Luminescent hybrid materials of lanthanide β-diketonate and mesoporous host through covalent and ionic bonding with anion metathesis

Article abstract of DOI:10.1039/c2dt30364g

Luminescent mesoporous materials were prepared by performing an anion metathesis reaction on ionic liquid modified SBA15, which has imidazolium chloride bridging units. The lanthanide β-diketonate complex anion was successfully anchored onto the SBA15 fra

Full text of DOI:10.1039/c2dt30364g

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PAPER
Luminescent hybrid materials of lanthanide β-diketonate and mesoporous
host through covalent and ionic bonding with anion metathesis†
Qiu-Ping Li and Bing Yan*
Received 16th February 2012, Accepted 23rd May 2012
DOI: 10.1039/c2dt30364g
Luminescent mesoporous materials were prepared by performing an anion metathesis reaction on ionic
liquid modified SBA15, which has imidazolium chloride bridging units. The lanthanide β-diketonate
complex anion was successfully anchored onto the SBA15 framework after the anion metathesis reaction.
The resulting materials were characterized by FTIR, TEM, TGA, small-angle X-ray powder diffraction
(SAXRD) and nitrogen adsorption–desorption isotherms. The photoluminescent properties of these
materials were investigated in detail, and the results reveal that these hybrid mesoporous SBA15, prepared
through this preparation approach, present favorable photoluminescent behavior such as high luminescent
quantum efficiencies and long luminescent lifetimes.
liquid such as negligible vapor pressure, thermal stability and
widely tunable cation or anion have aroused wide public concern
Introduction
The photophysical properties of lanthanide 1,3-diketonate
(β-diketonate) complexes have attracted considerable attention in
the past decades due to their potential applications in laser,
optical amplifier and organic electroluminescent devices.1 These
complexes not only would preserve the merits of lanthanides in
luminescence, such as high color purity and narrow band emis-
sion, but also can make use of the ligand to metal energy transfer
mechanism (antenna effect2) to enhance the quantum efficiency
of luminescence. Therefore, the synthesis of such luminescent
lanthanide materials has been extensively studied in recent years.
However, the practical applications of these compounds have
been limited by their low thermal and photochemical stability,
together with poor mechanical properties.3 Much work has been
done to overcome these drawbacks by incorporating lanthanide
β-diketonate complexes into inorganic matrices, such as silica-
based sol–gel matrices. The resulting organic–inorganic
materials are likely to combine the outstanding luminescent
properties of the lanthanide β-diketonate complexes with the
favorable thermal and mechanical characteristics of inorganic
networks.4 Among these materials, lanthanide β-diketonate com-
plexes functionalized ordered mesoporous silica is especially
studied due to unique properties such as good hydrothermal and
thermal stability, uniform pore structure and high surface area.5
Ionic liquids have attracted considerable attention within the
past decades, owing to their potential application as environmen-
tally benign solvents for chemical synthesis, separation, catalysis
and electrochemistry.6 The prominent properties of an ionic
in material chemistry. The introduction of a metal ion complex
anion can lead to a functional ionic liquid, which can combine
the properties of ionic liquids with those properties such as cata-
lytic, magnetic or photophysical properties that originate from
the corresponding metal ion.7 Several approaches have been
developed for preparing ionic liquid based hybrid materials
including in situ or post polymerization of monomers, sol–gel
processing in the ionic liquid, physical gelation, or impregnation
into as-prepared matrices.8 Among these approaches, the method
that anchors the ionic liquid to a specific supporting matrix fol-
lowed by anion metathesis has been proven to be an effective
way for constructing functional materials.9 Recently, a series of
work has been done around highly luminescent lanthanide-con-
taining ionic liquids, which show promising applications in con-
structing luminescent materials for use in photics and
photochemistry.10 Several kinds of materials have been syn-
thesized by introducing lanthanide β-diketonate complexes into
various matrices, which exhibit high quantum yields and good
photochemical stability.11
In this work, we put forward a novel method to construct
luminescent mesoporous materials. In this case, the ionic liquid
1-methyl-3-[3-(trimethoxysilyl)propyl]imidazolium chloride is
incorporated into the mesoporous SBA15 framework by an
in situ sol–gel processing. Then, an anion metathesis processing
is performed to introduce the lanthanide β-diketonate complexes
anion. Four kinds of commercially available β-diketonates are
used here; they are benzoyltrifluoroacetone (BTA), hexafluoro-
acetylacetone (HTA), thenoyltrifluoroacetone (TTA) and trifluoro-
acetylacetone (TAA). Subsequently, four europium based
mesoporous materials SBA15–IM+[Eu(L)4]− (L
=
TTA,
Department of Chemistry, Tongji University, Siping Road 1239,
Shanghai, 200092, China. E-mail: byan@tongji.edu.cn;
Fax: +86-21-65982287; Tel: +86-21-65984663
†Electronic supplementary information (ESI) available. See DOI:
10.1039/c2dt30364g
BTA, HTA or TAA) are obtained after anchoring the europium
β-diketonate complex anion onto the SBA15 framework.
