Conjugated and nonconjugated bipolar-transporting dinuclear europium(III) complexes involving triphenylamine and oxadiazole units: Synthesis, photophysical and electroluminescent properties

Article abstract of DOI:10.1016/j.tet.2013.03.092

Two novel bipolar-transporting dinuclear europium(III) complexes incorporating both hole-transporting triphenylamine (TPA) and electron-transporting oxadiazole (OXD) units into the dual phenanthroline ligands were successfully synthesized and characterize

Full text of DOI:10.1016/j.tet.2013.03.092

Tetrahedron xxx (2013) 1e8
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Conjugated and nonconjugated bipolar-transporting dinuclear
europium(III) complexes involving triphenylamine and oxadiazole
units: synthesis, photophysical and electroluminescent properties
,
a
b
a
a
a
Yu Liu a , Kai Chen , Kongqiang Xing , Yafei Wang , Haigang Jiang , Xiangping Deng ,
*
Meixiang Zhu a, Weiguo Zhu a
,
*
a Department of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in the Ministry of Education, Xiangtan University,
Xiangtan 411105, PR China
b Institute of Biological Science and Technology, Qionzhou College, Wuzhishan 572200, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Two novel bipolar-transporting dinuclear europium(III) complexes incorporating both hole-transporting
triphenylamine (TPA) and electron-transporting oxadiazole (OXD) units into the dual phenanthroline
ligands were successfully synthesized and characterized by IR, elemental analysis, electrochemical,
photophysical analysis, and thermogravimetric analysis, in which the OXD unit was attached onto TPA
unit by conjugated and unconjugated linkages for the europium complex A and B, respectively.
Compared with complex B, complex A exhibited higher thermal stability and quantum efficiency. A
maximum brightness of 296.3 cd/m2 at voltage of 8.5 V was obtained in the complex A-doped devices
with saturated red emission using a blend of poly(vinylcarbazole) and 2-(4-biphenyl)-5-(4-tert-butyl-
phenyl)-1,3,4-oxadiazole as a host matrix, which is about two-times higher than that from the complex
B-doped devices with the same configuration. To best of our knowledge, this is one of the best results
based on the dinuclear europium(III) complexes by spin-casting method.
Received 19 November 2012
Received in revised form 18 March 2013
Accepted 25 March 2013
Available online xxx
Keywords:
Bipolar-transporting
Dinuclear europium(III) complexes
Electroluminescence
Polymer light-emitting devices
Synthesis
Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction
transporting ligands, such as triphenylamine,11,12 carbazole,9,13 and
oxadiazole14,15 were designed and prepared for efficient devices.
In recent years, organic light-emitting diodes (OLEDs) have been
attracted considerable attention because of their potential applica-
tions in the large-area flat displays.1e4 Up to date, the organo-
lanthanide compounds as primary emitters have been intensively
studied due to their very sharp emission and theoretically quantum
efficiency up to 100%.5,6 Among the previously reported organo-
lanthanide compounds, europium(III) complexes have been exten-
sively studied because of their unique spectroscopic characteristics,
including pure red narrow emission,7 higher emission quantum
yield, and better thermal stability.8 However, satisfied electrolumi-
nescent (EL) performances of the red-emitting devices using euro-
pium(III) complexes as emitters were still not obtained yet due to the
low-energy transfer efficiency from the ligands to the central Eu3þ
ion.9,10 In order to improve the performance of devices, a suitable
strategy was used by introduction of functional units into anionic or
ancillary ligands to enhance the efficiency of energy transfer from
the ligands to the Eu3þ ion.9,10 Thus, numerous europium(III) com-
plexes containing various hole-transporting and/or electron-
Zhang et al. first reported an europium(III) complex containing both
oxadiazole and carbazole fragments in the anionic ligand, and the
resulting device exhibited a maximum brightness of 1845 cd/m2
with a maximum current efficiency of 2.62 cd/A.9 We also developed
a series of bipolar-transporting europium(III) complexes recently by
incorporating carbazole and oxadiazole units into phenanthroline
ligand at the same time for light emitting applications.16
Subsequently, some dinuclear lanthanide complexes have been
reported to enhance the luminescent properties because the emis-
sion quantum yield of dinuclear or trinuclear lanthanide complexes
is much higher than that of mononuclear lanthanide complexes.17e22
So it is possible to be expected to achieve high efficiency OLEDs by
careful designing the reasonable molecular structure of dinuclear
europium(III) complexes with proper ligands. Ma et al. reported
a dinuclear europium(III) complex-doped OLED with a brightness of
340 cd/m2 at a driving voltage of 19 V and a current density of
0.14 mA/cm2.19 Do et al. synthesized an unmodified dinuclear euro-
pium(III) complex of Eu2(tta)6(bpm) (tta¼trifluorothenoylacetone,
bpm¼2,20-dipyrimidine) andmade itsdopeddevices usinga blend of
poly(vinylcarbazole) (PVK) and 2-(4-biphenyl)-5-(4-tert-butyl-phe-
nyl)-1,3,4-oxadiazole (PBD) as host matrix with only a brightness of
* Corresponding authors. Tel.: þ86 731 58298280; fax: þ86 731 58292251; e-mail
addresses: liuyu03b@126.com (Y. Liu), zhuwg18@126.com (W. Zhu).
