
phenyl]anthracene (DNPA) were synthesized through a Suzuki–
Miyaura cross-coupling reaction[23] by introducing fluorene and
anthracene moieties, as shown in Scheme 1.
2.2. Physical Properties
The absorption spectra of the films, which were deposited by
high-vacuum (10−6 Torr) thermal evaporation on quartz, are
shown in Figure 1. The absorption peaks at around 335 nm for
DNPF and 332 nm for DNPA can be assigned to the π–π∗ tran-
sitions of the naphthyridine moiety. There were peaks at 362 nm
(shoulder), 382 nm, and 404 nm for DNPA, which can be attrib-
uted to absorptions of the anthracene moiety.[24] According to
calculations based on the absorption edges, the highest occu-
pied molecular orbital (HOMO)/lowest unoccupied molecular
orbital (LUMO) energy gap (Eg) of DNPF is 3.33 eV, and that
of DNPA is 2.94 eV, both wider than that of Alq3 (2.70 eV). The
energy gap of DNPF is even wider than that of BAlq (3.00 eV);[25]
the values are listed in Table 1. The HOMO energy levels of the
synthesized naphthyridine derivatives are deeper than that of
Alq3 (i.e., its Ip (5.85–5.93 eV), measured by atmospheric ultra-
violet (UV) photoelectron spectroscopy, is larger than that of
Alq3 (5.70 eV)). The ET level of DNPF was found to be 2.33 eV
from the onset of the phosphorescent spectrum in the film state
under excitation by a nitrogen laser (337 nm) at 4.2 K, which is
higher than those of 2.03 eV for Alq3 and 2.18 eV for BAlq. It is
therefore expected that the triplet exciton in the emitting layer
H3C CH3
H3C CH3
O
O
O
B
O
Br
Br Br
B
Br
2
1
H3C
CH3
CONH2
N
N
N
N
N
CHO
NH2
Br
N
N
N
N
N
N
N
DNPF
3
4
5
H3COC
H3COC
B(OH)2
4
+
Br
Br
N
N
N
N
COCH3
6
DNPA
Scheme 1. Syntheses of naphthyridine derivatives.
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1
A 30 nm layer of DNPF or BAlq (reference device) as the ET
and HB combined layer was then deposited on the surface of
the EM. Another HB layer, namely a 10 nm layer of BCP, and
a 20 nm layer of DNPF or DNPA as the ET layer were then
deposited on the EM surface. For comparison, a 10 nm layer
of BCP together with a 20 nm layer of Alq3 as the HB mate-
rial and ET material, respectively, were deposited as a reference
device. Finally, a 0.5 nm layer of LiF and a 100 nm layer of Al
were deposited as the cathode. The emitting area was defined,
using a shadow mask, to be 0.2 cm × 0.2 cm. The device archi-
tecture is shown in Figure 2.
DNPF
DNPA
0.8
0.6
0.4
0.2
0
The current-density–voltage curves are shown in Figure 3;
the current density of the DNPF device at low voltage is higher
than that of the BAlq device, and the turn-on voltage of the
DNPF device is 2.5 V, lower than that of 3.0 V for the BAlq
device. This indicates that the EI ability of DNPF is higher
than that of BAlq. The luminance of the DNPF device is also
higher than that of the BAlq device. The driving voltage of
3.7 V for the DNPF device at 100 cd m−2 is much lower than
that of 5.3 V for the BAlq device. Accordingly, the current effi-
ciency (CE) and PE are higher than those of the BAlq device,
especially at low current densities, as shown in Figure 4. The
maximum CE and PE of the DNPF device are 19.6 cd A−1 at
4.0 V and 18.9 lm W−1 at 3.0 V, respectively, much higher than
those of 12.5 cd A−1 at 12.0 V and 4.0 lm W−1 at 3.0 V for the
BAlq device. The device performances are listed in Table 2.
Although a 5.39% EQE was obtained for the DNPF device,
higher than that of 3.69% for the BAlq device, these values
are still not high enough for an electrophosphorescent device.