But for terbium based materials, because the triplet energy level
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of TTA and BTA does not match the 5D4 transition level of
terbium, we have only selected HTA and TAA for constructing
terbium based luminescent mesoporous materials. Moreover, all
the resulting materials have been investigated in detail with
respect to their characteristic and photoluminescent properties.
For NEt4Eu(TTA)4, typically: 0.888 g of TTA (4 equiv.) is dis-
solved in ethanol and deprotonated with 0.16 g NaOH (4 equiv.)
at 60 °C for 2 h followed by the dropwise addition of 1 equiv. of
EuCl3·6H2O in ethanol and by the addition of 1 equiv. of
NEt4Cl in ethanol. The mixture is kept at 60 °C for another 1 h.
Then, the mixture is filtered to remove sodium chloride, and the
filtrate is transferred to a flask for anion exchange. The synthesis
procedures for NEt4Eu(BTA)4, NEt4Eu(HTA)4, NEt4Eu(TAA)4,
NEt4Tb(HTA)4 and NEt4Tb(TAA)4 are similar to that for
NEt4Eu(TTA)4.
Experimental section
General
EuCl3·6H2O and TbCl3·6H2O are prepared according to a
modified procedure reported in the literature12 by dissolving
their corresponding oxides (Eu2O3, 99.9%, and Tb2O3, 99.9%,
Aladdin) in concentrated hydrochloric acid, followed by evapor-
ation until the solutions become a pasty material. After that, the
corresponding lanthanide chloride is collected by being filtered
and washed with ice water and diethyl ether several times, sub-
sequently. Tetraethoxysilane (TEOS, 99%, Aladdin) is distilled
and stored under nitrogen atmosphere. Pluronic P123
(EO20PO70EO20) is purchased from the Lancaster Company.
(3-Chloropropyl)trimethoxysilane (CPTMO, 98%, Aladdin),
1-methylimidazole (99%, Aladdin), tetraethylammonium chloride
(NEt4Cl, 98%, Aladdin), trifluoroacetylacetone (TAA, 98%,
Aladdin) and thenoyltrifluoroacetone (TTA, 98%, Aladdin),
benzoyltrifluoroacetone (BTA, 99%, Aladdin) and hexafluoro-
acetylacetone (HTA, 98%, Aladdin) are used as received. All the
other reagents are analytical pure and purchased from China
National Medicines Group.
Synthesis of lanthanide functionalized mesoporous materials
SBA15–IM+[Ln(L)4]− (Ln = Eu, Tb; L = TTA, BTA, HTA or
TAA). In order to perform anion exchange, the SBA15–IM+Cl−
is dispersed in the as-prepared NEt4Ln(L)4 solution and stirred
for 48 h at room temperature. The resulting material is collected
by filtration and washed with ethanol. The excess NEt4Ln(L)4 is
removed by Soxhlet extraction with ethanol under reflux for
12 h. The resulting solid is dried overnight at 50 °C under
vacuum condition and denoted as SBA15–IM+[Eu(TTA)4]−,
SBA15–IM+[Eu(BTA)4]−, SBA15–IM+[Eu(HTA)4]−, SBA15–
IM+[Eu(TAA)4]−, SBA15–IM+[Tb(HTA)4]− and SBA15–
IM+[Tb(TAA)4]−, respectively. The synthesis procedures and the
possible structures of the resulting materials are shown in
Scheme 1 and Fig. 1, respectively.
Synthesis procedure of 1-methyl-3-[3-(trimethoxysilyl)propyl]-
imidazolium chloride. The ionic liquid 1-methyl-3-[3-(tri-
methoxysilyl)propyl]imidazolium chloride is synthesized
according to the procedure reported in the literature13 as follows:
the 1-methylimidazole is mixed with one equiv. of CPTMO; the
mixture is stirred at 70 °C for 3 d under argon atmosphere. Then,
the pale-yellow viscous product is washed with anhydrous acetic
ester three times. The resulting ionic liquid is dried overnight at
70 °C to remove the excess acetic ester under vacuum condition
and denoted as IM+Cl−.
