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

Article abstract of DOI:10.1016/j.saa.2007.04.029

A novel bis-β-diketon ligand, 1,1′-(2,6-bispyridyl)bis-3-phenyl-1,3-propane-dione (L), was designed and synthesized and its complexes with Eu(III), Tb(III), Sm(III) and Gd(III) ions were successfully prepared. The ligand and the corresponding metal comple

Full text of DOI:10.1016/j.saa.2007.04.029

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Spectrochimica Acta Part A 69 (2008) 513–516
Investigations into the synthesis and fluorescence properties
of Eu(III), Tb(III), Sm(III) and Gd(III) complexes of
a novel bis-β-diketone-type ligand
Yi-Ming Luo∗, Zhe Chen, Rui-Ren Tang, Lin-Xiang Xiao, Hong-Jian Peng
School of Chemistry and Chemical Engineering, Central South University, Hunan 410083, PR China
Received 21 January 2007; received in revised form 29 March 2007; accepted 6 April 2007
Abstract
A novel bis-β-diketon ligand, 1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-propane-dione (L), was designed and synthesized and its complexes with
Eu(III), Tb(III), Sm(III) and Gd(III) ions were successfully prepared. The ligand and the corresponding metal complexes were characterized by
elemental analysis, and infrared, mass and proton nuclear magnetic resonance spectroscopy. Analysis of the IR spectra suggested that each of
the lanthanide metal ions coordinated to the ligand via the carbonyl oxygen atoms and the nitrogen atom of the pyridine ring. The fluorescence
properties of these complexes in solid state were investigated and it was discovered that all of the lanthanide ions could be sensitized by the ligand
(L) to some extent. In particular, the Tb(III) complex was an excellent green-emitter and would be a potential candidate material for applications
in organic light-emitting devices (OLEDs) and medical diagnosis.
© 2007 Elsevier B.V. All rights reserved.
Keywords: 1,1ꢀ-(2,6-Bispyridyl)bis-3-phenyl-1,3-propanedione; Bis-β-diketone; Synthesis fluorescence of lanthanide complexes; IR spectra
1. Introduction
The complexes of Eu(III), Tb(III), Sm(III) and Gd(III) ions
with ␲-conjugated ligands such as β-diketones have also been
examined as emitting materials for OLEDs [9–12] following
the reports by Okamoto and his coworkers [13]. However, in
most cases the quantum efficiency of most of these complexes
is still low, possibly due to inefficiency of triplet–triplet energy
transfer in these complexes. To combat this difficulty, chemists
have sought to design ligands that allow for improved energy
transfer to the lanthanide metal ion.
In the present work, we have designed and synthesized
a novel, pyridine-containing, bis-β-diketone-type ligand, 1,1ꢀ-
(2,6-bispyridyl)bis-3-phenyl-1,3-propanedione (Scheme 1), and
studied the fluorescence properties of its Eu(III), Tb(III), Sm(III)
and Gd(III) complexes in the solid state.
Since a brightly photoluminescent europium complex was
first reported by Weissman in 1942 [1], the properties of
lanthanide organic complexes have been extensively studied.
Complexes of lanthanide ions with organic ligands have been
doped in polymers for optical amplification [2,3], and lumines-
cent lanthanide complexes have been used in medical diagnosis
where they are used to detect small amounts of biomolecules
that reveal information about the physical state of a patient
[4,5]. More recently, organic electroluminescence (OEL) has
been investigated at length for applications because of its low
drive voltage, suitability for integrated circuit and potential
application for large flat panel display [6,7]. Several lanthanide
complexes have been utilized in the fabrication of OEL devices
(OLEDs), and significant progress has been made in improving
the brightness and electroluminescence efficiencies of OELDs
with Eu(III), Tb(III), Sm(III)and Gd(III) organic complexes as
luminescent centers [8].
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 [14,15].
∗
Corresponding author. 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.2007.04.029