0040-4020/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2013.03.092
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Y. Liu et al. / Tetrahedron xxx (2013) 1e8
30.2 cd/cm2 at 14 V.20 In the past few years, our group reported
bipolar-transporting dinuclear europium(III) complex of
Therefore, both bipolar-transporting dinuclear europium(III) com-
plexes were formed and confirmed by the IR data.
a
Eu2(DBM)6(FPhOXD6Cz-Phen2) and its doped PVKePBD (30 wt %)
devices with a brightness of 48.5 cd/m2 at 13.5 V.21 Furthermore,
some reported results show that the luminescence quantum effi-
ciency of dinuclear europium(III) complexes is higher than mono-
nuclear ones.18,22
2.3. Thermal stability
The thermal stability of europium(III) complexes were mea-
sured through thermogravimetic analysis (TGA) under N2 stream
with a scanning rate of 20 ꢁC/min and their TGA curves are shown
in Fig. 1. The dual europium(III) complexes A and B are thermally
stable and the decomposition temperatures are measured to be
304 and 252 ꢁC, which corresponded to a 5% weight loss, and rapid
weight loss occurs at above 342 and 331 ꢁC, respectively. Compared
to the known Eu(DBM)3Phen (Td¼297.0 ꢁC),16 both dinuclear
europium(III) complexes exhibited a better thermal stability. It
implies that this class of dinuclear europium(III) complexes has an
improved thermal stability.
High thermal stability is favorable for both europium(III) com-
plexes to be applied in the fabrication of OLEDs.27e29 Furthermore,
the better thermal stability was found for the complex A rather
than the complex B. It indicates that conjugated linkage of the bi-
polar-transporting units between the TPA and OXD units into the
europium(III) complex was found to improve the thermal stability
efficiently.29
In this paper, two novel functionalized dinuclear europium(III)
complexes incorporating both hole-transporting triphenylamine
(TPA) and electron-transporting oxadiazole (OXD) units into a dual
phenanthroline ligand were designed and synthesized, in
which the OXD unit was attached onto TPA unit by conjugated and
unconjugated linkages for the europium(III) complex A and B,
respectively. Their molecular structures and the synthetic routes
are shown in Scheme 1. Their thermal, electrochemical, and
optophysical properties were also investigated. By blending this
class of dinuclear europium(III) complexes into the PVKePBD
(30 wt %) host matrix, the single-layer polymer light-emitting
devices (PLEDs) were fabricated at dopant concentrations from
1 wt % to 8 wt %. As expected, all of these devices emitted red light
at 616 nm with a full width at half-maximum (FWHM) of 11 nm. A
maximum brightness of 296.3 cd/m2 at 8.5 V and 158.2 cd/m2 at
10.6 V was obtained for the complex A- and B-doped devices at
1
wt
%
doped concentration, respectively. Compared to
Eu2(tta)6(bpm),20 these bipolar-transporting dinuclear euro-
pium(III) complexes exhibited a much higher brightness in the
PLEDs. Accordingly, introducing the bipolar-transporting TPA and
OXD units into the dinuclear europium(III) complex is responsible
for the enhanced device performance.
2.4. UV absorption property
Normalized UVevis absorption spectra of both dinuclear euro-
pium(III) complexes in CH2Cl2 solution (1.5ꢂ10ꢀ6 mol/L) are de-
scribed in Fig. 2. An intense high-energy absorption band at about
279 nm and a strong medium-energy absorption band at about
339 nm for both europium(III) complexes, but a weak low-energy
absorption band at 417 nm for complex A and 397 nm for com-
plex B was observed, respectively. The high-energy absorption
2. Results and discussion
2.1. Syntheses and characterization
band is attributed to the singletesinglet
the phenanthroline unit. The medium-energy absorption band is
assigned to the singletesinglet electron transition of the TTA
anion ligand. The low-energy absorption band is attributed to the
pep electron transition of
4,40-[(4-Bromophenyl)azanediyl]dibenzaldehyde,
2-(4-
bromophenyl)-5-(4-(tert-butyl) phenyl)-1,3,4-oxadiazole (1) and
2-(4-(6-bromo-hexyloxy)-phenyl)-5-(4-(tert-butyl)phenyl)-1,3,4-
oxadiazole (6) and 5-amino-1,10-phenanthroline were synthesized
according to the reported method in the previous literature.23e26
Compound 7 was synthesized via a Williamson ether-forming
reaction. The neutral dual phenanthroline ligands of 8 and 9 con-
taining TPA and OXD units were synthesized via an aldehyde-
eammonia condensation reaction and confirmed by 1H NMR
spectroscopy and MALDI-TOF mass spectrometry, respectively
(Fig. S1). Both europium(III) complexes of A and B were prepared
according to the general coordination reaction and characterized by
element analysis and IR spectra.
