Characterization and spectroscopic studies of Eu3+ complexes with 3-phenyl-2,4-pentanedione

Article abstract of DOI:10.1016/j.jallcom.2003.11.079

Synthesis, characterization, and photoluminescent properties of the Eu(ppa)₃·2H₂O and Eu(ppa)₃·phen (ppa = 3-phenyl-2,4-pentanedione and phen = 1,10-phenanthroline) solid complexes are reported. The characterization was carried out by IR spectroscopy, thermogravimetric, and elementary analyses (C, H, N). The photophysical data of Eu(ppa)₃·phen reveal a highly luminescent material where the characteristic Eu³⁺ transitions are observed in the emission spectrum (λ_exc=350nm). Based on the luminescence spectra of the Eu(III) compounds, the Ω_λ experimental intensity parameters (λ=2 and 4) were calculated for the electronic transitions ⁵D₀→⁷F_2,4. The Ω₂ of Eu(ppa)₃·phen is higher than for the Eu(ppa)₃·2H₂O complex. The mono exponential decay curves reveal only one type of site for the Eu³⁺ in both complexes.

Full text of DOI:10.1016/j.jallcom.2003.11.079

Journal of Alloys and Compounds 374 (2004) 151–153
Characterization and spectroscopic studies of Eu3+ complexes
with 3-phenyl-2,4-pentanedione
Anderson Orzari Ribeiro, Paulo Sérgio Calefi, Ana Maria Pires, Osvaldo Antonio Serra∗
Depto. Qu´ımica, FFCLRP, USP, Av. Bandeirantes 3900, 14040-901, Ribeirão Preto, SP, Brazil
Abstract
Synthesis, characterization, and photoluminescent properties of the Eu(ppa)3·2H2O and Eu(ppa)3·phen (ppa = 3-phenyl-2,4-pentanedione
and phen = 1,10-phenanthroline) solid complexes are reported. The characterization was carried out by IR spectroscopy, thermogravimetric,
and elementary analyses (C, H, N). The photophysical data of Eu(ppa)3·phen reveal a highly luminescent material where the characteristic
Eu3+ transitions are observed in the emission spectrum (λexc = 350 nm). Based on the luminescence spectra of the Eu(III) compounds,
5
7
the Ω experimental intensity parameters (λ = 2 and 4) were calculated for the electronic transitions D0 → F2,4. The Ω2 of Eu(ppa)3·
␭
phen is higher than for the Eu(ppa)3·2H2O complex. The mono exponential decay curves reveal only one type of site for the Eu3+ in both
complexes.
© 2003 Elsevier B.V. All rights reserved.
Keywords: Europium(III); Luminescence; Complex; Pentanedione
Tb3+ ␤-diketonates have been studied with respect to the
substituent effects on intramolecular energy transfer, where
a theoretical study can be made to elucidate the parameters
that govern this energy transfer mechanism [7–9].
This work reports on the synthesis, characterization, and
photophysical properties of Eu(ppa)3·2H2O and Eu(ppa)3·
phen solid complexes (ppa = 3-phenyl-2,4-pentanedione
and phen = 1,10-phenanthroline). The ligand ppa is the
only ␤-diketone with a phenyl group attached to the cen-
tral carbon of the coordination ring (Fig. 1) and also is an
efficient antenna for the transfer of the absorbed energy
through the enolate-conjugated system [10].
1. Introduction
Compounds containing rare earth ions have been used
as phosphors, fluorescence sensors, and laser materials due
to their sharp intensely luminescent f–f electronic transi-
tions [1]. However, they are forbidden transitions, so the
rare earth ions absorb very little of the excitation energy.
On the other hand, rare earth organic complexes exhibit in-
tense narrow emission band via an effective intramolecular
energy transfer from the ligand to the metal ion, under exci-
tation by near ultraviolet light, which is called the antenna
effect [2,3]. The ␤-diketones have been shown as excellent
organic molecules to transfer energy for rare earth ions [4,5].
The efficiency of this energy transfer depends on the effi-
ciency of the organic ligand absorption, the ligand-to-metal
energy transfer and the rare earth luminescence. The energy
gap between the excited and ground state levels of the lan-
thanide and the rigidity of molecular structure contributes
to the enhancement of the luminescent intensity of the sys-
tem [6]. The 4f–4f luminescence intensity depends on the
balance between radiative and non-radiative process in the
compound. The luminescence properties of many Eu3+ and
2. Experimental section
2.1. Preparation of Eu(ppa)3·2H2O and Eu(ppa)3·phen
The Eu(ppa)3·2H2O compound was synthesized by the
addition of EuCl3 aqueous solution (from Aldrich oxide)
to an aqueous/methanolic solution of ppa, pH ∼ 7.0, un-
der stirring overnight. For the synthesis of ternary complex
Eu(ppa)3·phen, 1,10-phenanthroline was added to a solu-
tion by using the same procedures. The solid complexes
were filtered and dried at room temperature under reduced
pressure.
∗
Corresponding author. Tel.: +55-16-602-4376;
fax: +55-16-633-8151.
E-mail address: osaserra@usp.br (O.A. Serra).
0925-8388/$ – see front matter © 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2003.11.079

