Synthesis, optophysical and electrochemical properties of bipolar-transporting europium(III) complexes with carbazole and oxadiazole units

Article abstract of DOI:10.1016/j.tet.2010.07.003

A series of bipolar-transporting europium(III) complexes containing carbazole and oxadiazole units were synthesized and characterized. Two intense UV absorption bands at around 286 nm and 352 nm, and sharply red emissions peaked at 614 nm were observed for these europium complexes in dichloromethane. Importantly, the bipolar-transporting europium(III) complexes exhibited higher thermal stability, more intense UV absorption at 286 nm and twofold increased photoluminescent quantum yield compared to the reported red chromophore of tri(dibenzoylmethane) (1,10-phenanthroline) europium(III).

Full text of DOI:10.1016/j.tet.2010.07.003

Tetrahedron 66 (2010) 7411e7417
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Tetrahedron
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Synthesis, optophysical and electrochemical properties of bipolar-transporting
europium(III) complexes with carbazole and oxadiazole units
*
Liang Li, Yu Liu , Haipeng Guo, Yafei Wang, Yunbo Cao, Aihui Liang, Hua Tan, Hongrui Qi,
*
Meixiang Zhu, Weiguo Zhu
College of Chemistry, Key Lab of Environment-Friendly Chemistry and Application in Ministry of Education, Xiangtan University, Xiangtan 411105, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of bipolar-transporting europium(III) complexes containing carbazole and oxadiazole units were
synthesized and characterized. Two intense UV absorption bands at around 286 nm and 352 nm, and
sharply red emissions peaked at 614 nmwere observed for these europium complexes in dichloromethane.
Importantly, the bipolar-transporting europium(III) complexes exhibited higher thermal stability, more
intense UV absorption at 286 nm and twofold increased photoluminescent quantum yield compared to the
reported red chromophore of tri(dibenzoylmethane) (1,10-phenanthroline) europium(III).
Ó 2010 Elsevier Ltd. All rights reserved.
Received 20 April 2010
Received in revised form 11 June 2010
Accepted 2 July 2010
Available online 15 July 2010
Keywords:
Europium(III) complexes
Synthesis
Optophysical properties
Bipolar-transporting
Carbazole
1,3,4-Oxadiazole
1. Introduction
doped devices. The first approach is doping europium complexes
into polymers or small molecular compounds with high hole and/
Europium complexes have attracted a great deal of attention in
recent decades because of their highly monochromatic red
emission from f-orbit electron transition of europium(III) ion and
100% theoretical emission quantum efficiency resulting from both
singlet and triplet excitonic emission in organic and polymeric
light-emitting devices (OLEDs and PLEDs).1e6 The best europium
complex-doped OLED was developed by Kido group, which con-
sisted of tri(dibenzoylmethane) (4,7-biphenyl-1,10-phenanthro-
line) europium(___) dopant and presented an external quantum
efficiency (EQE) as high as 7.5% at 0.02 mA/cm2 with corre-
sponding luminous efficiency of 10 lm/W, as well as gave the
maximum brightness of 2797 cd/m2.7 The high-efficiency euro-
pium complex-doped PLEDs with an EQE of about 1.0% were
demonstrated by McGehee and our groups, respectively.8,9 These
performances of the europium complex-doped devices have not
met practical requirement yet.
or electron mobility. The second approach is grafting hole- or
electron-transporting units on anionic or ancillary ligands.11e14 The
grafted hole- and electron-transporting groups (i.e., carrier-trans-
porting groups) mainly contain carbazole,11e13 triphenylamine,14
and oxadiazole groups.15,16 Most of europium(III) complexes were
tuned by grafting one of two carrier-transporting units in the same
ligand. Recently, the first example of modification of europium
complex has been reported by Zhang group, in which both hole-
and electron-transporting units were, respectively, attached on
diketone and phenanthroline.17 This europium(III) complex in-
volving both hole- and electron-transporting functional groups
exhibited an significantly improved device performance with
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. It shows that incorporating
both hole- and electron-transporting units into europium com-
plexes is available for improving electroluminescent properties of
europium complexes.
