Bipolar-transporting dinuclear europium(III) complexes involving carbazole and oxadiazole units: Synthesis, photophysical and electroluminescent properties

Article abstract of DOI:10.1016/j.dyepig.2012.05.015

A class of bipolar-transporting dinuclear europium (III) complexes involving a carbazole and a oxadizaole fragments was synthesized and characterized by elemental analysis, infrared spectro-scopy and thermal analyses. Compared to the mononuclear europium (III) tris(dibenzoylmethane) (1,10-phenanthroline), these dinuclear europium (III) complexes presented better thermal stability, and similar UV-vis absorption and photoluminescence spectra in dichloromethane. Two intense UV-vis absorption bands at around 282 nm and 352 nm, and a characteris-tical red emission peak at 614 nm from the ⁵D₀ → ⁷F₂ transition of the central Eu³⁺ ions were observed under photo-excitation, respectively. Using the Eu₂(DBM)₆(FPhOXD6CzPhen₂) as emitters and a blend of poly (vinylcarbazole) and 2-(4-biphenyl)-5-(4-tert- butylphenyl)-1,3,4-oxadiazole as a host matrix, the single emitting-layer polymer light-emitting devices exhibited saturated red electrolumine- scent emission from Eu³⁺ ion at dopant concentrations from 1 wt % to 8 wt %.

Full text of DOI:10.1016/j.dyepig.2012.05.015

Dyes and Pigments 95 (2012) 322e329
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Dyes and Pigments
journal homepage: www.elsevier.com/locate/dyepig
Bipolar-transporting dinuclear europium(III) complexes involving carbazole and
oxadiazole units: Synthesis, photophysical and electroluminescent properties
*
*
Yu Liu , Jianyu Wang, Yafei Wang, Zhiyong Zhang, Meixiang Zhu, Gangtie Lei, Weiguo Zhu
Department of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in the Ministry of Education, Xiangtan University, Xiangtan 411105, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A class of bipolar-transporting dinuclear europium (III) complexes involving a carbazole and a oxadizaole
fragments was synthesized and characterized by elemental analysis, infrared spectro-scopy and thermal
analyses. Compared to the mononuclear europium (III) tris(dibenzoylmethane) (1,10-phenanthroline),
these dinuclear europium (III) complexes presented better thermal stability, and similar UV-vis
absorption and photoluminescence spectra in dichloromethane. Two intense UV-vis absorption bands
Received 12 January 2012
Received in revised form
15 May 2012
Accepted 15 May 2012
Available online 23 May 2012
at around 282 nm and 352 nm, and a characteris-tical red emission peak at 614 nm from the 5D0
/
7F2
transition of the central Eu3þ ions were observed under photo-excitation, respectively. Using the
Eu2(DBM)6(FPhOXD6CzPhen2) as emitters and a blend of poly (vinylcarbazole) and 2-(4-biphenyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiazole as a host matrix, the single emitting-layer polymer light-emitting
devices exhibited saturated red electrolumine- scent emission from Eu3þ ion at dopant concentrations
from 1 wt % to 8 wt %.
Keywords:
Dinuclear europium (III) complexes
Bipolar-transporting
Photophysical properties
Electroluminescence
Synthesis
Ó 2012 Elsevier Ltd. All rights reserved.
PLEDs
1. Introduction
of carrier-transporting groups were introduced into phenanthro-
line ligand to improve the carrier-transporting properties of their
Organic light-emitting diodes (OLEDs) have been intensively
studied throughout the world owing to their potential application
in the next generation of full-color flat panel displays. For
commercial application, three primary colors of blue, green and red
are basically required. As europium (III) complexes can emit highly
monochromatic red light at around 612 nm with narrow half peak
bandwidths of 5e10 nm [1,2] and have a 100% emission quantum
efficiency theoretically [3], they have developed to be a class of
promising red-emitting materials used in electroluminescent (EL)
devices fabricated by vacuum deposition [4e11] and by solution-
processing [12e16]. However, compared with the green- and
blue-emitting devices, the red-emitting devices using europium
complexes as emitters have not yet exhibited satisfying perfor-
mances although europium complexes presented excellent pho-
toluminescence (PL) properties. In order to improve EL
performance of the europium complexes-doped devices, the first
way is doping europium (III) complexes into polymers or small
molecules with high hole and/or electron mobility [12,17]. The
second approach is introducing hole- or electron-transporting units
into anionic or ancillary ligands [18e32]. As a result, a wide variety
corresponding europium complexes [20e31], in which the hole-
transporting groups mainly contain triphenylamine [21,22] and
carbazole units [23e27], and the electron-transporting groups
mainly are oxadiazole units [28,29]. Both hole- and electron-
transporting units sometime were attached into the mononuclear
europium (III) complexes [20,30e32]. For example, Zhang et al
reported a mononuclear europium (III) complex containing both
oxadiazole and triphenylamine units exhibited a high brightness of
1845 cd/m2 and current efficiency of 2.62 cd/A, as well as a low
turn-on voltage of 5.5 V in its OLEDs [31]. Our group successfully
achieved a series of bipolar-transporting europium (III) complexes
by incorporating both carbazole and oxadiazole units into phe-
nanthroline ligand [32]. These studies showed that these europium
(III) complexes bearing both hole- and electron-transporting
groups presented improved optophysical and EL properties.
