Synthesis, structures, photo- and electro-luminescent properties of novel oxadiazole-functionlized europium(iii) benzamide complexes

Article abstract of DOI:10.1039/b906470b

With N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide (HL₁) as the first ligand, three new luminescent europium complexes Eu₂Na ₂(L₁)₆(OH)₂·2C ₂H₅OH·2CHCl₃ (1),

Full text of DOI:10.1039/b906470b

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PAPER
www.rsc.org/dalton | Dalton Transactions
Synthesis, structures, photo- and electro-luminescent properties of novel
oxadiazole-functionlized europium(III) benzamide complexes†
Fuli Zhang,a Yanhui Hou,b Chenxia Du*a and Yangjie Wua
Received 1st April 2009, Accepted 2nd June 2009
First published as an Advance Article on the web 16th July 2009
DOI: 10.1039/b906470b
With N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide (HL1) as the first ligand, three new luminescent
europium complexes Eu2Na2(L1)6(OH)2·2C2H5OH·2CHCl3 (1), Eu(L1)3(phen) (phen =
1,10-phenanthroline) (2) and Eu2Na2(L1)6(L2)2(OH)2·8CHCl3 [L2 = 2,5-bis(2-pyridyl)-1,3,4-oxadiazole]
(3) have been prepared and structurally characterized. Each ligand L1 coordinates with a Eu3+ ion
through a carbonyl oxygen atom and oxadiazol nitrogen atom, the coordination geometry at europium
is a distorted square antiprism. Complexes 1 and 3 have centrosymmetric dimeric structural features
with OH- as the bridging ligand, while 2 is mononuclear with phen as the neutral ligand. All these
complexes show efficient red emissions typical of Eu3+ ions at room temperature, the overall quantum
yields of complexes 1, 2 and 3 were 0.15, 0.37 and 0.23, respectively. Using Eu(L1)3(phen) as the
emitting material, an electroluminescent device with the structure ITO/TPD (30 nm)/Eu(L1)3(phen):
TPD (1 : 3, 50 nm)/AlQ (30 nm)/Mg0.9Ag0.1/Ag was fabricated. The device emits sharp red light
originating from europium complex, demonstrating that Eu(L1)3(phen) is a promising red emitter with
good electron-transporting property.
complex could be greatly improved by using oxadiazole-modified
Introduction
b-diketone as the chelating ligand.9 This and other work seem
The design of efficient luminescent lanthanide(III) complexes
to indicate that the photo- and electro-luminescent properties of
continues to be an active area of research.1 One of the notable
recent interests in luminescent lanthanide complexes concerns
their application in electroluminescent display.2 Among the rare
earth complexes studied, europium(III) complexes appear to be
the most attractive in view of the high photoluminescent (PL)
efficiency and the narrow band red-emission ability, which are
widely exploited in full-colour displays.3 There are usually two
general approaches for attaining high luminescence. The first
involves the addition of an organic ligand capable of efficiently
transferring the energy absorbed by chromophores to the rare
earth ion. The second approach is to reduce the non-radiation
processes of the centre ion’s excited state by protecting the ion
from O–H oscillators. Much effort has been devoted to designing
such ligands, and a great variety of highly photoluminescent eu-
ropium(III) complexes are now available.4,5 However, high photo-
luminescence (PL) efficiency of europium(III) complexes does not
translate into electroluminescence (EL) efficiency of a comparable
magnitude. Several groups have studied using these lanthanide
complexes as emitters in electroluminescent devices with limited
success.6,7 It is generally believed that the poor carrier transporting
ability of the lanthanide complexes is a contributing factor.8
Wang et al. first reported that EL efficiency of lanthanide(III)
complexes could be greatly improved by the ligand substituents.10,11
Thus, manipulation of oxadiazole and other groups with good
electron-transporting ability may represent a promising venue
for the development of highly electroluminescent europium(III)
complexes.
N-(5-phenyl-1,3,4-oxadiazol-2-yl)-substituted amides have
been reported for their synthesis,12 electrochemical character13
and biological activities.14 To our knowledge, however, there
are no literatures about their coordination properties with
metal ions although this kind of compounds contain two
potential coordinating groups: a carbonyl and an oxadiazol.
In this paper, we synthesized three new europium complexes,
using N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide (HL1) as the
first ligand and 1,10-phenanthroline (phen), 2,5-bis(2-pyridyl)-
1,3,4-oxadiazole (L2) as the second ligand. Compared with
other oxadiazole-modified ligands of europium(III) complexes,
ligand HL1 is unique in that the oxadiazole group coordinates
directly with the centre ion, and what is more, it is very cheap
and convenient to synthesize. The complexes exhibit efficient
photoluminescence. Their binding nature was fully characterized
by X-ray structure analysis. The electroluminescent property of
Eu(L1)3(phen) was also investigated.
