Synthesis and luminescence properties of lanthanide complexes with a new aryl amide bifunctional bridging ligand

Article abstract of DOI:10.1016/j.saa.2005.07.062

A new aryl amide type bifunctional bridging ligand 4,4′-bis{[(2″-benzylaminoformyl)phenoxyl]methyl}-1,1′-biphenyl (L) and its complexes with lanthanide ions (Ln = Pr, Eu, Gd, Tb, Ho, Er) were synthesized and characterized by elemental analysis, infrared s

Full text of DOI:10.1016/j.saa.2005.07.062

Spectrochimica Acta Part A 64 (2006) 595–599
Synthesis and luminescence properties of lanthanide complexes
with a new aryl amide bifunctional bridging ligand
Yu-Lan Song, Yu Tang∗,
Wei-Sheng Liu, Min-Yu Tan
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, PR China
Received 28 June 2005; accepted 29 July 2005
Abstract
Anewarylamidetypebifunctionalbridgingligand4,4ꢀ-bis{[(2ꢀꢀ-benzylaminoformyl)phenoxyl]methyl}-1,1ꢀ-biphenyl(L)anditscomplexeswith
lanthanide ions (Ln = Pr, Eu, Gd, Tb, Ho, Er) were synthesized and characterized by elemental analysis, infrared spectra, conductivity measurements
and thermal analysis. At the same time, the luminescence properties of the Eu and Tb complexes in acetone solutions were investigated. Under the
excitation of UV light, these two complexes exhibited characteristic emission of europium and terbium ions. And the lowest triplet state energy
level T1 of this ligand matches better to the lowest resonance energy level of Tb(III) than to Eu(III) ion.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Aryl amide bifunctional bridging ligand; Lanthanide(III) complexes; Synthesis; Luminescence properties
1. Introduction
energy level T1 of the ligand to the resonance level of Ln(III)
[6]. And this energy transfer process is one of the most impor-
tant processes having influence on the luminescence properties
of the lanthanide complexes.
The emission of light from lanthanide ions arises from f–f
transitions, which result in emission bands with extremely nar-
row bandwidth and no theoretical cap on the quantum efficiency.
This makes lanthanide ions very attractive for a variety of appli-
cations, such as chromophores for LEDs and as probes and labels
in a variety of biological and chemical applications [1]. And
the probes based on europium and terbium ions are of special
interest because of the particularly suitable spectroscopic prop-
erties of these ions, such as their large stokes shift and narrow
emission profiles [2,3]. Since the f–f transitions are spin- and
parity-forbidden, the excited state of the lanthanide ion is pop-
ulated through intramolecular energy transfer from the ligand
or ligands, which, therefore, serve as sensitizers (antenna effect)
[4,5]. Theroleoftheligandsisononehand, tocollectthephotons
provided by the light source in order to allow an energy transfer
to the emitting levels of the Ln(III) ion and on the other to shield
it against the solvent in order to avoid non-radioactive deactiva-
tion processes. It is generally accepted that the energy transfer
from ligand to Ln(III) ion occurs from the lowest triplet state
Recently, polynuclear lanthanide complexes, especially with
luminescence properties, are attracting more and more chemists
[7]. Our group is interested in the supramolecular coordination
chemistry of lanthanide ions with amide type ligands that have
strong coordination capability to the lanthanide ions and termi-
nalgroupeffects[8]. Wehavedesignedaseriesofpolyfunctional
ligands having both selective ability to coordinate more than one
lanthanide ions and enhanced luminescence of lanthanide com-
plexes by providing some of proper conjugate absorption groups
suitable for energy transfer. In the present work, we designed
and synthesized a new aryl amide type bifunctional bridging
ligand 4,4ꢀ-bis{[(2ꢀꢀ-benzylaminoformyl)phenoxyl]methyl}-
1,1ꢀ-biphenyl (L) (Scheme 1) and studied the luminescence
properties of europium and terbium complexes with this new
ligand. Under the excitation of UV light, Eu and Tb complexes
exhibited characteristic emission of lanthanide ions. The lowest
triplet state energy level of the ligand was calculated from the
phosphorescence spectrum of the Gd complex at 77 K. The
result indicate that the triplet state enengy level of the ligand
matches better to the resonance level of Tb(III) than to Eu(III)
ion.
∗
Corresponding author. Tel.: +86 9318912552; fax: +86 9318912582.
E-mail address: tangyu@lzu.edu.cn (Y. Tang).
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.07.062