However, all of the mononuclear europium complexes exhibited
a concentration quench in their doped devices, which result in
lower efficiency and brightness at higher doping concentrations
and current density. As dinuclear lanthanide complexes have
higher quantum yield than mononuclear lanthanide complexes,
study on dinuclear europium complexes has been developed in
order to obtain high-efficiency europium complexes-doped
OLEDs in recent years [33e38]. For example, Ma group reported
a dinuclear europium (III) complex of (TTA)3Eu(PYO)2Eu(TTA)3
* Corresponding authors. Tel.: þ86 731 58298280; fax: þ86 731 58292251.
E-mail addresses: liuyu03b@126.com (Y. Liu), zhuwg18@126.com (W. Zhu).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.05.015

Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
323
(TTA ¼ thenoyltrifluoroacetonate, PYO ¼ pyridine N-oxide), and its
doped OLEDs exhibited a brightness of 340 cd/m2 at a driving
voltage of 19 V and a current efficiency of 2.4 cd/A at a current
density of 0.14 mA/cm2, respectively [34]. Gong group reported
a dinuclear europium (III) complex of Eu2(2,7-BTFDBC)3(phen)2
with high thermal stability and intense red emission under blue-
light excitation, where 2,7-BTFDBC is 2,7-bis(4’4’40-trifluoro-10,30-
dioxobutyl) carbazole [37]. Do group reported a dinuclear euro-
pium (III) complex of Eu2(tta)6(bpm)2 and its doped PVK-PBD
(30 wt %) only with a brightness of 30.2 cd/cm2 at 14 V [38]. To
our best knowledge, the bipolar-transporting dinuclear europium
(III) complexes incorporated both hole-transporting carbazole and
electron-transporting oxadiazole units have not been reported.
Moreover, the application of dinuclear europium (III) complexes in
OLEDs is rather limited to their photophysical characteristics and
thermal stability.
Eu2(DBM)6(RPhOXD6CzPhen2) is employed as a guest and a blend
of poly(N-vinyl-carbazole) (PVK) along with 2-(4-biphenyl)-5-(4-
tert-butylphenyl)-1,3,4-oxadiaole (PBD) is used as a host matrix.
The device electroluminescent performances were studied. A
maximum luminance of 48.5 cd/m2 at 13.3 V was obtained in the
device at 4 wt % dopant concentration.
2. Experimental
2.1. Reagents and apparatus
Dibenzoylmethane (HDBM), tetrahydrofuran (THF), N,N-
dimethylformamide (DMF), dichloromethane (DCM), ethanol,
Eu2O3 (99.5%), K2CO3 and KOH were analytic reagents and
purchased from some reagent companies in China. 4-(5-Aryl-1,3,4-
oxadiazole-2-yl)phenol derivatives were prepared according to
previous reports [39,40]. 5-Amino-1,10-phenanthroline was
prepared according to the literature procedure [41]. The interme-
diates of RPhOXD6Cz were synthesized according to our previous
work [32]. All of the other reagents were used without further
purification unless otherwise stated.
With the same mind, here we designed and synthesized a series
of bipolar-transporting dinuclear europium (III) complexes of
Eu2(DBM)6(RPhOXD6CzPhen2) containing
a bi-phenanthroline
derivatives linked with both hole-transporting carbazole and
electron-transporting oxadiazole units. The structure of the
Eu2(DBM)6(RPhOXD6CzPhen2)complexes and their synthetic route
are shown in Scheme 1. Their thermal, electrochemical and opto-
physical properties were primary investigated. The single-layer
polymer light-emitting devices (PLEDs) were fabricated, in which
All 1H NMR spectra were acquired at a Bruker Dex-400NMR
instrument using CDCl3 as solvent. Mass spectra (MS) were recor-
ded on a Bruker Autoflex TOF/TOF (MALDI-TOF) instrument using
dithranol as a matrix. Elemental analyses were carried out on
Scheme 1. Synthetic route of the bipolar-transporting dinuclear europium complexes.

324
Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
a Heraus CHN-rapid elemental analyzer. Thermogravimetric anal-
yses (TGA) were performed under a nitrogen atmosphere at
a heating rate of 20 ꢀC/min using a Perkin-Elmer TGA-7 thermal
analyzer. The FT-IR spectra were obtained on a Perkin-Elmer
spectrum one Fourier transform infrared spectrometer (KBr
pellet). Ultravioletevisible (UV-vis) spectra were measured with
a Lambda 25 spectrophotometer. Photoluminescence (PL) spectra
were conducted on a Perkin-Elmer LS55 lumine-scent spectrom-
eter with a xenon lamp as a light source. Cyclic voltammetry was
carried out on a CHI660A electrochemical workstation in a DCM
solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6)
(0.1 M) with a scan rate of 50 mV/s at room temperature (RT) under
nitrogen flow protection.
The single-layer PLEDs with a typical architecture of ITO/
PEDOT:PSS (50 nm)/PVK-PBD (30 wt %): europium (III) complex/LiF
(0.5 nm)/Al (150 nm) were initially fabricated. The doping concen-
trations of the europium (III) complex varied from 1 wt %, 2 wt%, 4 wt
% to 8 wt %. In the devices, ITO is used as the anode, poly(3,4-
ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) is
used as a hole-injection layer, and LiF/Al is employed as a cathode.
The emitting-layer consists of europium (III) complexes and the
PVK-PBD blend. The weight ratio of PBD is 30% in the blend.
Mp: 125.0e127.0 ꢀC. 1H NMR(400 MHz, CDCl3, TMS, dppm):
8.13e8.10 (m, 4H), 8.05e8.03 (d, J ¼ 8.6 Hz, 2H), 7.47e7.43 (m, 4H),
7.25e7.22 (m, 4H), 6.98e6.96 (d, J ¼ 8.4 Hz, 2H), 4.37e4.33 (t, 2H),
4.00e3.97 (t, 2H), 1.96e1.93 (t, 2H), 1.81e1.77 (t, 2H), 1.53e1.47
(m, 4H).