Scheme 1 Scheme of the synthesis procedures.
Synthesis of ionic liquid functionalized SBA15 (SBA15–IM+-
Cl−). The ionic liquid modified mesoporous material SBA15–
IM+Cl− is synthesized using a typical sol–gel processing14 with
the molar ratio of P123 : TEOS : IM+Cl− : HCl : H2O at
0.017 : 0.95 : 0.04 : 5.88 : 136. Typically, P123 (2 g) is dissolved
in deionized water (7.5 g) at room temperature. Then a 2 M HCl
solution (30 g) is added into the P123 solution. The mixture sol-
ution is then placed in a constant temperature (35 °C) oil bath. A
mixture of TEOS and IM+Cl− is added to the above solution
with stirring over 24 h and the mixture is transferred into a
Teflon bottle sealed in an autoclave, which is heated to 100 °C
for 2 d. The solid product is filtered off, washed with deionized
water, and dried at 60 °C overnight. Removal of the template
agent P123 is conducted by Soxhlet extraction with ethanol
under reflux for 2 d. The resulting product is dried overnight at
50 °C under vacuum condition and denoted as SBA15–IM+Cl−.
Synthesis of lanthanide complexes NEt4Ln(L)4 (Ln = Eu, Tb;
L = TTA, BTA, HTA or TAA). The lanthanide complexes
are synthesized by using a modified literature procedure.15
Fig. 1 Possible structure of the resulting materials.
8568 | Dalton Trans., 2012, 41, 8567–8574
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Physical measurement
TAA) indicates that the lanthanide complex anions have been
successfully anchored onto the SBA15 frameworks by anion
exchange. In addition, the broad band around 3400 cm−1 and the
band at 1633 cm−1 can be ascribed as the bending and stretching
vibrations of physically absorbed water,18 respectively.
Fourier transform infrared spectra (FTIR) are measured within
KBr slices from 4000–400 cm−1 using a Nexus 912 AO446
infrared spectrum radiometer. X-Ray powder diffraction patterns
(XRD) are acquired on a Rigaku D/max-Rb diffractometer
equipped with Cu anode; the data were collected within the 2θ
range of 0.6–6°. Nitrogen absorption and desorption isotherms
are measured using a Nova 1000 system under the liquid nitro-
gen temperature. The Brunauer–Emmett–Teller (BET) method is
used to calculate the surface of the mesoporous materials. Mean-
while, the Barrett–Joyner–Halenda (BJH) method is used to
evaluate the pore size distribution and the other pore parameters.
Thermogravimetric analysis (TG) is measured using a Netzsch
STA 449C system at a heating rate of 5 °C min−1 under a nitro-
gen atmosphere. Transmission electron microscopy (TEM)
experiments are performed using a JEOL2011 microscope oper-
ated at 200 kV. Luminescence excitation spectra, emission
spectra and lifetimes (τ) are measured on an Edinburgh Instru-
ments FLS 920 phosphorimeter. The diffuse reflectance UV-vis
spectra of the powdered samples are recorded by a BWS003
spectrophotometer.
One of the popular and efficient methods to characterize
highly ordered mesoporous materials with hexagonal symmetry
of the space group P6mm is X-ray diffraction analysis. Fig. 3
shows the small-angle X-ray powder diffraction (SAXRD) pat-
terns of precursor SBA15–IM+Cl− and the lanthanide complex
functionalized mesoporous materials. It can be observed that all
the patterns show three characteristic peaks of SBA15 in the 2θ
range of 0.6–2°, including a prominent peak indexed as (100) at
a low angle and two weaker peaks (110) and (200) at higher
angles, suggesting that the ordered mesostructures have been
well preserved in the final products. In addition, it is worth noti-
cing that these functionalized mesoporous materials appear have
decreased peak intensity, which is probably due to the decreasing
of the degree of order after the introduction of the ionic liquid
and the surface functionalization.
