Anion effects on the fluorescence spectra of europium and terbium complexes with a family of ether-amide type multipodal ligands

Article abstract of DOI:10.1016/j.saa.2006.04.008

Three multipodal ligands: 3,3,7,7-tetra[N-methyl-N-phenyl(acetamide)-2-oxymethyl]-5-oxanonane (L^a), 3,3,7,7-tetra[N-ethyl-N-phenyl(acetamide)-2-oxymethyl]-5-oxanonane (L^b), 3,3,7,7-tetra[N-benzyl-N-phenyl(acetamide)-2-oxymethyl]-5-ox

Full text of DOI:10.1016/j.saa.2006.04.008

Spectrochimica Acta Part A 66 (2007) 662–666
Anion effects on the fluorescence spectra of europium and terbium
complexes with a family of ether-amide type multipodal ligands
Ke-Wei Lei, Wei-Sheng Liu∗, Min-Yu Tan
Department of Chemistry and State Key Laboratory of Applied Organic Chemistry, College of Chemistry and
Chemical Engineering, Lanzhou University, Lanzhou 730000, China
Received 21 September 2005; received in revised form 2 December 2005; accepted 14 April 2006
Abstract
Three multipodal ligands: 3,3,7,7-tetra[N-methyl-N-phenyl(acetamide)-2-oxymethyl]-5-oxanonane (La), 3,3,7,7-tetra[N-ethyl-N-phenyl(aceta-
mide)-2-oxymethyl]-5-oxanonane (Lb), 3,3,7,7-tetra[N-benzyl-N-phenyl(acetamide)-2-oxymethyl]-5-oxanonane (Lc) and their europium and ter-
bium nitrate and perchlorate complexes were synthesized. The 12 complexes were characterized by elemental analysis, IR, fluorescence
spectroscopy and conductivity. The lanthanide atoms are coordinated by O atoms from C O, C–O–C. With the difference of the anion,
the fluorescence properties of anion effect for the complexes were investigated. Some factors that influencing the fluorescent intensity were
discussed.
© 2007 Published by Elsevier B.V.
Keywords: Ether-amide type multipodal ligand; Lanthanide; Fluorescence; Nitrate; Perchlorate; Anion effect
1. Introduction
suitable molecular structure: a chain with inflexible terminal
groups. Therefore, they are excellent reagents for activating
ion-selective electrodes and extracts of rare earth ions [11].
However, luminescent properties on open-chain crown ethers
with lanthanide complexes have been rarely reported [12]. So
we have designed a series of multi-functional ligands having
both selective ability to enhance luminescence of lanthanide
complexes by providing some of proper conjugate absorption
groups suitable for energy transfer, i.e. the coordinating groups
with cavities for lanthanide ions and amide-based crown ethers.
With several suitably designed arms, the multipodal ligands
could shield the encapsulated lanthanide ions from interaction
with the surroundings, and by deliberate incorporation of appro-
priate multiple absorption groups suitable for energy transfer,
they could be used to develop strong luminescent lanthanide
complexes (Scheme 1).
The development of luminescent probes and sensors [1] is the
subject of intensive research both in natural and medical science
[2,3]. Probes based on europium and terbium ions are of special
interest because of the particularly suitable spectroscopic prop-
erties of these ions [2,4,5]. The main advantages of lanthanide
chelates in fluorescence spectrometry are their large stokes
shifts, narrow emission profiles and long fluorescence lifetimes
[6]. These have led to use lanthanides as fluorescent probes
for highly sensitive time-resolved fluorimetric immunoassays,
metal ion coordination in proteins and structural studies of bio-
logical macromolecule [7]. And these properties have also been
used to improve the sensitivity for lanthanide estimation by
conventional spectrofluorimetry and laser induced spectroflu-
orimetry [8].
Amide-based crown ethers offer many advantages in extrac-
tion and analysis of the rare earth ions [9,10] because of
their ring-like coordination structure and terminal group effects.