2.2.5. Synthesis of 9-(6-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole-3,6-dicarbaldehyde
[ButPhOXD6Cz(CHO)2]
Phosphoryl chloride (4.39 g, 29.0 mmol) was added dropwise to
a mixture of DMF (3.8 mL) and 1,2-dichloroethane (3.0 mL) at 0 ꢀC.
ButPhOXD6Cz (0.76 g, 1.4 mmol) was then added under a vigorously
stirring and the resulting mixture was reacted for 48 h at 90 ꢀC.
After cooled to RT, the mixture was poured into distilled water
(50 mL) and extracted with chloroform (3 ꢁ 50 mL). The combined
organic layer was dried over anhydrous MgSO4 and filtered. The
filtrate was distilled under reduced pressure to remove the solvent,
and the residue was purified by silica-gel column chromatography
using DCM/ethyl acetate (V/V ¼ 15:1) as eluent to provide a yellow
powder (0.37 g) with a yield of 45%. Mp: 128e130 ꢀC. 1H NMR
(400 MHz, CDCl3), d (ppm): 10.13 (s, 2H), 8.67 (s, 2H), 8.10e8.03 (m,
6H), 7.57e7.55 (t, J ¼ 4.2 Hz, 4H), 6.97e6.95 (d, J ¼ 8.0 Hz, 2H),
4.44e4.41 (t, J ¼ 6.4 Hz, 2H), 3.99e3.98 (t, J ¼ 3.2 Hz, 2H), 2.04e1.96
(m, 4H), 1.81e1.77 (m, 4H), 1.37 (s, 9H).
2.2. Synthesis of ligands and complexes
2.2.1. Synthesis of 9-(6-(4-(5-(4-tert-butylphenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole [ButPhOXD6Cz]
2.2.6. Synthesis of 9-(6-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)
phenoxy)hexyl)-9H-carbazole-3,6-dicarbaldehyde
2-(4-Hydroxyohenyl)-5-(4-4-tert-butylphenyl)-1,3,4-
[CH3PhOXD6Cz(CHO)2]
oxadiazole (2.7 g, 10 mmol), 6-bromo-hexyl-9-H-carbazole (1.8 g,
8.8 mmol), K2CO3 (4.4 g, 30.0 mmol), DMF (35 mL) and KI (0.1 g)
were stirred at 100 ꢀC for 24 h under nitrogen atmosphere. The
resulting mixture was cooled to RT, poured into water (50 mL) and
then extracted with DCM (3 ꢁ 50 mL). The combined organic layer
was dried over anhydrous MgSO4 and filtered. The filtrate was
evaporated to remove the solvent and the residue was purified by
silica-gel column chromatography using petroleum ether/ethyl
acetate (V/V ¼ 5: 1) as eluent to provide a white solid (1.7 g) with
a yield of 60%. Mp: 118.0e119.0 ꢀC. 1H NMR(400 MHz, CDCl3, TMS,
dppm): 8.12e8.10 (d, J ¼ 8.4 Hz, 2H), 8.05e8.03 (m, 4H), 7.55e7.53 (d,
J ¼ 8.4 Hz, 2H), 7.46e7.40 (m, 4H), 7.23e7.25 (d, J ¼ 8.4 Hz, 2H),
6.97e6.95 (d, J ¼ 8.5 Hz, 2H), 4.36e4.33 (t, 2H), 4.00e3.96 (t, 2H),
1.98e1.75 (m, 4H), 1.58e1.48 (m, 4H), 1.37 (s, 9H).
CH3PhOXD6Cz(CHO)2 was prepared according to the synthetic
procedure of ButPhOXD6Cz(CHO)2. A yellow solid was obtained
with a yield of 43%. Mp: 129e130 ꢀC. 1H NMR (400 MHz, CDCl3),
d
(ppm): 10.14 (s, 2H), 8.68 (s, 2H), 8.11e8.00 (m, 6H), 7.58e7.56 (d,
J ¼ 8.2 Hz, 2H), 7.34e7.32 (d, J ¼ 8.0 Hz, 2H) 6.99e6.97 (d, J ¼ 8.8 Hz,
2H), 4.45e4.31 (t, J ¼ 8.4 Hz, 2H), 4.01e3.98 (t, J ¼ 6.2 Hz, 2H), 2.44
(s, 3H), 2.00e1.78 (m, 4H), 1.60e1.49 (m, 4H).
2.2.7. Synthesis of 9-(6-(4-(5-(4-methoxyphenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole-3,6-dicarbaldehyde
[MeOPhOXD6Cz(CHO)2]
MeOPhOXD6Cz(CHO)2 was prepared according to the synthetic
procedure of ButPhOXD6Cz(CHO)2. A yellow solid was obtained
with a yield of 42%. Mp: 131e132 ꢀC. 1H NMR(400 MHz, CDCl3),
d
(ppm): 10.14 (s, 2H), 8.68 (s, 2H), 8.11e8.02 (m, 6H), 7.58e7.56 (d,
2.2.2. Synthesis of 9-(6-(4-(5-(4-methylphenyl)-1,3,4-oxadiazol-2-
yl)phenoxy)hexyl)-9H-carbazole [MePhOXD6Cz]
J ¼ 8.4 Hz, 2H), 7.04e7.02 (d, J ¼ 8.8 Hz, 2H), 6.97e6.95 (d, J ¼ 8.4 Hz,
2H), 4.45e4.41 (t, J ¼ 8.6 Hz, 2H), 4.01e3.98 (t, J ¼ 6.6 Hz, 2H), 3.89
(s, 3H), 2.05e1.78 (m, 4H), 1.82e1.75 (m, 4H), 1.61e1.49 (m, 4H).
