Terminal group effects on the fluorescence spectra of europium(III) nitrate complexes with a family of amide-based 2,3-dihydroxynaphthalene derivatives

Article abstract of DOI:10.1016/j.saa.2005.10.028

Three ligands 2,2′-[2,3-naphthylenebis(oxy)]-bis(N,N-diethyl(acetamide)) (L^a), 2,2′-[2,3-naphthylenebis(oxy)]-bis(N,N-diisopropyl(acetamide)) (L^b) and 2,2′-[2,3-naphthylenebis(oxy)]-bis(N,N-dibutyl(acetamide)) (L^c) and their europium(III) nitrate complexes were synthesized. The complexes were characterized by elemental analysis, IR, fluorescence spectroscopy and conductivity. The europium atoms are coordinated by O-atoms from C{double bond, long}O, Ar-O-C. With the difference of the ligands, the solid fluorescent intensities of the Eu complexes vary regularly. Some factors that influencing the fluorescent intensity were discussed.

Full text of DOI:10.1016/j.saa.2005.10.028

Spectrochimica Acta Part A 65 (2006) 187–190
Terminal group effects on the fluorescence
spectra of europium(III) nitrate complexes with a family
of amide-based 2,3-dihydroxynaphthalene derivatives
Ke-Wei Lei, Wei-Sheng Liu∗
Department of Chemistry and State Key Laboratory of Applied Organic Chemistry,
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China
Received 22 August 2005; received in revised form 11 October 2005; accepted 11 October 2005
Abstract
Three ligands 2,2ꢀ-[2,3-naphthylenebis(oxy)]-bis(N,N-diethyl(acetamide)) (La), 2,2ꢀ-[2,3-naphthylenebis(oxy)]-bis(N,N-diisopropyl(acetam-
ide)) (Lb) and 2,2ꢀ-[2,3-naphthylenebis(oxy)]-bis(N,N-dibutyl(acetamide)) (Lc) and their europium(III) nitrate complexes were synthesized. The
complexes were characterized by elemental analysis, IR, fluorescence spectroscopy and conductivity. The europium atoms are coordinated by
O-atoms from C O, Ar–O–C. With the difference of the ligands, the solid fluorescent intensities of the Eu complexes vary regularly. Some factors
that influencing the fluorescent intensity were discussed.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Amide-based 2,3-dihydroxynaphthalene; Synthesis; Complexes; Fluorescent property
1. Introduction
functional ligands having selective ability to coordinate lan-
thanides ions. The luminescence of these lanthanide complexes
were enhanced by providing some of proper conjugate absorp-
tion groups suitable for energy transfer, i.e. naphthalene, and the
other coordinating groups with cavities for lanthanide ions, i.e.
amide-based open-chain crown ethers. In the present work, we
designed and synthesized three ligands and their Eu(III) nitrate
complexes. The effects of the terminal groups on the fluores-
cence properties of the solid complexes were investigated.
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 spec-
troscopic properties of these ions [2,4]. The main advantages
of lanthanide chelates in fluorescence spectrometry are their
large stokes shifts, narrow emission profiles and long fluores-
cence lifetimes [5]. These have led to use lanthanides as flu-
orescent probes for highly sensitive time-resolved fluorimetric
immunoassays, metal ion coordination in proteins and structural
studies of biological macromolecule [2]. And these properties
have also been used to improve the sensitivity for lanthanide
estimationbyconventionalspectrofluorimetryandlaser-induced
spectrofluorimetry [5].
2. Experimental
2.1. Materials
All commercially available chemicals were of A.R. grade and
all solvents used were purified by standard methods.
Amide-based open-chain crown ethers offer many advan-
tages in extraction and analysis of the rare earth ions [6,7]
because of their ring-like coordination structure and terminal
group effects [7,8]. So we have designed a series of multi-
2.2. Methods
The metal ions were determined by EDTA titration using
xylenol orange as an indicator. Carbon, nitrogen and hydrogen
analyses were determined using a Vario EL elemental analyzer.
Conductivity measurements were carried out with a DDSJ-308
type conductivity bridge using 1.0 × 10−3 mol dm−3 solution in
∗
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 © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.10.028

