Photophysical properties of lanthanide (Eu 3+, Tb 3+) hybrid soft gels of double functional linker of ionic liquid-modified silane

Article abstract of DOI:10.1111/php.12149

Four kinds of luminescent hybrid soft gels have been assembled by introducing the lanthanide (Eu³⁺, Tb³⁺) tetrakis β-diketonate into the covalently bonded imidazolium-based silica through electrostatic interactions. Here, the imidazolium-based silica matrices are prepared from imidazolium-derived organotriethoxysilanes by the sol-gel process, in which the imidazolium cations are strongly anchored within the silica matrices while anions can still be exchanged following application for functionalization of lanthanide complexes. The photoluminescence measurements indicated that these hybrid soft gels exhibit characteristic red and green luminescence originating from the corresponding ternary lanthanide ions (Eu ³⁺, Tb³⁺). Further investigation of photophysical properties reveals that these soft gels have inherited the outstanding luminescent properties from the lanthanide tetrakis β-diketonate complexes such as strong luminescence intensities, long lifetimes and high luminescence quantum efficiencies. Ternary lanthanide luminescent hybrid soft gels Gel-Im⁺[LaL₄]^- were prepared by linking the europium/terbium tetrakis β-diketonate complexes into the silicon framework, which provides a representative method to assemble lanthanide-functionalized porous materials with chemical bonds.

Full text of DOI:10.1111/php.12149

Photochemistry and Photobiology, 2014, 90: 22–28
Photophysical Properties of Lanthanide (Eu3+, Tb3+) Hybrid Soft Gels of
Double Functional Linker of Ionic Liquid–Modified Silane
Qiu-Ping Li1,2 and Bing Yan*1,2
1Department of Chemistry, Tongji University, Shanghai, China
2State Key Lab of Water Pollution and Resource Reuse, Tongji University, Shanghai, China
Received 30 May 2013, accepted 23 July 2013, DOI: 10.1111/php.12149
ABSTRACT
applications, such lanthanide complexes still have poor thermal sta-
bility and low mechanical strength (14). To overcome these, the
organic–inorganic hybrid matrix materials have attracted consider-
able attention because of the ability to integrate the photophysical
properties of the organic components and the favorable thermal and
mechanical properties of the inorganic networks (15–17).
Four kinds of luminescent hybrid soft gels have been
assembled by introducing the lanthanide (Eu3+, Tb3+) tetrakis
b-diketonate into the covalently bonded imidazolium-based sil-
ica through electrostatic interactions. Here, the imidazolium-
based silica matrices are prepared from imidazolium-derived
organotriethoxysilanes by the sol–gel process, in which the
imidazolium cations are strongly anchored within the silica
matrices while anions can still be exchanged following applica-
tion for functionalization of lanthanide complexes. The photo-
luminescence measurements indicated that these hybrid soft
gels exhibit characteristic red and green luminescence origi-
Room-temperature ionic liquids (RTILs) are salts which have
melting points below room temperature and compose entirely of
ions (18). Over the past decades, RTILs have received numerous
attentions for their potential application due to their low melting
points, negligible volatility, nonflammability, increased thermal
stability, high electrochemical window and ease of recycling, etc.
(19). For material field applications, one of the important strategies
for using ionic liquids is by confining them into some porous sol-
ids to obtain so-called “ionogels” (20). By confining the lantha-
nide-doped ionic liquid mixtures into porous silica network,
Binnemans et al. have prepared some highly luminescent inor-
ganic–organic hybrid ionogels (21,22). And they also prepared a
kind of flexible luminescent polymer film by confining europium
complexes–doped ionic liquid into the PMMA membrane (23). Vi-
oux et al. has been conscious of the applications of ionogels early
and discussed its properties, preparation and potential applications
in great details (24–26). An alternative approach is to covalently
immobilize the ionic liquids on silica supports via bridged sils-
esquioxanes formation for constructing functional materials
(27,28). Here, an N′-alkyl imidazolium-based silylated ionic
liquid, which contains the trialkoxysilyl groups, is mostly used
(29). Carlos et al. have successfully prepared several kinds of imi-
dazolium salt–immobilized silica-supported materials with euro-
pium tetrakis(b-diketonate) complex anion counterions assembled
through electrostatic interactions, which emit intense luminescence
of the trivalent europium ions (30). We have also successfully pre-
pared some kinds of luminescent materials using the imidazolium-
based silylated ionic liquid as building blocks (31,32).
nating from the corresponding ternary lanthanide ions (Eu3+
,
Tb3+). Further investigation of photophysical properties
reveals that these soft gels have inherited the outstanding
luminescent properties from the lanthanide tetrakis b-diketo-
nate complexes such as strong luminescence intensities, long
lifetimes and high luminescence quantum efficiencies.