A promising europium complex used as a red-emitting elec-
troluminescent material should have excellent optophysical and
electrochemical properties at least. In order to further study the
influence of bipolar-transporting units on these properties, we
designed and synthesized another class of novel bipolar-trans-
porting europium complexes by incorporating both hole-trans-
porting carbazole and electron-transporting oxadiazole units into
Low EQE and brightness for the europium complex-doped de-
vices were mainly attributed to the poor carrier-transporting ability
of europium complexes.10 Therefore, two main approaches have
been developed to improve properties of the europium complex-
* 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 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2010.07.003

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L. Li et al. / Tetrahedron 66 (2010) 7411e7417
phenanthroline ligand. The structure of europium(III) complexes
and their synthetic route are shown in Scheme 1. As dibenzoyl-
methane (HDBM) has relatively high PL efficiency and the bipolar-
transporting units have been confirmed to have an ability to
improve optoelectronic property in OLEDs,18 this class of europium
(III) complexes is expected to exhibit improved optophysical
properties. In research on thermal stability, photophysical, and
electrochemical properties of these europium complexes, we found
that this kind of europium complexes with both carbazole and
oxadiazole units presented higher thermal stability, more intense
UV absorption at high-energy area and higher emission quantum
efficiency compared to the reported tri(dibenzoylmethane) (1,10-
phenanthroline) europium(III) [Eu(DBM)3Phen].
a heating rate of 20 ꢀC/min using a PerkineElmer TGA-7 thermal
analyzer. The FT-IR spectra were obtained on a PerkineElmer
spectrum one Fourier transform infrared spectrometer (KBr pellet).
Ultraviolet-visible (UVevis) spectra were measured with a Lambda
25 spectrophotometer. Photoluminescence spectra were conducted
on a PerkineElmer LS55 luminescence spectrometer with a xenon
lamp as a light source. Cyclic voltammetry was carried out on
a CHI660A electrochemical work station in a solution of tetrabu-
tylammonium hexafluorophosphate (Bu4NPF6) (0.1 M) and aceto-
nitrile at a scan rate of 50 mV/s and room temperature under
nitrogen flow protection. These europium complexes were dis-
solved in the above solution (1 mg cmꢁ3). A platinum electrode was
used as the working electrode. A platinum wire was used as the
Scheme 1. Synthetic route of the bipolar-transporting europium complexes.
2. Experimental
2.1. General
counter electrode and a calomel electrode was used as the refer-
ence electrode. Ferrocene was employed as the reference of redox
system.
Carbazole, 1,6-dibromohexane, dibenzoylmethane (HDBM),
tetrahydrofuran (THF), N0N-dimethylformamide (DMF), dichloro-
methane (DCM), ethanol, Eu2O3 (99.5%), K2CO3 and KOH were an-
alytic reagents and purchased from some reagent plants in China.
4-(5-Aryl-1,3,4-oxadiazol-2-yl)phenol derivatives were prepared
according to previous report.19,20 5-Amino-1,10-phenanthroline
was prepared according to the literature procedure.21 All other
reagents were used from commercial sources, unless otherwise
stated.
2.2. Synthesis of 9-(6-bromohexyl)carbazole (1)
50 mL of 50% KOH aqueous solution was added to a solution of
12.0 g (41.1 mmol) carbazole, 9.6 mL (61.5 mmol) 1,6-dibromohex-
ane, a catalytic amount of tetrabutylammonium bromide, and
50 mL toluene under stirring in a 250 mL three-necked flask and
was refluxed for 24 h. The mixture was then evaporated to remove
the solvent under vacuum. The residue was extracted with DCM
(3ꢂ20 mL). The organic layer was washed with water, dried over
anhydrous MgSO4, and then filtered. The filtrate was evaporated
and the residue was purified by silica gel column chromatography
using hexanes/DCM (v/v¼4: 1) as eluent to provide 9.2 g white
solid with a yield of 66.8%. Mp: 70e72 ꢀC. 1H NMR (400 MHz,
All 1H NMR spectra were acquired at a Bruker Dex-400NMR
instrument using CDCl3 as solvent. Elemental analysis was carried
out on a Heraus CHN-Rapid elemental analyzer. Thermogravimetric
analysis (TGA) was performed under a nitrogen atmosphere at

L. Li et al. / Tetrahedron 66 (2010) 7411e7417
7413
CDCl3):
d
ppm 8.09e8.11 (d, J¼8.4 Hz, 2H), 7.39e7.48 (m, 4H),
2.8. Synthesis of 9-(6-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)
phenoxy)hexyl)-9H-carbazole-3-car-baldehyde (HPhOXD6Cz-
CHO)
7.21e7.25 (t, 2H), 4.30e4.33 (t, 2H), 3.34e3.37 (t, 2H), 1.79e1.92 (m,
4H), 1.44e1.53 (m, 4H).
