Synthesis and infrared and fluorescence spectral properties of luminescent terbium and europium complexes with open-chain carboxylate crown ethers.

Article abstract of DOI:10.1016/S1386-1425(03)00112-4

Two open-chain carboxylate crown ether ligands and their terbium(III) and europium(III) complexes were synthesized and characterized. The terbium and europium ions were found to coordinate to the carboxylate oxygens. The fluorescence properties of europium complexes in the solid state and terbium complexes in the solid state and in the organic solvent were studied in detail, respectively. Under the excitation of ultraviolet light, strong green fluorescence of solid terbium complexes and red fluorescence of solid europium complexes were observed. These observations show that the two ligands favor energy transfer to the emitting energy level of Tb(3+). Some factors that influencing the fluorescent intensity were also discussed.

Full text of DOI:10.1016/S1386-1425(03)00112-4

Spectrochimica Acta Part A 59 (2003) 3085ꢁ3092
/
www.elsevier.com/locate/saa
Synthesis and infrared and fluorescence spectral properties of
luminescent terbium and europium complexes with open-chain
carboxylate crown ethers
Wenwu Qin, Yanling Zhang, Weisheng Liu *, Minyu Tan
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
Received 5 June 2002; received in revised form 25 February 2003; accepted 25 February 2003
Abstract
Two open-chain carboxylate crown ether ligands and their terbium(III) and europium(III) complexes were
synthesized and characterized. The terbium and europium ions were found to coordinate to the carboxylate oxygens.
The fluorescence properties of europium complexes in the solid state and terbium complexes in the solid state and in the
organic solvent were studied in detail, respectively. Under the excitation of ultraviolet light, strong green fluorescence of
solid terbium complexes and red fluorescence of solid europium complexes were observed. These observations show
3
ꢀ
that the two ligands favor energy transfer to the emitting energy level of Tb . Some factors that influencing the
fluorescent intensity were also discussed.
#
2003 Elsevier Science B.V. All rights reserved.
Keywords: Synthesis; Complexes; Infrared and fluorescence spectral properties; Open-chain carboxylate crown ethers
1
. Introduction
vatives [5] and pyrazolone derivatives [6] have
attracted much attention mainly because they can
form highly stable and strongly luminescent ter-
bium and europium ion complexes.
Open-chain crown ethers offer many advantages
in separation and determination of lanthanide ions
Much attention has been paid to lanthanide
complexes because of their particular luminescence
properties. Especially europium(III) and terbiu-
m(III) ions have excellent luminescence properties,
but their absorption coefficients are very small. In
order to overcome these shortages, lanthanide ions
are usually chelated with ligands that have broad
intense absorption bands. Recently, podands [1,2],
macrocyclic compound [3,4], carboxylic acid deri-
[7ꢁ
open-chain crown ether molecules consists of a
chain of several oxygen atoms connected by (ꢀ
CH ) groups. When proper conjugate absorption
groups are introduced onto these (ꢀ ), they
/9]. The basic molecular frame of ordinary
/
ꢀ
/
2
/
CH2ꢀ
/
will serve as potential luminescent reagents. In the
present work, we designed and synthesized two
new and doubly functionalized open-chain carbox-
ylate crown ether ligands. They have both coordi-
*
E-mail address: liuws@lzu.edu.cn (W. Liu).
Corresponding author. Tel./fax: ꢀ
/
86-931-891-2582.
1386-1425/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1386-1425(03)00112-4

