Investigations into the synthesis and fluorescence properties of Tb(III) complexes of a novel bis-β-diketone-type ligand and a novel bispyrazole ligand

Article abstract of DOI:10.1016/j.saa.2007.12.025

A novel bis-β-diketone organic ligand, 1,1′-(2,6-bispyridyl)bis-3-(p-methoxyphenyl)-1,3-propanedione (L₁) and its derivatives, a novel bispyrazole ligand, 2,6-bis(5-(4-methoxyphenyl)-1H-pyrazol-3-yl)pyridine (L₂) were designed and synthesized and their complexes with Tb(III) ion were successfully prepared. The ligands and the corresponding metal complexes were characterized by elemental analysis, infrared, proton nuclear magnetic resonance spectroscopy and TG-DTA. Analysis of the IR spectra suggested that the lanthanide metal ion Tb(III) coordinated to the ligands via the nitrogen atom of the pyridine ring and the carbonyl oxygen atoms for ligand L₁ and the nitrogen atom of the pyrazole ring for ligand L₂. The fluorescence properties of the two complexes in solid state were investigated and it was discovered that the Tb(III) ions could be sensitized by both the ligand (L₁) and ligand (L₂) to some extent. In particular, the complex of ligand (L₂) is a better green luminescent material that could be used as a candidate material in organic light-emitting devices (OLEDs) since it could be much better sensitized by the ligand (L₂), and the fluorescence intensity of Tb(III) complex of L₂ are almost as twice strong as L₁'s.

Full text of DOI:10.1016/j.saa.2007.12.025

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 321–325
Investigations into the synthesis and fluorescence properties of Tb(III)
complexes of a novel bis-␤-diketone-type ligand
and a novel bispyrazole ligand
Lin-Xiang Xiao, Yi-Ming Luo∗, Zhe Chen, Jun Li, Rui-Ren Tang
School of Chemistry and Chemical Engineering, Central South University, Hunan 410083, PR China
Received 29 September 2007; received in revised form 18 December 2007; accepted 19 December 2007
Abstract
A novel bis-␤-diketone organic ligand, 1,1ꢀ-(2,6-bispyridyl)bis-3-(p-methoxyphenyl)-1,3-propanedione (L1) and its derivatives, a novel bispyra-
zole ligand, 2,6-bis(5-(4-methoxyphenyl)-1H-pyrazol-3-yl)pyridine (L2) were designed and synthesized and their complexes with Tb(III) ion were
successfully prepared. The ligands and the corresponding metal complexes were characterized by elemental analysis, infrared, proton nuclear
magnetic resonance spectroscopy and TG-DTA. Analysis of the IR spectra suggested that the lanthanide metal ion Tb(III) coordinated to the
ligands via the nitrogen atom of the pyridine ring and the carbonyl oxygen atoms for ligand L1 and the nitrogen atom of the pyrazole ring for
ligand L2. The fluorescence properties of the two complexes in solid state were investigated and it was discovered that the Tb(III) ions could be
sensitized by both the ligand (L1) and ligand (L2) to some extent. In particular, the complex of ligand (L2) is a better green luminescent material
that could be used as a candidate material in organic light-emitting devices (OLEDs) since it could be much better sensitized by the ligand (L2),
and the fluorescence intensity of Tb(III) complex of L2 are almost as twice strong as L1’s.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Bis-␤-diketone; Bispyrazole; Synthesis; Fluorescence; IR spectra; Lanthanide complexes
1. Introduction
form uniform thin film in vacuum vapor deposition and are rea-
sonably stable to heat, which is required for OEL displays [8].
Further investigations into the relationship between the struc-
tures of organic ligands and the energy levels of lanthanide ions
will give evidences for designing high luminescent lanthanide
organic complexes.
Organic electroluminescence (OEL) has been studied exten-
sively for its applications because of its low drive voltage,
suitability for integrated circuit and potential application for
large flat panel display [1,2]. As some lanthanide ions, e.g. Eu3+
and Tb3+, possess good luminescence characteristics (high color
purity) based on the transitions between the 4f energy levels, a
series of compounds activated Eu3+ and Tb3+ have been studied
for practical application as phosphors and laser materials [3,4].