aDepartment of Chemistry, Zhengzhou University, Zhengzhou, P. R. of
China. E-mail: dcx@zzu.edu.cn; Fax: +86 371 67763390; Tel: +86 371
67763390
Results and discussion
X-Ray crystal structures of the complexes Eu2Na2(L1)6(OH)2·
2C2H5OH·2CHCl3 (1), Eu(L1)3(phen) (2) and
Eu2Na2(L1)6(L2)2(OH)2·8CHCl3 (3)
bCollege of Materials Science and Chemical Engineering, Tianjin Polytechnic
University, Tianjin, P. R. of China; Fax: +86 371 67763390; Tel: +86 371
67763390
† CCDC reference numbers 705013 (complex 1), 705014 (complex 2) and
705018 (complex 3). For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b906470b
Single crystals of Eu2Na2(L1)6(OH)2·2C2H5OH·2CHCl3 (1),
Eu(L1)3(phen) (2) and Eu2Na2(L1)6(L2)2(OH)2·8CHCl3 (3) were
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Table 1 Crystallographic data for complexes 1–3
Complex
1
2
3
Empirical formula
Mr/g mol-1
C90H62Eu2Na2N18O14·2C2H5OH·2CHCl3
C57H38EuN11O6
1124.94
P21/c
15.7659(12)
15.5239(12)
19.8423(15)
90
94.425(1)
90
4841.9(6)
4
1.543
291(2)
0.71073
1.362
C114H78Eu2Na2N26O16·8CHCl3
2300.41
3372.96
¯
P1
15.772(3)
16.611(3)
17.450(4)
61.65(3)
64.05(3)
72.30(3)
3590.8(18)
1
1.559
291(2)
0.71073
1.387
1.42 < q < 27.53
46 267
Space group
I41/a
38.557(6)
38.557(6)
13.363(3)
90
90
90
19 866(6)
˚
a/A
˚
b/A
˚
c/A
a/◦
b/◦
g /◦
3
˚
V/A
Z
8
r
calcd/g cm-3
1.536
291(2)
0.71073
1.495
1.49 < q < 25.00
24 599
8215
623
T/K
˚
l/A
m/mm-1
q range/◦
2.35 < q < 28.41
44 719
12 054
676
1.020
0.0451
Reflections collected
Independent reflections
Parameters
16 267
849
1.033
0.0676
GOF on F2
1.147
0.0612
Rint
Final R [I > 2s(I)]ab
R (all data)
R1 = 0.0719, wR2 = 0.1433
R1 = 0.0370, wR2 = 0.0741
R1 = 0.0617, wR2 = 0.1687
R1 = 0.1355, wR2 = 0.1623
R1 = 0.0665, wR2 = 0.0849
R1 = 0.0670, wR2 = 0.1742
2
1/2
a R1 = RꢀFo| - |Fcꢀ/R|Fo|. b wR2 = {R[w(Fo - Fc2)2]/R[w(Fo2)2]}
.
◦
˚
Table 2 Selected bond lengths (A) and angles ( ) of complex 1
˚
Bond lengths/A
Eu(1)–O(7)
Eu(1)–O(5)
Eu(1)–O(7)#1
Eu(1)–O(3)
2.331(3)
2.339(4)
2.344(4)
2.372(4)
Eu(1)–O(1)
Eu(1)–N(5)
Eu(1)–N(8)
Eu(1)–N(2)
2.423(4)
2.577(4)
2.628(4)
2.551(5)
Na(1)–N(6)
2.443(5)
2.564(5)
2.241(4)
2.257(7)
Na(1)–O(1)#1
2.738(5)
Na(1)–N(9)
Na(1)–O(7)
Na(1)–O(8)
Bond angles/◦
O(7)–Eu(1)–O(5)
O(7)–Eu(1)–O(7)#1
O(5)–Eu(1)–O(7)#1
O(7)–Eu(1)–O(3)
O(5)–Eu(1)–O(3)
O(7)#1–Eu(1)–O(3)
O(7)–Eu(1)–O(1)
O(5)–Eu(1)–O(1)
O(7)#1–Eu(1)–O(1)
O(3)–Eu(1)–O(1)
141.11(14)
71.80(14)
146.93(13)
134.20(13)
76.77(15)
77.25(13)
122.94(14)
78.46(16)
77.36(13)
80.45(13)
O(7)–Eu(1)–N(2)
O(5)–Eu(1)–N(2)
O(7)#1–Eu(1)–N(2)
O(3)–Eu(1)–N(8)
O(1)–Eu(1)–N(8)
N(2)–Eu(1)–N(8)
N(5)–Eu(1)–N(8)
O(5)–Eu(1)–N(8)
O(7)#1–Eu(1)–N(8)
O(3)–Eu(1)–N(2)
72.74(14)
91.60(16)
98.77(14)
125.20(14)
126.13(14)
75.52(15)
80.01(14)
66.09(14)
146.94(13)
146.14(14)
O(1)–Eu(1)–N(2)
O(7)–Eu(1)–N(5)
O(5)–Eu(1)–N(5)
O(7)#1–Eu(1)–N(5)
O(3)–Eu(1)–N(5)
O(1)–Eu(1)–N(5)
N(2)–Eu(1)–N(5)
O(7)–Eu(1)–N(8)
O(7)–Na(1)–O(8)
O(7)–Na(1)–N(6)
66.00(14)
80.19(13)
98.22(16)
89.90(13)
66.67(13)
146.70(13)
147.15(14)
75.47(13)
145.8(3)
O(8)–Na(1)–N(6)
O(8)–Na(1)–N(9)
N(6)–Na(1)–N(9)
N(6)–Na(1)–O(1)#1
O(7)–Na(1)–N(9)
O(7)–Na(1)–O(1)#1
O(8)–Na(1)–O(1)#1
N(9)–Na(1)–O(1)#1
122.3(3)
107.5(2)
96.16(17)
89.98(15)
78.21(16)
72.74(14)
93.4(3)
150.24(18)
89.49(16)
Symmetry transformations used to generate equivalent atoms: #1 - x -1/2, -y + 3/2, -z + 1/2.