MePhOXD6Cz was prepared according to the synthetic proce-
dure of ButPhOXD6Cz. A white solid was obtained with a yield of
58%. Mp: 122.0e123.0 ꢀC. 1H NMR(400 MHz, CDCl3, TMS, dppm):
8.12e8.02 (m, 6H), 7.47e7.41 (m, 4H), 7.26e7.23 (d, J ¼ 9.4 Hz, 2H),
7.04e6.95 (m, 4H), 4.59e4.33 (t, 2H), 4.00e3.96 (t, 2H), 2.44 (s, 3H),
1.98e1.75 (m, 4H), 1.58e1.47 (m, 4H).
2.2.8. Synthesis of 9-(6-(4-(5-(4-fluorophenyl)-1,3,4-oxadiazol-2-
yl)phenoxy)hexyl)-9H-carbazole-3,6-dicarbaldehyde
[FPhOXD6Cz(CHO)2]
FPhOXD6Cz(CHO)2 was prepared according to the synthetic
procedure of ButPhOXD6Cz(CHO)2. A yellow solid was obtained
with a yield of 44%. Mp: 127e128 ꢀC. 1H NMR (400 MHz, CDCl3),
2.2.3. Synthesis of 9-(6-(4-(5-(4-methoxyphenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole [MeOPhOXD6Cz]
d
(ppm): 10.16 (s, 2H), 8.70 (s, 2H), 8.15e8.04 (m, 6H), 7.59e7.57 (d,
MeOPhOXD6Cz was prepared according to the synthetic
procedure of ButPhOXD6Cz. A white solid was obtained with a yield
of 60%. Mp: 122.0e123.0 ꢀC. 1H NMR(400 MHz, CDCl3, TMS, dppm):
8.12e8.02 (m, 6H), 7.47e7.41 (m, 4H), 7.26e7.23 (d, J ¼ 9.4 Hz, 2H),
7.04e6.95 (m, 4H), 4.36e4.33 (t, 2H), 4.00e3.96 (t, 2H), 3.89 (s, 3H),
1.98e1.75 (m, 4H), 1.58e1.47 (m, 4H).
J ¼ 8.4 Hz, 2H), 7.24e7.22 (d, J ¼ 8.4 Hz, 2H), 6.99e6.97 (d,
J ¼ 8.4 Hz, 2H), 4.47e4.43 (t, J ¼ 8.2 Hz, 2H), 4.03e4.00 (t, J ¼ 6.2 Hz,
2H), 2.01e1.80 (m, 4H), 1.56e1.49 (m, 4H).
2.2.9. Synthesis of ButPhOXD6CzPhen2
ButPhOXD6Cz(CHO)2 (0.3 g, 0.5 mmol) dissolved in CHCl3
(20 mL) and a catalytic amount of acetic acid were mixed under
stirring for about 20 min 5-amino-1,10-phenanthroline (0.23 g,
1.2 mmol) dissolved in CHCl3 (25 mL) was then added dropwise.
The resulting mixture was refluxed for another 24 h under nitrogen
protection and distilled to remove excess chloroform under
2.2.4. Synthesis of 9-(6-(4-(5-(4-fluorophenyl)-1,3,4-oxadiazol-2-
yl)phenoxy)hexyl)-9H-carbazole [FPhOXD6Cz]
FPhOXD6Cz was prepared according to the synthetic procedure
of ButPhOXD6Cz. A white solid was obtained with a yield of 60%.

Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
325
reduced pressure. Then the mixture was allowed to cool to RT and
a yellow solid was formed. The solid was collected and purified by
neutral aluminum column chromatography using DCM/ethyl
acetate (V/V ¼ 10:1) as eluent to give a yellow powder (0.25 g) with
calcd. for Eu2C152H123 N9O14 (2603.58): C, 70.12; H, 4.76; N, 4.84.
Found: C, 70.62; H, 4.95; N, 5.12.
2.2.14. Synthesis of Eu2(DBM)6(MePhOXD6CzPhen2)
a yield of 60%. Mp: 155e156 ꢀC. 1H NMR(400 MHz, CDCl3),
d
(ppm):
Eu2(DBM)6(MePhOXD6CzPhen2) was prepared according to the
synthetic procedure of Eu2(DBM)6(ButPhOXD6CzPhen2). A yellow
solid was obtained with a yield of 60.0%. Mp: 175e176 ꢀC. FT-IR
(KBr, cmꢂ1) 2927, 2345, 1611, 1595, 1550, 1518, 1478, 1458, 1412,
1309, 1256, 1220, 1177, 1068, 815, 738, 724, 521, 434. Anal. calcd. for
Eu2C149H117N9O14 (2561.5) : C, 69.87; H, 4.60; N, 4.92. Found: C,
69.60; H, 4.34; N, 4.85.
9.24e9.23 (d, J ¼ 7 Hz, 2H), 9.13e9.12 (d, J ¼ 7 Hz, 2H), 8.88 (s, 4H),
8.29e8.24 (t, J ¼ 9.6 Hz, 4H), 8.06e7.99 (q, 4H), 7.70e7.61 (m, 6H),
7.51e7.49 (d, J ¼ 8.6 Hz, 4H), 7.38 (s, 2H), 6.98e6.96 (d, J ¼ 8.2 Hz,
2H), 4.51e4.48 (t, J ¼ 6.4 Hz, 2H), 4.03e4.00 (t, J ¼ 6.2 Hz, 2H),
2.08e2.02 (m, 2H), 1.85e1.81 (t, J ¼ 8.4 Hz, 2H), 1.35 (s, 9H),
1.25e1.20 (m,4H). TOF-MS: 954.
