Highly efficient blue electrophosphorescent devices with a new series of host materials: Polyphenylene-dendronized oxadiazole derivatives

Article abstract of DOI:10.1039/b705515c

A new series of oxadiazole derivatives TPO, PPO, MTPO, MPPO, BTPO, and BPPO have been designed and synthesized in high yields by a convenient synthetic procedure. A single crystal structure of TPO exhibited that this polyphenylene-dendronized material has a sterically crowded structure which efficiently prevents π-π stacking and endows it with good thermal stability. The photophysical properties of the materials show strong emission between 376 and 390 nm in the films. In addition, good reversible reductive waves in the cyclic voltammograms implied that these materials might have good electron transport properties. Blue electrophosphorescent devices fabricated using these materials as the host materials and iridium(iii)bis[4,6-di- fluorophenyl-pyridinato-N,C^2′]picolinate as the blue phosphorescent dopant emitter show very high efficiencies. A maximum luminance of 4484 cd m^-2 and an external quantum efficiency of 6.20% were achieved under ambient conditions. The Royal Society of Chemistry 2007.

Full text of DOI:10.1039/b705515c

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www.rsc.org/materials | Journal of Materials Chemistry
Highly efficient blue electrophosphorescent devices with a new series of host
materials: polyphenylene-dendronized oxadiazole derivatives
Shiyan Chen, Xinjun Xu, Yunqi Liu,* Wenfeng Qiu, Gui Yu, Xiaobo Sun, Hengjun Zhang, Ting Qi, Kun Lu,
Xike Gao, Ying Liu and Daoben Zhu*
Received 11th April 2007, Accepted 13th July 2007
First published as an Advance Article on the web 27th July 2007
DOI: 10.1039/b705515c
A new series of oxadiazole derivatives TPO, PPO, MTPO, MPPO, BTPO, and BPPO have been
designed and synthesized in high yields by a convenient synthetic procedure. A single crystal
structure of TPO exhibited that this polyphenylene-dendronized material has a sterically crowded
structure which efficiently prevents p–p stacking and endows it with good thermal stability. The
photophysical properties of the materials show strong emission between 376 and 390 nm in the
films. In addition, good reversible reductive waves in the cyclic voltammograms implied that these
materials might have good electron transport properties. Blue electrophosphorescent devices
fabricated using these materials as the host materials and iridium(III)bis[4,6-di-fluorophenyl-
pyridinato-N,C29]picolinate as the blue phosphorescent dopant emitter show very high efficiencies.
A maximum luminance of 4484 cd m22 and an external quantum efficiency of 6.20% were
achieved under ambient conditions.
the morphologically stable and uniform amorphous thin films
Introduction
with typical processing techniques, we have to consider
Organic light-emitting diodes (OLEDs) have been widely
increasing the steric bulk of the molecules. Therefore, an
recognized as a technology for flat panel display products
exquisite and flexible molecular design is highly desired to
and for solid state white-light sources in potential lighting
satisfy these conflicting requirements for the host materials of
blue electrophosphorescent OLEDs.6,25
industries. Electrophosphorescent OLEDs have the potential
of offering 100% internal quantum efficiencies due to their
Most of the current highly fluorescent materials used as
ability to use both the singlet and triplet excitons for emission,
emissive materials in OLEDs are good p-type (hole-transport-
so they have rapidly developed in recent years. The only way to
ing) materials. They, however, have generally small electron
realize efficient electrophosphorescent OLEDs is to choose
suitable host materials which allow for exothermic energy
transfer between a host excited state and a lower energy guest
unoccupied orbital, and in turn results in an energetically
favorable excited state transition between molecules.1–3 To
date, a few works about efficient green or red electrophos-
phorescent OLEDs with high electroluminescence (EL)
efficiencies have been reported.3–20 As we know well, it is
easy to find suitable host materials to favor the triplet level
of the green or red electrophosphorescent materials which
have relatively low triplet levels. But, it is very difficult to
find high energy host materials for blue electrophosphorescent
OLEDs due to their high triplet levels. The blue-light-emitting
phosphorescent EL performance is far from perfect.21–24
However, designing effective host materials for blue electro-
phosphorescent OLEDs is a great challenge. The reason is that
the host materials must have rather large energy gaps. Usually,
enlargement of the energy gap can be realized by decreasing
the extent of conjugation of the molecule, that is to say, the
molecule size should be constrained. Nevertheless, to obtain
affinities and poor electron-transporting properties. Compared
with organic hole-transporting materials, electron-transport-
ing materials with low electron injection barriers and high
electron mobilities for improving the performance and the
durability of the devices are also required. The use of electron-
deficient heterocyclic small molecules or polymers as a
separate electron-transporting layer or as a blend component
in conjunction with the emissive material has been proved
very useful in improving balanced injection and recombination
in OLEDs.26–29 In this work, we report the synthesis of a
new series of host materials with large band gaps adopting
an oxadiazole moiety, which has been reported to have good
electron-transporting properties.30,31 In addition, we introduce
polyphenyl rings to increase the thermal properties and
amorphous properties of the materials, since sterically
crowded polyphenylphenyl (tetraphenylphenyl and pentaphe-
nylphenyl) moieties have been reported as effective bulky
groups used in light-emitting materials.32–34 It is a successful
example if the dendrons can inhibit the formation of molecular
aggregation or p–p stacking in the solid state. We investigated
the photophysical, thermal, electrochemical properties of the
compounds. The blue electrophosphorescent OLEDs employ-
ing these compounds as the host materials and one of the best
known triplet-state blue-light emitters, iridium(III)bis[4,6-di-
fluorophenyl-pyridinato-N,C29]picolinate (FIrpic), as the
dopant were described as well.