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Scheme 1. Synthesis of the ligand (L).
Physical properties of 1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-
propanedione(L):The 1H-NMRspectrumshowsthepresenceof
several keto-enol tautomers. In this paper, we describe only the
major tautomer, the bis(keto-enol) system, which formed yel-
low microcrystals from methanol, m.p. 176–178 ◦C; IR (KBr):
νmax(cm−1); 3422 (C C OH), 2924 (CH2), 1616 (C O), 1560
(C N); 1H-NMR (400 MHz, CDCl3): ␦16.5 (d, 2H, OH),
8.04–8.32(m, 3H, C5H3N), 7.25–7.75(m, 10H, C6H5); MS, m/z:
371(M+), 294, 266, 252, 224, 105, 77. Elemental analytical
(calc.) C% 74.26 (74.38), H% 4.78 (4.61), N% 3.90 (3.77).
2.1.1. Methods
Contents of carbon, hydrogen and nitrogen were determined
using an Elementar vario EL elemental analyzer. Content of
Ln(III) was determined by EDTA titration. Infrared spectra
(4000–400 cm−1) were recorded with samples as KBr discs
using a Nicolet NEXUS 670 FTIR spectrophotometer. 1H-NMR
spectra were measured using a Bruker-400 MHz nuclear mag-
netic resonance spectrometer with CDCl3 as solvent and TMS
as internal reference. Mass spectra were measured using a ZAB-
HS analyzer. Fluorescence measurements were made on a Hitich
F-4500 spectrometer.
2.3. Synthesis of the complexes
2.2. Synthesis of the ligand (L)
A solution of LnCl3·6H2O (Ln = Eu, Tb, Sm, Gd) (0.2 mmol)
in ethanol (5 ml) was added dropwise to a solution of the ligand
(0.6 mmol) in ethanol (15 ml) and the mixture stirred at 60 ◦C for
3 h. The resulting precipitate was collected by filtration, washed
three times each with ethanol and chloroform and dried in vacuo
to give a flaky solid (typically about 80% yield).
2.2.1. Synthesis of
1,1ꢀ-(2,6-bispyridyl)bis-3-phenyl-1,3-propanedione (L)
To a suspension of freshly cut sodium (1.0 g, 44 mmol) and
diethyl 2,6-pyridinedicarboxylate (4.90 g, 22 mmol) in dry ben-
zene (50 ml) was placed in a three-necked, round-bottom flask,
fitted with a condenser. The mixture in the flask was stirred,
while a solution of acetophenone (5.28 g, 44 mmol) in benzene
(25 ml) was added dropwise. The reaction mixture was incu-
bated and stirred at room temperature for about 30 min until the
yellow sodium salt had precipitated. The resulting suspension
was heated to 45 ◦C and incubated for about 2 h. The sodium salt
was collected by filtration, washed thoroughly with petroleum
ether and dried. The dried solid was added to dilute hydrochloric
acid (2 M, 50 ml) and the resulting precipitate was collected by
filtration. The crude products were recrystallized from methanol
to give the ligand (L) (5.89 g, 72%).
3. Results and discussion
3.1. Properties of the complexes
The results of elemental analysis (see Table 1) indicated that
the composition of the complexes conforms to Eu2L3·6H2O,
Tb2L3·4H2O, Sm2L3·6H2O and Gd2L3·8H2O, respectively. All
complexes were found to be soluble in H2O, DMF, DMSO,
slightly soluble in ethanol and acetone, and insoluble in benzene,
diethyl ether and tetrahydrofuran.

