luminescent organic–inorganic hybrid materials based
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O R I G I N A L P A P E R
Luminescent organicinorganic hybrid materials basedon lanthanide containing ionic liquids and sylilated b-diketone
Yige Wang Yu Feng Hongsheng Zhao
Quanying Gan Xiaoyan Yu
Received: 4 January 2011 / Accepted: 19 March 2011 / Published online: 2 April 2011
Springer Science+Business Media, LLC 2011
Abstract In this work, we report the luminescent
organicinorganic hybrid materials prepared by hydrolysisand condensation of sylilated b-diketone under acid con-
ditions in the presence of carboxyl-functionalized ionic
liquid in which Eu3? ions are coordinated to the oxygen
atoms of carboxylate groups from the ionic liquids. The
obtained materials were characterized with FT-IR, TG and
photoluminescence spectroscopy. FT-IR spectra imply that
Eu3? ions are still coordinated to the ionic liquid in the
hybrid materials. Excitation and emission spectra demon-
strate that the energy transfer occurs from the b-diketone
molecules covalently bonded with silica to Eu3? ions. The
Eu3? (5D0) quantum efficiency value of the hybrid mate-
rials has been estimated based on the emission spectrum
and the value of lifetime. A large value of ratio (16.44)
between the intensities of the 5D0?7F2 and
5D0?7F1
transition and high value of 5D0 quantum efficiency
(51.01%) are obtained.
Keywords Ionic liquids Lanthanide Solgel Photoluminescence
1 Introduction
The trivalent lanthanide ions display photoluminescence
properties that are favorable for optical applications such as
fiber amplifiers and solid-state lasers [13]. However, the
luminescence intensity of lanthanide ions is limited by its
poor light-absorbing ability due to the Laporte forbidden
character and intraconfigurational nature of the 4f transi-
tions. Organic ligands with large molar absorption coeffi-
cients are normally used to coordinate to lanthanide ions,
resulting in sensitized emission via the so called antenna
effect [4, 5]. These ligands can also protect the lanthanide
ions from molecules with high-energy vibrations such as
water molecules that can quench lanthanide ion lumines-
cence by radiationless deactivation. Some of the best
ligands for these purpose are b-diketonates having aro-
matic and fluorine substituents in terms of high harvest
emissions due to the effectiveness of the energy transfer
from the ligand to the Ln3? cations [6, 7]. However, in
spite of the interesting luminescence features, lanthanide
complexes have been excluded from practical applications
as tunable solid-state lasers or phosphor devices due to
their poor thermal stability and mechanical properties.
One acceptable solution to this problem is to immobilize
lanthanide complexes in solgel derived silica, resulting in
luminescent organicinorganic hybrid materials [814].
The interest in light-emitting lanthanide based organic
inorganic hybrid materials has grown considerably during
the last decade as they can find potential applications in
tunable lasers, amplifiers for optical communications,
emitter layers in multilayer light emitting diodes, efficient
light conversion molecular devices, UV dosimeters, and
light concentrators for photovoltaic devices [8]. Various
lanthanide complexes have been immobilized in the silica
matrices via the SiC covalent bond through the hydrolysis
Electronic supplementary material The online version of thisarticle (doi:10.1007/s10971-011-2450-7 ) contains supplementary
material, which is available to authorized users.
Y. Wang (&) Q. Gan X. YuSchool of Chemical Engineering and Technology,
Hebei University of Technology, 300130 Tianjin, China
e-mail: [email protected]
Y. Feng
The College of Environmental Science and Engineering,
Nankai University, 30071 Tianjin, China
H. Zhao
Chaoyang Health School, Chaoyang, Liaoning 12200, China
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J Sol-Gel Sci Technol (2011) 58:711715
DOI 10.1007/s10971-011-2450-7
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and condensation of TEOS and silylated organic ligands
that play double roles of both coordinating to lanthanide
ions or lanthanide complexes and acting as an organosilane
precursor to form the silica network. Among of them,
immobilization of b-diketonate complexes in silica and
mesoporous materials is highly intensive due to the reasons
mentioned above. This is normally achieved either by
adduct formation with a heterocyclic molecular immobi-lized on the matrices or through the modification of
b-diketone [1517].
We recently reported a facile way to prepare lumines-
cent organicinorganic hybrid materials through hydrolysis
and condensation of silylated bipyridine that can sensitize
the luminescence of europium (III) ions in the presence of a
carboxyl-functionalized ionic liquid in which the Eu3? ions
are coordinated to the oxygen atoms of carboxylate groups
[18]. A longer lifetime of Eu3? 5D0 excited state level was
obtained in comparison with similar materials without the
addition of ionic liquids [11]. In the present work, lantha-
nide b-diketonate complexes were immobilized in ionicliquid containing hybrid organosilica via SiC bond by
hydrolysis and condensation of TTA-Si (scheme 1) in the
presence of 3-(5-carboxypropyl)-1-methylimidazolium
bromide (IL, Scheme 1)) in which the Eu3? ions are
coordinated to the oxygen atoms of carboxylate groups.
The thermal stabilities and the luminescence properties of
the resulting hybrid materials are analyzed in detail.
2 Results and discussion
FT-IR spectrum was firstly employed to characterize the
obtained materials, the FT-IR spectra of TTA-Si and the
hybrid material Eu@IL-TTA-SiO2 are shown in Fig. 1.