Phosphorus oxychloride 2.85 mL (30.5 mmol) was added
dropwise into a solution of 1.02 mL (45.8 mmol) DMF and 30 mL
1,2-dichloroethane with stirring under ice bath and nitrogen at-
mosphere. The mixture was stirred for another 1 h at room
temperature, and 1.7 g (4.03 mmol) HPhOXD6Cz dissolved in DMF
(10 mL) was added into it. Then the reaction temperature was
raised to 85 ꢀC and the mixture was continued to be stirred for
24 h. The resulting mixture was permitted to cool to room tem-
perature and poured into ice water (50 mL), then extracted with
DCM (3ꢂ30 mL). The resulting organic layer was washed with
water, dried over anhydrous MgSO4, and filtered. The filtrate was
evaporated and the residue was purified by silica gel column
chromatography using hexanes/ethyl acetate (v/v¼3: 1) as eluent
2.3. Synthesis of 9-(6-(4-(5-phenyl-1,3,4-oxadiazol-2-yl)
phenoxy)hexyl)-9H-carbazole (HPhOXD6Cz)
9-(6-Bromohexyl)carbazole 3.4 g (8.0 mmol) was added to
a mixture of 2.4 g (10.0 mmol) 2-(4-hydroxyphenyl)-5-phenyl-
1,3,4-oxadiazol (HPhOXDOH), 4.4 g (30.0 mmol) dispersing K2CO3,
35.0 mL DMF and 0.1 g KI under stirring for 30 min at room tem-
perature. The mixture was refluxed for 24 h under nitrogen atmo-
sphere and allowed to cool to room temperature. Then the reaction
mixture was subsequently poured into ice water (100 mL) and
extracted with DCM (3ꢂ30 mL). The organic layer was washed with
water, dried over anhydrous MgSO4, and then filtered. The filtrate
was evaporated and the residue was purified by recrystallization
using THF and ethanol (v/v¼1: 2) as solvent to provide 1.7 g pale
yellow solid with a yield of 50.8%. Mp: 120e122 ꢀC. 1H NMR
to provide 0.94 g yellow solid with
a
yield of 60.2%. Mp:
ppm 10.11 (s, 1H),
100e100.5 ꢀC. 1H NMR (400 MHz, CDCl3):
d
8.63 (s, 1H), 8.19e8.02 (m, 6H), 7.55e7.47 (m, 6H), 6.99e6.97 (d,
J¼8.8 Hz, 2H), 4.41e4.37 (t, 2H), 4.02e3.98 (t, 2H), 2.01e1.93 (m,
2H), 1.82e1.78 (m, 2H), 1.56e1.49 (m, 4H).
(400 MHz, CDCl3):
d
ppm 8.14e8.10 (m, 4H), 8.07e8.04 (d, J¼9.2 Hz,
2H), 7.54e7.41 (m, 7H), 7.22e7.24 (d, J¼8.4 Hz, 2H), 6.98e6.96 (d,
J¼8.4 Hz 2H), 4.36e4.33 (t, 2H), 4.00e3.96 (t, 2H), 1.97e1.72 (m,
4H), 1.53e1.47 (m, 4H).
2.9. Synthesis of 9-(6-(4-(5-(4-fluorophenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole-2-carbaldehyde
(FPhOXD6Cz-CHO)
2.4. Synthesis of 9-(6-(4-(5-(4-fluorophenyl)-1,3,4-oxadiazol-
2-yl)phenoxy)hexyl)-9H-carbazole(FPhOXD6Cz)
FPhOXD6Cz-CHO was synthesized according to the method
described in Section 2.8. A yellow solid was obtained with a yield of
FPhOXD6Cz was synthesized according to the method described
in Section 2.3. A pale yellow solid was obtained with a yield of
51.2%. Mp: 98e99 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 10.11 (s,
56.8%. Mp: 125e126 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm
1H), 8.64 (s, 1H), 8.19e8.02 (m, 6H), 7.55e7.47 (m, 6H), 6.99e6.97
(d, J¼8.8 Hz, 2H), 4.42e4.38 (t, 2H), 4.02e3.99 (t, 2H), 2.00e1.79 (m,
4H), 1.56e1.49 (m, 4H).
8.13e8.10 (m, 4H), 8.05e8.03 (d, J¼8.6 Hz, 2H), 7.47e7.43 (m, 4H),
7.25e7.22 (m, 4H), 6.98e6.96 (d, J¼8.4 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).