The open-chain crown ethers containing amide groups possess
In the recent work, we designed and synthesized a family
of open-chain crown ether ligands, 3,3,7,7-tetra[N-methyl-N-
phenyl(acetamide)-2-oxymethyl]-5-oxanonane (La), 3,3,7,7-
tetra[N-ethyl-N-phenyl(acetamide)-2-oxymethyl]-5-oxanonane
(Lb), 3,3,7,7-tetra[N-benzyl-N-phenyl(acetamide)-2-oxyme-
thyl]-5-oxanonane (Lc) and 12 europium and terbium nitrate
and perchlorate complexes. The anion effects on fluorescence
properties of the complexes were investigated.
∗
Corresponding author. Tel.: +86 931 8912552; fax: +86 931 8912582.
E-mail address: liuws@lzu.edu.cn (W.-S. Liu).
1386-1425/$ – see front matter © 2007 Published by Elsevier B.V.
doi:10.1016/j.saa.2006.04.008

K.-W. Lei et al. / Spectrochimica Acta Part A 66 (2007) 662–666
663
residue was washed by column chromatography (silica gel,
2:1 CHCl3/CH3CO2Et) and evaporated in vacuum resulted a
yellow oil (4.3 g). Yield: 50%. La, 1H NMR spectrum (300 M,
CDCl3): δ = 7.13–7.38 (m, 20H); 3.73(s, 8H; 4O–CH2–C(O));
3.22(s, 12H; 4N–CH3); 3.21(s, 8H; 4C–CH2–O); 2.98(s,
4H; 2C–CH2–O); 1.23 (q, 4H; 2R–CH2–C); 0.70(t, 6H;
2CH3–R–C). IR: ν 1674s (C O), 1112s (Ar–O–C). Formula
weight of La: 839.45. Analytical data, C 68.67(Calc. 68.71); H
7.39(7.45); N 6.56(6.68) %.
1
Lb, H NMR spectrum (300 M, CDCl3): δ = 7.06–7.36 (m,
20H); 3.66(s, 8H; 4O–CH2–C(O)); 3.65(q, 8H; 4N–CH2–R);
3.17(s, 8H; 4C–CH2–O); 3.02(s, 4H; 2C–CH2–O); 1.21
(q, 4H; 2R–CH2–C); 1.04(t, 12H; 4CH3–R–N); 0.70(t, 6H;
2CH3–R–C). IR: ν 1674s (C O), 1122s (Ar–O–C). Formula
weight of Lb: 895.13. Analytical data, C 69.77(Calc. 69.53); H
7.88(7.97); N 6.26(6.42) %.
1
Lc, H NMR spectrum (300 M, CDCl3): δ = 6.92–7.26(m,
40H); 4.84(s, 8H; 4N–CH2–R); 3.75(s, 8H; 4O–CH2–C(O));
3.23(s, 8H; 4C–CH2–O); 3.11(s, 4H; 2C–CH2–O); 1.27 (q, 4H;
2R–CH2–C);0.76(t, 6H;2CH3–R–C). IR:ν 1675s(C O), 1108s
(Ar–O–C). Formula weight of Lc: 1142.58. Analytical data, C
75.77(Calc. 75.63); H 7.02(6.88); N 5.06(4.90) %.
Scheme 1. The ligands.
2.4. Synthesis of complexes
2. Experimental
An ethyl acetate solution of Ln(NO3)3·6H2O [Ln = Eu(III),
Tb(III), 0.2 mmol], Ln(ClO4)3·6H2O [Ln = Eu(III), Tb(III),
0.1 mmol], was added dropwise to a solution of the ligand L
(0.1 mmol) in the ethyl acetate (30 ml). The mixture was stirred
for 4 h and white precipitate formed. The precipitate was col-
lected and washed three times with ethyl acetate. Further drying
in vacuum afforded pale white powder, yield: 75–80%.
2.1. Materials
All commercially available chemicals were of A.R. grade and
all solvents used were purified by standard methods.