INTRODUCTION
It has long been well recognized that trivalent lanthanide elements
have excellent luminescent properties, which is due to the shield-
ing of the inner 4f orbital by the filled 5p66s2 subshells and the
abundant transition energy level (1). As a result, the photophysical
properties of trivalent lanthanide ions have attracted much atten-
tion due to their potential applications in the field of light-emitting
devices, optical amplifiers, laser materials, functional probes for
chemical or biological systems and biological imaging techniques
(2–8). However, the wide application of lanthanide ions them-
selves to luminescent materials has been limited for a long period,
which is mainly due to their low molar absorption coefficients as
the f–f transitions are Laporte forbidden (9), so that it is difficult to
generate efficient luminescence by direct excitation of the lantha-
nide ion. To resolve this problem, the lanthanide ions are usually
either doped into some special matrices such as oxides and oxysalts
or chelated with an adjacent strongly absorbing chromophore in the
UV region (10). For the latter, the usually used sensitizing ligands
mainly comprise b-diketones, aromatic carboxylic acids and hetero-
cyclic ligands (11). Among these molecular systems, the lanthanide
b-diketonate complexes are the most intensely studied ones due to
their intense emission bands, long luminescent lifetime and high
luminescence quantum efficiency (12,13). However, for practical
Here, we synthesized and performed characterization of a set
of luminescent soft gels by immobilizing the imidazolium-based
silylated ionic liquid to silica-based matrix followed by an anion
metathesis assembly. Finally, all the resulting materials have
been investigated in detail with respect to their characteristic and
photoluminescent properties.
MATERIALS AND METHODS
Materials. The EuCl3Á6H2O and TbCl3Á6H2O were prepared by dissolving
their corresponding oxides in concentrated hydrochloric acid followed by
evaporation. 1-Chlorohexane (98%), lithium bis(trifluoromethanesulfonyl)
*Corresponding author email: byan@tongji.edu.cn (Bing Yan)
© 2013 The American Society of Photobiology
22

Photochemistry and Photobiology, 2014, 90 23
imide (98%), 3-(Chloropropyl)triethoxysilane (98%, (OEt)3Si(CH2)3Cl),
1-Methylimidazole (99%), Thenoyltrifluoroacetone (98%, TTA),
1,1,1-Trifluoroacetylacetone (98%, TFA), hexafluoroacetylacetone (98%,
HFA) and 2-Naphthoyltrifluoroacetone (98%, NTA) are purchased from
Adamas reagent company and used as received. Formic acid (97%, FA),
Tetramethoxysilane (98%, TMOS) and Sodium hydroxide (98%, NaOH)
are purchased from Alfa Aesar and generally used as received. All the
other reagents were analytical pure and purchased from China National
Medicines Group.
Synthesis procedures of 1-methyl-3-[3-(triethoxysilyl)propyl]-imidazo-
lium chloride. The 1-methyl-3-[3-(triethoxysilyl)propyl]imidazolium chlo-
ride [Mtespim][Cl] was prepared using a previous reported method (33)
as following: the (OEt)3Si(CH2)3Cl was mixed with the same equivalent
1-methylimidazole and added to a two-necked, round-bottomed flask
equipped with a magnetic stirrer. Then, the mixture was kept at 70°C for
72 h under argon protection with vigorous stirring. The resulting pale
yellow viscous liquid was then cooled to room temperature and washed
with anhydrous cyclohexane. The upper cyclohexane was decanted
directly and the procedure was repeated three times. Finally, the residual
cyclohexane was removed by reduced pressure distillation using a rotary
evaporator.
remove the excess NEt4Ln(L)4. As before, the lanthanide complexes–
functionalized gel material was dried at 50°C overnight under vacuum
condition to remove the residual ethanol. Subsequently, a series of frag-
mented luminescent soft gels: Gel-Im+[Eu(TTA)4]À, Gel-Im+[Eu
(NTA)4]À, Gel-Im+[Tb(TFA)4]À and Gel-Im+[Tb(HFA)4]À were obtained.