Beijing National Laboratory for Molecular Sciences, Institute of
Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R. China.
E-mail: liuyq@mail.iccas.ac.cn; Fax: +86-10-62559373;
Tel: +86-10-62613253.
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Scheme 1 The synthetic route of the target compounds. Reagents and conditions: (i) (a) trimethylsilylacetylene, Pd(PPh3)2Cl2/CuI, THF/Et3N, rt,
12 h, (b) K2CO3, CH2Cl2/CH3OH, rt, 1 h. (ii) Phenylacetylene, Pd(PPh3)2Cl2/CuI, THF/Et3N, 50 uC, 18 h. (iii) Tetraphenylcyclopentadienone,
m-xylene, reflux, 12 h (TPO, MTPO, BTPO); tetraphenylcyclopentadienone, biphenyl ether, reflux, 12 h (PPO, MPPO, BPPO).
results demonstrate that all compounds have excellent
Results and discussion
amorphous glass state stability. The ability to form morpho-
Synthesis and thermal properties
logically stable and uniform amorphous films and good
thermal stability should be beneficial to the device prepared
The new series of materials were designed and synthesized via
by a vacuum-deposition method.
the procedure described in Scheme 1. First, Sonogashira
coupling of the oxadiazole derivatives 1a–c with trimethyl-
Crystal structure and optical properties
silylacetylene and phenylacetylene gave alkylenes 2a–c and
3a–c, respectively. Then, the final products were synthesized by
the Diels–Alder reaction of 2a–c or 3a–c with tetraphenyl-
cyclopentadienone with an overall yield of 80%. The benefits
of a convenient synthetic procedure and relatively high yields
are very promising in OLED materials. In addition, due to
their excellent solubility of the target compounds in common
organic solvents such as dichloromethane (CH2Cl2), trichloro-
methane (CHCl3), toluene and tetrahydrofuran (THF), the
separation and purification processes were very easy. Thus,
characterization such as MALDI-TOF mass spectroscopy,
elemental analysis, and NMR spectroscopy, especially 13C
NMR spectroscopy is easily accomplished.
Single crystals of TPO suitable for X-ray crystallography
were grown by layer-diffusion of hexane to their CH2Cl2
solution, and the X-ray single crystal determination provides
an insight into structure–property relationships. As shown in
Fig. 1(a), the phenyl rings a, b and the oxadiazole ring are
almost coplanar with dihedral angles of only 13.4u and 3.0u,
respectively. However, the central phenyl ring c and the five
phenyl rings attached to it are not coplanar with dihedral
angles of 48.6u, 50.2u, 64.9u, 66.4u and 67.8u, respectively. The
Table 1 The data of the optical and the thermal properties of the
compounds
The thermal properties of the compounds were determined
by differential scanning calorimetry (DSC) and thermogravi-
metric analysis (TGA) measurements and the data are
summarized in Table 1. We observed a glass transition
temperature (Tg) of 121 uC and 191 uC for TPO and BTPO,
respectively. Other compounds showed no obvious Tg. In
addition, it should be noticed that there is no indication of
melting in the DSC curve for BPPO. Their thermal decom-
position temperatures (Tds) are in the range 427–489 uC. The
labsmax/nm
lemmax/nm
Compounds Tg/uC Tm/uC Td/uC Solution Film Solution Film WFL
PPO
TPO
MPPO
MTPO
BPPO
BTPO
— 349
121 215
452 301
465 305
427 308
447 312
489 313
476 317
301 367
306 376
309 370
313 376
313 375
319 381
376 0.91
380 0.77
383 0.85
384 0.88
379 0.77
390 0.87
—
—
—
353
271
—
191 337
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the p–p* absorption of the oxadiazole skeleton. The remark-
able increase in absorption intensity at shorter wavelength
reflects the corresponding increase in the number of phenyl
units. In addition, the absorption bands of PPO, MPPO
and BPPO were observed to be blue-shifted by about 4 nm
compared to their corresponding tetraphenyl-substituted
oxadiazole derivatives TPO, MTPO and BTPO, the band
shifts can be explained by the decrease in p-conjugation which
comes from the larger torsion angles of the phenyl ring when
the polyphenyl group was increased from four to five. This
phenomenon was also reported in the literature.32 Compared
with those of PPO and TPO, the red shift of the absorption
bands of MPPO and MTPO was observed due to introducing
the methoxy group to the phenyl, which can be ascribed to the
electron-rich nature of the methoxy group. Additionally, a
slight red shift of the absorption bands occurs for compounds
BPPO and BTPO, which can be attributed to the increase in
the p-conjugation scale due to the incorporation of another
polyphenylphenyl group to the oxadiazole group.
