Studies on synthesis and infrared and fluorescence spectra of new europium and terbium complexes with an amide-based open-chain crown ether

Article abstract of DOI:10.1016/S1386-1425(01)00680-1

An amide-based open-chain crown ether ligand and its complexes with europium and terbium were synthesized. The complexes were characterized by elemental analysis, infrared spectra and conductivity. The europium and terbium ions were found to coordinate to

Full text of DOI:10.1016/S1386-1425(01)00680-1

Spectrochimica Acta Part A 58 (2002) 2153–2157
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Studies on synthesis and infrared and fluorescence spectra of
new europium and terbium complexes with an amide-based
open-chain crown ether
Yan-Ling Zhang, Wen-Wu Qin, Wei-Sheng Liu *, Min-Yun Tan, Ning Tang
College of Chemistry and Chemical Engineering, Lanzhou Uni6ersity, Lanzhou 730000, People’s Republic of China
Received 7 August 2001; received in revised form 13 November 2001; accepted 13 November 2001
Abstract
An amide-based open-chain crown ether ligand and its complexes with europium and terbium were synthesized.
The complexes were characterized by elemental analysis, infrared spectra and conductivity. The europium and
terbium ions were found to coordinate to the CꢀO oxygen atoms and pyridine nitrogen atoms. The fluorescence
properties of these complexes in DMF and CH3OH/CHCl3 were studied. Under the excitation of UV light, these
complexes exhibit characteristic fluorescence of europium and terbium ions. The solvent factors influencing the
fluorescent intensity are discussed. © 2002 Elsevier Science B.V. All rights reserved.
Keywords: Amide-based open-chain crown ether; Complexes; Synthesis; Fluorescence properties
ions by facilitating the well-known light conver-
sion process, the antenna effect [3]. Recently, cal-
ixarenes and terpyridine-like ligands have
attracted much attention mainly because they can
form highly stable and strongly luminescent eu-
ropium and terbium ion complexes [4,5].
Open-chain crown ethers offer many advan-
tages in extraction and analysis (ion-selective elec-
trodes) of the rare earth ions [6,7]. The open-chain
crown ethers containing amide groups possess
suitable molecular structure: a chain with inflex-
ible terminal groups. Therefore, they are excellent
reagents for activating ion-selective electrodes and
extracts of rare earth ions [8]. However, the lu-
minescent properties on amide-based open-chain
1. Introduction
The development of luminescent chemical
probes and sensors is the subject of intensive
research both in natural and medical science.
Probes based on europium and terbium ions are
of special interest because of the particularly suit-
able spectroscopic properties of these ions [1,2].
Moreover, chemists have realized that it is essen-
tial to design the encapsulating ligand to optimize
the luminescent properties of these lanthanide
* Corresponding author. Fax: +86-931-891-2582
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1386-1425/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved.
PII: S1386-1425(01)00680-1

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Y.-L. Zhang et al. / Spectrochimica Acta Part A 58 (2002) 2153–2157
crown ethers with lanthanide complexes have
been rarely reported [9]. Due to their low lumines-
cence, recently, we have designed a series of poly-
functional ligands having both selective ability to
coordinate lanthanide ions and enhanced lumines-
cence of lanthanide complexes, by providing
proper conjugate absorption groups suitable for
energy transfer. In the present work, we designed
and synthesized a new and doubly functionalized
open-chain crown ether ligand with pyridine
amide terminal groups, 1,7-bis[N-(2-pridinoyl ani-
line)]-4-p-toluene sulfonyl-1,7-dioxa-4-azaheptane
(Scheme 1), and studied the fluorescence proper-
ties of europium and terbium complexes with the
new ligand. The results indicated that the organic
solvent affected the fluorescence characteristics of
europium and terbium ions.
2. Experimental
2.1. Materials
All solvents used were purified by standard
methods.
2.2. Methods
The metal ions were determined by EDTA ti-
tration using xylenol orange as an indicator.
Combustion analyses were determined using a
Vario EL elemental analyzer. The IR spectra were
recorded on a Nicolet Avatar 360 FT-IR instru-
ment. Conductivity measurements were carried
out with a DDSJ-308 type conductivity bridge
using 1.0×10−3 mol dm−3 solutions in dimethyl-
Scheme 1. The synthesis of ligand.