Physicochemical characterization. Fourier transform infrared spectra
(FTIR) were measured within KBr slices from 4000 to 400 cmÀ1 using a
Nexus 912 AO446 infrared spectrum radiometer. X-ray powder diffrac-
tion patterns (XRD) were acquired on Rigaku D/max-Rb diffractometer
equipped with Cu anode; the data were collected within the 2h range
10°–70°. Scanning electronic microscope (SEM) images were obtained
with a Philips XL-30. Transmission electron microscope (TEM) experi-
ments were performed using
a JEOL2011 microscope operated at
200 kV. Thermogravimetric analysis (TG) was carried out using a Net-
zsch STA 449C system at a heating rate of 5°C/min under the nitrogen
protection. The diffuse reflectance UV–vis spectra of the powdered sam-
ples were recorded by a BWS003 spectrophotometer. Luminescence
excitation spectra, emission spectra, lifetimes and quantum efficiencies
are measured on an Edinburgh Instruments FLS 920 fluorescence
spectrometer.
Synthesis procedures of 1-hexyl-3-mexylimidazolium bis[(trifluorom-
ethyl)sulfonyl]imide. The ionic liquid 1-hexyl-3-methylimidazolium bis
(trifluoromethylsulfonyl)imide [C6mim][Tf2N] was synthesized from
RESULTS AND DISCUSSION
[C6mim][Cl] by
a metathesis reaction between lithium bis(trif-
The silica-based matrix is prepared through an ionic liquid–
luoromethylsulfonyl)imide Li[Tf2N]2 and [C6mim][Cl] according to the
previous literatures (34–36). Firstly, the 1-hexyl-3-methylimidazolium
chloride ([HMIM][Cl]) was prepared as follows: 4 mmol of 1-methylimi-
dazole and 4 mmol 1-chlorohexane were added to a two-necked, round-
bottomed flask equipped with a reflux condenser and a magnetic stirrer.
The reaction mixture was heated under argon protection at 80°C with
stirring for 1 week. The resulting viscous liquid was then cooled to room
temperature and washed with ethyl acetate by thorough mixing. The
upper ethyl acetate was decanted directly and the procedure was repeated
three times. Finally, the residual ethyl acetate was removed with the
rotary vacuum distillation apparatus by heating at 80°C. Subsequently, an
aqueous solution of 4.5 mmol lithium bis(trifluoromethanesulfonyl)imide
was added to [HMIM][Cl] with thorough mixing. Then, the mixture was
washed several times with aliquots of deionized water until no longer
chloride residues were detected by the AgNO3 test. The product was
dried at 80°C overnight under vacuum condition to afford an oil-like
[C6mim][Tf2N] at room temperature.
Synthesis of lanthanide complexes NEt4Ln(L)4 (for Ln = Eu, L = TTA
or NTA; while Ln = Tb, L = HFA or TFA). The lanthanide complexes
NEt4Eu(TTA)4 were synthesized by a method previously described by us
(32). Typically, in a two-necked, round-bottomed flask equipped with a
reflux condenser and a magnetic stirrer, 4 mmol of TTA was dissolved
in ethanol and this ligand was deprotonated with 4 mmol NaOH at 60°C
for 2 h. Subsequently, a portion of EuCl3Á6H2O solution (1 mmol in eth-
anol) was added to it dropwise, followed by the addition of 1 mmol
NEt4Cl in ethanol. The solution was reflexed at 60°C for another 1 h.
Then, the mixture was filtered to remove the white precipitate, sodium
chloride. The resulted filtrate was the ethanol solution of NEt4Eu(TTA)4
and was transferred into a flask for anion exchange. For NEt4Eu(NTA)4,
the same procedure was used except that the TTA was replaced by NTA.
The synthesis procedures for NEt4Tb(TFA)4 and NEt4Tb(HFA)4 are simi-
lar to that of NEt4Eu(TTA)4 except that the ligand TTA was replaced by
TFA and HFA and the EuCl3Á6H2O was replaced by TbCl3Á6H2O.
