Synthesis and electroluminescent property of novel europium complexes with oxadiazole substituted 1,10-phenanthroline and 2,2′-bipyridine ligands

Article abstract of DOI:10.1039/b9nj00461k

Several 1,10-phenanthroline derivatives (PhoR), 2,2′-bipyridine derivatives (BpoR) and related europium complexes Eu(TTA)₃PhoR and Eu(TTA)₃BpoR were synthesized (HTTA = 2-thenoyltrifluoroacetone). Single crystal X-ray diffraction of Eu(TTA)₃Php (Php = 2-(pyridyl)-1,10-phenanthroline) shows that Php acts as a tridentate N N N ligand, leading to a high stability of the complex for vacuum evaporation. When an oxadiazole moiety is incorporated into the ligand, the corresponding europium complexes show improved carrier-transporting abilities as well as thermal stabilities under vapor deposition for electroluminescence (EL) applications. Experiments revealed that these complexes have high photoluminescence (PL) quantum yields due to suitable triplet energy levels (E_T) of the ligands, between 19 724 and 22 472 cm^-1, for the sensitization of Eu(III) (⁵D₀: 17 500 cm ^-1). Utilizing Eu(TTA)₃PhoB (PhoB = 2-(5-phenyl-1,3,4- oxadiazol-2-yl)-1,10-phenanthroline) as the dopant emitter in CBP (4,4′-N,N′-dicarbazolebiphenyl), EL devices with a structure of TPD (4,4′-bis[N-(p-tolyl)-N-phenylamino] biphenyl, 30 nm)/Eu(TTA) ₃PhoB:CBP (7.5%, 20nm)/BCP (2,9-dimethyl-4,7-diphenyl-1,10- phenanthroline, 20 nm)/Alq₃ (tris(8-hydroxyquinoline), 30 nm) exhibited a pure emission from europium ions. The highest efficiency obtained was 5.5 lm W^-1, 8.7 cd A^-1 and the maximum brightness achieved was 1086 cd m^-2. At a practical brightness of 100 cd m ^-2, the efficiency remains above 2.0 cd A^-1. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010.

Full text of DOI:10.1039/b9nj00461k

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis and electroluminescent property of novel europium complexes
with oxadiazole substituted 1,10-phenanthroline and 2,20-bipyridine
ligandsw
Zhuqi Chen,z Fei Ding,z Feng Hao, Ming Guan, Zuqiang Bian,* Bei Ding and
Chunhui Huang
Received (in Montpellier, France) 7th September 2009, Accepted 10th November 2009
First published as an Advance Article on the web 13th January 2010
DOI: 10.1039/b9nj00461k
Several 1,10-phenanthroline derivatives (PhoR), 2,20-bipyridine derivatives (BpoR)
and related europium complexes Eu(TTA)3PhoR and Eu(TTA)3BpoR were synthesized
(HTTA = 2-thenoyltrifluoroacetone). Single crystal X-ray diffraction of Eu(TTA)3Php
(Php = 2-(pyridyl)-1,10-phenanthroline) shows that Php acts as a tridentate N^N^N ligand,
leading to a high stability of the complex for vacuum evaporation. When an oxadiazole moiety
is incorporated into the ligand, the corresponding europium complexes show improved
carrier-transporting abilities as well as thermal stabilities under vapor deposition for
electroluminescence (EL) applications. Experiments revealed that these complexes have high
photoluminescence (PL) quantum yields due to suitable triplet energy levels (ET) of the ligands,
between 19 724 and 22 472 cmꢀ1, for the sensitization of Eu(III) (5D0: 17 500 cmꢀ1). Utilizing
Eu(TTA)3PhoB (PhoB = 2-(5-phenyl-1,3,4-oxadiazol-2-yl)-1,10-phenanthroline) as the
dopant emitter in CBP (4,40-N,N0-dicarbazolebiphenyl), EL devices with a structure of TPD
(4,40-bis[N-(p-tolyl)-N-phenylamino] biphenyl, 30 nm)/Eu(TTA)3PhoB:CBP (7.5%, 20nm)/BCP
(2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 20 nm)/Alq3 (tris(8-hydroxyquinoline), 30 nm)
exhibited a pure emission from europium ions. The highest efficiency obtained was 5.5 lm Wꢀ1
,
8.7 cd Aꢀ1 and the maximum brightness achieved was 1086 cd mꢀ2. At a practical brightness of
100 cd mꢀ2, the efficiency remains above 2.0 cd Aꢀ1
.
