Studies on synthesis, infrared and fluorescence spectra of new europium (III) and terbium (III) complexes with an β-diketonate-type ligand

Article abstract of DOI:10.1016/j.saa.2004.07.006

A new β-diketonate-type ligand, N-(2-amino-6-methyl-pyridinyl) ketoacetamide (L) and its complexes with europium (III) and terbium (III) were synthesized. The complexes were characterized by elemental analysis, infrared spectra and conductivity. The europium (III) and terbium (III)-ions were found to coordinate to the CO oxygen atoms and pyridine nitrogen atoms. The fluorescence properties of these complexes in solid, DMF and CH₃OH were studied. The solvent factors influencing the fluorescent intensity are discussed.

Full text of DOI:10.1016/j.saa.2004.07.006

Spectrochimica Acta Part A 61 (2005) 1767–1770
Studies on synthesis, infrared and fluorescence spectra of new europium
(III) and terbium (III) complexes with an ␤-diketonate-type ligand
Xi-Shi Taia,∗, Min-Yu Tanb
a
Department of Chemistry, Weifang University, Weifang 261061, P.R. China
Department of Chemistry, Lanzhou University, Lanzhou 730000, P.R. China
b
Received 17 June 2004; accepted 12 July 2004
Abstract
A new ␤-diketonate-type ligand, N-(2-amino-6-methyl-pyridinyl)ketoacetamide (L) and its complexes with europium (III) and terbium (III)
were synthesized. The complexes were characterized by elemental analysis, infrared spectra and conductivity. The europium (III) and terbium
(III)-ions were found to coordinate to the C O oxygen atoms and pyridine nitrogen atoms. The fluorescence properties of these complexes
in solid, DMF and CH3OH were studied. The solvent factors influencing the fluorescent intensity are discussed.
© 2004 Elsevier B.V. All rights reserved.
Keywords: ␤-Diketonate-type ligand; Europium (III) nitrate complexes; Terbium (III) nitrate complexes; Fluorescence of lanthanide complexes; IR spectra
1. Introduction
fer property to the lanthanide metal ion. In the present
work, we designed and synthesized a new ␤-diketonate-
type ligand containing pyridine terminal groups, N-(6-(2-
methyl)pyridinyl)ketoacetamide (Scheme 1), and studied
the fluorescence properties of europium (III) and terbium
(III) complexes with the new ligand, N-(2-amino-6-methyl-
pyridinyl)ketoacetamide. The results indicated that the or-
ganic solvent affected the fluorescence characteristics of eu-
ropium (III) and terbium (III) ions.
Organiclight-emittingdevices(OLEDs)haveattractedex-
tensive attention due to their excellent performance with high
brightness, low power consumption, wide viewing angle, fast
response time and potentially low cost [1–5]. Since Tang and
VanSlyke [1] reported a low-voltage driven electrolumines-
cent (EL) device using tris-(8-hydroxyquinoline) aluminum
(Alq3) as an emitting material and electron transport material
produced strong electroluminescence in the green, the study
on using metal complex as organic light-emitting material
becomes more and more attention.
2. Experimental
The europium (III) and terbium (III) complexes with
␲-conjugated ligands such as ␤-diketonato also have been
studied as emitting materials for OLEDs [6–9], since the
reports by Okamoto and co-workers [10]. However, the
quantum efficiency of most these complexes are unfortu-
nately still low. This may be mostly due to inefficiency
of the energy transfer, particularly, triplet–triplet transfer
in these complexes. Chemists have realized that it is es-
sential to design ligands, which have better energy trans-
2.1. Materials
2-Amino-6-methylpyridine (A.R.) was purchased from
Aldrich, All other regents used were analytical grade and
used without further purification. Rare earth nitrates were
prepared according to the literature method [11].
2.2. Methods
∗
Carbon, hydrogen and nitrogen were determined using
a Elementar Vario EL elemental analyzer. Infrared spectra
Corresponding author. Tel.: +86 931 8619799; fax: +86 931 8912582.
E-mail address: taixishi@lzu.edu.cn (X.-S. Tai).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.07.006