The other popular and efficient method to characterize highly
ordered mesoporous materials is nitrogen adsorption–desorption
isotherms, which could be used to explore the surface area, pore
diameter and pore volume of the material. The N2 adsorption–
desorption isotherms of the precursor SBA15–IM+Cl− and the
lanthanide complex functionalized mesoporous materials are pre-
sented in Fig. 4A and B. According to the IUPAC classification,
SBA15 would show a type IV isotherm with a H1 type hysteresis
loop at the high relative pressure region.19 In Fig. 4, it can be
observed that all patterns show a slight deviation from the stan-
dard isotherm of pure SBA15, implying that the introduction of
an ionic liquid and anion exchange with the lanthanide com-
plexes slightly changes the pore regularities of the mesoporous
material.20 As shown in Fig. 4A, the N2 adsorption–desorption
isotherm for SBA15–IM+Cl− is just a little different to the other
ones for the lanthanide complex functionalized mesoporous
materials, indicating that the anion exchange processing hardly
changes the morphology of the resulting materials. The specific
area and the pore size have been calculated by using BET and
BJH methods and listed in Table 1 together with the other struc-
tural parameters. It is worth noticing that these lanthanide
Results and discussion
The FTIR spectra of the precursor SBA15–IM+Cl− and selected
lanthanide complex functionalized mesoporous materials are
shown in Fig. 2. As shown in the FTIR spectra of SBA15–
IM+Cl−, the characteristic absorbance for the bonds of an imida-
zole ring is found at 1570 cm−1, suggesting that the ionic liquid
IM+Cl− has been successfully immobilized onto the framework
of mesoporous SBA15.16 Meanwhile the vibration of –CH2–
appearing at about 2982 cm−1 further evidences the bonding of
the ionic liquid. The broad bands appearing at 1080 cm−1 (νas,
Si–O–Si), 795 cm−1 (νs, Si–O–Si) and 463 cm−1 (δ, Si–O–Si)
indicate the success of hydrolysis and copolycondensation pro-
cedures, and the band located at 961 cm−1 is associated with the
stretching vibrations of the silanol (Si–OH) of surface groups.17
Comparing the absorption spectrum of precursor SBA15–
IM+Cl− with those of the lanthanide complex functionalized
mesoporous materials, the new band at 1678 cm−1 (which is
caused by the –CvO group of β-diketone TTA, BTA, HTA or
complex functionalized mesoporous materials possessed
a
smaller BET surface area, pore volume and average pore
Fig. 2 The FTIR spectra for SBA15–IM+Cl− and different β-diketo-
nate based materials, and the lanthanide complex functionalized meso-
porous materials.
Fig. 3 XRD patterns for precursor SBA15–IM+Cl− and the resulted
materials.
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The TEM images for SBA15–IM+Cl− and SBA15–IM+-
[Eu(TTA)4]− shown in Fig. 5A and B, respectively. It can be
observed that the well-ordered hexagonal mesostructure is pre-
served in both SBA15–IM+Cl− and SBA15–IM+[Eu(TTA)4]−,
which is consistent with the results of the SAXRD patterns. As
shown in the pictures, there is a regular hexagonal array of
uniform two-dimensional channels, which also indicates that the
mesostructure of SBA15 can be substantially conserved after the
introduction of the ionic liquid and the subsequent anion
exchange reaction. In addition, the distance between the contigu-
ous centers of the mesopore is estimated to be about 10 nm,
which is consistent with the results calculated from the corre-
sponding SAXRD data (see Table 1). Furthermore, it can be also
concluded through the comparison of the two TEM images that
the anion exchange reaction has only a little effect on the mor-
phology of the resulting materials.
To investigate the thermal stability of the obtained mesoporous
materials, the thermogravimetric (TG) and the corresponding
derivative weight loss (DTG) analysis were performed. Fig. 6
presents the TG and DTG curves of the SBA15–IM+-
[Eu(TTA)4]− at a heating rate of 5 °C min−1 under nitrogen atmos-
phere. It is clear from the TG curve that three main degradation
steps are observed. Combining with the DTG analysis, the first
step of mass loss (about 15.0%) from 30 to 160 °C could be
divided into two sections, for which, the initial faster weight loss
of 4.5% observed up to 60 °C is due to the residual ethanol
loaded during the anion metathesis process, and the latter weight
loss of 10.5% is mainly attributed to the physically absorbed
water. With further heating, the second step of weight loss
Fig. 4 N2 adsorption–desorption isotherms for SBA15–IM+Cl− and
final products.