the EL efficiencies of europium complex-based OLEDs were
enhanced through the above methods. However, the perfor-
mances achieved are not satisfactory compared to organic
molecules and polymer-based LEDs. Therefore, further
improvement of OLEDs based on europium complexes is of
great importance. When carrier transporting groups are
introduced to the ligands, high molecular weights bring another
key problem: volatilities and thermal stabilities of europium
complexes during the vacuum evaporation.26,27 After all, while
many advances have been made, europium complexes including
high quantum efficiencies, good carrier transporting abilities
and thermal abilities for deposition are still in demand. Based
on these considerations, novel 1,10-phenanthroline and 2,20-
bipyridine derivatives and related europium complexes were
designed and synthesized. By single crystal X-ray diffraction
we confirm that 2-(pyridyl)-1,10-phenanthroline (Php) offers
three coordination atoms which contribute to a high stability
of the related complex.28,29 The pyridine moiety is further
replaced by an oxadiazole moiety, which have been proven to
be very effective in improving the injection and transport of
electrons.30–32 By comparing element analysis data of the
complex before and after sublimation, we figure that europium
complexes with these oxadiazole substituted ligands maintain
thermal stability for deposition. Furthermore, the triplet
energy levels (ET) of the ligands are suitable for complete
energy transfer to central ions, which leads to high quantum
yields of related complexes. The utilization of these novel
Introduction
For organic electroluminescence (EL) devices, the develop-
ment of high-performance red emission is still much in
demand. Rare earth complexes have been used as emitters
in organic light-emitting diodes (OLEDs) due to their sharp-
band emissions and high photoluminescence (PL) quantum
yields.1–5 Many europium complexes have been investigated
because they are known as excellent red phosphors that exhibit
intense sharp red light at around 613 nm while other red
organic emitting materials give broad emission spectra with
bandwidths of around 100 nm resulting dull colors.6–14
However, OLEDs based on europium(III) complexes do not
show satisfying brightness and efficiency due to their poor
carrier transporting abilities.2,11 There are two methods of
modification: one is introducing carrier transporting groups to
ligands,15–20 the other is doping europium complexes into host
materials with good semiconductor properties.11,21–25 Indeed,
Beijing National Laboratory for Molecular Sciences, State Key
Laboratory of Rare Earth Materials Chemistry and Applications,
College of Chemistry and Molecular Engineering, Peking University,
100871, Beijing, China. E-mail: bianzq@pku.edu.cn;
Fax: +86 10-6275-7156; Tel: +86 10-6275-7156
w Electronic supplementary information (ESI) available: Excitation
spectra of complexes Eu(TTA)3BpoR and Eu(TTA)3PhoR in CH2Cl2.
CCDC reference numbers 754692. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/b9nj00461k
z These authors contributed equally to this work.
ꢁc
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complexes as red emitters in EL devices are explored and
remarkable performances are achieved.
Table 1 Element analysis data of europium complexes
Products from Residue left
Calculated
valuesa
Complexes
sublimation
in crucible
Results and discussion
Eu(TTA)3PhoB
(C44H24EuF9N4O7S3)
C: 46.43
H: 2.27
N: 4.80
C: 40.23
H: 1.83
N: 2.43
C: 46.05
H: 2.10
N: 4.92
C: 50.12
H: 3.26
N: 1.39
C: 46.36
H: 2.12
N: 4.92
C: 44.49
H: 2.55
N: 2.73
C: 35.35
H: 1.48
N: —
As shown in Scheme 1, Compounds 1, 2 and 3 were synthe-
sized according to reported procedures.33,34 Reaction of
compound 3 with a variety of benzoyl chloride derivatives
afforded the neutral ligands. Then, based on the well-known
formation of eight-coordinate adducts between {Ln(dik)3}
units and N,N-bidentate chelates, europium complexes
Eu(TTA)3Php, Eu(TTA)3PhoR and Eu(TTA)3BpoR were
synthesized and characterized. Details were described in the
experimental section.
Eu(TTA)3Phen
(C36H20EuF9N2O6S3)
Eu(TTA)3
(C24H12EuF9O6S3)
a
According to the molecular formula.
evaporation of the complexes in vacuum. Previous reports
showed that the neutral ligand phenanthroline was slightly
dissociated in vapor deposition and the residue in the crucible
became dark.26,27 We compare the element analysis data of
both the products of europium complexes from sublimation
and residues left in the crucible. For Eu(TTA)3Phen, an
obvious decrease of C, H, and N contents of the product after
sublimation is observed (Table 1). The products were
presumed to be a mixture of Eu(TTA)3Phen and Eu(TTA)3.
The neutral ligand dissociates in the sublimation process.
Meanwhile, Eu(TTA)3PhoR remains as a white powder after
vacuum evaporation for many times. Taking Eu(TTA)3PhoB
as an example, we compared the elemental analysis data of
both the product from vacuum evaporation and the residue.
They were both coincident with calculated values. The melting
points (Tm) and the thermal decomposition temperatures (Td)
of the complexes are also measured and are listed in Table 2.
No glass transition temperature or crystallization temperature
is found in any of these complexes. Thus, with the substituted
1,3,4-oxadiazole group, Eu(TTA)3PhoR shows the desired
thermal stability for deposition. This stability makes devices
fabricated by vacuum evaporation more feasible.