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Y.-L. Song et al. / Spectrochimica Acta Part A 64 (2006) 595–599
Scheme 1. The synthetic route for the ligand L.
L, yield 80%, m.p. 132–134 ◦C; 1H NMR (DMSO, 300 MHz):
2. Experimental
4.46–4.49 (s, 4H), 5.29 (d, 4H), 7.02–7.09 (t, 2H), 7.18–7.30
(q, 12H), 7.43–7.75 (m, 12H), 8.66 (s, 2H). IR (KBr): 3398
(m), 3320 (m), 1639 (s, C O), 1600 (m), 1534 (m), 1486
(m), 1451 (m), 1299 (m), 1229 (m, Ar–O), 1107 (w, Ar–O),
807 (w), 755 (s), 700 (m). Analytical data, calculated for
L·H2O—C: 77.52, H: 5.89, N: 4.30. Found—C: 77.29, H: 5.41,
N: 4.14.
2.1. Materials
N-Benzylsalicylamide was prepared according to the litera-
ture methods [9]. Other chemicals were obtained from commer-
cial sources and used without further purification.
2.2. Methods
2.4. Preparation of the complexes
The metal ions were determined by EDTA titration using
xylenol orange as an indicator. Carbon, nitrogen and hydrogen
analyses were determined using an Elementar Vario EL (see
Table 1). Conductivity measurements were carried out with a
DDSJ-308 type conductivity bridge using 1.0 × 10−3 mol cm−3
solutions in acetone at 25 ◦C. IR spectra were recorded on
a Nicolet 360 FT-IR instrument using KBr discs in the
An ethyl acetate solution (5 cm3) of Ln(NO3)3·6H2O
(Ln = Pr, Eu, Gd, Tb, Ho, Er) (0.1 mmol) was added dropwise
to a solution of 0.1 mmol ligand L in the chloroform (5 cm3).
The mixture was stirred at room temperature for 8 h. And then
the precipitated solid complex was filtered, washed with ethyl
acetate and chloroform, dried in vacuo over P4O10 for 48 h and
submitted for elemental analysis, yield: 70%.
1
4000–400 cm−1 region. H NMR spectrum was measured on
a Varian Mercury-300B spectrometer in d-DMSO solution with
TMS as internal standard. Luminescence and phosphorescence
spectra were obtained on a Hitachi F-4500 spectrophotometer.
The excitation and emission slit widths were 10 nm.
3. Result and discussion
3.1. Properties of the complexes
2.3. Synthesis of the ligand L
Analytical data for the newly synthesized complexes, listed
in Table 1, indicate that the six nitrates complexes conform to a
2:3metal-to-ligandstoichiometryLn2(NO3)6L3·2H2O(Ln = Pr,
Eu, Gd, Tb, Ho, Er). All complexes are soluble in DMF and
DMSO, slightly soluble in acetone and acetonitrile, but spar-
ingly soluble in methanol, ethanol, chloroform and ethyl acetate.
Conductivitymeasurementsforthesecomplexesinacetonesolu-
tion (Table 1) indicate that all the complexes are non-electrolyte
[10].
The synthetic route for the ligand L is shown in
Scheme 1. N-Benzylsalicylamide (2.0 mmol), potassium car-
bonate (3.6 mmol) and DMF (20 cm3) were warmed to ca. 90 ◦C
and 4,4ꢀ-bisbromomethyl-1,1ꢀ-biphenyl (0.75 mmol) was added.
The reaction mixture was stirred at 90–95 ◦C for 8 h. After cool-
ing down, the mixture was poured into water (100 cm3). The
resulted solid was treated with column chromatography on sil-
ica gel [petroleum ether:ethyl acetate (1:1)] to get the ligand
Table 1
Analytical and molar conductance data of the complexes (calculated values in parentheses)
Complexes
Analysis (%)
C
Λm (S cm2 mol−1
)
H
N
Ln
Pr2(NO3)6L3·2H2O
Eu2(NO3)6L3·2H2O
Gd2(NO3)6L3·2H2O
Tb2(NO3)6L3·2H2O
Ho2(NO3)6L3·2H2O
Er2(NO3)6L3·2H2O
58.61 (58.47)
57.86 (57.98)
57.94 (57.74)
57.91 (57.67)
57.34 (57.41)
57.37 (57.31)
3.90 (4.36)
3.84 (4.32)
3.88 (4.31)
3.98 (4.30)
4.01 (4.28)
3.92 (4.27)
6.71 (6.49)
6.67 (6.44)
6.74 (6.41)
6.67 (6.41)
6.73 (6.38)
6.63 (6.36)
10.78 (10.89)
11.40 (11.64)
11.55 (12.00)
12.00 (12.11)
12.25 (12.51)
12.78 (12.67)
62.1
51.6
45.8
39.2
35.4
36.8

