Luminescent lanthanide (Eu3+, Tb3+) hybrids with 4-vinylbenzeneboronic acid functionalized Si-O bridges and beta-diketones

Article abstract of DOI:10.1111/j.1751-1097.2011.00928.x

4-Vinylphenylboronic acid ligand (VPBA) is functionalized with two crosslinking reagents (3-(triethoxysilyl)-propylisocyanate [TEPIC] and 3-(trimethoxysilyl) propyl methacrylate [TMPMA]) to achieve the two special molecular bridge VPBA-TEPIC and VPBA-TMPMA. Meanwhile, beta-diketone ligands (2-thenoyltrifluoroacetone [TTA], acetyl acetone [ACAC]) as the second ligands play the role of the main energy donor, which absorb abundant energy in ultraviolet-visible extent and then transfer the energy to the corresponding lanthanide ions (Eu³⁺_, Tb³⁺) to sensitize their emission of them. Eight binary and ternary Eu³⁺, Tb³⁺ hybrids with VPBA-TEPIC (VPBA-TMPMA) and TTA (ACAC) have been constructed, whose photoluminescence properties are studied in depth and suggest that the ternary hybrids show the favorable characteristic luminescent properties (longer lifetime and higher quantum efficiency). Eight binary and ternary lanthanide (Eu³⁺, Tb³⁺) hybrids with two linkages VPBA-TEPIC (VPBA-TMPMA) and beta-diketones (TTA, ACAC) have been constructed, whose photoluminescence properties are studied deeply and suggest that the ternary hybrids show favorable characteristic luminescent properties (longer lifetime and higher quantum efficiency).

Full text of DOI:10.1111/j.1751-1097.2011.00928.x

Photochemistry and Photobiology, 2011, 87: 786–794
Luminescent Lanthanide (Eu3+, Tb3+) Hybrids with 4-Vinylbenzeneboronic
Acid Functionalized Si-O Bridges and Beta-Diketones
Bing Yan*, Min Guo and Xiao-Fei Qiao
Department of Chemistry, Tongji University, Shanghai, China
Received 27 February 2011, accepted 28 March 2011, DOI: 10.1111/j.1751-1097.2011.00928.x
However, concerning the technological applicability, there
exist somewhat low thermal and photochemical stability,
ABSTRACT
4-Vinylphenylboronic acid ligand (VPBA) is functionalized with
two crosslinking reagents (3-(triethoxysilyl)-propylisocyanate
[TEPIC] and 3-(trimethoxysilyl) propyl methacrylate
[TMPMA]) to achieve the two special molecular bridge
VPBA-TEPIC and VPBA-TMPMA. Meanwhile, beta-diketone
ligands (2-thenoyltrifluoroacetone [TTA], acetyl acetone
[ACAC]) as the second ligands play the role of the main energy
donor, which absorb abundant energy in ultraviolet–visible
extent and then transfer the energy to the corresponding
together with the poor mechanical properties of lanthanide
complexes that represent some disadvantages. In order to
overcome these drawbacks, lanthanide complexes have been
encapsulated into polymers (11–13), liquid crystals (14,15) and
sol–gel-derived organic–inorganic hybrids (mostly siloxane-
based ones) (16–18). Organic–inorganic hybrid materials have
been developed in the past two decades affording materials with
desired properties, for they combine some advantages of
organic compounds (easy processing with conventional tech-
niques, elasticity and organic functionalities) with properties of
inorganic oxides (hardness, thermal and chemical stability,
transparency) and thus have attracted considerable attention
(19,20). These kinds of hybrids can be classified according to the
interaction established between organic and inorganic compo-
nents: the physically doping hybrids and chemically bonding
one (21). Carlos et al. and Binnemans both give overview of the
different types of lanthanide-based hybrid materials and
compared their respective advantages and disadvantages
(22,23). Our research team has presently done extensive work
on covalently grafting the ligands to the inorganic networks in
which the luminescent centers of lanthanide complexes are
bonded with a siloxane matrix through Si-O linkage and we
have realized six paths to construct the functional silylated
precursors. In addition, after the modification, we assembled
the above modified bridge ligands with lanthanide ions and
tetraethoxysilane (TEOS) to compose hybrid systems with
covalent bonds and obtained a series of stable and efficient
molecular hybrid materials in optical areas (24–31).
The special covalent linkage, named as functional molecular
bridge, is the key to construct these kinds of organometallic
hybrid systems. Based on the former work, 4-vinylphenylbo-
ronic acid (VPBA) containing a hydroxyl group and a carboxyl
group (32) can realize the hydrogen-transfer nucleophilic
addition reaction through the hydroxyl group with 3-(trieth-
oxysilyl) propyl isocyanate (TEPIC) and 3-(trimethoxysilyl)
propyl methacrylate (TMPMA) to achieve two kinds of
molecular bridge (VPBA-TEPIC and VPBA-TMPMA) and
coordinate to lanthanide ions (Eu3+ and Tb3+) through
carboxyl groups. Under this consideration, we construct eight
chemically binary and ternary bonded lanthanide organome-
tallic hybrids (RE-VPBA-RSi and L-RE-VPBA-RSi: RE = Eu
or Tb, RSi = TEPIC, TMPMA, L = TTA or ACAC), whose
physical characterization and photophysical properties are
reported in detail.
lanthanide ions (Eu3+ Tb3+) to sensitize their emission of
,
them. Eight binary and ternary Eu3+, Tb3+ hybrids with VPBA-
TEPIC (VPBA-TMPMA) and TTA (ACAC) have been con-
structed, whose photoluminescence properties are studied in
depth and suggest that the ternary hybrids show the favorable
characteristic luminescent properties (longer lifetime and higher
quantum efficiency).