pep
singletesinglet
pep electron transition of neutral ancillary ligand
containing binary phenanthroline units.30
On the other hand, it is easy to find that the europium(III)
complex A with conjugatedly linked OXD unit exhibited a larger
spectral overlap than the europium(III) complex B with non-
conjugatedly linked OXD unit between photoluminescence (PL)
spectrum of PVKePBD (30 wt %) and their intrinsic UV spectrum of
the dinuclear europium(III) complexes. This means that the energy
transfer between the PVKePBD host matrix and the complex A
instead of the complex B is more efficient.10,31 As a result, the
complex A is expected to exhibit better electroluminescent (EL)
performance than complex B in their OLEDs/PLEDs.
2.2. IR absorption spectrum
The IR spectra of both europium(III) complexes are shown in
Fig. S2, and their corresponding data are summarized in Table 1 for
comparison. The stretching vibration peaks from the C]O, C]C,
and C]N double bands were observed at 1601, 1537, and
2.5. Photoluminescence property
The normalized PL spectra of the dual phenanthroline ligands (8
and 9) and their resulting europium(III) complexes (A and B) in
CH2Cl2 (1.0ꢂ10ꢀ5 mol/L) are shown in Fig. 3, and the corresponding
PL data are summarized in Table 1. An intense sharp low-energy
emission peak at 613 nm with a narrow FWHM of 10 nm was ob-
served for both europium(III) complexes, which is corresponded to
5D0/7F2 transition of Eu3þ ion.32 In addition, a weak high-energy
emission band between 350 and 550 nm was also appeared at
the same time, which results from the dual phenanthroline neutral
ancillary ligand compared to the PL spectrum of the 8 and 9 ligands
under photo-excitation (lex¼339 nm).33 Furthermore, the high-
energy emission band is weaker for the complex A than the com-
plex B under this situation. It implies that the energy transfer is
nearly completed from these dual phenanthroline ligands to Eu3þ
1508 cmꢀ1, respectively, which indicate that
b
-diketone ligand of
TTA is coordinated to Eu3þ ion.27,28 On the other hand, the skeleton
vibration from phenanthroline ring was found at 1537 cm
ꢀ1 27 The
.
peaks at 826 and 722 cmꢀ1 were assigned to the CeH bending vi-
bration from phenanthroline ring. Additionally, a series of absorp-
tion peaks were observed at 1307, 787, and 581 cmꢀ1
corresponding to the CF3 vibrations of TTA.27 Another bending vi-
bration peak from EueO was also observed at 494 cmꢀ1, which
means that these biphenanthroline ligands were also coordinated
to Eu3þ ions.27,28 These results suggest that the coordination bonds
were formed between the Eu3þ ion and the ligands of TTA and
biphenanthroline in their corresponding europium(III) complexes.
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Y. Liu et al. / Tetrahedron xxx (2013) 1e8
3
Scheme 1. Synthetic route of the binuclear europium(III) complexes of A and B. Reagents and reaction conditions: (a) and (c): Pd(dppf)Cl2, KOAc, Bis(pinacolato)diboron, DMSO,
80 ꢁC, 24 h; (b) and (d): 4,40-[(4-bromophenyl)azanediyl]dibenzaldehyde, THF/K2CO3, Pd(PPh3)4, reflux, 24 h; (e): Cs2CO3, THF, reflux, 24 h; (f) and (g): CHCl3, CH3COOH, reflux, 24 h;
(h)and (i): NaOH/EuCl3, TTA, THF/ethanol, 65 ꢁC, 12 h. All reactions under dried nitrogen purge.
