N,N-diphenylpyridin-4-amine as a bipolar core structure of high-triplet-energy host materials for blue phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/asia.201200463

Got the blues: High-triplet-energy bipolar host materials based on a bipolar N,N-diphenylpyridin-4-amine (DPPA) core structure were developed for blue phosphorescent organic light-emitting diodes (PHOLEDs). Two DPPA-based host materials (see picture) showed high triplet energy over 2.70 eV and a high quantum efficiency of 15.6 %. Copyright

Full text of DOI:10.1002/asia.201200463

DOI: 10.1002/asia.201200463
N,N-Diphenylpyridin-4-amine as a Bipolar Core Structure of High-Triplet-
Energy Host Materials for Blue Phosphorescent Organic Light-Emitting
Diodes
Chil Won Lee and Jun Yeob Lee*[a]
High-triplet-energy bipolar host materials with both hole-
transport and electron-transport units in the molecular
structure have been developed to improve the quantum effi-
ciency of blue phosphorescent organic light-emitting diodes
(PHOLEDs) through charge balance in the emitting
layer.[1–3] The use of the bipolar host materials enabled the
fabrication of high-efficiency blue PHOLEDs with a quan-
tum efficiency above 20%.[4–7]
Several chromophores have been used as the core struc-
tures of bipolar host materials. Fluorene was one bipolar
core structure for host materials, and several derivatives of
fluorene were synthesized as bipolar host materials for blue
PHOLEDs.[8–10] Modification of fluorene with hole- and
electron-transport units yielded various bipolar host materi-
als.[10] Diphenylsilane was another common core structure
for high-triplet-energy bipolar host materials. Diphenylsi-
lane could be easily modified with hole- and electron-trans-
port moieties by nucleophilic substitution reactions.[11] The
other core structure which has been widely used is phenyl-
carbazole. As phenylcarbazole is a hole-transport core, it
was modified with an electron-transport substituent for
better electron-transport properties.[12–16] Phenylcarbazole-
based bipolar host materials were effective as host materials
for blue PHOLEDs.
DPPA was designed as a bipolar core structure with high
triplet energy. DPPA has a molecular structure with an elec-
tron-donating diphenylamine unit attached to an electron-
deficient pyridine unit. As DPPA has the electron-deficient
pyridine and electron-donating amine moieties, it can play
the role of a bipolar core structure. Moreover, it can exhibit
high triplet energy.
The synthetic route for DPPA-based host materials is
shown in Scheme 1. The halogenated DPPA core was pre-
pared by the reaction between 4-amino-2,6-dichloropyridine
and bromobenzene using palladium catalyst. The halogenat-
ed DPPA was treated with boronic acid of biphenyl and
phenylcarbazole to produce PyA-BP and PyA-PCz.
Although many bipolar host materials have been studied
for application in blue PHOLEDs, further development of
the core structure and its derivatives is required to improve
the device performances of blue PHOLEDs. Herein, N,N-di-
phenylpyridin-4-amine (DPPA) was developed as a high-
triplet-energy bipolar core structure for blue host materials,
and two derivatives of DPPA, 2,6-di(biphenyl-3-yl)-N,N-di-
phenylpyridin-4-amine (PyA-3BP) and 2,6-bis(3-(9H-carba-
Scheme 1. Synthesis of PyA-BP and PyA-PCz.
zol-9-yl)phenyl)-N,N-diphenylpyridin-4-amineACTHNUTRGNEUG(N PyA-PCz),
were synthesized as host materials. It was demonstrated that
PyA-3BP and PyA-PCz show bipolar charge-transport prop-
erties and high triplet energy for use in blue PHOLEDs.
