Synthesis, characterization and luminescent properties of lanthanide complexes with an unsymmetrical tripodal ligand

Article abstract of DOI:10.1002/bio.1215

Solid complexes of lanthanide nitrates with an novel unsymmetrical tripodal ligand, butyl-N,N-bis[(2′-benzylaminofomyl)phenoxyl)ethyl]-amine (L) have been synthesized and characterized by elemental analysis, infrared spectra and molar conductivity measurements. At the same time, the luminescent properties of the Sm(III), Eu(III), Tb(III) and Dy(III) nitrate complexes in solid state were also investigated. Under the excitation of UV light, these complexes exhibited characteristic emission of central metal ions. Copyright

Full text of DOI:10.1002/bio.1215

Research article
Received: 29 June 2009,
Revised: 26 January 2010,
Accepted: 12 March 2010,
Published online in Wiley Online Library: 21 May 2010
(wileyonlinelibrary.com) DOI 10.1002/bio.1215
Synthesis, characterization and luminescent
properties of lanthanide complexes with an
unsymmetrical tripodal ligand
Zhen-Zhong Yan,* Guo-Liang Dai, Hua-Ding Liang and Yong-Qiang Lin
ABSTRACT: Solid complexes of lanthanide nitrates with an novel unsymmetrical tripodal ligand, butyl-N,N-bis[((2Ј-
benzylaminofomyl)phenoxyl)ethyl]-amine (L) have been synthesized and characterized by elemental analysis, infrared spectra
and molar conductivity measurements. At the same time, the luminescent properties of the Sm(III), Eu(III), Tb(III) and Dy(III)
nitrate complexes in solid state were also investigated. Under the excitation of UV light, these complexes exhibited character-
istic emission of central metal ions. Copyright © 2010 John Wiley & Sons, Ltd.
Keywords: lanthanide nitrate complexes; unsymmetrical tripodal ligand; luminescent properties
complexes exhibited characteristic emission of corresponding
Introduction
lanthanide ions. The lowest triplet state energy level of the ligand
There has been much interest in luminescent lanthanide com-
was calculated from the phosphorescence spectrum of the Gd
plexes because of their attractive emission properties such as
complex at 77 K. The results indicate that the triplet state energy
long lifetime, large Stokes shift, and line-like emission (1–4).
level of the ligand matches better the resonance level of Tb (III)
These complexes can be used as analytical tools ranging from
than other lanthanide ions.
luminescent probes in time-resolved measurements (5–11) to
luminescent chemosensors for detection of gases, ions and mol-
ecules in solution (12–17). Lanthanide ions are photophysically
Experimental
inert because direct photoexcitation is difficult due to the f–f
transition (18). For effective excitation of lanthanide ions, an
Materials
organic chromophore, a so-called‘antenna’, is usually attached to
lanthanide complexes. In order to obtain strongly luminescent
complexes, the chromophoric ligands, which chelate to lan-
thanide ions, should be able to encapsulate and protect the lan-
thanide ion from the solvent molecules and to absorb energy and
transfer it efficiently to the central metal. More and more chem-
ists are attracted to designing the organized molecular architec-
tures containing trivalent lanthanide ions (Sm3+, Eu3+, Tb3+, Dy3+),
working as efficient light conversion devices (3,19).
Tripodal ligands have drawn much attention in recent years,
mainly due to the employment of self-assembly strategies for
lanthanide complexes. Zhang and co-workers have used tripodal
ligands as sensitizers for lanthanide luminescence (20). Tripodal
ligands have the advantages of selective coordinating capacity
and hard binding sites, thereby stabilizing their complexes,
acquiring novel coordination structure and shielding the encap-
sulated ion from interaction with the surroundings (21). Most of
these ligands are symmetrical with three identical arms while the
unsymmetrical tripodal organic ligands are rare (22).
N-benzylsalicylamide (23), b,bЈ-dichlordiethylamine hydrochlo-
ride salt (24) and lanthanide nitrates (25) were prepared accord-
ing to the literature methods. Other chemicals were obtained
from commercial sources and used without further purification.
