Synthesis, infrared and fluorescence spectra of lanthanide complexes with a new amide-based 1,3,4-oxadiazole derivative

Article abstract of DOI:10.1016/j.saa.2006.11.044

A new amide-based 1,3,4-oxadiazole derivative ligand 2,5-bis[2-(N,N-diethyl-1′-oxopropylamide)phenyl]-1,3,4-oxadiazole (L) and its complexes, Ln(NO₃)₃L (Ln = La, Eu, Gd, Tb, Er), were synthesized. The complexes were characterized by

Full text of DOI:10.1016/j.saa.2006.11.044

Spectrochimica Acta Part A 68 (2007) 349–353
Synthesis, infrared and fluorescence spectra of lanthanide complexes
with a new amide-based 1,3,4-oxadiazole derivative
Xiao-Liang Tanga, Wei Doua, Su-Wen Chena, Fang-Fang Danga, Wei-Sheng Liua,b,∗
a Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China
b State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China
Received 23 October 2006; received in revised form 20 November 2006; accepted 30 November 2006
Abstract
A new amide-based 1,3,4-oxadiazole derivative ligand 2,5-bis[2-(N,N-diethyl-1ꢀ-oxopropylamide)phenyl]-1,3,4-oxadiazole (L) and its com-
plexes, Ln(NO3)3L (Ln = La, Eu, Gd, Tb, Er), were synthesized. The complexes were characterized by elemental analysis, infrared spectra and
conductivity. The lanthanide ions were coordinated by O atoms from C O. The fluorescence properties of Eu(NO3)3L and Tb(NO3)3L in the solid
state and in different solvents were investigated. Under the excitation of UV light, these complexes exhibit characteristic fluorescence of europium
and terbium ions. The solvent factors influencing the fluorescent intensity were discussed.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Amide-based 1,3,4-oxadiazole derivative; Synthesis; Complexes; Fluorescent properties
1. Introduction
In recent years, it is growing rapidly in the application of some
1,3,4-oxadiazoles consisting of five-membered heterocyclic as
the bent shape bridging ligands in the construction of coordina-
tion chemistry [14,15]. Especially, the molecules of substituted
2,5-diaryl-1,3,4-oxadiazoles have an almost planar rigid non-
linear structure which comprises larger conjugate absorption
groups [14]. However, luminescent properties on these substi-
tuted 2,5-diaryl-1,3,4-oxadiazoles with lanthanide complexes
have been rarely reported.
On the other hand, amide-based open-chain crown ethers
offer many advantages in extraction and analysis of the rare earth
ions [16,17] because of their ring-like coordination structure and
terminal group effects [17,18]. So we have designed a series of
poly-functional amide-based open-chain crown ethers having
both selective ability to coordinate lanthanide ions and enhanced
luminescence of complexes by providing some of proper conju-
gate absorption groups suitable for energy transfer, i.e. phenyl.
As a branch of our systematic studies, we introduced
2,5-diaryl-1,3,4-oxadiazole as the basic molecular frame and
obtained a new amide-based open-chain crown ether lig-
and, 2,5-bis[2-(N,N-diethyl-1ꢀ-oxopropylamide)phenyl]-1,3,4-
oxadiazole (L). The fluorescence properties of europium and
terbium complexes with the new ligand were studied. The results
indicated that organic solvent notably affected the fluorescence
characteristics of europium and terbium ions.
Luminescent lanthanide complexes [1–3] have attracted
intense research interest from both material and biological sci-
ence [4,5] mainly due to their very narrow emission bands and
large Stokes shifts, etc. [6]. The probes based on europium and
terbium ions are of special focus because of the particularly suit-
able spectroscopic properties of these ions [4,7]. However, the
low f–f emission intensity of lanthanide ions demands activators,
i.e. appropriate ligands that can enhance the f–f electronic tran-
sitions through an intersystem energy transfer process [8–10].
Therefore, more and more chemists are paying attention to
designing a variety of organic molecular architectures contain-
ing trivalent lanthanide ions (Eu3+, Tb3+) to form highly and
strongly luminescent complexes as efficient light conversion
devices [1,11] by facilitating the well-known light conversion
process, the antenna effect [12].
Aromatic 1,3,4-oxadiazoles have been used as laser dyes,
photographic materials or scintillators during past decades [13].
∗
Correspondingauthorat:DepartmentofChemistryandStateKeyLaboratory
of Applied Organic Chemistry, College of Chemistry and Chemical Engineering,
Lanzhou University, Lanzhou 730000, China. Tel.: +86 931 8911809;
fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2006.11.044

