Preparation, characterization and luminescence properties of ternary europium complexes covalently bonded to titania and mesoporous SBA-15

Article abstract of DOI:10.1039/c1jm10388a

A novel multifunctional precursor Ti-MAB-S15 (MAB = meta-aminobenzoic acid) were prepared through the reaction of the carboxylic group with titanium alkoxide, and the amino group was modified with the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) and covalently bonded to mesoporous silica SBA-15. Then, novel organic-inorganic luminescent mesoporous hybrid titania materials, designated as Eu(Ti-MAB-S15)₂(NTA)₃, were obtained by introducing the Eu(NTA)₃·2H₂O (NTA = 1-(2-naphthoyl)-3,3,3-trifluoroacetonate) complex into the hybrid materials Ti-MAB-S15 via a ligand exchange reaction. FTIR, SAXRD, N₂ absorption measurements, TEM, SEM, and photoluminescent spectra were characterized, and the results reveal that it has high surface area, is uniform in the mesostructure, and has good crystallinity. In addition, compared to the pure complex Eu(NTA)₃·2H₂O, the mesoporous hybrid titania material Eu(Ti-MAB-S15)₂(NTA)₃ exhibits longer luminescent lifetime and higher quantum efficiency, which indicates that the introduction of the multifunctional ligand Ti-MAB-S15 can sensitize the luminescence emission of the Eu³⁺ ions. Moreover, the luminescent mesoporous hybrid titania materials containing terbium ions, designated as Tb(Ti-MAB-S15)₂(NTA)₃, were also prepared, and were found to emit green photoluminescence characteristic of terbium ions.

Full text of DOI:10.1039/c1jm10388a

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PAPER
Preparation, characterization and luminescence properties of ternary
europium complexes covalently bonded to titania and mesoporous SBA-15†
*
Yajuan Li and Bing Yan
Received 25th January 2011, Accepted 24th March 2011
DOI: 10.1039/c1jm10388a
A novel multifunctional precursor Ti-MAB-S15 (MAB ¼ meta-aminobenzoic acid) were prepared
through the reaction of the carboxylic group with titanium alkoxide, and the amino group was modified
with the coupling agent 3-(triethoxysilyl)-propyl isocyanate (TEPIC) and covalently bonded to
mesoporous silica SBA-15. Then, novel organic–inorganic luminescent mesoporous hybrid titania
materials, designated as Eu(Ti-MAB-S15)2(NTA)3, were obtained by introducing the Eu(NTA)3$2H2O
(NTA ¼ 1-(2-naphthoyl)-3,3,3-trifluoroacetonate) complex into the hybrid materials Ti-MAB-S15 via
a ligand exchange reaction. FTIR, SAXRD, N2 absorption measurements, TEM, SEM, and
photoluminescent spectra were characterized, and the results reveal that it has high surface area, is
uniform in the mesostructure, and has good crystallinity. In addition, compared to the pure complex Eu
(NTA)3$2H2O, the mesoporous hybrid titania material Eu(Ti-MAB-S15)2(NTA)3 exhibits longer
luminescent lifetime and higher quantum efficiency, which indicates that the introduction of the
multifunctional ligand Ti-MAB-S15 can sensitize the luminescence emission of the Eu3+ ions.
Moreover, the luminescent mesoporous hybrid titania materials containing terbium ions, designated as
Tb(Ti-MAB-S15)2(NTA)3, were also prepared, and were found to emit green photoluminescence
characteristic of terbium ions.
forces, or weak static effects) exist between the mesoporous silica
1. Introduction
materials and the lanthanide complexes. This will give little
control over the quenching effect of emitting centers, inhomo-
geneous dispersion of both phases, and leaching of the photo-
active molecules. Therefore, another appealing method in regard
to complexation of lanthanide ions using ligands that are cova-
lently bonded to the mesoporous silica network has emerged, and
the as-derived molecular-based materials exhibit a higher and
more homogeneous surface coverage of organosilane function-
alities and improved luminescent properties. In addition, among
all mesoporous materials, SBA-15 has certainly become one of
the most attractive hosts owing to its high hydrothermal stability
and the presence of hexagonally ordered large mesopores (P6mm
symmetry group) interconnected by complementary micro-
pores.7 Recently, Zhang8 and our group9 have done several
studies on the synthesis and photoluminescence properties
studies of lanthanide complexes covalently bonded to meso-
porous SBA-15. It is shown that promising visible-luminescent
properties can be obtained by linking the lanthanide complexes
to mesoporous materials.
It is well known that lanthanide complexes have a variety of
potential technological applications, such as in fluoroimmuno-
assays, optical amplification, light-conversion molecular devices
and organic light-emitting devices.1 This is mainly because they
exhibit high luminescence quantum efficiency, sharp and intense
emission lines, long lifetimes and high color purity upon ultra-
violet light irradiation, through the effective intramolecular
energy transfer from the coordinated ligands to the luminescent
central lanthanide ions (the so-called ‘‘antenna affect’’).2
However, they have so far been excluded from practical appli-
cations as optical devices mainly due to their poor thermal
stabilities and low mechanical strength. One solution is to
immobilize the complexes in stable rigid matrixes, for example,
polymer,3 liquid crystal,4 or silica-based materials.5 In the past
few years, ordered mesoporous silicas used as a support for
lanthanide complexes have attracted much attention.6 Most of
the earlier studies were mainly focused on doping mesoporous
silica materials with lanthanide complexes, in which only weak
physical interactions (typically hydrogen bonding, van der Waals
On the other hand, titania has started to attract much atten-
tion as a host matrix for lanthanide complexes because of its
peculiar and fascinating physicochemical properties and a wide
variety of potential uses in diverse fields, including solar cells,
energy conversion, environmental purification, and photo-
catalysis.10 Recently, Li and co-workers reported a novel method
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/c1jm10388a
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ꢀ
to immobilize lanthanide complexes on titania via modification
of the titanium alkoxide, and the as-derived materials exhibit
excellent photoluminescence properties.11
deionized water (7.5g) and 2 M HCl solution (30 g) at 35 C. A
mixture of MAB-Si and TEOS was added into the above solution
at 35 ꢀC with stirring for 24 h and transferred into a Teflon bottle
sealed in an autoclave, which was heated at 100 ꢀC for 48 h. Then
the solid product was filtered, washed thoroughly with deionized
Taking into account the above, it would be highly attractive to
investigate the luminescence properties of lanthanide meso-
porous complexes grafted to titania, which not only will result in
systems with interesting properties but also is a key study on the
chemical possibilities to tether metal complexes to mesoporous
silica and nonsilicate metal oxides simultaneously. To the best of
our knowledge, the research on this regard has not been reported
so far. In the present paper, we present a novel method to tether
lanthanide mesoporous materials to titania by employing meta-
aminobenzoic acid. This compound has a double function: (1)
the carboxylic acid group can react with titanium alkoxide to
moderate the reactivity towards hydrolysis and condensation; (2)
the amino group can be modified with TESPIC and covalently
bonded to mesoporous silica, and then coordinated to lanthanide
ions. In addition, in order to improve the photoluminescence
properties of the lanthanide materials, we have introduced
1-(2-naphthoyl)-3,3,3-trifluoroacetonate (NTA) to coordinate
with the lanthanide ions because the ligand NTA can act both as
an antenna to absorb and transfer energy to the metal ions and to
expel water molecules from the first coordination sphere.12
ꢀ
water, and air-dried for 12 h at 60 C. Removal of copolymer
surfactant P123 was conducted by Soxhlet extraction with ethanol
under reflux for 2 days to give the sample denoted as MAB-S15.