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/3092
Fig. 1. Scheme of the synthesis of the ligands.
nation selective abilities to lanthanide ions and
enhanced luminescence of lanthanide complexes.
The fluorescence properties of europium(III) and
terbium(III) complexes with new ligands were
studied in detail. IR spectra show that the ethereal
oxygens of the two ligands do not take park in
coordination. But the length of the chains of the
ethereal oxygens affects the energy transfer effi-
ciency between ligands and rare earth ions. The
role of solvents in the fluorescence intensities is
also reported.
Shanghai Instrument Factory DDSJ-308 type
conductivity bridge with approximately 1.0ꢂ
/
ꢃ3
ꢃ1
1
10
mol l
solutions of methanol at 25 8C. H
NMR spectra were recorded on a Varian FT-80A
spectrometer using chloroform-d as a solvent and
TMS as an internal standard. Mass spectra of the
ligands were recorded on a VG ZAB-HS mass
spectrometer. Fluorescence measurements were
made on a Shimadzu RF-540 spectrofluorophot-
ometer equipped with quartz curettes of 1 cm path
length at room temperature. The excitation and
emission slit widths were 10 nm.
2
. Experimental
I
II
.3. Preparation of ligands (L and L )
2
2
.1. Materials
Two ligands were prepared by the same method.
Compound 1 (Fig. 1) was prepared according to
the literature [10].
All material and solvents employed in this study
were analytical reagents. Absolute ethanol was
dried and distilled by standard method.
Metal sodium (0.30 g, 13 mmol) was slowly
added to a hot anhydrous ethanol (20 ml) solution.
Then ethyl salicylate (2.20 g, 13 mmol) was added.
Finally 4.3 mmol of 1 was added and refluxing for
20 h. The solvent was partially removed in vacuum
at about 40 8C The crude product was chromato-
graphed on silica gel (petroleum ether/
CH COOC H 5:1 v/v) to afford 2 as white oil.
2
.2. Physical measurements
Carbon and hydrogen analyses were performed
using a Vario EL elemental analyzer. The com-
plexes were decomposed in stove at about 500 8C
for 1 h. The residue was dissolved in HCl and then
metal ions were determined by EDTA titration
using xylenol orange as an indicator. Infrared (IR)
spectra were obtained with a Nicolet Nesus 670
FT-IR spectrophotometer with KBr discs. Con-
ductivity measurements were carried out with a
3
2
5
A mixture solution of 2 (2.5 mmol) and KOH
(1.4 g, 25 mmol) in CH OH (30 ml) and H O (12
3
2
ml) was refluxed for 6 h. Then concentrated HCl
was added and white precipitates formed. The
precipitates were collected by filtration, wash with

W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ
/3092
3087
I
water and dried under in vacuo to afford 3 (L and
II
L ) as white powder.
3. Results and discussion
3.1. Properties of the complexes
2
.3.1. 1,7-Bis(2?-carboxylphenyl)-1,4,7-
I
Elemental analyses and molar conductance of
the complexes are listed in Table 1. All complexes
are soluble in methanol, DMF, DMSO and THF/
triaoxaheptane(L , nꢄ
Yield, 80%. M.p. 106ꢁ
259 (OꢀH), 1734 (Cꢁ
phenyl CꢀC), 1244 (Arꢀ
/
0)
108 8C, IR (cmꢃ1, KBr),
O), 1603, 1489, 1456
OꢀC), 1168 (CꢀOꢀC),
/
3
/
/
H O (v/v, 1:1) moderately soluble in chloroform
2
(
/
/
/
/
/
and THF, and sparingly soluble in ethanol,
acetonitrile, acetone and ethyl acetate. The ele-
mental analyses of the complexes show that the
lanthanide ions coordinate to ligand with 2:3
ratios. Conductivity measurements for these com-
plexes in methanol solution indicate that all
complexes are a partial ionization in methanol
1
7
58 (phenyl subst). H NMR (CDCl , ppm): d 4.00
3
(
t, 4H, 2O
CH2OAr), 7.00ꢁ
m/zꢄ347 (Mꢀ
/
CH
/
CH OAr), 4.41 (t, 4H, 2OCH
/
2
2
2
/
/
8.16 (m, 8H, Ar-H). FABMS:
/
/1). Found: C, 62.18; H,
5
.56. Anal. Calc. for C H O : C, 62.43; H,
18 18 7
5
.24%.
[
11].
3
.2. IR spectra
2
.3.2. 1,10-Bis(2?-carboxylphenyl)-1,4,7,10-
II
tetraoxadecane(L , nꢄ
Yield, 82%. M.p. 116ꢁ
259 (OꢀH), 1716 (Cꢁ
phenyl CꢀC), 1243 (Arꢀ
61 (phenyl subst). H NMR (CDCl , ppm): d 3.78
/
1)
118 8C. IR (cmꢃ1, KBr),
O), 1602, 1484, 1461
OꢀC), 1166 (CꢀOꢀC),
IR spectra of the complexes are similar to each
other. The main infrared bands of the ligands and
their complexes are presented in Table 2 and Figs.
/
3
/
/
(
/
/
/
/
/
1
2
and 3 show main infrared data of the two
europium(III) complexes. The salicylic acids of
both free ligands give OꢀH stretching band forꢀ
7
3
(
s, 4H, O
/
CH
/
/
CH
/
2O), 3.93 (t, 4H, 2OH
/
CH
/
2OAr),
8.15 (m, 8H,
COOH signal of the
2
2
/
/
4
.38 (t, 4H, 2OCH
/
CH
/
2OAr), 6.99ꢁ
/
2
ꢃ1
COOH group at 3259 cm , disappearance of
which in the complexes are caused by the replace-
ment of hydrogen of the carboxylate group by the
metal. The disappearance of carbonyl stretching
Ar-H). In this condition, the ꢀ
two free ligands could not be observed in the H
NMR spectra either, FABMS: m/zꢄ391 (Mꢀ1),
Found: C, 61.39; H, 5.87. Anal. Calc. for
C H O : C, 61.53; H, 5.68%.
/
1
/
/
I
II
ꢃ1
frequency at 1734 (L ) or 1716 (L ) cm
carboxylic acid and appearance of symmetric and
asymmetric vibrations for ꢀCOO group at about
407ꢁ1411 and 1551ꢁ1561 cm , respectively,
confirms the above fact. The stretching frequency
in
2
0
22
8
/
ꢃ1
1
/
/
2
.4. Synthesis of complexes
ꢃ1
difference (Dnꢄ
carboxylate group is smaller in the complexes than
that of the corresponding sodium salt (about 205
cm ). This indicated that the coordinated car-
boxylates in the complexes are bidentate [12,13].
/
n ꢃ
/
ns) (140ꢁ
/
154 cm ) of the
as
I
To a solution of ligand L or L (0.3 mmol) in
II
4
added until ligands were dissolved completely and
.0 ml ethanolꢁwater (1:1 v/v), 1:1 NH3×/H O was
/
2
ꢃ1
the pH of the mixture was adjusted about 6.0.
Then
a
2.0 ml ethanolꢁ/water solution of
In addition, the n(Arꢀ
/
Oꢀ
/
C) and n(Cꢀ
/
Oꢀ
/
C)
Ln(NO ) ×
/
6H O [LnꢄEu(III) or Tb(III), 0.2
/
bands of the free ligands have hardly changed
compared with those of their complexes. This
shows that the ethereal oxygens of the ligands
3
3
2
mmol] was added dropwise to the solution of
ligands. The mixture was refluxed for 4 h, and then
a white precipitate was formed. The precipitate
was collected and washed three times with
have not been coordinated to the metal ion. The
ꢃ1
new bands in the 409ꢁ
/
413 cm
regions in the
O stretching
ethanolꢁ
a white powder in about 80%.
/water. Further drying in vacuo afforded
complexes may be assigned to Lnꢀ
vibration modes.
/