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K.-W. Lei, W.-S. Liu / Spectrochimica Acta Part A 65 (2006) 187–190
MeOH at 25 ◦C. IR spectra were recorded on a Nicolet Avatar
360 FT-IR instrument using KBr discs in the 400–4000 cm−1
region. H NMR spectra were measured on a FT-80A or FT-
Analytical data, Calc. for L: C, 70.85; H, 8.45; N, 6.38. Found:
C, 70.56%; H, 8.65%; N, 6.33%.
1
2.3.4. 2,2ꢀ-[2,3-naphthylenebis(oxy)]-
200A spectrometer in CDC13 solution, 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.
bis(N,N-dibutyl(acetamide)) (Lc)
Yield, 78%. 1H NMR (200 MHz, CDC13): δ 7.26–7.68 (m,
6H); 4.85 (s, 4H; 2O–CH2–C(O)); 3.35 (t, 8H; 2N–(CH2–R)2);
1.52 (m, 8H; 2N–(R–CH2–R)2) 1.30 (m, 8H; 2N–(R–CH2–R)2)
0.92 (t, 12H; 4R–CH3) IR: ν 1646(C O), 1172 (Ar–O–C). Ana-
lytical data, Calc. for L: C, 72.35; H, 9.24; N, 5.68. Found: C,
72.25%; H, 9.30%; N, 5.62%.
2.3. Synthesis of the ligand
2.3.1. General procedure for ligand synthesis
The three ligands were prepared by the same procedure.
Anhydrous K2CO3 (4.0 g) was added slowly to the DMF
solution of 2,3-dihydroxynaphthalene (1) (6.04 mmol) at
110 ◦C. An hour later, a DMF solution containing 2 (La: N,N-
diethylchloroacetamide; Lb: N,N-diisopropylchloroacetamide;
Lc: N,N-dibutylchloroacetamide) (13.0 mmol) was added
dropwise and slowly to the mixture. The reaction mixture was
stirred for 24 h. Fifty milliliters of water were poured into the
cooled mixture. Then the crude product was extracted by CHCl3
(3 × 40 ml). Thecombinedorganicphasesweredriedinvacuum.
The crude product was dissolved in EtOAc and then purified
by chromatography on silica gel until 3 was received as white
solid.
2.4. Synthesis of complexes
A solution of Eu(NO3)3·6H2O (0.1 mmol) in ethyl acetate
was added slowly to 20 ml of a solution of the ligand (0.1 mmol).
The mixture was stirred for 5 h and white precipitate formed.
The precipitate was collected and washed three times with ethyl
acetate. Further drying in vacuum resulted in a pale white pow-
der, yield: 80% Fig. 1.
3. Result and discussion
3.1. Properties of the complexes
Analytical data for the complexes are listed in Table 1, con-
formed to [Eu(NO3)3L]. All complexes are soluble in DMF,
DMSO, acetonitrile, THF, methanol and acetone, but little solu-
ble in chloroform and ethyl acetate. Conductivity measurement
for these complexes in methanol solution (Table 1) indicate the
presence of a non-dissociated compound [9].
2.3.2. 2,2ꢀ-[2,3-naphthylenebis(oxy)]-
bis(N,N-diethyl(acetamide)) (La)
Yield, 80%. 1H NMR spectrum (80 MHz, CDC13): δ
7.27–7.74 (m, 6H); 4.84 (s, 4H; 2O–CH2–C(O)); 3.41 (q, 8H;
4N–CH2–R), 1.20 (t, 12H; 4R–CH3). IR: ν 1640(C O), 1166
(Ar–O–C). Analytical data, Calc. for La: C, 66.30; H, 8.29; N,
7.73. Found: C, 66.10%; H, 9.21%; N, 7.20%.
3.2. IR spectra
2.3.3. 2,2ꢀ-[2,3-naphthylenebis(oxy)]-bis(N,N-
IR spectra of the complexes are similar to each other. The
main infrared bands of the ligands and their complexes are pre-
sented in Table 2. The IR spectrum of the free ligands show a
strong band in the region 1640–1661 cm−1, which can assigned
to the stretch vibration of the carbonyl group. The peak at
diisopropyl(acetamide)) (Lb)
Yield, 80%. 1H NMR (200 MHz, CDC13): δ 7.29–7.69 (m,
6H); 4.75 (s, 4H; 2O–CH2–C(O)); 3.42 (q, 2H; 2N–CH(R)2);
1.40 (d, 12H; 2R–(CH2)2) IR: ν 1661(C O), 1174 (Ar–O–C).
Fig. 1. Scheme of the synthesis of the ligands.