Table 1 Structural parameters for SBA15–IM+Cl− and the resulted
materials
d100
a0
S
V
D
t
Sample
(nm) (nm) (m2 g−1
)
(cm3 g−1
)
(nm) (nm)
SBA15–IM+Cl−
SBA15–
8.923 10.30 446.1
8.909 10.29 424.4
0.9895
0.9737
7.501 2.803
7.253 3.033
IM+[Eu(TTA)4]−
SBA15–
9.099 10.51 406.0
9.098 10.50 447.9
9.104 10.51 451.9
9.010 10.40 407.8
9.003 10.39 437.9
0.9094
0.8740
0.9077
0.9314
0.8918
6.578 3.929
6.821 3.687
7.053 3.459
6.268 4.137
6.952 3.444
IM+[Eu(BTA)4]−
SBA15–
IM+[Eu(HTA)4]−
SBA15–
IM+[Eu(TAA)4]−
SBA15–
Fig. 5 TEM images of SBA15–IM+Cl− and SBA15–IM+[Eu(TTA)4]−.
IM+[Tb(HTA)4]−
SBA15–
IM+[Tb(TAA)4]−
d100 is the d(1 0 0) spacing, a0 the cell parameter (a0 = 2d100 ÷ √3), S
the BET surface a, V the total pore volume, D the average pore diameter,
and t the wall thickness calculated by a0 − D.
diameter than those typically reported for pure SBA15,
suggesting that the incorporation of the ionic liquid and the sub-
sequent anion exchange would reduce the pore volume of the
samples, since the guests that are incorporated or coated inside
the channels would enhance the roughness of the pore surface of
the support. Additionally, the inflection positions have slightly
shifted toward lower P/P0 values, which also can be ascribed to
the loading of guests into SBA15 frameworks.
Fig. 6 TG and DTG curves for SBA15–IM+[Eu(TTA)4]−.
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(about 9.5%) from 160 to 524 °C can be observed, which may
also be associated with two different weight loss reasons according
to the DTG analysis. The weight loss from 160 to 227 °C can
probably be attributed to europium tris chelates that have not
been washed away, usually with the common formula
Eu(TTA)3·nH2O.21 The weight loss between 227 and 524 °C is
associated with the decomposition of the organic moieties,
which mainly refers to those europium tetrakis(β-diketonate)
attaching onto the mesoporous frameworks through electrostatic
interactions. Finally, the last weight loss of 3.2% beyond 524 °C
is attributed to the decomposition of those organic parts that
derived from the IM+Cl−, whose composition could be corre-
sponded to a formula of C7H12N2Cl. Comparing to the general
decomposition temperature range of pure lanthanide complexes,
our materials possess a wider decomposition temperature interval
ranging from 227 to 524 °C, indicating that the thermal stability
of the europium complex moieties was enhanced as they were
introduced into the mesoporous silicon–oxygen matrix.21,22 Such
enhanced thermal stability of lanthanide complexes when intro-
ducing them into an appropriate matrix has also been observed
in the literature.23 In addition, the residual weight of 75.5%
when the sample is heated above 524 °C could be seen as a com-
bined contribution of ionic liquid functionalized mesoporous
material and the residual europium oxide. The europium oxide
would occupy one third of the weight composition of the
europium complex. Therefore, the reasonable percentage for
residual europium oxide could take about half the weight loss
value of 9.5%. That is to say, the weight percentage of the ionic
liquid functionalized mesoporous material could be estimated to
a value of 71%. Basing on the above discussion, the possible
loading quantity of 4.5% of ionic liquid is deduced.
The diffuse reflectance UV-vis spectra measurements are per-
formed for all powdered materials. Fig. 7A–C show the spectra
of the precursor SBA15–IM+Cl− and all the lanthanide complex
functionalized mesoporous materials. It is clear from the figure
that most of the resulting materials show an intense broad band
adsorption among the ultraviolet region (250–400 nm), except
for the HTA modified materials, which is attributed to the
absorption of the β-diketone ligands (TTA, BTA or TAA). These
adsorption bands also partially overlap with their own photolu-
minescence excitation spectra shown in Fig. 8. It can be primar-
ily inferred that the energy levels between the β-diketone TTA
(or BTA, TAA) and the europium ion match well, so that the
organic ligand can sensitize the emission of the europium ion by
energy transfer after it absorbs the ultraviolet light, and so can
TAA for the terbium ion. Then, most of the final products can be
expected to have excellent photoluminescence properties under
the irradiation of ultraviolet light, which is proved in the lumi-
nescence spectra in Fig. 8. Unfortunately, it seems that the ligand
HTA exhibits a weak adsorption in the ultraviolet region,
although we can observe the luminescent spectra of HTA
modified materials. In addition, a peak at 612 nm (Fig. 7A and
B) also can be observed in the spectra, which is due to the corre-
sponding europium ion.