2.2.10. Synthesis of CH3PhOXD6CzPhen2
2.2.15. Synthesis of Eu2(DBM)6(MeOPhOXD6CzPhen2)
CH3PhOXD6CzPhen2 was prepared according to the synthetic
Eu2(DBM)6(MeOPhOXD6CzPhen2) was prepared according to
the synthetic procedure of Eu2(DBM)6(ButPhOXD6CzPhen2). A
yellow solid was obtained with a yield of 58%. Mp: 178e179 ꢀC. FT-
IR (KBr, cmꢂ1) 2930, 2344, 1610,1595, 1550, 1517, 1494, 1478, 1458,
1411, 1308, 1220, 1173, 1068, 1025, 839, 811, 723, 609, 519, 435. Anal.
calcd. for Eu2C149H117N9O15 (2577.5): C, 69.43; H, 4.58; N, 4.89.
Found: C, 70.02; H, 4.71; N, 4.85.
procedure of ButPhOXD6CzPhen2. A yellow solid was obtained with
a yield of 56%. Mp: 157e159 ꢀC. 1H NMR (400 MHz, CDCl3),
d (ppm):
9.25e9.23 (d, J ¼ 7.0 Hz, 2H), 9.15e9.14 (d, J ¼ 7.2 Hz, 2H), 8.89 (s,
4H), 8.32e8.26 (t, J ¼ 10.2 Hz, 4H), 8.10e7.94 (m, 4H), 7.70e7.61 (m,
6H), 7.51e7.49 (d, J ¼ 8.6 Hz, 4H), 7.39 (s, 2H), 6.98e6.96 (d, J ¼ 8 Hz,
2H), 4.51e4.48 (t, J ¼ 6.4 Hz, 2H), 4.03e4.00 (t, J ¼ 6.6 Hz, 2H),
2.08e2.02 (m, 2H), 1.85e1.81 (t, J ¼ 8.2 Hz, 2H), 1.35(s, 3H),
1.25e1.20 (m, 4H). TOF-MS: 912.
2.2.16. Synthesis of Eu2(DBM)6(FPhOXD6CzPhen2)
Eu2(DBM)6(FPhOXD6CzPhen2) was prepared according to the
synthetic procedure of Eu2(DBM)6(ButPhOXD6CzPhen2). A yellow
solid was obtained with a yield of 62.0%. Mp: 174e176 ꢀC. FT-IR
(KBr, cmꢂ1) 2927, 2343, 1611, 1595, 1550, 1518, 1494, 1478, 1458,
1412, 1310, 1176, 1068, 811, 745, 723, 520, 436. Anal. calcd. for
Eu2C148H114FN9O14 (2565.47): C, 69.29; H, 4.48; N, 4.91. Found: C,
69.52; H, 4.34; N, 4.86.
2.2.11. Synthesis of MeOPhOXD6CzPhen2
MeOPhOXD6CzPhen2 was prepared according to the synthetic
procedure of ButPhOXD6CzPhen2. A yellow solid was obtained with
a yield of 50%. Mp: 158e159 ꢀC. 1H NMR(400 MHz, CDCl3),
d (ppm):
9.26e9.25 (d, J ¼ 2.4 Hz, 2H), 9.15e9.14 (d, J ¼ 2.4 Hz, 2H), 8.89 (s,
4H), 8.30e8.26 (t, J ¼ 8.6 Hz, 4H), 8.06e7.99 (m, 6H), 7.72e7.62 (m,
6H), 7.39 (s, 2H), 6.98e6.96 (t, J ¼ 4.2 Hz, 4H), 4.53e4.49 (t,
J ¼ 8.8 Hz, 2H), 4.04e4.02 (t, J ¼ 4.6 Hz, 2H), 3.88 (s, 3H), 2.09e1.83
(m, 8H). TOF-MS: 928.
3. Results and discussion
3.1. IR absorption spectrum
2.2.12. Synthesis of FPhOXD6CzPhen2
FPhOXD6CzPhen2 was prepared according to the synthetic
These bipolar-transporting dinuclear europium complexes dis-
played similar IR spectra in Fig. 1. Three stretching vibration peaks
from the C]O, C]C and C]N double bands, company -ing with
a bending vibration in plane from the CeH bond, are observed at
1595 cmꢂ1, 1518 cmꢂ1, 1610 cmꢂ1 and 1410 cmꢂ1, respectively. It
procedure of ButPhOXD6CzPhen2. A yellow solid was obtained with
a yield of 58%. Mp: 154e156 ꢀC. 1H NMR(400 MHz, CDCl3),
d (ppm):
9.27e9.26 (d, J ¼ 2.8 Hz, 2H), 9.16e9.15 (d, J ¼ 2.8 Hz, 2H), 8.90
(s, 4H), 8.31e8.26 (t, J ¼ 9.6 Hz, 4H), 8.12e8.05 (m, 4H), 7.72e7.69
(m, 4H), 7.65e7.63 (t, J ¼ 4.2 Hz, 4H), 7.40 (s, 2H), 7.24e7.17
(t, J ¼ 12.6 Hz, 2H), 6.99e6.98 (d, J ¼ 7.2 Hz, 2H), 4.51e4.48
(t, J ¼ 6.6 Hz, 2H), 4.52e4.48 (t, J ¼ 8.2 Hz, 2H), 4.05e4.02
(t, J ¼ 6.6 Hz, 2H), 2.09e1.82 (m, 8H). TOF-MS: 916.
indicates that
ion. The strong bending vibration peaks out of plane from the CeH
bonds in phenanthroline ring are displayed at 811 and 723 cmꢂ1
b
-diketone ligand of HDBM is coordinated to Eu3þ
.