This might be interpreted in terms of the low ET of 2.33 eV
of the DNPF, deduced from the phosphorescence spectrum,
and that of 2.18 eV for BAlq, both lower than the 2.42 eV
for Ir(ppy)3, so triplet excitons cannot be blocked effectively
because of the longer diffusion paths of triplet excitons than
those of singlet excitons. The higher efficiency of the DNPF
device at low current densities than that of the BAlq device
might be because the ET level of DNPF is higher than that of
BAlq. In addition, holes cannot be effectively blocked because
of the insufficiently deep HOMO level of 5.9 eV for DNPF
and BAlq.[25]
300
400
500
600
700
800
Wavelength (nm)
Figure 1. UV–vis absorption spectra of naphthyridine films.
might be more effectively blocked by DNPF than by Alq3 and
BAlq.
The thermal properties of the naphthyridine derivatives were
measured using thermogravimetric analysis (TGA) and differ-
ential scanning calorimetry (DSC) (Table 1). The glass-transition
temperature (Tg) and decomposition temperature (Td) of DNPA
are 180 °C and 518 °C, respectively, both higher than those of
Alq3.[26] The Td of DNPF is higher than that of Alq3; although
the Tg is lower than that of Alq3 (175 °C), it is still as high as
150 °C. The Tg values of the synthesized naphthyridine deriva-
tives are much higher than that of BAlq (92 °C).[27] The higher
thermal stability of DNPA than that of DNPF can be attributed
to the higher thermal stability of the anthracene moiety com-
pared with that of the fluorene moiety. These results indicate
that the thermal stabilities of the synthesized naphthyridines
are high enough for them to be used in OLEDs.
2.3. Device Properties
To study the ET properties of the naphthyridine derivatives,
phosphorescent devices were fabricated using a 20 nm layer of
10:1 (wt) poly(arylene ether sulfone) containing tetraphenylben-
zidine (TPDPES) doped with tris(4-bromophenyl)ammonium
hexachloroantimonate (TBPAH) (TPDPES:TBPAH) as the
To improve the blocking effect for triplet excitons and
holes in the device, a 10 nm layer of BCP with an ET 2.5 eV
higher than that of Ir(ppy)3 and a much-deeper HOMO level
of 6.5 eV was inserted between the emitter and the ET layers.
For comparison, Alq3 was chosen as the ET material. From
the current-density–voltage curves (Figure 5), we found that
the current density of BCP/DNPA was larger than that of
hole-injecting material, a 30 nm layer of 4,4′-bis[N-(1-naphthyl)-
N-phenylamino]biphenyl (α-NPD) as the hole-transporting
material, and a 30 nm layer of emitting material (EM) consisting
of Ir(ppy)3 (6 wt%) as the phosphorescent emitter, codeposited
with 4,4′-N,N′-dicarbazolebiphenyl (CBP) as the host material.
T a b le 1 . Physical properties of naphthyridine derivatives.
Material
DNPF
DNPA
BAlq
Ea [eV]
2.60
2.91
2.90
3.00
3.00
Ip [eV]
5.93
5.85
5.90
5.70
5.60
Eg [eV]
3.33
2.94
3.00
2.70
2.60
ET [eV]
Reference
This work
This work
[6,25,27]
[7,26]
Tg [°C]
150
180
92
Td [°C]
510
2.33
518
2.18
2.03
2.42
Alq3
175
508
Ir(ppy)3
[8]
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Br
O
S
-
SbCl6
O
N
N
O
N+
n
O
Al (100 nm)
Br
Br
TPDPES
TBPAH
LiF (0.5 nm)
BAlq 30 nm or DNPF 30nm or
BCP (10 nm)/DNPF or DNPA or Alq3 (20 nm)
N
N
N
N
N
Ir
CBP:Ir(ppy)3 6% (30 nm)
3
CBP
-NPD
Ir(ppy)3
- NPD (30 nm)
N
TPDPES:TBPAH 10% (20 nm)
ITO
O
O
N
O
N
Al
Al
O
O
N
N
N
2
H3C
CH3
BAlq
Alq3
BCP
Figure 2. Chemical structures of organic materials used and OLED structure.
104
105
BAlq
DNPF
BAlq
DNPF
103
104
103
102
101
100
102
101
100
10-1
10-2
10-3
10- 1
0
2
4
6
8
10
12
14
0
5
10
15
20
Voltage (V)
Voltage (V)
Figure 3. Current-density–voltage (left) and luminance–voltage (right) curves of devices: ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BAlq 30 nm/LiF/Al (ꢀ); and ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/DNPF 30 nm/LiF/Al (ꢁ).