INTRODUCTION
Due to their unique photophysical properties that aid in
shielding 4f electrons from interactions with their surround-
ings by the filled 5s2 and 5p6 orbital, lanthanide ions have
been well known as important components in phosphors,
lasers and optical amplifiers (1–3). However, the direct Ln3+
photoexcitation is not very efficient, with the low molar
absorption coefficients limiting the light output. Some
organic ligands such as aromatic carboxylic acids etc. are
well known to be efficient sensitizers for the luminescence of
lanthanide ions, whose organic chromophores typically pres-
ent effective absorption and a much broader spectral range
than the corresponding Ln3+ ions can absorb energy to be
transferred to nearby Ln3+ ions by an effective intramolec-
ular energy transfer process. This process is called lanthanide
luminescence sensitization or antenna effect (4–7). These
chelates possessing the effective emission in the near-UV,
visible and NIR spectral regions are of great interest for a
wide range of optical applications, such as tunable lasers,
amplifiers for optical communications, components of the
emitter layers in multilayer organic light-emitting diodes
(OLEDs), light concentrators for photovoltaic devices and so
on (8–10).
*Corresponding author email: byan@tongji.edu.cn (Bing Yan)
ꢀ 2011 The Authors
Photochemistry and Photobiology ꢀ 2011 The American Society of Photobiology 0031-8655/11
786

Photochemistry and Photobiology, 2011, 87 787
Physical measurements. Infrared spectroscopy (FT-IR) were ob-
tained in KBr pellets and recorded on a Nexus 912 AO446 FT-IR
spectrophotometer in the range of 4000–400 cm)1. Ultraviolet absorp-
tion spectra of these samples (5 · 10)4 mol)1 DMF solution) and the
ultraviolet–visible diffuse reflection spectra of the powder samples were
recorded on an Agilent 8453 spectrophotometer and a BWS003
spectrophotometer, respectively. Differential scanning calorimetry
(DSC) and thermogravimetric analysis (TGA) were performed on a
NETZSCH STA 449C with a heating rate of 10 K min)1 under a
nitrogen atmosphere (flow rate: 40 mL min)1). Scanning electronic
microscope (SEM) images were obtained with a Philips XL-30. The X-
ray diffraction (XRD) measurements were carried out on powdered
samples via a BRUKER D8 diffractometer (40 mA ⁄ 40 kV) using
MATERIALS AND METHODS
Materials. VPBA, TEPIC and TMPMA were purchased from Lan-
caster Synthesis Ltd. 2-thenoyltrifluoroacetone (TTA) and acetyl
acetone (ACAC) were purchased from Shanghai chemical plant.
Benzoyl peroxide (BPO) and TEOS were supplied by China National
Medicines Group. Ln (NO3)3 (Ln = Eu, Tb) aqueous solution was
prepared by dissolving their respective oxides (Eu2O3 and Tb4O7) in
concentrated nitric acid (HNO3). The solvents were purified according
to literature procedures. All reagents were analytically pure.
Synthesis of molecular bridge precursors VPBA-TMPMA and
VPBA-TEPIC linkages. For VPBA-TMPMA: VPBA (1 mmol,
0.148 g) was first dissolved in 20 mL of tetrahydrofuran (THF)
solvent with stirring. Then, 2.0 mmol (0.496 g) of TMPMA was added
drop-wise. Then the mixtures were heated to reflux at 65ꢁC in a
covered flask for about 12 h under nitrogen atmosphere. The coating
liquid was concentrated to remove the solvent THF using a rotary
˚
monochromated CuKa1 radiation (k) 1.54 A over the 2h range 10–70ꢁ.
Luminescence excitation and emission spectra are obtained on a
Perkin–Elmer LS-55 spectrophotometer with different excitation and
emission slits. Luminescent lifetimes were recorded on an Edinburgh
FLS 920 phosphorimeter using a 450 W xenon lamp as excitation
source (pulse width, 3 ls). 1H NMR spectra were recorded on a Bruker
AVANCE-500 spectrometer with tetramethylsilane (TMS) as internal
reference.
vacuum evaporator, and
a colorless viscous liquid sample was
obtained. The colorless viscous sample was then dissolved in solvent
N,N-dimethylformamide (DMF) 20 mL and acrylic acid was added
with initiator BPO to the solution, and the polymers were then
constructed through addition polymerization under nitrogen atmo-
sphere for approximately 14 h. The solvent DMF was then removed
using a rotary vacuum evaporator. After isolation and purification, a
yellow viscous liquid was obtained. The 1H NMR data of VPBA-
TMPMA are as follows: 0.57(2H, t), 0.68(4H, t) 1.22(6H, d) 1.70 (4H,
m), 2.61(2H, m), 3.18(4H, t), 3.35 (9H, m), 3.74(4H, t), 3.58(18H, s),
6.83(1H, t), 7.2(2H, s), 7.1(2H, s).
RESULTS AND DISCUSSION
Figure 1 shows the scheme for the synthesis of the precursors
and the basic composition of the hybrid systems. The main
composition and coordination effect of the precursors can be
predicted according to the lanthanide coordination chemistry
principle and the functional groups of organic units. Consid-
ering the structure of the molecular bridge ligands VPBA-
TEPIC (TMPMA) with two modified carbonyl groups in this
article, it can be assumed that there exist two chelated oxygen
atoms of C=O groups and so the molar ratio of 1:3 between
RE3+ and VPBA-TEPIC (TMPMA) will occupy the six
coordination position around RE3+. For the binary hybrid
system, the remaining coordination space can be filled with
two or three H2O molecules to form the common eight or nine
coordination structure. For ternary hybrid systems, one beta-
diketone ligand (TTA or ACAC) can provide the two chelated
oxygen atoms to replace the two H2O molecules without
influence on the coordination behavior of RE3+; besides this
there still may exist coordinated water molecules. These
predictions have also been confirmed by the infrared spectra
and the estimation of the coordination water molecules from
the luminescent lifetimes and quantum efficiency afterward.