2.2.13. Synthesis of Eu2(DBM)6(ButPhOXD6CzPhen2)
Eu2O3 (0.037 g, 0.12 mmol) was dissolved in concentrated
hydrochloric acid (1 mL) at 80 ꢀC and formed white EuCl3$6H2O.
This europium chloride was cooled to RT and further dissolved in
ethanol (3.0 mL) for the following procedure. After a solution of
HDBM (0.161 g, 0.72 mmol) in 6 mL ethanol was neutralized to
pH ¼ 6.5e7 with 1 mol/L NaOH aqueous solution in a 25 mL
three-necked flask under stirring, the above europium chloride
solution was added dropwise into the HDBM solution. The
reaction mixture was stirred for 30 min under RT and a solution
of ButPhOXD6CzPhen2 (0.1 g, 0.1 mmol) in THF (2.0 mL) was
added. The resulting mixture was then carefully adjusted to
pH ¼ 6.5e7 again with 1 mol/L NaOH aqueous solution and
continued to be stirred for 12 h at 50 ꢀC. After cooled to RT, the
mixture was added dropwise into 25 mL ethanol to form
precipitate. The precipitate was collected and washed with water
and ethanol alternately, further purified by recrystallization with
a mixing solvent (THF and ethanol, V/V ¼ 1:5) to provide a yellow
solid (0.17 g) with a yield of 65.0%. Mp: 174e175 ꢀC. FT-IR (KBr,
cmꢂ1) 2926, 2342, 1610, 1595, 1550, 1517, 1478, 1458, 1411, 1308,
1220, 1176, 1068, 1024, 838, 810, 737, 723, 690, 521, 436. Anal.
Fig. 1. Infrared spectra of europium complexes.

326
Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
Fig. 2. TG curves of europium complexes recorded in dynamic nitrogen atmosphere
Fig. 3. Normalized UV-vis absorption spectra of europium complexes in dichloro-
methane solution (1.0 ꢁ 10ꢂ5 mol/L).
(50 mL/min) and heating rate of 20 ꢀC/min.
The additional new weak vibration bands at 520 and 435 cmꢂ1 are
ascribed to Eu-N and Eu-O vibrations, respectively. It further means
that the phenanthroline derivatives of RPhOXD6CzPhen2 were
together coordinated to Eu3þ ion [42]. The IR data of these euro-
pium (III) complexes were according with those of the other
europium (III) complexes reported in the previous reference [43].
Therefore, the Eu2(DBM)6(RPhOXD6CzPhen2) were confirmed to be
formed by the IR data.
3.3. UV absorption property
The normalized UV-vis absorption spectra of the bipolar-
transporting dinuclear complexes of Eu2(DBM)6(RPhOXD6Cz-
Phen2) and the mononuclear complex of Eu(DBM)3Phen in dilute
DCM solution (10ꢂ5 mol Lꢂ1) are illustrated in Fig. 3. All the bipolar-
transporting dinuclear europium complexes show an intense high-
energy absorption band at about 282 nm and a strong low-energy
absorption band at about 352 nm, in which the former absorption
3.2. Thermal stability
band is attributed to
derivatives of RPhOXD6CzPhen2 and the latter absorption band is
attributed to * transition of the DBM anion [28]. Compared to
Eu(DBM)3Phen, these bipolar-transporting europium complexes
exhibited a similar low-energy absorption band, and a red-shifted
high-energy absorption band [14]. This indicates that the incor-
porated bipolar-transporting units have a significant effect on UV-
vis absorption spectra of their europium complexes.
pep* transition of the bi-phenanthroline
The thermal properties of europium complexes were deter-
mined by thermogravimetric (TG) analysis under N2 stream with
a scanning rate of 20 ꢀC/min and their TG curves are shown in
Fig. 2. All these bipolar-transporting dinuclear europium
complexes of Eu2(DBM)6(RPhOXD6CzPhen2) exhibited high
thermal stability. Their decomposition temperature (Td) values
varied from 349.0 ꢀC to 374.0 ꢀC, which corresponds to a 5%
weight loss (Table 1). Compared to the known europium (III)
pep
tris(dibenzoylmethane)(1,10-phenanthroline)
[Eu(DBM)3Phen]
3.4. Photoluminescence property
(Td ¼ 297.0 ꢀC), these dinuclear europium complexes exhibited
better thermal stability. It implies that incorporating
a
bi-
The normalized PL spectra of europium complexes in DCM at RT
are shown in Fig. 4. All these bipolar-transporting dinuclear euro-
pium complexes display an intense sharp low-energy emission peak
at 614 nm and a broad high-energy emission band between 350 and
500 nm, in which the low-energy emission peak is corresponding to
the 5D0 / 7F2 transition of Eu (III) ion, and the high-energy emission
band is assigned to excited states of the bi-phenanthroline deriva-
phenanthroline derivative of RPhOXD6CzPhen2 with both carba-
zole and oxadiazole units into europium complex is favorable to
enhance its thermal stability. Furthermore, the substituent groups
in the oxadiazole unit have a different effect on the thermal
stability. An electron-donoring substituent is more efficient to
improve the thermal stability of its europium complexes.