We also examined the fluorescence spectra of the com-
pounds in CHCl3 (Fig. 3) and the solid films. The data are
listed in Table 1. They all show the most emission in the
ultraviolet (UV) region (below 400 nm), and the fluorescence
quantum yields in CHCl3 solution were measured to be more
than 77% for all compounds using quinine in 1.0 M H2SO4
(W313 = 0.48) as a calibration standard.35 The values are very
unusual for organic UV emitters. Furthermore, in the solid
state, photoluminescence (PL) is also very high because the
arrangement of the sterically crowded tetra- or pentaphenyl-
phenyl polycycle provides satisfying effects on inhibiting mole-
cular aggregation. Photoluminescence spectra of the materials
in the films are only red-shifted by 4–13 nm compared to those
in solution, indicating that the aggregation is suppressed.
Fig. 1 Crystal structure (a) and the molecular packing pattern (b) of
TPO. Hydrogen atoms are omitted for clarity.
steric interaction between the phenyl rings is so clearly evident
that the molecules in the crystal lattice pack in a relatively
loose way. Furthermore, from the molecular packing of TPO
[Fig. 1(b)], we observed that the head (the phenyl rings a) of
the molecule is oriented to the central phenyl ring c of the
neighboring molecule with a relatively large dihedral angle
59.9u. It is evident that there is no p–p stacking in the solid
state in the present TPO model compound. The result is
consistent with the discussion in the optical properties.
The electronic absorption spectra of the compounds were
examined in CHCl3 (Fig. 2) and in the solid films. The data are
listed in Table 1. In Fig. 2, two absorption bands are observed
for all of the compounds. One band at about lmax = 240 nm
can be assigned to the p–p* transition of the phenyl ring,
and another band at about lmax = 320 nm can be assigned to
Electrochemical properties
Fig. 4 shows the reduction cyclic voltammograms of the
compounds and the data are listed in Table 2. They all
exhibit good reversible cathodic reduction couples at about
22.0 V (E1/2), and compounds BPPO and BTPO show extra
Fig. 2 Normalized electronic absorption (at the maximum of the
Fig. 3 Normalized fluorescence emission spectra of the compounds in
longest wavelength) of the compounds in CHCl3.
CHCl3.
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to 22.33 eV. The observed reversible reductive process
suggests that these compounds have a potential for electron-
transporting properties. The value is higher in some sort
than that of tris(8-hydroxyquinolinolato)aluminium (Alq3)
(22.85 eV)37 (the representative electron-transporting mate-
rial). HOMO–LUMO gaps (Eg) of these compounds were
calculated by the equation: Eg = 1240/lonset eV, where the
lonset denotes the onset of UV absorption, and the Eg values
are over 3.5 eV (Table 2). Based on the LUMO energy levels
and the band gaps, the HOMO levels of the compounds are
between 25.73 and 25.94 eV. The values are lower than that
of 4,49-bis{4-[N-(1-naphthyl)-N-phenylamino]phenyl}biphenyl
(NPB) (25.46 eV) (the representative hole-transporting
material).37 It is very favorable for our device fabrication in
the subsequent section. Furthermore, these materials have
relatively high band gaps, which is very promising in the host
materials of blue, green or red organic electrophosphorescent
devices.
Fig. 4 Cyclic voltammograms of all compounds (1 mM) in 0.1 M
Bu4NPF6-THF, scan rate 50 mV s21
.