Y.-L. Song et al. / Spectrochimica Acta Part A 64 (2006) 595–599
597
Table 2
IR spectral data of the ligand and its complexes (cm−1
)
−
ν(NO3 )
Compounds
ν(C O)
ν(Ar–O–C)
ν1
ν4
ν2
ν3
ν1–ν4
L
1639
1610
1611
1612
1611
1612
1612
1229, 1107
1232, 1109
1231, 1108
1233, 1109
1231, 1108
1231, 1108
1231, 1108
Pr2(NO3)6L3·2H2O
Eu2(NO3)6L3·2H2O
Gd2(NO3)6L3·2H2O
Tb2(NO3)6L3·2H2O
Ho2(NO3)6L3·2H2O
Er2(NO3)6L3·2H2O
1483
1486
1484
1486
1485
1485
1300
1301
1298
1304
1301
1302
1030
1028
1029
1029
1030
1030
810
811
812
811
812
812
183
185
186
182
184
183
3.2. IR spectra
formula of these six nitrate complexes could be indicated as
[Ln2(NO3)6L3(H2O)2] (Ln = Pr, Eu, Gd, Tb, Ho, Er).
The most important IR peaks of the ligand and the complexes
are reported in Table 2. The IR spectra of the complexes are
similar.
3.4. Luminescence properties of the complexes
The IR spectrum of the free ligand shows bands at 1639,
1229 and 1107 cm−1, which may be assigned to ν(C O) and
ν(Ar–O–C), respectively. In the complexes, the low-energy band
remains unchanged, but the high-energy band red shifts to about
1611 cm−1 (ꢀν = 28 cm−1) as compared to its counterpart for
the “free” ligand, indicating that only the oxygen atom of C O
takes part in coordination to the metal ions.
Ligand-based excitations cause the structural emission of lan-
thanide complexes and the ligand luminescence is quenched
show that the ligand-to-metal energy transfer occurs [13]. The
ability to transfer energy from ligand-centered to metal-centered
is important in the design of lanthanide(III) supramolecular pho-
tonic devices [14]. As the complexes were stable in acetone, the
luminescent investigations were carried out in this solution.
Under identical experimental conditions, the luminescence
characteristics emissions of the complexes in acetone are listed
in Table 3. The emission spectra of the Eu and Tb complexes in
acetone solution are shown in Fig. 1. The ligand having multi-
ple aromatic rings has a strong antenna effect. So its complexes
could have strong luminescence properties. From Table 3 it can
be seen that great difference (87 nm) between the excitation
maxima of the europium complex and that of the terbium one.
The luminescence characteristic emission wavelengths of the
europium and terbium ions are observed. This indicates that the
ligand L is a good organic chelator to absorb energy and transfer
them to metal ions. In the spectrum of Eu complex, the relative
The characteristic frequencies of the coordinating nitrate
groups (C2v) appear at ca. 1485 cm−1 (ν1), 1300 cm−1 (ν4),
1030 cm−1 (ν2) and 810 cm−1 (ν3) cm−1, and the difference
between the two strongest absorptions (ν1 and ν4) of the nitrate
−
groups is about 180 cm−1, clearly establishing that the NO3
groups in the solid complexes coordinate to the lanthanide ion
as bidentate ligands [11,12]. Additionally, no bands at 1380, 820
and 720 cm−1 in the spectra of complexes indicates that free
nitrate groups (D3h) are absent, in agreement with the results of
the conductivity experiments.
3.3. Thermal analysis
7
7
intensity of 5D0 → F2 is more intense than that of 5D0 → F1,
showing that the Eu(III) ion does not lie in a centro-symmetric
coordination site [15].
The TG and DTA curves for the Pr and Er complexes show
that the complexes has no melting point and the water molecules
containing in the complexes are all coordinated water molecules,
becausetheonlyoneendothermicpeakappearatca. 125 ◦C. And
the percent of water is consistent with the results of elemental
analysis. The complexes are stable up to ca. 445 ◦C and then
decompose in the temperature range 445–800 ◦C, producing
two exothermic peaks. The residual weights are approximately
consistent with the values required for the metal oxide. So the
Due to the presence of a scatting signal near 490 nm, the peak
height at 547 nm for terbium was used to measure the lumines-
cence intensity. Comparing the spectra of Eu and Tb complexes
(Fig. 1), we can see that the luminescence intensity of terbium
complexat547 nmisgreatlystrongerthanthatofeuropiumcom-
plex at 618 nm. As we know, the luminescence of Ln3+ chelates
is related to the efficiency of the intramolecular energy transfer
Table 3
Luminescence data for the Eu and Tb complexes
Complexes
Concentration (×10−4 mol L−1
)
λex (nm)
λem (nm)
RFIa
Assignment
7
[Eu2(NO3)6L3(H2O)2]
1
397
592
618
38
83
5D0 → F1
7
5D0 → F2
7
[Tb2(NO3)6L3(H2O)2]
1
310
493
547
587
4244
8666
495
5D4 → F6
5D4 → F5
7
7
5D4 → F4
a
RFI: relative fluorescence intensity.

