Preparation, characterization and luminescent properties of lanthanide complexes with a new aryl amide bridging ligand

Article abstract of DOI:10.1016/j.saa.2008.03.013

A new aryl amide type bridging ligand 1,4-bis{[(2′-benzylaminoformyl)phenoxyl]ethoxyl}benzene (L) and its complexes with lanthanide ions (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) were synthesized and characterized by elemental analysis, infrared spectra and electronic spectra. At the same time, the luminescent properties of the Sm, Eu, Tb and Dy complexes in solid state and the Tb complex in solvents were also investigated. At room temperature, these four complexes exhibited characteristic luminescence emissions of the central metal ions under UV light excitation and could be significant in the field of supramolecular photonic devices.

Full text of DOI:10.1016/j.saa.2008.03.013

Spectrochimica Acta Part A 71 (2008) 1153–1157
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Preparation, characterization and luminescent properties of lanthanide
complexes with a new aryl amide bridging ligand
Ya-Fei Li, Kuan-Zhen Tang, Yu Tang∗, Wei-Sheng Liu, Min-Yu Tan
State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou University,
222 Tianshui South Road, Lanzhou 730000, PR China
a r t i c l e i n f o
a b s t r a c t
A new aryl amide type bridging ligand 1,4-bis{[(2ꢀ-benzylaminoformyl)phenoxyl]ethoxyl}benzene (L)
and its complexes with lanthanide ions (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Er) were synthesized and charac-
terized by elemental analysis, infrared spectra and electronic spectra. At the same time, the luminescent
properties of the Sm, Eu, Tb and Dy complexes in solid state and the Tb complex in solvents were also
investigated. At room temperature, these four complexes exhibited characteristic luminescence emissions
of the central metal ions under UV light excitation and could be significant in the field of supramolecular
photonic devices.
Article history:
Received 13 December 2007
Received in revised form 26 February 2008
Accepted 11 March 2008
Keywords:
Aryl amide bridging ligand
Lanthanide(III) complexes
Luminescent properties
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Recently, polynuclear lanthanide complexes, especially with
luminescent properties, are attracting more and more chemists
The emission of light from lanthanide ions arises from f–f tran-
sitions, which result in emission bands with extremely narrow
bandwidth and no theoretical cap on the quantum efficiency. This
makes lanthanide ions very attractive for a variety of applications,
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 properties of these ions, such as
their large stokes shift and narrow emission profiles [2,3]. Since
f–f transitions are Laporte forbidden [4], free Ln3+ have low extinc-
tion coefficients resulting in low luminescence intensity. Therefore,
it is necessary to sensitize these cations through a suitable chro-
mophore (“antenna effect”) [5], an area of research that has been
highly active in recent years. The role of the ligands is on one hand,
to collect the photons provided by the light source in order to allow
an energy transfer to the emitting levels of the Ln3+ ion and on
the other to shield it against the solvent in order to avoid non-
radioactive deactivation processes. It is generally accepted that the
energy transfer from ligand to Ln3+ ion occurs from the lowest
triplet state energy level T1 of the ligand to the resonance level of
Ln3+ [6]. And this energy transfer process is one of the most impor-
tant processes having influence on the luminescent properties of
the lanthanide complexes.
[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-
nal group effects [8]. We have designed a series of polyfunctional
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 bridging ligand 1,4-bis{[(2ꢀ-
benzylaminoformyl)phenoxyl]ethoxyl}benzene (L) (Scheme 1) and
studied the luminescent properties of samarium, europium, ter-
bium and dysprosium complexes with this new ligand. Under the
excitation of UV light, Sm, Eu, Tb and Dy complexes all can exhibit
characteristic emission of central lanthanide ions. The lowest triplet
state energy level of the ligand was calculated from the phospho-
rescence spectrum of the Gd complex at 77 K. The result indicate
that the triplet state enengy level of this ligand matches better to
the resonance level of Tb(III) than other lanthanide ions.
2. Experimental
2.1. Materials
Lanthanide nitrates [9] and N-benzylsalicylamide [10] were
prepared according to the literature methods, respectively. Other
chemicals were obtained from commercial sources and used with-
out further purification.