152
A.O. Ribeiro et al. / Journal of Alloys and Compounds 374 (2004) 151–153
O
O
Eu(ppa)3.2H2O
Eu(ppa)3.phen
Fig. 1. Molecular structure of ligand ppa (3-phenyl-2,4-pentanedione).
2.2. Apparatus
250
300
350
Wavelength / nm
400
450
Infrared spectra were recorded (Perkin Elmer FTIR
1600) from 4000 to 400 cm−1 in KBr pellets. TGA were
carried out (Thermal analyst 2100-TA Instruments STD
2690-Simultaneous DTA–TGA) in air with a heating rate
of 10 ◦C/min from 25 to 1000 ◦C. Luminescence data were
obtained at room temperature (∼300 K) and in N2 liquid
(77 K), using a spectrofluorimeter SPEX Fluorolog III Triax
550. Elemental analyses (C, H, N) were performed by Carlo
Erba CE Instruments, EA 1110.
Fig. 3. Excitation (λem = 610 nm) spectra of Eu(ppa)3·2H2O and
Eu(ppa)3·phen, at room temperature.
phen-free ligand IR spectrum and it is shifted to 848 cm−1
in Eu(ppa)3·phen complex, because the nitrogen atoms must
coordinate with Eu3+, indicating the presence of phen in
the complex.
3.2. Luminescence spectroscopy
Fig. 3 shows the excitation spectra of Eu(ppa)3·2H2O
3. Results
5
7
and Eu(ppa)3·phen monitoring the D0 → F2 transition at
610 nm. The maximum observed is the same for both the
complexes i.e. 350 nm. However, this energy position of the
maximum excitation is different from the maximum absorp-
tion of ppa-free ligand (298 nm in methanol), indicating that
the ppa takes part of coordination sphere in the Eu3+ com-
plexes synthesized.
3.1. Characterization of the ppa-complexes
The Eu3+ percentages in complexes were determined
by thermogravimetric analysis. The analytical data of
C, H, N and Eu3+ percentages (found/calculated) for
Eu(ppa)3·2H2O are C: 55.3/55.44, H: 5.0/5.20, Eu:
21.7/21.32 and for Eu(ppa)3·phen are C: 62.4/63.00, H:
5.1/4.80, and N: 3.3/3.26, Eu: 17.5/17.73. These results
confirm the molar ratio 1:3 (Eu:ppa) for both complexes, as
well as two water and one 1,10-phenanthroline molecules
in order to complete the coordination sphere.
The emission spectra of the complexes under 350 nm ex-
citation at N2 liquid temperature, 77 K, are shown in Fig. 4.
The characteristics set of transitions from 5D0 → FJ (J =
7
0-0
Eu(ppa) .2H O
The IR spectra (Fig. 2) show the displacement of carbonyl
3
2
0-2
Eu(ppa) .phen
−1
3
=
stretching frequency (C O) from 1600 cm
(ppa-free
ligand) to 1584 cm−1 in both complexes providing good
evidence that the metal ion is coordinated through the oxy-
gen atoms. The vibrational mode at 864 cm−1, assigned
to C–H out-of-plane deformation [11], is detected in the
0
570
573
576
579
582
Wavelength / nm
0-0
0-4
0-3
C=O stretching
Eu(ppa)3.phen
1607
ppa
1584
Eu(ppa) .2H 0
3
2
Eu(ppa) .phen
3
Eu(ppa)3.2H2O
C-H out-of-plane
deformation
550
600
650
700
750
2000
1800
1600
1400
1200
1000
800
600
Wavelength / nm
Wavenumber (cm-1)
Fig. 4. Emission (λex = 350 nm) spectra of Eu(ppa)3·2H2O and Eu
Fig. 2. Infrared spectra of ppa ligand, Eu(ppa)3·2H2O and Eu(ppa)3·phen,
(ppa)3·phen at 77 K. The upper right inset shows 5D0
→
7F0 Eu3+
in KBr pellets.
transition in detail for both the complexes.

