Optical properties of a tetradentate bis(β-diketonate) europium(III) complex

Article abstract of DOI:10.1016/j.saa.2007.10.041

Eu₂(BPOPB)₃H₂O, an europium complex chelated with bis(β-diketone), was synthesized. Its properties have been investigated by absorption spectrum, emission spectrum and luminescence lifetime measurement. The complex display

Full text of DOI:10.1016/j.saa.2007.10.041

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Spectrochimica Acta Part A 70 (2008) 1203–1207
Optical properties of a tetradentate bis(␤-diketonate)
europium(III) complex
Biao Chena,b,c,∗, Yanhua Luob, Hao Liangb,d, Jie Xub,e, Fuquan Guof,
Yizhong Zhangc, Aibin Linc, Xuan Liuc
a Department of History of Science and Technology and Archaeometry, University of Science and Technology of China, Hefei, Anhui 230026, PR China
b Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China
c Knowledge Management Institute, University of Science and Technology of China, Hefei, Anhui 230026, PR China
d Department of Chemistry Engineering, Huizhou University, Huizhou, Guangdong 516001, PR China
e Key Lab of Green Processing & Functionalisation of New Textile Materials, Ministry of Education, Wuhan University of Science & Engineering,
Wuhan, Hubei 430073, PR China
f Department of Materials Science and Engineering, Luoyang Institute of Science and Technology, Luoyang, Henan 471023, PR China
Received 5 September 2007; received in revised form 17 October 2007; accepted 31 October 2007
Abstract
Eu2(BPOPB)3H2O, an europium complex chelated with bis(␤-diketone), was synthesized. Its properties have been investigated by absorption
spectrum, emission spectrum and luminescence lifetime measurement. The complex displays strong red luminescence upon irradiation at the
ligand band around 355 nm, which indicates that the bis-␤-diketonate ligand BPOPB is an efficient sensitizer. The Judd–Ofelt parameters obtained
from the emission spectrum of Eu2(BPOPB)3H2O have been used to calculate the total spontaneous emission probabilities (A), the radiative
lifetime (τrad), the fluorescence branching ratio (␤) and the stimulated emission cross-sections (σ). The luminescence lifetimes are determined to
be 402 and 169 ␮s for Eu2(BPOPB)3H2O and Eu(DBM)3(H2O)2, respectively. The relationship between the structures of rare-earth complexes and
luminescence lifetimes was analyzed. The radiative properties reveal that Eu2(BPOPB)3H2O is potential to be an efficient luminescent material.
© 2007 Elsevier B.V. All rights reserved.
Keywords: bis(β-Diketonate) Europium(III) complex; Luminescence; Radiative properties; Judd–Ofelt theory
1. Introduction
ally populated by energy transfer from the triplet state of the
ligand (the sensitizer). One approach to increase the lumi-
nescence output signal is to introduce ligands with multiple
binding sites [5,7]. 1,3-bis(3-phenyl-3-oxopropanoyl)benzene
(BPOPB), a bis-(␤-diketonate) ligand, was chosen for the for-
mation of dinuclear lanthanide complexes. Ligand BPOPB
bears two benzoyl ␤-diketonate sites joined by a 1,3-phenylene
spacer unit. The 1,3-phenylene spacer is ideal for formation
of helicate metal complexes. At the same time, dinuclear lan-
thanide diketonate complexes have received scant attention.
Eu3+ is a well-known rare-earth ion with high red fluores-
In recent years, optical properties of lanthanide ion com-
plexes continue to be an active area of research. Luminescence
of lanthanide ions usually originates from the forbidden char-
acter of their intra-4f electronic transitions, which results
in long lifetimes of the excited states of lanthanide ions
but unfortunately result in low absorption coefficients [1–3].
For this reason, the design of efficient lanthanide complexes
has become an important research goal, working with many
different classes of ligands, such as amidic ligands [4], ␤-
diketones [5,6], etc. In the efficient lanthanide complexes, the
excited state of a luminescent lanthanide(III) ion is gener-
7
cence intensity at 613 nm (5D0 → F2) and has attracted a lot
of attention because of its potential applications in optoelec-
tronics.