The main changes upon hydrolysis and condensation of
TTA-Si in europium (III)-containing ionic liquid with
respect to the FT-IR spectrum of TTA-Si is the disap-
pearance of characteristic bands of SiOCH2CH3 groups at
960 and 1,172 cm-1, indicating the complete hydrolysis of
the TTA-Si precursor. The presence of the broad band at
1,1201,000 cm-1 indicates the formation of siloxane
bonds. No aborption band at 1,729 cm-1 attributed to the
COOH (Fig. 1c) groups from the ionic liquid can be
observed, implying that Eu3? ions are still coordinated to
the ionic liquid in the hybrid materials. The band at
1,690 cm-1 can be ascribed to the absorption of CONHgroups from Si-TTA.
Thermogravimetric analysis was also performed to
determine the thermal stability of the hybrid materials.
Figure 2 presents the thermogravimetric weight loss curve
(TG). Three main degradation steps can be observed from
the TG curve. The first step of weight loss of ca.17.4%
below 200 C could be attributed to the desorption of
physically absorbed water and residual solvents. The sec-
ond weight loss of 45.4% in the range of 200600 C can
be ascribed to the decomposition of ionic liquid and the
organic moieties from Si-TTA. Finally, the slight weight
loss beyond 600 C is ascribed to the release of water
Scheme 1 Ionic liquid (IL, a) and silylated ligand (TTA-Si, b) used
in this study and the digital photo of luminescent hybrid materials
irradiated under uv lamp (kmax = 365 nm, c)
Fig. 1 FT-IR spectrum of TTA-Si (a), the hybrid material Eu@IL-
TTA-SiO2 (b) and the ionic liquid (c)
Fig. 2 TGA curve of Eu@IL-TTA-SiO2
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formed from the further condensation of silanols in the
silica framework.
The organicinorganic hybrid materials show an red
photoluminescence upon irradiation with UV radiation
(scheme 1). The excitation and emission spectrum of the
obtained material were measured at room temperature and
are shown in Fig. 3. The excitation spectrum obtained by
monitoring the 5D0?7F2 emission at 612 nm displays alarge broad band between 225 and 450 nm resulting from
the p?p* transition of TTA superimposed with a sharp
line characteristic of Eu3? energy level. The relatively low
intensity of the intra-4f6 transition compared with that of
the broad band in the excitation spectrum indicates that
Eu3? ions are essentially excited by a sensitized process
rather than by direct population of the intra-4f6 levels. The
luminescence spectrum was measured with 325 nm as the
excitation wavelength, several narrow peaks can be
observed and are attributed to the transitions between the5D0 excited state and the different J levels of the ground
term 7FJ, (J= 04). The 5D0?7F2 emission line at 612 nmdominates the spectrum and this luminescence line is
responsible for the red luminescence color. The presence of
the forbidden 5D0?7F0 transition indicates that Eu
3? ions
are located in a coordination sphere with low symmetry
[19]. The 5D0?7F1 transition corresponds to a parity-
allowed magnetic dipole transition that is independent of
the environment. The hypersensitive 5D0?7F2 transition
varies strongly with the local surrounding around the Eu3?
ions. Its intensity increases when the lattice environment is
distorted and contains certain components of noninversion
symmetry. [20] The ratio (R) between the intensities of the5D0?
7F2 and5D0?
7F1 transition therefore can be used as
a parameter to probe the asymmetry of the Eu3? sites. The
R value here is determined to be 16.44, indicating that the
local symmetry groups of the Eu3? chemical environment
is not characterized by an inversion centre. The typical
decay curve of the hybrid material was measured and can
be described as a single exponential (supporting informa-
tion, Fig. 1s), indicating that all Eu3? ions occupy the same
average coordination environment, and the luminescence
lifetime is determined to be 0.531 0.001 ms.
We also determine the emission quantum efficiency (q)
of the 5D0 excited state based on the emission spectra andthe lifetime of the Eu3? first excited level by using the
following equations according to reference [21]. q can be
defined by Eq. 1 if we assuming that only nonradiative and
radiative processes are involved in the depopulation of the5D0 state.
q kr
kr knr1
where kr and knr are the radiative and nonradiative
probabilities, respectively. The radiative contribution may
be calculated from the relative intensities of the 5D0?7FJ
(J= 04) and can be expressed by Eq. 2. The 5D07F5,6transitions are not taken into account for the calculus of the
efficiency of the 5D0 level, probably due to their low
intensity (compared with the other lines).
KrA01E01
S01
X4
J0
S0J
E0J2
where A01 is Einsteins coefficient of spontaneous emis-
sion between the 5D0 and7F1 level, usually considered to
be equal to 50 s-1 when an average index of refraction n
equal to 1.506 was considered. E0J and S0J are the energy
and the integrated intensity of the
5
D0?
7
FJ transitions,respectively. The obtained data are summarized in
Table 1.The q value for the sample is 51.01% that is much
higher than the luminescent hybrid materials reported
Fig. 3 Excitation (a) and emission (b) spectrum of the hybrid
material Eu@IL-TTA-SiO2
Table 1 Experimental 5D0 lifetime, calculated radiative and nonra-
diative 5D0 decay rate, and5D0 quantum efficiency value
Sample s (ms) kr (ms-1) knr (ms
-1) q (%)
Eu@IL-TTA-SiO2 0.53 0.96 0.92 51.01
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