Methods
The Ln(III) ion was determined by EDTA titration using xylenol-
orange as an indicator. Carbon, nitrogen and hydrogen were
determined using an Elementar Vario EL. Conductivity measure-
ments were carried out with a DDS-307 type conductivity bridge
using 1.0 ¥ 10-3 mol cm-3 solutions at 25°C. The IR spectra were
recorded in the 4000–400 cm-1 region using KBr pellets and a
Nicolet Nexus 670 FTIR spectrometer. 1H NMR spectra were mea-
sured on a Varian Mercury 200 spectrometer in D-chloroform
solutions, with TMS as internal standard. Luminescence and
phosphorescence spectra were obtained on a Hitachi F-4500
spectrophotometer.
In our research, we designed and prepared a new con-
formationally flexible unsymmetrical butyl- and (2′-
benzylaminoformyl)phenoxyl-containing amide type tripodal
* Correspondence to: Zhen-Zhong Yan, Department of Pharmaceutics and
Chemical Engineering, Taizhou University, Linhai 317000, People’s Republic
of China. E-mail: yanzhzh@tzc.edu.cn
ligand
butyl-N,N-bis[(2′-benzylaminofomyl)phenoxyl)ethyl]-
amine (L) (Scheme 1) and studied the luminescent properties of
samarium, europium, terbium and dysprosium complexes with
this ligand. Under the excitation of UV light, Sm, Eu, Tb and Dy
Department of Pharmaceutics and Chemical Engineering, Taizhou Univer-
sity, Linhai 317000, People’s Republic of China
Copyright © 2010 John Wiley & Sons, Ltd.
Luminescence 2011; 26: 218–222

Properties of lanthanide complexes
Preparation of the Unsymmetrical Tripodal Ligand L
0.1 mmol ligand L in 5 cm3 of ethyl acetate. The mixture was
stirred at room temperature for 8 h. Then the precipitated solid
complex was filtered, washed with ethyl acetate, dried in vacuo
over P4O10 for 48 h and submitted for elemental analysis, yield
65%.
The synthetic route for the unsymmetrical tripodal ligand L is
shown in Scheme 1. The b, b′-dichlordiethylamine hydrochloride
salt (1 mmol) and potassium carbonate (2 mmol) were refluxed in
acetone (25 cm3) for 30 min, then the 1-bromobutane (1 mmol)
was added to the solution. The reaction mixture was refluxed for
12 h and the hot solution was filtered off. The collected organic
phase was evaporated in vacuum. Then the obtained product
was added to a mixture of N-benzylsalicylamide (2.0 mmol),
potassium carbonate (4 mmol) and DMF (20 cm3), which was
warmed to ca 95°C. The reaction mixture was stirred at 98–100°C
for 8 h. After cooling down, the mixture was poured into water
(100 cm3). The resulting solid was treated with column chroma-
tography on silica gel (petroleum ether : ethyl acetate, 2:1) to get
the ligand L, yield 70%, m.p. 102–104°C; 1H NMR (CDCl3,
300 MHz): 0.88–0.94 (t, 3H), 1.64–1.75 (q, 2H), 1.82–1.96 (q, 2H),
2.26–2.38 (q, 2H), 2.60–2.69 (t, 4H), 3.87–3.93 (t, 4H), 4.57–4.61 (d,
4H), 6.67–7.49 (m, 18H), 8.20–8.26 (t, 2H); anal. calcd for
C36H41O4N3: C, 74.58%; H, 7.13%; N, 7.25%. Found: C, 74.42%; H,
7.46%; N, 7.52%.
Results and discussion
Properties of the complexes
Analytical data for the newly synthesized complexes, listed in
Table 1, indicate that the five nitrate complexes all conform to a
1:1 metal-to-ligand stoichiometry, [LnL(NO3)3]·H2O (Ln = Sm, Eu,
Gd, Tb, Dy). All the complexes are soluble in DMF and DMSO,
slightly soluble in methanol, ethanol, acetone and acetonitrile,
but sparingly soluble in chloroform, ethyl ether and ethyl acetate.