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X.-L. Tang et al. / Spectrochimica Acta Part A 68 (2007) 349–353
Scheme 1. Synthesis of the ligand.
2. Experimental
2 (0.9 g, 6.0 mmol) was added dropwise to the mixture (excess of
2 for improving the yield of the ligand 3). The reaction mixture
was held at reflux for another 6 h. The precipitate was obtained
by adding the mixture to ice-water (100 mL). The crude prod-
uct was filtered off, dried and chromatographed on silica gel
[1:1 (v/v) petroleum ether/ethyl acetate] to afford 3 as white
2.1. Materials
All commercially available chemicals were of A.R. grade and
were used without further purification.
1
solid, yield: 87%; m.p. 70–71 ◦C. H NMR (CDCl3, ppm): δ
2.2. Methods
8.02–8.05 (2H, d, Ar–H); 7.46–7.51 (2H, t, Ar–H); 7.10–7.19
(4H, m, Ar–H); 4.88 (4H, s, 2O–CH2–C(O)); 3.43–3.51 (4H, q,
2N–CH2–); 3.34–3.42 (4H, q, 2N–CH2–); 1.07–1.13 (12H, m,
–CH3). IR (cm−1, KBr): 1659 (C O), 1541 (oxadiazole, C N),
1247 (N–N), 1070 (oxadiazole, C–O–C), 1168 (Ar–O). MS:
m/z = 480.
The metal ions were determined by EDTA titration using
xylenol orange as an indicator. Carbon, nitrogen and hydro-
gen analyses were determined using a Vario EL elemental
analyzer. Conductivity measurements were carried out with a
DDSJ-308 type conductivity bridge using 1.0 × 10−3 mol dm−3
solution in acetonitrile at 25 ◦C. IR spectra were recorded on
a Nicolet Avatar 360 FT-IR instrument using KBr discs in the
2.4. Synthesis of complexes
400–4000 cm−1 region. H NMR spectra were measured on a
A solution of 0.10 mmol L in 10 mL of ethyl acetate was
added dropwise to a solution of 0.10 mmol Ln(NO3)3·6H2O
(Ln = La, Eu, Gd, Tb, Er) in 10 mL of ethyl acetate. The mixture
was stirred at room temperature for 4 h and precipitate formed.
The precipitate was collected, washed three times with ethyl
acetate and dried in vacuum over P2O5 for 48 h. These com-
plexes were obtained as pale white powders except a light pink
powder Er(NO3)3L, yield: 60%.
1
Varian Mercury plus 300BB spectrometer in CDCl3 solution
with TMS as internal standard. MS were performed on a HP-
5988A spectrometer (EI at 70 eV). Fluorescence measurements
were made on a Hitachi F-4500 Fluorescence spectrophotome-
ter equipped with quartz curettes of 1 cm path length at room
temperature.
2.3. Synthesis of the ligand
3. Result and discussion
Compounds 1 and 2 (Scheme 1) were prepared as described
in literatures [19,20].
3.1. Properties of the complexes
2.3.1. Procedure for the ligand
Analytical data for the newly synthesized complexes, listed
in Table 1, indicate that the five complexes of lanthanum nitrates
all conform to a 1:1 metal-to-ligand stoichiometry Ln(NO3)3L
(Ln = La, Eu, Gd, Tb, Er).
Anhydrous potassium carbonate (4.0 g) was added to a solu-
tion of 1 (0.51 g, 2.0 mmol) in acetone (40 mL), and the mixture
was held at reflux. An hour later, an acetone solution containing
Table 1
Elemental analytical and molar conductance data for the complexes
Complexes
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Ln (%) found (calc.)
ꢀm (s cm2 mol−1
)
La(NO3)3L
Eu(NO3)3L
Gd(NO3)3L
Tb(NO3)3L
Er(NO3)3L
38.93 (38.76)
38.51 (38.14)
38.06 (37.90)
38.28 (37.82)
37.72 (37.44)
3.64 (3.98)
3.96 (3.91)
3.59 (3.89)
4.18 (3.88)
3.44 (3.84)
12.32 (12.18)
11.52 (11.98)
11.72 (11.90)
11.67 (11.88)
11.90 (11.76)
17.67 (17.26)
18.93 (18.58)
19.34 (19.11)
19.57 (19.26)
20.49 (20.08)
62.2
58.6
56.3
65.5
61.3