20
15
10
5
20
15
10
5
BAlq
DNPF
BAlq
DNPF
0
10-3
10-2
10-1
100
101
102
103
104
0.001
0.01
0.1
1
10
100
1000
104
Current Density (mA/cm2)
Current Density (mA/cm2)
Figure 4. CE–current-density (left) and PE–current-density (right) curves of devices. ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BAlq 30 nm/LiF/Al (ꢀ); and ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/DNPF 30 nm/LiF/Al (ꢁ).
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T a b le 2 . Performances of devices without additional HB layer.
1000
100
10
DNPF 90 nm
DNPA 90 nm
Alq 90nm
ET
Turn-on
Max Luminance
[cd m−2 @ V]
Max PE
Max CE Max EQE [%]
voltage [V]
[lm W−1 @ V] [cd A−1 @ V]
1
DNPF
BAlq
2.5
3.0
65817 @ 14.5
70143 @ 13.5
18.9 @ 3.0
4.0 @ 3.0
19.6 @ 4.0
5.39
3.69
0.1
0.01
1E-3
1E-4
1E-5
1E-6
12.5 @ 12.0
BCP/Alq3, and the current density of BCP/DNPF increased
to be larger than that of BCP/Alq3with increasing operating
voltage. These results indicate that the ET/EI abilities of naph-
thyridines are comparable or even higher than that of Alq3.
These results might be explained by the presence of the
electron-withdrawing naphthyridine moiety.[21] To confirm
this, electron-only devices of DNPF, DNPA, and Alq3 were
fabricated with the structure of indium tin oxide (ITO)/BCP
10 nm/DNPF, DNPA, or Alq3 90 nm/LiF/Al; BCP acted as an
HB layer to prevent hole injection from the ITO layer to the
ET layer. The electron-current densities of both DNPF and
DNPA were higher than that of Alq3 (Figure 6). This indicates
that the ET capabilities of DNPF and DNPA are definitely
higher than that of Alq3. This is consistent with the result that
the electron mobility of a 1,5-naphthyridine Al(III) chelate is
10−4 cm2 V−1 s−1 , higher than that of 10−5 cm2 V−1 s−1 for Alq3,
as previously reported.[21]
The luminance–voltage curves (Figure 5) show that the BCP/
DNPA device exhibits a higher luminance than that of the BCP/
Alq3 device, and the BCP/DNPF device shows a comparable
luminance to that of the BCP/Alq3 device. The naphthyridine
devices show maximum luminances of 115 880 cd m−2 at
12.0 V for the DNPA device and 102 480 cd m−2 at 13.0 V for
the DNPF device, both higher than that of 99 704 cd m−2 , at a
higher voltage of 13.5 V, for the Alq3 device.
-2 0
2
4
6
8 10 12 14 16 18 20 22
Votage (V)
Figure 6. Current-density–voltage curves of devices: ITO/BCP10 nm/
DNPF (ꢀ), DNPA (ꢁ), or Alq3 (ꢂ) 90 nm/LiF/Al.
device at low current densities. The BCP/DNPA device
showed a comparable performance to that of the BCP/Alq3
device; the device performances are listed in Table 3. Max-
imum EQEs of 13.2–13.7% (Figure 8) were obtained for the
naphthyridine devices, corresponding to maximum CEs of
48.2–52.4 cd A−1 and maximum PEs of 50.2–54.4 lm W−1 ;
these values are comparable to or even higher than those
of the BCP/Alq3 device. Because the ET of 2.3 eV for DNPF
is higher than that of 2.0 eV for Alq3, triplet excitons in the
device might be more effectively blocked in the BCP/DNPF
device than in the BCP/Alq3 device as a result of the longer
diffusion paths of the triplet excitons than those of the sin-
glet excitons, as a result of diffusion to the ET layer, giving
a higher efficiency. The turn-on voltages of the BCP/DNPF
and BCP/DNPA devices are the same as that of the BCP/Alq3
device, and no additional peak was observed for the naphthy-
ridine device, the same as in the case of the BCP/Alq3 device
(Figure 9). These results indicate that naphthyridines are
suitable for use as ET materials for green electrophosphores-
cent devices.
