Synthesis, characterization, photophysical and oxygen-sensing properties of a novel europium(III) complex

Article abstract of DOI:10.1016/j.saa.2010.05.025

In this paper, we report the synthesis, characterization, crystal structure, and photophysical properties of a novel Eu³⁺ complex of Eu(DBM)₃IPD, where DBM = 1,3-diphenyl-propane-1,3-dione and IPD = 4-(1H-imidazo[4,5-f][1,10]phenanth

Full text of DOI:10.1016/j.saa.2010.05.025

Spectrochimica Acta Part A 77 (2010) 292–296
Contents lists available at ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Synthesis, characterization, photophysical and oxygen-sensing properties of a
novel europium(III) complex
Nan Feng , Jing Xie , Dawei Zhangb
a
a,∗
a
Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, PR China
Tsinghua University, Beijing 100084, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this paper, we report the synthesis, characterization, crystal structure, and photophysical prop-
erties of a novel Eu complex of Eu(DBM)3IPD, where DBM = 1,3-diphenyl-propane-1,3-dione and
Received 27 March 2010
Received in revised form 28 April 2010
Accepted 15 May 2010
3+
IPD = 4-(1H-imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline. Its elementary application for
oxygen-sensing application is also investigated by doping it into a silica matrix of MCM-41. Experimen-
tal data suggest that the 20 mg/g doped Eu(DBM)3IPD/MCM-41 system exhibits a high sensitivity of 3.6
Keywords:
2
towards molecular oxygen with a good linear relationship of R = 0.9987. In addition, the 20 mg/g doped
Europium complexes
Crystal structure
Photoluminescence
Oxygen-sensoring
Eu(DBM)3IPD/MCM-41 system owns a quick response of 8 s towards oxygen, along with its excellent
atmosphere insensitivity and photobleaching resistance. All these results suggest that both Eu(DBM)3IPD
and Eu(DBM)3IPD/MCM-41 systems are promising candidates for oxygen-sensing optical sensors.
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
netic filed as well as easy to transmit over a long distance [5,6].
For practical applications in optical oxygen-sensing devices, it is
necessary to embed sensors into a solid matrix acting as a support-
ing medium, allowing oxygen transportation from surroundings.
The commonly used matrixes are silica-based materials, polymers,
and Langmuir–Blodgett films. Indeed, the support may have quite
stringent criteria for suitable performances [7]. For example, a high
diffusion coefficient is necessary for a rapid response, while, a
highly locally effective quenching around the sensor molecule is
necessary for good sensitivity, and long distance on-line monitoring
necessitates a high degree of photostability. Thus, the exploration
for sensors with intense luminescence, excellent antijam ability,
high photostability, and good compatibility with supporting matrix
is still a challenge for analytical chemistry.
It is an important analytical problem to determine the molecular
oxygen concentration in gas phase, liquid phase, or both in differ-
ent branches of chemical and food industries, medicine, analytical
chemistry, and environmental monitoring [1,2]. Traditionally, oxy-
gen concentration measurements are based on the Clark electrode
and the Winkler titration approach which suffer from drawbacks
such as oxygen consumption during sensing process, long response
times and the tendency of electrodes to be poisoned by sam-
ple constituents (e.g., H S, proteins, and certain anesthetics) [3,4].
2
In addition, both techniques necessitate the use of sophisticated
instrumentations and require complicated pretreatment proce-
dures, not suitable for on-line or in-field monitoring. Given these
limitations, the search for new oxygen-sensing systems and their
fabrication methods has attracted considerable interest in the past
few decades.
Sensor technology is much simpler in instrumental implemen-
tation and sample preparation. Owing to the advantages of simple,
rapid, and non-destructive characteristics, many oxygen-sensing
systems have been reported. Among them, the development of
optical oxygen sensors has attracted considerable attention in
recent years, as such sensors can offer advantages in terms of size,
electrical safety, costs, not requiring a reference element, and the
fact that analytical signals is free of influence from electromag-
It seems that rare-earth (RE)-based emitters which are usu-
ally excellent emitters can well satisfy the above requirements.