The molar conductances of the complexes in acetone (see
Table 1) indicate that all complexes act as noneletrolytes (26),
implying that all nitrate groups are in coordination sphere.
IR Spectra
The most important IR peaks of the ligand L and the complexes
are reported 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.
Preparation of the complexes
An ethyl acetate solution (5 cm3) of Ln(NO3)3·6H2O (Ln = Sm, Eu,
Gd, Tb, Dy) (0.1 mmol) was added dropwise to a solution of
Cl
N
H
.
Cl
NH HCl
N
O
O
O
O
OH O
Cl
N
N
N
K2CO3 acetone
DMF
K2CO3
Br
H
H
Cl
Scheme 1. The synthetic route for the ligand L.
Table 1. Analytical and molar conductance data of the complexes (calculated values in parentheses)
Complexes
Analysis (%)
Lm (cm2 W-1 mol-1)
C
H
N
Ln
[SmL(NO3)3]·H2O
[EuL(NO3)3]·H2O
[GdL(NO3)3]·H2O
[TbL(NO3)3]·H2O
[DyL(NO3)3]·H2O
46.36(46.29)
46.02(46.21)
45.64(45.95)
45.97(45.87)
45.72(45.69)
4.81(4.64)
4.78(4.63)
4.58(4.61)
4.76(4.60)
4.76(4.58)
9.05(9.00)
8.92(8.98)
9.06(8.93)
8.96(8.91)
8.68(8.88)
15.87(16.10)
16.44(16.24)
16.96(16.71)
16.93(16.86)
16.98(17.17)
8.4
7.8
7.6
7.0
6.8
Table 2. The most important IR spectral data of the ligand L and complexes (cm-1)
-
Compounds
n(C=O)
n(C–O–C)
n(NO3 )
n1
n4
n2
n3
n1–n4
L
1638
1607
1608
1610
1609
1610
1106
1113
1113
1111
1112
1110
[Sm2L3(NO3)6]·H2O
[Eu2L3(NO3)6]·H2O
[Gd2L3(NO3)6]·H2O
[Tb2L3(NO3)6]·H2O
[Dy2L3(NO3)6]·H2O
1486
1483
1482
1483
1484
1301
1302
1298
1299
1302
1045
1046
1044
1043
1042
814
815
813
814
812
185
181
184
184
182
Luminescence 2011; 26: 218–222
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Z. Z. Yan et al.
The IR spectrum of the free ligand L shows bands at 1638 and
1106 cm-1, which are attributable to the stretch vibration of the
carbonyl group [n(C=O)] of the amide group and n(C–O–C),
respectively. In the complexes, the low-energy band remains
unchanged, but the high-energy band red shifts to about
1610 cm-1 (Dn = 28 cm-1) as compared with its counterpart for the
‘free’ ligand, thus indicating that only the oxygen atom of C=O
takes part in coordination to the lanthanide ions.
The characteristic frequencies of the coordinating nitrate
groups (C2v) appear at ca 1484 cm-1 (n1), 1300 cm-1 (n4), 1045 cm-1
(n2) and 812 cm-1 (n3), and the difference between two strongest
absorptions (n1 and n4) 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 (27,28).
Additionally, the absence 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 experi-
ments. In addition, broad bands at ca 3400 cm-1 indicate that
water molecules are existent in the complexes, confirming the
elemental results (29).
Table 3. Luminescence data for the ligand and complexes
in solid state at room temperature
Compounds
lex
(nm)
lem
(nm)
RIa
Assignment
L
332
314
466
561
595
592
617
490
545
582
482
574
2218
66
82
140
368
4355
8678
450
289
276
6
[Sm(NO3)3L]·H2O
4G5/2 → H5/2
6
4G5/2 → H7/2
7
[Eu(NO3)3L]·H2O
[Tb(NO3)3L]·H2O
397
312
5D0 → F1
7
5D0 → F2
7
5D4 → F6
7
5D4 → F5
Luminescent properties of the complexes
7
5D4 → F4
6
[Dy(NO3)3L]·H2O
308
4F9/2 → H5/2
Excited by the absorption band at 332 nm, the‘free’unsymmetri-
cal tripodal ligand exhibits broad emission bands (lmax = 466 nm)
in solid state (the excitation and emission slit widths were 1.0 and
6
4F9/2 → H13/2
aRI, relative intensity.