X.-L. Tang et al. / Spectrochimica Acta Part A 68 (2007) 349–353
351
Table 2
IR spectral data of the free ligand and its complexes (cm−1
)
−
ν(NO3 )
Compound
ν(C O)
ν(Ar–O–C)
ν(C N)
ν(N–N)
ν(C–O–C)
ν1
ν4
ν2
ν3
|ν1–ν4|
L
1660
1614
1629
1633
1636
1627
1168
1169
1168
1170
1171
1170
1541
1545
1546
1538
1542
1543
1247
1232
1231
1230
1234
1232
1070
1072
1069
1069
1070
1069
La(NO3)3L
Eu(NO3)3L
Gd(NO3)3L
Tb(NO3)3L
Er(NO3)3L
1496
1501
1499
1500
1497
1296
1312
1304
1306
1298
1031
1032
1031
1030
1033
817
810
814
814
815
200
189
195
194
199
All complexes are soluble in DMF, acetonitrile, acetone,
THF, methanol and ethanol, but sparingly soluble in water and
ethyl acetate. The molar conductance measurements for these
complexes in acetonitrile solution (in Table 1) indicate that
all complexes act as non-electrolytes, implying that all nitrate
groups are in coordination sphere [21].
atom of heterocyclic can not coordinate to the metal ion.
Notably, thevibrationν(N–N)ofoxadiazoleringat1247 cm−1 is
shifted towards lower frequency by 17–19 cm−1, and ν(C N) at
1541 cm−1 is erratically shifted a few towards higher frequency
in the complexes as compared to counterpart for the free ligand.
This may be due to coordination of the oxygen atoms of L to
Ln(III) which causes the fixing of L conformation so that N–N
of oxadiazole ring faces towards interior of open-chain crown
ether and gets closer to the lanthanide ion. So the change of
the electronic cloud density in N–N results in the changes of
vibration ν(N–N) and ν(C N).
The absorption bands assigned to the coordinated nitrates
groups (C2v) were observed at about 1500 cm−1 (ν1), 1305 cm−1
(ν4), 1031 cm−1 (ν2) and 814 cm−1 (ν3) [23] for the com-
plexes, respectively. In addition, the separation of two strongest
frequency bands |ν1–ν4| is approximately 195 cm−1, clearly
establishing that the NO3− groups in the solid complexes coor-
dinate to the lanthanide ion as bidentate ligands [24]. And no
noteworthy bands at 1380, 820 and 720 cm−1 in the spectra of
complexes indicate that free nitrate groups (D3 h) are absent, in
agreement with the results of the conductivity experiments.
3.2. IR spectra
The main infrared bands of the ligand and their complexes
are presented in Table 2. The IR spectra of the complexes are
similar to each other. The comparison of the IR spectra of the
free ligand and La(NO3)3L has been shown in Fig. 1.
The IR spectrum of the free ligand shows strong band at
1660 cm−1, which is attributable to stretch vibration of the
carbonyl group of amide (ν(C O)). The peaks at 1168, 1541
and 1070 cm−1 can be assigned to ν(Ar–O–C), ν(C N) and
␯(C–O–C) of oxadiazole ring, respectively. In addition, the
middle peak at 1247 cm−1 is assigned to the N–N stretch of
oxadiazole ring, and peaks at 1431 and 1471 cm−1 are assigned
to the bending vibration of –CH2–CO– [22].
In the IR spectra of the lanthanum complexes, the ν(C O)
shifts by 25–40 cm−1 towards lower wave numbers, but
ν(C–O–C) of oxadiazole ring and ν(Ar–O–C) almost remain
unchanged. Thus, the data indicate that only carbonyls take
part in coordination to the metal ion, ether O atoms and O
3.3. Fluorescence studies
Under identical experimental, the fluorescence characteris-
tics of the complexes in the solid state and in EtOH, acetone and
acetonitrile solutions (concentration: 1.0 × 10−4 mol L−1) are
listed in Table 3. The ligand having multiple aromatic rings with
planar rigid non-linear structure has a strong antenna effect, so
its europium and terbium complexes have stronger fluorescence.
Comparing the fluorescence characteristic emission wave-
lengths in the solid state, we can see that the fluorescence
intensities of terbium complex at 546 nm are much stronger
than those of europium complex at 617 nm in Fig. 2. Addition-
ally, it is easy to see four fluorescence characteristic emission
wavelengths of terbium complex both in the solid state and in
acetonitrile solution clearly in such small slit width (2.5 nm).
The intensity of the luminescence of Ln3+ chelates is related to
the efficiency of the intramolecular energy transfer between the
triple level of the ligand and the emitting level of the ions, which
depends on the energy gap between the two levels [25,26]. To
the terbium complex in the solid state, as we know, the very dif-
ferent energy of the emitting levels exists in between europium
(III) and terbium (III). Therefore, the energy gap between the
ligand triple level and the emitting level of the terbium ion may
Fig. 1. The IR spectra of the free ligand (1) and La(NO3)3L (2).

