Triazole and pyridine hybrid molecules as electron-transport materials for highly efficient green phosphorescent organic light-emitting diodes

Article abstract of DOI:10.1002/ijch.201400064

A series of triazole and pyridine hybrid molecules, with a triazole core and pyridine periphery, were designed and synthesized as an electron-transport layer (ETL) and a hole/exciton-block layer for green phosphorescent organic light-emitting diodes. Compared with the widely-used electron-transport material (ETM) of 3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) with a triazole core, lower-lying HOMO and LUMO energy levels were obtained with the introduction of pyridine rings onto the periphery of the molecules, giving improved electron injection and carrier confinement. Significantly reduced driving voltages were achieved in a device structure of ITO/HATCN (5 nm)/TAPC (40 nm)/CBP:8 wt % Ir(PPy)₃ (10 nm)/ETL (40 nm)/LiF (1 nm)/Al (90 nm), giving a maximum power efficiency of 72.2 lm W ^-1 and an external quantum efficiency of 21.8 %, due to the improved electron injection and transport and thus, more balanced carrier recombination, which are much higher than those of the device based on TAZ.

Full text of DOI:10.1002/ijch.201400064

Full Paper
DOI: 10.1002/ijch.201400064
Triazole and Pyridine Hybrid Molecules as Electron-
Transport Materials for Highly Efficient Green
Phosphorescent Organic Light-Emitting Diodes
Xiang-Long Li,[a,+] Hua Ye,[a,+] Dong-Cheng Chen,[a] Kun-Kun Liu,[a] Gao-Zhan Xie,[a] Yi-Fan Wang,[b]
Chang-Cheng Lo,[b] A. Lien,[c] Junbiao Peng,[a] Yong Cao,[a] and Shi-Jian Su*[a]
Abstract: A series of triazole and pyridine hybrid molecules,
with a triazole core and pyridine periphery, were designed
and synthesized as an electron-transport layer (ETL) and
a hole/exciton-block layer for green phosphorescent organic
light-emitting diodes. Compared with the widely-used elec-
tron-transport material (ETM) of 3-(biphenyl-4-yl)-5-(4-tert-
butylphenyl)-4-phenyl-4H-1,2,4-triazole (TAZ) with a triazole
core, lower-lying HOMO and LUMO energy levels were ob-
tained with the introduction of pyridine rings onto the pe-
riphery of the molecules, giving improved electron injection
and carrier confinement. Significantly reduced driving vol-
tages were achieved in a device structure of ITO/HATCN
(5 nm)/TAPC (40 nm)/CBP:8 wt% Ir(PPy)3 (10 nm)/ETL
(40 nm)/LiF (1 nm)/Al (90 nm), giving a maximum power
efficiency of 72.2 lmWꢀ1 and an external quantum efficiency
of 21.8%, due to the improved electron injection and trans-
port and thus, more balanced carrier recombination, which
are much higher than those of the device based on TAZ.
Keywords: charge carrier injection · electron transport · hybrid molecules · organic light-emitting diodes · triazole derivatives
1. Introduction
The performance of organic light-emitting diodes
(OLEDs) has been greatly improved[1] since Tang and his
coworkers made a huge breakthrough by advancing the
efficiency of OLEDs to 1%.[2] OLEDs with high efficien-
cy have been reported by many groups[3] since then. Phos-
phorescent OLEDs using phosphorescent dyes doped
into charge-transporting hosts as emissive layers
(EMLs)[4] have attracted extensive attention due to their
highly efficient emission, compared with conventional flu-
orescent OLEDs, on account of collecting both singlet
and triplet excitons. Generally, balanced carrier recombi-
nation plays a critical role in achieving high efficiency
OLEDs. Since the electron-transport ability of organic
materials is usually poorer than their hole-transport abili-
ty, electron-transport materials (ETMs), with high elec-
tron-injection and -transport abilities, are required to give
balanced carriers, and thus improve OLED per-
formance.[5] Kido et al. have developed a highly conduc-
tive n-doped electron-transport layer (ETL) by co-evapo-
rating ETMs and highly active alkaline metals such as
Li[6] to obtain an effective electron injection and transport
in OLEDs. In this case, an exciton-block layer (EBL) is
required to prevent penetration of n-dopants into the
EML and thus exciton quenching. The additional layer
will lead to the complexity of the device fabrication, and
this may increase the cost for applications in flat-panel
displays and lighting.