Electroluminescent properties
We explored the performance characteristics of the devices
using these compounds as the host and FIrpic as the dopant.
The data are listed in Table 3. As shown in Fig. 5a, the PL
spectra of the thin films (host:FIrpic) show an emission at
480 nm with a shoulder emission at 500 nm which is consistent
with the EL spectra (Fig. 5b) of the devices. From the EL
spectra, no host emission at about 380 nm was observed which
demonstrated complete energy transfer from the host to the
dopant molecules. The emission from the dopant is similar
to the previous reports.20,21 In addition, an undesired weak
emission of the hole-transporting material NPB was observed
at about 420 nm, which indicates that the devices need to be
further optimized. The current–voltage–luminance (I–V–L)
characteristics and the external quantum efficiency versus
current density characteristics of the devices are shown in
Fig. 6a and 6b, respectively. Obviously, the best performance
was achieved for the device using BTPO as the host material
with a maximum luminance of 4484 cd m22 and an external
quantum efficiency of 6.20%. The result can be attributed to
the lower energy barrier of electron injection compared with
those of other compounds (the lowest LUMO level of BTPO
in these compounds is show in Table 2). Further optimization
of the devices such as varying the thickness of the layers,
adopting a hole-blocking layer material other than 2,9-
dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), substitut-
ing FIrpic with other blue emitters, or modifying the electrode
to enhance the electron/hole-injection to realize more efficient
energy transfer from host to dopant, eliminating other
Table 2 Electrochemical data of the compounds. E1/2 = 1/2(Epa
Epc), where Epa and Epc are the anodic and cathodic peak potentials.
Eredonset= onset reduction potential
+
1
2
onset
E1/2
V
/
E1/2
V
/
Ered
V
/
EHOMO
eV
/
ELUMO
eV
/
Eg/
eV
Compounds
PPO
TPO
MPPO
MTPO
BPPO
BTPO
22.07
21.96
22.14
22.04
22.07 22.39 22.01
21.94 22.23 21.88
21.98
21.90
22.08
21.96
25.92
25.94
25.73
25.83
25.76
25.83
22.23
22.31
22.13
22.25
22.20
22.33
3.69
3.63
3.60
3.58
3.56
3.50
reduction couples at 22.39 and 22.23 V, respectively. This
result suggests that these materials may have good charge
transport properties. We observed that the reduction value of
PPO, MPPO, and BPPO shifts more negative than that of
TPO, MTPO, and BTPO. This phenomenon may come from
the better symmetry and stability of PPO, MPPO, and BPPO.
The difference in the effective conjugation as revealed in their
absorption spectra might also cause the difference in their
reduction potentials. Therefore, the series of pentaphenyl-
phenyl oxadiazole derivatives are more difficult to be reduced
than the series of tetraphenylphenyl ones. The energy levels of
all compounds were calculated using the ferrocene (EFOC) value
of 24.8 eV36 as the standard, while the EFOC was calibrated to
be 0.59 V versus the Ag/AgCl electrode in a ferrocene solution.
So the electron affinity (EA) (LUMO level) derived from the
onset reduction potentials (EA = Eredonset + 4.21 eV) are 22.20
Table 3 EL data of the devices. Lmax = maximum luminance, gc.max= maximum current efficiency, gext,
efficiency.
= maximum external quantum
max
Compounds
Turn-on voltage/V
Peak position/nm
Lmax/cd m22, voltage/V
g
c.max/cd A21
gext,max (%)
CIE x, y
PPO
TPO
MPPO
MTPO
BPPO
BTPO
4.82
4.97
4.97
4.83
4.68
4.19
481
481
481
483
481
484
5113, 15.0
3749, 17.0
4136, 16.0
5999, 16.0
2846, 15.5
4484, 14.0
5.02
4.00
7.71
10.8
6.72
11.9
2.61
2.08
4.00
5.59
3.49
6.20
0.18, 0.37
0.18, 0.37
0.20, 0.38
0.19, 0.42
0.18, 0.34
0.19, 0.44
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Fig. 6 Luminance versus applied electric field characteristics of the
devices, the inset shows current density versus applied electric field
characteristics (a) and external quantum efficiency versus current
density characteristics of the devices (b).
Fig. 5 The PL spectra of the thin films (host:FIrpic) and the EL
spectra of the devices.
emission thus increasing the device performance and realizing
pure blue emission will be further pursued in future studies.
Furthermore, these materials would be good UV emitters for
their high band gaps and high fluorescence quantum yields,
and the work will be reported in the future.
materials with a high band gap for blue, green or red
electrophosphorescent devices.