The plots of CE versus current density (Figure 7) showed
a higher CE for the BCP/DNPF device than for the BCP/Alq3
103
106
105
102
BCP/Alq
BCP/DNPF
BCP/DNPA
104
BCP/Alq
101
BCP/DNPF
103
BCP/DNPA
100
10-1
10-2
10-3
102
101
100
10- 1
0
2
4
6
8
10
12
14
0
5
10
15
20
Voltage (V)
Voltage (V)
Figure 5. Current-density–voltage (left) and luminance–voltage (right) curves of devices: ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BCP 10 nm/Alq3 20 nm/LiF/Al (ꢀ); ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/DNPF 20 nm/LiF/Al (ꢁ);
ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/DNPA 20 nm/LiF/Al (ꢂ).
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60
50
40
30
20
10
0
100
10
1
BCP/Alq
BCP/DNPF
BCP/DNPA
BCP/Alq
BCP/DNPF
BCP/DNPA
0.001
0.01
0.1
1
10
100
1000
0.001
0.01
0.1
1
10
100
1000
Current Density (mA/cm2)
Current density (mA/cm2)
Figure 7. CE–current-density (left) and PE–current-density (right) curves of devices: ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BCP 10 nm/Alq3 20 nm/LiF/Al (ꢀ); ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/DNPF 20 nm/LiF/Al (ꢁ);
ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/DNPA 20 nm/LiF/Al (ꢂ).
20
15
10
5
700
600
500
400
300
200
100
0
BCP/Alq
BCP/DNPF
BCP/DNPA
BCP/Alq
BCP/DNPF
0
300
400
500
600
700
800
0.001
0.01
0.1
1
10
100
1000
Wavelength (nm)
Current Density (mA/cm2)
Figure 9. EL spectra of devices: ITO/TPDPES:TBPAH 20 nm/α-NPD
30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/Alq3 20 nm/LiF/Al (dashed
line); ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BCP 10 nm/DNPF 20 nm/LiF/Al (solid line).
Figure 8. EQE–current-density curves of devices: ITO/TPDPES:TBPAH
20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3 30 nm/BCP 10 nm/Alq3 20 nm/
LiF/Al (ꢀ); ITO/TPDPES:TBPAH 20 nm/α-NPD 30 nm/CBP:6% Ir(ppy)3
30 nm/BCP 10 nm/DNPF 20 nm/LiF/Al (ꢁ); ITO/TPDPES:TBPAH 20
nm/α-NPD 30 nm/CBP:6%Ir(ppy)3 30 nm/BCP 10 nm/DNPA 20 nm/
LiF/Al (ꢂ).
comparable to or even higher than those of the BCP/Alq3
device, were obtained using the naphthyridine derivatives as ET
materials. Because of the higher ET of 2.33 eV for DNPF com-
pared with that of 2.03 eV for Alq3, triplet excitons in the device
might be more effectively blocked in the BCP/DNPF device
3. Conclusions
A series of 1,8-naphthyridine derivatives were synthesized and
their ET/EI properties in green electrophosphorescent devices
were investigated. The turn-on voltage of the DNPF device was
lower than that of the BAlq device as a result of the higher EI
ability of the naphthyridine derivative DNPF compared with
that of the conventional ET material BAlq. The maximum CE
and PE of the DNPF device were 19.6 cd A−1 and 18.9 lm W−1 ,
much higher than those of 12.5 cd A−1 and 4.0 lm W−1 for the
BAlq device; this might be the result of the higher ET. With the
aid of an HB and exciton-blocking layer of BCP, 13.2–13.7%
maximum EQEs with maximum PEs of 50.2–54.5 lm W−1 ,
T a b le 3 . Performances of devices with HB layer of BCP.
ET
Turn-on
voltage [V]
Max Luminance
[cd m−2 @ V]
Max PE
Max CE
Max EQE
[%]
[lm W−1 @ V] [cd A−1 @ V]
Alq3
3.0
3.0
3.0
99704 @ 13.5
102480 @ 13.0
115880 @ 12.0
53.4 @ 3.0
54.5 @ 3.0
50.2 @ 3.0
50.1 @ 3.5
52.4 @ 3.5
48.2 @ 3.5
13.6
13.7
13.2
DNPF
DNPA
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1328 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Synthesis of 2-(3-Bromophenyl)-1,8-naphthyridine (5) : A Friedlander
reaction[22] was used for the synthesis of 5. Sodium methoxide
(1.08 g, 0.02 mol) was added to a solution of 4 (1.22 g, 0.01 mol) and
3-bromoacetylbenzene (1.99 g, 0.01 mol) in absolute ethanol (80 mL).