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W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ3092
/
Table 1
Analytical and molar conductance data for the complexes
Lm (S cm molꢃ1)
2
Compound
C (%) Found (Calc.)
H (%) Found (Calc.)
Ln (%) Found (Calc.)
I
Eu
2
Tb
2
Eu
2
Tb
2
L
L
L
L
3
I
3
II
3
×
×
×
/
6H
2
O
O
44.65 (44.89)
44.51 (44.46)
45.93 (45.69)
45.01 (45.29)
4.05 (4.19)
4.12 (4.15)
4.34 (4.60)
4.87 (4.56)
21.33 (21.03)
21.51 (21.79)
19.02 (19.27)
19.64 (19.98)
65.9
65.0
61.3
59.7
/6H
2
/
6H
2
O
O
II
3
×6H
2
/
I
3
3
.3. Fluorescence spectra
Under identical experimental conditions, the
fluorescence characteristics of the complexes in
The emission spectra of the Tb L ×
/
6H O complex
2
2
are shown in Fig. 5.
The fluorescence intensities of the terbium
complex at 545 nm are stronger than those of the
europium complex at 613 nm in the solid state (see
solid and in organic solvent at various concentra-
II
3
tions (1.0ꢁ
/0.1 mM) are listed in Table 3. The
Table 3). The fluorescence intensities of Tb L ×
6H O are about 100 times than those of europium
/
2
emission spectra of the two europium complexes in
solid are similar to each other (Fig. 4). They both
exhibit three emission bands at about 592, 613 and
2
complexes. The fluorescence intensities of com-
II
I
/
plexes are in the order of Tb L ×
/
6H Oꢀ
/
Tb L ×
2
II
3
2
2 3
5
7
5
7
I
3
6
80 nm, corresponding to D 0
/
F , D 0
/
F2
F emission
6H Oꢀ
/
Eu L ×
/
6H Oꢀ
/
Eu L ×
/
6H O. Based on
3 2
0
5
1
7
0
2
2
2
2
5
7
and D 0
/
F transitions. The D 0
/
the theory of antenna effect [14,15], the intensity
ꢀ
0
4
0
3
3
peaks were too weak to be observed at about 645ꢁ
/
of the luminescence of Ln
complexes is related
6
60 nm range. Among these transitions, the
7
0
to the efficiency of the intramolecular energy
transfer between the triplet energy level of the
ligand and the emitting level of the central ions,
which depends on the energy gap between the two
levels. In the solid state, probably the energy gap
between the ligands triplet levels and the emitting
level of the terbium favors the energy transfer
process for terbium. This leads to the terbium
complexes that show the most intense green
fluorescence.
5
D 0
/
F around 613 nm is the strongest and,
2
therefore, the peak height at 613 nm was used to
measure the fluorescence intensities of an euro-
pium complex. From Table 3, we can see that
terbium complexes are strongly fluorescent in the
solid state and in organic solution. The emission
spectra of the terbium complexes in solid and
solution are similar to each other and are all
characteristic emission bands of a terbium(III) ion
at about 490, 545 and 585 nm. The emission peak
at 545 nm is the strongest, followed by the one at
According to the results in Table 3, the relative
fluorescence intensity of terbium complexes in
II
powder is highest for ligand L . Whereas in
solutions, the fluorescence intensity of terbium
4
90 nm. Therefore, the peak height at 545 nm for
terbium complexes was chosen to detect the
fluorescence intensities of a terbium complex.
I
complexes for ligand L is stronger than that for
Table 2
IR spectral data of the free ligands and its complexes (cmꢃ1)
Compound
n (Oꢀ/H)
n (Cꢁ
/
O)
n
as (COOꢃ
)
n
s
(COOꢃ
)
n (Arꢀ
/
Oꢀ/C)
n (Cꢀ
/
Oꢀ
/
C)
n (LnꢀO)
/
I
L
3259
3431
3387
3259
3425
3425
1734
1244
1243
1243
1243
1245
1243
1168
1165
1169
1166
1162
1162
I
3
I
Eu
Tb
2
L
L
×
×
/
6H
2
O
O
1553
1551
1407
1411
409
410
2
3
/6H
2
II
L
1716
II
3
II
Eu
Tb
2
L
L
×
/
6H
2
O
O
1558
1561
1407
1407
413
412
2
3
×6H
2
/