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Y. Liu et al. / Tetrahedron xxx (2013) 1e8
Table 1
Optophysical, thermal, and electrochemical properties of the dinuclear europium(III) complexes
Compounds
Complex A
lAbs (nm)
max/dm3 molꢀ1 cmꢀ1
)
lem (nm)
Ff (%)
Td (ꢁC)
Eg (eV)
Eox (V)
Ered (V)
ꢀ1.76
EHOMO (eV)
ꢀ6.14
ELUMO (eV)
ꢀ2.64
(
3
279(310666)
339(250812)
417(144126)
279(202857)
339(159285)
397(113333)
493, 613
10.48
304
3.50
1.86
Complex B
396, 613
6.30
252
3.66
1.88
ꢀ1.78
ꢀ6.26
ꢀ2.60
Fig. 3. Normalized PL spectra of the dual phenanthroline ligands and their corre-
sponding europium(III) complexes in CH2Cl2 solution (1.0ꢂ10ꢀ5 mol/L), respectively.
Fig. 1. TG curves of both europium(III) complexes recorded in dynamic nitrogen at-
mosphere (50 mL/min) and heating rate of 20 ꢁC/min.
these triphenylamine and oxadiazole-functionalized dinuclear euro-
pium(III) complexes presented higher emission quantum yields.
Furthermore, the europium(III) complex A exhibited higher level
than the europium(III) complex B. It convincingly supports the sug-
gestion that extending the conjugation degree of the dual phenan-
throline ligands could efficiently improve the PL quantum yields of
their corresponding europium(III) complexes.8,10,35
2.6. Electrochemical property
Both europium(III) complexes presented a reversible reduction
waves (Ered) at ꢀ1.5 to ꢀ2 V versus a saturated calomel electrode
(SCE), but their oxidation waves were not observed. The oxidation
potential (Eox) was calculated from the energy band gap (Eg) and Ered
,
in which Eg was obtained from the UVevis absorption edge. As a re-
sult, the highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) energy levels (EHOMO and
ELUMO) of europium(III) complexes were calculated according to the
empirical formula: EHOMO¼ꢀ(Eoxþ4.38) and ELUMO¼ꢀ(Eredþ4.38).36
As shown in Table 1, the complex A displayed the HOMO and LUMO
energy levels of ꢀ6.14 and ꢀ2.64 eV, respectively. However, the
complex B exhibited a decreased EHOMO level by 0.12 eV and an in-
creased ELUMO level by 0.04 eV. The high EHOMO and low ELUMO levels
are available to facilitate hole/electron injection, transportation, and
finally trap for the complex A.
Fig. 2. UVevis absorption spectra of both europium(III) complexes and normalized
emission spectrum of the PVKePBD (30 wt %) blend in CH2Cl2 solution (1.5ꢂ10ꢀ6 mol/L).
ion in both dinuclear europium(III) complexes. The complex A
exhibited more efficient energy transfer from the ligand to the
central Eu3þ ion than the complex B. This phenomenon indicates
that the conjugated rather than the unconjugated linkage between
the TPA and OXD units in the dual phenanthroline ligand is avail-
able to improve intramolecular energy transfer efficiency for its
resultant europium(III) complexes.10
2.7. Electroluminescent property
In order to further understand the influence of the linked mode
between the TPA and OXD units on emission property in both
dinuclear europium(III) complexes, the emission quantum yields
were measured using EuCl3$6H2O (Ff¼0.073% in water) as a standard
at rt.34 The measured emission quantum yield values of the com-
plexes A and B were 11.48 and 8.30%, respectively. Compared with the
unmodified mononuclear europium(III) complex of Eu(DBM)3Phen,16
To investigate the EL properties of both europium(III) com-
plexes, the single-emissive-layer (SEL) PLEDs with a device con-
figuration of ITO/PEDOT:PSS (50 nm)/PVKePBD (30 wt %):x wt %
europium(III) complexes (70 nm)/LiF(0.5 nm)/Al(150 nm) were
fabricated. The dependence of EL spectra and CIE chromaticity
diagrams on the dopant concentrations of 1, 2, 4, and 8 wt % are
shown in Fig. 4a for the complex B-doped devices and Fig. 4b for the
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5
Fig. 4. Normalized EL spectra of the dinuclear europium(III) complex B-doped devices
(a) and A-doped devices (b) at different dopant concentrations from 1 to 8 wt % under
bias of 15 V, respectively.