obtained by slow evaporation of ethanol and chloroform mixed
solution at room temperature, and characterized by single-crystal
X-ray crystallography. Details of crystal data and data collection
parameters for complexes 1, 2 and 3 are listed in Table 1. The
complex 1 crystallizes in tetragonal space group I41/a, featuring a
centrosymmetric dimeric structure. The selected bond lengths and
bond angles for complex 1 are listed in Table 2. As shown in Fig. 1,
each europium atom is eight-coordinated by three carbonyl oxygen
atoms and three oxadiazol nitrogen atoms of ligand L1, two oxygen
atoms from the bridging hydroxide anions. As shown in Fig. 2,
the coordination geometry of europium ion can be described as
distorted square antiprism. The O(1), O(7), N(2), O(7A) and O(3),
N(5), N(6), O(5) form the two facial planes, which being parallel to
within 6◦ and staggered by 40.5◦. The distances between Eu centre
˚
and the two faces are 1.355 and 1.380 A, respectively. The Eu–O
˚
distances are in the range of 2.331(3) to 2.423(4) A and Eu–N
˚
distances are in the range of 2.551(5) to 2.628(4) A, consisting
with those reported for b-diketone lanthanide complexes.15 Each
sodium ion is five-coordinated by one ketone oxygen atom from L1,
one bridging hydroxide anion, one hydroxyl group of ethanol and
two oxadiazol nitrogen atoms from L1, resulting in a distorted
trigonal bipyramidal geometry. The atoms of O(1) and N(9) seem
to occupy the axial sites with angle O(1)–Na(1)–N(9) 150.24(18)◦,
showing a great deviation from linearity, while O(7) and O(8)
7360 | Dalton Trans., 2009, 7359–7367
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Compound 2 was designed and synthesized based on the
concept of synergistic coordination effect. The coordination of
the neutral ligand 1,10-phenanthroline (phen) with the europium
ion replaces the chelating water molecules, and as a result a
mononuclear complex Eu(L1)3(phen) was obtained. Its crystal
structure is shown in Fig. 3. The europium ion is chelated with
three oxygen atoms and three nitrogen atoms from the ligand L1,
two nitrogen atoms from the neutral ligand phen, resulting in a
eight coordinated geometry of distorted square antiprism. The
˚
average Eu–O bond length is 2.326(2) A. The Eu–N bond lengths
involving the nitrogen atoms of ligand L1 are a little shorter
˚
(2.4945(19)–2.5484(18) A) than those with the nitrogen atoms of
˚
phen ligand (2.5750(18)–2.5810(18) A) (Table 3).
Fig. 1 A diagram showing the molecular structure of complex 1.
Hydrogen atoms and solvent molecules were omitted for clarity.
Fig. 3 A diagram showing the molecular structure of complex 2.
Hydrogen atoms were omitted for clarity.
When L2 was selected as a synergistic ligand and reacted with
HL1 and EuCl3 in 3: 1:1 (HL1: L2: EuCl3) ratio, a new compound,
3, was obtained. The complex 3 crystallizes in triclinic with space
Fig. 2 Coordination environment of europium and sodium ions in the
crystal of complex 1.
¯
group P1, also featuring a centrosymmetric dimeric structure
(Fig. 4). Each europium ion is chelated by three oxadiazole-
modified acylamide ligands and two bridging hydroxide anions.
The coordination geometry and the bonding parameters around
the europium centre are essentially identical with that of complex
1. However, in complex 3, the neutral ligand L2 doesn’t coordinate
with europium ion directly, but links with europium center by
chelating with sodium ion, although it has the potential to chelate
with rare earth ions as a heterocyclic nitrogen donor, this special
binding mode is probably caused by steric effect of L2. The overall
coordination sphere around each sodium ion consists of one
carbonyl oxygen atom of ligand L1, one bridging hydroxide anion,
two oxadiazol nitrogen atoms of L1 ligands and two nitrogen atoms
of L2 ligands, resulting in a six-coordinated distorted trigonal
anti-prismatic geometry (Fig. 5). The Na–O distances range from
and the nitrogen atom N(6) reside on the equatorial plane with
the bond angles to the axial Na(1)–O(1) or Na(1)–N(9) vectors
ranging from 72.74(14) to 107.5(2)◦. The Na–O distances are in
˚
the range of 2.241(4) to 2.738(5) A, and the Na–N distances are
˚
in the range of 2.443(5) to 2.564(5) A, the longest contact being to
the ketone oxygen atom (O(1)) of L1. All the L1 ligands feature a
near co-planar relationship between their oxadiazole and phenyl
rings, the dihedral angles between the two phenyl rings being 7, 3,
5◦ for the O(1), O(3) and O(5) containing ligands, respectively. For
ligand L1, both nitrogen and oxygen atoms in the oxadiazol group
have the possibility to chelate with europium ion. However, only
nitrogen atom was observed to coordinate with europium ion,
although the oxygen atom shows stronger coordination ability
than the nitrogen atom, as usually indicated by the shorter Eu–O