Synthesis procedures of the lanthanide complexes–functionalized soft
gel. The ionogel was prepared according to a modified published method
using a nonhydrolytic sol–gel route (37). In a Teflon vial, 1 mmol
[C6mim][Tf2N] and 26 mmol FA were stirred well at room temperature,
and then a mixture of [Mtespim][Cl] and TMOS was added to the solu-
tion (molar ratio FA/TMOS/[Mtespim][Cl]/[C6mim][Tf2N] = 7.8/0.9/0.1/
0.3). After stirring well, the mixture was exposed at ambient temperature
for 2 weeks and then a transparent monolith bulk ionogel was obtained.
Unfortunately, the monolithic ionogels fragmentized during the following
experimentation. After a 48 h extraction in Soxhlet extractor with reflux-
ing acetonitrile, the entrapped ionic liquid [C6mim][Tf2N] was removed.
Then, the gel material was dried at 50°C overnight under vacuum condi-
tion to remove the residual acetonitrile. After that, the gel material was
directly immersed into the previous solution of NEt4Ln(L)4 for anion
exchange at room temperature. After a week, the gel material was taken
out and transferred into Soxhlet extractor with refluxing ethanol to
mediated nonhydrolytic sol–gel route, using
a mixture of
TMOS, FA, 1-hexyl-3-mexylimidazolium bis[(trifluoromethyl)
sulfonyl]imide and 1-methyl-3-[3-(triethoxysilyl)propyl]-imidazo-
lium chloride. The formic acid used here serves as a solvent,
water source and catalyst for both hydrolysis and condensation.
After the completion of gelation, the confined ionic liquid was
removed by Soxhlet extraction. Then, different lanthanide tetra-
kis(b-diketonate) complex anions are assembled into the matrix
through electrostatic interactions after a simple anion metathesis
reaction. The synthetic route is shown in the Scheme 1 and the
detailed synthetic procedures are given in the following experi-
mental section.
The FTIR spectra of lanthanide complexes–functionalized soft
gels are shown in Fig. 1. As shown in the FTIR spectra, a weak
characteristic absorbance for the ring stretching of imidazolium
moiety is found at 1578 cmÀ1, suggesting that the ionic liquid
[Mtespim][Cl] has been successfully immobilized onto the frame-
work of silica matrix (38). The imidazolium moiety also exhibits
the characteristic bands (39) of aromatic C–H stretching at 3162,
2953 and 2882 cmÀ1. The absorptions at 1690 cmÀ1 are assign-
able to the stretching vibration of carbonyl groups, which come
from the residual formic acid confined deeply in the soft gels. For
the lanthanide complexes moieties, there are only very weak
absorption peaks that appear at about 1626 cmÀ1, which could be
assigned to the carbonyl stretching band of the b-diketone ligands
coordinated to the lanthanide ions. Meanwhile, the band appearing
at 1459 cmÀ1 can be ascribed to the scissoring vibration of
-(CH2)3- groups originated from 3-(chloropropyl)triethoxysilane
(40), which further evidences the immobilization of [Mtespim]
[Cl]. The asymmetric stretching of Si–O–Si appears as a very
strong band at 1067 cmÀ1, whereas the bending stretching vibra-
tion of it appears at 798 and 451 cmÀ1. The medium intensity
band at 950 cmÀ1 and the shoulder at about 570 cmÀ1 are associ-
ated with the Si–O stretching vibrations of the silanol (Si–OH) of
surface groups (41). Moreover, the broad band around 3600 cmÀ1
and the weak band at 1630 cmÀ1 can be assigned to the bending
and stretching vibrations of -OH of the physical adsorbed water. It
was worth noticing that the Gel-Im+[Tb(HFA)4]À shows a rela-
tively stronger band than the other ones, which may be owing to
the higher content of fluorine for the HFA ligand.

24 Qiu-Ping Li and Bing Yan
Figure 2 shows the room-temperature XRD patterns of the all
lanthanide complexes–functionalized soft gels from 10° to 70°.
All curves exhibit a similar single broad hump centered at 23.4°,
which demonstrates the amorphous nature of the soft gels.