∗
Corresponding author. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail address: tangyu@lzu.edu.cn (Y. Tang).
1386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.03.013

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Y.-F. Li et al. / Spectrochimica Acta Part A 71 (2008) 1153–1157
Table 1
washed by distilled water to afford pure ligand L as a white
solid, yield 63%, m.p. 168–170 ◦C. 1H NMR (CDCl3, 300 MHz): ı:
4.16(t, J = 9.0 Hz, 4H), 4.40(t, J = 9.3 Hz, 4H), 4.57(d, J = 5.7 Hz, 4H),
6.64(s, 4H), 6.96–7.47(m, Ar–H, 16H), 8.23(d, J = 1.8 Hz, 1H), 8.26(d,
J = 1.8 Hz, 1H), 8.42(b, 2H); IR (KBr pellet, cm−1) ꢀ: 3296(w, N–H),
3032(m, Ar–H), 1632(s, C O), 1601(s), 1539(m), 1474(m), 1452(m),
1233(s, Ar–O), 1110(s, Ar–O), 760(s) and 694(m).
Analytical data of the complexes (calculated values in parentheses)
Complexes
Analysis (%)
C
H
N
Ln
Pr2L3(NO3)6·2H2O
53.91 (53.59)
4.44 (4.39)
4.43 (4.05)
4.41 (4.29)
4.41 (4.06)
4.39 (4.03)
4.38 (4.07)
4.37 (4.14)
4.35 (4.58)
6.62 (6.33)
6.60 (6.34)
6.57 (6.33)
6.56 (6.34)
6.53 (6.28)
6.52 (6.32)
6.51 (6.40)
6.48 (6.47)
11.13 (11.10)
11.30 (11.33)
11.77 (11.75)
11.86 (11.86)
12.25 (12.22)
12.37 (12.34)
12.56 (12.58)
12.87 (12.90)
Nd2L3(NO3)6·2H2O 53.77 (53.50)
Sm2L3(NO3)6·2H2O 53.51 (53.23)
Eu2L3(NO3)6·2H2O 53.44 (53.20)
Gd2L3(NO3)6·2H2O 53.22 (52.96)
2.4. Preparation of the complexes
Tb2L3(NO3)6·2H2O
53.15 (52.93)
Dy2L3(NO3)6·2H2O 53.01 (52.87)
A solution of 0.15 mmol L in 10 cm3 of ethyl acetate was added
drop wise to a solution of 0.1 mmol lanthanide nitrate (Ln = Pr, Nd,
Sm, Eu, Gd, Tb, Dy, Er) in 10 cm3 of ethyl acetate. The mixture was
stirred at room temperature for 4 h. Then diethyl ether was added
to the mixture and the precipitated solid complex was filtered,
washed with ethyl acetate and diethyl ether and dried in vacuo over
P4O10 for 48 h and submitted for elemental analysis, yield: 72%.
Er2L3(NO3)6·2H2O
52.81 (52.47)
Table 2
The IR spectra data (cm−1
)
−
ꢀ(C O) ꢀ(Ar–O–C) ꢀ(NO3 )
Compounds
ꢀ1
ꢀ4
ꢀ2
ꢀ5
ꢀ1 − ꢀ4
L
1632
1610
1233, 1110
3. Results and discussion
Pr2L3(NO3)6·2H2O
1224, 1112 1485
1225, 1112 1486
1225, 1112 1484
1226, 1113 1480
1226, 1113 1482
1226, 1112 1481
1225, 1112 1482
1225, 1113 1486
1294
1029
1029
1029
1030
1031
1031
1031
1032
816
816
815
814
815
815
814
817
191
190
184
178
179
178
177
178
Nd2L3(NO3)6·2H2O 1610
1296
1300
1302
1303
1303
1305
1308
Sm2L3(NO3)6·2H2O 1610
3.1. Properties of the complexes
Eu2L3(NO3)6·2H2O
Gd2L3(NO3)6·2H2O
Tb2L3(NO3)6·2H2O
Dy2L3(NO3)6·2H2O
Er2L3(NO3)6·2H2O
1610
1611
1609
1611
1612
Analytical data for the newly synthesized complexes, listed
in Table 1, indicate that these eight complexes of lanthanide
nitrate all conform to a 2:3 metal-to-ligand stoichiometries,
Ln2L3(NO3)6·2H2O. All complexes are soluble in DMF, DMSO and
methanol, but slightly soluble in ethanol, ethyl acetate, acetonitrile,
acetone, chloroform, THF and diethyl ether.