distances in comparison with Eu–N distances. The steric effect
may be responsible for the binding mode.
˚
2.354(2) to 2.424(3) A, and the Na–N distances range from
˚
2.514(3) to 2.626(3) A (Table 4). These data are comparable to
those observed in sodium ketoiminate complexes.16
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◦
˚
Table 3 Selected bond lengths (A) and angles ( ) of complex 2
˚
Bond lengths/A
Eu(1)–O(5)
Eu(1)–O(3)
2.3123(14)
2.3255(14)
Eu(1)–O(1)
Eu(1)–N(2)
2.3409(15)
2.4945(19)
Eu(1)–N(8)
Eu(1)–N(5)
2.5081(17)
2.5484(18)
Eu(1)–N(10)
Eu(1)–N(11)
2.5750(18)
2.5810(18)
Bond angles/◦
O(5)–Eu(1)–O(3)
O(5)–Eu(1)–O(1)
O(3)–Eu(1)–O(1)
O(5)–Eu(1)–N(2)
O(3)–Eu(1)–N(2)
O(1)–Eu(1)–N(2)
O(5)–Eu(1)–N(8)
149.70(5)
83.72(5)
116.59(5)
91.28(6)
77.81(6)
67.43(6)
69.12(5)
O(3)–Eu(1)–N(8)
O(1)–Eu(1)–N(8)
N(2)–Eu(1)–N(8)
O(5)–Eu(1)–N(5)
O(3)–Eu(1)–N(5)
O(1)–Eu(1)–N(5)
N(2)–Eu(1)–N(5)
80.85(5)
134.53(6)
77.15(6)
142.43(5)
67.72(5)
73.53(6)
106.39(6)
O(5)–Eu(1)–N(11)
O(3)–Eu(1)–N(11)
N(8)–Eu(1)–N(5)
O(5)–Eu(1)–N(10)
O(3)–Eu(1)–N(10)
O(1)–Eu(1)–N(10)
N(2)–Eu(1)–N(10)
76.44(6)
125.70(6)
146.33(5)
101.03(6)
76.45(6)
143.40(5)
147.37(6)
N(8)–Eu(1)–N(10)
N(5)–Eu(1)–N(10)
O(1)–Eu(1)–N(11)
N(2)–Eu(1)–N(11)
N(8)–Eu(1)–N(11)
N(5)–Eu(1)–N(11)
N(10)–Eu(1)–N(11)
79.26(6)
81.84(6)
82.96(5)
149.13(6)
122.33(6)
71.50(6)
63.49(5)
◦
˚
Table 4 Selected bond lengths (A) and angles ( ) of complex 3
˚
Bond lengths/A
Eu(1)–O(7)#1
Eu(1)–O(1)
Eu(1)–O(7)
Eu(1)–O(3)
2.3480(19)
2.359(2)
2.358(2)
2.367(2)
Eu(1)–O(5)
Eu(1)–N(5)
Eu(1)–N(8)
Eu(1)–N(2)
2.415(2)
2.574(3)
2.585(3)
2.615(3)
Na(1)–O(7)
Na(1)–O(5)
Na(1)–N(6)#1
Na(1)–N(3)#1
2.354(2)
2.424(3)
2.514(3)
2.580(3)
Na(1)–N(11)
Na(1)–N(10)
2.598(3)
2.626(3)
Bond angles/◦
O(7)#1–Eu(1)–O(1)
O(7)#1–Eu(1)–O(7)
O(1)–Eu(1)–O(7)
O(7)#1–Eu(1)–O(3)
O(1)–Eu(1)–O(3)
O(7)–Eu(1)–O(3)
O(7)#1–Eu(1)–O(5)
O(1)–Eu(1)–O(5)
O(7)–Eu(1)–O(5)
O(3)–Eu(1)–O(5)
O(7)#1–Eu(1)–N(5)
144.31(8)
71.63(8)
84.57(9)
136.91(8)
74.44(9)
148.79(7)
120.55(8)
77.54(8)
77.34(7)
75.77(9)
77.87(8)
O(1)–Eu(1)–N(5)
O(7)–Eu(1)–N(5)
O(3)–Eu(1)–N(5)
O(5)–Eu(1)–N(5)
O(7)#1–Eu(1)–N(8)
O(1)–Eu(1)–N(8)
O(7)–Eu(1)–N(8)
O(3)–Eu(1)–N(8)
O(5)–Eu(1)–N(8)
N(5)–Eu(1)–N(8)
O(7)#1–Eu(1)–N(2)
112.17(10)
144.70(6)
66.16(8)
135.17(8)
75.05(7)
138.74(7)
106.45(9)
77.00(9)
66.93(8)
81.78(9)
82.94(7)
O(1)–Eu(1)–N(2)
O(7)–Eu(1)–N(2)
O(3)–Eu(1)–N(2)
O(5)–Eu(1)–N(2)
N(5)–Eu(1)–N(2)
N(8)–Eu(1)–N(2)
O(7)–Na(1)–O(5)
O(7)–Na(1)–N(6)#1
O(5)–Na(1)–N(6)#1
O(7)–Na(1)–N(3)#1
O(5)–Na(1)–N(3)#1
66.66(8)
80.03(9)
111.15(9)
139.04(8)
78.98(9)
153.32(8)
77.22(8)
84.15(9)
158.61(9)
88.33(9)
85.61(10)
N(6)#1–Na(1)–N(3)#1
O(7)–Na(1)–N(11)
O(5)–Na(1)–N(11)
N(6)#1–Na(1)–N(11)
N(3)#1–Na(1)–N(11)
O(7)–Na(1)–N(10)
O(5)–Na(1)–N(10)
N(6)#1–Na(1)–N(10)
N(3)#1–Na(1)–N(10)
N(11)–Na(1)–N(10)
83.43(10)
155.23(9)
92.49(10)
108.73(12)
113.59(10)
94.07(10)
99.61(11)
92.02(11)
174.62(13)
65.15(10)
Symmetry transformations used to generate equivalent atoms: #1 - x, -y, -z.
Fig. 5 Coordination environment of europium and sodium ions in the
crystal of complex 3.
Fig. 4 A diagram showing the molecular structure of complex 3.
Hydrogen atoms and solvent molecules were omitted for clarity.
the ketonic form. In the IR spectrum of the complex 1, the uC=
O
IR spectra of the free ligand HL1 and the complex 1
vibration at 1713 cm-1 disappears, and the uC= vibration of the
N
The free ligand HL1 shows strong C–O stretch vibration at
1713 cm-1. The absence of O–H stretch vibration in the range
of 3200–3500 cm-1 indicates that the free ligand mainly exits in
heterocyclic ring at 1618 cm-1 moves to 1547 cm-1, showing that
the nitrogen of the oxadiazole ring coordinates with Eu(III) ion,
which is in accordance with the X-ray analysis results.
7362 | Dalton Trans., 2009, 7359–7367
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Electronic spectroscopy and photoluminescence studies
Eu(L1)3(phen) features a very broad band in the range of 250–
380 nm with intensity dropping slowly toward lower wavelength,
the maximum peak is at 337 nm. Furthermore, the relative
intensity of the narrow band at 464 nm corresponding to the
UV-vis absorption spectra of the ligands (HL1, L2, phen) and
their Eu(III) complexes measured in methanol are shown in Fig. 6.
The broad band at 273 nm for HL1 can be assigned to p–p*
1
5
7F0 → D2 hypersensitive transition of complex 2 is significantly
transition, while the shoulder band at 290 nm can be identified
as n–p* transition of the free ligand owing to its low absorption
intensity. Upon coordinating with europium ion, the absorption
at 273 nm shifts to 307 nm, 305 nm and 302 nm for complex 1, 2
and 3, respectively. The hump observed around 272 and 286 nm
larger than that of complexes 1 and 3. This result indicates that
the asymmetric part of the crystal field is larger in complex 2
compared with complexes 1 and 3, which is in accordance with
the X-ray analysis results.