(a)
(b)
(c)
(d)
Figure 1. Emission spectra of the complexes [LnL(NO3)3]·H2O in solid state at room temperature: (a) [SmL(NO3)3]·H2O; (b)[EuL(NO3)3]·H2O; (c) [TbL(NO3)3]·H2O;
(d) [DyL(NO3)3]·H2O.
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Properties of lanthanide complexes
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2.5 nm, respectively). The luminescence emission spectra of
[LnL(NO3)3]·H2O (Ln = Sm, Eu, Tb, Dy) in solid state (the excitation
and emission slit widths were 1.0 and 2.5 nm, Table 3, Fig. 1a–d)
were recorded at room temperature. The efficient energy transfer
from ligand to center ions (antenna effect) is one of key factors to
achieve lanthanide characteristic luminescence (30,31). It is
shown in Fig. 1 that these four complexes all show the character-
istic emissions of Sm3+, Eu3+, Tb3+ or Dy3+. This indicates that the
ligand L is a good organic chelator to absorb and transfer energy
to lanthanide ions. The ligand has multiple aromatic rings with a
semirigid skeleton structure, so it is a strong luminescence sub-
stance(32). InthespectrumofEucomplex, therelativeintensityof
5D0 → 7F2 is more intense than that of 5D0 → 7F1; the most intensity
ratio value h(5D0 → 7F2/5D0 → 7F1) is 2.63, showing that the Eu (b)
ion does not lie in a centro-symmetric coordination site (33).
A triplet excited state T1 is localized on one ligand only and is
independent of the lanthanide nature (34). In order to acquire the
triplet excited stateT1 of the ligand L, the phosphorescence spec-
trum 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 calculated from the shortest-wavelength
phosphorescence band (35), was 23,923 cm-1. This energy level is
4
above the lowest excited resonance level G5/2 of Sm(III)
(17,924 cm-1), 5D0 of Eu(III) (17,286 cm-1), 5D4 of Tb(III)
(20,545 cm-1) and 4F9/2 of Dy(III) (21,144 cm-1). Thus, the absorbed
energy could be transferred from ligand to the Sm, Eu, Tb or Dy
ions. We can deduce that the triplet state energy level T1 of this
ligand L matches better the lowest resonance level of Tb(III) (Dn =
3378 cm-1) than Sm(III) (Dn = 5999 cm-1), Eu(III) (Dn = 6637 cm-1)
and Dy(III) (Dn = 2779 cm-1) ions, because such large or small Dn
could result in the non-radiative deactivation of the lanthanide
emitting state and quench the luminescence of the complexes
(36).
Conclusion
According to the data and discussion above, the novel
unsymmetrical
tripodal
ligand
butyl-N,N-bis[(2′-
benzylaminofomyl)phenoxyl)ethyl]-amine (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 coordi-
nated to the C=O oxygen atoms of the ligand L. The character-
ization of these complexes demonstrates 1:1 (M : L) type
coordination stoichiometries. Thus, the lanthanide ion could be
effectively encapsulated and protected by the coordinated
ligands. The luminescent properties of the Sm, Eu, Tb and Dy
complexes in solid state were investigated. Under UV light exci-
tation, the complexes exhibited characteristic luminescence of
samarium, europium, terbium and dysprosium ions. This indi-
cates that the ligand L is a good organic chelator to absorb and
transfer energy to lanthanide ions. The lowest triplet state energy
level of the ligand indicates that the triplet state energy level (T1)
of the ligand matches better the resonance level of Tb(III) than
other lanthanide ions.
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