Novel four-pyridylbenzene-armed biphenyls as electron-transport materials for phosphorescent OLEDs

Article abstract of DOI:10.1021/ol7030872

A series of four-pyridylbenzene-armed biphenyl derivatives were designed and synthesized as an electron-transport and exciton- and hole-block layer for the fac-tris(2-phenylpyridine)iridium (lr(PPy)₃)-based green phosphorescent organic light-em

Full text of DOI:10.1021/ol7030872

ORGANIC
LETTERS
2008
Vol. 10, No. 5
941-944
Novel Four-Pyridylbenzene-Armed
Biphenyls as Electron-Transport
Materials for Phosphorescent OLEDs
Shi-Jian Su,*,†,‡ Daisaku Tanaka,† Yan-Jun Li,† Hisahiro Sasabe,†
Takashi Takeda,† and Junji Kido*,†,‡
Optoelectronic Industry and Technology DeVelopment Association, Bunkyo-ku,
Tokyo 112-0014, Japan, and Department of Polymer Science and Engineering,
Faculty of Engineering, Yamagata UniVersity, 4-3-16 Jonan, Yonezawa,
Yamagata 992-8510, Japan
sushijian@hotmail.com; kid@yz.yamagata-u.ac.jp
Received December 29, 2007
ABSTRACT
A series of four-pyridylbenzene-armed biphenyl derivatives were designed and synthesized as an electron-transport and exciton- and hole-
block layer for the fac-tris(2-phenylpyridine)iridium (Ir(PPy)3)-based green phosphorescent organic light-emitting devices (OLEDs), giving improved
efficiency in comparison to that with both the electron-transport layer of tris(8-hydroxyquinoline)aluminum (Alq3) and the exciton- and hole-
block layer of 2,9-dimethyl-4,7-diphenylphenathroline (BCP).
Since the first report of multilayered organic light-emitting
devices (OLEDs),1 many studies have focused on improving
the device efficiency of OLEDs.2 Recently, phosphorescent
OLEDs using phosphorescent dyes doped into charge-
transporting hosts as emissive layers (EMLs) have attracted
intensive attention due to the highly efficient emission
compared to conventional fluorescent OLEDs, through
radiative recombination of both singlet and triplet excitons.3
Triplets typically have long diffusion lengths.4 To prevent
exciton quenching by the electron-transport layer (ETL) and
hole leakage into the ETL, an exciton- and/or hole-block
layer was generally inserted between the EML and ETL,3c,5
and it was reported that no efficient emission can be obtained
if there is not a hole- and exciton-block layer, which has
electron-transport properties.3b,6 This may induce more
complexity of device structure and thus higher cost for
applications to flat-panel displays and lighting. As such, it
† Optoelectronic Industry and Technology Development Association.
‡ Yamagata University.
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10.1021/ol7030872 CCC: $40.75
© 2008 American Chemical Society
Published on Web 02/01/2008