The emission spectra of the three complexes at room tem-
perature show efficient red emission typical of Eu3+ ion and no
emission bands from organic ligands were observed (Fig. 8),
indicating that the energy transfer from the ligands to the Eu(III)
centre is very efficient. The assignment of observed bands to
appropriate f–f transitions is straightforward, the five sharp
peaks at 580, 590, 612, 651 and 701 nm correspond to the
1
in complexes 2 and 3, can be attributed to p–p* absorptions of
the neutral ligands. The spectral shapes of the Eu(III) complexes
in methanol are similar to those of the corresponding ligands,
indicating that the coordination of the europium ion does not
significantly influence the energy of the singlet state of ligands.
However, the red shift observed in the absorption maximum of all
the complexes is due to the perturbation induced by the Eu(III)
coordination.
7
5
7
5
7
5
7
5
7
5D0 → F0, D0 → F1, D0 → F2, D0 → F3 and D0 → F4
transitions, respectively. The 5D0 → F1 magnetic dipole transition
7
is independent of the coordination sphere, while the electric dipole
transition 5D0
→
7F2 is extremely sensitive to the nature and
symmetry of the coordinating environment. The intensity ratio
5
7
5
7
of D0 → F1 and D0 → F2 transitions (I7F2/I7F1) is a good
measure of the nature and symmetry of the first coordination
sphere.17,18 In a centrosymmetric environment, the magnetic dipole
7
5D0 → F1 transition of Eu3+ is dominating, whereas distortion of
the symmetry around the ion cause an intensity enhancement of
the hypersensitive 5D0 → F2 transition.19 In all those complexes,
7
5
7
the domination of the D0 → F2 is a typical feature that no
inversion centre is occupied by Eu(III) ion in the complexes (Fig. 8).
The I7F2/I7F1 ratios for complex 1, 2 and 3 are 7.6, 9.5 and 7.9,
respectively. The singlet of the 5D0
→
7F0 transition indicates
the presence of only single site symmetry of Eu3+ ion, which
is in good agreement with the structure analysis results. It is
interesting to note that the I7F2/I7F1 ratios of these complexes
are smaller compared with the reported literature values for tris-
(b-diketone) complexes.19 Judd considered that the polarizability
of the coordinating groups play a crucial role in the hypersensiti-
vity of certain lanthanide(III) transitions.20 The nitrogen donor
atoms of L1 are less polarizable than the coordinating carbonyl
groups of b-diketonates, resulting in a decrease in the intensity
Fig. 6 UV spectra of ligand HL1, L2, phen and the complexes 1–3 in
methanol solution at room temperature.
The fluorescence spectra of complexes 1, 2 and 3 in solid state
were recorded at room temperature. The excitation spectra of
7
complexes 1 and 3, obtained by monitoring 5D0 → F2 transition
of Eu(III) ion at 612 nm, were found to be very similar (Fig. 7).
They are all dominated by strong bands in the range of 300–
380 nm with the maximum absorptions at 365 nm and 355 nm,
respectively. However, the excitation spectrum of complex 2
shows apparent differences from that of complexes 1 and 3. By
introducing the second ligand phen, the excitation spectrum of
5
7
of the hypersensitive D0 → F2 transition. As shown in Fig. 8,
the complex Eu(L1)3(phen) is more strongly luminescent in the
Fig. 7 Excitation spectra of complexes 1–3 in solid state at room
temperature.
Fig. 8 Emission spectra of 1 (lex = 365 nm), 2 (lex = 337 nm) and 3 (lex =
355 nm) in solid state at room temperature.
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©

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red than the other complexes when excited with near–ultraviolet
radiation. This result demonstrates that the coordination of the
phen can not only erase the H–O vibration quenching from the
of water molecules and improve the rigidity of the complex, but
also can act as a good light-harvesting group for sensitized Eu3+
luminescence.
The photophysical model that explains the sensitization path-
way in a luminescent lanthanide complex is well established.
The overall quantum yield (Utot) of ligand-sensitized lanthanide
emission, determined experimentally, is the product of the ligand
qualitatively in term of the synergistic sensitization effects as well
as the antenna-Eu(III) distances. In complex 1, only ligand L1
acts as energy sensitizer for Eu(III) center. In complexes 2 and
3, both ligand L1 and neutral ligands (L2 or Phen) act as energy
sensitizers for Eu(III) center. Since the distances between L1 ligands
and europium ions are comparable in complexes 2 and 3, we
will focus on the distances between donating neutral ligands and
Eu(III) centers. Given the steep distance-dependence of energy
transfer mechanisms, even a small change in the distance has a
25
significant effect on hsens
.
In complex 3, the neutral ligand L2
sensitization efficiency (hsens) and the intrinsic quantum yield (ULn
)
doesn’t coordinate with Eu(III) ion directly, but links with Eu(III)
center through chelating with Na+ ion. The relative longer distance
between L2 and Eu(III) center may cause apparent decrease of
sensitization efficiency of L2. In complex 2, besides the good
sensitizing capabilities of ligand L1 and phen, these antennas
also contribute positively to the overall luminescence quantum
yield (Utot) by increasing the intrinsic quantum yield of Eu(III)
ion (ULn).
of the lanthanide luminescence according to:21
Utot = hsensULn
.
(1)
The energy transfer efficiency (hsens) is the product of the
two processes involving intersystem crossing (ISC) from the first
excited singlet state of the ligand to the triplet state and energy
transfer (LET) to the lanthanide. The intrinsic quantum yield of
the lanthanide luminescence step (ULn) can be evaluated on the
basis of observed luminescence lifetime (tobs) and pure radiative
Energy transfer between ligands and europium(III)
5
7
lifetime (tR) of the Eu(III) D0 → FJ (J = 0–4) transitions by
using:22
In an effort to demonstrate the energy transfer process, the
phosphorescence spectra of the complexes Gd(L1)3(H2O)2 and
Gd(NO3)3L2 were measured at 77 K for triplet energy level data
of the ligand L1 and L2, respectively. Since the lowest excited
ULn = tobs/tR
(2)
1/tR = AMD,0n3(Itot/IMD).