It is generally believed that Judd–Ofelt theory can be used to
provide insight into the nature of the chemical bonding presented
in the rare-earth complexes because the intensity parameter ꢀ2
is sensitive to the changes of the chemical properties and the
ligand field around the rare-earth ion centers. At the same time,
∗
Corresponding author at: Department of History of Science and Technology
andArchaeometry, UniversityofScienceandTechnologyofChina, Hefei, Anhui
230026, PR China. Tel.: +86 551 3600803; fax: +86 551 3600803.
E-mail address: chenbiao@ustc.edu.cn (B. Chen).
1386-1425/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2007.10.041

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B. Chen et al. / Spectrochimica Acta Part A 70 (2008) 1203–1207
Judd–Ofelt theory can be used to estimate quantitatively the
optical properties of rare-earth ions [8–10].
Due to these reasons, in this work, Eu2(BPOPB)3H2O, an
europium complex chelated with bis(␤-diketone), was synthe-
sized and its emission spectrum, luminescence lifetime were
measured. The relationship between the structures of rare-
earth complexes and luminescence lifetimes was analyzed. The
radiative properties, such as transition probabilities, emission
cross-section, fluorescence branching ratios and radiative life-
time were calculated and analyzed according to the Judd–Ofelt
analysis based on the emission spectrum.
2. Experimental
The ligand, 3-bis(3-phenyl-3-oxopropanoyl)benzene, was
obtained by the Claisen condensation following a modifica-
tion of the reported preparation [7]. A solution of dimethyl
isophthalate (0.02 mol) and acetophenone (0.01 mol) in freshly
distilled tetrahydrofuran (60 ml) was added to NaH (0.06 mol)
at 0–5 ◦C. The mixture was stirred at this temperature for 2 h and
for a further 2 h at room temperature. The product was recrys-
tallized from a mixture solvent of chloroform and petroleum
ether (1:1). C24H18O4 found: C, 77.7; H, 4.81%, requires C,
77.8; H, 4.86%. Eu2(BPOPB)3H2O was prepared by reacting
BPOPB with EuCl3 (2:3) in ethanol. The mixture was washed
with CHCl3, methanol, H2O and ether and dried under vac-
uum to give the desired product (Found: C, 60.4; H, 3.25%.
C72H48O12Eu2·H2O requires C, 60.6; H, 3.53%).
Fig. 1. Absorption spectrum of 1.0 × 10−5 mol dm−3 Eu2(BPOPB)3H2O in
DMSO.
Elemental analyses were performed using a Perkin-Elmer
2400 CHN elemental analyzer. UV–vis absorption spectrum was
recordedatroomtemperaturewithaShimadzuUV-2401PC. The
emission spectrum was analyzed with a 44 W monochromator
equipped with a Hamamatsu R456 photomultiplier. The lumi-
nescence decay curve was recorded by a Tektronix TDS2024
oscilloscope interfaced with a computer.
3. Results and discussions
Fig. 2. Emission spectrum of Eu2(BPOPB)3H2O.
3.1. Absorption of Eu chelates in DMSO
Fig. 1 shows the absorption spectrum of 1.0 × 10−5
mol dm−3 Eu2(BPOPB)3H2O in DMSO at room tempera-
ture. The spectrum exhibited intense band with λmax = 360 nm
(ε = 1.4 × 105 dm3 mol−1 cm−1) for Eu2(BPOPB)3H2O, which
was attributed to singlet–singlet π → π* of the ligand absorp-
tion. As it is known that the absorption spectrum of BPOPB
exhibited intense bands with λmax = 357 nm, the red shift shows
the formation of the coordinating bond between Eu3+ and
BPOPB [11].
3.2. Emission spectrum of Eu2(BPOPB)3H2O
The emission spectrum of Eu2(BPOPB)3H2O is shown in
Fig. 2.