becomes particularly challenging to develop electron-
transport materials (ETMs) that can also act as an exciton-
and hole-block layer. The object ETM should possess an
appropriate lowest unoccupied molecular orbital (LUMO)
level to give a low electron injection barrier, a low-lying
highest occupied molecular orbital (HOMO) level to block
holes from EML, and a high triplet energy level to block
triplet excitons formed in the EML. To match these require-
ments, the conjugation length of the ETM must be extremely
confined to achieve a wide energy gap and a high triplet
energy (ET) level. It is well-known that the π-electron
delocalizations are extended along the elongated molecular
axis for para linkage with ET decreasing with the number of
phenyl and interrupted at the meta linkage.7 As an example,
the ET of m-terphenyl (2.81 eV) is close to that of biphenyl
(2.84 eV) and much higher than that of p-terphenyl (2.55
eV).8 ET of a typical phosphorescent emitter of fac-tris(2-
phenylpyridine) iridium (Ir(PPy)3) is 2.55 eV. It seems that
a molecule with a π-electron delocalization longer than
p-terphenyl might not be appropriate to block its triplet
excitons formed in the EML for the Ir(PPy)3-based phos-
phorescent OLEDs. To give an ETM with improved electron
mobility, heteroaromatics like phenanthroline,9 pyridine,10
pyrimidine,11 quinoline,12 triazole,13 etc. were incorporated
into the π-conjugated systems. However, more delocalized
π-conjugation may induce a narrower energy gap and a lower
triplet energy level.
We report in this letter syntheses of a series of four-
pyridylbenzene-armed biphenyls with controlled π-conjuga-
tion and application for the Ir(PPy)3-based green phospho-
rescent OLEDs as an electron-transport and exciton- and
hole-block layer. Device efficiency was improved signifi-
cantly in comparison to the devices with both electron-
transport and exciton- and hole-block layers. They were
designed and synthesized by introducing pyridyls on the
periphery of the molecules started from bromopyridine or
pyridylboronate. All the pyridyls and phenyls were combined
with each other at the meta position for 10a and 10b in
comparison to 11a and 11b which contain a more elongated
π-conjugation of 4-pyridyl-1,1′-biphenyl. Scheme 1 shows
synthetic routes of pinacol boronates of pyridylbenzene.
Pyrid-3-yl-containing 1 and 2 were synthesized by Suzuki-
Miyaura coupling reaction between chlorophenylboronic acid
and 3-bromopyridine in high yields. In comparison, pyrid-
4-yl-containing 5 and 6 were synthesized from bromoiodo-
Scheme 1
benzene and pinacol pyrid-4-ylboronate in a moderate yield.
Cross-coupling reaction of bis(pinacolato)diboron with 1 and
2 catalyzed by bis(dibenzylideneacetone) palladium(0) (Pd-
(dba)2) with the ligand of tricyclohexylphosphine (PCy3)
gives the corresponding arylboronates of 3 and 4 in the
presence of potassium acetate, respectively.14 Pyrid-4-yl-
containing arylboronates of 7 and 8 were synthesized by a
modified cross-coupling reaction of bis(pinacolato)diboron
with 5 and 6 catalyzed by 1,1′-bis(diphenylphosphino)-
ferrocene dichloropalladium(II) (PdCl2(dppf)).15
3,3′,5,5′-Tetrabromobiphenyl (9) was synthesized by the
lithiation of 1,3,5-tribromobenzene followed by oxidative
coupling with CuCl2.16 Following Suzuki-Miyaura coupling
reaction of 9 with arylboronates of 3, 4, 7, and 8 gives
3,5,3′,5′-tetra(m-pyrid-3-yl)phenyl[1,1′]biphenyl (10a), 3,5,3′,5′-
tetra(p-pyrid-3-yl)phenyl[1,1′]biphenyl (11a), 3,5,3′,5′-tetra-
(m-pyrid-4-yl)phenyl[1,1′]biphenyl (10b), and 3,5,3′,5′-tetra-
(p-pyrid-4-yl)phenyl[1,1′]biphenyl (11b), respectively (Scheme
2).
As shown in Table 1, HOMO levels of both the pyrid-3-
yl- and pyrid-4-yl-containing biphenyl derivatives determined
by atmospheric ultraviolet photoelectron spectroscopy are
around 6.60 eV. They are much lower than that of well-
known ETM of tris(8-hydroxyquinoline)aluminum (Alq3)5a
and equal to that of well-known hole-block material of 2,9-
dimethyl-4,7-diphenylphenathroline (BCP),6 indicating their
improved hole-block property. LUMO levels of 10a and 10b
calculated from their HOMO levels and the lowest energy
absorption edges of the UV-vis absorption spectra are 2.57
and 2.53 eV, respectively, in contrast to 2.94 and 3.04 eV
for 11a and 11b. Much lower lying LUMO levels for 11a
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942
Org. Lett., Vol. 10, No. 5, 2008