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X.-S. Tai, M.-Y. Tan / Spectrochimica Acta Part A 61 (2005) 1767–1770
Scheme 2. The keto and enol tautomerism of L.
Scheme 1. The synthesis of ligand (L).
3. Result and discussion
(4000–400 cm−1) were recorded with KBr optics on a Nico-
let NEXUS 670 FTIR spectrophotometer. The molar con-
ductance values were determined on a DDS-11A conduc-
tivity meter with methanol as solvent (10−3 mol L−1 solu-
tion) at 25 ◦C. 1H NMR and 13C NMR were measured with a
Bruker-300MHz nuclear magnetic resonance instrument us-
ing CDCl3 as solvent and TMS as internal reference. Mass
spectrum was measured using a ZAB-HS analyzer. Fluores-
cence measurements were made on a Hitich F-4500 spec-
trometer equipped with quartz curettes of 1 cm path length.
3.1. 1H NMR and 13C NMR of the ligand (L)
In chloroform solution, the ligand displays keto enol tau-
tomerism as judge by 1H NMR and 13C NMR spectroscopy
(Table 1, Scheme 2).
3.2. Properties of the complexes
Analytical data for the complexes, presented in Table 2,
The results of elemental analyses indicated that the com-
position of the complexes conform to Eu(NO3)3L2 and
Tb(NO3)3L2, and that all the complexes conform to a 1:2
metal-to-ligand stoichiometry.
2.3. Synthesis of the ligand
All complexes are soluble in DMF, DMSO, methanol, a
little soluble in ethanol and acetone, insoluble in benzene,
diethyl ether and tetrahydrofuran. The molar conductance
values of the complexes measured in CH3OH solution (1 ×
10−310−3 mol L−1) at 25 ◦C are 168 and 172 S cm2 mol−1
(Table 2), indicating 1:2 electrolytes [13] and that uncoordi-
nated nitrates are in the complexes, which is in accordance
with the results of IR spectra of the complexes.
The ligand was prepared by reaction of 2-amino-6-
methylpyridine with ethyl acetoacetate by a method similar to
literature [12]. The ligand was obtained as a white flake solid.
IR: cm−1 (KBr, pellet). ν3240 (m, N H), 1722 (s, CH3C O),
1664 (s, amide C=O), 1553 (s, C N). Elemental analytical:
(calc.) C% 62.40 (62.49), H% 6.40 (6.25), N% 14.48 (14.57).
MS (FAB): 193 (M + 1).
3.3. IR spectra
2.4. Synthesis of the complexes
The IR spectra of all complexes are similar, indicating
that the complexes have identical structure. Table 3 gives the
characteristic bands of ligand and its complexes. The IR spec-
An ethanol solution of Ln (NO3)3·6H2O (Ln = Eu and Tb)
(0.2 mmol) was added dropwise to a solution of the ligand
(0.6 mmol) in chloroform (15 ml). The mixture was stirred
for 3 h, and white precipitate formed. The precipitate was
collected by filtration and washed three times with ethanol
and chloroform, respectively. Further drying in vacuum af-
forded a white flake solid, yield ca. 80%.
trum of the free ligand shows bands at 1722 and 1664 cm−1
,
the former due to the CH3C O moiety and the latter to the
amide C O. The band at 1553 cm−1 in the free ligand can
be assigned to ν (C N) of pyridine ring. In the complexes,
Table 1
Chemical shifts (ppm) of the ligand (L) in CDCl3
Tautomer
1H NMR
C (9) H
N (5) H
9.12
C (7) H
O (1) H
13.5
C (1) H
Py-H
Keto
Enol
2.32
1.97
3.58
5.03
2.45
6.89-7.96
C (4)
13C NMR
C (6)
C (7)
C (1)
24.2
C (2)
119.8
C (3)
C (9)
31.2
Keto
Enol
51.6
91.7
150.3
138.9
111.3