2.2. Methods
The metal ions were determined by EDTA titration using
xylenal orange as indicator. C, H and N were determined
using an Elementar Vario EL. Conductivity measurements were
carried out with a DDS-307 type conductivity bridge using
10−3 mol dm−3 solutions in acetone at 25 ◦C. IR spectra were
recorded on Nicolet FT-170SX instrument using KBr discs in
3. Result and discussion
3.1. Properties of the complexes
Analytical data for the complexes listed in Table 1
conform to an 2:6:1 metal-to-nitrate-to-L stoichiometry
[LnL]·[Ln(NO3)5]·(NO3), 1:3:1 metal-to-perchlorate-to-L sto-
ichiometry [LnL]·(ClO4)3. All complexes are soluble in DMF,
acetonitrile, THF, MeOH, acetone, dioxane andCHCl3, butspar-
ingly soluble in water and ethyl acetate. The molar conductances
of the complexes in MeOH (see Table 1) indicate that all nitrate
complexes act as 1:2 electrolytes, and all perchlorate complexes
act as 1:3 electrolytes [13].
1
the 400–4000 cm−1 region, H NMR spectra were measured
on a Varian Mercury plus 300 M spectrometer in CDCl3 solu-
tion with TMS as internal standard. Fluorescence measurements
were made on a Hitachi F-4500 spectrophotometer equipped
with quartz curettes of 1 cm path length at room temperature.
The excitation and emission slit widths were 2.5 nm.
2.3. Synthesis of the ligand
3.2. IR spectra
A
solution of 3,3,7,7-tetrahydroxymethyl-5-oxanonane
(2.5 g, 10 mmol) in THF was added dropwise into a THF solu-
tion that was suspended with NaH (1.6 g, 60%, 40 mmol), and
the mixture was stirred under nitrogen at room temperature until
no gas appeared. Then a solution of N-ethyl-N-phenylchlo-
roacetamide (8.7 g, 44 mmol) (N-methyl-N-phenylchloro-
acetamide for La, N-benzyl-N-phenylchloroacetamide for
Lc) in THF was added dropwise into the mixture. After the
mixture was refluxed for 6 h, the THF was evaporated and the
The main infrared bands of the ligand and its complexes are
presented in Table 2. The IR spectrum of free ligand shows bands
at 1674–1675 and 1122–1108 cm−1, which may be assigned to
ν (C O) and ν (C–O–C), respectively. In the IR spectra of all
the lanthanide complexes, these bands shift by about 50 and
25 cm−1 toward lower wavenumbers, thus indicating that the
carbonyl and ethereal oxygen atoms take part in coordination to
the metal ion.

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K.-W. Lei et al. / Spectrochimica Acta Part A 66 (2007) 662–666
Table 1
Analytical and molar conductance data for the complexes (calculated values in parentheses)
Complexes
C
H
N
Ln
Λm (s cm2 mol−1
)
[EuLa]·[Eu(NO3)5]·(NO3)
[TbLa]·[Tb(NO3)5]·(NO3)
[EuLb]·[Eu(NO3)5]·(NO3)
[TbLb]·[Tb(NO3)5]·(NO3)
[EuLc]·[Eu(NO3)5]·(NO3)
[TbLc]·[Tb(NO3)5]·(NO3)
[EuLa]·(ClO4)3
38.14(38.05)
38.12(37.71)
39.85(39.75)
39.51(39.40)
48.01(47.53)
47.06(47.17)
44.85(44.71)
44.41(44.47)
46.48(46.42)
46.12(46.18)
54.19(54.26)
54.12(54.02)
4.23(4.12)
4.12(4.09)
4.55(4.49)
4.37(4.45)
4.39(4.32)
4.20(4.29)
4.72(4.83)
4.80(4.82)
5.28(5.24)
5.28(5.22)
4.89(4.93)
4.22(4.91)
9.16(9.25)
9.08(9.16)
8.85(8.92)
8.89(8.84)
7.85(7.70)
7.52(7.64)
4.24(4.35)
4.50(4.32)
4.20(4.16)
4.18(4.14)
3.57(3.52)
3.54(3.50)
20.89(20.06)
20.65(20.79)
19.28(19.35)
19.98(20.05)
16.58(16.71)
17.69(17.34)
11.26(11.79)
12.31(12.26)
11.24(11.29)
11.85(11.75)
9.57(9.54)
180
176
193
185
167
171
321
342
332
348
336
327
[TbLa]·(ClO4)3
[EuLb]·(ClO4)3
[TbLb]·(ClO4)3
[EuLc]·(ClO4)3
[TbLc]·(ClO4)3
9.87(9.93)
5
7
The absorption bands assigned to the coordinated nitrates
were observed at about 1493 cm−1 (ν1), 1311 cm−1 (ν4) and
816 cm−1(ν3) for the complexes, respectively, implying that
coordinated nitrate groups in the complexes are bidentate lig-
ands [14]. The absorption band of about 1384 cm−1 suggests
that there is free nitrate in the complexes, which is in agreement
with the results of the conductivity experiments.