The emission spectrum was recorded from 570 to 720 nm
under excitation at 355 nm which is close to the maximum
absorption of Eu2(BPOPB)3H2O. As seen from Fig. 3, the
Fig. 3. The most possible energy transfer mechanism for Eu2(BPOPB)3H2O.
ISC: intersystem crossing and ET: intramolecular energy transfer.

B. Chen et al. / Spectrochimica Acta Part A 70 (2008) 1203–1207
1205
Table 1
Radiative properties of Eu2(BPOPB)3H2O
Emission transition
ν (cm−1
)
Aed (s−1
)
Amd (s−1
)
A (s−1
)
β (%)
σ (10−22 cm2)
7
5D0 → F0
17271
16920
16313
15337
14205
0
0
0
0
0
0
5.60
175.30
0
7
5D0 → F1
66.50
66.50
846.63
0
6.88
87.65
0
7
5D0 → F2
846.63
0
52.82
0
0
0
5D0 → F3
7
7
5D0 → F4
52.82
5.47
8.61
AT (s−1
)
965.95
Radiative lifetime τrad (ms) = 1.035.
of Eu3+ ion into Eq. (1), the ꢀt can be determined from the
effective absorption populates the excited singlet S1 from the
ground singlet state S0 in the ligand of BPOPB. Competing
with the fast intersystem crossing (ISC) conversion are organic
fluorescence and non-radiative deactivation of S1. Intramolec-
ular energy transfers (ET) from ligands lead to a population
7
ratios of intensities of 5D0 → F2,4,6 transitions to the intensity
7
of 5D0 → F1 transition as follows:
ꢁ
IJ (ν)dν
AJ
e2 νJ3 (n2 + 2)2
2
ΩtꢃU(t)ꢃ
(3)
5
ꢁ
=
=
of D1 level (lifetime < 4 ␮s [12]) and long-lived metastable
νm3 d
Imd(ν)dν
Amd
Smd
9n2
5D0 level of the Eu3+ ion giving a rise of the Eu3+ emission
7
to the ground multiplet FJ (J = 0–4). Therefore, five emission
Using Eq. (3), the Judd–Ofelt parameters have been deter-
mined and the values of ꢀ2 and ꢀ4 are 19.25 × 10−20 and
2.51 × 10−20 cm2, respectively. The ꢀ6 intensity parameter was
peaks are centered at 581, 595, 613, 652, 703 nm, and can be
7
7
7
7
assigned to the 5D0 → F0, 5D0 → F1, 5D0 → F2, 5D0 → F3,
and 5D0 → F4, respectively.
7
not determined because the 5D0 → F6 transition could not be
7
experimentally detected. This indicates that the ꢀ6 is not impor-
3.3. Judd–Ofelt analysis
tant here.
It is well-known that the 5D0 → F2,4,6 transitions of Eu3+ are
7
allowed by induced electric dipole mechanisms due to selection
rules. According to the Judd–Ofelt theory, the spontaneous emis-
sion probability of an electric dipole transition between initial J
manifold |(S, L) Jꢀ to terminal manifold |(Sꢁ, Lꢁ) Jꢁꢀ is given by:
3.4. Radiative properties of Eu2(BPOPB)3H2O
After the Judd–Ofelt parameters have been determined, the
radiative properties, such as spontaneous transition probability
of electric dipole transition Aed and magnetic dipole transi-
tion Amd, the total transition probability AT, the fluorescence
branching ratio β, the stimulated emission cross-section σ and
the radiative lifetime τrad, can be calculated with the obtained
emission spectrum and are presented in Table 1.