Synthesis of organic ligand grafted onto mesoporous silica
matrix and Ti–O network simultaneously (Ti-MAB-S15). The
mesoporous material MAB-S15 was soaked in ethanol with
stirring and an equimolar amount of Ti(OCH(CH3)2)4 was added
into the solution. The mixture was refluxing at 65 ꢀC for 3 h,
followed by filtration and extensive washing with EtOH. After
drying in a vacuum at 65 ꢀC for 10 h, the solid hybrid material Ti-
MAB-S15 was obtained.
Synthesis of the lanthanide complexes Ln(NTA)3$2H2O (Ln ¼
Eu, Tb). Ln(NTA)3$2H2O complexes were prepared for Ln ¼ Eu,
Tb, according to the published procedure as described below.13
Under stirring, 1 mmol of Ln(NO3)3$6H2O ethanol solution was
added dropwise to a solution of NTA (3 mmol) in ethanol (20 mL)
that has been neutralized ꢀwith 3 mmol sodium hydroxide. The
mixture was heated to 80 C for 4 h, then filtered, washed with
ethanol, and finally dried in a vacuum overnight. The purity of the
compounds was verified by CHN elemental analysis.
2. Experimental section
Chemicals and procedures
Reagents. Tetraethoxysilane (TEOS), 1-(2-naphthoyl)-3,3,3-
trifluoroacetonate (NTA), 3-(triethoxysilyl)-propyl isocyanate
(TEPIC), triblock copolymer poly(ethylene glycol)-block-poly
(propylene glycol)-block-poly-(ethylene glycol) (Pluronic P123,
EO20PO70EO20), meta-aminobenzoic acid (MAB), tetraiso-
propyl titanate (Ti(OCH(CH3)2)4) and tetraethoxysilane (TEOS)
were purchased from Aldrich and used without further purifi-
cation. Terbium and europium nitrate were obtained by dis-
solving their respective oxides (Tb4O7 and Eu2O3) in
concentrated nitric acid. The other chemicals were all commer-
cially available and used as received.
Synthesis of ternary hybrid materials containing Ti–O network
and mesoporous silica network (Ln(Ti-MAB-S15)2(NTA)3, Ln ¼
Eu, Tb). The Ti-MAB-S15-derived hybrid material was prepared
as follows. Firstly, the mesoporous material Ti-MAB-S15 was
ꢀ
soaked in ethanol with stirring at 65 C for 3 h, then an appro-
priate amount of Ln(NTA)3$2H2O ethanol solution (molar ratio
of Ti-MAB-S15 : Ln(NTA)3$2H2O ¼ 2 : 1) was added dropwise
into the above solution, and an appropriate amount of H2O was
added. The stirring was continued for another 24 h to yield
a precipitate, which was recovered by filtration and extensive
washing with EtOH. The resulting material Ln(Ti-MAB-
ꢀ
S15)2(NTA)3 was dried at 65 C under vacuum overnight. The
Synthesis of cross-linking precursor containing Si–O chemical
bonds (MAB-Si). A typical procedure for the preparation of the
modified precursor MAB-Si was as follows: 1.0 mmol MAB was
firstly dissolved in 20 mL of CHCl3. 1.0 mmol (0.25 g) of TEPIC
was add dropwise into the solution with stirring. The mixture
detailed synthesis process and the predicted structure of Eu(Ti-
MAB-S15)2(NTA)3 are outlined in Scheme 1.
Synthesis of binary hybrid materials containing Ti–O network
and mesoporous silica network (Eu(Ti-MAB-S15)4). The synthesis
procedure of the hybrid material Eu(Ti-MAB-S15)4 was similar
to that of Eu(Ti-MAB-S15)2(NTA)3 except that Eu
(NTA)3$2H2O ethanol solution was replaced by Eu
(NO3)3$6H2O ethanol solution, and the molar ratio of Ti-MAB-
S15 : Eu(NO3)3$6H2O ¼ 4 : 1. The predicted structure of Eu(Ti-
MAB-S15)4 was obtained and is outlined in Scheme S1.†
ꢀ
was heated at 70 C in a covered flask for approximately 10 h
under a nitrogen atmosphere. Then cold hexane was added to
precipitate the white powder. The powder was obtained by
1
filtration, purified in hexane and dried in a vacuum. H NMR
(CDCl3, 400 MHz): d 1.24 (18H, m), 3.73 (m, 12H), 0.64 (4H, t),
1.83 (4H, m), 3.82 (4H, t), 8.63 (1H, s), 6.98 (1H, d), 7.90 (1H, m),
7.75 (1H, d). Therefore, we could infer that the precursor MAB-
Si has been synthesized successfully as proved by the data.