about 616 nm from D0 → F2 electronic dipole transition is
the strongest, suggesting low symmetry around the Eu(III) ion
[15]. Therefore, the peak height at 619 nm is used to measure
the fluorescence intensities of europium complexes. We also
see characteristic emission bands of Tb(III) ion at about 492,
5
5
5
546 and 586 nm, assigned to D4–7F6, D4–7F5 and D4–7F4.
The peak height of the strongest emission at about 546 nm from
5D4–7F5 electronic dipole is used to measure the fluorescence
intensities of Tb complexes. We can notice solid luminescence
intensity of each Eu perchlorate complex is stronger than that
of Eu nitrate complex with the same ligand, the case is the
same with the Tb complexes (Fig. 1). This is due to the anion
effect [16]. In the studied examples, perchlorate anion distinctly
increase the Ln(III) luminescence intensity. We know that
the complexes conform to [LnL]·[Ln(NO3)5]·(NO3) (nitrate
complexes) and [LnL]·(ClO4)3 (perchlorate complexes). In
every complex, a ligand chelates a Ln ion with nine O atoms,
but the counter-anions are different: [Ln(NO3)5]2− and (NO3)−
in nitrate complexes, (ClO4)3 in perchlorate complexes. Based
on antenna effect [17,18], the intensity of the luminescence of
Ln3+ complexes is related to the efficiency of the intramolecular
3.3. Fluorescence studies
Under identical experimental, the fluorescence characteris-
tics of the solid complexes are listed in Table 3. The ligands have
strong antenna effect. So its terbium and europium complexes
have strong fluorescence. The fluorescence characteristic
emission wavelengths of the terbium and europium ions were
observed (Fig. 1). As shown in Fig. 1, the emission spectra
of the six Eu complexes at room temperature are similar to
each other and show characteristic emission bands of Eu (III)
ion at about 594 and 616 nm, assigned to 5D0 → F1 and
7
5D0 → F2. The D0 → F3 emission peaks are too weak to
7
5
7
be observed at about 645–660 nm range [2]. The emission at
Table 2
IR spectral data of the free ligands and their complexes (cm−1
)
−
−
−
−
−
ν4 (ClO4 )
Compound
ν (C O)
ν (C–O–C)
ν1 (NO3
)
ν3 (NO3
)
ν4 (NO3
)
ν (free)
ν3 (ClO4
)
La
1675
1623
1624
1623
1624
1112
1090
1091
1096
1097
[EuLa]·[Ln(NO3)5]·(NO3)
[TbLa]·[Ln(NO3)5]·(NO3)
[EuLa]·(ClO4)3
1490
1492
816
816
1309
1310
1384
1384
1097
1098
942
946
[TbLa]·(ClO4)3
Lb
1675
1618
1920
1618
1619
1122
1095
1095
1098
1099
[EuLb]·[Ln(NO3)5]·(NO3)
[TbLb]·[Ln(NO3)5]·(NO3)
[EuLb]·(ClO4)3
1488
1493
813
815
1308
1310
1384
1384
1099
1100
962
962
[TbLb]·(ClO4)3
Lc
1676
1616
621
1619
1620
1108
1093
1076
1094
1096
[EuLc]·[Ln(NO3)5]·(NO3)
[TbLc]·[Ln(NO3)5]·(NO3)
[EuLc]·(ClO4)3
1492
1495
816
826
1310
1306
1384
1384
1095
1098
926
922
[TbLc]·(ClO4)3

K.-W. Lei et al. / Spectrochimica Acta Part A 66 (2007) 662–666
665
Fig. 1. (a) Emission spectra of the solid [EuLa]·(ClO4)3 (1) (λex = 398 nm) and [EuLa]·[Eu(NO3)5]·(NO3) (2) complex (λex = 399 nm) at room temperature. (b)
Emission spectra of the solid [TbLa]·(ClO4)3 (1) (λex = 309 nm) and [TbLa]·[Tb(NO3)5]·(NO3) (2) complex (λex = 319 nm) at room temperature. (c) Emission
spectra of the solid [EuLb]·(ClO4)3 (1) (λex = 397 nm) and [EuLb]·[Eu(NO3)5]·(NO3) (2) complex (λex = 399 nm) at room temperature. (d) Emission spectra of
the solid [TbLb]·(ClO4)3 (1) (λex = 306 nm) and [TbLb]·[Tb(NO3)5]·(NO3) (2) complex (λex = 321 nm) at room temperature. (e) Emission spectra of the solid
[EuLc]·(ClO4)3 (1) (λex = 397 nm) and [EuLc]·[Eu(NO3)5]·(NO3) (2) complex (λex = 399 nm) at room temperature. (f) Emission spectra of the solid [TbLc]·(ClO4)3
(1) (λex = 354 nm) and [TbLc]·[Tb(NO3)5]·(NO3) (2) complex (λex = 320 nm) at room temperature.