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X.-L. Tang et al. / Spectrochimica Acta Part A 68 (2007) 349–353
Table 3
Fluorescence data for the complexes
Compound
Eu(NO3)3L
Solvent
Slit (nm)
2.5
λex (nm)
λem (nm)
RFIa
Assignment
Solid state
332
593
617
594
617
593
617
593
618
224
879
158
341
144
561
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
5D0–7F1
5D0–7F2
Acetonitrile
Acetone
5.0
5.0
5.0
325
340
337
Ethanol
3.7
4.3
Tb(NO3)3L
Solid state
Acetonitrile
Acetone
2.5
2.5
2.5
2.5
343
330
340
335
493
546
586
621
492
546
586
622
493
546
586
622
492
546
586
622
1706
5D4–7F6
5D4–7F5
5D4–7F4
5D4–7F3
5D4–7F6
5D4–7F5
5D4–7F4
5D4–7F3
5D4–7F6
5D4–7F5
5D4–7F4
5D4–7F3
5D4–7F6
5D4–7F5
5D4–7F4
5D4–7F3
4229
208
100
1519
3672
185
76
198
450
30
13
19.6
16.3
5.2
4.5
Ethanol
a
RFI is relative fluorescence intensity.
be in favor of the energy transfer process. In addition, the emis-
uptake of a lanthanide ion, but this ajar cavity could not prevent
absolutely the solvent molecules from entering. Thus, the oscil-
latory different solvent molecules consume different energy,
which the ligand triple level transfers to the emitting level of the
lanthanide ion, so that the fluorescence intensities in different
solvents are different. Therefore, the fluorescence of the com-
plexes in EtOH solution can be quenched easily because of the
O–Hhigh-energyoscillatorsandthepossibilityofthecomplexes
to form hydrogen bonds with ethanol or with water contained
in ethanol when the solvent molecules enter this ajar cavity of
the complexes. Meanwhile, because the solvents have different
capabilities of consuming intramolecular energy, the strongest
5
7
sion at 617 nm from D0 → F2 electronic dipole transition is
the strongest, suggesting a low symmetry for the electrostatic
field around the Eu (III) ion [22].
From Table 3 it can be seen that there is little differences
between the excitation maxima because of influences of the dif-
ferent solvents. As shown in Figs. 3 and 4 for the emission,
the europium complex has the strongest fluorescence in acetone
solution, and then in acetonitrile, EtOH solutions. However, the
fluorescence intensities for the terbium complex become weaker
from acetonitrile, acetone to EtOH. This is mainly due to the
coordinating effects of solvents [27]. The amide-based portion
of the ligand forms a caverned conformation suitable for the
Fig. 3. Emission spectra of Eu(NO3)3L in different solutions at room tempera-
ture. Concentration: 1.0 × 10−4 mol L−1. (1) EtOH, (2) acetonitrile, (3) acetone.
Fig. 2. Emission spectra of Tb(NO3)3L (1) and Eu(NO3)3L (2) in the solid state
in slit width 2.5 nm.

X.-L. Tang et al. / Spectrochimica Acta Part A 68 (2007) 349–353
353
ous. Based on those results, a series of new ligands could be
designed and synthesized to optimize the luminescent properties
of these lanthanide ions.
Acknowledgements
We are grateful to the NSFC (Grants 20371022, 20431010,
20021001 and J0530190) and the Specialized Research Fund
for the Doctoral Program of Higher Education (200307300015)
for financial support.
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4. Conclusion
According to the data and discussion above, the new
amide-based crown ether ligand containing 2,5-diaryl-1,3,4-
oxadiazole could form stable complexes with lanthanide nitrates
(L:metal = 1:1). In the complexes, the lanthanide ions were coor-
dinated to the C O oxygen atoms and influencing oxadiazole
ring. Scheme 2 shows the most probable coordination structure
with lanthanide ions, and the coordination number of lanthanide
ions is 8.
The europium and terbium complexes exhibited strong char-
acteristic fluorescence of europium and terbium ions. The
solvent factors influencing the fluorescent intensity were obvi-
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