Figure 2A shows the Fourier transform infrared spectra of
the precursor VPBA and the two modified molecular bridges
VPBA-TEPIC (TMPMA). For the IR spectrum of VPBA-
TEPIC, the peak at 2275 cm)1 to the O=C=N group of
TEPIC disappears while the peaks at 2965, 2930 and
2890 cm)1 corresponding to the stretching vibration of meth-
ylene groups of VPBA-TEPIC appear, suggesting that TEPIC
has been grafted onto VPBA. Furthermore, the peaks located
at 1082 and 465 cm)1 derived from the stretching and bending
vibrations of the Si-O group have not been changed to the
broad band at 1150–1000 cm)1, which indicates that the
molecular precursor has not hydrolyzed to form the silicon-
oxygen network, consistent with the 1H NMR data. The peaks
at 1330 and 1260 cm)1 corresponding to the stretching and
bending vibrations of the Si-C group reveal that no silicon-
carbon bond (Si-C) cleavage occurred during the synthesis of
the precursor (33). The absorption peak for stretching vibra-
tion of the carbonyl group is located at 1690 cm)1, and the
broad peak at 3345 cm)1 has accounted for the existence of the
For VPBA-TEPIC (32): VPBA (1 mmol, 0.148 g) was first
dissolved in 20 mL of THF solvent with stirring. Then, 2.0 mmol
(0.495 g) of TESPIC and hydrochloric acid (0.11 mmol) were added
drop-wise. Further steps were similar to those of VPBA-TMPMA (32).
The 1H NMR data of VPBA-TEPIC are as follows: d0.58(2H, t),
0.64(4H, t), 1.244(18H, t), 1.56(4H, m), 3.2(2H, t) 3.35(9H, m)
3.78(12H, q), 3.83(12H, m), 6.76(2H, t), 7.2(2H, s), 7.1(2H, s).
Synthesis of eight kinds of hybrids with lanthanide complexes
imbedded into Si-O networks and organic carbon chains through
chemical bonds. The organic cross-linking precursor VPBA-TE-
PIC ⁄ TMPMA was dissolved in DMF solvent and a stoichiometric
amount of Ln (NO3)3Æ6H2O (0.33 mmol, 0.149 g [Ln = Eu], 0.150 g
[Ln = Tb]) was added to the solution while stirring. After 6 h, a
stoichiometric amount of TEOS and H2O was added to the mixed
solution after completion of the coordination reaction between the
organic cross-linking precursor VPBA-TEPIC ⁄ TMPMA and the
europium ion, which accompanied the addition of one drop of dilute
hydrochloric acid to promote hydrolysis. The molar ratio of Ln
(NO3)3Æ6H2O:VPBA-TEPIC ⁄ TMPMA:TEOS:H2O was 1:3:6:24. After
hydrolysis, an appropriate amount of hexamethylenetetramine was
added to adjust the pH to 6–7. The mixture was agitated magnetically
to achieve a single phase in a covered Teflon beaker, and then it was
aged at 65.5ꢁC until the onset of gelation in about 5 days. The gels
VPBA-TEPIC ⁄ TMPMA-Eu and VPBA-TEPIC ⁄ TMPMA-Tb were
collected as monolithic bulks and were ground into powdered samples
for photophysical studies.
TTA-Eu-VPBA-TEPIC (TMPMA) and ACAC-Tb-VPBA-TEPIC
(TMPMA): The precursor VPBA-TEPIC (TMPMA) was dissolved in
DMF solvent and
a stoichiometric amount of Ln(NO3)3Æ6H2O
(0.33 mmol, 0.149 g [Ln = Eu], 0.150 g [Ln = Tb]) was added to
the solution while stirring. After 2 h, the terminal ligand TTA (or
ACAC) (0.33 mmol, 0.0734 g [or 0.033 g]) was added to the mixture,
and then another 6 h later, a stoichiometric amount of TEOS and H2O
was added to the mixed solution, after completion of the coordination
reaction between precursors and europium ions, which accompanied
the addition of one drop of dilute hydrochloric acid to promote
hydrolysis. The molar ratio of Ln (NO3)3Æ6H2O:VPBA-TE-
PIC ⁄ (TMPMA):TTA(ACAC):TEOS:H2O is 1:3:1:6:24 (TEOS
2 mmol, 0.417 g, and H2O 0.144 g). After hydrolysis, an appropriate
amount of hexamethylenetetramine was added to adjust the pH to 6–7.
The mixture was agitated magnetically to achieve a single phase in a
covered Teflon beaker, and then it was aged at 65.5ꢁC until the onset of
gelation in about 6 days. The gels TTA-Eu-VPBA-TEPIC (TMPMA)
and ACAC-Tb-VPBA-TEPIC (TMPMA) were collected as monolithic
bulks and were ground into powdered samples for photophysical
studies.

788 Bing Yan et al.
Figure 2. The FT-IR spectra (A) and UV–visible absorption spectra
(B) of VPBA and the two linkages VPBA-TEPIC and VPBA-
TMPMA.
Figure 1. The selected scheme for the synthesis of linkage VPBA-
TMPMA (A) and the predicted composition and structure of binary
and ternary lanthanide hybrids (B).
reaction between VPBA and TEPIC, the hydroxyl group of
VPBA is modified to ester group O-C=O and the ultraviolet
absorption peak of VPBA is redshifted to 271 nm, which
indicates that major p-p* electronic transitions occurred and
the formation of VPBA decreases the energy level difference
among electron orbits, and enlarges the planar conjugated
effect of double bonds. A difference can be observed when
comparing the ultraviolet absorption spectra of VPBA and
VPBA-TMPMA. The ultraviolet absorption spectrum of
VPBA-TMPMA shows the blueshift to 242 nm for the
completion of the addition polymerization reaction between
VPBA and the TMPMA, suggesting that the planar conju-
gated effect has been further decreased and the blueshift at
wavelength 7 nm has been found. It is indicated from the
figure that the two precursors have been synthesized as they
have different electron distribution from the raw materials,
which can be reflected by the ultraviolet spectra.
stretching vibration of the grafted -NH- group and the
residual-associated hydroxyl group, both of which have
proved the formation of the group O=C-NH- through the
hydrogen-transfer nucleophilic addition reaction (34). For
the IR spectrum of VPBA-TMPMA, the absorption band in
the 3010–3050 cm)1 range for double C=C group of TMPMA
is not observed, which is due to the addition reaction of it and
hydroxyl group of VPBA. Besides, the absorption peaks within
the 1000–1500 cm)1 range become complicated, suggesting
that the ester group exists in the product VPBA-TMPMA. The
other feature is similar to the IR spectrum of VPBA-TEPIC.