K.-W. Lei, W.-S. Liu / Spectrochimica Acta Part A 65 (2006) 187–190
189
Table 1
Elemental analytical and molar conductance data for the complexes
Complexes
Found (calc. %)
C
Λm (s cm2 mol−1
)
H
N
Eu
[Eu(NO3)3La]
[Eu(NO3)3Lb]
[Eu(NO3)3Lc]
37.36 (38.02)
40.76 (39.86)
43.72 (44.21)
4.50 (4.35)
5.19 (5.24)
5.80 (5.84)
9.47 (9.56)
8.80 (8.75)
8.22 (8.36)
20.55 (21.06)
19.10 (19.26)
17.84 (18.26)
68.2
63.5
58.6
Table 2
IR spectral data of the free ligands and their complexes (cm−1
)
−
−
)
Compound
ν(C O)
ν(Ar–O)
ν1(NO3
)
ν4(NO3
La
1640
1623
1661
1618
1646
1620
1166
1159
1174
1158
1172
1161
[Eu(NO3)3La]
Lb
1482
1483
1489
1303
1308
1316
[Eu(NO3)3Lb]
Lc
[Eu(NO3)3Lc]
1166–1174 cm−1, is attributed to the ν of the Ar–O bond. In
the IR spectra of the Eu complexes, the ν(C O) and ν(Ar–O)
are shifted by 20–40 and 6–14 cm−1, respectively.
The absorption bands assigned to the coordinated nitrates
were observed as two bands at about 1483 cm−1 (ν1) and
1308 cm−1 (ν4), 817 cm−1 for the complexes. This indicates
that coordinated nitrate groups in the complex are bidentate
in agreement with the results of the conductivity measure-
ments.
Fig. 2. Emission spectra of the solid europium complexes and Eu nitrate at room
temperature: (1) [Eu(NO3)3La] λex = 466 nm; (2) [Eu(NO3)3Lb] λex = 466 nm;
(3) [Eu(NO3)3Lc] λex = 466 nm; (4) Eu(NO3)3·6H2O λex = 466 nm.
(619 nm) decrease continuously with the decrease of the termi-
nal groups, i.e. in the series dibutylamide, diisopropylamide,
diethylamide. We can also see the fluorescence intensity of
Eu(NO3)3·6H2O is weaker than that of the complexes. The for-
mation of complex is helpful to enhance fluorescence intensity
of Eu ion. Based on antenna effect [11,12], the intensity of the
luminescence of Ln3+ complexes is related to the efficiency of
the intramolecular 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. We consider that the
group having a positive effect to the fluorescence intensity of the
Eu(III)—complexes is mainly the naphthalene group, as can be
deduced by the excited wave numbers. The terminal group has a
negative effect to the fluorescence intensity of the Eu(III) com-
plex. The ligand with the shorter alkyl terminal group increases
the ability of O-atoms of the C O and Ar–O–C to coordinate to
lanthanide ions and decreases the fluorescence intensity of the
Eu(III)-complexes.
3.3. Fluorescence studies
Under identical experimental conditions, the fluorescence
characteristics of the solid complexes at room temperature were
studied. They are summarized in Table 3. The maximum of exci-
tation was observed at a wavelength of 466 nm for all complexes.
As shown in Fig. 2, the emission spectra of the three complexes
at room temperature are similar to each other and show char-
acteristic emission bands of Eu(III) ion at about 593, 617 nm,
7
7
7
assigned to 5D0 → F1, 5D0 → F2, 5D0 → F3, emission peaks
are too weak to be observed at about 645–660 nm range [2]. The
emission at 619 nm from 5D0 → F2, electronic dipole transition
7
is the strongest, suggesting low symmetry around the Eu(III)
ion [10]. Therefore, the peak height at 619 nm is used to mea-
sure the fluorescence intensities of three europium complexes.
5
7
As shown in Fig. 2, the fluorescence intensities of D0 to F2
Table 3
4. Conclusion
Fluorescence data for the complexes
Complex
λex (nm)
λem (nm)
RFI
590
127
904
229
1252
273
Assignment
According to the data and discussion above, obvious changes
in IR spectra were observed, after formation of complexes
between the ligands and Eu(III). In the complexes, Eu ions
were coordinated to the C O oxygen atoms and ether oxygen
atoms. The complexes emitted the characteristic fluorescence of
Eu(III) ions. The terminal group may affect the fluorescence of
Eu ions. A series of new amide based 2,3-dihydroxynaphtahlene
[Eu(NO3)3La]
466
616
593
617
592
618
591
5D0 → F2
7
5D0 → F1
7
[Eu(NO3)3Lb]
[Eu(NO3)3Lc]
466
466
5D0 → F2
7
7
5D0 → F1
7
5D0 → F2
7
5D0 → F2

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K.-W. Lei, W.-S. Liu / Spectrochimica Acta Part A 65 (2006) 187–190
derivates was synthesized to optimize the luminescence proper-
ties of lanthanide ions.
[2] F.S. Richardson, Chem. Rev. 82 (1982) 514.
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Labelling Techologies, Wallac Oy, Turku, 1995.
Acknowledgements
We gratefully acknowledge the NSFC (Grants 20371022,
20431010 and 20021001), the Specialized Research Fund for
theDoctoralProgramofHigherEducation(200307300015), and
the Key Project of the Ministry of Education of China (Grant
01170) for financial support.
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