2.2. Methods
The lanthanide ions were determined by EDTA titration using
xylenol orange as an indicator. Carbon, nitrogen and hydrogen were
determined on an Elementar Vario EL analyzer (see Table 1). The FT-
IR spectra were recorded on a Nicolet 360 FT-IR spectrometer using
KBr pellets in the range of 4000–400 cm−1 (see Table 2). 1H NMR
spectrum was measured on a Varian Mercury-300B spectrometer
in d-chlorform solution with TMS as internal standard. Electronic
spectra were recorded with a Varian Cary 100 spectrophotometer
in methanol solution. Luminescence and phosphorescence spectra
were obtained on a Hitachi F-4500 fluorescence spectrophotome-
ter.
3.2. IR spectra
The main infrared bands of the ligand and the complexes are pre-
sented in Table 2. The complexes have similar IR spectra, of which
the characteristic bands have similar shifts (see Table 2), suggesting
they have a similar coordination structure.
The IR spectrum of the free ligand shows bands at 1632, 1233
and 1110 cm−1, which may be assigned to ꢀ(C O) and ꢀ(Ar–O–C),
respectively. In the complexes, the low-energy band remains
unchanged, but the high-energy band red shifts to about 1610 cm−1
(ꢁꢀ = 22 cm−1) as compared to its counterpart for the “free” lig-
and, indicating that only the oxygen atom of C O takes part in
coordination to the metal ions.
2.3. Synthesis of the ligand L
The characteristic frequencies of the coordinating nitrate groups
(C2v) appear at about 1485 cm−1 (ꢀ1), 1030 cm−1 (ꢀ2), 815 cm−1 (ꢀ3)
and 1300 cm−1 (ꢀ4) and the difference between the two highest
frequency bands (ꢀ1−and ꢀ4) is about 180–190 cm−1, clearly estab-
lishing that the NO3 groups in the solid complexes coordinate to
the lanthanide ion as bidentate ligands [13]. Additionally, no bands
at 1380, 820 and 720 cm−1 in the spectra of complexes indicates
that free nitrate groups (D3h) are absent.
The synthetic route for the ligand L is shown in Scheme 1.
The intermediate A [11] and B [12] were prepared according
to literature methods. N-Benzylsalicylamide (5 mmol), potassium
carbonate (5 mmol) and acetone (50 cm3) were warmed to ca.
60 ◦C, then the intermediate B (15 mmol) was added. The reac-
tion mixture was stirred and refluxed for 12 h. After cooling down,
the mixture was filtered to give the crude product, which was
Scheme 1. The synthetic route for the ligand L.

Y.-F. Li et al. / Spectrochimica Acta Part A 71 (2008) 1153–1157
1155
Table 3
assessed by Sinha’s parameter (ı), the nephelauxetic ration (ˇ) and
the bonding parameter (b1/2) [15].
Electronic spectral data and covalent parameters of Pr, Nd and Er complexes
Complexes
Frequency (cm−1
)
Assignment
Covalent parameters
Absorption spectra of the Pr(III), Nd(III) and Er(III) complexes
were registered in methanol solutions at room temperature and
the covalent parameters were calculated and listed in Table 3. The
values of ˇ, ı and b1/2 indicate that the interaction between the
trivalent lanthanide ions and the ligands is essentially electrostatic
and that there is a very small participation of 4f orbitals in bonding
[16]. Moreover, the magnitudes of covalency and bonding param-
eters increase from Pr(III) to Er(III), indicating that the extent of
electrostatic character of the metal–ligand bond increases with
atomic number (in agreement with the lanthanide contraction).