Crystal and molecular structure
Single crystals of Eu(TTA)3Php were isolated from acetone
and ethanol solutions and analyzed by X-ray diffraction. The
X-ray diffraction data were collected on a Rigaku MicroMax-007
CCD diffractometer by the o scan technique at 131 K using
graphite-monochromated Mo/Ka (l = 0.71073 A) radiation.
An absorption correction by multiscan was applied to
the intensity data. The structures were solved by the direct
method, and the heavy atoms were located from the E-map.
The remaining nonhydrogen atoms were determined from the
successive difference Fourier syntheses. The non-hydrogen
atoms were refined anisotropically, whereas the hydrogen
atoms were generated geometrically with isotropic thermal
parameters. The structures were refined on F2 by full-matrix
least-squares methods using the SHELXTL-97 program package.
The europium ion was surrounded by six oxygen atoms
from the bidentate b-diketonate ligands and three nitrogen
atoms from the neutral ligand Php (Fig .1). The coordination
polyhedron can be described as a distorted square antiprism.
The lengths of Eu–N are 2.596(4), 2.650(4) and 2.603(4) A.
They are shorter than those in previous reports of N^N^N
ligand based europium complexes (2.68, 2.70 and 2.72 A)
in the literature.35–37 Shorter bond lengths suggest higher
stability, which is presumed by the rigid structure of the
phenanthroline moiety.
Photophysical studies
For further research on EL properties of corresponding
europium complexes, their photophysical properties were
investigated. We focused more on the oxadiazole substituted
Thermal stability for deposition
The chemical weight of materials increasing upon the intro-
duction of modification groups usually leads to difficulty in
Table 2 Thermal stabilities and photophysical data of Eu complexes
lmax(abs)/nm
(e/dm3 molꢀ1 cmꢀ1
)
Complexes
Td/1C
Tm/1C
Eu(TTA)3Php
290.5
242
303 (35 971)
341 (33 854)
304 (43 040)
343 (45 710)
269 (31 820)
308 (45 520)
343 (55 030)
266 (30 260)
345 (46 480)
271 (42 210)
345 (40 610)
274 (53 760)
343 (46 980)
275 (40 910)
345 (52 980)
Eu(TTA)3PhoA
Eu(TTA)3PhoB
282.8
327.2
239
261
Eu(TTA)3PhoC
Eu(TTA)3BpoA
Eu(TTA)3BpoB
Eu(TTA)3BpoC
307.9
281.4
285.6
293.9
262
224
227
231
Scheme 1 Synthetic routes, chemical structures and corresponding
abbreviations of the neutral ligands and europium complexes.
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Fig. 3 PL spectra of europium complexes in CH2Cl2 (10ꢀ5 mol Lꢀ1).
Fig. 1 An ORTEP view of the crystal structure of Eu(TTA)3Php with
partial atomic labeling. Thermal ellipsoids are drawn at the 30%
probability level.
Fig.
4
Fluorescence spectra of the ligands (a, b) in CH2Cl2
(10ꢀ5 mol Lꢀ1) and phosphorescence spectra of the related gadolinium
complexes in ethanol (10ꢀ5 mol Lꢀ1) at 78 K (c, d).
Fig. 2 Absorption spectra of europium complexes in CH2Cl2
(10ꢀ5 mol Lꢀ1).
Since the distortion of the symmetry around the central ion causes
an intensity enhancement of the hypersensitive 5D0
ligands (PhoR and BpoR) rather than on Php, which had been
reported before.34,38 The absorption spectra of europium
complexes in CH2Cl2 (10ꢀ5 mol Lꢀ1) at room temperature
are shown in Fig. 2. Similar absorption bands of
Eu(TTA)3PhoR were observed at about 264 nm and
305 nm, while those of Eu(TTA)3BpoR were observed at
275 nm and 322 nm. These broad absorption bands are due
to both TTA and neutral ligands.14 All of the complexes show
the same lowest absorption maximums at about 340 nm and
their low energy absorption edges at 390–400 nm. This distinct
absorption band is probably assigned to the p - p* transition
of the anionic ligand TTA.39
-
7F2
transition and the intensity of the 5D0
-
7F1 transition is
independent of the coordination sphere, the high I7F2/I7F1
indicates that the symmetry of the coordination sphere is low.41
It is known that the sensitization pathway in lanthanide
complexes generally consists of excitation of the ligand into its
singlet excited state, subsequent intersystem crossing to its
triplet state and energy transfer from the triplet state of the
ligand to the lanthanide ion.40,42,43 The singlet energy levels
(ES) and triplet energy levels (ET) of the ligands and the PL
Table 3 Singlet and triplet state energy levels of the ligands and
quantum yields (F) of related europium complexes
The PL spectra of the complexes show characteristic
-
7F0),
europium ion emissions at l = 579 nm (5D0
7
7
588–597 nm (5D0 - F1), 613–620 nm (5D0 - F2), 650 nm
Ligand
ES/cmꢀ1
ET/cmꢀ1
ET–5D0/cmꢀ1
Fa
7
7
7
(5D0 - F3) and 692 nm (5D0 - F4) (Fig. 3). Since the F0
PhoA
PhoB
PhoC
BpoA
BpoB
BpoC
28 329
28 169
26 525
29 070
28 902
28 409
20 492
19 841
19 724
22 472
22 075
21 882
2992
2341
2224
4972
4574
4382
30%
40%
42%
16%
22%
26%
state of Eu3+ is nondegenerate, the D0 - 7F0 transition can
5
not be split by the ligand field. Therefore, the single peak at
579 nm in the emission spectrum indicates that there is only
one luminescent Eu3+ species in solution.40 The splitting of
the 5D0 - 7F1 and 5D0 - 7F2 transition, caused by the ligand
a
Measured in CH2Cl2 by using Ru(bpy)3Cl2 distilled water as a
reference compound (ref. 44).