According to the literatures (42,43), these broad peaks can be
associated with the presence of amorphous siliceous domains in
soft gels. Furthermore, the absence of any sharp and strong peak
in the diffraction curve of these samples correlates well with the
absence of a large crystalline region. In addition, none of the
hybrid materials contains measurable amounts of phases corre-
sponding to the pure lanthanide tetrakis(b-diketonate) complexes,
which is an indication of the formation of true homogeneous lan-
thanide complexes–functionalized soft gels. SEM and TEM of
the most efficient luminescent hybrid soft gel Gel-Im+[Eu
(TTA)4]À, which demonstrate a homogeneous materials were
shown in Fig. 3. As clearly shown in the SEM images (A, B), a
relatively homogeneous fine porous structure is observed on the
surface of this material. The main reason for the porosity prop-
erty is probably the confinement and extraction effect of the
ionic liquid [C6mim][Tf2N] through the sol–gel processes and
Soxhlet extraction. Accordingly, this kind of fine porous struc-
ture also can be seen in other hybrid soft gels. As we discussed
above, however, the hybrid soft gels discussed in this study are a
kind of noncrystalline state materials with amorphous nature.
Therefore, there exists no exact long-range ordered microstruc-
ture that we can see from its TEM images (Fig. 3C,D), which is
also a coincidence with the results of XRD analysis.
To investigate the thermal stability of the obtained hybrid
materials, the TG and differential thermogravimetric trace (DTG)
are performed on the most efficient luminescent hybrid soft gel
Gel-Im+[Eu(TTA)4]À as a representative example under a nitro-
gen atmosphere at a heating rate of 5°C/min. As shown in the
Fig. 4, the TG curve could be divided into four main weight loss
procedures with the help of DTG curve analysis. The first weight
loss procedure before 175°C can be attributed to the loss of
physically absorbed water and residual solvent. When we con-
tinue heating, the gel material may further crack and release
those deeply wrapped solvent confined during the sol–gel pro-
cesses, which is associated with to the second step (ranging from
175°C to 258°C) of weight loss. While the third procedure of
weight loss between 258°C and 430°C could be associated with
the decomposition of the europium complexes attaching onto the
soft gel matrices through electrostatic interactions (30), it is also
an evidence for the success of anion exchange. Finally, the last
weight loss beyond 430°C is probably caused by the decomposi-
tion of imidazolium moieties that covalently bonded to the
siliceous matrixes.
Figure 5 shows the diffuse reflectance spectra of different soft
gels, which provide an intuitive way for the observation of the
absorbance of powdered materials. It is observed that all these
materials exhibit a broad absorption band in the ultraviolet region
except the Gel-Im+[Tb(HFA)4]À and a similar stable absorption
band above 400 nm. These broad absorption bands correspond
to the absorbance of the b-diketone ligands, which chelate with
the trivalent lanthanide ions. Beside the Gel-Im+[Tb(HFA)4]À,
these absorption bands mostly overlap with their corresponding
photoluminescence excitation spectra as shown in Figs. 6 and 7.
It can be predicted that the hybrid soft gels can absorb abundant
energy in the ultraviolet range, this energy can be transferred to
the trivalent lanthanide ions through the “antenna effect” and
then sensitize them. Thus, we expect that those lanthanide com-
plexes–functionalized soft gels would possess excellent photolu-
minescence properties under the irradiation of ultraviolet light,
which has been confirmed by their corresponding emission spec-
tra as shown in Figs. 6 and 7. Besides, it is worthy pointing out
that all the soft gels have a poor absorption to the ultraviolet
wave around 250 nm.
The luminescent excitation (A, C) and emission (B, D) spectra
of the europium complexes–functionalized soft gels are shown in
Fig. 6. The excitation spectra are collected by monitoring the
strongest emission band of Eu3+ ion at 612 nm, which presents
a broad band centered at 350 nm for Gel-Im+[Eu(TTA)4]À
and 341 nm for Gel-Im+[Eu(NTA)4]À, respectively. These broad
excitation spectra in ultraviolet range can be both ascribed to the
characteristic absorption of the lanthanide complexes, it is in
accordance with the conclusion of the UV–vis diffuse reflectance
absorption analysis. For the emission spectra of Gel-Im+[Eu
(TTA)4]À and Gel-Im+[Eu(NTA)4]À, the emission lines located
at 579, 591, 612, 652 and 703 nm can be assigned to the charac-
teristic 5D0?7F0, 5D0?7F1, 5D0?7F2, 5D0?7F3 and 5D0?7F4
5
transitions, respectively. As we know that the D0?7F2 transition
is a typical electric dipole transition, which is strongly relying on
(OEt)3Si(CH2)3Cl
H3C
CH3
N
N
N
N
N
N
(OEt)3Si(CH2)3
70 °C, 72 h, Ar
Cl
CH3
Si
(CH2)3
Gel
FA, TMOS, [C6mim][Tf2N] -[C6mim][Tf2N]
Soxhlet extraction
gelation, 2 weeks, r.t.
