Synthesis of multiaryl-substituted pyridine derivatives and applications in non-doped deep-blue OLEDs as electron-transporting layer with high hole-blocking ability

Article abstract of DOI:10.1002/adma.200902430

Two multiaryl-substituted pyridine derivatives, applied as electron transport materials (ETMs; see figure) with good holeblocking ability in high-performance deep-blue organic light-emitting diodes (OLEDs), are characterized. The maximum current efficienc

Full text of DOI:10.1002/adma.200902430

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Synthesis of Multiaryl-Substituted Pyridine Derivatives
and Applications in Non-Doped Deep-Blue OLEDs as
Electron-Transporting Layer with High Hole-Blocking
Ability
By Na Li, Pengfei Wang,* Shiu-Lun Lai, Weimin Liu, Chun-Sing Lee,*
Shuit-Tong Lee, and Zengtao Liu
In recent years, increasing demands for full-color flat-panel
displays have accelerated the development of different display
technologies. Among these, organic light-emitting devices
(OLEDs) have attracted much attention since the report by Tang
and Vanslyke[1] for their promising applications. Much effort has
been made to improve the performance of OLEDs from many
aspects. For example, new materials have been designed,
synthesized, and applied in OLEDs. In comparison with the
development of many high-performance light-emitting materials
(LEMs) and hole-transporting materials (HTMs), there are
relatively few reports on good electron-transporting materials
(ETMs).[2–4] After three decades of research since Tang and
Vanslyke’s work,[1] tris(8-hydroxyquinoline)aluminum (Alq3) is
still one of the most widely used ETMs even though its electron
drift mobility is more than one order lower than the hole drift
mobility of typical HTMs.[5–7] In addition, another traditional
hole-blocking material is often required for use with Alq3 to
improve electron charge balance, because of its poor hole-blocking
ability. This would inevitably increase the device fabrication
complexity. For some applications, such as in phosphorescent
OLEDs or other OLEDs with hosts for light-emitting dopants, a
good hole-blocking ability is an important additional requirement
for the ETMs in order to simplify the device fabrication.[8,9]
Materials that can fulfil these requirements are so far limited.
1,3,5-Tris(phenyl-2-benzimidazolyl)benzene (TPBI) is one such
material with good electron-transporting properties and hole-
blocking properties. Unfortunately, TPBI molecules in an
amorphous film tend to crystallize and can lead to stability
problems.[10] 4,7-Diphenyl-1,10-phenanthroline (BPhen) is another
commonly used ETM that does have a good hole-blocking ability.
However, its potential for commercial applications is again low
because of its poor stability. Thus, it is important to explore ETMs
with excellent electron-transporting and hole-blocking properties
and good thermal stability. Indeed, extensive efforts have been
made to increase the highest occupied molecular orbital (HOMO)
energies of materials to enhance their hole-blocking ability. For
example, Suzuki et al. reported a series of perfluorinated
phenylene dendrimers with electron-transporting properties
and large HOMO values in the early 20th century.[11] Su and
Kido have recently reported several ETMs with hole-blocking
ability.[12–14] However, an increase in the value of the HOMO is
often accompanied by a decrease in the value of the lowest
unoccupied molecular orbital (LUMO). Inevitably, smaller LUMO
values, which can increase the barrier between the cathode and
ETMs, suggest that electron injection from the cathode to the
ETMs would become more difficult. However, herein, two
multiaryl-substituted pyridine derivatives have been designed
and synthesized to have large HOMO and LUMO values (i.e.,
energy levels are pulled down as a whole in the electron energy
diagram). It is thus a challenge to synthesize such materials from
the point of view of molecular design.
In this work, two novel multiaryl-substituted pyridine deriva-
tives, namely 4,40-(1,4-phenylene) bis(2-phenyl-6-p-tolylni-
cotinonitrile) (p-PPtNT) and 6,60-(1,4-phenylene) bis(2-phenyl-
4-p-tolylnicotinonitrile) (p-PPtNN) were synthesized and applied
as ETMs with good hole-blocking properties. They were found to
have suitable LUMO values (>3.50 eV) to allow efficient electron
injection. They also have HOMO values as large as 6.87 eV, which
are among the highest reported for molecular semiconductor
materials.[12–14] By introducing a methyl group, adverse effects
because of molecular aggregation and crystallization can be
effectively avoided. Furthermore, the new materials do show good
electron-transporting and hole-blocking abilities. These properties
facilitate their applications as electron-transporting hosts in
high-performance OLEDs.