Due to the unique excitation mechanism of antenna effect and
f–f radiative transition, RE3+ based emitters can generate char-
acteristically sharp and narrow emissions without being affected
by surrounding environment or reagents, offering an excellent
antijam ability. In addition, RE3+ based emitters’ good solubil-
ity in common solvents allow themselves to be easily loaded
into supporting matrixes. All these excellent characters make
3+
RE
based emitters promising candidates for oxygen-sensing
materials. Guided by above results, in this paper, we report
the synthesis, characterization, crystal structure, and photo-
physical properties of a novel Eu3+ complex of Eu(DBM) IPD,
3
∗ Corresponding author. Tel.: +86 22 27401021; fax: +86 22 27401021.
E-mail address: fengnan 1978@yahoo.com.cn (J. Xie).
where DBM = 1,3-diphenyl-propane-1,3-dione and IPD = 4-(1H-
imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline. Its
1
386-1425/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2010.05.025

N. Feng et al. / Spectrochimica Acta Part A 77 (2010) 292–296
293
Scheme 1. A synthetic procedure for IPD diimine ligand, Eu(DBM)3IPD, and Eu(DBM)3IPD/MCM-41 oxygen-sensing systems.
elementary application for oxygen-sensing application is also
investigated by doping it into a silica matrix of MCM-41. Experi-
A typical synthetic procedure for Eu(DBM) IPD/MCM-41 sys-
3
tems is described as follows. Eu(DBM) IPD was dissolved in 5 mL
3
mental data suggest that the 20 mg/g doped Eu(DBM) IPD/MCM-41
of CH Cl2 to form a light yellow transparent solution, then 1 g of
3
2
system exhibits a high sensitivity of 3.6 towards oxygen with a
good linear relationship of R = 0.9987. In addition, the 20 mg/g
MCM-41 was added. The solution was stirred at room temperature
for 5 h and filtered to give a yellowish powder. The crude powder
2
doped Eu(DBM) IPD/MCM-41 system owns a quick response of 8 s
was washed with CH Cl and dried under N atmosphere at room
3
2 2 2
towards oxygen, along with its excellent atmosphere insensitivity
and photobleaching resistance. All these results suggest that both
Eu(DBM) IPD and Eu(DBM) IPD/MCM-41 systems are promising
temperature.
.2. Methods and measurements
Single-crystal data were collected on a Siemens P4 single-crystal
3
3
2
candidates for oxygen-sensing optical sensors.
X-ray diffractometer with a Smart CCD-1000 detector and graphite-
monochromated Mo K␣ radiation, operating at 50 kV and 30 A at
2
. Experimental
2
98 K. Elemental analysis was performed on a Carlo Erba 1106
elemental analyzer. Luminescence lifetimes were obtained with a
55 nm light generated from the third-harmonic-generator pump,
2
.1. Synthesis
3
A synthetic procedure for the diimine ligand of 4-(1H-
using a pulsed Nd:yttrium aluminium garnet (YAG) laser as the
excitation source. The Nd:YAG laser possesses a line width of
imidazo[4,5-f][1,10]phenanthrolin-2-yl)-N,N-diphenylaniline
IPD) and its corresponding Eu3 complex with 1,3-diphenyl-
+
(
−
1
1
1
.0 cm , a pulse duration of 10 ns, and a repetition frequency of
0 Hz. All photoluminescence (PL) spectra were measured with a
propane-1,3-dione (DBM) as the first ligand is shown in
Scheme 1.
Triphenylamine (TPA), 1,10-phenanthroline (Phen), and 1,3-
diphenyl-propane-1,3-dione (DBM) were purchased from Aldrich
Chemical Co. and used without further purification.
-(Diphenylamino)benzaldehyde
phenanthroline-5,6-dione (Phen-O),
synthesized according to the literature procedures [7,8].