The resultant solution was refluxed under nitrogen for 4.5 h, until no 4
was detected by thin-layer chromatography. Water was added to quench
the reaction. Then the reaction mixture was extracted using chloroform,
and purified on a column containing silica gel, with 9:1 chloroform:ethyl
acetate as the eluent. Solid 5 was obtained in a 47% yield and identified
using NMR spectroscopy. The synthesis is shown in Scheme 1. 1H NMR
(270 MHz, CDCl3, δ): 9.171 (dd, 1H, J = 1.9 Hz, 4.2 Hz), 8.541 (t, 1H, J =
1.8 Hz), 8.317 (s, 1H), 8.285 (s, 1H), 8.234 (dd, 1H, J = 2.2 Hz, 8.4 Hz),
8.004 (d, 1H, J = 8.4 Hz), 7.629 (ddd, 1H, J = 1.2 Hz, 1.0 Hz, 8.1 Hz),
7.515 (dd, 1H, J = 4.2 Hz, 8.0 Hz), 7.413 (t, 1H, J = 8.0 Hz).
than in the BCP/Alq3 device, resulting in a higher efficiency.
The naphthyridine derivatives have high thermal stabilities,
even higher than those of Alq3, and have potential applications
as ET materials for green electrophosphorescent devices.
4. Experimental Section
General: The 1H NMR spectra were recorded using a JEOL 270
(270 MHz) spectrometer. Electron-ionization mass spectrometry
(EI-MS) was performed using a JEOL JMS-K9 mass spectrometer. UV–vis
spectra were measured using a Shimadzu UV-3150 UV–vis/near-IR
spectrophotometer. Photoluminescence spectra were obtained using a
FluroMax-2 (Jobin–Yvon–Spex) luminescence spectrometer. DSC was
performed on a Perkin–Elmer Diamond DSC Pyris instrument under a
nitrogen atmosphere at a heating rate of 20 °C min−1 . TGA was performed
using a SEIKO EXSTAR 6000 TG/DTA 6200 unit under a nitrogen
atmosphere at a heating rate of 10 °C min−1 . The HOMO levels were
determined using atmospheric UV photoelectron spectroscopy (Rikken
Keiki AC-1). Phosphorescence spectra were measured using a streak
camera (Hamamatsu Photonics, C4334) at 4.2 K. Electroluminescence
(EL) spectra were obtained using an optical multichannel analyzer PMA
10 (Hamamatsu). The luminance was measured using a Topcon BM-8
luminance meter at room temperature under an ambient atmosphere.
Synthesis of 2,7-Dibromo-9,9-dimethylfluorene (1) : The synthesis
procedure was a modified version of a previously reported method.[28]
Methyl iodide (32.8 g, 0.231 mol) was added dropwise to a solution of
2,7-dibromofluorene (26.6 g, 0.082 mol) and sodium methoxide (12.5 g,
0.231 mol) in dimethyl formamide (500 mL), under nitrogen at 0–5 °C.
Then the reaction mixture was stirred using a mechanical stirrer for
12 h. Water was added to quench the reaction, the mixture was extracted
with ethyl acetate and finally purified on a column with n-hexane as the
solvent; 1 was obtained as a white solid in an 86% yield, and identified
using NMR spectroscopy. The synthesis is shown in Scheme 1. 1H NMR
(270 MHz, CDCl3, δ): 7.538 (dd, 4H, J = 3.7 Hz, 5.7 Hz), 7.465 (d, 2H,
J = 8.2 Hz), 1.464 (s, 6H).
Synthesis of 2,7-Bis[3-(2-phenyl)-1,8-naphthyridinyl]-9,9-dimethylfluorene
(DNPF): A standard Suzuki–Miyaura cross-coupling reaction[23] was
used to synthesize DNPF. Ethanol (100 mL) and Na2CO3 solution
(2 M, 100 mL) were added to a solution of 2 (0.89 g, 2 mmol) and 5
(1.25 g, 4.4 mmol) in toluene (200 mL) under nitrogen with agitation.