To obtain high electron-injection and -transport materi-
als, pyrimidine,[7] phenanthroline[8], pyridine[5,7d,e,9] and
other heteroaromatic groups with high electron affinity
were introduced into p-conjugated systems. As a com-
pound with a five-membered heteroaromatic triazole
core, TAZ [3-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-4-
phenyl-4H-1,2,4-triazole] is widely used as an ETM.[10]
However, good performance could be hardly achieved for
the devices with TAZ as an ETL because of its poor elec-
tron-transport ability.[11] This drawback limits its further
application in high performance OLEDs. It was found
that the LUMO (lowest unoccupied molecular orbital)
energy level of the six-membered pyridine ring is deeper
[a] X.-L. Li,+ H. Ye,+ D.-C. Chen, K.-K. Liu, G.-Z. Xie, J. Peng, Y. Cao,
S.-J. Su
State Key Laboratory of Luminescent Materials and Devices and
Institute of Polymer Optoelectronic Materials and Devices
South China University of Technology
Guangzhou 510640 (China)
e-mail: mssjsu@scut.edu.cn
[b] Y.-F. Wang, C.-C. Lo
Shenzhen China Star Optoelectronics Technology Co., Ltd
Shenzhen 518132 (China)
[c] A. Lien
TCL Corporate Research
Shenzhen 518052 (China)
[+] These authors contributed equally to this work.
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than that of the C=N containing five-membered heteroar-
omatic compounds, leading to better electron injection.[12]
The phenylpyridine[7d,9b] moiety can be used to construct
ETMs with high electron-transport ability[9d,13] and high
triplet energy (ET).[5] In addition, excellent ETMs were
developed with the combination of pyridine ring as the
periphery and benzene[5] or triazine[14] as the core, and
the ETMs with a pyrid-3-yl periphery were confirmed to
give better device performance.[14,15]
2. Results and Discussion
2.1 Thermal and Photophysical Properties
As can be clearly seen from Figure 2, TPyTAZm and
TPyBnTAZm possess distinct glass-transition tempera-
tures (Tg) of 77 and 948C, respectively. In contrast, when
the three meta-connected benzene rings are changed to
para-connection, the enhanced molecular rigidity of
TPyBnTAZp leads to difficulty in the movement of the
molecular segments, and thus its Tg increases to 1128C.
Similar to TAZ, TPyTAZp exhibits an amorphous state
up to 2008C, maybe due to its most rigid molecular struc-
ture. In addition, the current triazole and pyridine hybrid
molecules exhibit excellent thermal stability, with decom-
position temperatures (Td) ranging from 450 to 5108C,
which is far higher than that of TAZ (2988C).
In this article, we report a series of triazole and pyri-
dine hybrid molecules with triazole as the core and pyri-
dine as the periphery (Figure 1), which exhibit obviously
As shown in Figure 3, the absorption spectrum of TPy-
TAZp, based on a p-triphenyltriazole core, peaks at
300 nm in toluene solution. With three more meta-con-
nected benzene rings, the absorption peak of TPyBn-
TAZp blue-shifts to 285 nm, accompanied with a shoulder
at 300 nm, due to the weakened conjugation. For the two
molecules based on a m-triphenyltriazole core, the ab-
sorption peak of TPyTAZm with a peripheral pyridine
ring is at 283 nm. In comparison, TPyBnTAZm, with a pe-
ripheral pyridyl-benzene, red-shifts to 290 nm. This phe-
nomenon indicates that TPyBnTAZm, with peripheral
meta-connected benzene rings, have longer a conjugated
length than TPyTAZm. Compared with the absorption
spectra in toluene solutions, the absorption peaks of all
the molecules in thin solid films do not change obviously.
Meanwhile, TPyTAZm and TPyBnTAZm have relatively
narrower absorption ranges than TPyTAZp and TPyBn-
TAZp, because of their poorer conjugation.
Figure 1. Molecular structures of the designed triazole and pyri-
dine hybrid molecules TPyTAZp, TPyTAZm, TPyBnTAZp, and TPyBn-
TAZm. Also shown the molecular structure of TAZ for comparison.
better electron injection and transport than TAZ. Highly
efficient green phosphorescent OLEDs, with reduced effi-
ciency roll-off, were achieved with these molecules as the
ETLs, due to their good confinement of both carriers and
excitons within the EML, giving a maximum power effi-
ciency (PEmax) of 72.2 lmWꢀ1, and an external quantum
efficiency (EQEmax) of 21.8%. In addition, extremely re-
duced driving voltages were obtained for the hybrid mol-
ecules, in comparison with TAZ. The lowest turn-on volt-
age (Von) (at the luminance of 1 cdmꢀ2) of 2.5 V was ach-
ieved for the device based on TPyTAZp, which is much
lower than that of the device based on TAZ (4.1 V).
However, when a p-doped hole injection layer is not
used, the current Von value is even lower than that of the
device based on a p-doped polymer hole injection layer
and a classic ETM of TpPyPB (1,3,5-tri(p-pyrid-3-yl-phe-
nyl)benzene) with a benzene core.[5] Their structure-prop-
erty relationships, including the thermal and electrochem-
ical properties; electron-transport properties in electron-
only devices; and current density-voltage-luminance (J-V-
L) characteristics of phosphorescent OLEDs, were stud-
ied comprehensively.