Terbium(III) has four narrow emission bands corresponding to
Since the 1960s, the complexes of Eu(TTA)3Phen,
Sm(TTA)3Phen have attracted considerable attention because
of their high fluorescence emission efficiency, which owes to
high absorption coefficient of HTTA and the synergistic effect of
1,10-phenanthroline [9,10]. The complexes of Eu(III), Tb(III),
Sm(III) and Gd(III)ions with ␲-conjugated ligands such as
␤-diketones have also been examined as emitting materials
for OLEDs [11] following the reports by Okamoto and his
co-workers. Yan and Song put more emphasis on the Eu3+,
Tb3+, Sm3+, Dy3+ and Gd3+ complexes with 2,2ꢀ-bipyridine-
N,Nꢀ-dioxide [12], 5-bromonicotinic acid [13], 2,6-pyridine
dicarboxylic acid and phthalate, [14,15], etc., the correspond-
ing coordination behavior and symmetry of central metal ions
were investigated, and the outstanding luminescence properties
were discussed especially. However, the quantum efficiency of
5
7
the D4 → Fj transitions, where j = 6, 5, 4, 3. The strongest
transition, 5D4 → F5, occurs at approximately 544 nm, the nar-
7
row intense green terbium luminescence [5]. However, several
ternary rare earth complexes applied to prepare OEL devices
showed weak visible emission and low electroluminescence effi-
ciency [6,7]. In principle, electroneutral metal complexes may
∗
Corresponding author. Tel.: +86 731 8876675; fax: +86 731 8879616.
E-mail address: ymluo@mail.csu.edu.cn (Y.-M. Luo).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.12.025

322
L.-X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 321–325
most of these complexes is unfortunately still low. Chemists
have realized that it is essential to design ligands, which
have better energy transfer property to the lanthanide metal
ion.
2.2. Methods
Contents of carbon, hydrogen and nitrogen were determined
using an Elementar vario EL elemental analyzer. Content of
Tb(III) was determined by EDTA titration. Infrared spectra
(4000–400 cm−1) were recorded with samples as KBr dis-
In our earlier work, we have designed and synthe-
sized a novel, pyridine-containing, bis-␤-diketone-type ligand,
1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-propanedione, and stud-
ied the fluorescence properties of its Eu(III), Tb(III),
Sm(III) and Gd(III) complexes in the solid state [16]. The
result showed that the complexes with Tb(III) exhibited an
strong green luminescent, but the quantum efficiency of
these complexes are not high enough. In an attempt to
increase the intensity of the lanthanide fluorescence exploiting
the antenna effect, in this paper, on the basis of 1,1ꢀ-
(2,6-bispyridyl)bis-3-phenyl-1,3-propanedione, another novel
bis-␤-diketone organic ligand, 1,1ꢀ-(2,6-bispyridyl)bis-3-(p-
methoxyphenyl)-1,3-propanedione (L1), and its derivative,
bispyrazole ligand, 2,6-bis(5-(4-methoxyphenyl)-1H-pyrazol-
3-yl)pyridine(L2)weresynthesized(Scheme1), thecorrespond-
ing terbium complexes were synthesized, characterized and the
luminescence properties for the two terbium complexes were
investigated as well.
1
cussing a Nicolet NEXUS 670 FTIR spectrophotometer. H
NMR spectra was measured by using a Bruker-400 MHz nuclear
magnetic resonance spectrometer with CDCl3 or DMSO as
solvents and TMS as internal reference. Differential thermal
analysis (DTA) was carried out using a Shimadzu DT-40.
FluorescencemeasurementsweremadeonaHitichF-4500spec-
trometer.