5
7
field, is about 200 cmꢀ1 and the intensity ratio of D0 - F2
transition and 5D0 7F1 transition (I7F2/I7F1) is about 19.
-
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quantum yields (F) of the related complexes were measured
(Fig. 4) and are listed in Table 3. The singlet energy levels
were determined by fluorescence spectra of the ligands
(10ꢀ5 mol Lꢀ1 in CH2Cl2, room temperature). The
phosphorescence spectra of the related gadolinium complexes
(10ꢀ5 mol Lꢀ1 in ethanol, 78 K) reveal the lowest triplet energy
performances of devices are listed in Table 4. Compared
with previous reports of non-doped OLEDs based on
Eu(TTA)3Phen,11,46
a
higher brightness of 32 cd mꢀ2
(160 mA cmꢀ2) is achieved at a lower applied voltage (14 V).
The EL spectra at high voltages show broad peaks at 660 nm
(Fig. 5). Device A2 was fabricated with a CBP film comple-
menting as a hole-transporting layer (HTL). Compared to
device A1, the intensity of the broad peak at high wavelength
6
levels of the ligands because the lowest excited state P7/2
energy of Gd(III) is too high to accept energy from the ligands.
PhoA with three fluoro substituents as an electron drawing
group shows higher ET (20 492 cmꢀ1) than that of PhoB
(19 841 cmꢀ1) while PhoC with tert-butyl as an electron
pushing group shows lower ET (19 724 cmꢀ1). All of three
ligands show a lower ET than phenanthroline (21 480 cmꢀ1),41
as the p-system of the ligand is extended by the oxadiazole
group. Moreover, the fluorescence peaks of PhoA and PhoC
show hypsochromic and bathochromic shifts compared with
that of PhoB, respectively. This confirms that the addition of
the electron-pushing tert-butyl group or the electron-
withdrawing fluorine substituents on the phenyl ring alters
the electronic density of the p-system; therefore, they reduce or
increase both the singlet and the triplet energy level of the
ligand. A similar phenomenon is observed in bipyridine
derivatives and all of them show a lower ET than that of
bipyridine (22 900 cmꢀ1).44 As all of these complexes have
similar coordination environments, the suitability of the
energy gap between the triplet energy level of the ligand
and the main luminescent energy level of the central ion
(5D0 for Eu3+: 17 500 cmꢀ1) is therefore critical for the
efficiency of the energy transfer. By comparing the energy
gaps and the quantum yields (Table 3), a general tendency is
observed that a smaller energy gap (ET–5D0) leads to a higher
quantum yield of the related complex.
Electroluminescent properties
By introducing TPD and Alq3 as hole and electron transport-
ing materials, respectively, and BCP as a hole blocking
material, EL device A1 was fabricated. Eu(TTA)3(PhoB)
was chose as the emitter for its high PL quantum yield as
well as its good thermal stability. The structures and the
Fig. 5 EL spectra of device A1 (a) and A2 (b) at different voltages.