2.10. Synthesis of 9-(6-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)
phenoxy)hexyl)-9H-carbazole-2-carbaldehyde
(MePhOXD6Cz-CHO)
2.5. Synthesis of 9-(6-(4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)
phenoxy)hexyl)-9H-carbazole (MePhOXD6Cz)
MePhOXD6Cz-CHO was synthesized according to the method
described in Section 2.8. A yellow solid was obtained with a yield of
MePhOXD6Cz was synthesized according to the method de-
scribed in Section 2.3. A pale yellow solid was obtained with a yield
49.2%. Mp: 99e100 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 10.11 (s,
of 50.8%. Mp: 122e123 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm
1H), 8.63 (s, 1H), 8.19e8.01 (m, 6H), 7.57e7.47 (m, 3H), 7.34e7.33 (d,
J¼7.9 Hz, 3H), 6.98e6.96 (d, J¼8.6 Hz, 2H), 4.40e4.14 (t, 2H),
4.08e3.80 (t, 2H), 2.45 (s, 3H), 2.09e1.80 (m, 4H),1.58e1.37 (m, 4H).
8.12e8.02 (m, 6H), 7.47e7.41 (m, 4H), 7.26e7.23 (d, J¼9.4 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).
2.11. Synthesis of 9-(6-(4-(5-(4-methoxyphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)hexyl)-9H-carbazole-2-carbaldehyde
(MeOPhOXD6Cz-CHO)
2.6. Synthesis of 9-(6-(4-(5-(4-methoxyphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)hexyl)-9H-carbazole (MeOPhOXD6Cz)
MeOPhOXD6Cz was synthesized according to the method de-
scribed in Section 2.3. A white solid was obtained with a yield of
MeOPhOXD6Cz-CHO was synthesized according to the method
described in Section 2.7. A yellow solid was obtained with a yield of
49.2%. Mp: 123e125 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm
51.2%. Mp: 99e101 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 10.11 (s,
8.12e8.02 (m, 6H), 7.47e7.41 (m, 4H), 7.26e7.23 (d, J¼9.4 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).
1H), 8.64 (s, 1H), 8.19e8.02 (m, 6H), 7.55e7.47 (m, 4H), 7.06e7.04
(d, J¼8.6 Hz, 2H), 6.98e6.96 (d, J¼8.6 Hz, 2H), 4.41e4.38 (t, 2H),
4.02e3.98 (t, 2H), 3.91 (s, 3H), 2.01e1.96 (m, 2H), 1.82e1.75 (m, 4H),
1.56e1.49 (m, 4H).
2.7. Synthesis of 9-(6-(4-(5-(4-tert-butylphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)hexyl)-9H-carbazole (ButPhOXD6Cz)
2.12. Synthesis of 9-(6-(4-(5-(4-tert-butylphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)hexyl)-9H-carbazole-2-carbaldehyde
(ButPhOXD6Cz-CHO)
ButPhOXD6Cz was synthesized according to the method de-
scribed in Section 2.3. A pale yellow solid was obtained with a yield
of 54.0%. Mp: 118e119 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm
8.12e8.10 (d, J¼8.4 Hz, 2H), 8.05e8.03 (m, 4H), 7.55e7.53 (d,
J¼8.4 Hz, 2H), 7.46e7.40 (m, 4H), 7.23e7.25 (d, J¼8.4 Hz, 2H),
6.97e6.95 (d, J¼8.5 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).
ButPhOXD6Cz-CHO was synthesized according to the method
described in Section 2.8. A yellow solid was obtained with a yield of
57.2%. Mp: 97e99 ꢀC. 1H NMR (400 MHz, CDCl3):
d
ppm 10.10 (s,
1H), 8.62 (s, 1H), 8.18e8.00 (m, 6H), 7.55e7.46 (m, 6H), 6.97e6.95

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L. Li et al. / Tetrahedron 66 (2010) 7411e7417
(d, J¼8.7 Hz, 2H), 4.40e4.36 (t, 2H), 4.00e3.97 (t, 2H), 2.04e1.75 (m,
4H), 1.56e1.49 (m, 4H), 1.37 (s, 9H).
6.98e6.96 (d, J¼8.4 Hz, 2H), 4.44e4.40 (t, 2H), 4.01e3.98 (t, 2H),
2.00e1.98 (d, J¼8.7 Hz, 2H), 1.82e1.80 (t, 2H), 1.59e1.52 (m, 13H).