Table 1
Thermal, optophysical and electrochemical properties of europium complexes.
UV-vis absorptiona
l
/nm
)
lemb (nm)
Td (ꢀC)
Eox (V)
Eredd (V)
EHOMOe(eV)
ELUMOf (eV)
Eg (eV)
c
c
Complexes
(εmax/dm3molꢂ1cmꢂ1
Eu(DBM)3Phen
352 (29945)
614
297
374
371
361
349
1.25
1.93
1.91
2.10
2.00
ꢂ1.96
ꢂ1.65
ꢂ1.65
ꢂ1.61
ꢂ1.68
ꢂ5.63
ꢂ6.31
ꢂ6.29
ꢂ6.84
ꢂ6.38
ꢂ2.42
ꢂ2.73
ꢂ2.73
ꢂ2.77
ꢂ2.70
3.21
3.58
3.56
3.71
3.68
Eu2(DBM)6(ButPhOXD6CzPhen2)
Eu2(DBM)6(MePhOXD6CzPhen2)
Eu2(DBM)6(MeOPhOXD6CzPhen2)
Eu2(DBM)6(FPhOXD6CzPhen2)
286 (179445), 352 (158196)
286 (134540), 352 (139426)
286 (130828), 352 (137503)
286 (128755), 352 (125415)
424, 614
421, 614
435, 614
420, 614
a
Measured in CH2Cl2 at a concentration of 10ꢂ5 mol/L at 298 K.
Measured in CH2Cl2 at 298 K.
b
c
d
e
f
Estimated according to the reduction potential and the UV-vis absorption spectrum.
Calculated using equation: E1/2 ¼ (Epa þ Epc)/2.
EHOMO ¼ ꢂ(4.38 þ Eox1/2) eV.
ELUMO ¼ ꢂ(4.38 þ Ered1/2) eV.

Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
327
europium complexes. The reduction and the oxidation peaks
occurred at around ꢂ1.55 V and 2.0 V, respectively. The higher
oxidation peaks were observed in these dinuclear europium
complexes rather than the mononuclear Eu(DBM)3Phen. It means
that these dinuclear europium complexes containing bipolar-
transporting units are more difficult to be oxidized than the
mononuclear europium complex. The highest occupied molecular
orbital (HOMO) and the lowest unoccupied molecular orbital
(LUMO) energy levels (EHOMO and ELUMO) are calculated according
to the following empirical formula: EHOMO ¼ ee(Eoxþ4.38) and
ELUMO ¼ ee(Eredþ4.38), proposed by Brédas group [46]. The
received electrochemical data of europium complexes are listed in
Table 1. The HOMO and LUMO energy levels of these bipolar-
transporting complexes are ꢂ6.29 to ꢂ6.48 eV and ꢂ2.70
to ꢂ2.77 eV, respectively. Compared to the Eu(DBM)3Phen, these
dinuclear complexes exhibited a significant decreased HOMO and
LUMO energy levels. The decreased LUMO energy levels are avail-
able to facilitate electrons injection and transportation from
cathode to emitters of europium complexes. Therefore, incorpo-
rating both hole-transporting carbazole and electron- transporting
oxadiazole units into the bi-phenanthroline derivatives can
significantly tune electrochemical and carrier-transporting prop-
erties of their dinuclear europium complexes.
Fig. 4. Normalized photoluminescence spectra of europium complexes in dichloro-
methane solution (1.0 ꢁ 10ꢂ5 mol/L, lex ¼ 350 nm).
tives of RPhOXD6CzPhen2 or/and the ligand-to-metal charge
transfer (LMCT) transition resulting from the interaction between
the central Eu3þ ion and the ligand [38]. Compared to
Eu(DBM)3Phen, these bipolar-transporting dinuclear europium
complexes exhibited an enhanced high-energy emission. Further-
more, the Eu2(DBM)6(ButPhOXD6CzPhen2) and Eu2(DBM)6(FPh-
OXD6CzPhen2) presented maximum and minimum high-energy
emission band in DCM, respectively. Therefore, incorporating an
electron-accepting substituent group into oxadiazole unit is avail-
able for inhibiting the emission of its bi-phenanthroline derivatives
in the europium complexes. In order to further understand the
emissionproperty of these bipolar-transporting dinucleareuropium
(III) complexes, the emission quantum yields were measured using
EuCl3$6H2O (Vf ¼ 0.073% in water) as standard at RT in DCM based
on the literatures [44]. The measured emission quantumyield values
3.6. Electroluminescent property
To investigate EL property of these dinuclear europium
complexes-doped PLEDs, we used the Eu2(DBM)6(FPhOXD6Cz-
Phen2) as emitter and made the single-layer PLEDs with a configu-
ration of ITO/PEDOT:PSS (50 nm)/PVK-PBD:europium(III) complex/
LiF (0.5 nm)/Al (150 nm) because Eu2(DBM)6(FPhOXD6CzPhen2) has
the highest PL quantum yield. The normalized EL spectra of the
Eu2(DBM)6(FPhOXD6CzPhen2)-doped PVK-PBD devices are shown
in Fig. 5 with different doping concentrations from 1 wt % to 8 wt % at
voltage of 13 V. Extremely sharp red emission centered at around
616 nm is observed for these devices, which is attributed to the
europium (III) ion’s characteristic emission. At the same time,
a minor high-lying emission band around 400e480 nm is also
observed at doping concentrations of 1 wt % and 2 wt %, but almost
disappear at doping concentrations of 4 wt % and 8 wt %. This high-
lying emission is attributed to the emission from exciplex between
of
Eu(DBM)3Phen,
Eu2(DBM)6(ButPhOXD6CzPhen2),
Eu2(DBM)6(MeOPhOXD6CzPhen2)
Eu2-
(DBM)6(MePhOXD6CzPhen2),
and Eu2(DBM)6(FPhOXD6CzPhen2) were 0.78%, 10.3%, 10.1%, 9.5%
and 10.5%, respectively. It is obvious that these dinuclear europium
complexes exhibited twelvefold higher emission quantum yields
than the mononuclear Eu(DBM)3Phen complexes. Among these
dinuclear europium complexes, the Eu2(DBM)6(FPhOXD6CzPhen2)
presented the highest quantum yield. This indicates that the bi-
phenanthroline derivatives bridged with a bipolar-transporting
unit can enhance emission quantum yield of their europium
complexes [45]. Especially, attaching an electron-accepting fluorine
substituent into the oxadiazole unit is more available to improve
emission efficiency of its bipolar-transporting dinuclear europium
complex.