W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ
/3092
3089
I
2 3 2
Fig. 2. The IR spectra of the Eu L /6H O complex.
×
II
ligand L . Although the ethereal oxygens of the
result in increasing the distance between the ligand
II
ligands do not take park in the coordination, the
length of chains affects energy transfer efficiency.
In the solid state the molecules are closely packed
L
between the two in solid state. The increasing of
and Tb(III) ion compared with the distance
II
distance between ligand L and Tb(III) ion can
II
[
16] so that the ligand L may has the optimum
decrease the energy transfer efficiency from the
aromatic ring of the ligand to Tb(III) ion resulting
length suitable for transferring energy to the
emitting level of terbium ion. However, the com-
plex molecules are isolated in solutions. This can
II
in fluorescence intensity of ligand L lower than
I
that of ligand L . That is to say, ligand L may
I
II
3
Fig. 3. The IR spectra of the Eu
2
L
×
/
6H
2
O complex.

3090
W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ3092
/
Table 3
Fluorescence data of the complexes at room temperature
a
Complexes
Solve or state
Concentration (0.1 mM)
lex (nm)
l
em (nm)
RFI
I
3
II
3
I
3
Eu
Eu
Tb
2
2
2
L
L
L
×
/
6H
2
O
Solid
Solid
396
396
318
305
613
613
545
545
49.0
35.0
×
/
6H
2
O
×
/
6H
2
O
Solid
DMSO
600.0
453.1
415.6
378.1
362.5
306.3
406.3
361.3
315.6
275
10.0
7
5
2
1
.5
.0
.5
.0
CH OH
3
10.0
315
310
310
545
545
545
7
5
2
1
.5
.0
.5
.0
243.8
235.0
202.5
158.7
116.9
86.9
DMF
10.0
7
5
2
1
.5
.0
.5
.0
THF/H
2
O
10.0
111.7
98.8
85.6
7
5
2
1
.5
.0
.5
.0
75.0
60.3
II
3
Tb
2
L
×
/
6H
2
O
Solid
DMSO
312
308
545
545
4400
362.5
334.4
305.0
266.9
229.3
316.3
278.1
221.9
198.1
145.6
208.8
171.2
148.8
123.4
116.9
100.8
92.5
10.0
7
5
2
1
.5
.0
.5
.0
CH OH
3
10.0
308
310
310
545
545
545
7
5
2
1
.5
.0
.5
.0
DMF
10.0
7
5
2
1
.5
.0
.5
.0
THF/H
2
O
10.0
7
5
2
1
.5
.0
.5
.0
81.3
63.2
52.8
a
RFI is relative fluorescence intensity.
have the favorable distance to transfer energy to
Tb(III) ion under solution condition. The fluores-
cence intensities of terbium complexes in organic
solution are weaker than that of powder. This may
be due to the quench process of solvent molecules
in the solutions.
The influence of solvent on the fluorescence
intensities of two terbium complexes was investi-