Y.-M. Luo et al. / Spectrochimica Acta Part A 69 (2008) 513–516
515
Table 1
Elemental analytical data for the complexes
Complex
M (%) found (calc.)
C (%) found (calc.)
H (%) found (calc.)
N (%) found (calc.)
Eu2L3·6H2O
Tb2L3·4H2O
Sm2L3·6H2O
Gd2L3·8H2O
19.23 (19.92)
21.15 (21.13)
19.86 (19.75)
20.05 (20.00)
54.08 (54.30)
55.60 (55.10)
54.51 (54.41)
53.00 (52.70)
2.83 (2.75)
2.86 (2.79)
4.36 (4.17)
4.21 (4.30)
4.08 (4.16)
4.02 (3.95)
2.87 (2.76)
2.89 (2.67)
Table 2
Characteristic IR bands (cm−1) of the ligand and its complexes
Compounds
C
OH
C
O
C
N
Eu
N
Eu O
L
3422
3401
3396
3376
3393
1616
1608
1611
1610
1611
1560
1572
1574
1571
1573
Eu2L3·6H2O
Tb2L3·4H2O
Sm2L3·6H2O
Gd2L3·8H2O
526
529
525
525
431
436
432
434
3.2. IR spectra
The IR spectra of all four complexes are similar, indicating
that they are structurally alike. Table 2 summarises the charac-
teristic bands observed for the ligand and its metal complexes.
The IR spectrum of the free ligand shows bands at 3422, 1616
and 1560 cm−1, which can be assigned as (C OH), ν(C O) and
ν(C N)oftheligand, respectively. Inthecomplexes, thesebands
are shifted upfield by 21–46 cm−1 for ν(C OH) and 5–8 cm−1
for ν(C O) and downfield by 11–14 cm−1 for ν(C N). In each
case, the shifts suggested that the relevant oxygen and nitro-
gen atoms of the ligand were involved in coordination to the
metal centre. The absorption bands assigned to the Ln O and
Ln N stretching frequencies of the complex were observed at
431–436 cm−1 and 525–529 cm−1, respectively. The results of
elemental analysis and IR spectroscopy indicate that coordina-
tion only involves the enol-forms of the ligand.
Fig. 1. The excitation and emission spectrum of the Tb(III)complex. The exci-
tation and emission slit widths were 2.5 nm in solid state and the drive voltage
was 400 V.
(λex) of the Eu(III), Tb(III), Sm(III) and Gd(III) complexes were
276 281, 272 and 274 nm, respectively (Table 3). Fluorescent
spectra for the Eu(III), Sm(III) and Gd(III) complexes were mea-
sured at two different drive voltages, 400 V (Fig. 1) and 700 V
(Figs. 2–4). At 400 V, the fluorescent intensity observed from
Eu(III), Sm(III) and Gd(III) complexes were very weak (spectra
not shown), while very strong fluorescence was observed from
the Tb(III) complex (Fig. 1) under the same experimental condi-
3.3. Fluorescence studies
The fluorescence data for each of the complexes in the solid
state is listed in Table 3. The maximum excitation wavelengths
Table 3
Fluorescence data for the complexes
Complexes
λex (nm)
λem (nm)
RFI
746
Assignment
Eu2L3·6H2O
276
593
616
5D0-7F1
5D0-7F2
2616
Tb2L3·4H2O
Sm2L3·6H2O
281
272
493
545
584
2208
3896
385
5D4-7F6
5D4-7F5
5D4-7F4
487
532
545
584
648
2985
439
3780
549
44
4I9/2
4F3/2
4G5/2
4G5/2
4G5/2
Gd2L3·8H2O
274
493
545
586
1238
939
214
4I9/2
4G5/2
4G5/2
Fig. 2. The excitation and emission spectrum of the Eu(III) complex. The exci-
tation and emission slit widths were 2.5 nm in solid state and the drive voltage
was 700 V.

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Y.-M. Luo et al. / Spectrochimica Acta Part A 69 (2008) 513–516
cence intensity is the energy gap between the two energy states
involved. Since the fluorescence intensity of the Tb(III) complex
at 545 nm was much stronger than that of the Eu(III), Sm(III)
and Gd(III) complexes, it can be inferred that the energy differ-
ence between the ligand triplet states and the emitting energy
state of Tb(III) is more favorable for energy transfer than those
of the other three rare earth ions.
4. Conclusion
We have successfully synthesized a novel bis-β-diketo-type
ligand and shown that it can form stable complexes with Eu(III),
Tb(III), Sm(III) and Gd(III) ions. Differences in the IR spectra of
the free ligand and the metal complexes indicated that coordina-
tion of each of the four rare earth ions to the ligand was occurring
at the oxygen atoms of the carbonyl and enol groups and the
nitrogen atom of the pyridine ring. The complexes each exhib-
ited characteristic fluorescence, with the Tb(III) complex being
an excellent green luminescent material that could be considered
as a candidate material for applications in organic light-emitting
devices (OLEDs) and medical diagnosis. Based on our results,
a series of novel bis-β-diketo-type ligands could be synthesized
and screened to optimize the luminescent properties of these
lanthanide ions complexes.
Fig. 3. The excitation and emission spectrum of the Sm(III) complex. The exci-
tation and emission slit widths were 2.5 nm in solid state and the drive voltage
was 700 V.
Acknowledgement
This work is supported by Science Foundation of Hunan
Province.
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Fig. 4. The excitation and emission spectrum of the Gd(III) complex. The exci-
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