[*] Prof. P. Wang, Dr. N. Li, Dr. W. Liu
Key Laboratory of Photochemical Conversion and Optoelectronic
Materials
Technical Institute of Physics and Chemistry
Chinese Academy of Sciences
Beijing, 100190 (P. R. China)
Two multiaryl-substituted pyridine derivatives, p-PPtNT and
p-PPtNN, are reported here. Synthetic routes and molecular
structures are shown in Scheme 1. The two pyridine derivatives
were synthesized as described in the literature.[15–17] They were
synthesized by cyclocondensation of p-PPtNT I and p-PPtNN I
with benzoylacetonitrile in the presence of ammonium acetate,
respectively, and their spectroscopic and thermal data are listed in
Table 1. From Table 1 they are found to have relatively high
E-mail: wangpf@mail.ipc.ac.cn
Prof. C.-S. Lee, Prof. S.-T. Lee, Dr. S.-L. Lai, Dr. Z. Liu
Center of Super-Diamond and Advanced Films (COSDAF)
Department of Physics and Materials Science
City University of Hong Kong, Hong Kong SAR (P. R. China)
E-mail: apcslee@cityu.edu.hk
DOI: 10.1002/adma.200902430
Adv. Mater. 2010, 22, 527–530
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Scheme 1. The molecular structures and synthetic route of the two
pyridine compounds.
glass-transition temperatures (Tgs) (138.5 8C for p-PPtNT,
101.5 8C for p-PPtNN), because of the methyl group introduced
to the molecules, in comparison with that (60 8C) of BPhen. Films
of the new compounds should thus have good thermal stability
against crystallization. It is also noted from Table 1 that the
LUMO value for p-PPtNT is 3.57 eV, while that for p-PPtNN is
3.69 eV. As is well known, the LUMO values for traditional ETMs,
such as Alq3, TPBI, and BPhen, are all lower than 3.0 eV (i.e., the
energy levels are higher).[18–20] It is more difficult for electrons to
inject from the cathode to the ETMs, because of the large barrier
(>0.7 eV) between the cathode (the energy level of the MgAg alloy
is 3.70 eV) and ETM. Therefore, the lower LUMO energy levels
for the two pyridine derivatives, which imply no barrier, facilitate
electron injection from the cathode to the organic layer. p-PPtNT
and p-PPtNN have HOMO values (6.81 and 6.87 eV, respectively)
higher than those of BCP (6.7 eV),[18] TPBI (6.7 eV),[19] and BPhen
(6.4 eV).[20] These imply that the compounds can be used as hole
blockers. From Table 1, it can be found that the HOMO and
the LOMO values of p-PPtNN are a little larger than those of
p-PPtNT. It is considered that the sp-hybrid cyano group in the
para-orientation in p-PPtNN has a higher electron withdrawing
ability and thus lowers the energy levels. This is also the reason
for the red shift of the absorption and the emission peaks of
p-PPtNN (labmax: 290/355 nm; lemmax: 427 nm) in contrast with
Figure 1. Chemical structures of the employed materials and energy level
diagram for devices (The two pyridine compounds are used as ETL
......
respectively). — p-PPtNT;
p-PPtNN
The ET value of p-PPtNN is a little smaller than that of p-PPtNT. It
is also interpreted that the sp-hybrid cyano group in the
para-orientation in p-PPtNN has a higher electron withdrawing
ability and thus lowers the energy levels.
In comparison with green- and red-emitting OLEDs,[21,22] the
performance of blue-emitting OLEDs is still lagging behind. In
particular, there is still much room for improvement for both the
efficiency and stability of deep-blue OLEDs. In this work, simple
deep-blue OLEDs based on 9,10-di(2-naphthyl)anthracene (ADN)
have been fabricated with a device structure as shown in Figure 1:
indium-tin oxide (ITO)/a-napthylphenylbiphenyl diamine
(NPB) [60 nm]/ADN [30 nm]/p-PPtNT or p-PPtNN [30 nm]/LiF
[1 nm]/MgAg [100 nm 10: 1]. In the devices, ITO and the MgAg
alloy were used as the anode and cathode, respectively. The
organic layers included NPB as the hole-transporting layer, ADN
as the light-emitting layer (LEL), and p-PPtNT or p-PPtNN as the
ETL with a hole-blocking property to confine the emission zone in
the ADN LEL.
those of p-PPtNT in the solid film (labmax: 294/330 nm; lem
:
max
407 nm). The triplet energy (ET) values of p-PPtNT and p-PPtNN
are 3.04 and 2.99 eV, respectively (Table 1). The values are
higher than a typical phosphorescent emitter, such as iridium(III)
bis[(4,6-difluorophenyl)-pyridinato-N,C20] picolinate (FIrpic,
2.65 eV) and fac-tris(2-phenylpyridine) iridium (Ir(PPy)3,
2.55 eV). The high ET levels can effectively confine the triplet
excitons in the phosphorescent emissive layer in the application
of phosphorescent OLEDs as electron-transporting layer (ETL).