Then Pd(PPh3)4 (102 mg, 0.088 mmol) was added, and the resultant
solution was refluxed for 12 h. Water was added to quench the reaction,
and the mixture was then extracted with ethyl acetate, and purified by
column chromatography with an eluent of ethyl acetate:chloroform = 1:9.
A pale-yellow solid, DNPF, was obtained in an 83% yield (corresponding
to an overall yield of 43.2% from 2,7-dibromofluorene). 1H NMR
(270 MHz, CDCl3, δ): 9.184 (dd, 2H, J = 2.1 Hz, 4.4 Hz), 8.659 (t, 2H, J
= 1.7 Hz), 8.337 (s, 1H), 8.305 (s, 2H), 8.271 (d, 2H, J = 2.0 Hz), 8.241
(d, 1H, J = 1.7 Hz), 8.149 (s, 1H), 8.118 (s, 1H), 7.890 (s, 1H), 7.861 (s,
1H), 7.841 (t, 1H, J = 1.4 Hz), 7.809 (d, 3H, J = 0.9 Hz), 7.760 (d, 1H,
J = 1.5 Hz), 7.731 (d, 1H, J = 1.6 Hz), 7.653 (t, 2H, J = 7.7 Hz), 7.515
(dd, 2H, J = 4.1 Hz, 8.0 Hz), 1.581 (s, 6H). EIMS m/z: [M+]calcd. for
C
43H30N4, 602.73; found: 603. Anal. calcd. for C43H30N4: C 85.69%, H
5.02%, N 9.30%; found: C 85.45%, H 5.26%, N 9.13%.
Synthesis of 9,10-Di(m-diacetylphenyl)anthracene (6) : A Suzuki–Miyaura
cross-coupling reaction similar to that described above was used for
the synthesis of 6 from 9,10-dibromoanthracene (3.12 g, 9 mmol) and
m-acetylphenylboronic acid (3.35 g, 20 mmol). The product was purified
by column chromatography with an eluent of ethyl acetate:toluene = 1:20;
a pale-yellow solid 6 was obtained in 90% yield. 1H NMR (270 MHz,
CDCl3, δ): 8.205 (t, 1H, J = 1.9 Hz), 8.179 (t, 1H, J = 1.9 Hz), 8.085 (s,
2H), 7.740 (q, 4H, J = 7.6 Hz), 7.635 (dd, 4H, J = 3.0 Hz, 6.8 Hz), 7.374
(dd, 4H, J = 3.4 Hz, 7.2 Hz), 2.667 (s, 6H).
Synthesis
of
2,2′-(9,9-Dimethyl-9H-fluorene-2,7-diyl)bis(4,4,5,5-
tetramethyl-1,3,2-dioxaborolane) (2): 1 (10.56 g, 0.03 mol) was dissolved
in 400 mL of dehydrated diethyl ether with agitation under a flow of
nitrogen. Then n-butyl lithium (2.44 M in n-hexane, 31 mL, 0.075 mol) was
added to the solution after cooling to −78 °C using dry-ice. 2-Isopropoxy-
4,4,5,5-tetramethyl-1,3,2-dioxaborolane (16 mL, 0.078 mol) was added to
the reaction solution after 2 h at −78 °C. The reaction was continued for
12 h after the temperature was back up to room temperature. Finally,
100 mL of water were added to quench the reaction. The product was
extracted with diethyl ether, and purified by column chromatography
(solvent: ethyl acetate:n-hexane = 1:9), and identified by 1H NMR
spectroscopy. The product 2 was obtained as a white solid in a 60.5%
Synthesis
of
9,10-Bis[3-(1,8-naphthyridin-2-yl)phenyl)]anthracene
(DNPA): The naphthyridine derivative DNPA was obtained via
a
Friedlander reaction as follows. Anhydrous ethanol (20 mL) and
NaOCH3 (0.432 mg, 8 mmol) were added to a suspension of 4 (0.513 g,
4.2 mmol) and 6 (0.829 mg, 2 mmol) in o-dichlorobenzene (100 mL).