2.3. Synthesis of the ligands
2.3.1. Synthesis of 1,1ꢀ-(2,6-bispyridyl)bis-3-(p-
methoxyphenyl)-1,3-propanedione
(L1)
To a suspension of freshly cut sodium (1.0 g, 44.0 mmol)
in anhydrous toluene (50.0 ml) was placed in a three-necked,
round-bottom flask, fitted with a condenser, heated up with sil-
icone oil till the sodium was molten. The mixture was then
cooled to 110 ◦C and dimethyl 2,6-pyridinedicarboxylate(4.3 g
22.0 mmol) wad added into the mixture. The mixture in the flask
was stirred, meanwhile, a solution of p-methoxy acetophenone
(6.6 g, 44.0 mmol) in toluene (25.0 ml) was added dropwise. The
reaction mixture was incubated and stirred at the temperature
of 110 ◦C for about 2 h until the dark yellow sodium salt pre-
cipitated. The sodium salt was collected by filtration, washed
thoroughly with petroleum ether and dried. The dried solid was
2. Experimental
2.1. Materials
Pyridine-2,6-dicarboxylic acid and other reagents used were
purchased and used as analytical grade. Rare earth chlorides
were prepared according to the literature method [17] and
[18].
Scheme 1. Synthesis of the ligands (L1 and L2).

L.-X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 321–325
323
Table 1
Elemental analytical data for the complexes
Complex
Tb (%) found (calc.)
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Tb2(L1)3·5H2O
Tb(L2)3·4H2O
18.71 (18.69)
10.76 (10.60)
53.10 (52.92)
60.13 (60.00)
3.98 (4.29)
4.58 (4.73)
2.62 (2.47)
14.36 (14.00)
added to dilute hydrochloric acid (2.0 M, 50.0 ml) and the result-
ing precipitate was collected by filtration. The crude products
were recrystallized from methanol to give the ligand (L1) (8.1 g,
85.5%).
filtration, purified by washing for several times with ethanol and
chloroform and dried for 24 h in a vacuum to give a flaky solid,
the complex of L1 is yellow power and the complex of L2 is
white power.
1
Physical properties of L1: The H NMR spectrum shows
the presence of several keto-enol tautomers. In this paper,
we described only the major tautomer, the bis(keto-enol) sys-
tem, which formed yellow microcrystals from methanol, mp
220–221 ◦C; IR (KBr): νmax(cm−1); 3422 (C C–OH), 2924
(CH2), 2845 (–OCH3), 1616 (C C), 1570 (C N), 1533 (C C),
1250 (Ar–O), 833 (Phen); 1H NMR (400 MHz, CDCl3): δ16.5
(d, 2H, OH), 8.05–8.35 (m, 3H, Py), 7.04–7.45 (m, 8H, Ph), 3.92
(s, 6H, OCH3); Elemental analytical (calc.) C%69.70 (69.62),
H% 4.92 (4.87), N% 3.43 (3.25).
3. Result and discussion
3.1. Properties of the complexes
The results of elemental data for the complexes presented in
Table 1 indicated that the composition of the complexes con-
forms to Tb2(L1)3·5H2O, and Tb(L2)3·4H2O, respectively. The
both complexes were found to be soluble in H2O, DMF, DMSO
and acetone, slightly soluble ethanol, and Tb2(L1)3·5H2O solu-
ble in CHCl3 and methanol, but Tb(L2)3·4H2O slightly soluble
in methanol and CHCl3, they were both insoluble in benzene,
diethyl ether and tetrahydrofuran, and may be kept in air for a
long time.
2.3.2. Synthesis of
2,6-bis(5-(p-methoxyphenyl)-1-H-pyrazol-3-yl) pyridine
(L2)
Hydrazine hydrate (4.0 ml, 85.0%) was added dropwise to
a solution of 1,1ꢀ-(2,6-bispyridyl)bis-3-(p-methoxyphenyl)-1,3-
propanedione (1.72 g, 40.0 mmol) in acetic acid (50.0 ml) under
stirring at room temperature for 30 min, The mixture was heated
to 60 ◦C and incubated for about 4 h until the precipitate van-
ished, then was cooled to room temperature and ice water
(20.0 g)was added to the mixture, a white precipitate formed.