64π4e2ν3 n(n2 + 2)2
Aed = A[(S, L)J; (Sꢁ, Lꢁ)Jꢁ] =
Sed
3h(2J + 1)
9
64π4e2ν3 n(n2 + 2)2
(1)
ꢀ
2
=
Ωt|ꢂ(S, L)JꢃU(t)ꢃ(Sꢁ, Lꢁ)Jꢁꢀ|
3h(2J + 1)
9
t=2,4,6
The luminescence decay curve of Eu2(BPOPB)3H2O moni-
tored at 613 nm, which was shown in Fig. 4, was fitted to a single
where h is the Plank’s constant, m is the mass of the elec-
tron, c is the velocity of light, n is the refractive index of the
medium, ν is the wavenumber of the transition, J is the total
2
angular momentum of the ground state and the ꢃU(t)ꢃ are
the squared reduced matrix elements of the rank t = 2,4,6. The
5
7
squared reduced matrix elements ꢂ D0ꢃU(2)ꢃ F2ꢀ = 0.0032 and
ꢂ D0ꢃU(2)ꢃ F4ꢀ = 0.0023 in Eq. (1) were taken from ref. [13].
5
7
7
It is well-known that the 5D0 → F1 of Eu3+ ion is a magnetic
dipole transition which is independent of the environment and
canbeusedasareference. Thespontaneousemissionprobability
of magnetic dipole transition Amd is [14]:
64π4e2ν3
Amd
=
n3Smd
(2)
3h(2J + 1)
Smd refers to the strength of the magnetic dipole line strength of
7
the 5D0 → F1 transition, which is a constant and independent
of the medium.
Due to selection rules and the unique nature of transition
intensitiesofEu3+ ion, anyoneoftheꢃU(t)ꢃ parametersdecided
2
the intensities of the transition since the remaining two are zero
Fig. 4. The decay curve of (a): Eu2(BPOPB)3H2O and (b): Eu(DBM)3(H2O2)
(solid: experiment decay curve and dotted: fitting decay curve).
[14]. With the help of Eqs. (1) and (2) and inserting the ꢃU(t)ꢃ
2

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B. Chen et al. / Spectrochimica Acta Part A 70 (2008) 1203–1207
exponential of the form:
transition into two components (Fig. 2), though the high-energy
ꢂ
ꢃ
component has not been well-resolved. At the same time, the
␤-diketonates are expected to have covalency in the com-
pounds.
−t
I(t) = I(0) exp
(4)
τ
where I(t) is the intensity at time t after the excitation flash, I(0) is
the initial intensity at t = 0 and τ is the luminescence lifetime. In
our case there is only one type of chromophore BPOPB, thereby
single exponential decay function is used to extract the lifetime,
which is about 402 ␮s.
4.3. Radiative properties of Eu2(BPOPB)3H2O
In our previous study, the lifetime of Eu(DBM)3(H2O)2
was found to be 169 ␮s (Fig. 4). The luminescence life-
time of Eu2(BPOPB)3H2O is much longer than that of
Eu(DBM)3(H2O)2. The energy of the triplet state of BPOPB
and DBM are almost the same, the triplet state differences
are not significant. Therefore, the much longer luminescence
lifetime of Eu2(BPOPB)3H2O is attributed to the effect of
an additional Eu3+ lumophor in the dinuclear complexes,
which enhances its luminescence efficiency. At the same time,
Eu2(BPOPB)3H2O only has one water molecule in the inner
coordination sphere of Eu3+, less than Eu(DBM)3(H2O)2,
which decreases the non-radiative dissipation of energy on
the high-energy O–H vibrations too. The non-radiative decay
As the energy gap between 5D0 and 7FJ levels is very
large (∼12,300 cm−1), non-radiative decay due to multiphonon
relaxation is negligible. The difference between predicted and
experimental lifetimes may be due to Eu3+ ion–Eu3+ ion inter-
action.
The non-radiative decay rates in the present Eu chelate are
estimated using the following expression [14,15]
1
τexp
Wnr
=
− Arad
(5)
5
The non-radiative decay rate for D0 of Eu2(BPOPB)3H2O is
1522 s−1
5
rates can be calculated using Eq. (5), and the values for D0
.
of Eu(DBM)3(H2O)2 and Eu2(BPOPB)3H2O are 6006 and
1522 s−1, respectively. From these results, it is clear that non-
radiative decay rate of Eu2(BPOPB)3H2O is much smaller than
that of Eu(DBM)3(H2O)2.