Fig. 5. Normalized EL spectra and CIE 1931 chromaticity diagrams of the dinuclear
europium(III) complex B-doped devices (a) and A-doped devices (b).
remarkable that the complex A-doped devices exhibited fewer
change for the CIE coordinates than the complex B-doped devices
This should be related to the more intense charge trapping of the
europium(III) complex A than B as the dominant EL mechanism in
OLEDs.37 In this situation, it is suggested that the excitons are easier
to be directly trapped on the europium(III) complex A than B
resulting from direct recombination of holes and electrons in the
SEL, as well as the device A exhibited lower turn-on voltage.38
The current densityevoltageeluminance (JeVeL) curves of the
dinuclear europium(III) complex B- and A-doped devices at differ-
ent dopant concentrations from 1 to 8 wt % were shown in Fig. 6a
and b, respectively. Theircorresponding data are also summarized in
Table 2. The maximum brightness of 296.3 cd/m2 at 69.1 mA/cm2 in
the complex A-doped device and 158.2 cd/m2 at 45.2 mA/cm2 in the
complex B-doped device was observed at 1 wt % doping concen-
tration. Obviously, both bipolar-transporting dinuclear euro-
pium(III) complexes-doped devices exhibited an improvement
performance than the unfunctional dinuclear Eu2(tta)6(bpm)-
doped devices.20 Furthermore, the complex A-doped device
exhibited two-times brightness level than the complex B-doped
device. Here, the larger spectral overlap between the PL spectrum of
the PVKePBD host matrix and the absorption spectrum of the
complex A than B play an important role in improving EL proper-
ties.20,31 However, with increasing dopant concentrations from 1 to
8 wt %, both dinuclear europium(III) complexes-doped devices
complex A-doped devices, respectively. An intense red low-energy
emission peak at 616 nm with narrow FWHM of 11 nm at different
dopant concentrations, a weak medium-energy emission peak at
550 nm, and a minor high-energy emission peak about 440 nm are
observed for all recorded EL spectra, in which the low-energy peak
is attributed to the Eu3þ ion’s emission, the medium-energy
emission peak is assigned to the dual phenanthroline ligands and
the high-energy peak results from the host matrix’s emission.
Compared to the complex B-doped devices, the complex A-doped
devices exhibited weaker medium- and high-energy emissions.
This implies that not only the intermolecular energy transfer is
more efficient from the PVKePBD host matrix to the complex A
instead of to the complex B, but also the intramolecular energy
transfer is more efficient from the dual phenanthroline ligand 8
rather than 9 to europium ion under electric field. This firmly
suggests that the conjugated rather than the unconjugated linkage
between the TPA and OXD units in the dual phenanthroline ligands
is available to improve EL for its resultant europium(III) complexes-
doped devices.
The substantial difference between EL spectra of the complex A-
and B-doped devices is related to the different inter-and intra-
molecular energy transfer. In order to further understand EL spectra
stability of the dinuclear europium(III) complexes-doped devices,
their corresponding CIE chromaticity coordinates (X, Y) at 15 V with
different dopant concentrations are listed in Table 2, as well as the
dependence of EL spectra and CIE chromaticity diagrams at 1 wt %
doping concentrations under different applied voltages are shown
in Fig. 5. Minor changes for the CIE coordinates are observed in the
complex A- and B-doped devices at 1% dopant concentration under
different applied voltages. It indicates that their EL emission here
are almost completely dominated by the Eu3þ ion emission and the
CIE coordinates are almost independent of the voltages. It is
exhibited
a decreasing luminance due to their luminescent
quenching.12 Therefore, the device optimization should be carried
out in the further.
Table 2
Device performances of the dinuclear europium(III) complexes-doped PVKePBD
(30 wt %) devices at different dopant concentrations from 1 to 8 wt %
Compound Doping
Turn-on
Maximum brightness
CIE
rate (wt %) voltages (V)
Ja (mA/cm2)
B
b (cd/m2) (X, Y)
Complex A 1%
8.5
10.2
11.5
13.8
10.6
12.1
14.8
16.7
69.1
59.5
56.2
31.8
45.2
11.5
33.3
15.3
296.3
224.5
170.2
100.5
158.2
78.3
(0.592, 0.321)
(0.621, 0.332)
(0.633, 0.332)
(0.644, 0.333)
(0.537, 0.281)
(0.596, 0.318)
(0.577, 0.302)
(0.633, 0.319)
2%
4%,
8%
Complex B 1%
2%
4%,
8%
62.5
36.3
a
J¼current density.
Fig. 6. JeVeL curves of the dinuclear europium(III) complexes B-doped devices (a) and
A-doped devices (b) at different dopant concentrations.
b
B¼brightness.
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Y. Liu et al. / Tetrahedron xxx (2013) 1e8
3. Conclusion
at 80 ꢁC for 24 h under nitrogen atmosphere. The resulting mixture
was cooled to rt, poured into ice-water (100 mL), and then extracted
with CH2Cl2 (60 mL). The combined organic layer was dried over
anhydrous MgSO4 and filtered. The filtrate was evaporated, and the
residue was purified by column chromatography over silica gel using
CH2Cl2/ethyl acetate (v/v¼5:1) as eluent to provide 1.33 g with a yield
of 61% as a white solid. Mp: 74e76 ꢁC. 1H NMR (400 MHz, CDCl3, TMS,
dppm): 8.16e8.14 (d, J¼7.6 Hz, 2H), 8.11e8.09 (d, J¼7.6 Hz, 2H),
7.99e7.97 (d, J¼7.6 Hz, 2H), 7.58e7.56 (d, J¼8.0 Hz, 2H), 1.39 (s, 21H).