Cl
OEt
Gel-Im+[Eu(TTA)4]-
-
Gel-Im+[Eu(NTA)4]
CH3
Si
(CH2)3
N
N
Gel
NEt4Ln(L)4, in ethonal
Gel-Im+[Tb(TFA)4]-
Gel-Im+[Tb(HFA)4]-
anion exchange, 1 week, r.t.
[Ln(L)4]
OEt
Scheme 1. Synthesis of hybrid soft gels with silica-supported imidazolium salt bridge and lanthanide tetrakis(b-diketonate) luminescence center.

Photochemistry and Photobiology, 2014, 90 25
1067
451
950
570
798
1459
1578
1690
+
-
Gel-Im [Eu(TTA) ]
4
+
-
Gel-Im [Eu(NTA) ]
4
+
-
Gel-Im [Tb(HFA) ]
4
+
-
Gel-Im [Tb(TFA) ]
4
4000 3500 3000 2500 2000 1500 1000 500
Wavenumber / cm-1
Figure 4. TG and differential thermogravimetric trace analysis curves
for the Gel-Im+[Eu(TTA)4]À.
Figure 1. The Fourier transform infrared spectra for the lanthanide com-
plexes–functionalized soft gels.
the local symmetry environment of Eu3+ ions, therefore, the most
predominant transition at 612 nm can be seen as a sign of low
symmetry environment around the Eu3+ ions. As the 5D0?7F1
transition is a parity-allowed magnetic dipole type which is prac-
tically independent of the surroundings of Eu3+ ions, the integra-
tion ratio of transition I(5D0?7F2)/I(5D0?7F1) (red/orange ratio)
has been widely taken as an indicator of Eu3+ ions site symme-
try. The intensity ratio of the two emission peaks are about 14.8
and 9.5 for Gel-Im+[Eu(TTA)4]À and Gel-Im+[Eu(NTA)4]À,
respectively, suggesting that the Eu3+ is not at the center of
an asymmetric coordination field. This is due to the fact that
the strong coordination interactions took place between the
b-diketones and the Eu3+ ions (44). In addition, we have found
that the Gel-Im+[Eu(TTA)4]À show better luminescent properties
than Gel-Im+[Eu(NTA)4]À under the same test conditions, which
is coincidental with the results of luminescent lifetimes and
quantum efficiencies as shown in Table 1.
Figure 7 illustrates the typical photoluminescence spectra of
the terbium complexes–functionalized soft gels. The excitation
spectra of the two hybrid soft gels Gel-Im+[Tb(TFA)4]À and
Gel-Im+[Tb(HFA)4]À are similar, which are collected by moni-
toring the corresponding emission wavelength of the Tb3+ ions
at 545 nm. Both of them are dominated by a broad band, which
center at 312 and 314 nm for Gel-Im+[Tb(TFA)4]À and Gel-
Im+[Tb(HFA)4]À, respectively. Subsequently, their corresponding
emission spectra are collected by choosing the same wavelength
(313 nm) to excite the two terbium complexes–functionalized
soft gels. As shown in the Fig. 7, four emission peaks are
observed at about 489, 545, 588 and 618 nm in the spectrum of
Gel-Im+[Tb(HFA)4]À, which can be assigned to 5D4?7F6,
Figure 2. The XRD patterns of different lanthanide complexes–
functionalized soft gels.