Figure 2B illustrates the ultraviolet absorption spectra of
VPBA and linkages VPBA-TEPIC and VPBA-TMPMA.
From the spectra, it is observed that VPBA has the charac-
teristic absorption peak of an aromatic ring at 248 nm. After
the completion of the hydrogen-transfer nucleophilic addition
The IR spectra of binary and ternary lanthanide hybrids
with VPBA-TEPIC linkage can be seen in Fig. 3A. The spectra

Photochemistry and Photobiology, 2011, 87 789
Figure 4. Select X-ray diffraction patterns of binary and ternary
lanthanide hybrids.
Certainly, strong absorption to the Si-O-Si framework also can
be found at about 1100–1200 cm)1 (mas, Si-O) for the hydro-
lysis and condensation reactions. In addition, the introduction
of TTA and ACAC ligands in the ternary hybrid systems of
both linkages produces some absorption peaks in their IR
spectra.
The room-temperature X-ray diffraction patterns of the
hybrid materials in Fig. 4 reveal that all the materials with
10ꢁ £ 2h £ 70ꢁ are totally amorphous, suggesting the noncrys-
talline state of the hybrids. All diffraction curves show similar
broad peaks centered around 22ꢁ, which is attributed to the
‘‘amorphous hump’’ and it is a typical characteristic of
amorphous silica backbone materials (35). For amorphous
solids, the position of the first sharp diffraction peak can be
related, via a reciprocal relation, to a distance in the real space
between the structural units (Bragg law: 2dsinh = nk). So the
Figure 3. The IR spectra of binary and ternary lanthanide hybrids
with VPBA-TEPIC linkage (A) and VPBA-TMPMA linkage (B).
˚
structural unit distance is approximately 4.04 A, similar to the
˚
4.2 A reported for vitreous SiO2 (36). The small narrow peaks
are weak, suggesting the contents of the simple Si-O compo-
nents are less for the incompleteness of hydrolysis–condensa-
tion reactions between the excessive TEOS molecule. Besides,
the formation of the true covalent-bonded molecular hybrid
system proves that none of the hybrid materials contains
measurable amounts of phases corresponding to the pure
organic compound or free lanthanide nitrate.
of these hybrid materials with different lanthanide ions are
similar. After coordination of Eu3+ and Tb3+ ions in the
hybrid system, the m (C=O) vibrations are shifted to lower
frequencies (Dm = 10–16 cm)1) compared with that of the
VPBA-TEPIC linkage, which indicates the coordination inter-
action between Ln3+ with the oxygen atom (16). Besides, the
spectra of the hybrid material indicate the formation of the Si-
O-Si framework, which is evidenced by the broad bands
located at about 1110–1065 cm)1 (mas, Si-O). It is attributed to
the achievement of hydrolysis and condensation reactions (16).
The m (O-H) vibration at around 3200 cm)1 can be observed,
which means the existence of the H2O molecule. And the qx
(H2O) stretching vibration at 425 cm)1 is further evidence of
participation of water molecules in coordination in these
hybrids. Figure 3B shows the IR spectra of binary and ternary
lanthanide hybrids with VPBA-TMPMA linkage, which shows
a character similar to that of Fig. 3A. The shifts in the m
(C=O) vibrations compared with that of the VPBA-TEPIC
linkage reveal that the coordination interaction takes place
between Ln3+ with the oxygen atom of carbonyl groups.
Figure 5 presents select TG diagraphs of binary and ternary
.
lanthanide hybrids conducted at a heating rate of 5.0ꢁC min)1
The three hybrids have lost the mass from 70ꢁC to 200ꢁC,
corresponding to the loss of the solvent ethanol and DMF.
The mass changes of three hybrids are about 4.8% for TTA-
Eu-VPBA-TMPMA and TTA-Eu-VPBA-TEPIC and 5.5%
for Eu-VPBA-TEPIC, respectively. Furthermore, the second
weight losses for the three hybrids are about 14.5% for TTA-
Eu-VPBA-TMPMA, 24.9% for TTA-Eu-VPBA-TEPIC and
25.2% for Eu-VPBA-TEPIC beyond 350ꢁC, which is related to
the decomposition of ligands. In addition, ternary hybrids
show higher thermal stability than binary ones.
SEM of the lanthanide hybrid materials demonstrate that
the homogeneous materials are obtained (Fig. 6). On the

790 Bing Yan et al.
surface of the materials, there are many linear stripes.
Moreover, new small branches emerge at the end of each
trunk stripe and the stripes continue to grow according to the
direction of these branches to form the final structure. In the
sol-gel process between the linkage and TEOS, the cohydro-
lysis and copolycondensation reaction is not equivalent for the
influence of organically modified groups, which influence the
hydrolysis and polycondensation of TEOS to form the Si-O
host. After the introduction of lanthanide ions, the coordina-
tion reaction between Ln3+ and ligands or linkage also affects
the sol-gel process, resulting in the dominant tendency to
special direction (trunk or stripe). Subsequently, a huge
complicated molecular system is obtained containing a func-
tional bridge ligand with strong covalent bonds between the
inorganic and organic phases. In addition, there still exists
some purity over the surface of the hybrids, which may be the
sole product of TEOS.
Figure 7A shows select excitation spectra of binary and
ternary europium hybrids. All these excitations are obtained
by monitoring the maximum emission of the Eu3+ ions at
Figure 5. Select TG diagraphs of binary and ternary lanthanide
hybrids.