2.13. Synthesis of N-((9-((4-(5-phenyl-1,3,4-oxadiazol-2-yl)
phenoxy)methyl)-9H-carbazol-3-yl)methylene)-1,10-
phenanthrolin-5-amine (HPhOXD6CzPhen)
2.18. Synthesis of Eu(DBM)3(HPhOXD6CzPhen)
Eu2O3 0.036 g (0.1 mmol) was dissolved in 1 mL concentrated
hydrochloric acid was heated to 70e80 ꢀC to form white
EuCl3$6H2O. This europium chloride was dissolved in ethanol
(2.0 mL) for the following procedure. Then, a solution of 0.136 g
(0.6 mmol) HDBM in 5 mL ethanol was added to a 25 mL three-
necked flask and neutralized to pH¼6 with 1 mol/L NaOH
aqueous solution under stirring. The above europium chloride
solution was then added dropwise into the HDBM solution with
stirring. The reaction mixture was stirred for 30 min in room
temperature and a solution of 0.150 g (0.2 mmol) HPhOXD6Cz-
Phen in 2 mL THF was added to it. The mixture was then care-
fully adjusted to pH¼6.5e7 again and continued to be stirred for
5 h at 50 ꢀC. The resulting mixture was allowed to cool to room
temperature and then added dropwise into 20 mL ethanol. The
precipitate was formed and washed with water and ethanol al-
ternately, further was purified by recrystallization with mixing
solvents (THF and ethanol, v/v¼1: 5) to provide 0.16 g yellow
A mixture of 0.1 g (1.05 mmol) 5-amino-1,10-phenanthroline,
0.5 g (1.2 mmol) HPhOXD6Cz-CHO, 15 mL CHCl3, and a catalytic
amount of acetic acid was refluxed for 6 h. After evaporated solvent,
a yellow solid was formed. The solid crude was collected and
washed with methanol, and then was purified by recrystallization
from ethanol to provide 0.32 g yellow solid with a yield of 70%. Mp:
81e82 ꢀC. 1H NMR (400 MHz, CDCl3):
d
ppm 9.24e9.22 (d, J¼8.4 Hz,
1H), 9.13e9.12 (d, J¼8.4 Hz, 1H), 8.87e8.75 (t, 2H), 8.23e8.20 (m,
2H), 8.06e8.01 (q, 5H), 7.36e7.26 (m, 8H), 6.98e6.96 (d, J¼8.6 Hz,
2H), 4.44e4.40 (t, 2H), 4.01e3.98 (t, 2H), 1.99e1.97 (m, 4H),
1.83e1.80 (m, 4H).
2.14. Synthesis of N-((9-((4-(5(4-fluorophenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)methyl)-9H-carbazole-3-yl)
methylene)-1,10-phenanthrolin-5-amine (FPhOXD6CzPhen)
solid with a yield of 60.0%. Mp: 118e119 ꢀC. FT-IR (KBr, cmꢁ1
)
FPhOXD6CzPhen was synthesized according to the method de-
scribed in Section 2.13. A yellow solid was obtained with a yield of
2923, 1610, 1594, 1549, 1517, 1496, 1477, 1458, 1411, 1308, 1219,
1175, 1067, 1024, 837, 808, 738, 723, 690, 520. Anal. Calcd for
EuC90H69N6O8 (1514.96): C, 71.37; H, 4.59; N, 5.55. Found: C,
70.62; H, 5.04; N, 5.38.
62%. Mp: 82e83 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 9.24e9.23 (d,
J¼5.4 Hz, 1H), 9.13e9.12 (d, J¼5.4 Hz, 1H), 8.85e8.75 (t, 2H),
8.23e8.20 (m, 2H), 8.06e8.03 (q, 5H), 7.36e7.26 (m, 8H), 6.98e6.96
(d, J¼8.4 Hz, 2H), 4.44e4.41 (t, 2H), 4.02e3.99 (t, 2H), 2.06e1.98 (d,
J¼8.6 Hz, 4H), 1.82e1.79 (t, 4H).
2.19. Synthesis of Eu(DBM)3(FPhOXD6CzPhen)
Eu(DBM)3(FPhOXD6CzPhen) was synthesized according to the
method described in Section 2.19. A yellow solid was obtained with
a yield of 58.0%. Mp: 118e120 ꢀC. FT-IR (KBr, cmꢁ1) 2924, 1610,
1595, 1550, 1518, 1490, 1477, 1458, 1412, 1384, 1308, 1179, 1069, 841,
744, 737, 518. Anal. Calcd for EuC90H68FN6O8 (1532.5) C, 70.54; H,
4.47; N, 5.48. Found: C, 69.92; H, 4.34; N, 5.35.
2.15. Synthesis of N-((9-((4-(5-p-tolyl-1,3,4-oxadiazol-2-yl)
phenoxy)methyl)-9H-carbazol-3-yl)methylene)-1,10-
phenanthrolin-5-amine (MePhOXD6CzPhen)
MePhOXD6CzPhen was synthesized according to the method
described in Section 2.12. A yellow solid was obtained with a yield
of 68%. Mp: 82e83 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 9.24e9.23
(d, J¼5.4 Hz, 1H), 9.13e9.12 (d, J¼6.4 Hz, 1H), 8.87e8.75 (t, 2H),
8.23e8.20 (m, 2H), 8.06e8.01 (q, 5H), 7.36e7.26 (m, 8H), 6.98e6.96
(d, J¼8.4 Hz, 2H), 4.44e4.40 (t, 2H), 4.01 (s, 2H), 2.45 (s, 3H),
2.00e1.70 (m, 8H).