Experimental
Materials and instruments
Conclusions
Trimethylsilylacetylene, phenylacetylene and tetraphenyl-
cyclopentadienone were purchased from Acros, and used as
received. THF and triethylamine (TEA) was distilled from
sodium–benzophenone under a nitrogen atmosphere. N,N9-
dicarbazolyl-4,49-biphenyl (CBP) and BCP were purchased
from Acros. Alq3,38 NPB,39 FIrpic40 were prepared by
published methods. NPB, CBP, Alq3 and FIrpic were purified
by sublimation prior to use. All reactions were carried out
under a nitrogen atmosphere.
1H NMR and 13C NMR spectra were obtained on a Bruker
DMX 300 or 400 NMR Spectrometer. 1H and 13C chemical
shifts are reported in ppm downfield from a tetramethylsilane
(TMS) reference, using the residual protonated solvent
resonance as an internal standard. The UV-Vis and fluores-
cence spectra were obtained on a Hitachi U-3010 and Hitachi
F-4500 spectrometer, respectively. MS (MALDI-TOF-MS)
In summary, a new series of polyphenylene-dendronized
oxadiazole derivatives has been synthesized via an easy
synthetic procedure with high yields. Analysis of the single
crystal structure of the model compound shows that it is
very efficient to prevent the p–p aggregation by introducing
the polyphenylphenyl to the materials. In addition, the
materials have high thermal properties and relatively high
band gaps of over 3.5 eV. They all show the most emissions in
the UV region and have high fluorescence quantum yields.
Blue electrophosphorescent devices fabricated using these
materials as host materials and FIrpic as the dopant show
very high efficiencies. The best device performance was
achieved with a maximum luminance of 4484 cd m22 and an
external quantum efficiency of 6.20% under ambient condi-
tions. We set up a new alternative approach to the design of
stable and efficient ultraviolet-emitting materials and host
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were performed on a Bruker BIFLEX III Mass Spectrometer.
Elemental analyses were carried out on a Carlo-Erba 1160
elemental analyzer. TGA was carried out using a Perkin-Elmer
thermogravimeter (Model TGA7) under a dry nitrogen gas
flow at a heating rate of 10 uC min21. Tg and melting
temperatures (Tm) were determined by DSC at a heating rate
of 10 uC min21 using a Perkin-Elmer differential scanning
calorimeter (DSC7).
atmosphere. The solvents were evaporated in vacuo and the
resulting solid purified by column chromatography on silica
gel using a mixture of CH2Cl2–petroleum ether (1 : 2) as the
eluent. The product, 567 mg, was obtained as a white solid in
94% yield. MS (MALDI-TOF): m/z 603.1 (M+); 1H NMR
(CDCl3, 300 MHz): d = 8.13, 8.11 (dd, J1 = 7.9 Hz, J2 = 1.5 Hz,
2H), 7.95 (d, J = 8.5 Hz, 2H), 7.61 (s, 1H), 7.50–7.54 (m, 3H),
7.34 (d, J = 8.2 Hz, 2H), 7.18 (brs, 5H), 6.78–6.97 (m, 15H).
13C NMR (CDCl3, 75 MHz): d = 164.5, 164.4, 145.4, 142.0,
141.4, 141.0, 140.0, 139.7, 139.5, 139.2, 131.6, 131.4, 131.0,
130.6, 129.9, 129.0, 127.6, 127.1, 126.9, 126.8, 126.7, 126.4,
126.2, 125.9, 125.7, 125.4, 123.9, 121.7. Elemental analysis (%)
calcd for: C44H30N2O C 87.68, H 5.02, N 4.65; found: C 87.28,
H 5.08, N 4.77.
2-(4-Iodophenyl)-5-phenyl-[1,3,4]oxadiazole (1a). Yield: 80%.
1
EI: m/z 348 (M+); H NMR (CDCl3, 300 MHz): d = 8.14 (d,
J = 7.0 Hz, 2H), 7.91 (d, J = 8.6 Hz), 7.86 (d, J = 8.6 Hz), 7.55
(d, J = 6.5 Hz, 3H).
2-(4-Iodophenyl)-5-(4-methoxyphenyl)-[1,3,4]oxadiazole (1b).
1
Yield: 85%. EI: m/z 378 (M+); H NMR (CDCl3, 300 MHz):
2-Phenyl-5-[4-(2,3,4,5,6-pentaphenyl)phenyl]phenyl-
[1,3,4]oxadiazole (PPO). Compound 3a (322 mg, 1.0 mmol),
tetraphenylcyclopentadienone (422 mg, 1.1 mmol), and 10 mL
of diphenyl ether were deoxygenated and then heated to reflux
overnight under nitrogen atmosphere. After being cooled to
room temperature, the diphenyl ether was removed under
reduced pressure and the residue was purified by column
chromatography using CH2Cl2 as the eluent affording a pale
yellow solid, which was further purified by recrystallization
from methanol–CH2Cl2 to give a white powder (624 mg, 92%).