Physical measurements
Synthesis of meta-aminobenzoic acid-functionalized SBA-15
material (MAB-S15). MAB-functionalized SBA-15 mesoporous
material was synthesized from an acidic mixture with the
following molar composition: 0.0172 P123 : 0.96 TEOS : 0.04
MAB-Si : 6 HCl : 208.33 H2O. P123 (1.0 g) was dissolved in
1H NMR spectra were recorded in CDCl3 on a BRUKER
AVANCE-400 spectrometer with tetramethylsilane (TMS) as
internal reference. FTIR spectra were measured within the 4000–
400 cmꢁ1 region on an Nicolet model 5SXC FT-IR spectropho-
tometer with the KBr pellet technique. The ultraviolet
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reaction, and the synthesis process and the predicted structure of
this hybrid are shown in Scheme 1. Generally it is very difficult to
prove the exact structure of this kind of non-crystalline hybrid
material and it is hardly possible to solve the coordination
behaviour of lanthanide ions. However, the main composition
and coordination effect according to the lanthanide coordination
chemistry principle and the configuration of the organic func-
tional groups can be predicted. The rare earth positive ions are
hard Lewis acids so they readily coordinate with hard bases
containing oxygen and nitrogen atoms. Additionally, most of the
lanthanide complexes hold polar covalent bonds as the lantha-
nide ions always bond through the 6s, 6p, and 5d electronic
orbitals, whose electron count is nine. Therefore, it is most stable
for the lanthanide complex to exist with the coordination number
of eight or nine. Furthermore, in our experiment we synthesize
the material by adding the appropriate and accurate proportion
of reagent into the system [Ti-MAB-S15 : Ln(NTA)3$2H2O ¼
2 : 1] to obtain the fixed model of lanthanide complex.
Furthermore, in view of the spatial steric hindrance effect, we
predicted the optimum coordination structure for the hybrid
material.
The selection of the ligand is the key point for the preparation
of this hybrid material. Herein, the organic ligand MAB is
selected as a multifunctional linker, which not only can be
covalently bonded to the framework of mesoporous silica and
coordinate to lanthanide ions as well as sensitize the lumines-
cence of them, but also can modify the reactivity of the titanium
precursor and introduce titania matrix to the hybrid system. The
infrared spectra of MAB (A), MAB-Si (B), MAB-S15 (C), and
Ti-MAB-S15 (D) are shown in Fig. 1. From A to B, it can be
observed that there are new bands located at 2970, 2925, and
2886 cmꢁ1, which originated from the three methylene groups of
TEPIC. In addition, the spectrum of MAB-Si is dominated by n
(C–Si, 1168 cmꢁ1) and n(C–Si, 1080 cmꢁ1) absorption bands,
characteristic of trialkoxylsilyl functions, and the band centered
at 3420 cmꢁ1 corresponds to the stretching vibration of grafted
–NH– groups and the hydroxyl groups of adsorbed water
Scheme 1 Synthesis procedure and predicted structure of the ternary
mesoporous hybrid titania materials Eu(Ti-MAB-S15)2(NTA)3.
absorption spectra were taken with an Agilent 8453 spectro-
photometer. The X-ray diffraction (XRD) measurements were
carried out using powder samples via a BRUKER D8 diffrac-
tometer (40 mA/40 kV), using monochromated Cu-Ka1 radia-
together in B. Moreover, the bending vibration (dNH, 1564 cmꢁ1
)
ꢀ
tion (l ¼ 1.54 A) in a 2q range from 0.6 to 6ꢀ. Nitrogen
adsorption/desorption isotherms were measured at the liquid
nitrogen temperature, using a Nova 1000 analyzer. The samples
were outgassed for 3 h at 150 ꢀC before the measurements.
Surface areas were calculated by the Brunauer–Emmett–Teller
(BET) method and pore size distributions were evaluated from
the desorption branches of the nitrogen isotherms using the
Barrett–Joyner–Halenda (BJH) model. The fluorescence excita-
tion and emission spectra were obtained on a SHIMADZU
RF-5301 spectrophotometer. Luminescence lifetime measure-
ments were determined on an Edinburgh Instrument FLS920
phosphorimeter using a 450 W xenon lamp as the excitation
source (pulse width, 3 ms). Scanning electronic microscopy
(SEM) was performed on a Philips XL-30. Transmission electron
microscope (TEM) experiments were conducted on a JEOL2011
microscope operated at 200 kV or on a JEM-4000EX microscope
operated at 400 kV.
ꢀ
further proves the formation of amide groups. New peaks at
1640 cmꢁ1 in B were attributed to the absorptions of the –CONH
3. Results and discussion
A novel organic-inorganic mesoporous hybrid titania materials
Fig. 1 The infrared spectra of MAB (A), MAB-Si (B), MAB-S15 (C),
Eu(Ti-MAB-S15)2(NTA)3 was prepared via a ligand exchange
and Ti-MAB-S15 (D).
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group deriving from the cross-linking reagent TEPIC, proving
that TEPIC was successfully grafted onto the ligand MAB.14 It is
worth noting that the bands at 1661 and 1556 cmꢁ1 ascribed to
the vibrations of NH–CO–NH can be also observed in Fig. 1(C),
suggesting that MAB-Si was successfully incorporated into the
mesoporous silica SBA-15. Furthermore, the formation of a Si–
O–Si framework is clearly evidenced by the broad bands located
at 1089 cmꢁ1 (nas, Si–O–Si), 803 cmꢁ1 (ns, Si–O–Si), and 462 cmꢁ1
(d, Si–O–Si). The spectrum of Ti-MAB-S15 (D) is similar to that
of C, and the characteristic absorption of Ti–O is not clearly
shown in the spectrum, which is mainly attributed to the lower
content in the material or the shielding effect of the host material.