Figure 2. Thermogravimetric analysis (TGA) and differential scan-
ning calorimetry (DSC) (inset) characteristics of TPyTAZp, TPyTAZm,
TPyBnTAZp, and TPyBnTAZm.
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Figure 4. Cyclic voltammograms of TPyTAZp, TPyTAZm, TPyBn-
TAZp, and TPyBnTAZm in dichloromethane solutions.
Figure 3. Room temperature UV-vis absorption (solid) and photo-
luminescent (PL) (open) spectra of TPyTAZp (!!), TPyTAZm
(**), TPyBnTAZp (~~) and TPyBnTAZm (&&) in toluene solu-
tions (bottom) and thin solid films (top) on quartz substrates.
tials (Ered) for TPyTAZp, TPyTAZm, TPyBnTAZp and
TPyBnTAZm were found to be 1.87/ꢀ1.67, 1.84/ꢀ1.63,
1.82/ꢀ1.62, and 1.89/ꢀ1.70 V, respectively. Under the
same experimental conditions, the redox potential (E1/2)
of ferrocene/ferrocenium (Fc/Fc+) was measured to be
0.40 V. The redox potential of Fc/Fc+ has an absolute
energy level of ꢀ4.80 eV, compared with the vacuum
level.[16] Therefore, the HOMO (highest occupied molecu-
lar orbital) and LUMO energy levels of the current mate-
rials are calculated according to the following equations:
HOMO=ꢀe (Eox +4.40) V, LUMO=ꢀe (Ered +4.40) V.
Due to the introduction of strong electrophilic pyridine
rings onto the periphery of the molecules, much lower
LUMO energy levels are achieved in comparison with
the ꢀ2.7 eV of TAZ. Such deep LUMO energy levels are
beneficial for electron injection and transport.
Similar to the absorption spectra, bathochromic shifts
were also observed for the PL spectra recorded in both
toluene solutions and thin solid film states. PL spectra of
TPyTAZp and TPyBnTAZp peak at 372 and 385 nm in
toluene solution, respectively. While the emission peaks
of TPyTAZm and TPyBnTAZm are at 349 and 348 nm,
respectively, which show smaller stoke shifts and narrow-
er full-widths at half maximum than TPyTAZp and
TPyBnTAZp. As compared with the PL spectra in tolu-
ene solutions, the PL spectra of TPyTAZm and TPyBn-
TAZm in thin films are greatly red-shifted, which may be
ascribed to the intense aggregation in solid films.
2.2 Electrochemical Properties and DFT Calculations
To get further insights into the molecular orbitals of
the pyridine and triazole hybrid molecules, density func-
tional theory (DFT) calculations were also performed
with the B3LYP/6-311G+dp basis set, using the Gaussian
suite of programs (Gaussian 03W). The calculated
The electrochemical properties of the triazole and pyri-
dine hybrid molecules were measured by cyclic voltam-
metry in dichloromethane solutions (Figure 4). The
onsets of oxidation potentials (Eox) and reduction poten-
Table 1. Physical properties of the triazole-core-containing materials.
opt [c]
[f]
[f]
cal [g]
cal [g]
Tg[a]
Td[b]
Eg
HOMO[d]
(eV)
LUMO[e]
(eV)
EHOMO
(eV)
ELUMO
(eV)
Eg
ET
Materials
(oC)
(oC)
(eV)
(eV)
(eV)
TPyTAZp
TPyTAZm
TPyBnTAZp
TPyBnTAZm
TAZ
–
77
112
94
458
450
498
510
298
3.32 (3.59)
3.80 (4.05)
3.42 (3.62)
3.72 (3.95)
–
ꢀ6.27
ꢀ6.24
ꢀ6.22
ꢀ6.29
ꢀ6.3[1]
ꢀ2.73
ꢀ2.77
ꢀ2.78
ꢀ2.70
ꢀ2.7[1]
ꢀ6.40
ꢀ6.22
ꢀ6.04
ꢀ6.24
ꢀ5.97
ꢀ2.03
ꢀ2.09
ꢀ1.95
ꢀ1.86
ꢀ1.62
4.37
4.12
4.09
4.38
4.35
2.57
2.77
2.57
2.78
2.58
–
[a] Tg was measured by DSC. [b] The decomposition temperature (Td) was measured by TGA with 5% weight-loss. [c] The optical band gap
corresponding to the lowest-energy absorption edge of the UV-vis absorption spectrum of the material in thin solid film and in toluene solu-
tion (in parentheses). [d] HOMO=ꢀe (4.40+Eox) V. [e] LUMO=ꢀe (4.40+Ered) V. [f] HOMO and LUMO energy levels obtained by DFT cal-
culations (B3LYP/6-311G+dp) (Gaussian 03W). [g] Calculated energy band gap (Egcal = jEHOMO jꢀjELUMO j) and triplet energy (ETcal).