MS (MALDI-TOF): m/z 679.6 (M++1); 1H NMR (CDCl3,
300 MHz): d = 8.06, 8.04 (dd, J1 = 1.8 Hz, J2 = 7.8 Hz, 2H),
7.66 (d, J = 8.3 Hz, 2H), 7.48 (m,3H), 7.02 (d, J = 8.2 Hz, 2H),
6.85 (brs, 25H). 13C NMR (CDCl3, 100 MHz): d = 164.5,
164.1, 144.7, 140.8, 140.4, 140.2, 140.1, 140.0, 139.0, 131.9,
131.4, 131.2, 128.8, 126.7, 126.6, 126.5, 125.4, 125.2, 123.8,
120.5. Elemental analysis (%) calcd for: C50H34N2O C 88.47, H
5.05, N 4.13; found: C 88.37, H 5.09, N 4.34.
d = 8.07 (d, J = 9.0 Hz, 2H), 7.88 (d, J = 8.6 Hz, 2H), 7.84 (d,
J = 7.0 Hz, 2H), 7.04 (d, J = 9.0 Hz, 2H), 3.88 (s, 3H).
2,5-Bis-(4-iodophenyl)-[1,3,4]oxadiazole (1c). Yield: 78%. EI:
1
m/z 474 (M+); H NMR (CDCl3, 300 MHz): d = 7.91 (d, J =
8.5 Hz, 4H), 7.86 (d, J = 8.6 Hz, 4H).
2-Phenyl-5-(4-ethynylphenyl)-[1,3,4]oxadiazole (2a).
Compound 1a (696 mg, 2.0 mmol) and trimethylsilylacetylene
(0.3 mL, 2.2 mmol) in a solution of THF (20 mL) and TEA
(20 mL) were stirred at room temperature overnight under a
nitrogen atmosphere with the catalyst CuI (38 mg, 0.2 mmol)
and Pd(PPh3)2Cl2 (70 mg, 0.1 mmol). The reaction mixture
was poured on a silica pad and eluted with CH2Cl2. The
solvent was removed, and the residue was dissolved in a
mixture of 40 mL CH2Cl2–CH3OH (1 : 1) and K2CO3 (1.38 g,
20 mmol). The reaction mixture was stirred for one hour
at room temperature and then 50 mL of ethyl acetate
was added. The mixture was washed with water, brine
and dried with anhydrous magnesium sulfate. After removal
of the solvent, a pale yellow solid 443 mg was obtained in 90%
yield. EI: m/z 246 (M+); 1H NMR (CDCl3, 300 MHz): d = 8.13
(t, 4H), 7.65 (d, J = 8.2 Hz, 2H), 7.51–7.56 (m, 3H), 3.26 (s,
1H).
2-(4-Methoxy)phenyl-5-(4-ethynyl)phenyl-[1,3,4]oxadiazole
(2b). The procedure is analogous to that described for 2a
1
(yield: 90%). EI: m/z 276 (M+); H NMR (CDCl3, 300 MHz):
d = 8.06–8.10 (m, 4H), 7.64 (d, J = 8.1 Hz, 2H), 7.03 (d, J =
8.7 Hz, 2H), 3.90 (s, 3H), 3.25 (s, 1H).
2-(4-Methoxy)phenyl-5-(4-phenylethynyl)phenyl-[1,3,4]oxa-
diazole (3b). The procedure is analogous to that described
for 3a (yield: 97%). EI: m/z 352 (M+); 1H NMR (CDCl3,
300 MHz): d = 8.11 (t, J = 8.3 Hz, 4H), 7.68 (d, J = 8.3 Hz,
2H), 7.57 (q, 2H), 7.38 (t, 3H), 7.05 (d, J = 8.8 Hz, 2H), 3.90
(s, 3H).
2-Phenyl-5-(4-phenylethynyl)phenyl-[1,3,4]oxadiazole (3a). A
flame-dried flask with a magnetic stirrer was loaded with 1a
(696 mg, 2 mmol), Pd(PPh3)2Cl2 (70 mg, 0.1 mmol), and CuI
(38 mg, 0.2 mmol). Under a nitrogen atmosphere, dry THF
(20 mL) and TEA (20 mL) were added, and the mixture was
stirred for 20 min, yielding a yellow transparent solution.