Figure 6. Select scanning electron microscope images of the lanthanide
hybrids.
Figure 7. Select excitation spectra of europium hybrids (A) and
terbium hybrids (B).

Photochemistry and Photobiology, 2011, 87 791
616 nm and appear as a dominant broad band from 300 to
400 nm except for the distinct maximum excitation peaks,
which can be due to the absorption of photoactive VPBA unit
in the organically modified Si-O network. The strong absorp-
tion of photoactive VPBA unit behaves as the main energy
absorption for the whole hybrid systems. With the introduc-
tion of beta-diketones (TTA), the whole excitation bands
become stronger, suggesting that the energy absorption of the
whole ternary system is more effective for the energy transfer
to Eu3+. The sharp excitation bands corresponding to the f-f
transition of Eu3+ cannot be observed for they are too weak.
Undoubtedly, the broad band excitation is favorable for
sensitizing the luminescence of Eu3+. The excitation spectra
for the selected terbium hybrid materials show a similar feature
(Fig. 7B at 545 nm emission wavelength). The wide excitation
bands in the 300–400 nm range are attributed to the absorp-
tion of VPBA-TMPMA-modified Si-O host. The excitation
bands of the ternary hybrid system ACAC-Tb-VPBA-
TMPMA become wider and stronger than that of the binary
one. ACAC is beneficial for the luminescence of Tb3+. Here, it
needs to be mentioned that the organically modified VPBA Si-
O networks play a dual role for the luminescence of Ln3+ in
the hybrid systems: one is the host for which Ln3+ are
immobilized in it through the covalently bonded linkage, and
the other is the ligand for they are coordinated to Ln3+
through the oxygen atoms.
Figure 8A,B presents the emission spectra of binary euro-
pium and terbium hybrids with two linkages. Both of them
show the weak characteristic emission of europium and
terbium ions within the four hybrid systems. For binary
europium hybrids (Fig. 8A), only two emission bands can be
7
observed, 590 and 616 nm, corresponding to the 5D0 fi F1
7
and 5D0 fi F2 transition, respectively. Besides, there exists
the high baseline in the short wavelength <575 nm, which can
be ascribed to the emission of the organically modified host (or
ligand). For binary terbium hybrids (Fig. 8B), two weak
emission bands can also be observed: 488 nm (5D4 fi F6) and
7
Figure 8. Emission spectra of binary europium hybrids (A) and
terbium hybrids (B).
7
545 nm (5D4 fi F5). Here, the emissions of the organically
modified host (or ligand) become clear (especially Tb-VPBA-
TESPIC): a broad emission band covering the 400–650 nm
range, which is overlapped with the characteristic emission of
terbium ions. There are three distinct energy transfer pathways
that can be figured out (37): (1) the emitting centers of the
hybrid host transfer energy to the ligand excited states (hybrid-
to-ligand energy transfer) or (2) directly to the Ln3+ ions
(hybrid-to-Ln3+ energy transfer), and (3) the excited ligand
states transfer energy to the Ln3+ ions (ligand-to-Ln3+ energy
transfer). Here, a broad band in the blue ⁄ green spectral region
to extensive visible region, which is ascribed to the emitting
levels of the hybrid host and already observed in similar
organic–inorganic hybrids, results from a convolution of the
emission that originates in the NH ⁄ C=O groups of the urea
bridges with electron-hole recombination occurring in siloxane
nanoclusters. From the luminescence of the binary hybrids, it
can be predicted that the energy transfer to Eu3+ or Tb3+ is
not effective within these systems. The two linkages VPBA-
TEPIC (TMPMA) are not suitable for the luminescence of
Eu3+ (Tb3+). So the introduction of secondary ligands such as
beta-diketones may be necessary.
europium hybrids, the very sharp peaks of the lines are
located at 578, 590, 616, 652 and 695 nm for TTA-Eu-VPBA-
TEPIC (578, 590, 615, 650 and 697 nm for TTA-Eu-VPBA-
7
TMPMA), assigned to the 5D0 fi FJ (J = 0, 1, 2, 3, 4)
transitions of Eu3+, respectively. The ternary hybrids exhibit
much stronger luminescence intensity than the binary ones,
suggesting that the introduction of a second TTA ligand
improves the sensitization degree of Eu3+ greatly. Further-
more, among these emission peaks of each material, the red
emission peak at 616 (615) nm corresponding to the electric
5
7
dipole transition of D0 fi F2, which is hypersensitive to the
coordinative environment around Eu3+, shows higher inten-
sities than the orange emission peak at 590 nm, which is the
5
7
magnetic dipole transition of D0 fi F1 and independent of
the environment around. The domination of electric dipole
transition demonstrates that Eu3+ occupies the coordination
environment site without inversion symmetry (38) and the
asymmetric microenvironment will induce the polarization of
Eu3+ due to the influence of the electric field of the ligand with
the probability for electric dipole transition. The nonsymmetry
The emission spectra of the ternary europium and terbium
hybrid materials are displayed in Fig. 9. In Fig. 9A for
5
7
and the intensities of the electric dipole transition D0 fi F2

792 Bing Yan et al.
will increase as the interaction of the rare-earth complex with
its local chemical environment is completed more entirely, so
asymmetric environment. In addition, the I02 ⁄ I01 value of both
the ternary hybrids is much larger than those of the binary
ones. Therefore, the introduction of TTA is beneficial for the
improvement of the I02 ⁄ I01 value, from which it can be
expected that the ternary hybrids will obtain higher lumines-
cent quantum yields.