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HOMO and LUMO energy levels, energy band gaps
(Egcal), and triplet energy levels (ETcal) of the molecules
are listed in Table 1. Though the calculated values are
somewhat different from the experimental values, a simi-
lar trend is observed.
As shown in Table 2, the device based on TPyTAZp ex-
hibits a low Von of 2.5 V, and driving voltages of 3.0 and
3.7 V at 100 and 1000 cdmꢀ2, respectively. All are much
lower than the previously reported values based on
Ir(PPy)3 with the widely used ETM of 1,3,5-tri((3-pyrid-
yl)-phen-3-yl) benzene (TmPyPB)[5] as the ETL, probably
due to the improved electron injection. Its Von is even
lower than that of the device based on TpPyPB,[5] al-
though no p-doped hole injection layer was used for the
current devices. Compared with the device based on TPy-
TAZp, the Von of the devices based on TPyTAZm,
TPyBnTAZp and TPyBnTAZm increase to 2.9, 4.0, and
3.0 V, with driving voltages of 3.6, 5.4, and 4.0 V at
100 cdmꢀ2, and 4.5, 6.5, and 5.1 V at 1000 cdmꢀ2, respec-
tively. To further understand this phenomenon, electron-
only devices were fabricated with a configuration of ITO/
LiF (1 nm)/ETMs (50 nm)/LiF (1 nm)/Al (80 nm). Their
current density-voltage (J-V) characteristics are shown in
Figure 6. The devices based on the pyridine-containing
ETMs show much higher current density than that of the
device based on TAZ at the same bias, indicating they
2.3 OLED Characterization
To evaluate the electron-injection/-transport abilities of
the developed triazole and pyridine hybrid molecules,
green phosphorescent OLEDs were fabricated with
Ir(PPy)3 as the phosphorescent emitter. Table 2 gives
a summary of the device performance. Figure 5 presents
the current density-voltage-luminance (J-V-L) character-
istics. Compared with the device based on TAZ, the J-V
and L-V curves of the other devices feature steeper incre-
ments, indicating the improved electron injection and
transport by the developed ETMs.
Figure 5. Current density and luminance versus driving voltage (J-
V-L) characteristics of the green phosphorescent OLEDs in a struc-
ture of ITO/HATCN (5 nm)/TAPC (40 nm)/CBP:8 wt% Ir(PPy)3
(10 nm)/TPyTAZp (!!), TPyTAZm (**), TPyBnTAZp (~~) and
TPyBnTAZm (&&) or TAZ (^^) (40 nm)/LiF (1.0 nm)/Al(90 nm).
Figure 6. Current density-voltage (J-V) characteristics of the elec-
tron-only devices based on TPyTAZp (!), TPyTAZm (*), TPyBn-
TAZp (~) and TPyBnTAZm (&), TAZ (^) in a structure of ITO/LiF
(1 nm)/ETMs (50 nm)/LiF (1 nm)/Al (80 nm).
Table 2. Summary of the electroluminescent performance of the devices in a structure of ITO/HATCN (5 nm)/TAPC (40 nm)/CBP:8 wt%
Ir(PPy)3 (10 nm)/ETMs (40 nm)/LiF (1 nm)/Al (90 nm).
at 100 cdmꢀ2
V (V) CE
at 1000 cdmꢀ2
[a]
ETMs
Von
CEmax
PEmax
EQEmax
(%)
PE
EQE (%) V (V) CE
(cd Aꢀ1
PE
EQE (%)
(V)
(cd Aꢀ1
)
(lm Wꢀ1
)
(cd Aꢀ1
)
(lm Wꢀ1
)
)
(lm Wꢀ1
)
TPyTAZp
TPyTAZm
TPyBnTAZp
TPyBnTAZm 3.0
TAZ 4.1
2.5
2.9
4.0
51.9
73.6
30.5
52.8
57.3
58.2
72.2
30.1
53.3
36.0
15.5
21.8
9.1
16.1
17.1
3.0
3.6
5.4
4.0
5.6
51.7
53.3
58.3
17.9
39.8
30.4
15.4
20.0
9.0
15.1
16.0
3.7
4.5
6.5
5.1
7.2
46.7
60.2
29.5
44.2
43.9
40.1
42.3
14.1
27.3
19.1
13.9
17.9
8.8
13.1
13.1
67.8
30.4
50.8
53.9
[a] at the luminance of 1 cdmꢀ2
.