(3)
6
state P7/2 of Gd(III) ion is too high to accept energy from
a ligand, the data obtained from the phosphorescence spectra
of the complexes actually reveal the triplet energy level of the
corresponding ligand.26 The triplet state energy (3pp*) levels of L1
and L2 based on our measurement were found to be 21 270 cm-1
(470 nm) and 20 260 cm-1 (496 nm), respectively. The singlet
state energy levels of L1 and L2 were estimated by referencing
their absorbance edges, which were 27 397 cm-1 (365 nm) and
31 056 cm-1 (322 nm), respectively. The singlet and triplet energy
(1pp*) levels of DBM (28 300 and 20 520 cm-1), bpy (29 900 and
22 900 cm-1) and phen (31 000 and 22 100 cm-1) were taken from
the literatures.27,28
where AMD,0 = 14.65 s-1 is the spontaneous emission probability
5
7
of the D0 → F1 transition of Eu(III), n, the refractive index of
the medium, Itot/IMD, the ratio of the integrated total area of the
corrected Eu(III) emission spectrum to the area of magnetic dipole
7
5D0 → F1 transition.
Table 5 presents the radiative lifetimes (tR), intrinsic quantum
yields (ULn), sensitization efficiencies (hsens), the experimentally
determined luminescence lifetimes (tobs) and overall quantum
yields (Utot) of the three Eu(III) complexes. Intrinsic quantum
yields (ULn) of complex 1 and complex 3 are similar to each
other due to their comparable coordination environments.23 These
values are smaller than that calculated for complex 2. This
phenomenon can be attributed to two aspects. Firstly, the presence
of O–H oscillators of OH- in the first coordination sphere of
Eu(III) centre efficiently quenches the luminescence via vibrational
relaxation. Secondly, the energy transfer between Eu3+ ions (with
Generally, the excited energy of the organic ligand undergoes an
intramolecular energy transfer from the triplet state of the ligand
to an excited state of Eu(III), and then the emission occurs when
the energy transfer from the lowest excited state 5D0 to the ground
7
state FJ (J = 0–4) of Eu(III).29 Therefore, matching the energy
levels of the triplet state of the ligands to 5D0 of Eu3+ is one of the
key factors that affect the luminescent properties of the europium
complexes.
˚
the Eu–Eu seperation of 3.78(7) A) can also be a key factor, which
may cause concentration quenching when the distance between
Eu3+ ions themselves is less than 8.4 A.24 The efficiencies of energy
˚
Based on the above experimental results, the triplet energy levels
of L1 (21 270 cm-1) and L2 (20 260 cm-1) are obviously higher than
the 5D0 level (17 500 cm-1) of Eu3+, and their energy gaps DE(3pp*–
5D0) between ligand and metal-centered levels are too high to allow
an effective back energy transfer. According to Latva’s empirical
rule,30 an optimal ligand-to-metal energy transfer process for Eu3+
needs DE(3pp*–5D0) > 2500 cm-1 and hence the energy transfer
process is effective for complexes 1–3. We also noticed that the
transfer from the ligands to the Eu3+ ion, hsens, are 0.25, 0.53
and 0.38 for complexes 1, 2 and 3, respectively. The differences
in sensitization efficiencies of those complexes can be explained
Table 5 Radiative lifetimes (tR), intrinsic quantum yields of the lan-
thanide luminescence step (ULn), sensitization efficiencies (hsens), the exper-
imentally determined luminescence lifetimes (tobs) and overall quantum
yields (Utot) of complexes 1–3
1
3
energy gaps between the pp* and pp* levels are 6127 and
10 796 cm-1 for L1 and L2, respectively. According to Reinhoudt’s
empirical rule,1b the intersystem crossing process becomes effective
when DE(1pp*–3pp*) is at least 5000 cm-1; thus L1 and L2 are
effective sensitizers for Eu3+ and the intersystem crossing processes
in complexes 1–3 are effective.
Complex
tR/ms
t
obs/ms
ULn (%)
hsens (%)
Utot (%)
1
2
3
1.50
1.42
1.56
0.87
0.99
0.95
58
70
61
25
53
38
15
37
23
7364 | Dalton Trans., 2009, 7359–7367
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Electroluminescence property of complex 2
With the aim of developing new bright and efficient red
Organic light emitting diodes (OLED). The EL properties of
the complexes were also investigated. Of the three complexes
studied, only Eu(L1)3(phen) is a mononuclear complex and
can be used as an emitter in OLED through vacuum deposi-
tion. Using Eu(L1)3(phen) doped into N,N-bis(3-methylphenyl)-
N,N -diphenyl)-benzidine (TPD) as the emitting layer, N,N-
bis(3-methylphenyl)-N,N -diphenyl)benzidine (TPD) as the hole–
transporting layer, tris(8-quinolinolato)aluminium(III) complex
(AlQ) as the electron-transporting layer, a three layer device with
the structure of ITO/TPD (30 nm)/Eu(L1)3(phen): TPD (1:3,
50 nm)/AlQ (30 nm)/Mg0.9Ag0.1/Ag was fabricated. With hole-
transporting compound TPD as the host in the emitting layer, the
oxadiazole-modified europium complex Eu(L1)3(phen) is expected
to exhibit the combined properties of europium-based complex
and electron-transporting property of oxadiazole moiety in the
OLED device. Fig. 9 shows the EL spectrum of the resulting device
at 12 V. The EL spectrum is dominated by a sharp red emission at
Fig. 10 The current–voltage (I–V) and luminance-voltage (L–V) charac-
teristics of the ITO/TPD (30 nm)/Eu(L1)3(phen): TPD (1:3, 50 nm)/AlQ
(30 nm)/Mg0.9Ag0.1/Ag device.