the relative intensity ratio (I02 ⁄ I01) of D0 fi F2 ⁄ 5D0 fi F1
transition is widely used as an indicator of Eu3+ site
symmetry. The relative intensity ratio (I02 ⁄ I01) of
5D0-7F2 ⁄ 5D0-7F1 transition for these europium hybrids is
summarized in Table 1 and TTA-Eu-VPBA-TEPIC possesses
the largest value, indicating Eu3+ is located in the most
5
7
7
The peaks of the lines for ternary terbium hybrids in Fig. 9B
5
7
are assigned to the D4 fi FJ (J = 6, 5, 4, 3) transitions at
about 488 (488), 545 (544), 582 (583) and 619 nm, respectively,
for Tb3+. The green emission at 545 (544) nm possesses the
strongest intensity among all the emissions. It can be found
that the emission intensity of ACAC-Tb-VPBA-TMPMA is
stronger than that of ACAC-Tb-VPBA-TEPIC, whereas the
emission intensity of TTA-Eu-VPBA-TMPMA weaker than
that of TTA-Eu-VPBA-TEPIC. This can be explained by the
energy match and energy transfer mechanism between the
ligands and lanthanide ions (39–41). The intramolecular
energy transfer efficiency from ligands to lanthanide ions
mainly depends on the energy match between them. The
energy match for Eu3+ and Tb3+ is different and generally
opposite for the difference of the two ions, resulting in the
different luminescence order for their hybrids (39–41).
The typical decay curves of the Eu and Tb hybrid materials
are measured and they can be described as a single exponential
(Ln(S(t) ⁄ S0) = )k1t = )t ⁄ s), indicating that all Eu3+ and
Tb3+ ions occupy the same average coordination environ-
ment. The resulting lifetime data of Eu and hybrids are given
in Table 1. It is found that the lifetimes of all binary and
ternary hybrids show no apparent distinction. The luminescent
lifetimes of ternary hybrids are slightly longer than those of
binary ones. And the luminescent lifetimes of binary europium
hybrids are a bit longer than those of binary terbium. These
facts agree with the corresponding luminescent intensities. But
the difference is not apparent and the luminescent quantum
yields will be determined with the luminescent intensity ratio.
Furthermore, we selectively determined the emission quantum
yields of the 5D0 excited state of europium ion for Eu3+
hybrids on the basis of the emission spectra and lifetimes of the
5D0 emitting level; the detailed luminescent data are shown in
Table 1. To investigate the luminescence efficiency of these
hybrids, we selectively determined the emission quantum
efficiencies of the 5D0 excited state of europium ion on the
5
basis of the emission spectra and lifetimes of the D0 emitting
level for Eu3+ hybrids. The detailed principles and methods
are adopted from references (42–45).
Figure 9. Emission spectra of ternary europium hybrids (A) and
terbium hybrids (B).
sexp ¼ ðAr þ AnrÞꢀ1
g ¼ Arsexp
ð1Þ
ð2Þ
ð3Þ
ð4Þ
Table 1. Photoluminescence data for binary and ternary lanthanide
hybrids.
s
Arad Anrad
)
g
(%) nw
Ar ¼ RA0J ¼ A00 þ A01 þ A02 þ A03 þ A04
A0J ¼ A01ðI0J=I01Þðt01=t0JÞ
Hybrids
I02 ⁄ I01 (ls) (s)1 (s)1
)
Eu-VPBA-TEPIC
Eu-VPBA-TMPMA
TTA-Eu-VPBA-TEPIC
TTA-Eu-VPBA-TMPMA
Tb-VPBA-TEPIC
Tb-VPBA-TMPMA
ACAC-Tb-VPBA-TEPIC
ACAC-Tb-VPBA-TMPMA
2.52 298 181 3180
4.35 303 279 3020
5.4 ꢁ3
8.5 ꢁ3
15.9
14.3
321 922 2200 29.5 ꢁ2
313 887 2380 27.1 ꢁ2
As shown in Table 1, the quantum yields of the europium
hybrid materials can be determined in the order: Eu-VPTA-
TEPIC < Eu-VPTA-TMPMA < TTA-Eu-VPTA-TMPMA <
TTA-Eu-VPTA-TEPIC. The luminescent quantum yields of
ternary hybrids (29.5%, 27.1%) are much higher than those of
247
289
329
347

Photochemistry and Photobiology, 2011, 87 793
8. Mech, A., A. Monguzzi and F. Meinardi (2010) Sensitized NIR
erbium(III) emission in confined geometries: A new strategy for
light emitters in telecom applications. J. Am. Chem. Soc. 132,
4574–4576.
9. Chen, Z. Q., F. Ding, F. Hao, M. Guan, Z. Q. Bian, B. Ding and
C. H. Huang (2010) Synthesis and electroluminescent property of
novel europium complexes with oxadiazole substituted 1,10-phe-
nanthroline and 2,2¢-bipyridine ligands. New J. Chem. 34, 487–
494.
binary ones (5.37%, 8.46%) in that TTA is in a better position
to sensitize the emission of the Eu3+ ions.
To study the coordination environment surrounding lan-
thanide ions, according to Horrocks’ previous research (46,47),
it is therefore expected that probable number of coordinated
water molecules (nw) can be calculated according to following
equation:
10. Terai, T., K. Kikuchi, S. Iwasawa, T. Kawabe, Y. Hirata, Y.
Urano and T. Nagano (2006) Modulation of luminescence inten-
sity of lanthanide complexes by photoinduced electron transfer
and its application to a long-lived protease probe. J. Am. Chem.
Soc. 128, 6938–6946.
11. Du, C. X., L. Ma, Y. Xu and W. L. Li (1997) Synthesis and
fluorescent properties of europium-polymer complexes containing
naphthoate and 1.10-phenanthroline ligands. J. Appl. Polym. Sci.
66, 1405–1410.
12. Bekiari, V., G. Pistolis and P. Lianos (1999) Intensely luminescent
materials obtained by combining lanthanide ions, 2,2¢-bipyridine,
and poly(ethylene glycol) in various fluid or solid environments.
Chem. Mater. 11, 3189–3194.
13. Wang, Q. M. and B. Yan (2006) Assembly of luminescent hybrids
from co-polymers bearing functional 4-vinyl pyridine and euro-
pium aromatic carboxylate. J. Photochem. Photobiol. A, Chem.