The small-angle X-ray diffraction (SAXRD) patterns and
nitrogen adsorption/desorption isotherms are popular and effi-
cient methods to characterize highly ordered mesoporous mate-
rial with hexagonal symmetry of the space group p6mm. Fig. 2
presents the SAXRD patterns of a pure SBA-15 mesoporous
silica (A), Eu(Ti-MAB-S15)4 (B), Eu(Ti-MAB-S15)2(NTA)3 (C),
and Tb(Ti-MAB-S15)2(NTA)3 (D). The pure SBA-15 meso-
porous silica exhibits three characteristic reflections in the 2q
range of 0.6–6ꢀ, which are indexed as (100), (110) and (200)
diffractions of the hexagonal mesostructure. As shown in Fig. 2,
the Bragg peaks indexed as (100) are observed in the lanthanide
hybrid material (B–D), which suggests the ordered hexagonal
mesoporous structure of SBA-15 are substantially conserved
after the introduction of the lanthanide complex. However, it is
worth noting that the intensities of these hybrid material Bragg
peaks are reduced to the extent that the (110) and (200) reflection
are either absent or very weak. This can be attributed to the
decrease of structure ordering degree which might be due to
the reduction of scattering contrast between the channel walls of
the matrices and the covalently bonded lanthanide complexes in
the mesoporous materials.15 Meanwhile, the hexagonal meso-
structure of Eu(Ti-MAB-S15)2(NTA)3 is further confirmed by
TEM micrographs (see Fig. 3). As shown in the figure, the hybrid
material Eu(Ti-MAB-S15)2(NTA)3 exhibits a regular hexagonal
array of uniform channels, which suggests that the mesostructure
of the resulting material can substantially be conserved after the
modification with Ti–O network and complexation process,
Fig. 3 TEM images of Eu(Ti-MAB-S15)2(NTA)3 recorded along the
[100] (A) and [110] (B) zone axes.
coincident with the results of XRD patterns. The distance
between the centers of the mesopores is estimated to be around
11 nm, which is in good agreement with the value determined
from the corresponding XRD analysis (see Table 1).
The N2 adsorption–desorption isotherms for pure SBA-15 (A),
Eu(Ti-MAB-S15)4 (B), Eu(Ti-MAB-S15)2(NTA)3 (C), and Tb
(Ti-MAB-S15)2(NTA)3 (D) are shown in Fig. 4. They all display
Type IV isotherms with H1-type hysteresis loops at high relative
pressure according to the IUPAC classification,16 characteristic
of mesoporous materials with highly uniform size distributions.
The specific area and the pore size have been calculated by using
Brunauer–Emmett–Teller (BET) and Barrett–Joyner–Halenda
(BJH) methods, respectively. The structure data of all these
mesoporous materials (BET surface area, total pore volume,
pore size, etc.) are summarized in Table 1. It can be clearly seen
that pure SBA-15 has a high BET surface area (1197 m2 gꢁ1),
a large pore volume (1.81 cm3 gꢁ1), and a BJH pore diameter of
7.39 nm, indicative of its potential application as a host in
luminescent materials. It is obvious that the surface areas, pore
volumes and pore diameters of the lanthanide hybrid materials
decrease after immobilization of lanthanide complexes into SBA-
15, suggesting that the complexes are grafted inside the
mesopores.15
A scanning electron micrograph of mesoporous hybrid mate-
rial Eu(Ti-MAB-S15)4(NTA)3 is shown in Fig. 5. It can be
observed that the sample consists of relatively uniform globe-like
particles. It demonstrates that a homogeneous, molecular-based
material was obtained because of strong covalent bonds between
the organic ligand MAB and the inorganic mesoporous silica and
titania matrix, and the coordinate bonds between organic ligand
b-diketone or mesoporous hybrid material containing Ti–O
network (Ti-MAB-S15) and rare earth ions, which belong to
a complicated huge molecular system in nature. Compared with
the hybrid materials with doped lanthanide complexes generally
experiencing phase separation phenomena, in this paper, the
inorganic and organic phases can exhibit their distinct properties
together in the hybrid materials we obtained containing covalent
bonds.
The fluorescence excitation and emission spectra of the
resulting ternary europium mesoporous hybrid titania material
Eu(Ti-MAB-S15)2(NTA)3, binary europium mesoporous hybrid
titania material Eu(Ti-MAB-S15)4, and pure complex Eu(NTA)
3$2H2O, are displayed in Fig. 6a and b, respectively (A for Eu
(NTA)3$2H2O, B for Eu(Ti-MAB-S15)2(NTA)3, and C for Eu
(Ti-MAB-S15)4). The normalized excitation spectra of these
Fig. 2 SAXRD patterns of pure SBA-15 mesoporous silica (A), Eu(Ti-
MAB-S15)4 (B), Eu(Ti-MAB-S15)2(NTA)3 (C), and Tb(Ti-MAB-
S15)2(NTA)3 (D).
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a
Table 1 Structural parameters of SBA-15, Eu(Ti-MAB-S15)4, Eu(Ti-MAB-S15)2(NTA)3, and Eu(Ti-MAB-S15)2(NTA)3
Sample
d100/nm
a0/nm
SBET/m2 gꢁ1
V/cm3 gꢁ1
D/nm
SBA-15
10.76
10.90
10.64
10.64
12.42
11.71
11.82
11.82
1197
572
546
597
1.81
0.97
0.92
0.94
7.39
5.68
5.56
5.30
Eu(Ti-MAB-S15)4
Eu(Ti-MAB-S15)2(NTA)3
Tb(Ti-MAB-S15)2(NTA)3
a
d100 is the d(100) spacing, a0 the cell parameter (a0 ¼ 2 d100/O3), SBET the BET surface area, V the total pore volume, DBJH the average pore diameter.
mesoporous hybrid material.18 For europium binary mesoporous
hybrid Eu(Ti-MAB-S15)4, the excitation is dominated by
a broad band and a narrow peak centered at 393 nm, the former
band is attributed to the absorption of the organic ligand and the
latter narrow peak is ascribed to the absorption transition
5
(7F0 / L6) of the f–f transition of Eu3+. The f–f transition is
weaker than the absorption of the organic ligand, which indicates
that luminescence sensitization via the excitation of the ligand is
much more efficient than the direct excitation of the Eu3+ ion
absorption level. For europium ternary mesoporous hybrid Eu
(Ti-MAB-S15)2(NTA)3, the non-observation of Eu3+ intra-4f6
lines indicates that the energy transfer from the ligands to
Fig. 4 N2 adsorption–desorption isotherms for pure SBA-15 meso-
porous silica (A), Eu(Ti-MAB-S15)4 (B), Eu(Ti-MAB-S15)2(NTA)3 (C),
and Tb(Ti-MAB-S15)2(NTA)3 (D).