Table 4 Performances of Eu complexes based OLEDsa
Turn-on
voltage/V
L
Zc
Zp
CIE,
Device
A1
Device structure
[cd m2, V]
[cd Aꢀ1, V]
[lm Wꢀ1, V]
10 cd mꢀ2(x, y)
TPD (30 nm)/Eu(TTA)3PhoB (30 nm)/
BCP (5 nm)/Alq3 (30 nm)
9
32, 14
0.05,10
0.53, 7
3.0, 10
2.56, 9
1.37,17
2.6, 10
8.7, 6
0.017,10
0.24,7
0.65, 0.35
0.65, 0.33
0.65, 0.32
0.63. 0.31
0.63. 0.31
0.65, 0.32
0.64, 0.32
0.65, 0.32
A2
B1
B2
B3
C1
C2
C3
TPD (30 nm)/CBP (5 nm)/Eu(TTA)3PhoB
(30 nm)/BCP (5 nm)/Alq3 (30 nm)
NPB (25 nm)/Eu(TTA)3PhoB:CBP
(3%, 25 nm)/BCP (15 nm)/Alq3 (35 nm)
NPB (25 nm)/Eu(TTA)3PhoC:CBP
(3%, 25 nm)/BCP (15 nm)/Alq3 (40 nm)
NPB (25 nm)/Eu(TTA)3BpoB:CBP
(3%, 25 nm)/BCP (15 nm)/Alq3 (30 nm)
TPD (30 nm)/Eu(TTA)3PhoB:CBP
(2.2%, 20 nm)/BCP(20 nm)/Alq3 (30 nm)
TPD (30 nm)/Eu(TTA)3PhoB:CBP
(4.2%, 20 nm)/BCP (20 nm)/Alq3 (30 nm)
TPD (30 nm)/Eu(TTA)3PhoB:CBP
(7.5%, 20 nm)/BCP (20 nm)/Alq3 (30 nm)
6
67, 14
9
703, 18
421, 20
424, 53
1099, 20
877, 19
1086, 17
0.90,10
0.89, 9
0.26,17
0.8, 10
3.9, 6
9
10
6
5
4
8.7, 5
5.5, 5
a
The data for brightness (L), current efficiency (Zc), and power efficiency (Zp) are the maximum values of devices.
ꢁc
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Fig. 6 EL spectra of device B1, C1 and C2 at similar current
densities.
decreases but an emission centered at 420 nm arises at high
voltages in device A2. Thus, we presume that the broad peak
at 660 nm is attributable to an exciplex of Eu(TTA)3PhoB and
TPD. Moreover, it also demonstrates that the substituted
1,3,4-oxadiazole group is favorable to increase the electron-
transporting ability of the complex, thus the carriers are
mainly recombined between EML and HTL in these devices.
To avoid the exciplex at high voltages, NPB was used
instead of TPD as a hole transporting material. Three
devices B1–B3 with Eu(TTA)3PhoB, Eu(TTA)3PhoC, and
Eu(TTA)3BpoB as dopant emitters in CBP (3%) were fabri-
cated, respectively. All of the EL spectra of these devices show
a strong emission of europium ions at 612 nm while an
emission centered at 420 nm is observed at high voltages,
similar to the spectrum of device A2. This additional purple-
blue emission can be attributed to the direct emission from
CBP.11 In device B1 a maximum brightness of 703 cd mꢀ2 at
18 V, a current efficiency of 3.0 cd Aꢀ1 and a power efficiency
of 0.90 lm Wꢀ1 at 10 V are achieved. In device B2 and B3,
Eu(TTA)3PhoC and Eu(TTA)3BpoB are used instead of
Eu(TTA)3PhoB, respectively. Device B2 shows a similar
performance with a maximum brightness of 421 cd mꢀ2 at
20 V, a current efficiency of 2.56 cd Aꢀ1 and a power efficiency
of 0.89 lm Wꢀ1 at 9 V. However, the current efficiency and the
Fig. 7 EL performance of device C1–C3: brightness (a), current
density (b), power efficiency (c) and current efficiency (d) versus
voltage.
Fig. 8 EL spectra of device C3 at different applied voltages.
higher EL efficiencies. Hence, the power efficiency and the
current efficiency increase from 0.8 lm Wꢀ1 and 2.6 cd Aꢀ1
(device C1) to 5.5 lm Wꢀ1 and 8.7 cd Aꢀ1 (device C3)
(Table 4). Device C3 shows a purer emission that originates
from the europium complex. Even at the highest applied
voltage, only a trace emission at 450 nm is observed (Fig. 8).
This reveals that carriers are well confined within the
emitting layer and the energy transfer between CBP and
Eu(TTA)3PhoB is complete in this device. Thus, a remarkable
performance of device C3 is achieved.
power efficiency of device B3 is 1.37 cd Aꢀ1 and 0.26 lm Wꢀ1
.
This performance is consistent with their different PL
quantum yields (Table 3). Strong roll-off characteristics of
efficiencies at high voltages are observed in all devices (Fig. 7).
The phenomenon is thought to be a result of some processes
quenching the excited states.10,11,47
Using Eu(TTA)3PhoB as the guest emitter with concentrations
from 2.2% to 7.5% in CBP, devices C1–C3 are fabricated.
The structure was ITO/TPD (30 nm)/Eu(TTA)3PhoB:CBP
(X%, 20 nm)/BCP (20 nm)/AlQ (30 nm)/Mg0.9Ag0.1. EL
spectra of device C1 and C2 show a characteristic red emission
of Eu(TTA)3PhoB with a weak emission at 420 nm (Fig. 6).
All of the devices have maximum brightness around
1000 cd mꢀ2. As the concentration of Eu(TTA)3PhoB is
increased from 2.2% (device C1) to 7.5% (device C3), more
excitons are contributed to the lanthanide luminescence
process. A higher PL quantum yield of the lanthanide
complexes (100% theoretical quantum efficiency) leads to
Experimental
Methods and instruments
Alq3 was synthesized according to literature procedures48 and
sublimed twice prior to use. CBP, NPB, TPD and BCP
were purchased from Aldrich and used after sublimation.