177, 1–5.
nw ¼ 1:05ðAexp ꢀ ArÞ
ð5Þ
On the basis of the results, the coordination number of
water molecules in the binary and ternary europium hybrids
can be estimated to be about 3 and 2, respectively. So the total
coordination number for Eu3+ in the binary and ternary
hybrids is around 9 and 10, which correspond to the
coordination chemistry behavior of lanthanide ions. Besides,
the coordinated water molecules produce the vibration of the
hydroxyl group, resulting in nonradiative transition and
decreased luminescent efficiency.
CONCLUSIONS
14. Binnemans, K. and C. Gorller-Walrand (2002) Lanthanide-con-
taining liquid crystals and surfactants. Chem. Rev. 102, 2303–2345.
15. Van Deun, R., D. Moors, B. De Fre and K. Binnemans (2003)
Near-infrared photoluminescence of lanthanide-doped liquid
crystals. J. Mater. Chem. 13, 1520–1522.
16. Franville, A. C., D. Zambon, R. Mahiou and Y. Troin (2000)
Luminescence behavior of sol-gel-derived hybrid materials
resulting from covalent grafting of a chromophore unit to differ-
ent organically modified alkoxysilanes. Chem. Mater. 12, 428–435.
17. Li, H. R., J. Lin, H. J. Zhang, H. C. Li, L. S. Fu and Q. G. Meng
(2001) Novel covalently bonded hybrid materials of europium
(terbium) complexes with silica. Chem. Commun. 1212–1213.
18. Bekeari, V., P. Lianos, U. L. Stangar, B. Orel and P. Judeinstein
(2000) Optimization of the intensity of luminescence emission
from silica ⁄ poly(ethylene oxide) and silica ⁄ poly(propylene oxide)
nanocomposite gels. Chem. Mater. 12, 3095–3099.
In summary, eight binary and ternary luminescent lanthanide
organometallic hybrid material systems are assembled through
two novel bridge molecules and covalent linkages (VPBA-
TEPIC and VPBA-TMPMA). These hybrids have been
characterized in detail, especially the photophysical properties.
Within the hybrid systems, the connection of inorganic and
organic parts on a molecular level is to exhibit homogeneous
microstructures. The binary hybrids show weak luminescence
and the introduction of beta-diketone ligands (TTA and
ACAC) in the ternary hybrid systems shows the greatest
improvement in luminescence (intensity, red ⁄ orange ratio,
quantum yields).
19. Bermudez, V. D., L. D. Carlos and L. Alcacer (1999) Sol-gel de-
rived urea cross-linked organically modified silicates. 1. Room
temperature mid-infrared spectra. Chem. Mater. 11, 569–580.
20. Carlos, L. D., V. D. Bermudez, R. A. S. Ferreira, L. Marques and
M. Assuncao (1999) Sol-gel derived urea cross-linked organically
modified silicates. 2. Blue-light emission. Chem. Mater. 11, 581–
588.
Acknowledgements—This work was supported by the National Nat-
ural Science Foundation of China (20971100) and Program for New
Century Excellent Talents in University (NCET 2008-08-0398).
21. Sanchez, C. and F. Ribot (1994) Design of hybrid organic-inor-
ganic materials synthesized via sol-gel chemistry. New J. Chem. 18,
1007–1047.
22. Carlos, L. D., R. A. S. Ferreira, V. D. Bermudez and S. J. L.
Ribeiro (2009) Lanthanide-containing light-emitting organic-
inorganic hybrids: A bet on the future. Adv. Mater. 21, 509–534.
23. Binnemans, K. (2009) Lanthanide-based luminescent hybrid
materials. Chem. Rev. 109, 4283–4374.
REFERENCES
1. Silver, J. and R. Withnall (2004) Probes of structural and elec-
tronic environments of phosphor activators: Mossbauer and
Raman spectroscopy. Chem. Rev. 104, 2833–2855.
2. Kido, J. and Y. Okamoto (2002) Organo lanthanide metal com-
plexes for electroluminescent materials. Chem. Rev. 102, 2357–
2368.
24. Wang, Q. M. and B. Yan (2004) Novel luminescent terbium
molecular-based hybrids with modified meta aminobenzoic acid
covalently bonded with silica. J. Mater. Chem. 14, 2450–2455.
25. Wang, Q. M. and B. Yan (2005) A novel way to luminescent
terbium molecular-scale hybrid materials: Modified heterocyclic
ligands covalently bonded with silica. Cryst. Growth Des. 5, 497–
503.
3. Chen, Z. G., H. L. Chen, H. Hu, M. X. Yu, F. Y. Li, Q. Zhang, Z.
G. Zhou, T. Yi and C. H. Huang (2008) Versatile synthesis
strategy for carboxylic acid-functionalized upconverting nano-
phosphors as biological labels. J. Am. Chem. Soc. 130, 3023–3029.
4. Sabbatini, N., M. Guardigli and J. M. Lehn (1993) Luminescent
lanthanide complexes as photochemical supramolecular devices.
Coord. Chem. Rev. 123, 201–228.
26. Liu, J. L. and B. Yan (2008) Lanthanide (Eu3+, Tb3+) centered
hybrid materials using modified functional bridge chemical bon-
ded with silica: Molecular design, physical characterization and
photophysical properties. J. Phys. Chem. B 112, 10898–10907.
27. Lu, H. F., B. Yan and J. L. Liu (2009) Functionalization of
calix[4]arene as molecular bridge to assemble novel luminescent
lanthanide supramolecular hybrid systems. Inorg. Chem. 48, 3966–
3975.
5. Justel, T., H. Nikol and C. Ronda (1998) New developments in the
field of luminescent materials for lighting and displays. Angew.
Chem. Int. Edn. 37, 3085–3103.