13C NMR (100 MHz, CDCl3, TMS, dppm): 164.58, 164.16, 155.12, 135.10,
126.56, 126.05, 125.72, 125.80, 120.02, 84.06, 34.97, 30.92, 24.67. TOF-
MS (Hþ): 404.
Two novel dinuclear europium(III) complexes containing TPA
and OXD units with a conjugated linkage for complex A and another
unconjugated linkage for complex B in the biphenanthroline li-
gands were successfully obtained. Complex A exhibited better
thermal stability, higher emission quantum efficiency, and im-
proved EL performance. All of their doped devices emitted sharp
light at 616 nm with a narrow FWHM of 11 nm even at 1 wt %
dopant concentration. A maximum brightness of 296.3 cd/m2 at
69.1 mA/cm2, as well as a low turn-on voltage of 8.5 V was achieved
in the complex A-doped PVKePBD devices at 1 wt % dopant con-
centration. This brightness level is about two-times higher than
that in the complex B-doped devices. Our primary results indicated
that the conjugated than unconjugated linkage between the TPA
and OXD units into the dual phenanthroline ligands is available to
improve photophysical and EL properties for its resulting euro-
pium(III) complex.
4.2.2. 4,40-((40-(5-(4-(tert-Butyl)phenyl)-1,3,4-oxadiazol-2-yl)-[1,10-
biphenyl]-4-yl)azanediyl)dibenzaldehyde (3). To a mixture of 4,40-
((4-bromophenyl)azanediyl)dibenzaldehyde18 (0.50 g, 1.31 mmol),
compound
2 (0.64 g, 1.64 mmol) and Pd(PPh3)4 (60 mg,
0.052 mmol) was added a mixture of THF (40 mL) and K2CO3
aqueous solution (2 M, 15 mL). The mixture was refluxed for 24 h
under a dry nitrogen protection. After cooled to rt, the mixture was
poured into water (50 mL), and then extracted with CH2Cl2 (60 mL).
The combined organic layer was dried over anhydrous MgSO4 and
filtered. The filtrate was evaporated, and the residue was purified
by column chromatography over silica gel using petroleum ether/
ethyl acetate (v/v¼2:1) as eluent to provide 0.54 g with a yield of
72% as a yellow solid. Mp: 105e107 ꢁC. 1H NMR (400 MHz, CDCl3,
TMS, dppm): 9.94 (s, 2H), 8.25e8.23 (d, J¼7.6 Hz, 2H), 8.11e8.09 (d,
J¼8.0 Hz, 2H), 7.84e7.82 (d, J¼7.6 Hz, 4H), 7.79e7.77 (d, J¼8.0 Hz,
2H), 7.70e7.68 (d, J¼8.0 Hz, 4H), 7.59e7.57 (d, J¼8.0 Hz, 4H),
7.45e7.38 (m, 2H), 1.39 (s, 9H). 13C NMR (100 MHz, CDCl3, TMS,
dppm): 190.43, 151.63, 144.69, 133.29, 131.72, 131.40, 131.32, 130.17,
128.98, 128.56, 128.29, 128.06, 127.76, 127.08, 126.62, 126.14, 125.48,
123.03,122.79,121.35,119.25, 35.09, 31.16, 29.24, 29.11, 28.91, 25.90,
25.85, 25.77. TOF-MS (Hþ): 577.
4. Experimental section
4.1. General
Tetrahydrofuran (THF) was distilled over sodium before used.
Dimethylsulfoxide (DMSO) was distilled over calcium hydride be-
fore used. The other reagents were directly used without further
purification. All reactions were performed under purified nitrogen
protection and were monitored by thin-layer chromatography (TLC).
Flash column chromatography and preparative TLC were carried out
using silica gel from Merck (200e300 mesh). All 1H NMR and 13C
NMR spectra were acquired at a Bruker Dex-400NMR instrument
using CDCl3 as solvent. Mass spectra (MS) were recorded on a Bruker
Autoflex TOF/TOF (MALDI-TOF) instrument using dithranol as
a matrix. Elemental analysis was carried out with a Harrios ele-
mental analysis instrument. Thermogravimetric analyses (TGA)
were performed under a nitrogen atmosphere at a heating rate of
20 ꢁC/min using a PerkineElmer TGA-7 thermal analyzer. The FTIR
spectra were obtained on a PerkineElmer spectrum one Fourier
transform infrared spectrometer (KBr pellet). UVevis absorption
and fluorescence spectra were recorded with a Shimadzu UV-265
spectrophotometer and a PerkineElmer LS-50 luminescence spec-
trometer, respectively. Cyclic voltammetry was carried out on
a CHI660A electrochemical workstation in a solution of tetrabuty-
lammonium hexafluorophosphate (Bu4NPF6) (0.1 M) and CH2Cl2
with a scan rate of 50 mV/s at rt under nitrogen flow protection.