Phenylacetylene (0.24 mL, 2.2 mmol) was added via a syringe
and the reaction mixture was stirred vigorously for 18 h at
50 uC. After removal of the solvent, the residue was purified by
flash column chromatography using CH2Cl2 as the eluent
affording a pale yellow solid (618 mg, 96%). EI: m/z 322 (M+);
1H NMR (CDCl3, 300 MHz): d = 8.13–8.17 (m, 4H), 7.70 (d,
J = 8.4 Hz, 2H), 7.55–7.60 (m, 5H), 7.38 (t, 3H).
2-(4-Methoxy)phenyl-5-[4-(2,3,4,5-tetraphenyl)phenyl]phenyl-
[1,3,4]oxadiazole (MTPO). The procedure is analogous to that
described for TPO (yield: 95%). MS (MALDI-TOF): m/z
633.2 (M+);1H NMR (CDCl3, 300 MHz): d = 8.04 (d, J =
8.7 Hz, 2H), 7.92 (d, J = 8.1 Hz, 2H), 7.60 (s, 1H), 7.32 (d, J =
8.2 Hz, 2H), 7.17 (brs, 5H), 7.00 (d, J = 8.7 Hz, 2H), 6.78–6.94
(m, 15H) 3.86 (s, 3H). 13C NMR (CDCl3, 75 MHz): d = 164.4,
164.1, 162.3, 145.2, 142.0, 141.4, 141.0, 140.0, 139.7, 139.5,
139.3, 131.4, 131.1, 130.6, 129.9, 128.6, 127.6, 127.1, 127.0,
126.7, 126.4, 126.1, 125.9, 125.7, 125.4, 121.8, 116.4, 114.4,
55.4. Elemental analysis (%) calcd for: C45H32N2O2 C 85.42, H
5.10, N 4.43; found: C 85.44, H 5.22, N 4.59.
2-Phenyl-5-[4-(2,3,4,5-tetraphenyl)phenyl]phenyl-[1,3,4]oxa-
diazole (TPO). A mixture of compound 2a (246 mg, 1.0 mmol)
and tetraphenylcyclopentadienone (422 mg, 1.1 mmol) in
m-xylene (10 mL) was stirred at 150 uC for 12 h under nitrogen
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2-(4-Methoxy)phenyl-5-[4-(2,3,4,5,6-pentaphenyl)phenyl]phe-
nyl-[1,3,4]oxadiazole (MPPO). The procedure is analogous to
that described for PPO (yield: 95%). MS (MALDI-TOF): m/z
709.0 (M+ + 1); 1H NMR (CDCl3, 300 MHz): d = 7.97 (d, J =
8.6 Hz, 2H), 7.64 (d, J = 8.0 Hz, 4H), 6.99 (t, J = 8.5 Hz, 4H),
6.85 (brs, 25H), 3.86 (s, 3H). 13C NMR (CDCl3, 100 MHz):
d = 164.0, 162.1, 144.5, 140.7, 140.4, 140.2, 140.1, 140.0, 139.0,
131.9, 131.2, 128.4, 126.7, 126.5, 125.4, 125.2, 125.0,
120.6, 116.4, 114.3, 55.3. Elemental analysis (%) calcd for:
C51H36N2O2 C 86.41, H 5.12, N 3.95; found: C 86.74, H 5.24,
N 3.96.
LEDs fabrication and measurements
The substrates were indium tin oxide (ITO) coated glass with
a sheet resistance of 20 V square21. The ITO-coated glass
substrates were etched, patterned, and washed with detergent,
deionized water, acetone, and ethanol in turn. The multi-
layered devices of configuration ITO/NPB (20 nm)/CBP
(20 nm)/Host:FIrpic (3 y 6%) (40 nm)/BCP (20 nm)/Alq3
(5 nm)/LiF (1 nm)/Al (100 nm) were fabricated, where NPB,
CBP, BCP, and Alq3 serve as the hole-transporting layer, the
second hole-transporting layer, the hole-blocking layer, and
the electron transport layer, respectively, LiF/Al acts as the
cathode. The materials used as the emission and electron-
transporting layers and all organic layers were successively
deposited onto the ITO/glass substrates at a pressure of
3 6 1024 Pa. The active area of the devices was about
5 mm2. All device testing was carried out under an ambient
atmosphere at room temperature. EL spectra of LEDs
2,5-Bis-(4-ethynyl)phenyl-[1,3,4]oxadiazole (2c). The proce-
dure is analogous to that described for 2a (yield: 94%). EI: m/z
270 (M+); 1H NMR (CDCl3, 300 MHz): d = 8.11 (d, J = 8.1 Hz,
4H), 7.66 (d, J = 8.2 Hz, 4H), 3.27 (s, 2H).