Fig. 7 The diffuse reflectance UV-vis spectra for SBA15–IM+Cl− and
the resulting lanthanide complex functionalized materials.
of the europium ion at 612 nm and presented in Fig. 8A and B.
It is clear from the picture that both the materials have a broad
band absorption in the range of 240–400 nm, suggesting the
effective absorption of the materials. But, the maximum peaks
for SBA15–IM+[Eu(TTA)4]−, SBA15–IM+[Eu(BTA)4]−, SBA15–
IM+[Eu(HTA)4]− and SBA15–IM+[Eu(TAA)4]− centers at about
352, 357, 283 and 342 nm, respectively, which is attributed to
the difference between the energy levels of different ligands. As
a result, all the europium complex functionalized materials
possess similar emission spectra that come from the character-
istic europium ion emission. As shown in Fig. 7A and B, the
emission lines located at around 578, 590, 612, 654 and 701 nm
The photoluminescence properties of the obtained lanthanide
complex functionalized mesoporous materials have been investi-
gated at room temperature and are shown in Fig. 8A–C. The
excitation spectra of the europium complex functionalized
materials are obtained by monitoring the characteristic emission
5
7
are assigned to D0 → FJ (where J = 0, 1, 2, 3, 4). The spectra
are dominated by a very intense 5D0 → 7F2 transition at 612 nm,
which is typical for europium β-diketonate complexes. It is well
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Table 2 Luminescence data for the lanthanide complex functionalized
mesoporous materials
I02/I01 τb (ms) 1/τ (s−1) Ar
Anr
ηc (%)
a
Samples
SBA15–
14.68 685.2 1459
815.6 643.4 55.9
853.9 517.1 62.3
82.8 12 077 576.6 11 500 4.8
IM+[Eu(TTA)4]−
SBA15–
15.83 729.2 1371
IM+[Eu(BTA)4]−
SBA15–
9.82
IM+[Eu(HTA)4]−
SBA15–
14.32 317.5 3149
800.1 2349
25.4
IM+[Eu(TAA)4]−
SBA15–
18.5
44.8
IM+[Tb(HTA)4]−
SBA15–
IM+[Tb(TAA)4]−
5
7
a The emission intensity ratios of the D0 → F1 tr ansition (I01) and the
5D0
→
7F2 transition (I02). b Lifetimes (τ) of 5D0 energy levels of the
Eu3+ excited state. c (η value) is the luminescence quantum efficiency
value from the measurement (i.e. outer luminescence quantum efficiency
with error 10%).
because they possesses a more asymmetrical molecular structure
than TAA and HTA, in accordance with the above discussion.27
In the same way, the excitation spectra of the terbium complex
functionalized materials are shown in Fig. 8C, by monitoring the
characteristic emission of the terbium ion at 545 nm. Both the
SBA15–IM+[Tb(TAA)4]− and SBA15–IM+[Tb(HTA)4]− also
possess a broad excitation spectrum, which is centered at 284
and 268 nm, respectively, it also can be associated with the
ligand-to-metal charge transfer transition. The corresponding
emission spectra of the terbium ion are presented and while no
obvious f–f transitions are observed, four peaks between 450
and 650 nm can be clearly seen from it. These peaks transition at
487, 545, 582 and 621 nm, and are assigned to 5D4
→
7FJ
(where J = 6, 5, 4, 3, respectively). Among these peaks, the peak
7
located at 545 nm (5D4 → F5), which gives the green lumines-
cence, is the most intense one. Besides, we have found that the
luminescence intensity of SBA15–IM+[Tb(HTA)4]− is slighter
weaker than that for SBA15–IM+[Tb(TAA)4]−, which is prob-
ably due to the HTA exhibiting a weaker adsorption in the ultra-
violet region than TAA, according to the diffuse reflectance UV-
vis spectra presented in Fig. 7C. Besides, we have found that the
luminescence intensities of SBA15–IM+[Tb(TAA)4]− and
SBA15–IM+[Tb(HTA)4]− are much weaker than those for
SBA15–IM+[Eu(TAA)4]− and SBA15–IM+[Eu(HTA)4]− from
the spectra data, indicating that the TAA and HTA are not very
suited as sensitizers for the terbium ion, which can be ascribed
to the mismatch of the triplet energy level of the ligands and the
corresponding transition level of the terbium ion. In fact, a
β-diketone group ligand is not the best one for sensitizing the
luminescence of a terbium ion, by comparison with a carboxyl
Fig. 8 Excitation and emission spectra for the lanthanide complex
functionalized materials.