The resultant mixture was refluxed for 36 h, quenched using ice-water,
extracted with chloroform, and purified by column chromatography
with an eluent of ethyl acetate:chloroform = 1:9. DNPA was obtained
in a 51% yield (corresponding to an overall yield of 45.9% from
1
yield. The synthesis is shown in Scheme 1. H NMR (270 MHz, CDCl3,
δ): 7.883 (s, 2H), 7.809 (d, 2H, J = 7.8 Hz), 7.759 (d, 2H, J = 8.0 Hz),
1.537 (s, 6H), 1.377 (s, 24H).
1
9,10-dibromoanthracene). H NMR (270 MHz, CDCl3, δ): 9.161 (d, 2H,
J = 5.0 Hz), 8.656 (d, 2H, J = 9.3 Hz), 8.397 (s, 2H), 8.257 (m, 4H),
8.100 (dd, 2H, J = 5.7 Hz, 8.6 Hz), 7.799 (m, 6H), 7.666 (d, 2H, J =
7.2 Hz), 7.499 (dd, 2H, J = 3.6 Hz, 7.9 Hz), 7.359 (dd, 4H, J = 2.9 Hz,
7.2 Hz). Anal. calcd. for C42H26N4: C 85.98%, H 4.47%, N 9.55%; found:
C 85.61%, H 4.40%, N 9.36%. EIMS m/z: [M+]: calcd. for C42H26N4,
586.68; found, 587.
Fabrication of Organic Electrophosphorescent Devices and Measurements:
Organic layers were deposited by high-vacuum (10−6 Torr) thermal
evaporation, as previously reported.[30] Firstly, a 20 nm layer of 10:1
by weight of TPDPES:TBPAH was deposited via a dichloroethane
solution as the hole-injecting layer before it was loaded into the
evaporation system. Then, hole-transporting, EM, and HB/ET layers
were sequentially deposited on the surface via thermal evaporation in
a vacuum of 10−6 Torr. Finally, a 0.5 nm layer of LiF together with a
100 nm layer of Al were deposited as the cathode under a vacuum of
2 × 10−5 Torr. The emitting area was defined, using a shadow mask, to
Syntheses
of
2-(3’-Pyridyl)pyrido[2,3-d]pyrimidine
(3)
and
2-Aminonicotinaldehyde (4) : 3 and 4 were synthesized as described in the
literature.[29] A mixture of nicotinamide (73 g, 0.6 mol) and ammonium
sulfamate (104 g, 0.9 mol) was heated to melting. After the solid was
completely melted, the temperature was raised slowly to 200 °C. After
complete solidification, the material was kept at 200 °C for 6 h. Water
(300 mL) was used to wash the resultant material, followed by diethyl
ether to remove nicotinonitrile; 3 was obtained as a white solid. The
product 3 was refluxed in 2 N HCl for 8 h, NaOH was added to make
the mixture alkaline, and the mixture was extracted with diethyl ether. The
resulting 4 was obtained in a 16% yield. 1H NMR (270 MHz, CDCl3, δ,
ppm): 3, 9.926 (dd, 1H, J = 0.7 Hz, 2.4 Hz), 9.589 (s, 1H), 9.336 (dd, 1H, J
= 2.0 Hz, 4.3 Hz), 9.011 (m, 1H), 8.785 (dd, 1H, J = 2.0 Hz, 4.9 Hz), 8.381
(dd, 1H, J = 2.2 Hz, 8.1 Hz), 7.653 (dd, 1H, J = 4.3 Hz, 8.2 Hz), 7.509 (m,
1H); 4, 9.851 (s, CHO), 8.239 (dd, 1H, J = 2.1 Hz, 4.7 Hz), 8.001 (dd, 1H,
J = 2.1 Hz, 7.7 Hz), 7.555 (s, NH2), 6.738 (dd, 1H, J = 4.6 Hz, 7.5 Hz).
©
wileyonlinelibrary.com
Adv. Funct. Mater. 2013, 23, 1323–1330
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1329

www.afm-journal.de
www.MaterialsViews.com
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Acknowledgements
This study was partly financially supported by the National Basic Research
Program of China (2009CB930504), the NSFC (grants 61177020,
10934001, 60907015, and 11121091), the Beijing Municipal Science and
Technology Project (Z101103050410002), and NEDO (Japan) through
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©
1330 wileyonlinelibrary.com
2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2013, 23, 1323–1330