2,5-Bis-(4-phenylethynyl)phenyl-[1,3,4]oxadiazole (3c). The
procedure is analogous to that described for 3a (yield: 90%).
EI: m/z 422 (M+); 1H NMR (CDCl3, 300 MHz): 1H NMR
(CDCl3, 300 MHz): d = 8.15 (d, J = 8.3 Hz, 4H), 7.70 (d, J =
8.2 Hz, 4H), 7.57 (q, 5H), 7.38 (t, 7H).
were recorded on
a Hitachi F-4500 spectrophotometer.
Current–voltage characteristics for the LEDs were measured
with a PA meter/DC voltage source (HP4140B). The EL light
output was recorded by a NewPort 2835-C multifunction
optical meter.
2,5-Bis-[4-(2,3,4,5-tetraphenyl)phenyl]phenyl-[1,3,4]oxadia-
zole (BTPO). The procedure is analogous to that described for
TPO (yield: 93%). MS (MALDI-TOF): m/z 983.4 (M+); 1H
NMR (CDCl3, 300 MHz): d = 7.92 (d, J = 8.1 Hz, 4H), 7.60 (s,
2H), 7.32 (d, J = 8.2 Hz, 4H), 7.18 (brs, 10H), 6.79–6.96 (m,
30H). 13C NMR (CDCl3, 75 MHz): d = 164.4, 145.3, 142.0,
141.4, 141.0, 139.7, 139.5, 139.2, 131.4, 131.0, 130.6, 129.9,
127.6, 127.1, 126.9, 126.7, 126.4, 126.2, 125.9, 125.7, 125.4,
121.7. Elemental analysis (%) calcd for: C74H50N2O C 90.40, H
5.13, N 2.85; found: C 90.18, H 5.22, N 3.17.
X-Ray structure analysis
The single crystals of TPO were obtained from CH2Cl2–
hexane solutions and the data were collected on a Rigaku
R-Axis Rapid IP diffractometer with graphite-monochro-
mated Mo-Ka radiation. Data were processed on a personal
computer (PC) using the SHELXL-97 software package.41 The
structure was solved by direct methods. All non-hydrogen
atoms were refined anisotropically.
Selected crystal data of TPO: C44H30N2O, M = 602.70,
crystal dimensions 0.306 6 0.156 6 0.07 mm, triclinic, a =
˚
9.405(3), b = 10.934(4), c = 17.343(6) A, a = 84.339(5), b =
2,5-Bis-[4-(2,3,4,5,6-pentaphenyl)phenyl]phenyl-[1,3,4]oxadia-
zole (BPPO). The procedure is analogous to that described for
PPO (yield: 90%). MS (MALDI-TOF): m/z 1135.4 (M++1);
1H NMR (CDCl3, 300 MHz): d = 7.54 (d, J = 8.2 Hz, 4H),
6.95 (d, J = 8.2 Hz, 4H), 6.83 (brs, 50H), 6.79–6.96 (m, 30H).
13C NMR (CDCl3, 100 MHz): d =164.3, 144.7, 140.9, 140.1,
139.2, 132.0, 131.3, 132.0, 131.3, 126.8, 126.6, 125.6, 125.3,
125.2, 120.6 ; Elemental analysis (%) calcd for: C86H58N2O C
90.97, H 5.15, N 2.47; found: C 91.10, H 5.22, N 2.90.
86.416(6), c = 70.879(5)u, U = 1676.0(10), T = 294 K, space
group P1, Z = 2, Dc = 1.194 g cm23, F(000) = 636, 8252
¯
reflection collected, 5833 independent (Rint = 0.0316), which
were used in all calculations. The final wR(F2) was 0.1667 (all
data). CCDC reference number 290529. For crystallographic
data in CIF or other electronic format see DOI: 10.1039/
b705515c.
Acknowledgements
Cyclic voltammetry (CV) measurements
The present research was financially supported by National
Natural Science Foundation (20472089, 90206049, 20421101,
20573115, 60671047, 50673093), the Major State Basic
Research Development Program and Chinese Academy of
Sciences.
Cyclic voltammetric measurements were carried out in a
conventional three-electrode cell using
a glass carbon
working electrode, a platinum wire counter electrode, and a
Ag/AgCl reference electrode on a computer-controlled EG&G
Potentiostat/Galvanostat model 283 at room temperature.
Freshly distilled THF was used to prepare a solution of all
compounds (1 mM) containing Bu4NPF6 (0.1 M) as a
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