The precipitate was collected by filtration and washed with
water. Recrystallization from acetone gave the ligand (L2) (1.1 g,
65%). mp 240–241 ◦C. IR (KBr): νmax(cm−1): 3270 (N–H),
2835 (–OCH3), 1675, 1582, 1529, 1466, 1315 (skeleton of Ph,
Py and Pyraz), 1248 (C–N), 1H NMR (DMSO-d6, ppm): δ13.45
(s, 2H, NH), 8.05–7.82 (m, 3H, Py), 7.05–7.23 (m, 8H, C6H5),
6.05 (s, 2H, Pyraz.), 3.06 (s, 6H, OCH3); Elemental analytical
(calc.) C% 71.08 (70.92), H% 4.82 (4.96), N% 16.84 (16.55).
3.2. IR spectra
Table 2 describes the characteristic bands of ligands and their
complexes. The ν(C O), ν(C C) and νpyridine ring (C N) vibra-
tions of the free ligand (L1) are at 1616, 1533 and 1570cm−1
,
The band at 3422 in the free ligand (L1) can be assigned to
ν( C–OH). In the complex of Tb2(L1)3·5H2O, these bands
are shifted downfield by 10 cm−1for ν(C O) and 15 cm−1 for
ν(C C) and 21 cm−1for ν( C–OH) and shifted upfield by 65
cm−1for ν(C N). The ν(N–H), νpyridine ring (C N) and νpyraz ring
(C–N) vibrations of the free ligand (L2) are at 3270, 1582 and
1248 cm−1. In the complex, these bands are shifted downfield
by 66 cm−1 for ν(N–H) and shifted upfield by 77 cm−1 for
νpyridine ring (C N) and 88 cm−1 for νpyraz ring (C–N). All the
shifts indicate that carbonyl oxygen atoms, pyridine nitrogen
atoms and pyrazole nitrogen atoms take part in coordination.
The absorption bands assigned to the coordinated Tb–O and
Tb–N were observed at about 431 and 530 cm−1 for the com-
plexes respectively.
2.4. Synthesis of the complexes
The ligand L1 (3.0 mmol, 1.293 g)/L2 (6.0 mmol, 2.538 g)
and the TbCl3·6H2O (2.0 mmol) were added to the ethanol
(50.0 ml). The mixtures were stirred at 60 ◦C for 24 h at
room temperature. A yellow/white compounds precipitated, the
Tb(III) complexes was separated from the solution by suction
The results of elemental analysis and IR spectroscopy indi-
cate that coordination only involves the enol-forms of the ligand
(L1).
Table 2
Characteristic IR bands (cm−1) of the ligands and their complexes
Compounds
νC–OH
νN–H
ν–OCH3
νC
O
νC
N
νC=C
νC–N
νO–Ar
νTb–O
νTb–N
L1
L2
3422
2845
2835
2841
2833
1616
1606
1570
1582
1635
1659
1533
1250
1268
1255
1278
3270
3204
1248
1336
Tb2(L1)3·5H2O
3401
1518
431
Tb(L2)3·4H2O
530

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L.-X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 321–325
Fig. 1. Thermal behavior of Tb2(L1)3·5H2O.
Fig. 2. The excitation and emission spectrum of the complex of Tb2(L1)3·4H2O.
The excitation and emission slit widths were 2.5 nm in solid state and the drive
voltage was 400.0 V.
3.3. Thermogravimetric analysis
In order to examine the thermal stability of the complexes,
thermal gravimetric (TG) and differential thermal analyses
(DTA) were carried out between 30 to 700 ◦C in the static
atmosphere of air (Fig. 1). The complex of Tb2 (L1)3·5H2O
is stable up to 450 ◦C at which temperature it begins to melt.
The TG curve of Tb2(L1)3·5H2O indicates the release of 5.0
water molecules up to 120 ◦C (observed 5.10%, calc. 5.29%) to
give the product of water-free Tb2(L1)3, which is stable up to
450 ◦C (Fig. 1). The weight loss of 75.21% (calc. 76.05%) from
450 to 650 ◦C is equivalent to the loss of 3 coordinated ligands
(L1). The solid residue formed at around 650 ◦C is suggested
to be Tb2O3. Similarly, the complex of Tb(L2)3·4H2O is stable
up to 460 ◦C at which temperature it begins to decompose. The
TG curve of Tb(L2)3·4H2O indicates the release of ligands(L2)
which takes place at 460 ◦C (observed 83.23%,calc. 84.60%).