7
612 nm due to 5D0 → F2 transition of the Eu3+ ion and no broad
emission at 520 nm from the AlQ appears. This result confirms that
the complex Eu(L1)3(phen) has good electron-transporting ability,
and as a result the electron-hole recombination is successfully
confined within the Eu(L1)3(phen) layer.31
Fig. 11 The EL efficiency in lm W-1 and cd A-1 characteristics of the
ITO/TPD (30 nm)/Eu(L1)3(phen): TPD (1:3, 50 nm)/AlQ (30 nm)/
Mg0.9Ag0.1/Ag device.
the present device is optimized. More efforts will be taken to
develop highly luminescent europium acylamide complexes useful
in electroluminescent devices.
Conclusions
Three new luminescent europium complexes Eu2Na2(L1)6(OH)2·
2C2H5OH·2CHCl3 (1), Eu(L1)3(phen) (2) and Eu2Na2(L1)6-
(L2)2(OH)2·8CHCl3 (3) have been synthesized and structurally
characterized. For complexes 1, 2 and 3, the overall quantum
yield were 0.15, 0.37 and 0.23, respectively. The decay times of
complexes 1, 2 and 3, as measured at room temperature, were
0.87, 0.99 and 0.95 ms, respectively. The lowest triplet state energy
levels of the ligands L1 and L2 indicate that the triplet state energy
levels of the ligands match perfectly to the resonance energy
level of Eu(III). All the complexes exhibit strong characteristic
emission of Eu3+ ion, with Eu(L1)3(phen) being the most efficient.
Using Eu(L1)3(phen) as the emitting material, pure red emission
at 612 nm for the ITO/TPD (30 nm)/Eu(L1)3(phen): TPD (1:3,
50 nm)/AlQ (30 nm)/Mg0.9Ag0.1/Ag device was observed with
the maximum luminance of 107 cd m-2 and the maximum power
efficiency of 0.16 lm W-1. The compound 2 exhibits good electron-
transporting property and is a promising red emitter for EL device.
Efforts are being taken by our group to develop new kinds of
luminescent europium acylamide complexes for EL devices.
Fig. 9 The EL spectrum of the ITO/TPD (30 nm)/Eu(L1)3(phen): TPD
(1:3, 50 nm)/AlQ (30 nm)/Mg0.9Ag0.1/Ag device.
The current–voltage (I–V) and luminance–voltage (L–V) char-
acteristics of the device are shown in Fig. 10. The EL efficiencies
as a function of the voltage are presented in Fig. 11. The turn
on voltage is as low as 6 V. Although the brightness of the
device increases with increasing current density, the EL spectrum
was independent of the bias voltage. The maximum emission
intensity of 107 cd m-2 is observed at 16 V and 74 mA cm-2. The
maximum EL efficiency is 0.16 lm W-1 at 9 V. It is thus concluded
that the sublimable complex, Eu(L1)3(phen), can be used as a
red emitting material with good electron-transporting ability in
electroluminescent devices. Comparing with other conventional
three-layer devices based on b-diketonates europium complexes,
the performance of this device is only moderate.7,8 However,
Eu(L1)3(phen) is the first oxadiazole-modified europium benza-
mide complex used as the emitter in the EL device, furthermore,
neither the configurational nor the compositional structure of
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1 mmol 1,10-phenanthroline (phen). The mixture was stirred and
neutralized with NaOH aqueous solution. The solution was then
evaporated to a small volume. The product was obtained by
filtration. The pale yellow crystals were obtained in 45% yield by
re-crystallization from ethanol/hexane solution. Anal. calcd for
C57H38EuN11O6 (%): C 60.86, H 3.40, N 13.70. Found: C 60.79,
H 3.41, N 13.68.
Experimental
Preparation and composition analysis
All the chemicals and solvents were commercial grade, and were
purified or dried by standard methods when required. NMR
spectra were recorded on Bruker DPX-400MHz spectrometer in
DMSO. Chemical shifts (d) are expressed in ppm relative to Me4Si
for 1H and 13C NMR. IR spectra were recorded as KBr pellets with
a Bruker Vector22 FT-IR spectrophotometer. Elemental analysis
for carbon, hydrogen and nitrogen were performed with a Carlo-
Erba1106 Elemental Analyzer.
Complex Eu2Na2(L1)6(L2)2(OH)2·8CHCl3 (3). 1 mmol EuCl3
in ethanol was added to a ethanol solution (30 mL) of 3 mmol
HL1 and 1 mmol L2. The following steps are similar to those of
1 Yield: 28%. Anal. calcd for C114H78Eu2Na2N26O16·8CHCl3 (%):
C 40.60, H 2.33, N 10.80. Found: C 40.64, H 2.39, N 10.72.
Benzaldehyde semicarbazone (I). A solution of benzaldehyde
(10 mL, 0.10 mol) in methanol (10 mL) was added slowly to
a solution of semicarbazide hydrochloride (12 g, 0.11 mol) in
methanol (60 mL) and water (240 mL) with stirring; and a white
solid appeared at once. Then the slurry was stirred at 70 ◦C
for 2 h. The white precipitate was collected and dried under
vacuum. Crystallization from ethanol aqueous solution gave a
white crystalline solid. Yield: 9.78 g, 60%. M.p. 221–223 ◦C. Anal.
calcd for C8H9N3O (%): C 58.88, H 5.56, N 25.75. Found: C 58.42,
H 5.63, N 26.03. IR (KBr): 3406, 3289, 1689, 1623, 1595, 1356,
759 cm-1.
The X-ray crystal structure determination
Single crystals of complexes 1–3 suitable for X-ray diffraction
analysis were grown by solvent evaporation of their solutions.
Data collections were performed at ambient temperature on a
Rigaku Raxis-IV area detector with graphite-monochromated Mo
Ka radiation (l = 0.071073 nm). In each case the data were
corrected for Lorentz and polarization effects. An absorption
correction was applied (SADABS)34 to the collected reflections.