Fig. 5 SEM image of mesoporous ternary hybrid titania material Eu(Ti-
MAB-S15)2(NTA)3.
5
7
materials were obtained by monitoring at the D0 / F2 emis-
sion lines (614 nm). As shown in Fig. 6a, the excitation spectrum
of the pure Eu(NTA)3$2H2O complex exhibits a broad excitation
band between 220 and 450 nm (lmax ¼ 380 nm), which can be
assigned to the p–p* states of the organic ligand.17 Compared
with the pure complex Eu(NTA)3$2H2O, the excitation band of
the mesoporous hybrid material Eu(Ti-MAB-S15)2(NTA)3
becomes narrower and the maximum excitation wavelength
shifted from 380 nm to 342 nm. The blue shift of the excitation
bands upon introduction of organic ligands with Ti–O network
and mesoporous silica into the pure complex is due to a hyp-
sochromic effect resulting from the change in the polarity of the
environment surrounding the europium complex in the
Fig. 6 Room temperature excitation (a) and emission (b) spectra of (A)
pure complex Eu(NTA)3$2H2O, (B) ternary europium mesoporous
hybrid titania material Eu(Ti-MAB-S15)2(NTA)3, and (C) binary euro-
pium mesoporous hybrid titania material (Ti-MAB-S15)4. All spectra are
normalized to a constant intensity at the maximum.
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Eu3+ ion is more efficient in Eu(Ti-MAB-S15)2(NTA)3 than in
the binary hybrid Eu(Ti-MAB-S15)4.
Fig. 6b shows the normalized emission spectra for the obtained
Eu3+-containing materials as solids at room temperature. They
all reveal the characteristic emission bands of Eu3+ ion centered
at about 577, 589, 612, 650 and 696 nm, corresponding to the 5D0
7
/ FJ transitions (J ¼ 0, 1, 2, 3 and 4, respectively). Among
5
7
these transitions, the D0 / F2 transition at about 612 nm
shows the most prominent emission. It is well known that 5D0 /
7F2 belongs to a typical electric dipolar transition and strongly
varies with the local symmetry of Eu3+ ions, whereas the 5D0 /
7F1 transition is a parity-allowed magnetic dipolar transition,
which is independent of the host material. Therefore, the relative
intensity ratio (RI) of 5D0 / 7F2 to5D0 / 7F1 is sensitive to the
symmetry around the Eu3+ ion and gives valuable information
about the chemical microenvironment change of anions coordi-
nating the Eu3+ ion.19 The RI values for three kinds of materials
are listed in Table 2. By comparison, it can be observed that the
RI value of mesoporous hybrid Eu(Ti-MAB-S15)2 (NTA)3 (7.8)
is much higher than that of Eu(NTA)3$2H2O (5.4), which
suggests the introduction of organic ligand Ti-MAB-S15 into the
complex may make the material have high polarity, which may
affect the centrosymmetric environment of the Eu3+ ions, and
thus influence the RI value.
Fig. 7 Luminescence decay curve of the ternary mesoporous hybrid
titania material Eu(Ti-MAB-S15)2(NTA)3.
nonradiative and radiative processes are essentially involved in
5
the depopulation of the D0 state, h can be expressed as:20
h ¼ Ar/(Ar + Anr)
(1)
Here, Ar and Anr are radiative and nonradiative transition rates,
respectively. Ar can be obtained by summing over the radiative
The luminescence lifetimes and quantum yields, which are two
important parameters for the estimation of the efficiency of the
emission process of the complexes, have also been determined.
The decay lifetime values of 5D0 excited states were measured at
room temperature under the excitation wavelength that maxi-
mizes the emission intensity and monitored by the most intense
emission line at 613 nm. The luminescence decay curve of the
ternary mesoporous hybrid titania material Eu(Ti-MAB-
S15)2(NTA)3 is shown in Fig. 7, and the decay curves of materials
Eu(NTA)3$2H2O and Eu(Ti-MAB-S15)4 are also given in the
ESI (Fig. S1(A) and (B)†). The lifetime profiles for three samples
are fitted with single exponentials, demonstrating that all the
Eu3+ ions are located in the same local environment in the
obtained hybrid materials. The resulting lifetimes (shown in
Table 2) are on the same order of magnitude for all samples. On
the basis of the emission spectra and lifetimes of the 5D0 emitting
5
7
5
rates A0J for each D0 / FJ (J ¼ 0–4) transitions (the D0 /
7F5,6 branching ratios are neglected due to their poor relative
intensity with respect to that of the remaining 5D0 / 7F0–4 lines).
5
7
The D0 / F1 transition does not depend on the local ligand
field and thus may be used as a reference for the whole spectrum.
An effective refractive index of 1.5 was used leading to
A01 z 50 sꢁ1 21
, where A01 stands for the Einstein’s coefficient of
5
7
spontaneous emission between the D0 and the F1 Stark levels.
The lifetime, radiative (Ar), and nonradiative (Anr) transition
rates are related through the following equation:
sexp ¼ (Ar + Anr)ꢁ1
(2)
On the basis of the above discussion, the parameters Ar, Anr and
5
the quantum efficiency values, h, for the D0 Eu3+ ion excited
state in the three materials can be obtained, as shown in Table 2.