2.20. Synthesis of Eu(DBM)3(MePhOXD6CzPhen)
Eu(DBM)3(MePhOXD6CzPhen) was synthesized according to
the method described in Section 2.18. A yellow solid was obtained
with a yield of 65.0%. Mp: 117e117.5 ꢀC. FT-IR (KBr, cmꢁ1) 2921,
1610, 1594, 1549, 1517, 1490, 1477, 1457, 1410, 1383, 1308, 1219, 1174,
1067, 814, 737, 722, 519. Anal. Calcd for EuC91H71N6O8 (1528.54) C,
71.50; H, 4.68; N, 5.50. Found: C, 70.60; H, 4.34; N, 5.41.
2.16. Synthesis of N-((9-((4-(5(4-methoxyphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)methyl)-9H-carbazol-3-yl)
methylene)-1,10-phenanthrolin-5-amine
(MeOPhOXD6CzPhen)
2.21. Synthesis of Eu(DBM)3(MeOPhOXD6CzPhen)
MeOPhOXD6CzPhen was synthesized according to the method
described in Section 2.13. A yellow solid was obtained with a yield
Eu(DBM)3(MeOPhOXD6CzPhen) was synthesized according to
the method described in Section 2.19. A yellow solid was obtained
with a yield of 71.0%. Mp: 121e123 ꢀC. FT-IR (KBr, cmꢁ1) 2937, 1611,
1594, 1549, 1517, 1496, 1477, 1458, 1411, 1384, 1219, 1175, 1067, 1024,
835, 807, 723, 608, 515. Anal. Calcd for EuC91H71N6O9 (1544.54): C,
70.76; H, 4.63; N, 5.44. Found: C, 73.52; H, 4.71; N, 4.45.
of 50%. Mp: 84e85 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm 9.36e9.35
(d, J¼5.4 Hz, 1H), 9.13e9.12 (d, J¼6.4 Hz, 1H), 8.85e8.75 (t, 2H),
8.23e8.20 (m, 2H), 8.06e8.01 (q, 5H), 7.36e7.26 (m, 8H), 7.00e6.96
(m, 2H), 4.41e4.38 (t, 2H), 4.01e3.98 (t, 2H), 3.90 (s, 3H), 1.99e1.79
(m, 8H).
2.17. Synthesis of N-((9-((4-(5-(4-tert-butylphenyl)-1,3,4-
oxadiazol-2-yl)phenoxy)methyl)-9H-carbazol-3-yl)
2.22. Synthesis of Eu(DBM)3(ButPhOXD6CzPhen)
methylene)-1,10-phenanthrolin-5-amine(ButPhOXD6CzPhen)
Eu(DBM)3(ButPhOXD6CzPhen) was synthesized according to
the method described in Section 2.18. A yellow solid was obtained
with a yield of 57.0%. Mp: 117e118 ꢀC. FT-IR (KBr, cmꢁ1) 2930, 1610,
1594, 1549, 1517, 1496, 1477, 1458, 1411, 1384, 1219, 1175, 1067, 1023,
840, 808, 737, 721, 520. Anal. Calcd for EuC94H77N6O8 (1570.62): C,
71.88; H, 4.94; N, 5.35. Found: C, 71.62; H, 5.34; N, 5.39.
ButPhOXD6CzPhen was synthesized according to the method
described in Section 2.13. A yellow solid was obtained with a yield
of 74.6%. Mp: 79e81 ꢀC. 1H NMR (400 MHz, CDCl3):
d ppm
9.24e9.23 (d, J¼5.2 Hz, 1H), 9.13e9.12 (d, J¼5.6 Hz, 1H), 8.87e8.75
(t, 2H), 8.23e8.20 (m, 2H), 8.06e8.01 (q, 5H), 7.36e7.26 (m, 8H),

L. Li et al. / Tetrahedron 66 (2010) 7411e7417
7415
3. Results and discussion
3.1. IR absorption spectrum
These bipolar-transporting europium complexes displayed
similar IR spectra. The stretching vibration peaks from the C]O,
C]C, and C]N double bands were observed at 1594 cmꢁ1
,
1517 cmꢁ1, and 1549 cmꢁ1, respectively. It indicates that
b-diketone
ligand of HDBM is coordinated to Eu3þ ion. The skeleton vibration
from phenanthroline ring was found at 1538 cmꢁ1. Two CeH
bending vibration peaks from phenanthroline ring were displayed
at 812 cmꢁ1 and 722 cmꢁ1. Another bending vibration peak from
EueN was also observed at 519 cmꢁ1, which means that the phe-
nanthroline derivatives of RPhOXD6CzPhen are also coordinated to
Eu3þ ion.22 Therefore, the Eu(DBM)3 (RPhOXD6CzPhen) complexes
were confirmed to be formed by IR data.