Eu(TTA)3Phen and Eu(TTA)3(H2O)2 were synthesized
according to a previous report.49 Chromatographic separation
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was carried out on silica gel (200–300 mesh) for purification.
1
The H NMR spectra were recorded on a Varian 400 MHz
8.23–8.20 (q, 2H), 8.16–8.11 (q, 2H), 7.88–7.85 (d, 1H),
7.72–7.69 (m, 3H), MS (m/z): calcd. for C20H12N4O 324,
found 324. Anal. calcd. for C20H12N4O: C, 74.06; H, 3.73;
N, 17.27. Found: C, 74.09; H, 3.76; N, 17.04. 13C NMR
(400 MHz, CDCl3), 165.99 (C), 164.22 (C), 150.62 (CH),
145.86 (C), 145.55 (C), 143.25 (C), 137.33 (C), 136.29 (C),
131.92 (CH), 129.48 (C), 129.03 (CH), 128.82 (CH), 128.37
(CH), 127.56 (CH), 126.06 (CH), 123.50 (CH), 123.44 (CH),
122.13 (CH).
instrument using Me4Si as an internal reference. Mass spectra
were recorded on a ZAB-HS and BIFLEX III. Elemental
analyses were performed on a VARIO EL analyzer. Thermo-
gravimetric and differential thermal analysis were carried out
on Q600SDT and Q100DSC instruments at an elevation
temperature rate of 10 1C minꢀ1 under 100 mL minꢀ1 nitrogen
flow. EL and PL spectra were recorded with an Edinburgh
Analytical Instruments FLS920 spectrometer after removing
oxygen in the solution by a vacuum technique (3 or 4 freeze–
pump–thaw cycles). Absorption spectra were measured on
a Shimadzu UV-3100 UV-vis spectrometer. Solvents for
reactions were distilled prior to use. Solvents used in photo-
physical experiments were of spectroscopic grade.
2-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-1,10-phenanthro-
line (PhoC). Yield: 60%. 1H NMR (400 MHz, CDCl3),
9.30–9.29 (q, 1H), 8.70–8.67 (d, 1H), 8.47–8.45 (d, 1H),
8.34–8.29 (m, 3H), 7.94–7.88 (q, 2H), 7.94–7.88 (q, 2H),
7.74–7.71 (q, 1H), 7.58–7.56 (m, 2H), 1.39 (s, 9H), MS (m/z):
calcd. for C24H20N4O 380, found 380. Anal. calcd. for
C24H20N4O: C, 75.77; H, 5.30; N, 14.73. Found: C, 75.66;
H, 5.26; N, 14.72. 13C NMR (400 MHz, CDCl3), 166.02 (C),
163.98 (C), 155.44 (C), 150.59 (CH), 145.83 (C), 145.58 (C),
143.30 (C), 137.20 (C), 136.07 (C), 129.32 (C), 128.91 (CH),
128.23 (CH), 127.72 (CH), 125.92 (CH), 125.70 (CH), 123.36
(CH), 122.03 (CH), 120.54 (CH), 34.91 (C), 30.92 (CH3).
General procedure for the synthesis of the neutral ligands
2-(pyridyl)-1,10-phenanthroline, 1,10-phenanthroline 1-oxide
(1) and 2-cyano-1,10-phenanthroline (2) were synthesized
according to reported procedures.33,34 Compound 2 (4.1 g,
20 mmol), sodium azide (2.93 g, 45 mmol) and ammonium
chloride (2.4 g, 45 mmol) in 30 ml of DMF was heated at
100 1C for 24 h. After cooling to room temperature the
mixture was poured into 400 ml of water and acidified with
dilute HCl. The resulting white precipitate was collected,
washed with water, and dried in vacuum at 60 1C, yielding
2-(1H-tetrazol-5-yl)-1,10-phenanthroline (3).50 The benzoyl
chloride derivative (22 mmol) was then added to a solution
of 3 (4.96 g, 20 mmol) in 25 ml of pyridine, and the mixture
was refluxed for 2 h under nitrogen flow. 200 mL of water was
added to the cooled reaction mixture affording a white
precipitate. The precipitate was isolated, washed with water
and dried in vacuum at 60 1C. The 5-(1H-tetrazol-5-yl)-2,20-
bipyridine was synthesized with the same procedure of 3.
Bipyridine derivatives were then synthesized with the same
benzoyl chloride derivatives. The individual yields and important
spectral data of these products are listed below.