EL spectra were recorded with an Insta-Spec IV CCD system
(Oriel). Luminance was measured with a Si photodiode and calibrated
by a PR-705 spectrascan spectrophotometer (Photo Research).
The single-layer PLEDs were fabricated with a structure of ITO/
PEDOT:PSS(50 nm)/PVK-PBD (30 wt %):europium(III) complex
(70 nm)/LiF (0.5 nm)/Al (150 nm), inwhich indium-tin oxide (ITO) and
poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)
4.2.3. 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)phenol
(4). Compound 4 was synthesized according to the method described
in Section 4.2.1. The residue was purified by column chromatography
over silica gel using CH2Cl2/petroleum ether (v/v¼2:1) as eluent with
1
a yield of 78% as a white solid. Mp: 54e56 ꢁC. H NMR (400 MHz,
DMSO, TMS, dppm): 7.72e7.70 (d, J¼8.1 Hz, 2H), 6.83e6.81 (d,
J¼8.2 Hz, 1H), 1.33 (s, 12H). 13C NMR (100 MHz, CDCl3, TMS, dppm):
158.66, 136.81, 117.35, 114.98, 83.26, 25.00, 24.82. TOF-MS (Hþ): 219.
4.2.4. 4,40-((40-Hydroxy-[1,10-biphenyl]-4-yl)azanediyl)dibenzalde-
hyde (5). Compound 5 was synthesized according to the method
described in Section 4.2.2. The residue was purified by column
chromatography over silica gel using CH2Cl2/ethyl acetate (v/
v¼2:1) as eluent with a yield of 77% as a yellow solid. Mp:
85e86 ꢁC. 1H NMR (400 MHz, CDCl3, TMS, dppm): 9.90 (s, 2H),
7.80e7.78 (d, J¼8.4 Hz, 4H), 7.56e7.54 (d, J¼8.0 Hz, 2H), 7.49e7.47
(d, J¼8.4 Hz, 2H), 7.26e7.19 (m, 6H), 6.93e6.91 (d, J¼8.4 Hz, 2H).
13C NMR (100 MHz, CDCl3, TMS, dppm): 190.74, 152.06, 143.99,
131.44, 131.32, 128.22, 128.19, 127.25, 123.09, 122.86, 115.93. TOF-
MS (Hþ): 393.2.
(PEDOT:PSS) (Bayer AG) are used as the anode and hole-injection
layer, as well as lithium fluoride (LiF) and Al are employed as the
electron-injection layer and cathode, respectively. The emitting layer
consists of europium(III) complexes and host matrix of the PVKePBD
blend, in which the weight ratio of PBD is 30%. Doping weight con-
centrations of europium(III) complexes vary from 1, 2, 4 to 8 wt %.
4.2.5. 4,40-((40-((6-(4-(5-(4-(tert-Butyl)phenyl)-1,3,4-oxadiazol-2-
yl)phenoxy)hexyl)oxy)[1,10-biphenyl]-4-yl)azanediyl)dibenzalde-
hyde (7). A mixture of compound 5 (0.60 g, 1.52 mmol), 2-(4-(6-
bromo-hexyloxy)phenyl)-5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazole
(6) (1.39 g, 3.05 mmol), Cs2CO3 (2.45 g, 7.60 mmol), and THF (40 mL)
was stirred at 65 ꢁC for 24 h under nitrogen protection. The mixture
was cooled to rt, poured into water (100 mL), and then extracted with
CH2Cl2 (100 mL). The combined organic layer was dried over anhy-
drous magnesium sulfate and filtered. The filtrate was evaporated to
remove the solvent and the residue was purified by silica gel column
4.2. Synthesis of the ligands and complexes
4.2.1. 2-(4-(tert-Butyl)phenyl)-5-(4-(4,4,5,5-teramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)-1,3,4-oxadiazole (2). 2-(4-Bromophenyl)-
5-(4-(tert-butyl)phenyl)-1,3,4-oxadiazole (1) (2.0 g, 5.56 mmol),
bis(pinacolato)diboron (1.56 g, 6.16 mmol), KOAc (1.65 g,16.80 mmol),
and Pd(dppf)Cl2 (138 mg, 0.17 mmol) and DMSO (80 mL) were stirred
Please cite this article in press as: Liu, Y.; et al., Tetrahedron (2013), http://dx.doi.org/10.1016/j.tet.2013.03.092