3.2. Thermal stability
Figure 1. UVevis absorption spectra of europium complexes in DCM.
The thermal properties of europium complexes were de-
termined by TGA and the resulting TGA curves are shown in
Figure S1. All the bipolar-transporting europium complexes pre-
sented an onset degradation temperature (Td) for 5% weight loss as
high as 300 ꢀC. For comparison, the Td data are listed in Table 1.
Obviously, these bipolar-transporting europium complexes
exhibited better thermal stability than the known Eu(DBM)3Phen
complex. This implies that incorporating both carbazole and oxa-
diazole units into the phenanthroline ligand is favorable to enhance
the thermal stability of their corresponding europium complexes.
Furthermore, the substituent groups in oxadiazole units had a dif-
ferent effect on their thermal stability. Incorporating electron donor
substituent group into oxadiazole unit is more available for the
enhancement of thermal stability of the europium complexes.
3.4. Photoluminescence property
The normalized photoluminescence spectra of europium com-
plexes in DCM and neat film at room temperature are shown in
Figure 2a and Figure 2b, respectively. All these bipolar-transporting
europium complexes displayed an intense sharp low-energy emis-
sion peak at 614 nm and a broad weak high-energy emission band
between 350 and 550 nm, in which the low-energy emission peak is
corresponding to 5D0/7F2 transition of Eu(III) ion, and the high-en-
ergy emission band is related to excited states of the phenanthroline
derivatives of RPhOXD6CzPhen or to the ligand-to-metal charge
transfer (LMCT) transitions resulting from the interaction between
Table 1
Thermal stability, optophysical and electrochemical properties of europium complexes
UVevis absorptiona
l
/nm [3max/dm3 molꢁ1 cmꢁ1
]
lemb [nm]
Td [ꢀC]
Eoxc[V]
Eredd[V]
EHOMO [eV]
ELUMOf[eV]
Egc[eV]
e
Complexes
Eu(DBM)3Phen
Complex-1
Complex-2
Complex-3
Complex-4
Complex-5
352(29,945)
614
433, 614
403, 614
429, 614
407, 614
419, 614
297
375
312
375
375
376
1.25
1.25
1.35
1.28
1.26
1.51
ꢁ1.96
ꢁ1.80
ꢁ1.70
ꢁ1.84
ꢁ1.94
ꢁ1.53
ꢁ5.63
ꢁ5.63
ꢁ5.73
ꢁ5.66
ꢁ5.64
ꢁ5.91
ꢁ2.42
ꢁ2.58
ꢁ2.68
ꢁ2.54
ꢁ2.44
ꢁ2.87
3.21
3.05
3.05
3.12
3.20
3.04
292(32,274), 344(17,824)
286(52,479), 352(65,845)
286(38,540), 352(22,234)
286(49,564), 352(43,799)
286(65,423), 352(66,788)
a
Measured in CH2Cl2 at a concentration of 10ꢁ5 mol/L at 298 K.
Measured in CH2Cl2 at 298 K.
b
c
d
e
f
Estimated according to the reduction potential and the UVevis absorption spectrum.
Calculated using equation: Ered¼(EpaþEpc)/2.