W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ
/3092
3091
are arranged in the order of 46.7 (DMSO)ꢀ
/
32.63
(
CH OH)ꢀ7.0 (THF). The order of fluorescence
/
3
intensities in these solvents is in agreement with
the o values of them. This indicates that the polar
of solvents effect the fluorescent intensities of these
complexes.
Further investigation shows that the intensities
in DMF, DMSO and CH OH solutions are much
3
stronger than those in THF/H O solution. This
2
may be due to the Oꢀ/H oscillators of H O
2
molecules. It is well known that the excited state
of the lanthanide ions is efficiently quenched by
interactions with high-energy vibrations like Oꢀ
/H
groups. Solvent molecules have strong coordina-
tion effect and can replace coordinated water. The
I
3
Fig. 4. The fluorescence emission spectra of the Eu
2
L
×
/
6H
II
2 3 2
×/6H
L O (line 2) complexes in the solid state at
2
O
replacement can avoid the high frequency Oꢀ
/H
(line 1) and Eu
bond vibration and thus greatly decrease the
energy loss through the coordination water to
obtain much stronger fluorescence intensity. This
indicates that environment plays an important role
in determining the fluorescence intensity of the
complexes [17].
room temperature.
gated. As given in Table 3, the fluorescence
intensities sequence both have been shown to be
DMSOꢀ
/
CH OHꢀ
/
DMFꢀTHF/H O at the
same concentration. The o values of the four
solvents are 46.7 (DMSO), 32.63 (CH OH), 37.6
/
3
2
In addition, the fluorescence spectra of euro-
pium complexes also provide information about
the geometry of the complexes. Although there are
3
(DMF) and 7.0 (THF). It can be see that the o
values of these solvents that contain oxygen atom
I
3
Fig. 5. The fluorescence emission spectra of the Tb
THF/H
2
L
×
/
6H
2
O complex in DMSO (line 1), in CH
ꢃ4
3
OH (line 2), in DMF (line 3) and in
ꢃ1
2
O (1:1 v/v) (line 4) solution at room temperature. Concentration: 5.0ꢂ
/10
mol l
.

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W. Qin et al. / Spectrochimica Acta Part A 59 (2003) 3085ꢁ3092
/
[
3] M. Pietraszkiewicz, T. Karpiuk, A.K. Rout, Pure Appl.
short of the information about the number of lines
resulting from the splitting of an individual
transition due to the resolving power of instru-
ment. We have a tentative conclusion a low site
symmetry for the europium ions, based on the fact
Chem. 65 (1993) 563.
[
[
4] A. Beeby, S. Faulkner, Chem. Phys. Lett. 266 (1997) 116.
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5
7
that D 0
/
F transition could not be observed
0
0
5
7
5
7
[7] Y.S. Yang, Y.Z. Ding, G.Z. Tan, J.Z. Xu, Z.Q. Yao, F.S.
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[
18] and the value of D 0
/
F / D 0
/
F1 (the
are also
0
2
0
5
7
fluorescence intensities of D 0
/
F
1
0
[8] Y.Z. Ding, J.Z. Xu, Y.S. Yang, G.Z. Tan, J.Z. Xu,
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[
Acknowledgements
2
27.
This work is supported by the National Natural
Science Foundation of China (Project 20071015),
Foundation for University Key Teacher by the
Ministry of Education of China and Key Project
of the Ministry of Education (Project 01170).
[
13] K. Nakamoto, Infrared and Raman spectra of Inorganic
and Coordination Compounds, third ed., John Wiley, New
York, 1978, p. 227.
[
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