4. Discussions
4.1. Emission spectrum of Eu2(BPOPB)3H2O
The luminescence lifetime of the transition involved is an
important parameter in consideration of the pumping require-
ment for the threshold of laser action and the luminescence
lifetime of Eu2(BPOPB)3H2O is comparable with those of
europium laser glasses [18]. At the same time, the decay curve
can be fit with a single exponential, which indicates that there is
only one site symmetry from the Eu3+ ion [16].
5
7
It is well-known that D0 → F2 transition of Eu3+ ion is a
hypersensitive one (ꢁJ = 2) and its intensity is very sensitive to
the local environment in which the Eu3+ ion is located. There-
fore, the emission spectrum can provide some information about
the microstructure.
The presence of only one 5D0 → F0 line indicates that the
7
7
From Table 1, the transition 5D0 → F2 showed the highest
Eu3+ ion occupies only a single site and a single chemical
environment exists around it [16]. Compared to those of other
␤ value. It has already established that an emission level with
␤ value near 50% becomes a potential laser emission transi-
transitions, the much stronger intensity of 5D0 → F2 indicates
7
7
tion [19]. The stimulated emission cross-section of 5D0 → F2
that the ligand field surrounding the Eu ion is highly polarizable
and the Eu3+ ion is in a site without a center of inversion [17],
and this will be discussed further in the following sections.
The emission spectrum was recorded under excitation at
355 nm, which matches the corresponding absorption spectrum,
confirming that energy transfer takes place from the ligand to the
Eu(III) ion. Indeed, the energy of the ligand-centered triplet state
is 20,408 cm−1 [7], which is around 3000 cm−1 above the 5D0
level of Eu3+, confirming the suitability of the ligand BPOPB as
a sensitizer for Eu3+.
is 175.3 × 10−22 cm2 which is much higher than Eu3+ doped
tellurite glass and fluorophosphates glass [20]. In addition to
7
that, the luminescence lifetime of 5D0 → F2 is comparable
with those of europium laser glasses. All these showed that
Eu2(BPOPB)3H2Ocanbeconsideredasanefficientluminescent
material.
5. Conclusion
In conclusion, Eu2(BPOPB)3H2O, an europium complex
chelated with bis(␤-diketone), was synthesized. It displays
strong red luminescence upon irradiation at the ligand band
around 355 nm, confirming the suitability of the ligand
BPOPB as a sensitizer for Eu3+. The Judd–Ofelt parameters,
ꢀ2 and ꢀ4 were obtained from the emission spectrum of
Eu2(BPOPB)3H2O. The relationship between the structures of
rare-earth complexes and luminescence lifetimes was analyzed.
The evaluation of the radiative properties of Eu2(BPOPB)3H2O
showed that it has potential to be an efficient luminescent
material.
4.2. Judd–Ofelt analysis
The large value of ꢀ2 indicates the asymmetric environ-
ment of Eu3+ and the presence of covalent bonding between
the Eu3+ ions and the surrounding ligands. The much stronger
hypersensitive transition 5D0 → F2 accounts for such ꢀ2
7
values. These results show, in spite of having three sym-
metric BPOPB ligands, Eu2(BPOPB)3H2O has no center of
symmetry due to one coordinated water. The asymmetry of
Eu2(BPOPB)3H2O is confirmed by the splitting of 5D0 → F1
7

B. Chen et al. / Spectrochimica Acta Part A 70 (2008) 1203–1207
1207
The optical properties of Eu2(BPOPB)3H2O can be
improved, suggesting H2O is substituted by synergetic ligands,
which have lower vibrational frequencies, such as Phen, TOPO,
TPPO, etc. This work is undergoing.
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This work was supported by China Postdoctoral Sci-
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thanks to the referees for critically reviewing the manuscript
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