Y. Liu et al. / Tetrahedron xxx (2013) 1e8
7
chromatography using hexane/ethyl acetate (v/v¼2:1) as eluent to
provide 0.95 g with a yield of 83% as a white solid. Mp: 99e101 ꢁC. 1H
NMR (400 MHz, CDCl3, TMS, dppm): 9.90 (s, 2H), 8.07e8.03 (t, 4H),
7.79e7.77 (d, J¼8.0 Hz, 4H), 7.57e7.51 (m, 6H), 7.25e7.21 (t, 6H),
581, 494. Anal. Calcd for Eu2C110H69F18N9O13S6: C, 51.55; H, 2.71; N,
4.92. Found: C, 51.40; H, 2.63; N, 4.85.
4.2.9. Complex B. Complex B was synthesized according to the
method described in Section 4.2.8. A light-yellow powder was ob-
tained with a yield of 46%. FTIR (KBr, cmꢀ1) 2928, 2359, 1602, 1537,
1508, 1424, 1412, 1356, 1307, 1246, 1187, 1142, 1062, 934, 859, 826,
721, 641, 581, 494. Anal. Calcd for Eu2C122H85F18N9O15S6: C, 53.18; H,
3.11; N, 4.58. Found: C, 53.12; H, 3.07; N, 4.50.
7.03e6.97 (m, 4H), 4.08e4.03 (m, 4H), 1.87 (s, 4H), 1.36 (s, 13H). 13
C
NMR (100 MHz, CDCl3, TMS, dppm): 190.46, 164.38, 164.24, 161.86,
158.96, 155.16, 151.98, 144.04, 138.79, 132.34, 131.40, 131.34, 128.67,
128.19, 127.96, 127.21, 126.81, 126.79, 126.69, 126.08, 126.02, 123.08,
123.02, 122.86, 121.33, 116.46, 114.98, 113.17, 69.25, 68.13, 67.96, 35.09,
31.16, 29.24, 29.11, 28.91, 25.90, 25.85, 25.76. TOF-MS (Hþ): 770.
Acknowledgements
4.2.6. Ligand 8. Compound 3 (0.45 g, 0.78 mmol) dissolved in
CHCl3 (20 mL) and a catalytic amount of acetic acid were mixed and
stirred for 20 min, then 5-amino-1,10-phenanthroline (0.38 g,
1.94 mmol) dissolved in CHCl3 (25 mL) was added dropwise into
the above mixture. The mixture was refluxed for further 24 h under
a dry nitrogen protection. After cooled to rt, poured into ice-water
(50 mL), extracted with CHCl3 (60 mL), and then washed with
saturated brine. The combined organic layer was dried over anhy-
drous MgSO4 and filtered. The filtrate was evaporated, and the
residue was purified by column chromatography over silica gel
using CH2Cl2/ethyl acetate (v/v¼10:1) as eluent to give 0.4 g with
a yield of 55% as a light-yellow powder. Mp: 111e112 ꢁC. 1H NMR
(400 MHz, CDCl3, TMS, dppm): 9.25e9.23 (d, J¼10.4 Hz, 2H), 9.15 (s,
2H), 8.83e8.79 (t, 2H), 8.69e8.67 (d, J¼8.8 Hz, 2H), 8.32e8.25 (m,
4H), 8.12e7.99 (t, 6H), 7.83e7.79 (t, 2H), 7.73e7.51 (m, 10H),
7.38e7.34 (t, 6H), 1.40 (s, 9H). 13C NMR (100 MHz, CDCl3, TMS,
dppm): 26.93, 31.56, 122.24, 122.80, 123.24, 123.56, 123.79, 126.09,
126.25, 126.27, 126.82, 127.34, 127.38, 127.49, 128.51, 129.06, 130.68,
131.15, 132.73, 135.56, 145.25, 148.03, 149.18, 150.18, 150.64, 160.30.
TOF-MS (Hþ): 932.
The authors are deeply grateful to the National Natural Science
Foundation of China (Project No. 51273168, 50973093, and
21172187), the Specialized Research Fund for the Doctoral Program
of Higher Education (20094301110004), the Hunan Provincial
Natural Science Foundation of China (11JJ3061), the Scientific
Research Fund of Hunan Provincial Education Department (10B112,
10A119, 11CY023), the Provincial Natural Science Foundation of
Hunan (12JJ7002), and the Provincial Natural Science Foundation of
Hainan (808168).
Supplementary data
Supplementary data associated with this article can be found in
the online version, at http://dx.doi.org/10.1016/j.tet.2013.03.092.
References and notes
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