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have better electron-injection and -transport abilities than
TAZ due to their deeper LUMO energy levels, as well as
stronger electron affinity. Von shows the same trend as the
electron-injection and -transport abilities of the pyridine-
containing ETMs.
Although TPyTAZp has the best electron-transport
ability in both light-emitting devices and electron-only
devices, its phosphorescent OLEDs exhibit a maximum
current efficiency (CEmax) of only 51.9 cdAꢀ1 and a maxi-
mum power efficiency (PEmax) of 58.2 lmWꢀ1; both are
much lower than those of the devices based on TPy-
TAZm (73.6 cdAꢀ1 and 72.2 lmWꢀ1). This may be attrib-
uted to the reduced charge-carrier balance, since TPy-
TAZp has excellent electron-injection and -transport abil-
ities, which may lead to excess electrons in the EML.
Among them, TPyTAZm shows the best performance
(see Figure 7), and its EQEmax approaches 21.8%. In ad-
CBP and TPyBnTAZm is 0.20 eV. Thereby, the electron
transport into the EML from TPyTAZm may be easier
than that from TPyBnTAZm. Accordingly, current densi-
ty in the light-emitting devices exhibits the same behav-
ior, to give improved carrier balance and thus, improved
efficiency.
Similar to typical phosphorescent OLEDs, efficiency
roll-off at high driving-current density is also observed
here. However, power efficiencies for the TPyTAZp-
based device remain above 53 and 40 lmWꢀ1, at the prac-
tical brightnesses of 100 and 1000 cdmꢀ2, respectively,
which are much higher than those of the device based on
TAZ (Figure 7). The higher power efficiencies for the
device based on TPyTAZp should arise from its signifi-
cantly reduced driving voltage at the same brightness,
owing to its better electron injection and transport. Power
efficiencies are further improved to 58.3 lmWꢀ1 at
100 cdmꢀ2 and 42.3 lmWꢀ1 at 1000 cdmꢀ2 when the link-
age position of benzene rings changes from the para posi-
tion to the meta position. As shown in Table 1, TPy-
cal
TAZm possesses higher ET than TPyTAZp. The im-
proved efficiency may also be attributed to the improved
confinement of Ir(PPy)3 triplet excitons by the adjacent
TPyTAZm, compared with TPyTAZp, as well as the in-
creased carrier balance. Power efficiency of 39.8 lmWꢀ1
was achieved at the luminance of 100 cdmꢀ2 for the
TPyBnTAZm-based device. Athought it is lower than
that of the devices based on TPyTAZp or TPyTAZm, it
still shows a high EQE over 15%. As shown in Figure 7,
reduced roll-off in EQE can be achieved by the devel-
oped ETMs, compared with that of TAZ. As mentioned
above, since the triplet excitons could be well confined
within the EML, when the developed ETMs were used as
an ETL as well as a hole/exciton-block layer, the reduced
efficiency roll-off may also have arisen from good exciton
confinement. It is quite interesting that reduced efficiency
roll-off can be achieved solely by using an ETM with en-
hanced triplet exciton confinement as an ETL and a hole/
exciton-block layer. This suggests that efficiency roll-off
may also arise from triplet-exciton or carrier-exciton
quenching by the adjacent ETMs, as well as from the usu-
ally reported triplet-triplet annihilation.
Figure 7. Current and power efficiency versus luminance (top) and
external quantum efficiency versus luminance characteristics
(bottom) of the green phosphorescent OLEDs in a structure of ITO/
HATCN (5 nm)/TAPC (40 nm)/CBP:8 wt% Ir(PPy)3 (10 nm)/TPyTAZp
(!!), TPyTAZm (**), TPyBnTAZp (~~) and TPyBnTAZm (&&)
or TAZ (^^) (40 nm)/LiF (1.0 nm)/Al(90 nm).
All of the current devices exhibit similar EL spectra,
which peak at 512 nm (Figure 8), and no emission from
the ETL was found, further indicating that carriers and
excitons are completely confined within the EML.
dition, the EQE is as high as 17.9%, even at a brightness
of 1000 cdmꢀ2 (4.5 V). Aside from the balanced carrier
flux, considering the relatively higher ET of TPyTAZm,
cal
the improved performance may be also attributed to the
improved exciton confinement within the emission layer,
even at high current density. Compared with TPyTAZm,
TPyBnTAZm, with three more meta-connected phenyls,
has almost the same electron-injection and -transport
abilities and ETcal, but gives much lower efficiency. Taking
the LUMO energy levels of the emitting layer into ac-
count, the LUMO-LUMO offset at the interface between
CBP (LUMO=ꢀ2.9 eV)[17] and TPyTAZm is 0.13 eV,
while the LUMO-LUMO offset at the interface between
3. Conclusion
A series of triazole and pyridine hybrid molecules were
successfully synthesized using typical Suzuki cross-cou-
pling reactions, and their thermal, photophysical and elec-
trochemical properties were thoroughly studied. Their
HOMO and LUMO energy levels were strongly depen-
dent on the linkage position and the number of benzene
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were recorded on a FL-3 spectrofluorometer (HORIBA).