21,277
20,661
16,611
3H4
3H4
3H4
→
→
→
1I6
ˇ = 0.9940
ı = 0.2506
Pr2L3(NO)6·2H2O
3P1
1D2
b
1/2 = 0.05657
17,212
13,369
12,547
4I9/2
4I9/2
4I9/2
→
→
→
2G7/2
4S3/2
2H9/2
ˇ = 0.9969
ı = 0.3110
Nd2L3(NO)6·2H2O
Er2L3(NO)6·2H2O
b
1/2 = 0.03937
26,525
20,534
19,194
15,337
4I15/2
4I15/2
4I15/2
4I15/2
→
2G11/2
ˇ = 1.0067
ı = −0.6631
→
→
→
4F7/2
2H11/2
4F9/2
3.4. Luminescent properties of the complexes
In the IR spectra of the complexes, broad bands at 3350 cm−1
occur from the O–H stretching vibrations of water and the absence
of a band for ꢂr(H2O) at ca. 590 cm−1 shows that the water
molecules in the complexes are not coordinated water [14].
Excited by the absorption band at 323 nm, the “free” ligand
exhibits broad emission bands (ꢃmax = 442 nm) in solid state (the
excitation and emission slit widths were 2.5 nm). The luminescence
emission spectra of Ln2L3(NO3)6·2H2O (Ln = Sm, Eu, Tb, Dy) in solid
state (the excitation and emission slit widths were 1.0 nm, Table 4
and Fig. 1a–d) were recorded at room temperature. It is shown
in Fig. 1 that these four complexes all can show the characteris-
tic emissions of the central metal ions, indicating that the ligand L
is a good organic chelator to absorb and transfer energy to metal
ions. Intramolecular energy transfer from the triplet state of the
ligand to the resonance level of the Ln(III) ion is one of the most
important processes influencing the luminescence quantum yields
3.3. Electronic spectra
The electronic spectra in the visible region of the Ln(III) com-
plexes exhibit alternations in intensity and shifts in position of the
absorption bands relative to the corresponding Ln(III) aquo ions.
The shift has been attributed by Jørgensen to the effect on the
crystal field of interelectronic repulsion between the 4f electrons,
and is related to the covalent character of the metal–ligand bond,
Fig. 1. Emission spectra of the complexes Ln2L3(NO)6·2H2O in solid state at room temperature: (a) Sm2L3(NO)6·2H2O; (b) Eu2L3(NO)6·2H2O; (c) Tb2L3(NO)6·2H2O; (d)
Dy2L3(NO)6·2H2O.

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Y.-F. Li et al. / Spectrochimica Acta Part A 71 (2008) 1153–1157
The luminescence emission spectra of the Tb3+ complex in DMF,
Table 4
Luminescence data for Sm, Eu, Tb and Dy complexes
methanol, ethanol, acetonitrile, acetone and ethyl acetate solutions
(concentration: 1.0 × 10−3 mol L−1, the excitation and emission slit
widths were 5.0 nm, Fig. 2) were recorded at room temperature. It
could be seen from Fig. 2 that in methanol solution the Tb com-
plex has the strongest luminescence, and then in DMF, ethanol,
acetone, acetonitrile and ethyl acetate. This is due to the coordinat-
ing effects of solvents, namely solvate effect [23]. Together with the
raising coordination abilities of methanol, DMF, ethanol, acetone,
acetonitrile and ethyl acetate for the lanthanide ions, the oscillatory
motions of the entering molecules consume more energy which the
ligand triplet level transfer to the emitting level of the lanthanide
ion.
Compounds
ꢃex (nm)
323
ꢃem (nm)
RLIa
Assignment
L
442
8087
320
561
594
230
340
4G5/2
4G5/2
→
→
6H5/2
6H7/2
Sm2L3(NO)6·2H2O
396
323
323
579
591
617
108
441
1259
5D0
5D0
5D0
→
7F0
Eu2L3(NO)6·2H2O
Tb2L3(NO)6·2H2O
→
→
7F1
7F2
490
544
582
1173
2726
153
5D4
5D4
5D4
→
→
→
7F6
7F5
7F4
479
572
145
128
4F9/2
→
6H5/2
Dy2L3(NO)6·2H2O
4F9/2
→
6H13/2
a
RLI: relative luminescence intensity.