5
7
known that the D0 → F2 transition for the europium ion is a
typical electric dipole transition, which is very sensitive to even
small changes in their local environment;24 while the parity-
5
7
allowed magnetic dipole transition D0 → F1 is independent of
the local environment of the europium ion.25 As a result, the
intensity ratios I(5D0
→ →
7F2)/I(5D0 7F1) have been widely
used as an indicator of the local environment of the europium
ion. Based on the data, the intensity ratios 5D0
→
7F2 to
5D0 → F1 of all the europium complex functionalized materials
are calculated and listed in Table 2, indicating the non-existence
of an inversion center around the europium ion. In addition, we
can find that the luminescent intensities of SBA15–IM+-
[Eu(TAA)4]− and SBA15–IM+[Eu(HTA)4]− are weaker
than those of SBA15–IM+[Eu(TTA)4]− and SBA15–
IM+[Eu(BTA)4]−, indicating that the TTA and BTA ligands are
the favorable sensitizers for the europium ion,26 probably
group ligand, because the D4 transition level of the terbium ion
7
5
is higher than the triplet level of most β-diketone group
ligands.28
In order to further explore the luminescence properties
of these obtained materials, the typical decay curves of the
lanthanide complex functionalized mesoporous materials were
measured by using their own maximum excitation wavelength at
room temperature. It can be expressed as an exponential
8572 | Dalton Trans., 2012, 41, 8567–8574
This journal is © The Royal Society of Chemistry 2012

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formation [In(St/S0) = −k1t = −t/τ], indicating that all the lantha-
nide ions occupy the same average coordination environment.
The resulting luminescence lifetime values are listed in Table 2.
As shown in the Table 2, the luminescence lifetimes of SBA15–
IM+[Eu(TTA)4]− and SBA15–IM+[Eu(BTA)4]− are higher than
the ones for SBA15–IM+[Eu(TAA)4]− and SBA15–IM+[Eu-
(HTA)4]−, which is consistent with the results we concluded
from their emission spectra. In addition, the luminescence life-
time of SBA15–IM+[Tb(HTA)4]− is slightly lower than SBA15–
IM+[Tb(TAA)4]−, which can be also attributed to its low adsorp-
tion in the ultraviolet region.
For the europium based materials, the emission quantum
5
efficiency (η) of the emitting D0 level can be determined on the
basis of the emission spectra together with the lifetimes and can
be calculated using the following equation29 (detailed calculation
process is given in ESI†).
Ar
η ¼
Ar þ Anr
The quantum efficiencies of the europium based materials are
shown in Table 2. It can be clearly seen that the luminescence
quantum efficiencies of SBA15–IM+[Eu(TTA)4]− (55.9%) and
SBA15–IM+[Eu(BTA)4]− (62.3%) are higher than those we pre-
viously reported for europium based mesoporous materials,30
suggesting that this method is a preferable way for constructing
europium based luminescent mesoporous materials. Meanwhile,
the relatively high quantum efficiency of these materials shows
that the SBA15 is an excellent host for the europium complexes.
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Conclusions
In general, we have successfully designed and synthesized a
series of luminescent mesoporous materials by incorporating an
ionic liquid into the SBA15 framework in the synthesis pro-
cedure followed by anion exchange, which provides a novel
method for assembling luminescent materials. All mesoporous
materials are characterized and discussed in detail. The results
demonstrate that all the resulting materials only show a slight
change in their structure and morphology. Further investigation
of the luminescence properties of these materials reveals that the
lanthanide complex ions can be effectively anchored to the
SBA15 framework through an imidazolium molecular bridge,
and preserve their excellent characteristic emission that origi-
nated from the corresponding lanthanide ion. Efforts to research
these kinds of materials modified by ionic liquid are currently
under way in our group.
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Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (20971100, 91122003) and Program for New
Century Excellent Talents in University (NCET-08-0398).
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8574 | Dalton Trans., 2012, 41, 8567–8574
This journal is © The Royal Society of Chemistry 2012