DSC measurements were carried out with a NETZSCH
DSC 204 under a nitrogen flow at a heating rate of
108Cminꢀ1. TGAs were conducted on a NETZSCH TG
209 under a nitrogen flow rate of 20 mlminꢀ1 at a heating
rate of 108Cminꢀ1.
4.2 Materials
All materials were obtained from commercial suppliers
and used without further purification unless otherwise
noted. Scheme 1 depicts the synthetic routes of the de-
signed triazole and pyridine hybrid molecules.
Figure 8. EL spectra of the green phosphorescent OLEDs in a struc-
ture of ITO/HATCN (5 nm)/TAPC(40 nm)/CBP:8 wt% Ir(PPy)3
(10 nm)/TPyTAZp (!), TPyTAZm (*), TPyBnTAZp (~) and TPyBn-
TAZm (&) or TAZ(^) (40 nm)/LiF (1.0 nm)/Al (90 nm) measured at
a driving current density of 10 mAcmꢀ2
.
rings. By using these molecules as an ETL and a hole/ex-
citon-block layer in green phosphorescent OLEDs, much
better electron-injection/-transport abilities were ach-
ieved, which gave significantly reduced driving voltages,
compared with the widely used ETM of TAZ, due to the
introduction of strongly electrophilic pyridine rings on
the periphery of the molecules. The device based on TPy-
TAZm exhibited the highest efficiency, with a PEmax of up
to 73.6 lmWꢀ1 (72.2 cdAꢀ1, 21.8%), due to the matched
energy levels, high electron-transport ability, and more
balanced carrier recombination. In addition, compared
with the device based on TAZ, it showed much higher
PEs of 58.3 and 42.3 lmWꢀ1, at the practical brightnesses
of 100 and 1000 cdmꢀ2, due to its good triplet exciton
confinement ability and reduced efficiency roll-off. The
current findings are beneficial to the design of triazole-
based ETMs for the further improvement of phosphores-
cent OLEDs.
Scheme 1. Synthetic routes of TPyTAZp, TPyTAZm, TPyBnTAZp,
and TPyBnTAZm. Reagents and conditions: i) N2H4·H2O, NMP, r. t. ;
ii) PCl5, toluene, reflux; iii) p-bromoaniline or m-bromoaniline, N,N-
dimethylaniline, 1358; iv) Pd(PPh3)4, 2 M K2CO3, toluene/ethanol,
958.
1,2-bis(4-bromophenyl)hydrazine (2): 4-Bromobenzoyl
chloride (2.6 g, 12 mmol) was dissolved in 30 ml of N-
methylpyrrolidone (NMP) in a double-necked flask, and
then hydrazine hydrate (0.30 g, 6 mmol) was added drop-
wise into the solution. The reaction mixture was stirred at
room temperature overnight and then poured into 200 ml
deionized water. The precipitate was collected by filtra-
tion through a funnel. The collected solid was washed
using deionized water, ethyl acetate, and anhydrous etha-
nol in sequence, then dried under vacuum to give 2
(1.83 g, 76.6%) as a white solid. 1H NMR (300 MHz,
DMSO-d6, d, ppm): 10.65 (s, 2H), 7.84 (d, 4H ), 7.74 (d,
4H).
1,2-bi(chloro(4-bromophenyl)methylene)hydrazine (3):
2 (0.84 g, 2.1 mmol) and phosphorus pentachloride (PCl5)
(0.96 g, 4.62 mmol) were dissolved in 20 ml of anhydrous
toluene with agitation and refluxed under a flow of nitro-
gen for 5 h. After being cooled to room temperature,
water was added to quench the reaction, and the mixture
4. Experimental Section
4.1 General
1H and 13C nuclear magnetic resonance (NMR) spectra
were measured using a Bruker AV-300 spectrometer.
Mass spectrometry (MS) data were obtained on a Waters
TQD. Cyclic voltammetry (CV) measurements were car-
ried out in dichloromethane by using Bu4NPF6 (0.1 M) as
an electrolyte on a CHI600D electrochemical workstation
with a graphite working electrode, a platinum wire coun-
ter electrode, and a saturated calomel electrode (SCE) as
a reference electrode. UV-vis absorption spectra were re-
corded on a HP 8453 spectrophotometer. PL spectra
Isr. J. Chem. 2014, 54, 971 – 978
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was extracted with dichloromethane. The combined or-
ganic phase was washed with saturated brine and dried
over MgSO4. The subjection of the crude product to silica
gel chromatography afforded 3 (0.73 g, 80.0%) as a pale
1H NMR (300 MHz, CDCl3, d, ppm): 8.27 (s, 2H), 8.06 (d,
2H ), 7.67 (d, 2H), 7.36 (t, 2H).