ox
EHOMO ¼ ꢁð4:38 þ E Þ eV:
1=2
red
ELUMO ¼ ꢁð4:38 þ E Þ eV:
1=2
3.3. UV absorption property
the central Eu3þ ion and the ligand’s first coordination shell.24,25
While the substituent group is butyl, the Eu(DBM)3(ButPhOXD6Cz-
Phen) complex exhibited maximum high-energy emission band in
DCM and neat film. In contrast, while the substituent group is fluo-
rine, the Eu(DBM)3(FPhOXD6CzPhen) complex exhibit the minimum
high-energy emission band in DCM and neat film. Compared to Eu
(DBM)3Phen, Eu(DBM)3(FPhOXD6CzPhen) exhibited almost identical
emission spectrum in DCM. Therefore, incorporating electron-ac-
ceptor substituent group into oxadiazole unit is available for the en-
hancement of Eu3þ emission in europium complexes. In order to
further understand the emission property of these bipolar-trans-
porting europium complexes, the emission quantum yields were
measured using EuCl3$6H2O (Vf¼0.73% in water) as the standard at
room temperature based on the literature.26 The measured emission
quantum yield values of Eu(DBM)3(Phen), Eu(DBM)3(HPhOXD6Cz-
Phen), Eu(DBM)3(FPhOXD6CzPhen), Eu(DBM)3(MePhOXD6CzPhen),
The normalized UV absorption spectra of europium complexes
in DCM are illustrated in Figure 1. All the bipolar-transporting
europium complexes showed an intense high-energy absorption
band at about 288 nm and a strong low-energy absorption band
at about 352 nm, in which the high-energy absorption band is
attributed to
nanthroline ancillary ligand and low-energy absorption band is
attributed to the singlet-singlet transition of the -diketone
pep transition of the bipolar-transporting phe-
*
p
ep
*
b
ligand.23 Compared to Eu(DBM)3Phen, the bipolar-transporting
europium complexes of Eu(DBM)3(RPhOXD6CzPhen) exhibit
similar low-energy absorption band, but red-shifted high-energy
absorption band. This indicates that these bipolar-transporting
units have a significant effect on UV absorption spectra of their
europium complexes.

7416
L. Li et al. / Tetrahedron 66 (2010) 7411e7417
Figure 2. Photoluminescence spectra of europium complexes in CH2Cl2 (a) and in their neat (b).
Eu(DBM)3(MeOPhOXD6-CzPhen), and Eu(DBM)3(ButPhOXD6Cz-
Phen) were 0.78%, 9.9%, 10.5%, 10.1%, 9.5% and 10.3%, respectively.
These values are about 12 fold higher emission quantum yields than
that of the Eu(DBM)3Phen complex. Among these bipolar-trans-
bipolar-transporting europium complexes in DCM solution. The
improved optophysical properties are available for this class of
europium complexes to become promising red-emitting electro-
luminescent materials. As for research in electroluminescent
properties of these bipolar-transporting europium complexes, we
are in progress.
porting
europium
complexes,
Eu(DBM)3(FPhOXD6CzPhen)
presented highest quantum yield. This indicates incorporating
abipolar-transportingunitintoligandofphenanthrolinecanfacilitate
the enhancement of emission quantum yield of its europium com-
plex. Especially, attaching a fluorine substituent group with electron
acceptor property into the bipolar-transporting unit is more available
to improve emission property.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (20772101 and 20872124), the Science Foun-
dation of Hunan Province (2009FJ2002 and 2007FJ3017), Teachers
Fund for the Doctoral Program of Higher Education of China
(200805301013), the Postgraduate Science Foundation for In-
novation in Hunan Province (S2008 yjscx09 and CX2009B124) and
the Hunan Undergraduate Innovation Experiment Plan.
3.5. Electrochemical property
All bipolar-transporting europium complexes presented re-
versible reduction waves (Ered) at ꢁ1.53 to ꢁ1.94 V versus the sat-
urated calomel electrode (SCE), but their oxidation waves were
disappeared. The oxidation potential (Eox) had to be estimated
based on energy band gap (Eg) and Ered, in which Eg was estimated
based on the UVevis absorption edge. Thus, the highest occupied
molecular orbital (HOMO) and the lowest unoccupied molecular
orbital (LUMO) energy levels (EHOMO and ELUMO) of europium
complexes are calculated according to the empirical relationship
formula: EHOMO¼ꢁe(Eoxþ4.38) and ELUMO¼ꢁe(Eredþ4.38), pro-
posed by Brédas group.27 All the electrochemical data of these
europium complexes are listed in Table 1. The HOMO and LUMO
energy levels of the bipolar-transporting europium complexes are
ꢁ5.63 eV to ꢁ5.91 eV and ꢁ2.44 eV to ꢁ2.87 eV, respectively.
Compared to the Eu(DBM)3Phen complex, these bipolar-trans-
porting complexes exhibited a decrease in the HOMO and LUMO
energy levels. The decreased LUMO energy levels are available to
facilitate electron injection and transportation from cathode to
emitters of europium complexes. Therefore, incorporating both
hole-transporting carbazole and electron-transporting oxadiazole
unit into ligand can significantly tune electrochemical and carrier-
transporting property of its europium complex.
Supplementary data
Figure S1. TG curves of europium complexes recorded in dy-
namic nitrogen atmosphere (50 mL/min) and heating rate of 20 ꢀC/
min. Supplementary data associated with article can be found in
online version at doi:10.1016/j.tet.2010.07.003.
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