Y.-L. Zhang et al. / Spectrochimica Acta Part A 58 (2002) 2153–2157
2155
Table 1
Elemental analytical and molar conductance date for the complexes
Complex
C (%) found
(Calc.)
H (%) found
(Calc.)
N (%) found
(Calc.)
Ln (%) Found
(Calc.)
Lm
(S cm2 mol−1
)
L
64.48 (64.50)
40.51 (40.98)
4.98 (5.10)
3.55 (3.64)
10.70 (10.75)
11.07 (10.92)
Eu(NO3)3
15.13 (14.82)
15.78 (15.39)
79
73
L · 2H2O
Tb(NO3)3
40.39 (40.71)
3.44 (3.61)
11.05 (10.85)
L · 2H2O
formamide (DMF) at 25 °C. 1H NMR spectra
were measured on a FT-80A spectrometer in
CDCl3 solution, with TMS as internal standard.
Fluorescence measurements were made on a Shi-
madzu RF-540 spectrfluorophotometer equipped
with quartz curettes of 1 cm path length. The
excitation and emission slit widths were 10 nm.
collected and washed three times with ethyl ace-
tate. Further drying in vacuum afforded a pale
white powder, yield 50%.
3. Result and discussion
3.1. Properties of the complexes
2.3. Synthesis of the ligand
Analytical data for the complexes, presented in
Table 1, conform to Eu (NO3)3L · 2H2O and Tb
(NO3)3L · 2H2O.
1, 2, 3, (Scheme 1) were prepared according to
the literature [10,11].
All complexes are soluble in DMF, acetenitride,
tetrahydrofuran, methanol/chloroform (1:1), but
sparingly soluble in water, methanol, ethanol and
acetone. Conductivity measurements for these
complexes in DMF solution (Table 1) indicate
that all complexes are 1:1 ionic compounds [12].
2.3.1. Preparation of 4
A benzene solution containing 3 (3.47 mmol)
was added dropwise to another benzene solution
of pyridinoyl chloride (10.4 mmol) and anhydrous
pyridine (2 ml). The mixture was stirred at 50 °C
for 6 h. After solvent evaporation in vacuum, the
crude was chromatographed on silica gel (CHCl3/
EtOAc 5:1) to afford the ligand 4 as a pale white
3.2. IR spectra
1
solid. H NMR: d (CDCl3) 10.5 (s, 2H, 2NH);
The IR spectra of the complexes are similar.
Table 2 gives the characteristic bands of ligand
and its complexes. The IR spectrum of the free
ligand shows bands at 1685 and 1531 cm−1 which
may be assigned to w(CꢀO) and w(CꢀN), respec-
tively. In the complexes, these bands for w(CꢀO)
and w(CꢀN) are shifted by about 46 (from 1685 to
1639) and 21 (from 1531 to 1552) cm−1, respec-
tively, indicating that carbonyl oxygen atoms and
pyridine nitrogen atoms take part in coordination.
The absorption bands assigned to the coordi-
nated nitrates were observed at about 1492 (was)
and 814 (ws) cm−1 for the complexes. The w3(E%)
free nitrates appear at approximately 1384 cm−1
in the spectra of the complexes [13], in agreement
with the results of the conductivity experiments.
8.6–7.8 (m, 8H, PyꢁH); 7.4–6.9 (m, 8H, ArꢁH);
4.5–4.3 (q, 8H, 4CH2); 2.3 (s, 3H, CH3) IR: cm−1
(KBr, pellet). w 3326 (m, NꢁH), 1685 (s, CꢀO),
1531 (S, CꢀN), 1156 (S, CꢁOꢁC), l 619 (m,
Py-ring, in-plane). Elemental analytical: (calc.)
C% 64.48 (64.50), H% 4.98 (5.10), N% 10.70
(10.75).
2.4. Synthesis of the complexes
An ethyl acetate solution of Ln (NO3)3 · 6H2O
LnEu3 and Tb3 (0.15 mmol) was added dropwise
to a solution of the ligand (0.1 mmol) in ethyl
acetate (20 ml). The mixture was refluxed for 4 h,
and white precipitate formed. The precipitate was