Electroluminescent (EL) characteristics of the devices using the
two pyridine compounds as the ETL are shown in Figure 2. The
maximum current efficiencies of the devices based on p-PPtNT
Table 1. Spectroscopic and thermal data of the two pyridine compounds.
Compound
Td/Tm/Tg [a] [8C].
HOMO/LUMO energy level [eV]
labmax.[b] [nm]
lem.max.[c] [nm]
ET[d] [eV]
p-PPtNT
p-PPtNN
388/363/138.5
375/383/101.5
6.81/3.57
6.87/3.69
294/330
290/355
407
427
3.04
2.99
[a] Td: decomposition temperature, Tm: melting point temperature, Tg: glass transition temperature. [b] labmax: maximum wavelength of absorption, measured in the solid
film (vacuum deposited on quartz substrates. [c] lemmax: maximum wavelength of emission, measured in the solid film (vacuum deposited on quartz substrates). [d] ET: triplet
energy level.
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20 mA cmꢀ2 and a slightly lower current efficiency of 2.40 cd Aꢀ1
at 80 mA cmꢀ2. Compared with the device based on p-PPtNT, the
current efficiency of the device based on p-PPtNN shows a little
decrease. However, the value is still higher than the maximum
current efficiencies of 1.6, 1.3, and 1.3 cd Aꢀ1 reported in the
devices with similar structure, based on Alq3 (as ETL), BCP (as
hole-blocking layer)/Alq3 (as ETL), and TPBI (as ETL), respec-
tively.[23–28] The power efficiencies of the two devices are also
higher than the values reported in the literature.[23–28] The
external quantum efficiencies (EQEs) for the corresponding
devices based on p-PPtNT and p-PPtNN are 2.44% and 1.28%,
respectively (Fig. 2a). It is also noted that the EQEs show a mild
decrease as the current density increases. The higher current and
power efficiencies may be attributed to the lower LUMO energy
levels, and good electron-transporting ability and hole-blocking
ability of the two pyridine compounds. The lower LUMO energy
levels facilitate electron injection from the cathode to the organic
layer, while the good electron-transporting ability and hole-
blocking ability can jointly assure electron charge balance. The
efficiencies of the device based on p-PPtNTare higher than that of
the device based on p-PPtNN, possibly because of a better
electron-transporting ability.
As shown in Figure 2b, the devices have low turn-on voltages
(defined as the voltage at a luminance of 1 cd mꢀ2) of 4.1 V
(p-PPtNT) and 3.7 V (p-PPtNN) in comparison with those of
devices based on Alq3 (as ETL), BCP (as hole-blocking layer)/Alq3
(as ETL), and TPBI (as ETL).[23–28] It is considered that the smaller
turn-on voltages are attributed to the lower LUMO energy levels of
the two pyridine compounds, which facilitates electron injection
from the cathode to the organic layer. Moreover, besides the
turn-on voltage, the driving voltage of the device based on
p-PPtNN is smaller than that of the device based on p-PPtNT. For
instance, the driving voltages for p-PPtNN are only 8.6 and 9.9 V
at 100 and 200 mA cmꢀ2, while those for p-PPtNT are 10.0 and
11.4 V, respectively. The smaller driving voltage and turn-on
voltage are closely related to the lower LUMO energy level of
p-PPtNN.
Figure 2c shows the EL spectra and luminance (L)–current
density (J) characteristics of the devices based on the two pyridine
compounds. The EL peak positions (and Commission Inter-
nationale de L’Eclairage (CIE) coordinates) of the devices based on
p-PPtNT and p-PPtNN are 452 nm (0.15, 0.12) and 464 nm (0.16,
0.20), respectively. It should also be pointed out that the full
widths at half-maximum (lFWHM) for the two devices are 73 and
94 nm, respectively. The CIE coordinates and narrower lFWHM of
the device based on p-PPtNT show a color purity considerably
better than reported for a Alq3-based device (CIE 0.17, 0.17).[23–28]
Deep-blue emission of the device based on p-PPtNT should be
attributed to the electroluminescence of ADN. The better color
purity is attributed to the good hole-blocking property of the
pyridine derivatives, which helps to confine the emission zone in
the ADN layer. The lFWHM for the device based on p-PPtNN is
obviously broader than that for p-PPtNT. It may be attributed to
the formation of an exciplex in the ADN/p-PPtNN interface,
Figure 2. EL characteristics of devices using the new pyridine derivatives
as ETL. Device structure: ITO/NPB (60 nm)/ADN (30 nm)/p-PPtNT or
p-PPtNN (30 nm)/LiF (1nm)/MgAg (100 nm, 10: 1). a): Current efficiency
(hL)–current density (J)–power efficiency (hP)–and EQE. b) J–voltage
(V)–luminance (L). c) EL spectra, inset: L–J.
and p-PPtNN are 2.54 and 2.18 cd Aꢀ1, respectively (Fig. 2a).