3,4,5-tri(3-bromophenyl)-4H-1,2,4-triazole (8): 8 was
1
synthesized in a similar manner to 4. Yield: 77.5%. H
1
yellow solid. H NMR (300 MHz, CDCl3, d, ppm): 8.00
NMR (300 MHz, CDCl3, d, ppm): 7.80 (s, 3H), 7.53 (d,
2H), 7.41–7.32 (m, 2H), 7.30–7.11 (m, 5H). 13C NMR
(75 MHz, CDCl3, d, ppm): 153.49, 135.71, 133.49, 133.11,
131.86, 131.45, 130.64, 130.04, 128.16, 126.98, 126.52,
123.56, 122.74.
(d, 4H), 7.60 (d, 4H).
3,4,5-tri(4-bromophenyl)-4H-1,2,4-triazole
(4):
3
(0.86 g, 2.0 mmol) and p-bromoaniline (0.34 g, 2.0 mmol)
were dissolved in 15 ml of N,N-dimethylaniline and
stirred at 1358C for 12 h under a flow of nitrogen. After
being cooled to room temperature, HCl (30 ml, 2 M) was
added with agitation for 30 min, and then the mixture
was filtered through a Buchner funnel. The subjection of
the crude product to silica gel chromatography afforded 4
(0.63 g, 58.6%) as a white solid. 1H NMR (300 MHz,
CDCl3, d, ppm): 7.60 (d, 2H), 7.49 (d, 4H), 7.25 (d, 4H),
7.02 (d, 2H). 13C NMR (75 MHz, CDCl3, d, ppm): 153.94,
133.68, 133.61, 131.95, 130.19, 129.16, 125.30, 124.71,
124.25.
Synthesis of 3,4,5-tri(3-pyrid-3-yl-phenyl)-4H-1,2,4-tria-
zole (TPyTAZm): TPyTAZm was synthesized in a similar
1
manner to TPyTAZp. Yield: 87.1%. H NMR (300 MHz,
CDCl3, d, ppm): 8.64–8.55 (m, 6H), 7.82–7.78 (d, 1H),
7.76 (t, 1H), 7.74 (t, 1H), 7.73–7.70 (m, 2H), 7.69 (s, 1H),
7.67 (s, 1H), 7.65–7.62 (m, 1H), 7.60 (t, 1H), 7.56 (t, 1H),
7.53 (m, 1H), 7.49 (s, 1H), 7.46 (s, 1H), 7.45–7.43 (m, 1H),
7.37–7.30 (m, 4H). 13C NMR (75 MHz, CDCl3, d, ppm):
154.44, 149.58, 148.91, 148.09, 148.03, 140.43, 138.15,
136.08, 135.54, 134.48, 134.38, 134.27, 131.11, 129.45,
128.60, 128.54, 128.36, 127.59, 127.50, 127.35, 126.49,
123.81, 123.62. Calcd. C35H24N6 528.6, APCI+-MS (m/z):
529.2 (M+).
3,4,5-tri(4-pyrid-3-yl-phenyl)-4H-1,2,4-triazole
(TPy-
TAZp): 4 (1.10 g, 2.1 mmol), pyrid-3-yl boronic acid
(1.08 g, 8.64 mmol), Pd(PPh3)4 (84 mg, 0.073 mmol), and
2 M aqueous potassium carbonate (20 ml) in toluene
(50 ml) and ethanol (20 ml) was stirred at 958C for 24 h
under argon atmosphere. After being cooled to room
temperature, water was added to quench the reaction,
and the mixture was extracted with dichloromethane. The
combined organic phase was washed with saturated brine
and dried over MgSO4. The subjection of the crude prod-
uct to silica gel chromatography afforded TPyTAZp
(0.94 g, 94.7%) as a white solid. 1H NMR (300 MHz,
CDCl3, d, ppm): 9.10–8.47 (m, 5H), 7.96 (d, 1H), 7.88 (d,
2H), 7.73 (d, 2H), 7.62–7.52 (m, 9H), 7.48–7.30 (m, 5H).
13C NMR (75 MHz, CDCl3, d, ppm): 154.44, 149.43,
148.96, 148.08, 139.41, 139.11, 134.93, 134.35, 134.23,
129.41, 128.70, 128.55, 127.19, 126.48, 123.84. Calcd.
C35H24N6 528.6, APCI+-MS (m/z): 529.3 (M+).