According to the relationship between the quantum efficiency
radiative and nonradiative transition rates, it can be found the
5
level, the emission quantum efficiency (h) of the D0 europium
ion excited state can be determined. Assuming that only
Table 2 Photoluminescence data of Eu(NTA)3$2H2O, Eu(Ti-MAB-S15)2(NTA)3, and Eu(Ti-MAB-S15)4 as solidsa
Eu(NTA)3$2H2O
Eu(Ti-MAB-S15)2(NTA)3
Eu(Ti-MAB-S15)4
R
5.4
0.353 ꢂ 0.004
7.8
4.7
0.306 ꢂ 0.004
t (ms)c
0.464 ꢂ 0.002
2155
531
1624
24.6
1.4
tꢁ1(sꢁ1
Ar (sꢁ1
Anr (sꢁ1
h (%)
nw
)
2833
360
3268
328
)
)
2473
12.7
2.3
8.9
0.31
2940
10.0
2.9
7.7
0.31
U2 (ꢃ10ꢁ20 cm2)
12.6
0.70
U4 (ꢃ10ꢁ20 cm2)
a
RI, intensity ratios of 5D0 / 7F2 to 5D0 / 7F1; t, decay time; Ar, radiative decay rates; Anr, nonradiative decay rates; h, the emission quantum efficiency
of the 5D0 Eu3+ excited state calculated from the decay time and the emission intensities; nw, the number of water molecules coordinated to the
Eu3+ ion; Ul, the experimental intensity parameters.
8134 | J. Mater. Chem., 2011, 21, 8129–8136
This journal is ª The Royal Society of Chemistry 2011

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value h mainly depends on the values of two factors: one is
lifetime and the other is relative intensity ratio R (I02/I01). As we
can see from Table 2, hybrid material Eu(Ti-MAB-S15)2(NTA)3
exhibits longer lifetime and higher R value than that of pure
complex Eu(NTA)3$2H2O, so the quantum efficiency of hybrid
Eu(Ti-MAB-S15)2(NTA)3 (24.6%) is much higher than that of
Eu(NTA)3$2H2O (12.7%). This is mainly because the carbonyl
oxygen atoms located in the organic ligand Ti-MAB-S15 coor-
dinated to Eu3+ ions could replace the water molecules from the
first coordination sphere of europium ions, the energy loss and
clustering of the emitting centers caused by the vibration of the
hydroxyl groups of coordinated water molecules could be avoi-
ded. It is also worth noting that compared with binary meso-
porous hybrid Eu(Ti-MAB-S15)4 (10.0%), the quantum
efficiency of ternary mesoporous hybrid Eu(Ti-MAB-
S15)2(NTA)3 (24.6%) exhibits obvious enhancement, which is the
main reason why the presence of b-diketonate ligands can shield
the europium ions from water molecules and can help to absorb
the excitation energy and to transfer it to the central metal ions.22
In order to elucidate the negative influence of vibration caused
by the water molecules and further study the coordination
environment surrounding the rare earth ions in the mesoporous
hybrid materials, we further selectively estimated the number of
coordination molecules for Eu hybrid material systems. The
number of water molecules coordinated to the Eu3+ ions (nw) can
be determined with the empirical formula reported by Supkowski
and Horrocks:23
intensity parameters (Ul, l ¼ 2, 4, 6) can be calculated using the
following formula:25,26
AED(J / J0) ¼ [(64p4e2y3)/3h(2J + 1)]{[n(n2 + 2)2/9]
SUl|<FJkU(l)kF0J0>|2
(5)
where |<FJkU(l)kF0J0>|2 are the RMEs of the unit tensor. The
5D0 / 7F6 transition could not be experimentally detected and it
is not necessary to determine its experimental intensity parame-
ters. According to the emission spectra (Fig. 6), the experimental
intensity parameters U2, U4 were determined and are listed in
Table 2. It can be observed that the ternary hybrid mesoporous
titania material Eu(Ti-MAB-S15)3(NTA)2 possesses a relative
high value of the U2 intensity parameter, which may be inter-
preted as a consequence of the hypersensitive behavior of the 5D0
7
/ F2 transition. The dynamic coupling mechanism is, there-
fore, dominant, indicating that the Eu3+ ion is in a highly
polarizable chemical environment and suggesting an improve-
ment of the luminescence in Eu(Ti-MAB-S15)3(NTA)2 when
compared with that exhibited by the Eu(NTA)3$2H2O complex.8
Furthermore, the europium(III) ions can be replaced in this
system by other lanthanide ions that show luminescence with
different colors. The hybrid material Tb(Ti-MAB-S15)2(NTA)3,
which shows green photoluminescence upon radiation with
ultraviolet light (254 nm and 365 nm), was also obtained. The
normalized excitation and emission spectra of the hybrid Tb(Ti-
MAB-S15)2(NTA)3 are shown in Fig. 8. The excitation spectrum
of the material was obtained by monitoring the emission of the
Tb3+ ions at 545 nm. The spectrum presents a broad band ranging
from 250 nm to 400 nm with the maximum intensity at 350 nm,
which is due to the characteristic absorption of the organic
ligands, and no f–f transitions could be observed. This indicates
that an effective energy transfer occurs from the ligands to the
central Tb3+ ions. Excitation at 350 nm provides the typical
luminescence of Tb3+ ion at 488, 543, 583, and 620 nm, corre-
sponding to 5D4 / 7FJ transitions (J ¼ 6, 5, 4 and 3, respectively).
The typical green color of terbium emission is mostly ascribed to
the strongest transition (5D4 / 7F5) centered at 545 nm.
nw ¼ 1.11 ꢃ (sꢁ1 ꢁ Ar ꢁ 0.31)
(3)
All the calculated results are shown in Table 2. Based on the
results, the coordination numbers of water molecules in materials
Eu(NTA)3$2H2O, Eu(Ti-MAB-S15)4, and Eu(Ti-MAB-S15)2
(NTA)3 are 2.3, 1.4, and 2.9, respectively. The water molecules
produce the severe vibration of hydroxyl group, resulting in the
large non-radiative transition and decreasing the luminescent
efficiency.