Molecular simulation of DPPA was carried out to under-
stand its basic material properties. The Gaussian 03 program
suite and the nonlocal density functional of Beckeꢀs 3-pa-
rameters employing Lee–Yang–Parr functional (B3LYP)
with 6-31G* basis sets were used for the simulation.[17]
Figure 1 shows simulated highest occupied molecular orbital
(HOMO)/lowest unoccupied molecular orbital (LUMO)
levels and HOMO/LUMO orbital distribution of DPPA.
Molecular simulation results for DPPA were compared with
that of a common triphenylamine (TPA). The simulated
HOMO/LUMO level of DPPA was À5.38/À0.59 eV com-
pared with À4.95/À0.30 eV of TPA. Both HOMO and
[a] C. W. Lee, J. Y. Lee
Department of Polymer Science and Engineering
Dankook University
126, Jukjeon-dong, Suji-gu, Yongin-si, Gyeonggi-do, 448-701 (Korea)
Fax : (+82)31-8005-3585
E-mail: leej17@dankook.ac.kr
Supporting information for this article, including synthetic procedures
and details of characterization and device fabrication, is available on
the WWW under http://dx.doi.org/10.1002/asia.201200463.
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Figure 1. HOMO and LUMO distributions of TPA and DPPA.
LUMO levels of DPPA were stabilized by replacing one
phenyl unit with a pyridine unit due to electron deficiency
of the pyridine unit. This finding indicates that DPPA shows
better electron-transport properties than DPA owing to the
electron-withdrawing character of the pyridine unit. There-
fore, the DPPA core is more suitable as a bipolar core struc-
ture for bipolar host materials. Additionally, the simulated
triplet energy of DPPA was higher than that of TPA by
0.08 eV, implying that DPPA can be effective as a high-trip-
let-energy core for blue host materials. The HOMO distri-
bution of DPPA was similar to that of TPA, but the LUMO
distribution was different because of electron deficiency of
the pyridine unit. The LUMO distribution was rather local-
ized on the pyridine unit in DPPA, while it was uniform in
TPA.
As DPPA showed potential as a core structure of high-
triplet-energy bipolar host materials, it was modified with bi-
phenyl and phenylcarbazole moieties. Two biphenyl or phe-
nylcarbazole moieties were attached to the 2- and 6-posi-
tions of pyridine. Both units were substituted through meta
linkage to obtain high triplet energy. Biphenyl was intro-
duced as a bipolar substituent, while phenylcarbazole was
used as a hole-transport substituent. Two host materials
were synthesized by Suzuki coupling reaction of 2,6-di-
chloro-N,N-diphenylpyridin-4-amine with boronic acid of bi-
phenyl or phenylcarbazole.
Photophysical properties of PyA-BP and PyA-PCz were
analyzed using ultraviolet-visible (UV/Vis) and photolumi-
nescence (PL) spectroscopy. PyA-BP exhibited a broad ab-
sorption peak between 230 and 350 nm, which is assigned to
p–p* transition of the DPPA core (Figure 2). In the case of
PyA-PCz, phenylcarbazole absorption peaks at 325 and
339 nm were also observed in addition to the broad absorp-
tion peak of DPPA. Optical bandgaps were calculated from
the edge of UV/Vis absorption peak, which were 3.46 and
3.52 eV for PyA-BP and PyA-PCz, respectively. Solution PL
emission of PyA-BP and PyA-PCz was observed at 414 and
421 nm, respectively. Low-temperature PL measurement
was carried out in liquid nitrogen to measure triplet energy
of host materials. The triplet energy calculated from the first
phosphorescence emission peak was 2.76 eV for both com-
pounds. The triplet energy of the two compounds is deter-
Figure 2. UV/Vis, solution PL, and low-temperature PL spectra of
a) PyA-BP and b) PyA-PCz. UV/Vis and solution PL spectra were mea-
sured in tetrahydrofuran. Low-temperature PL was measured in liquid
nitrogen.
mined by the central phenyl-pyridine-phenyl structure as
this is the unit with the lowest triplet energy in the molecu-
lar structure. The two compounds had the same phenyl-pyri-
dine-phenyl structure, resulting in the same triplet energy.