X.-S. Tai, M.-Y. Tan / Spectrochimica Acta Part A 61 (2005) 1767–1770
1769
Table 2
Elemental analytical and molar conductance data for the complexes
Complex
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Λm (S cm2 mol−1
)
)
[Eu(NO3)L2](NO3)2
[Tb(NO3)L2](NO3)2
32.98 (33.24)
32.68 (32.92)
3.16 (3.32)
3.20 (3.29)
13.76 (13.57)
13.60 (13.44)
168
172
Table 3
The relevant characteristic IR bands (cm−1
)
−
−
−
Compound
ν (CH3C=O)
ν (amide C=O)
ν (C=N)
ν1 (A1, NO3
)
ν5 (B2, NO3
)
ν3 (E’, NO3
L
1722
1654
1655
1664
1617
1617
1553
1567
1567
[Eu(NO3)L2](NO3)2
[Tb(NO3)L2](NO3)2
1492
1496
1306
1302
1385
1384
these bands for ν (CH3C O), ν (amide C O) and ν (C N)
of pyridine ring are shifted by about 68 (from 1722 to 1654),
47 (from 1664 to 1617) and 14 (from 1553 to 1567) cm−1
,
respectively, indicating that the oxygen atoms of CH3C O,
amide C O and pyridine nitrogen atoms take part in coordi-
nation.
The absorption bands assigned to the coordinated nitrates
were observed at about 1492 (νas) and 814 (νs) cm−1 for the
complexes. The ν (D3h) free nitrates appear at approximately
1385 cm−1 in the spectra of the complexes, in agreement
with the results of the conductivity experiments. In addition,
the separation of two highest frequency bands |ν1 − ν4| is
approximately 200 cm−1, thus the coordinated NO3− ions in
the complexes are bidentate ligands [14].
The results of elemental analysis and IR spectra indicate
that the coordination only involves the keto-form of the lig-
and.
Fig. 1. The emission spectrum of the europium complex in solid. The exci-
tation and emission slit widths were 2.5 nm.
europium complexes (λex = 308 nm) in solid and in organic
solutions (λex = 396 nm) are quite different, and the excitation
wavelengths of terbium complexes are also different between
in solid and in organic solutions. The europium and terbium
complexes display a fluorescence emission maximum at 616
and 546 nm in solid (Fig. 1, Fig. 3), respectively. Compared
3.4. Fluorescence studies
The fluorescence characteristics of all complexes in solid
and in DMF, methanol solutions are listed in Table 4. From
Table 4 it could be seen that the excitation wavelengths of the
Table 4
Fluorescence data for the complexes
Solvent λex (nm) λem (nm) RFIa
Assignment
7
[Eu(NO3)L2](NO3)2, Solid
DMF
308
396
616
3717 5D0 → F2
7
591
616
16.4 5D0 → F1
32.1 5D0 → F2
7
7
CH3OH 396
591
616
4.5 5D0 → F1
6.4 5D0 → F2
7
7
[Tb(NO3)L2](NO3)2 Solid
387
337
495
546
588
2795 5D4 → F6
7
8659 5D4 → F5
179 5D4 → F4
7
7
DMF
490
545
584
896 5D4 → F6
7
2705 5D4 → F5
145 5D4 → F4
7
7
CH3OH 378
491
546
584
223 5D4 → F6
7
648 5D4 → F5
7
32 5D4 → F4
Fig. 2. The emission spectrum of the europium complex in solution. The
excitation and emission slit widths were 5 nm.
a
RFI: relative fluorescence intensity.

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X.-S. Tai, M.-Y. Tan / Spectrochimica Acta Part A 61 (2005) 1767–1770
ropium and terbium ions. Obvious IR spectrum changes were
observedaftertheligandformedcomplexeswiththetwoions.
In the complexes, europium and terbium ions were coordi-
nated to the oxygen atoms of CH3C O, amide C O and pyri-
dine nitrogen atoms. The complexes exhibited characteristic
fluorescence of europium and terbium ions. Based on those
results, a series of new ␤-diketonate-type ligand could be
designed and synthesized to optimize the luminescent prop-
erties of these lanthanideions.
Acknowledgements
This work is support by Science Foundation of Weifang
University.
Fig. 3. The emission spectrum of the terbium complex. The excitation slit
widths were 1 nm (in solid) and 2.5 nm (in solution). The excitation and
emission slit widths were 2.5 nm (in solid) and 5 nm (in solution).
with those in solid, the emission maximum is identical with
that in organic solutions (Fig. 2). However, from Table 4, we
can see that the fluorescence intensities of all the complexes
in solid are stronger than that in DMF and CH3OH solution.
This may be due to the excited state of the lanthanide ions is
efficiently quenched by interactions with high-energy vibra-
tions groups of organic solvent [15].
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539.
We also can see the fluorescence intensities of terbium
(III) complex at 546 nm are stronger than those of europium
complexes at 616 nm, either in solid or in organic solutions.
The luminescence of Ln3+ chelates is related to the efficiency
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the energy gap between the two levels. Probably the energy
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According to the data and discussion above, the ␤-
diketonate-type ligand has formed stable complex with eu-