Moreover, the experimental intensity parameters (Ul, l ¼ 2
and 4) for the europium material can be calculated by the method
originally proposed by Krupke24 and subsequently used by
others.25 This method takes advantage of the fact that the
emission intensities of the 5D0 / 7F2, 5D0 / 7F4, and 5D0 / 7F6
transitions are solely dependent on the U2, U4, and U6 parame-
5
7
ters, respectively. The radiative transition rate of D0 / F1,
which possesses solely magnetic dipole character and is insensi-
tive to the site symmetry, is used as a reference to scale the
5
7
absolute ED transition rates of D0 / FJ (J ¼ 2, 4, 6) by the
equation:25
AMD(J / J0) ¼ (64p4e2n3y3)/[3h(2J + 1)](e-/2mc)2|<FJkL
+ 2SkF0J0>|2
(4)
where e is electronic charge; n is the effective index of refraction,
as will be mentioned in the luminescence quantum yield section
(n ¼ 1.5); y is the average transition energy in cmꢁ1; |<FJkL +
2SkF0J0>|2 are the reduced matrix elements (RME) of the MD
operator, which were calculated based on the intermediate-
coupling wave functions. Once the ED radiative transition rates
Fig. 8 Room temperature excitation spectra (left) and emission spectra
of ternary terbium mesoporous hybrid titania material Tb(Ti-MAB-
S15)2(NTA)3 (right). Both spectra are normalized to a constant intensity
at the maximum.
5
7
of D0 / FJ (J ¼ 2, 4, 6) are determined, the experimental
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 8129–8136 | 8135

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A. Popitsch, B. F. Chmelka and G. D. Stucky, Chem. Commun.,
2002, 2474–2475.
4. Conclusions
7 D. Y. Zhao, J. Y. Sun, Q. Z. Li and G. D. Stucky, Chem. Mater.,
2000, 12, 275–279; D. Y. Zhao, Q. S. Huo, J. P. Feng, N. Melosh,
G. H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science, 1998,
279, 548–552; J. Feng, H. J. Zhang, C. Y. Peng, J. B. Yu,
R. P. Deng, L. N. Sun and X. M. Guo, Microporous Mesoporous
Mater., 2008, 113, 402–410.
8 C. Y. Peng, H. J. Zhang, J. B. Yu, Q. G. Meng, L. S. Fu, H. R. Li,
L. N. Sun and X. M. Guo, J. Phys. Chem. B, 2005, 109, 15278–
15287; L. S. Li, Y. Zhang, J. B. Yu, C. Y. Peng and H. J. Zhang, J.
Photochem. Photobiol., A, 2008, 199, 57–63; C. Y. Peng,
H. J. Zhang, Q. G. Meng, H. R. Li, J. B. Yu, J. F. Guo and
L. N. Sun, Inorg. Chem. Commun., 2005, 8, 440–443.
9 Y. Li, B. Yan and H. Yang, J. Phys. Chem. C, 2008, 112, 3959–3968;
B. Yan and Y. Li, Dalton Trans., 2010, 39, 1480–1487; Y. J. Li and
B. Yan, Inorg. Chem., 2009, 48, 8276–8285; L. L. Kong, B. Yan and
Y. Li, J. Solid State Chem., 2009, 182, 1631–1637; L. L. Kong,
B. Yan and Y. Li, J. Alloys Compd., 2009, 481, 549–554; Y. J. Li,
B. Yan and Y. Li, Chem.–Asian J., 2010, 5, 1642–1651; Y. J. Li,
B. Yan and Y. Li, Microporous Mesoporous Mater., 2010, 131, 82–
88; Y. J. Li, L. Wang and B. Yan, J. Mater. Chem., 2011, 21, 1130–
1138.
In summary, ternary europium luminescent mesoporous hybrid
titania material Eu(Ti-MAB-S15)2(NTA)3 was successfully
prepared by linking lanthanide complexes Eu(NTA)3$2H2O
complex to the mesoporous hybrid titania material Ti-MAB-S15
via a ligand exchange reaction, which provides a representative
method for assembling luminescent lanthanide molecular-based
hybrid materials containing ordered mesoporous Si–O network
and amorphous Ti–O network simultaneously. The obtained
material preserves the ordered mesoporous structures and shows
highly uniform pore size distributions. Further investigation on
the luminescence properties confirm that the europium meso-
porous hybrid titania material Eu(Ti-MAB-S15)2(NTA)3
5
exhibits higher D0 luminescence quantum efficiency and longer
lifetime than the pure Eu(NTA)3$2H2O complex and binary
mesoporous hybrid titania material Eu(Ti-MAB-S15)4. In addi-
tion, this reaction principle is not limited to europium
compounds but can be extended to other lanthanide compounds
that emit in the visible and near-infrared regions, thus opening
a door for the development of new materials. The excellent
luminescent properties of this kind of material, together with the
highly ordered mesoporous structures containing Ti–O network
will expand their applications in optical or electronic areas.
10 C. Sanchez, B. Julian, P. Belleville and M. Popall, J. Mater. Chem.,
2005, 15, 3559–3592; P. Wang, C. Klein, R. Humphry,
€
S. Zakeeruddin and M. Gratzel, J. Am. Chem. Soc., 2005, 127, 808–
809.
11 P. Liu, H. R. Li, Y. G. Wang, B. Y. Liu, W. J. Zhang, Y. J. Wang,
W. D. Yan, H. J. Zhang and U. Schubert, J. Mater. Chem., 2008,
18, 735–737; H. R. Li, P. Liu, Y. G. Wang, L. Zhang, J. B. Yu,
H. J. Zhang, B. Y. Liu and U. Schubert, J. Phys. Chem. C, 2009,
113, 3945–3949.
€
12 K. Binnemans, P. Lenaerts, K. Driesen and C. Gorller-Walrand, J.
Mater. Chem., 2004, 14, 191–195.
Acknowledgements
13 J. B. Yu, H. J. Zhang, L. S. Fu, R. P. Deng, L. Zhou, H. R. Li,
F. Y. Liu and H. L. Fu, Inorg. Chem. Commun., 2003, 6, 852–854.
14 Q. M. Wang and B. Yan, J. Mater. Chem., 2004, 14, 2450–2454.
15 H. P. Wang, Y. F. Ma, H. Tian, N. Tang, W. S. Liu, Q. Wang and
Y. Tang, Dalton Trans., 2010, 39, 7485–7492.
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) and Developing
Science Funds of Tongji University.
16 M. Kruk and M. Jaroniec, Chem. Mater., 2001, 13, 3169–3183;
W. H. Zhang, X. B. Lu, J. H. Xiu, Z. L. Hua, L. X. Zhang,
M. Robertson, J. L. Shi, D. S. Yan and J. D. Holmes, Adv. Funct.
Mater., 2004, 14, 544–552.