The solid residue formed at around 560 ◦C is suggested to be
Tb2O3.
plexes were measured at the drive voltage of 400 V. The emission
spectrums of the solid Tb3+ complexes consist of four main
lines at approximate 493 nm (5D4 → F6), 545 nm (5D4 → F5),
7
7
7
7
584 nm (5D4 → F4), and 621 nm (5D4 → F3) (Fig. 1). Due to
the presence of a scattering signal near 490 nm, the peak height
at 545 nm for terbium was used to measure the fluorescence
intensities. We can see that the emission band (5D4 → F5) is
7
obviously stronger than the other emission bands (5D4 → F6,
7
5D4 → F4, 5D4 → F3) from the Figs. 2 and 3.
7
7
According to the literature [16], we knew that very strong
fluorescence could be observed from the Tb(III) complex. Com-
paring the complexes of Tb2(L1)3·5H2O, Tb(L2)3·4H2O and
the Tb(III) complexes of 1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-
propane-dione [16] under the same experimental conditions,
we can see that the fluorescence intensity observed from
Tb(L2)3·4H2O are the strongest and is twice as strong as
that of Tb2(L1)3·5H2O, and the intensity of Tb2(L1)3·5H2O
is much stronger than that of the Tb(III) complexes of 1,1’-
(2,6-bispyridyl)bis-3-phenyl-1,3-propane-dione. It appears that
the energy-transfer from the organic ligand (L2) to the cen-
tral Tb(III) ions is the most effective compared to the ligand
(L1) and ligand, 1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-propane-
dione that we have reported[16], which could be attributed to
the following factors. Firstly, the ligand L1 has two activating
groups (–OCH3) which donate electrons to the aromatic rings
3.4. Fluorescence studies
The solid complexes have characteristics line emission of f–f
transition of metal ions when they are excited by UV light. The
fluorescence characteristics of the complexes in solid state are
listed in Table 3. The maximum excitation wavelengths (λex) of
the Tb2(L1)3·5H2O and Tb(L2)3·4H2O complexes were 271 and
282 nm, respectively. Fluorescent spectra for the Tb(III) com-
Table 3
Fluorescence data of the complexes at room temperature
Complexes
State
Solid
Slit (nm)
2.5
λex (nm)
λem (nm)
RFIa
Assignment
7
Tb2(L1)3·5H2O
282
493
545
584
623
2245
4774
435
5D4 → F6
7
5D4 → F5
7
5D4 → F4
7
118
5D4 → F3
7
Tb(L2)3·4H2O
Solid
2.5
271
492
547
586
622
1311
9135
261
5D4 → F6
5D4 → F5
7
7
5D4 → F4
7
113
5D4 → F3

L.-X. Xiao et al. / Spectrochimica Acta Part A 71 (2008) 321–325
325
dine ring and the pyrazole ring. The complexes each exhibited
characteristic fluorescence and the fluorescence intensities of
Tb(L2)3·4H2O are twice as strong as those of Tb2(L1)3·5H2O.
The complex with ligand (L2) is an very good green luminescent
material that could be used as a candidate material in organic
light-emitting devices (OLEDs). Based on the results, a series of
bis-pyrazoles can be synthesized and screened to optimize the
luminescent properties of these lanthanide ions complexes.
Acknowledgements
We are grateful to the National Natural Science Foundation
of China and the Science Foundation of Hunan Province.
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Fig. 3. The excitation and emission spectrum of the complex of Tb(L2)3·5H2O.
The excitation and emission slit widths were 2.5 nm in solid state and the drive
voltage was 400.0 V.
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4. Conclusion
We have successfully synthesized a novel bis-␤-diketo-type
and a novel bis-pyrazole ligand and shown that they can form
stable complexes with Tb3+ ion. Differences in the IR spectra of
the free ligands and the metal complexes indicate that coordina-
tion of each of the ligands was occurring at the oxygen atoms
of the carbonyl, enol groups, the nitrogen atoms of the pyri-