F-4500 fluorescence spectrophotometer. In the measurement of
Stern–Volmer plots, oxygen and nitrogen were mixed at different
concentrations via gas flow controls and passed directly into sealed
gas chamber. UV–vis absorption spectra were recorded using a HP
4
(TPA-CHO),
and MCM-41
1,10-
were
1
8
453 UV-Vis-NIR diode array spectrophotometer. H NMR spectra
were obtained with the use of a Varian INOVA 300 spectrometer.
All measurements were carried out in the air at room temperature
without being specified.
IPD was synthesized as follows. A mixture of 2.1 g of Phen-O,
2
.86 g of TPA-CHO, 15.4 g of NH Ac, and 30 mL of HAc was stirred
4
◦
at 90 C for 4 h. Then the mixture was poured into cold water and
extracted with CH Cl . The crude product was further purified by
recrystallization from EtOH. H NMR (300 Hz, CDCl ): ı 9.17 (t, 2H),
3. Results and discussion
2
2
1
3
7
.72 (m, 2H), 7.62 (m, 2H), 7.33–7.14 (m, 14H).
3.1. Crystal structure of Eu(DBM) IPD
3
Eu(DBM) IPD was synthesized according to the classic litera-
3
ture procedure [9]. Anal. Calcd. for C76H54N5O Eu: C, 71.02; H,
Fig.
1
shows the molecular structure obtained from
6
3+
4
.23; N, 5.45. Found: C, 71.16; H, 4.34; N, 5.33. The identity of
Eu(DBM) IPD single crystal. Clearly, the Eu
adopts a square
3
Eu(DBM) IPD was further confirmed by its single-crystal structure
antiprism coordination sphere. Selected geometric parameters
listed in Table 1 suggest that the central Eu3+ is coordinated by
3
(
CCDC 734798).

2
94
N. Feng et al. / Spectrochimica Acta Part A 77 (2010) 292–296
Fig. 1. ORTEP drawing of Eu(DBM)3IPD. All hydrogen atoms are not shown for clarity.
six oxygen atoms from three DBM molecules with Eu–O bond
lengths ranging from 2.347 Å to 2.368 Å and two nitrogen atoms
from one IPD molecule with Eu–N bond lengths ranging from
can use ultraviolet InGaN/GaN LEDs as excitation sources, which
is much helpful to further simplify instrumental implementation.
Considering that the excitation spectrum extends to visible region
of 420 nm, we may choose lower-energy LEDs to depress pho-
tobleaching phenomenon which is defined as the photochemical
destruction of a fluorophore.
2
.572 Å to 2.583 Å. It is found that the Eu–N coordination bond
lengths are obviously longer than the corresponding bond lengths
in a tetrahedral coordination environment (∼2.1 Å), such as in
Cu(I) complexes [10,11]. In addition, the N–Eu–N bite angle in
Eu(DBM) IPD is also much smaller than the corresponding bite
3
3.3. PL properties of Eu(DBM) IPD
3
◦
angle in a tetrahedral coordination environment (∼80 ) [10,11].
The crowded coordination environment around Eu3 center may
further distort the coordination sphere, leading to the above-
mentioned longer Eu–N bond lengths and smaller N–Eu–N bite
angle. In DBM moiety, the average bond lengths of C–C and C–O are
+
Fig. 2 shows the PL spectra of Eu(DBM) IPD in solid state
under pure N2 and O2 atmospheres upon excitation wavelength
3
3+
of 350 nm. No surprise, characteristic lines from Eu center peak-
5
7
5
7
ing at 578 nm for D → F , 590 nm for D → F , and 610 nm
0
0
0
1
1
.348 Å and 1.243 Å, respectively, lying between typical single- and
5
7
for D → F are observed. Even though the emission intensity
0
2
double-bond distances, reflecting the delocalization of ␲-electrons
in the chelate ring system. As for IPD moiety, it is observed
that there is a distortion between Phen and TPA planes, with a
under N2 atmosphere (intensity = 1.00 at 610 nm) is nearly two
times stronger than that under O2 atmosphere (intensity = 0.51 at
6
10 nm), their emission band shapes are exactly the same, along
◦
dihedral angle of 31.06 between Phen and TPA planes, making the
with the identical emission peak, which can be explained as fol-
conjugation between Phen and TPA moieties somewhat inefficient.