References
1 M. D. McGehee, T. Bergstedt, C. Zhang, A. P. Saab, M. B. O’Regan,
G. C. Bazan, V. I. Srdanov and A. J. Heeger, Adv. Mater., 1999, 11,
1349–1354; J. M. Lehn, Angew. Chem., 1990, 102, 1347–1362;
J. M. Lehn, Angew. Chem., Int. Ed. Engl., 1990, 29, 1304–1319;
C. Koeppen, S. Yamada, G. Jiang, A. F. Garito and L. R. Dalton,
J. Opt. Soc. Am. B, 1997, 14, 155–162; X. M. Guo, H. D. Guo,
L. S. Fu, R. P. Deng, W. Chen, J. Feng, S. Dang and H. J. Zhang,
J. Phys. Chem. C, 2009, 113, 2603–2610.
17 H. R. Li, J. Lin, H. J. Zhang, L. S. Fu, Q. G. Meng and S. B. Wang,
Chem. Mater., 2002, 14, 3651–2655; P. C. R. Soares-Santos,
ꢁ
ꢁ
H. I. S. Nogueira, V. Felix, M. G. B. Drew, R. A. Sa Ferreira,
L. D. Carlos and T. Trindade, Chem. Mater., 2003, 15, 100–108.
18 L. N. Sun, H. J. Zhang, C. Y. Peng, J. B. Yu, Q. G. Meng, L. S. Fu,
F. Y. Liu and X. M. Guo, J. Phys. Chem. B, 2006, 110, 7249–7258.
€
19 E. Guillet, D. Imbert, R. Scopelliti and J. C. G. Bunzli, Chem. Mater.,
2004, 16, 4063–4070; F. Renaud, C. Piguet, G. Bernardinelli,
€
2 D. J. Zhang, X. M. Wang, Z. A. Qiao, D. H. Tang, Y. L. Liu and
J. C. G. Bunzli and G. Hopfgartner, Chem.–Eur. J., 1997, 3, 1660–
ꢁ
Q. S. Huo, J. Phys. Chem. C, 2010, 114, 12505–12510; G. F. DeSa,
O. L. Malta, C. De Mello Donega, A. M. Simas, R. L. Longo,
1667; W. Q. Fan, J. Feng, S. Y. Song, Y. Q. Lei, G. L. Zheng and
H. J. Zhang, Chem.–Eur. J., 2010, 16, 1903–1910.
ꢁ
P. A. Santa-Cruz and E. F. da Silva Jr., Coord. Chem. Rev., 2000,
196, 165–195; N. Sabbatini, M. Guardingli and J. M. Lehn, Coord.
Chem. Rev., 1993, 123, 201–228; M. Q. Tan, Z. Q. Ye, G. L. Wang
and J. L. Yuan, Chem. Mater., 2004, 16, 2494–2498; P. Escribano,
20 M. H. V. Werts, R. T. F. Jukes and J. W. Verhoeven, Phys. Chem.
Chem. Phys., 2002, 4, 1542–1548; E. E. S. Teotonio,
J. G. P. Espinola, H. F. Brito, O. L. Malta, S. F. Oliveria,
D. L. A. de Faria and C. M. S. Izumi, Polyhedron, 2002, 21, 1837–
1844; L. D. Carlos, V. D. Bermudez, R. A. S. Ferreira, L. Marques
and M. Assuncao, Chem. Mater., 1999, 11, 581–588; L. N. Sun,
H. J. Zhang, C. Y. Peng, J. B. Yu, Q. G. Meng, L. S. Fu, F. Y. Liu
and X. M. Guo, J. Phys. Chem. B, 2006, 110, 7249–7258.
21 J. C. Boyer, F. Vetrone, J. A. Capobianco, A. Speghini and
M. Bettinelli, J. Phys. Chem. B, 2004, 108, 20137–20144.
22 H. R. Li, P. Liu, Y. G. Wang, L. Zhang, J. B. Yu, H. J. Zhang,
B. Y. Liu and U. Schubert, J. Phys. Chem. C, 2009, 113, 3945–3949.
23 R. M. Supkowski and W. D. Horrocks, Inorg. Chim. Acta, 2002, 340,
44–48.
ꢁ
ꢁ
ꢁ
B. Julian-Lopez, J. Planelles-Arago, E. Cordoncillo, B. Viana and
C. Sanchez, J. Mater. Chem., 2008, 18, 23–40.
3 L. H. Wang, W. Wang, W. G. Zhang, E. T. Kang and W. Huang,
Chem. Mater., 2000, 12, 2212–2218; L. D. Carlos and
A. L. L. Videira, Phys. Rev. B: Condens. Matter, 1994, 49, 11721–
11728.
ꢁ
4 R. Van Deun, D. Moors, B. De Fre and K. Binnemans, J. Mater.
Chem., 2003, 13, 1520–1522.
5 D. Sendor and U. Kynast, Adv. Mater., 2002, 14, 1570–1574;
H. Maas, A. Currao and G. Calzaferri, Angew. Chem., Int. Ed.,
2002, 41, 2495–2497; P. Minoofar, R. Hernandez, S. Chia,
B. Dunn, J. I. Zink and A. C. Franville, J. Am. Chem. Soc., 2002,
124, 14388–14396.
24 W. F. Krupke, Phys. Rev., 1966, 145, 325–327.
25 Y. S. Liu, W. Q. Luo, R. F. Li, G. K. Liu, M. R. Antonio and
X. Y. Chen, J. Phys. Chem. C, 2008, 112, 686–694.
6 Q. H. Xu, L. S. Li, X. S. Liu and R. R. Xu, Chem. Mater., 2002, 14,
549–555; M. H. Bartl, B. J. Scott, H. C. Huang, G. Wirnsberger,
26 Q. B. Xiao, Y. S. Liu, L. Q. Liu, R. F. Li, W. Q. Luo and X. Y. Chen,
J. Phys. Chem. C, 2010, 114, 9314–932.
8136 | J. Mater. Chem., 2011, 21, 8129–8136
This journal is ª The Royal Society of Chemistry 2011