To confirm their triplet exciton confinement property, the
transient photoluminescence decay of 3% (by weight) Ir-
(PPy)3 doped into typical four-pyridylbenzene-armed biphe-
nyls of 10a and 11a was measured (Figure 1). As expected,
Scheme 2
Figure 1. Transient photoluminescence decay curves of 10a:3 wt
% Ir(PPy)3 and 11a:3 wt % Ir(PPy)3 under excitation by a nitrogen
laser (λ ) 337 nm, 50 Hz, 800 ps pulses) at rt.
both the 10a:Ir(PPy)3 and 11a:Ir(PPy)3 films exhibit almost
monoexponential decays with relatively long lifetimes of 1.26
and 1.58 µs, respectively, indicating the triplet energy transfer
from Ir(PPy)3 to 10a and 11a was successfully suppressed
to confine the triplet excitons on the guest molecules.17 This
is of importance as an exciton-block layer to confine triplet
excitons in the EML leading to highly efficient phospho-
rescent OLEDs.
and 11b arising from their more delocalized π-conjugation
To evaluate their electron-transport and hole- and exciton-
block properties, green phosphorescent OLEDs based on Ir-
(PPy)3 were fabricated with 10a and 11a as an ETL without
inserting any hole- or exciton-block layers between EML
and ETL. Poly(arylene amine ether sulfone)-containing
tetraphenylbenzidine (TPDPES) doped with 10 wt % tris-
(4-bromophenyl)aminium hexachloroantimonate (TBPAH)
was spun onto the precleaned indium-tin oxide (ITO)-coated
glass substrate from its dichloroethane solution to form a
20-nm-thick polymer buffer layer.18 Then, a 30-nm-thick hole
transport layer of 1,1-bis[4-[N,N-di(p-tolyl)amino]phenyl]-
cyclohexane (TAPC) was deposited. Next, 7 wt % Ir(PPy)3
was co-deposited with N,N′-dicarbazolyl-4,4′-biphenyl (CBP)
to form the 10-nm-thick EML. Finally, a 50-nm-thick ETL
consisting of either 10a or 11a was deposited to block holes
and to confine triplet excitons in the emissive zone. For
comparison, a device with BCP as the hole- and exciton-
block layer (10 nm) and Alq3 as the ETL (40 nm) was also
fabricated. Cathodes consisting of a 0.5-nm-thick layer of
LiF followed by a 100-nm-thick layer of Al were patterned
using a shadow mask with an array of 2 mm × 2 mm
openings.
may induce a lower electron-injection barrier. In addition,
Table 1. Physical Properties of 10a,b and 11a,b
c
d
d
e
HOMOa LUMOb
λmax
(nm)
Tg
Tm
Td
materials
(eV)
(eV)
(°C) (°C) (°C)
10a
10b
11a
11b
6.66
6.67
6.58
6.68
2.57
2.53
2.94
3.04
258, 357 107
262, 358 128
293, 372
191
265
319
382
503
505
523
500
298, 391
a Determined by atmospheric ultraviolet photoelectron spectroscopy.
b Calculated from the HOMO level and the lowest energy absorption edge
of the UV-vis absorption spectrum. c Absorption and photoluminescent
spectra in vacuum deposited film on quartz substrate. d Glass transition
temperature (Tg) and melting temperature (Tm) obtained from differential
scanning calorimetry (DSC) measurement. 11a and 11b crystallized during
the heating-cooling cycles, and no Tg was detected for them. e Decompo-
sition temperature (Td) obtained from thermogravimetric analysis (TGA).
all-meta-combined 10a and 10b have relatively high glass
transition temperatures (Tg), proving the high morphologic
stability of the amorphous phase in a deposited film, which
is a prerequisite for the application in OLEDs. Tg of pyrid-
4-yl-containing 10b is 20 °C higher than that of pyrid-3-yl-
containing 10a due to its higher polarity and thus stronger
molecular interactions.
(17) Baldo, M. A.; Adachi, C.; Forrest, S. R. Phys. ReV. B 2000, 62,
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Org. Lett., Vol. 10, No. 5, 2008
943