5-(5-(2,3,4-trifluorophenyl)-1,3,4-oxadiazol-2-yl)-2,20-bipyridine
1
(BpoA). Yield: 57%. H NMR (400 MHz, CDCl3), 8.73–8.71
(d, 1H), 8.66–8.61 (q, 2H), 8.32–8.30 (d, 1H), 8.06–8.00
(m, 2H), 8.00–7.8 8(t, 1H), 7.40–7.37 (t, 1H), 7.22–7.18
(m, 1H). MS (m/z): calcd. for C18H9F3N4O 354, found 354.
Anal. calcd. for C18H9F3N4O: C, 61.02; H, 2.56; N, 15.81.
Found: C, 61.09; H, 2.58; N, 15.77.
5-(5-phenyl-1,3,4-oxadiazol-2-yl)-2,20-bipyridine (BpoB).
Yield: 75%. 1H NMR (400 MHz, CDCl3), 8.74–8.73
(d, 1H), 8.67–8.63 (t, 2H), 8.33–8.31 (d, 1H), 8.26–8.24
(m, 2H), 8.06–7.02 (t, 1H), 7.95–7.91 (t, 1H), 7.62–7.55
(m, 3H), 7.42–7.39 (t, 1H). MS (m/z): calcd. for C18H12N4O
300, found 300. Anal. calcd. for C18H12N4O: C, 71.99; H, 4.03;
N, 18.66. Found: C, 71.86; H, 4.08; N, 18.28. 13C NMR
(400 MHz, CDCl3), 165.30 (C), 163.85 (C), 156.52 (C),
154.84 (C), 149.10 (CH), 142.85 (C), 137.92 (C), 136.92
(CH), 131.83 (CH), 128.94 (CH), 127.11 (CH), 124.21 (CH),
123.64 (CH), 122.90 (CH), 122.88 (CH), 121.41 (CH).
2-(1H-tetrazol-5-yl)-1,10-phenanthroline (3). Yield: 72%. 1H
NMR (400 MHz, DMSO-d6), 9.44–9.42 (q, 1H), 9.28–9.26 (d, 1H),
8.92–8.90 (d, 1H), 8.65–8.63 (d, 1H), 8.38–8.33 (m, 3H), MS
(m/z): calcd. for C13H8N6 248, found 248.
5-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl)-2,20-bipyridine
5-(1H-tetrazol-5-yl)-2,20-bipyridine. Yield: 78%. 1H NMR
(400 MHz, DMSO-d6), 8.84–8.82 (d, 1H), 8.75–8.74
(d, 1H), 8.60–8.57 (d, 1H), 8.28–8.20 (m, 2H), 8.09–8.05
(t, 1H),7.55–7.54 (t, 1H), MS (m/z): calcd. for C11H8N6 224,
found 224.
1
(BpoC). Yield: 81%. H NMR (400 MHz, CDCl3), 8.73–8.71
(d, 1H), 8.66–8.60 (q, 2H), 8.31–8.29 (d, 1H), 8.18–8.15
(d, 2H), 8.04–8.01 (t, 1H), 7.93–7.88 (t, 1H), 7.59–7.57
(d, 2H), 7.40–7.37 (t, 1H), 1.39 (s, 9H), MS (m/z): calcd. for
C22H20N4O 356, found 356. Anal. calcd. for C22H20N4O: C,
74.14; H, 5.66; N, 15.72. Found: C, 74.09; H, 5.63; N, 15.67.
Anal. calcd. for C18H12N4O: C, 71.99; H, 4.03; N, 18.66.
Found: C, 71.86; H, 4.08; N, 18.28. 13C NMR (400 MHz,
CDCl3), 165.37 (C), 163.64 (C), 156.45 (C), 155.41 (C), 154.86
(C), 149.06 (CH), 142.93 (C), 137.86 (C), 136.87 (CH), 126.94
(CH), 125.90 (CH), 124.15 (CH), 122.82 (CH), 122.74 (CH),
121.37 (CH), 120.81 (CH), 34.69 (C), 30.97 (CH3).
2-(5-(2,3,4-trifluorophenyl)-1,3,4-oxadiazol-2-yl)-1,10-phenanthro-
line (PhoA). Yield: 60%. 1H NMR (400 MHz, CDCl3),
9.31–9.29 (q, 1H), 8.70–8.6 7(d, 1H), 8.51–8.49 (d, 1H),
8.37–8.34 (q, 1H), 8.19–8.13 (m, 1H), 7.97–7.91 (q, 2H),
7.77–7.74 (q, 1H), 7.26–7.18 (m, 1H), MS (m/z): calcd. for
C20H9F3N4O 378, found 378. Anal. calcd. for C20H9F3N4O: C,
63.50; H, 2.40; N, 14.81. Found: C, 63.50; H, 2.49; N, 14.88.
General procedure for the synthesis of europium complexes51
2-(5-phenyl-1,3,4-oxadiazol-2-yl)-1,10-phenanthroline (PhoB).