The structures were solved by direct methods, and the non-
hydrogen atoms were refined anisotropically with the full matrix
least–squares based on F2. Hydrogen atoms were included but not
refined. These structures of these molecules were solved by direct
methods and expanded by standard difference Fourier syntheses
with the software SHELXTL.35 The crystallographic data and
other experimental details are summarized in Table 1. Selected
bond lengths and angles of complexes 1–3 are listed in Tables 2–4,
respectively.
2-Amino-5-phenyl-1,3,4-oxadiazole (II). This preparation is
based on a published method.32 Yield: 78%. M.p.: 240–242 ◦C
(lit. m.p. 241–243 ◦C). Anal. calcd for C8H7N3O (%): C 59.62, H
4.38, N 26.07. Found: C 59.54, H 4.42, N 26.13.
N-(5-phenyl-1,3,4-oxadiazol-2-yl)-benzamide (HL1). Benzoyl
chloride (3.68 mL, 0.032 mol) was dropped slowly into the stirred
slurry of compound II (5.64 g, 0.035 mol) in 50 mL pyridine.
Compound II dissolved gradually and gave a buff solution.
After 2 h, the solution was poured into water; and the white
precipitate was collected and dried under vacuum. Pure product
was obtained by re-crystallization from ethanol, m.p. 203–205 ◦C.
Yield: 6.28 g, 74%. Anal. calcd for C15H11N3O2 (%): C 67.92,
Spectroscopic measurements
UV absorption spectra were obtained from methanol solu-
tions with a HP-8453 UV-vis spectrophotometer. Excitation and
emission spectra were recorded on HORIBA JOBIN YVON
FluoroMax-P spectrophotometer and powder samples were used
for this purpose. The decay times of complexes 1, 2 and 3 were mea-
1
H 4.18, N 15.84. Found: C 68.09, H 4.13, N 15.67. H NMR
(DMSO, 400 MHz) d/ppm: 7.55–8.05 (m, 10 H), 12.14 (s, 1 H)
ppm. 13C NMR (DMSO, 400 MHz): d/ppm: 165.18, 161.43,
158.14, 133.20, 132.39, 132.00, 129.70, 128.84, 128.45, 126.29,
123.59 ppm. IR (KBr): 1713, 1618, 1582, 1391, 1293, 1245, 1023,
694 cm-1.
7
sured monitoring the main F2 Stark component (lex = 365 nm,
337 nm and 355 nm, respectively) by HORIBA JOBIN YVON
FluoroMax-P spectrofluorometer system at room temperature.
Mono-exponential functions has been used to fit experimental
data and the lifetimes were obtained. The luminescence quantum
yields were determined at room temperature on a FL3–2-iHR221-
NIR-TCSPC transient and steady-state fluorescence spectrometer
using a standard integrating sphere, which coated on the inside
with PTFE. The aperture ratio of integrating sphere was less than
or equal to 3%. Instrument excitation and emission slits were set
at 1 and 1.5 nm, respectively.
2,5-Bis(2-pyridyl)-1,3,4-oxadiazole (L2). This compound was
synthesized according to the reference.33 Yield: 25%. M.p. 151–
153 ◦C (lit. m.p. 153–154 ◦C). Anal. calcd for C12H8N4O (%):
C 64.30, H 3.59, N 24.97. Found: C 64.32, H 3.55, N 24.91.
Complex Eu2Na2(L1)6(OH)2·2C2H5OH·2CHCl3 (1). 1 mmol
EuCl3 in ethanol was added to a ethanol solution (30 mL) of
3 mmol HL1, and neutralized with NaOH aqueous solution
(30 mL) under stirring. Then the mixture was stirred at 60 ◦C for
3 h. The crude product was collected by filtration. The colourless
crystals of 1 were obtained in 55% yield by re-crystallization
from ethanol and chloroform mixed solution. Anal. calcd for
C90H62Eu2Na2N18O14·2C2H5OH·2CHCl3 (%): C 46.99, H 2.72,
N 10.96. Found: C 46.92, H 2.76, N 10.89.
The overall luminescence quantum yield, defined by:
number of photons emitted
Ftot
=
number of photons absorbed
was measured at room temperature using the technique for
powdered samples described by Bril et al.,36 through the following
formula:
Complex Eu(L1)3(phen) (2). An ethanol solution of EuCl3
(1 mmol) was added to a ethanol solution of 3 mmol HL1 and
7366 | Dalton Trans., 2009, 7359–7367
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⎛
⎜
⎜
⎞⎛
1−rst ⎜ Ax
⎞
⎟
⎟
⎟
⎟
⎠
⎟
⎟
Ftot
=
F
⎜
⎟
st
⎜
⎜
⎝
⎜
⎜
A
⎟
1−r
x ⎠⎝
st
where rst and rx are the amount of exciting radiation reflected by the
standard and by the sample, respectively, and Ust is the quantum
yield of the standard phosphor (sodium salicylate, Merck). The
terms Ax and Ast represent the areas under the complex and
standard emission spectra, respectively. Three measurements were
carried out for each sample, so that the presented Utot value
corresponds to the arithmetic mean value.
Fabrication of electroluminescent device
The EL device with Eu(L1)3(phen) as the emitter layer was
fabricated on a patterned indium-tin oxide (ITO) substrate with
a sheet resistance of 50 X m-2. The ITO glass is cleaned by
ultrasonication in detergent solution, pure water, acetone and
chloroform in sequence before using it. All the organics layers
and the top cathode layer (Mg0.9Ag0.1) were successively vacuum
deposited onto an indium tin oxide (ITO) substrate below a
pressure of 1 ¥ 10-5 Torr. The layer thickness is controlled in vacuo
with a quartz crystal monitor. The EL spectra were measured
by HORIBA JOBIN YVON FluoroMax-P spectrometer. The
luminanceof theEL devices was measured with aspectraPritchard
photometer, model 1980 A, at room temperature under ambient
atmosphere.
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