Their current density versus driving voltage characteristics
(Figure 2) indicate that the driving voltages of the devices
Alq3 (84.4 cd/A) and those of the previously reported devices
based on both ETL and hole- and exciton-block layers
(Figure 2, inset).3b,5,6 At the practical brightness of 100 cd/
m2, their current efficiencies remain at 92.6 and 82.4 cd/A,
respectively, which are higher than that of the device based
on BCP and Alq3 (79.2 cd/A). Much higher luminous
efficiencies of 89.8 and 85.9 lm/W were achieved at 100
cd/m2 for 10a- and 11a-based devices, respectively, due to
increased carrier balance and decreased driving voltage,
which are even higher than that of the device with an
n-doping ETL and an exciton-block layer.5b At a much
brighter emission of 1000 cd/m2, their luminous efficiencies
remain at 68.5 and 67.2 lm/W, respectively, which are even
higher than that of the device based on BCP and Alq3 at a
low luminance of 100 cd/m2. It is quite interesting that their
current efficiency remains over 60 cd/A even when the
brightness reaches 9000 cd/m2. The reduced efficiency roll-
off at a high current density should arise from the balanced
injection of both holes and electrons and the efficient
confinement of triplet excitons generated in the EML.
Moreover, no emission from 10a and 11a was observed in
the electroluminescent (EL) spectra, further proving that
holes and excitons are well confined within the EML by the
current ETMs.
In conclusion, a series of four-pyridylbenzene-armed
biphenyls were designed and synthesized as ETMs for
efficient green phosphorescent OLEDs. Our findings indicate
that highly efficient emission can be achieved with finely
designed ETMs as a single ETL instead of generally used
ETL combined with hole- and exciton-block layers. Further
modification of the central skeleton to create other pyridyl-
benzene-containing ETMs from the pinacol boronates of
pyridylbenzene developed here to finely tune their energy
levels and carrier mobility and exploration of their structure-
performance correlation are currently under investigation and
will be reported in due course.
Figure 2. Current density vs driving voltage characteristics for
ITO/TPDPES:TBPAH (20 nm)/TAPC (30 nm)/CBP:7 wt % Ir-
(PPy)3 (10 nm)/10a (circles) or 11a (diamonds) (50 nm)/LiF (0.5
nm)/Al (100 nm) and ITO/TPDPES:TBPAH (20 nm)/TAPC (30
nm)/CBP:7 wt % Ir(PPy)3 (10 nm)/BCP (10 nm)/Alq3 (40 nm)/
LiF (0.5 nm)/Al (100 nm) (triangles). Inset: efficiency vs luminance
characteristics.
based on 10a and 11a are much lower than that of the device
based on BCP and Alq3. The LUMO level of 10a is slightly
higher than those of CBP (-2.7 eV) and Ir(PPy)3 (-2.8
eV),5a facilitating efficient electron transfer from the ETL
to the EML. In contrast, the LUMO level of BCP is 0.3 eV
lower than that of CBP and 0.2 eV lower than that of Ir-
(PPy)3, and it may induce an electron injection barrier
between the EML and the hole- and exciton-block layer of
BCP. The driving voltage of the 11a-based device further
decreased due to its lower lying LUMO level and more
delocalized π-conjugation, which may induce an increased
electron-injection and transport. Besides their appropriate
LUMO levels, the coordination effect of the nitrogen atom
in periphery pyridyls with lithium cations may also induce
an effective electron injection and thus a low driving
voltage.19 Peak current efficiencies of 96.5 and 90.2 cd/A
were obtained for 10a- and 11a-based devices, respectively,
which are higher than that of the device based on BCP and
Acknowledgment. This work was financially supported
by the New Energy and Industrial Technology Development
Organization (NEDO) through the “Advanced Organic
Device Project”.
Supporting Information Available: Experimental pro-
cedures, analytical and spectroscopic data of compounds,
UV-vis and PL spectra of 10a,b and 11a,b, device fabrica-
tion, and summary of device characteristics. This material
is available free of charge via the Internet at http://pubs.acs.org.
(19) Oyamada, T.; Yoshizaki, H.; Sasabe, H.; Adachi, C. Chem. Lett.
2004, 33, 1034.
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