1
Yield: 80%. H NMR (400 MHz, CDCl3), 9.23–9.22 (q, 1H),
To a 100 ml side-arm flask under nitrogen, 2-thenoyltrifluoro-
ancetone (0.916 g, 3 mmol), 2,20-bipyridine or 1,10-phenanthroline
8.79–8.77 (d, 1H), 8.62–8.60 (d, 1H), 8.58–8.56 (q, 1H),
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(1 mmol) derivative (BpoR or PhoR), sodium hydroxide
(0.12 g, 3 mmol) and 30 ml ethanol were added. The mixture
was stirred at 60 1C, while 10 ml ethanol solution of
EuCl3ꢂ6H2O (0.366 g, 1.00 mol) was added dropwise. The
reaction mixture was heated and stirred at 60 1C for 2 h,
and was then cooled to room temperature. The resulting
precipitate was filtered off, then washed with water and cool
ethanol in turn. The crude product was purified by recrystalli-
zation from ethanol solution. The yields and element analysis
data of these products are presented below.
OLED fabrication and measurement
EL devices were fabricated by sequentially depositing organic
layers using thermal evaporation in one run under high
vacuum (o8 ꢃ 10ꢀ5 Pa) onto a cleaned indium tin oxide
(ITO) glass substrate with a sheet resistance of 7 O sqꢀ1. The
active device area was 3 ꢃ 3 mm. The cathode was made of a
200 nm thick layer of Mg0.9Ag0.1 alloy with an 80 nm thick Ag
cap. The thickness of the deposited layer and the evaporation
speed of the individual materials were monitored with
quartz crystal monitors. Deposition rates were maintained at
0.05–0.2 nm sꢀ1 for organic materials and 0.2–0.3 nm sꢀ1 for
the Mg0.9Ag0.1 alloy. All electrical testing and optical
measurements were performed under ambient conditions.
The EL spectra were measured with a Spectra Scan PR650.
The current–voltage (I–V) and luminance–voltage (L–V)
characteristics were measured with a computer controlled
Keithley 2400 Sourcemeter unit with a calibrated silicon diode.
Eu(TTA)3(Php). Yield: 83%, Anal. calcd. for C41H23EuF9-
N3O6S3: C, 45.90; H, 2.16; N, 3.92. Found: C, 45.79; H, 2.13;
N, 3.93.
Eu(TTA)3(PhoA). Yield: 83%, Anal. calcd. for C44H21EuF9-
N4O7S3: C, 44.27; H, 1.77; N, 4.69. Found: C, 44.19; H, 1.78;
N, 4.78.
Eu(TTA)3(PhoB). Yield: 76%, Anal. calcd. for C44H24EuF9-
N4O7S3: C, 46.36; H, 2.12; N, 4.92. Found: C, 46.44; H, 2.09;
N, 4.97.
Conclusions
In summary, we report the design, synthesis, and EL applica-
tion of several europium complexes with phenanthroline
derivatives or bipyridine derivatives as neutral ligands. Our
results suggest that using tridentate neutral ligand is a viable
strategy for improving the stability of lanthanide complex for
deposition. By incorporating an oxadiazole moiety into the
ligand with further modifying, we achieved novel complexes
with characteristic emissions and high quantum yields,
improved carrier transporting abilities and thermal stabilities
for vapor deposition. These in turn lead to pure EL emissions
and high efficiencies in appropriately configured devices. The
modification of proper tridentate ligands with functional
groups provides a useful guide for future work on lanthanide
based OLEDs.
Eu(TTA)3(PhoC). Yield: 83%, Anal. calcd. for C48H32EuF9-
N4O7S3: C, 48.21; H, 2.70; N, 4.68. Found: C, 48.15; H, 2.74;
N, 4.75.
Eu(TTA)3(BpoA). Yield: 85%, Anal. calcd. for C42H21EuF9-
N4O7S3: C, 43.12; H, 1.81; N, 4.79. Found: C, 43.05; H, 1.85;
N, 4.84.
Eu(TTA)3(BpoB). Yield: 79%, Anal. calcd. for C42H24EuF9-
N4O7S3: C, 45.21; H, 2.17; N, 5.02. Found: C, 45.13; H, 2.18;
N, 4.97.
Eu(TTA)3(BpoC). Yield: 80%, Anal. calcd. for C46H32EuF9-
N4O7S3: C, 47.14; H, 2.75; N, 4.78. Found: C, 47.02; H, 2.80;
N, 4.88.
General procedure for the synthesis of gadolinium complexes
Acknowledgements
2,20-Bipyridine or 1,10-phenanthroline (1 mmol) derivative
(PhoR or BpoR) and GdCl3ꢂ6H2O (0.370 g, 1.00 mol) were
heated and stirred at 60 1C for 2 h, and then cooled to room
temperature. The product was purified by recrystallization
from an ethanol–water mixed solution. The element analysis
data of these products are presented below.
The authors thank the National Basic Research Program
(2006CB601103) and the NNSFC (50772003, 20671006,
20821091, 90922004, 20971006) for financial support.
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494 | New J. Chem., 2010, 34, 487–494