6. Parker, D., R. S. Dickins, H. Puschmann, C. Crossland and J. A.
K. Howard (2002) Being excited by lanthanide coordination
complexes: Aqua species, chirality, excited-state chemistry, and
exchange dynamics. Chem. Rev. 102, 1977–2010.
7. Eliseevaa, S. V. and J. C. G. Bunzli (2010) Lanthanide lumines-
cence for functional materials and bio-sciences. Chem. Soc. Rev.
39, 189–227.
28. Yan, B. and Q. M. Wang (2008) First two luminescent molecu-
lar hybrids composed of bridged Eu(III)-b-diketone chelates

794 Bing Yan et al.
covalently trapped in silica and titanate gels. Cryst. Growth Des. 6,
1484–1489.
europium(III): A quantitative assessment by light emission spec-
troscopy. J. Phys. Chem. C 111, 17627–17634.
29. Li, Y., B. Yan and H. Yang (2008) Construction, characterization
and photoluminescence of mesoporous hybrids containing
europium (I) complexes covalently bonded to SBA-15 directly
functionalized by modified b-diketone. J. Phys. Chem. C 112,
3959–3968.
38. Hasegawa, Y., M. Yamamuro, Y. Wada, N. Kanehisa, Y. Kai and
S. Yanagida (2003) Luminescent polymer containing the Eu(III)
complex having fast radiation rate and high emission quantum
efficiency. J. Phys. Chem. A 107, 1697–1702.
39. Sato, S. and W. Mada (1970) Relations between intramolecular
energy transfer efficiencies and triplet state energies in rare earth
b-diketone chelates. Bull. Chem. Soc. Jpn. 43, 2403–2410.
40. Dexter, D. J. (1953) A theory of sensitized luminescence in solids.
J. Chem. Phys. 21, 836–850.
41. Dean, C. R. S. and T. M. Shepherd (1975) Evaluation of the
intramolecular energy transfer rate constants in crystalline
Eu(hfaa)4ButNH3. J. Chem. Soc. Faraday Trans. II 71, 146–150.
42. Werts, M. H. V., R. T. F. Jukes and J. W. Verhoeven (2002) The
emission spectrum and the radiative lifetime of Eu3+ in luminescent
lanthanide complexes. Phys. Chem. Chem. Phys. 4, 1542–1548.
43. Teotonio, E. E. S., J. G. P. Espynola, H. F. Brito, O. L. Malta, S.
F. Oliveria, D. L. A. de Foria and C. M. S. Izumi (2002) Influence
of the N-[methylpyridyl]acetamide ligands on the photolumines-
cent properties of Eu(III)-perchlorate complexes. Polyhedron 21,
1837–1844.
44. Malta, O. L., H. F. Brito, J. F. S. Menezes, F. R. Goncalves e
Silva, S. Alves, F. S. Farias and A. V. M. Andrade (1997) Spec-
troscopic properties of a new light-converting device Eu(thenoyl-
trifluoroacetonate), 2(dibenzyl sulfoxide). A theoretical analysis
based on structural data obtained from a sparkle model. J. Lumin.
75, 255–268.
30. Li, Y. J. and B. Yan (2009) Lanthanide (Eu3+, Tb3+) ⁄ beta-
diketone modified mesoporous SBA-15 ⁄ organic polymer hybrids:
Chemically bonded construction, physical characterization, and
photophysical properties. Inorg. Chem. 48, 8276–8285.
31. Yan, B. and X. F. Qiao (2007) Rare earth ⁄ inorganic ⁄ organic
polymeric hybrid materials: Molecular assembly, regular micro-
structure and photoluminescence. J. Phys. Chem. B 111, 12362–
12374.
32. Qiao, X. F. and B. Yan (2011) Rare earth (Eu3+, Tb3+) centered
polymeric hybrids: Composite assembly of radical addition poly-
merization and condensation reaction, physical characterization
and photoluminescence. New J. Chem. 35, 568–575.
33. Yu, Y. Y., C. Y. Chen and W. C. Chen (2003) Synthesis and
characterization of organic–inorganic hybrid thin films from
poly(acrylic) and monodispersed colloidal silica. Polymer 44, 593–
601.
34. Binnemans, K., P. Lenaerts, K. Driesen and C. Gorller-Walrand
(2004) A luminescent tris (2-thenoyltrifluoroacetonato) euro-
pium(III) complex covalently linked to a 1,10-phenanthroline-
functionalised sol-gel glass. J. Mater. Chem. 14, 191–195.
35. Hoffmann, H. S., P. B. Staudt, T. M. H. Costa, C. C. Moro and E.
V. Benvenutti (2002) FTIR study of the electronic metal-support
interactions on platinum dispersed on silica modified with titania.
Surf. Interf. Anal. 33, 631–634.
36. Goncalves, M. C., V. D. Z. Bermudez, R. A. S. Ferreira, L. D.
Carlos, D. Ostrovskii and J. Rocha (2004) Optically functional
di-urethanesil nanohybrids containing Eu3+ ions. Chem. Mater.
16, 2530–2543.
45. Carlos, L. D., Y. Messaddeq, H. F. Brito, R. A. S. Ferreira, V. D.
Bermudez and S. J. L. Ribeiro (2000) Full-color phosphors from
europium (III)-based organosilicates. Adv. Mater. 12, 594–598.
46. Horrocks W. D., Jr. and D. R. Sudnick (1979) Lanthanide ion
probes of structure in biology. Laser-induced luminescence decay
constants provide a direct measure of the number of metal-coor-
dinated water molecules. J. Am. Chem. Soc. 101, 334–340.
47. Horrocks W. D., Jr. and D. R. Sudnick (1981) Lanthanide ion
luminescence probes of the structure of biological macromole-
cules. Acc. Chem. Res. 14, 384–392.
37. Lima, P. P., S. S. Nobre, R. O. Freire, S. A. Junior, R. A. S.
Ferreira, U. Pischel, O. L. Malta and L. D. Carlos (2007) Energy
transfer mechanisms in organic–inorganic hybrids incorporating