3+
lows. Eu emissions originate from f–f radiative transitions, and
the partially filled 4f orbitals are perfectly shielded from environ-
mental influence by the filled 5d and 5p orbitals, leading to the
immunity of emissions to various atmospheres. Since sensor tech-
3
.2. UV–vis and excitation spectra of Eu(DBM) IPD
3
Fig. 2 shows the UV–vis absorption and excitation spec-
tra of Eu(DBM) IPD in CH Cl solution with a concentration of
3
2
2
−
4
1
× 10 mol/L. Eu(DBM) IPD renders a broad absorption band cen-
3
tering at 350 nm which is attributed to ␲ → ␲* transition of DBM
moiety [9]. Eu(DBM) IPD excitation spectrum ranging from 300 nm
3
to 420 nm fits well with its absorption spectrum as shown in
Fig. 2, with a maximum of 352 nm. It is observed that the optimal
excitation window of Eu(DBM) IPD overlaps the emission region
3
of commercially available ultraviolet InGaN/GaN light-emitting
diodes (LEDs), suggesting that Eu(DBM) IPD based optical sensors
3
Table 1
Selected geometric parameters of Eu(DBM)3IPD.
◦
Bond length
(Å)
Bond angle
( C)
Eu–O1
Eu–O2
Eu–O3
Eu–O4
Eu–O5
Eu–O6
Eu–N1
Eu–N2
2.373
2.360
2.362
2.347
2.355
2.347
2.583
2.572
O1–Eu–O2
O3–Eu–O4
O5–Eu–O6
N1–Eu–N2
70.86
71.10
71.61
63.63
Fig. 2. Absorption and excitation spectra recorded at room temperature of
−
4
Eu(DBM)3IPD in CH2Cl2 solution with a concentration of 1 × 10 mol/L, along with
PL spectra of Eu(DBM)3IPD in solid state under pure N2 and O2 atmospheres upon
excitation wavelength of 350 nm. Inset: luminescence decay curves of 5D0 → F2
transition under pure N2 and O2 atmospheres.
7

N. Feng et al. / Spectrochimica Acta Part A 77 (2010) 292–296
295
onal mesostructure [14]. SAXRD curves of the composite materials
shown in Fig. 3 display an identical pattern with that of MCM-
4
1, indicating that the hexagonal arrangement of channels in
MCM-41 remains after the incorporation procedure. The absorp-
tion spectra of Eu(DBM) IPD/MCM-41 systems in CH Cl exhibit
3
2
2
characteristic absorption peaks of Eu(DBM) IPD as shown by the
3
inset of Fig. 3. Combined with the characteristic emission lines of
Eu(DBM) IPD/MCM-41 systems which will be discussed later, it
3
is safe to say that Eu(DBM) IPD is successfully incorporated into
3
MCM-41 matrix.
3.4.2. Sensitivity
The sensitivity (I /I100, where I0 is the luminescence inten-
0
sity under pure N2 atmosphere and I100 is that under pure
O2 atmosphere) values of Eu(DBM) IPD/MCM-41 systems with
3
various loading concentrations are 2.7 for 10 mg/g doped
Eu(DBM) IPD/MCM-41, 3.6 for 20 mg/g doped Eu(DBM) IPD/MCM-
3
3
4
1, and 3.4 for 30 mg/g doped Eu(DBM) IPD/MCM-41, respectively.
3
Fig. 3. SAXRD curves of pure MCM-41 and Eu(DBM)3IPD/MCM-41 systems with
various doping concentrations. Inset: absorption spectra of Eu(DBM)3IPD/MCM-41
systems.