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Y.-L. Song et al. / Spectrochimica Acta Part A 64 (2006) 595–599
Fig. 1. The emission spectra of Eu complex (A) and Tb complex (B) in acetone solutions.
between the triplet excited state T1 of the ligand and the lowest
excited resonance level of the lanthanide ions, which depends
on the energy gap between the two levels. So we may deduce
that the energy gap between the ligand triplet excited state T1
and the lowest excited state of the terbium ion may be in favour
of the energy transfer process. A triplet excited state T1 which is
localised on one ligand only and is independent of the lanthanide
nature [16]. In order to acquire the triplet excited state T1 of the
ligand L, the phosphorescence spectrum of the Gd(III) complex
was measured at 77 K in a methanol–ethanol mixture (v/v, 1:1).
The triplet state energy level T1 of the ligand L, which was calcu-
lated from the shortest-wavelength phosphorescence band [6],
is 22,523 cm−1. This energy level is above the lowest excited
Scheme 2. The chemical formula of the ligand LI.
5
resonance level D0 of Eu(III) (17,286 cm−1) and the lowest
benzylaminoformyl)phenoxyl]methyl}benzene (Scheme 2)
[18].
excited resonance level 5D4 (20,545 cm−1) of Tb(III). Thus, the
energy could be transferred from ligand to the Eu and Tb ions.
And this result also indicates that the triplet state energy level
T1 of this ligand L matches better to the lowest resonance level
Based on the comparability of these two ligands and the sto-
ichiometries of the complexes, we deduce that ligand L also
acts as a bi(unidentate) bridging ligand connecting two lan-
thanide(III) ions in these complexes. Scheme 3 shows the most
probable coordination structure with the lanthanide ions. And
the coordination number of the lanthanide ions is 10.
of Tb(III) (ꢀν = 1978 cm−1) than to Eu(III) (ꢀν = 5237 cm−1
)
ion [16,17].
When the ligand formed the lanthanide complexes, obvi-
ous changes in IR spectra were observed. In the complexes,
lanthanide ions were coordinated to the C O oxygen atoms
of the ligand L. The luminescence properties of the Eu and
Tb complexes in acetone solutions were investigated. Under
the excitation of UV light, these complexes exhibited charac-
teristic emission of europium and terbium ions. The lowest
triplet state energy level T1 of this ligand L matches better
4. Conclusion
According to the data and discussion above, the ligand
could form stable polynuclear complexes with lanthanide
ions (metal:L = 2:3). We have reported a novel 1D metal-
losupramolecular polymer {[Eu(NO3)3]2LI }n generated
3
from the reaction of Eu(NO3)3 with LI, 1,4-bis{[(2ꢀ-
Scheme 3. The most probable coordination structure of the complexes.

Y.-L. Song et al. / Spectrochimica Acta Part A 64 (2006) 595–599
599
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Acknowledgements
We are grateful to the National Natural Science Foundation
of China (project 20401008) and the Reseach Foundation for
the Young Teachers Possessing Doctor’s Degree of Lanzhou
University.
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