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K.-W. Lei et al. / Spectrochimica Acta Part A 66 (2007) 662–666
Table 3
4. Conclusion
Fluorescence data for the complexes
According to the data and discussion above, the amide-based
multipodal ligand could form complexes with lanthanide ions
and exhibit a caverned conformation. The complexes exhib-
ited characteristic fluorescence of europium and terbium ion,
respectively. The different anions may affect the fluorescence
of europium and terbium ions. Based on those results, a series
of new amide-based multipodal derivatives could be designed
and synthesized to optimize the luminescent properties of these
lanthanide ions.
Complexes
λex (nm)
λem (nm)
RFI
34
Assignment
[EuLa]·[Eu(NO3)5]·(NO3)
399
398
319
595
617
5D0 → F1
7
7
121
5D0 → F2
[EuLa]·(ClO4)3
594
616
285
865
5D0 → F1
7
5D0 → F2
7
7
[TbLa]·[Tb(NO3)5]·(NO3)
493
546
586
1179
2252
189
5D4 → F6
7
5D4 → F5
5D4 → F4
7
7
[TbLa]·(ClO4)3
309
492
546
585
4807
7998
822
5D4 → F6
7
5D4 → F5
7
5D4 → F4
Acknowledgements
7
[EuLb]·[Eu(NO3)5]·(NO3)
[EuLb]·(ClO4)3
399
397
321
595
618
35
131
5D0 → F1
5D0 → F2
7
We are grateful to the NSFC (Grants 20371022, 20431010
and 20021001), the Specialized Research Fund for the Doctoral
Program of Higher Education (200307300015), and the Key
Project of the Ministry of Education of China (Grant 01170)
for financial support.
7
594
617
105
298
5D0 → F1
7
5D0 → F2
7
[TbLb]·[Tb(NO3)5]·(NO3)
492
547
586
413
848
82
5D4 → F6
7
5D4 → F5
7
5D4 → F4
References
7
[TbLb]·(ClO4)3
[EuLc]·[Eu(NO3)5]·(NO3)
306
492
1385
5D4 → F6
7
546
586
2574
310
5D4 → F5
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7
5D4 → F5
7
5D4 → F4
[TbLc]·(ClO4)3
492
546
586
433
1041
81
5D4 → F6
7
7
5D4 → F5
7
5D4 → F4
RFI is relative fluorescence intensity.
energy transfer between the triplet energy level of the ligand
and the emitting level of the central ion, which depends on the
gap between the two levels. As we know luminescence intensity
of solid Ln (Ln = Eu, Tb) nitrate is quite weak, we consider that
efficiency of energy transfer from NO3 to Ln in [Ln(NO3)5]
is very poor, which results in incomplete energy transfer in the
whole nitrate complex molecule though that from L to Ln is
efficient. The [Ln(NO3)5] group is negatively effective to the
fluorescence intensity of the nitrate complexes.
M. Latva, H. Takalo, K. Simberg, J. Kankare, Polyhedron 16 (1997) 1381.