The HOMO levels of PyA-BP and PyA-PCz were mea-
sured using a surface analyzer, and the LUMO level was
calculated from the HOMO and the bandgap from UV/Vis
absorption measurement. HOMO/LUMO levels of PyA-BP
and PyA-PCz were À5.76/À2.28 eV and À5.81/À2.29 eV, re-
spectively.
HOMO and LUMO simulation of PyA-BP and PyA-PCz
was carried out to study the effect of different substituents
on the molecular orbital distribution. The HOMO and
LUMO distribution of PyA-BP and PyA-PCz is shown in
Figure 3. The HOMO of PyA-BP was localized on the
DPPA core due to the strong electron-donating character of
the diphenylamine unit, but the HOMO of PyA-PCz was
dispersed over the phenylcarbazole unit. This result indi-
cates that hole transport of PyA-BP is dominated by the
DPPA core, while that of PyA-PCz is dependent on phenyl-
carbazole. The LUMO was localized on the pyridine and
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N,N-Diphenylpyridin-4-amine
Figure 3. HOMO and LUMO distribution of PyA-BP and PyA-PCz.
Figure 5. Energy-level diagram of the blue-light-emitting PHOLED.
N1,N1’-(biphenyl-4,4’-diyl)bis(N1-phenyl-N4,N4-di-mtolylbenzene-1,4-dia-
mine (DNTPD), N,N’-bis(naphthalen-1-yl)-N,N’-bisACTHNUGRTENUNG(phenyl)-benzidine
(NPB), 1,3-bis(carbazol-9-yl)benzene (mCP), diphenylphosphine oxide-4-
(triphenylsilyl) (TSPO1).
phenyl units, and the LUMO distribution was similar in
both host materials due to the electron-accepting properties
of pyridine.
Hole-only and electron-only devices were fabricated to
compare hole and electron current density of the two host
materials. Figure 4 shows current-density–voltage curves of
ilar energy barriers for electron injection in the two host
materials. The energy-level diagram is shown in Figure 5.
Therefore, the high hole current density of PyA-PCz implies
that PyA-PCz has better hole-transport properties than
PyA-BP, which can be explained by the molecular-orbital
distributions of PyA-BP and PyA-PCz. The HOMO of PyA-
PCz was localized on hole-transporting phenylcarbazole,
while that of PyA-BP was dispersed over the bipolar DPPA
core. As phenylcarbazole is more suitable as a hole-trans-
port unit than bipolar DPPA, better hole-transport proper-
ties were observed in PyA-PCz. The LUMO distribution
was similar in the two host materials, resulting in similar
electron density in electron-only devices. In terms of bipolar
charge-transport properties, PyA-PCz was better than PyA-
BP. Although electron current density was higher than hole
current density, the difference of hole and electron current
density was small in PyA-PCz.
Blue PHOLEDs were fabricated to evaluate PyA-BP and
PyA-PCz as host materials for blue phosphorescent dopant
materials. Figure 6 represents current-density–voltage–lumi-
Figure 4. Hole-only device (a) and electron-only device (b) data of PyA-
BP and PyA-PCz.
hole- and electron-only devices of PyA-BP and PyA-PCz.
Hole current density was high in PyA-PCz, while electron
current density was similar in the two host materials. The
hole and electron current density is generally decided by an
energy barrier for charge injection and by charge-transport
properties. Considering the energy barrier for charge injec-
tion, there are no energy barriers for hole injection and sim-
Figure 6. Current-density–voltage–luminance curves of PyA-BP- and
PyA-PCz-based blue PHOLEDs.
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Chil Won Lee and Jun Yeob Lee
COMMUNICATION
nance curves of PyA-BP and PyA-PCz devices. The current
density and luminance of the PyA-PCz device were higher
than those of the PyA-BP device at the same driving voltage
due to the better hole-transport properties of PyA-PCz (see
Figure 4). The driving voltage of PyA-PCz device was lower
than that of PyA-BP by 0.5 V.