3.5. Electrochemical property
Cyclic voltammetry (CV) experiments were conducted to detect
the electrochemical properties of the europium complexes. All
measurements were carried out at RT in a nitrogen-saturated DCM
solution of tetrabutylammonium hexafluorophosphate (Bu4NPF6,
0.1 M) at a scan rate of 50 mV/s. A platinum rod and a platinum wire
were used as the working and counter electrode, respectively. A
calomel electrode was used as the reference electrode. Ferrocene
was employed as a reference of redox system. The measured
concentration of europium complexes was about 2 ꢁ 10ꢂ4 M. A
quasi-reversible reduction wave was observed for these dinuclear
Fig. 5. Normalized EL spectra of the Eu2(DBM)6(FPhOXD6CzPhen2)-doped devices at
different dopant concentrations from 1 wt % to 8 wt %. Inset: CIE 1931 chromaticity
diagrams of these corresponding devices.

328
Y. Liu et al. / Dyes and Pigments 95 (2012) 322e329
4 wt % dopant concentration under a driving voltage of 13.5 V.
Compared to the unfunctionlized dinuclear europium complex of
Eu2(dbm)6(bpm), this bipolar-transporting dinuclear europium
complex of Eu2(DBM)6(FPhOXD6CzPhen2) exhibited
a higher
brightness in the PLEDs with the same device structure. The results
further indicate that an introduction of bipolar-transporting groups
into neutral ligand can improve the performance of its europium
(III) complex-doped PLEDs.
4. Conclusion
In summary, a series of bipolar-transporting dinuclear europium
complexes of Eu2(DBM)6(RPhOXD6CzPhen2) with both carbazole
and oxadiazole units were obtained. These dinuclear europium
complexes presented higher thermal stability, more intense UV-vis
absorption at high-energy area as well as higher emission quantum
yield in DCM solution. Compared to the reported the mononuclear
Eu(DBM)3Phen, a red sharp emission at 614 nm was obtained for
the Eu2(DBM)6(FPhOXD6CzPhen2)-doped PVK devices at different
dopant concentrations and applied voltages. The results demon-
strate that incorporation of a bipolar-transporting moiety into the
europium (III) complex can improves the performance of the PLEDs.
Fig. 6. Normalized EL spectra of the Eu2(DBM)6(FPhOXD6CzPhen2)-doped device at
2
wt % dopant concentration under different bias. Inset: CIE 1931 chromaticity
diagrams of the corresponding device.
Eu2(DBM)6(FPhOXD6CzPhen2) and PVK [29]. The inset Fig. 5 shows
the CIE coordinates diagram for these devices at doping concen-
trations from1 wt % to8 wt %. Significantly different CIE coordinateis
displayed with increasing the doping concentrations from 1 wt % to
8 wt %.
To further investigate EL property of the Eu2(DBM)6(FPhOXD6Cz-
Phen2)-doped devices, we measured the EL spectra of the device at
2 wt% dopantconcentrationunderdifferentapplied voltages,whichis
showninFig. 6. The ELspectraexhibitlittlechangeatthegivenapplied
voltages, which contain a sharp red emission at 614 nm with a full
width at half maximum (FWHM) of 8 nm. The corresponding CIE
coordinates also present a minor change from (0.56, 0.32) to (0.53,
0.34) with increasing driving voltages from 10 V to 16 V, as shown in
Fig. 6 (inset). This means that stable red emissions were obtained in
the Eu2(DBM)6(FPhOXD6CzPhen2)-doped devices at 2 wt % dopant
concentration under different bias.
Acknowledgments
The authors thank the National Natural Science Foundation of
China (Project No.20872124, 50973053 and 21172187), the
Specialized Research Fund and New Teacher Fund for the Doctoral
Program of Higher Education (20094301110004, 200805301013),
the Open Project Program of Key Laboratory of Environmentally
Friendly Chemistry and Applications of Ministry of Education
(09HJYH06), the Scientific Research Fund of Hunan Provincial
Education Department (11CY023, 10A119, 10B112), the Hunan
Provincial Natural Science Foundation of China (11JJ3061), the
Postgraduate Science Foundation for Innovation in Hunan Province
(CX2011263) and Hunan Undergraduate Investigated-Study and
Innovated-Experiment Plan for the financial support of this
research.
Fig. 7 exhibits the current density-voltage-brightness (J-V-B)
curves of the Eu2(DBM)6(FPhOXD6CzPhen2)-doped devices at
different doping concentrations from 1 wt % to 8 wt %. The
maximum brightness of 48.5 cd/m2 was obtained in the device at
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