5
5
5D4?7F5, D4?7F4 and D4?7F3 transitions, respectively. Sim-
ilar pattern can also be found in the emission spectrum of Gel-
5
Im+[Tb(TFA)4]À, except the D4?7F5 transition that has divided
into two shoulder peaks (542 and 548 nm). Besides, the most
striking green luminescence (5D4?7F5) is observed in Fig. 7
due to the fact that this emission is the most intense one. We
also have found that the Gel-Im+[Tb(TFA)4]À shows better
luminescent properties than Gel-Im+[Tb(HFA)4]À according to
the elevatory baseline in the emission spectrum of Gel-Im+[Tb
(HFA)4]À, which is also proved by the results shown in
Table 1.
Figure 3. Scanning electronic microscope (A, B) and Transmission elec-
tron microscope (C, D) images of the hybrid soft gel Gel-Im+[Eu
(TTA)4]À.

26 Qiu-Ping Li and Bing Yan
Figure 5. The UV-vis diffuse reflectance absorption spectra of the soft gels.
Figure 6. Excitation (A, C) and emission (B, D) spectra of the europium complexes–functionalized soft gels.
To further investigate the luminescence properties of these
the lanthanide ions occupy the same average local environment
hybrid soft gels, we have measured the luminescence decay
curves for 5D0 energy level of Eu3+ and 5D4 energy level of
Tb3+ of the resulting materials with the Edinburgh FLS920 phos-
phorimeter. The resulting lifetimes of the Eu3+ and Tb3+ func-
tionalized hybrids are listed in Table 1, which confirms that all
within each sample (45).
Moreover, the outer luminescence quantum efficiencies for
5D0 energy level of Eu3+ and D4 energy level of Tb3+ are also
5
determined using an integrating sphere from the Edinburgh
FLS920 phosphorimeter. It can be clearly seen from the Table 1

Photochemistry and Photobiology, 2014, 90 27
Figure 7. Excitation (A, C) and emission (B, D) spectra of the terbium complexes–functionalized soft gels.
Table 1. Luminescent data for the lanthanide complexes–functionalized
soft gels.
cal bonds. During the synthesized process, silica-based matrix is
prepared through an ionic liquid–mediated nonhydrolytic sol–gel
route in advance, and then the lanthanide complexes anion are
assembled into the matrix through electrostatic interactions after a
simple anion metathesis reaction. The properties of these hybrid
soft gels are characterized in detail. Further investigation on the
luminescence properties show that all hybrid soft gels present
strong luminescence intensities, long lifetimes and high lumines-
cence quantum efficiencies. The outstanding luminescent proper-
ties of these hybrid soft gels inherit from the lanthanide
complexes, suggesting that the method used here can be copied
across the other lanthanide complexes functional materials. Ulti-
mately, this work not only enriches the types of lanthanide-func-
tionalized luminescent hybrid materials but also extends the
method for preparing lanthanide composite materials.
Samples
I
02/I01
*
s(ls)†
g(%)‡
Gel-Im+[Eu(TTA)4]À
Gel-Im+[Eu(NTA)4]À
Gel-Im+[Tb(TFA)4]À
Gel-Im+[Tb(HFA)4]À
14.8
9.5
–
784
532
543
179
14.81
7.57
14.44
8.83
–
*The emission intensity ratios of the 5D0?7F1 transition (I01) and the
5D0?7F2 transition (I02) for europium complexes–functionalized soft
gels; †lifetimes (s) of 5D0 energy level and 5D4 energy level for Eu3+
and Tb3+ excited state, respectively, error Æ10%; ‡the luminescent quan-
tum efficiency (g) value of 5D0 Eu3+ and 5D4 Tb3+ excited state deter-
mined by an integrating sphere from the Edinburgh FLS920
phosphorimeter, error Æ10%.
that the luminescent quantum efficiency of Gel-Im+[Eu(TTA)4]À
is higher than those of Gel-Im+[Eu(NTA)4]À, which coincide
with the values of integration ratio of transition red/orange ratio
and luminescence lifetimes. Meanwhile, we have noticed that
both the europium complexes–functionalized soft gels show
longer lifetimes than the terbium-containing materials, suggesting
that the method used in this study is better suited to prepare
europium-functionalized luminescent materials. Besides, the table
also indicates that the resulted soft gels possess some of the out-
standing luminescent properties of lanthanide complexes: rela-
tively high quantum efficiency and long luminescent lifetime.
Acknowledgements—This work was supported by the National Natural
Science Foundation of China (20971100) and Program for New Century
Excellent Talents in University (NCET-08-0398).
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