Sensitivity values of Eu(DBM) IPD/MCM-41 systems are found to
be larger than that of pure Eu(DBM) IPD in solid state (1.00/0.51 as
mentioned), suggesting that MCM-41 provides an excellent sup-
3
3
porting matrix for Eu(DBM) IPD molecules. The 20 mg/g doped
3
nology works on the basis of emission intensity measurements at a
certain wavelength, the immunity to various atmospheres of emis-
Eu(DBM) IPD/MCM-41 system exhibits a higher sensitivity than
3
the others, which means that the sensitivity is not simply propor-
tional to the loading concentration. We are giving an explanation as
follows. There may be at least two opposite factors affecting sample
sensitivity: emission intensity from probe molecules and adverse
interaction between probe molecules (aggregation, for example)
sion peak makes Eu(DBM) IPD a promising candidate for oxygen
3
sensors.
Correspondingly, the luminescence decay data shown by the
inset of Fig. 2 confirm that Eu(DBM) IPD emission is sensitive
3
5
7
towards O : the D → F lifetime decreases largely from 590 ␮s
2
0
2
[
7,14]. When loading concentration is low, emission from the probe
under pure N2 atmosphere to 400 ␮s under pure O2 atmosphere.
According to the previous report, there are three potential inter-
molecular energy transfer mechanisms in RE(III) complexes [12].
The one that seems to agree with most of the experimental data
is described as follows. After an efficient intersystem crossing
between the lowest singlet and triplet excited states of the ligand,
energy transfer from triplet excited state of the ligand to a lower-
energy state of the RE(III) ion, leading to the f–f radiative decay of
central RE(III) ion. Even though the intermolecular energy transfer
experiences a ligand’s triplet state which is generally believed to
is weak, leading to a low sensitivity. On the other hand, a much high
loading concentration may accelerate the intermolecular aggrega-
tion which also decreases the sensitivity. The two opposite factors
may achieve a balance in the 20 mg/g doped Eu(DBM) IPD/MCM-41
3
system, showing the highest sensitivity of 3.6.
Despite the long 5D → F lifetime of Eu center (590 ␮s),
7
3+
0
2
the sample’s sensitivity is lower than those of sensors based on
Ru(II) and Cu(I) complexes whose excited lifetimes are shorter
than 100 ␮s [15,7]. We believe the luminescence mechanism dif-
ference between Eu(III)-based and Cu(I)-based emitters should be
responsible for this phenomenon. As for a Eu(III)-based emitter,
the emission originates from metal-centered (MC) f–f transitions.
The emissive center is thus covered by surrounding ligands and
the outer orbitals of Eu(III) ion, which prevents molecular oxygen
from closing in, leading to the low sensitivity. On the other hand,
as for a typical Cu-based emitter, the highest occupied molecular
orbital has a predominant Cu d character, while the lowest unoccu-
pied molecular orbital is essentially ␲* orbital localized on diimine
ligand. The photoluminescence corresponds to the lowest triplet
T1 and is thus assigned as a character of metal-to-ligand-charge-
be vulnerable to O , the energy transfer process is so fast that the
2
triplet state is hardly to be quenched by O [13]. Thus, combined
2
with the largely decreased 5D → F lifetime of Eu center, we
7
3+
0
2
come to a conclusion that O molecules directly quench the excited
2
3
+
state Eu center instead of excited state ligands. The quenching
mechanism is then assigned as a dynamic one, and can be described
as follows:
∗
∗
2
Eu(III) + O → Eu(III) + O
(1)
2
where “*” denotes an excited state.
.4. Oxygen-sensing performances of Eu(DBM) IPD/MCM-41
3
transfer MLCT [d(Cu) → ␲*(diimine ligand)] [10]. The excited state
3
electron localizes on diimine ligand and thus is open for molecular
oxygen attack, leading to a high sensitivity, even though the excited
state lifetime is shorter than that of the Eu(III)-based emitter.
3
systems
Oxygen-sensing performances of Eu(DBM) IPD are initially
3
tested by physically incorporating it into mesoporous silica MCM-
4
1 (referred as Eu(DBM) IPD/MCM-41) with various loading
3.4.3. Stern–Volmer plots
3
concentrations of 10 mg/g, 20 mg/g, 30 mg/g, respectively. Their
oxygen-sensing performances are discussed on the basis of lumi-
nescence intensity quenching.