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Y.-L. Zhang et al. / Spectrochimica Acta Part A 58 (2002) 2153–2157
Table 2
The relevant characteristic IR bands (cm−1
)
Compound
w(CꢀO)
w(CꢀN)
w1(NO−3
)
w3(NO−3
)
w4(NO−3
)
w(NO−3
)
L
1685
1639
1641
1531
1552
1553
Eu(NO3)3L · 2H2O
Tb(NO3)3L · 2H2O
1492
1507
815
814
1307
1295
1384
1385
Table 3
Fluorescence data for the ligand and complexes
Solve
uex (nm)
uem (nm)
RFI
Assignment
L
DMF
CH3OH/CHCl3
DMF
370
380
417
435
591
616
591
616
488
545
590
488
545
590
1126
242
16.3
23.5
8.0
4.9
165.2
58.5
6.6
26.5
9.3
Eu(NO3)3L · 2H2O
5D0“7F1
5D0“7F2
5D0“7F1
5D0“7F2
5D4“7F6
5D4“7F5
5D4“7F4
5D4“7F6
5D4“7F5
5D4“7F4
CH3OH/CHCl3
395
370
Tb(NO3)3L · 2H2O
DMF
CH3OH/CHCl3
380
1.6
In addition, the separation of the two highest
frequency bands ꢀw4−w1ꢀ is approximately 200
cm−1, thus the coordinated NO−3 ions in the
complexes are bidentate ligands [14].
the excitation wavelength of terbium complex is
the same as that of the ligand in DMF or in
CH3OH/CHCl3 solvent, respectively. But its emis-
sion wavelength was shifted to 488, 545 and 590
nm (Fig. 2).
3.3. Fluorescence studies
The fluorescence characteristics of the ligand
and all complexes in DMF and CH3OH/CHCl3
(1:1 v/v) solutions are listed in Table 3. The ligand
having multiple aromatic rings with a rigid planar
structure, so it is a strong fluorescence substance.
It displays a fluorescence excitation maximum at
370 nm and an emission maximum at 417 nm in
DMF solvent. But in CH3OH/CHCl3 solution,
the excitation maximum and emission maximum
of the ligand are red-shifted by approximately 10
and 18 nm compared with those in DMF solvent,
respectively. From Table 3 it could be seen that
the excitation and emission wavelengths of the
europium complexes in organic solutions are quite
different from those of the ligand, its uex shift to
395 nm and uem to 591, 616 nm (Fig. 1). However,
Fig. 1. The emission spectrum of the europium complex.
Concentration: 5.0×10−4 mol l−1. One in DMF and two in
CH3OH/CHCl3 (1:1) solution.

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2157
amide-base open-chain crown ether could form
stable complex with europium and terbium ions.
Obvious IR spectrum changes were observed after
the ligand formed complexes with the two ions. In
the complexes, europium and terbium ions were
coordinated to the CꢀO oxygen atoms and pyridine
nitrogen atoms. The complexes exhibited charac-
teristic fluorescence of europium and terbium ions.
Based on those results, a series of new amide-based
open-chain crown ethers could be designed and
synthesized to optimize the luminescent properties
of these lanthanide ions.
Fig. 2. The emission spectrum of the terbium complex. Con-
centration: 5.0×10−4 mol l−1. One in DMF and two in
CH3OH/CHCl3 (1:1) solution.
Acknowledgements
This work is support by the National Natural
Science Foundation of China (Project 20071015)
and Foundation for University Key teacher by the
Ministry of Education (China).
Due to the presence of a scattering signal near
490 nm, the peak height at 545 nm for terbium was
used to measure the fluorescence intensities. From
Table 3, we can see that the fluorescence intensity
of terbium complex at 545 nm are stronger than
those of europium complexes at 616 nm, either in
DMF or in CH3OH/CHCl3 solution. The lumines-
cence of Ln3+ chelates is related to the efficiency
of the intramolecular energy transfer between the
triplet level of ligand and the emitting level of the
ions, which depends on the energy gap between the
two levels. In the organic solution, probably the
energy gap between the ligand triplet levels and the
emitting level of the terbium favor to the energy
transfer process for terbium.
We also can see the fluorescence intensities for
the complexes in DMF solution are stronger than
those in CH3OH/CHCl3 solution. We consider that
this is due to the OꢁH oscillators of CH3OH
molecules. It is well know that the excited state of
the lanthanide ions is efficiently quenched by inter-
actions with high-energy vibrations like OꢁH
groups. Therefore, the fluorescence of the com-
plexes in CH3OH/CHCl3 solution can be quenched
easily because of the OꢁH oscillators.
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4. Conclusion
According to the data and discussion above, the