A.O. Ribeiro et al. / Journal of Alloys and Compounds 374 (2004) 151–153
153
Table 1
4. Conclusions
Experimental intensity parameters Ω2 and Ω4 for Eu(ppa)3·2H2O and
Eu(ppa)3·phen complexes
The present study reports a new complex of Eu3+ with
a ␤-diketone with a phenyl group attached to the center of
the coordination ring, which represents an efficient antenna
molecule for the transfer of the absorbed energy to rare earth
ion. The ternary complex Eu(ppa)3·phen synthesized present
a strong luminescence, with the characteristic very sharp
Compound
Ω2 (10−20 cm2)
Ω4 (10−20 cm2)
Eu(ppa)3·2H2O
Eu(ppa)3·phen
23.7
24.6
8.9
11.4
5
7
bands of the transitions D0 → FJ (J = 0–4) of Eu3+
0–4) level of Eu3+ at 578, 591, 610, 651, and 702 nm are ob-
served for both complexes. The number of Stark levels [12]
5
7
(bandwidths of D0 → F2 at 610.6 nm = 15 cm−1 and
7
5D0 → F0 at 578.8 nm = 12 cm−1), becoming a promising
5
7
corresponding to D0 → F0,1,2 set of transitions observed
in the spectra are 1, 3, and 4 for hydrated complex and 1, 3,
and 5 for phen ternary complex, respectively. The different
number of Stark levels for Eu(ppa)3·phen is a result of the
change in the coordination site symmetry and an increase
in the rigidity of the complex, attributed by the presence
of the 1,10-phenanthroline molecule. A single symmetrical
candidate as luminescent material for photoluminescence
applications.
Acknowledgements
peak is observed for the 5D0 → F0 transition in both com-
7
CAPES, CNPq, FAPESP are gratefully acknowledged.
plexes, assuring that only one Eu3+ species is present in the
samples.
References
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5D0 to the ground state F2 of Eu3+ in the complexes con-
7
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sist in mono exponential decays with lifetimes values of
0.38 ms for Eu(ppa)3·2H2O and 0.51 ms for Eu(ppa)3·phen
( 0.01 ms). The mono exponential decay curves, in agree-
5
7
ment with the single peak observed for D0 → F0 transi-
tion, also indicate that Eu3+ occupies only one site in both
complexes.
Based on luminescence spectra of the Eu3+ compounds,
Ω experimental intensity parameters (λ = 2 and 4) were
␭
calculated (Table 1) [13]. The Ω2 and Ω4 values for
Eu(ppa)3·phen are higher than the ones for Eu(ppa)3·2H2O
complex. Therefore, the substitution of water molecules
for phen increases the polarizability of Eu3+ chemical en-
vironment, also increasing the hypersensitive behavior of
the 0 → 2 transition. Moreover, the area values related to
each set of transitions for the different complexes shows
that Eu(ppa)3·phen emission is more intense than that of
Eu(ppa)3·2H2O. The phen ligand displaces the two wa-
ter molecules of the Eu3+ coordination sphere, which are
strong luminescence quenchers, explaining the increasing
of luminescence.