The emission spectra of 20 mg/g doped Eu(DBM) IPD/MCM-
3
41 system under various oxygen concentrations from 0% to 100%
are presented in Fig. 4. With increasing oxygen concentrations,
the luminescence intensity at 610 nm decreases smoothly with no
affection on emission band or peak. It has been reported that the
quenching behavior of a probe is affected by the microstructure of
matrix and micro-environment in which the probe is located. In a
homogeneous media with a single-exponential decay, the inten-
sity form of Stern–Volmer equations with dynamic quenching is
3.4.1. Confirmation of sensor systems
As shown in Fig. 3, powder small angle X-ray diffraction
(
SAXRD) measurements reveal that blank MCM-41 owns three
well-resolved broad Bragg reflections that can be indexed as d1 0 0
d1 1 0, and d2 0 0, which is the characteristic of a well-ordered hexag-
,

2
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N. Feng et al. / Spectrochimica Acta Part A 77 (2010) 292–296
emission intensity when changed from 100% N2 atmosphere to
00% O2 atmosphere, and 95% recovery time as the time taken
when changed from 100% O atmosphere to 100% N atmosphere.
1
2
2
Correspondingly, 20 mg/g doped Eu(DBM) IPD/MCM-41 system
3
renders a response time of only 8 s and a recovery time of 15 s.
The quick response towards O2 and N2 suggests that 20 mg/g
doped Eu(DBM) IPD/MCM-41 system is highly sensitive towards
3
O . In addition, it is found that the recovery time is obviously
2
longer than the response time, which can be explained by the
diffusion-controlled dynamic response and recovery behavior of
a hyperbolic-type sensor reported by Mills and Lepre [17]. 20 mg/g
doped Eu(DBM) IPD/MCM-41 system exhibits reversible time
3
response plots, with slight intensity decrease which is called photo-
bleaching. For future studies, we are hoping to further improve the
photostability and eliminate photobleaching by covalently grafting
the probe into silica matrix.
Fig. 4. Emission spectra recorded at room temperature of 20 mg/g doped
Eu(DBM)3IPD/MCM-41 system under various oxygen concentrations from 0% to
4. Conclusion
1
00% with an interval of 10%.
In this paper, we report the synthesis, characterization, crys-
tal structure, and photophysical properties of Eu(DBM) IPD. Its
3
elementary application for oxygen-sensing application is also
investigated by doping it into a silica matrix of MCM-41. Experi-
mental data suggest that the 20 mg/g doped Eu(DBM) IPD/MCM-41
3
system exhibits a high sensitivity of 3.6 towards oxygen with a
2
good linear relationship of R = 0.9987. In addition, the 20 mg/g
doped Eu(DBM) IPD/MCM-41 system owns a quick response of 8 s
3
towards oxygen, along with its excellent atmosphere insensitivity
and photobleaching resistance. All these results suggest that both
Eu(DBM) IPD and Eu(DBM) IPD/MCM-41 systems are promising
3
3
candidates for oxygen-sensing optical sensors.
Acknowledgment
The research has been partly supported by National Natural Sci-
ence Foundation of China, under the Grant No. 70901054.
Fig. 5. Stern–Volmer plots of 20 mg/g doped Eu(DBM)3IPD/MCM-41 system at
various oxygen concentrations. Inset: PL intensity response of 20 mg/g doped
Eu(DBM)3IPD/MCM-41 system when exposed to periodically varied 100% N2 and
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.saa.2010.05.025.
1
00% O2 atmospheres.
described as follows [16]:
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I0
=
1 + KSV [O2]
(2)
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[
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in the absence of quencher, KSV is the Stern–Volmer constant, and
[
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2
[
3
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3.4.4. Response time and photostability
[
[
The inset of Fig. 5 demonstrates a PL intensity response of
2
0 mg/g doped Eu(DBM) IPD/MCM-41 system when exposed to
3
periodically varied 100% N2 and 100% O2 atmospheres. Here,
we define 95% response time as the time taken for 20 mg/g
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doped Eu(DBM) IPD/MCM-41 system to lose 95% of its initial
3