3,4,5-tri(3-(3-pyrid-3-yl-phenyl)-phenyl)-4H-1,2,4-tria-
zole (TPyBnTAZm): TPyBnTAZm was synthesized in
1
a similar manner of TPyTAZp. Yield: 79.7%. H NMR
(300 MHz, CDCl3, d, ppm): 8.78 (s, 2H), 8.75 (s, 1H),
8.63–8.57 (m, 3H), 7.88 (d, 2H), 7.85–7.77 (m, 2H), 7.68
(s, 4H), 7.67–7.64 (m, 3H), 7.54–7.45 (m, 11H), 7.43–7.38
(m, 4H), 7.35–7.28 (m, 4H). 13C NMR (75 MHz, CDCl3,
d, ppm): 154.57, 148.47, 148.14, 143.12, 141.05, 140.86,
139.77, 138.69, 138.45, 136.44, 136.08, 134.58, 134.47,
130.72, 129.82, 129.60, 129.31, 128.63, 128.54, 128.20,
127.59, 127.43, 127.05, 126.82, 126.76, 126.66, 126.50,
125.85, 123.58. Calcd. C53H36N6 756.9, APCI+-MS (m/z):
757.4 (M+).
4.3 Device Fabrication and Characterization
3,4,5-tri(4-(3-pyrid-3-yl-phenyl)-phenyl)-4H-1,2,4-tria-
zole (TPyBnTAZp): TPyBnTAZp was synthesized in
a similar manner to TPyTAZp. Yield: 76.5%. H NMR
Glass substrates pre-coated with a 170 nm, thin layer of
indium tin oxide (ITO) with a sheet resistance of 10 W
per square meter were thoroughly cleaned in an ultrason-
ic bath of acetone, isopropyl alcohol, detergent, deionized
water, isopropyl alcohol, and treated with O2 plasma, for
20 min in sequence. Organic layers were deposited onto
the ITO-coated substrates by high-vacuum (<5ꢀ10ꢀ4 Pa)
thermal evaporation. A 5 nm, thin layer of dipyrazino
(2,3-f :2’,3’-h)quinoxaline-2,3,6,7,10,11-hexacarbonitrile
(HATCN) was deposited to improve the hole injection
from the anode. Then, a 40 nm, thin hole-transport
layer of di-(4-(N,N-ditolyl-amino)-phenyl)cyclohexane
1
(300 MHz, CDCl3, d, ppm): 8.88 (s, 3H), 8.62 (s, 3H),
7.98–7.92 (m, 3H), 7.84 (s, 1H), 7.80 (s, 1H), 7.76 (s, 3H),
7.72–7.68 (m, 1H), 7.62–7.60 (m, 12H), 7.57 (s, 3H), 7.55
(s, 1H), 7.45–7.38 (m, 4H), 7.35 (s, 1H). 13C NMR
(75 MHz, CDCl3, d, ppm): 154.60, 148.84, 148.67, 148.30,
142.10, 141.90, 140.88, 140.00, 138.81, 138.55, 136.44,
136.27, 134.59, 134.52, 129.89, 129.68, 129.29, 128.72,
128.37, 127.26, 127.10, 126.87, 126.83, 126.66, 126.08,
126.02, 125.95, 123.68, 123.63. Calcd. C53H36N6 756.9,
APCI+-MS (m/z): 757.4 (M+).
1,2-bis(3-bromophenyl)hydrazine (6): 6 was synthesized
in a similar manner to 2. Yield: 75.3%. 1H NMR
(300 MHz, DMSO-d6, d, ppm): 10.71 (s, 2H), 8.09 (s, 2H),
7.92 (d, 2H), 7.82 (d, 2H), 7.51 (t, 2H).
1,2-bi(chloro(3-bromophenyl)methylene)hydrazine (7):
7 was synthesized in a similar manner to 3. Yield: 98.3%.
(TAPC)
was
deposited.
Next,
8 wt%
tris(2-
phenylpyridine)iridium(III) (Ir(PPy)3) was co-deposited
with 4,4’-bis(carbazol-9-yl)biphenyl (CBP) to form
a 10 nm, thin EML. Finally, a 40 nm, thin ETL of TAZ,
TPyTAZp, TPyTAZm, TPyBnTAZp or TPyBnTAZm
was deposited to transport electrons, block holes and to
Isr. J. Chem. 2014, 54, 971 – 978
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www.ijc.wiley-vch.de
977

Full Paper
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Acknowledgements
The authors greatly appreciate the financial support from
the National Natural Science Foundation of China
(91233116 and 51073057), the Ministry of Science and
Technology (2011AA03A110), the Ministry of Education
(NCET-11-0159), the Guangdong Natural Science Foun-
dation (S2012030006232), and the Fundamental Research
Funds for the Central Universities (2013ZG0007).
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Received: March 15, 2014
Accepted: April 2, 2014
Published online: June 17, 2014
Isr. J. Chem. 2014, 54, 971 – 978
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.ijc.wiley-vch.de
978