4. Conclusions
According
aryl
to
amide
the
type
data
bridging
and
discussion
ligand
above,
of lanthanide complexes [17]. This ligand has multiple aromatic
rings with a semirigid skeleton structure, so it is a strong lumi-
nescence substance [18]. The ligand-based emission band in the
Tb complex is invisible, which means the ligand-to-metal energy
transfer in the Tb3+ complex is most efficient in these complexes.
the
1,4-bis{[(2ꢀ-
benzylaminoformyl)phenoxyl]ethoxyl}benzene (L) can form
stable solid complexes with lanthanide nitrates. When the ligand
formed the lanthanide complexes, obvious changes in IR spectra
were observed. In the complexes, lanthanide ions were coordinated
to the C O oxygen atoms of the ligand L. Based on the structure
of the ligand and the stoichiometries of the complexes, we deduce
that ligand L acts as a bi(unidentate) bridging ligand connecting
two lanthanide(III) ions in these complexes and the coordination
polymers were generated. The luminescent properties of the Sm,
Eu, Tb and Dy complexes in solid state were investigated. Under the
UV light excitation, the complexes exhibited characteristic lumi-
nescence of samarium, europium, terbium and dysprosium ions.
This indicates that the ligand L is a good organic chelator to absorb
and transfer energy to metal ions. The lowest triplet state energy
level of the ligand indicates that the triplet state energy level (T1)
of the ligand matches better to the resonance level of Tb(III) than
other lanthanide ions. Thus, the present results demonstrate that
7
In the spectrum of Eu complex, the relative intensity of 5D0 → F2 is
7
more intense than that of 5D0 → F1, showing that the Eu(III) ion lie
in a non-centrosymmetric coordination site [19]. And the appear-
7
ance of the 5D0 → F0 transition is common when europium exists
in a non-centrosymmetric ligand field allowing both electric- and
magnetic-dipole transitions to mix resulting in transitions to the
“forbidden” state [20].
A triplet excited state T1 which is localized on one ligand only
and is independent of the lanthanide nature [21]. In order to
acquire the triplet excited state T1 of the ligand L, the phospho-
rescence spectrum of the Gd3+ complex was measured at 77 K in a
methanol–ethanol mixture (1:1, v/v). And the triplet state energy
level T1 of the ligand L, which was calculated from the shortest-
wavelength phosphorescence band [6], is 24,331 cm−1. This energy
the
1,4-bis{[(2ꢀ-benzylaminoformyl)phenoxyl]ethoxyl}benzene
levelisabove thelowest excited resonancelevel 4G5/2 (17,924 cm−1
)
complexes of lanthanide nitrates may find potential applications
in the field of photonic devices.
of Sm3+ 5D0 (17,286 cm−1) of Eu3+ 5D4 of (20,545 cm−1) Tb3+ and
, ,
4F9/2 (21,144 cm−1) of Dy3+. Thus the absorbed energy could be
transferred from the ligand to the Sm, Eu, Tb or Dy ions. And we may
deduce that the triplet state energy level T1 of this ligand L matches
Acknowledgements
better to the lowest resonance level of Tb3+ (ꢁꢀ = 3786 cm−1
)
than to Sm3+ (ꢁꢀ = 6407 cm−1), Eu3+ (ꢁꢀ = 7045 cm−1) and Dy3+
(ꢁꢀ = 3187 cm−1) ions, because such large or small ꢁꢀ could result
in the non-radiative deactivation of the lanthanide emitting state
and quench the luminescence of the complexes [22].
This work was supported by the National Natural Science Foun-
dation of China (Project No. 20401008), the program for New
Century Excellent Talents in University (NCET-06-0902) and the
Natural Science Foundation of Gansu Province (Project No. 3ZS061-
A25-003).
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