Quantum-efficiency–luminance curves of PyA-BP and
PyA-PCz devices are shown in Figure 7. Bis(3,5-difluoro-2-
Figure 7. Quantum-efficiency–luminance curves of PyA-BP- and PyA-
PCz-based blue PHOLEDs.
Figure 8. Hole-only device (a) and electron-only device (b) data of
FIrpic-doped PyA-BP and PyA-PCz.
(2-pyridyl)phenyl-(2-carboxypyridyl)iridium
G
(FIrpic)
was doped as a blue phosphorescent dopant in the emitting
layer. The quantum efficiency of the PyA-PCz device was
higher than that of the PyA-BP device for the whole lumi-
nance range measured. Maximum quantum efficiency of the
PyA-PCz device was 15.1%, and quantum efficiency at
1000 cdmÀ2 was 12.0%. The higher quantum efficiency of
the PyA-PCz device can be explained by the balance of
holes and electrons in the emitting layer. This can be con-
firmed by hole and electron density of the FIrpic-doped
PyA-BP and PyA-PCz emitting layers (Figure 8). Hole cur-
rent density of FIrpic-doped PyA-PCz was much higher
than that of FIrpic-doped PyA-BP, while electron current
density of both FIrpic-doped host materials was similar.
Comparing relative hole and electron current density of the
two host materials, PyA-PCz and PyA-BP showed relative
ratio of about 2.0 and 0.25. Therefore, balance of holes and
electrons in the FIrpic-doped emitting layer is better in
PyA-PCz than in PyA-BP, resulting in high quantum effi-
ciency in PyA-PCz. Device performances of PyA-BP and
PyA-PCz are summarized in Table 1.
Figure 9. Electroluminescence (EL) spectra of PyA-BP- and PyA-PCz-
based blue PHOLEDs.
Electroluminescence (EL) spectra of PyA-BP and PyA-
PCz blue PHOLEDs are shown in Figure 9. Both devices
showed only an FIrpic emission peak, thus indicat-
ing effective energy transfer from host to FIrpic
Table 1. Device performances of PyA-BP and PyA-PCz devices.
dopant material.
Material
Quantum effi-
ciency [%]
15.1,[a] 12.0[b]
14.5,[a] 10.5[b]
Current effi-
Power effi-
Color coor-
dinate
ciency [cdAÀ1
]
ciency [lmWÀ1
]
In conclusion, bipolar host materials based on the
DPPA core were developed as high-triplet-energy
host materials for blue PHOLEDs. High triplet
energy above 2.70 eV and bipolar charge-transport
PyA-BP
PyA-PCz
28.0,[a] 21.3[b]
25.6,[a] 18.1[b]
21.6,[a] 9.4[b]
17.7,[a] 7.6[b]
0.15, 0.33
0.15, 0.31
[a] Maximum efficiency. [b] Efficiency at 1000 cdmÀ2
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Keywords: bipolar hosts
·
electron transport
·
high
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Organic Materials
Chil Won Lee, Jun
Yeob Lee*
&&&& — &&&&
N,N-Diphenylpyridin-4-amine as
a Bipolar Core Structure of High-
Triplet-Energy Host Materials for Blue
Phosphorescent Organic Light-Emit-
ting Diodes
Got the blues: High-triplet-energy
bipolar host materials based on a bipo-
lar N,N-diphenylpyridin-4-amine
(DPPA) core structure were developed
for blue phosphorescent organic light-
emitting diodes (PHOLEDs). Two
DPPA-based host materials (see pic-
ture) showed high triplet energy over
2.70 eV and a high quantum efficiency
of 15.6%.
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ꢁ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Asian J. 0000, 00, 0 – 0
ÝÝ These are not the final page numbers!