These are considerably higher than the maximum current
efficiencies of 1.6, 1.3, and 1.3 cd Aꢀ1 reported in the devices with
similar structure, based on Alq3 (as ETL), BCP (as hole-blocking
layer)/Alq3 (as ETL), and TPBI (as ETL), respectively.[23–28] In
addition, the current efficiencies show only a mild decrease as
the current density increases. For example, the device based on
p-PPtNT exhibits a higher current efficiency of 2.51 cd Aꢀ1 at
which can induce
a red-shift on account of the more
electron-deficient molecular structure of p-PPtNN. In addition,
the L–J curve of the device based on p-PPtNT is above that of the
device based on p-PPtNN (the inset in Fig. 2c). The formation of
an exciplex in the ADN/p-PPtNN interface can also take charge of
Adv. Mater. 2010, 22, 527–530
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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of Hong Kong. This work is financially supported by the National Natural
Science Foundation of China (Grant No. 60118033), the National High
Technology Research and Development Program of China (863 Program)
(Grant No. 2008AA03A327), the National Basic Research Development
Program of China (973 Program) (Grant No. 2006CB933000), the Croucher
Foundation of Hong Kong, and RGC of Hong Kong (Grant No: CityU
101508). Supporting Information is available online from Wiley Inter-
Science or from the author.
the weaker luminance of the device based on p-PPtNN. This is
another possible reason for the lower efficiencies of the device
based on p-PPtNN compared to that of the device based on
p-PPtNT (Fig. 2a).
To summarize, two multiaryl-substituted pyridine derivatives,
namely p-PPtNT and p-PPtNN, have been synthesized and
applied as ETMs with good hole-blocking ability in high-
performance deep-blue OLEDs. The maximum current efficiency
Received: July 21, 2009
Published online: October 13, 2009
of the devices based on the new compounds is above 2.1 cd Aꢀ1
,
which is over 70% higher than for previously reported devices
using Alq3 and TPBI instead. The CIE coordinates of the
device based on p-PPtNT are close to the National Television
System Committee (NTSC) standards of 0.14, 0.08[29] for blue
emission.
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p-PPtNN are shown in Scheme 1. They were prepared using methods
similar to those described in the literature [15–17]. Taking p-PPtNN as an
example, 1,4-diacetylbene (1.62 g, 10 mmol) was first added to an alcohol
solution (50 mL) of p-methyl benzaldehyde (3.00 g, 25 mmol) under
stirring. A 10% NaOH solution (10 mL) was then dropped into the alcohol
solution under rapid stirring [15]. Stirring was continued for another 12 h.
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from alcohol. Yellow acicular crystals (p-PPtNN I) (3.2 g, 80% yield) were
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(1.45 g, 10 mmol), and anhydrous ammonium acetate (1 g) were added
into a three-necked flask [17]. Glacial acetic acid (50 mL) was then injected
in under a nitrogen atmosphere. The mixture was refluxed for 24 h under
stirring. The whole process was completed under a nitrogen atmosphere.
After cooling to room temperature, the reaction mixture was filtered. A
pale yellow powder was recrystallized from 1,2-diclorobenzene. A white
solid of p-PPtNN was obtained. p-PPtNT were recrystallized with the same
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4,40-(1,4-Phenylene)bis(2-phenyl-6-p-tolylnicotinonitrile) (p-PPtNT): 1H
NMR (400 MHz, CDCl3, d): 8.14 (d, J ¼ 8.16 Hz, 4H), 8.08–8.05 (m,
4H), 7.90 (s, 2H), 7.88 (s, 4H), 7.58–7.56 (m, 6H), 7.35 (d, J ¼ 8.08 Hz,
4H), 2.44 (s, 6H); 13C NMR (400 MHz, CDCl3, d): 162.6, 159.6, 154.4,
141.3, 138.4, 138.2, 134.7, 130.3, 130.0, 129.6, 128.7, 127.7, 118.4, 118.0,
103.9, 21.6. HRMS (EI, m/z): [M]þ calcd. for C44H30N4: 614.25; found,
614.2466. Anal. calcd. for C44H30N4: C 85.97, H 4.92, N 9.11; found:
C 83.56, H 4.497, N 8.671.
6,60-(1,4-Phenylene)bis(2-phenyl-4-p-tolylnicotinonitrile) (p-PPtNN): 1H
NMR (400 MHz, CDCl3, d): 8.32 (s, 4H), 8.06–8.04 (m, 4H), 7.87
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C 85.78, H 4.727, N 9.473.
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Acknowledgements
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The authors thank the Center of Super-Diamond and Advanced Film
(COSDAF), Department of Physics and Materials Science, City University
530
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2010, 22, 527–530