Rare-earth (Eu3+, Tb3+) hybrids through amide bridge: Chemically bonded self-assembly and photophysical properties

Article abstract of DOI:10.1016/j.jorganchem.2009.11.005

This work focuses on the construction of a series of chemically bonded rare-earth/inorganic/organic hybrid materials (TCH-Si-Ln, TCH-Si-Ln-Phen and TCH-Si-Ln-Bipy: Phen = 1,10-phenanthroline, Bipy = 2,2′-bipyridyl) using TCH-Si as an organic bridge molecule that can both coordinate to rare-earth ions (Eu³⁺ and Tb³⁺) and form an inorganic Si-O-Si network with tetraethoxysilane (TEOS) after cohydrolysis and copolycondensation through a sol-gel process. All of these hybrid materials exhibit homogeneous microstructures and morphologies, suggesting the occurrence of self-assembly of the inorganic network and organic chain. Measurements of the photoluminescent properties of these materials show that the ternary europium systems present stronger luminescent intensities than the binary hybrids, indicating that the introduction of the second ligands can sensitize the luminescence emission of the europium hybrid systems. However, in the terbium systems, this phenomenon was not observed.

Full text of DOI:10.1016/j.jorganchem.2009.11.005

Journal of Organometallic Chemistry 695 (2010) 580–587
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Rare-earth (Eu3+, Tb3+) hybrids through amide bridge: Chemically bonded
self-assembly and photophysical properties
*
Jinliang Liu, Bing Yan
Department of Chemistry, Tongji University, Siping Road 1239, Shanghai 200092, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
This work focuses on the construction of a series of chemically bonded rare-earth/inorganic/organic
hybrid materials (TCH–Si–Ln, TCH–Si–Ln–Phen and TCH–Si–Ln–Bipy: Phen = 1,10-phenanthroline,
Bipy = 2,20-bipyridyl) using TCH–Si as an organic bridge molecule that can both coordinate to rare-earth
ions (Eu3+ and Tb3+) and form an inorganic Si–O–Si network with tetraethoxysilane (TEOS) after cohydro-
lysis and copolycondensation through a sol–gel process. All of these hybrid materials exhibit homoge-
neous microstructures and morphologies, suggesting the occurrence of self-assembly of the inorganic
network and organic chain. Measurements of the photoluminescent properties of these materials show
that the ternary europium systems present stronger luminescent intensities than the binary hybrids,
indicating that the introduction of the second ligands can sensitize the luminescence emission of the
europium hybrid systems. However, in the terbium systems, this phenomenon was not observed.
Ó 2009 Elsevier B.V. All rights reserved.
Received 1 September 2009
Received in revised form 30 October 2009
Accepted 6 November 2009
Available online 12 November 2009
Keywords:
Rare-earth hybrid material
Chemically bonded self-assembly
Photophysical property
1. Introduction
components at the molecular or nanometer level and thus they
can be specially tailored to give desirable properties or to suppress
Luminescent lanthanide complexes have attracted special
attention in recent years due to their academic interests and po-
tential applications in a wide variety of photonic utilities [1–3].
Among the Ln3+ compounds, Eu3+ and Tb3+ ions are the most two
popular Ln3+ ions because of their sharp characteristic emissions
in red and green region, which can be used as emitters of electro-
luminescent devices and photoluminescent markers, labels of fluo-
roimmunoassay and so on [4,5]. In those compounds, the emission
of Eu3+ or Tb3+ complexes usually results from the so-called ‘‘an-
tenna effect”, which is defined as a light conversion process via
an energy absorption, energy-transfer and emission sequence. In
such a process, the organic ligands act as photosensitizers or an-
tenna chromophores to absorb and transfer the energy of light to
excite the lanthanide ions via an energy-transfer process [6,7].
Due to the versatility in organic synthesis, the luminescent proper-
ties of lanthanide complexes can be custom-tailored depending on
the desired application. The research in chromophores’ design
seems to be shifting to the optimization of a variety of other
parameters, i.e. thermal stability, chemical resistance and optical
transparence [8–10]. As a consequence, it comes up the possibility
to generate new materials which are known as organic–inorganic
hybrid materials [11,12]. Organic–inorganic hybrid materials have
attracted special attention in recent years due to their interesting
properties because they combine the two or more integrating
undesirable ones for wide-ranging applications [13,14]. The silica
network can provide good mechanical resistance, extraordinary
thermal stability and entirely amorphous systems, so the incorpo-
ration of organic compounds into silica gels and glasses is of the ut-
most interest for a variety of organic–inorganic hybrid materials,
including optical devices [15–17]. According to the types of chem-
ical interactions between the organic and the inorganic compo-
nents, the hybrids can be categorized into two main classes [18]:
Class I materials concerns the conventional doping systems in
which only weak interactions (such as hydrogen bonding, van
der Waals forces, or electrostatic forces) exist between organic
and inorganic moieties. Class II materials belong to the molecule-
based composite materials in which the organic and inorganic
phases are linked together through strong chemical interactions
(covalent, ion or coordination bonds). In these materials, the or-
ganic group is covalently bonded to the trifunctional silicon groups
through Si–C bonds and then embedded into the Si–O–Si matrixes
after hydrolysis and polycondensation processes, thus makes the
organic group an integral part of the materials. So, the design of
a functional bridge molecule (precursor), which can behave double
functions of coordinating to rare-earth ions and allowing sol–gel
process to constitute covalent Si–O networks, by the grafting reac-
tion, has become the key procedure to construct such molecule-
based materials [19–23]. Our research group recently put more
emphasis on the preparation of functional systems covalently
connecting both the siliceous backbone and photoactive units.
We have successfully realized six paths to construct the functional
* Corresponding author. Tel.: +86 21 65984663; fax: +86 21 65982287.
E-mail address: byan@tongji.edu.cn (B. Yan).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.11.005

J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
581
silylated precursors, which are the modifications of amine groups
[24], carboxyl groups [25], hydroxyl groups [26–28], sulfonic
groups [29], mercapto groups [30,31] and methylene groups [32].
After the modification, we assembled the above modified bridge li-
gands with lanthanide ions and tetraethoxysilane (TEOS) to com-
pose hybrid systems with covalent-bonds and obtained a series
of stable and efficient molecular hybrid materials in optical areas.
Based on the former work, in this paper, we first synthesized
the organic compound thiocarbohydrazide (denoted as TCH) con-
taining two amine groups. And then we grafted the compound
TCH to 3-(triethoxysilyl)propyl isocyanate (denoted as TESPIC) by
amine group modification to achieve a molecular precursor (de-
noted as TCH–Si) through the hydrogen-transfer nucleophilic addi-
tion reaction between the amine group of TCH and the isocyanate
group of TESPIC. Then, we prepared a series of chemically bonded
rare-earth/inorganic/organic hybrid materials (denoted as TCH–Si–
Ln) using TCH–Si as the bridge molecule that can both coordinate
to rare-earth ions (Eu3+ and Tb3+) and form inorganic Si–O–Si net-
works with tetraethoxysilane (TEOS) after cohydrolysis and
copolycondensation processes. Further, in order to study the influ-
ence of the addition of the second ligands (1,10-phenanthroline,
denoted as Phen, and 2,20-bipyridyl, denoted as Bipy) to the lumi-
nescent intensities of the above binary hybrids, we also prepared
two types of ternary rare-earth/inorganic/organic hybrids (denoted
as TCH–Si–Ln–Phen and TCH–Si–Ln–Bipy) by the introduction of
the second ligands (Phen and Bipy) into the above system. Compar-
ison of performances of TCH–Si–Lanthanide systems with those of
our previous materials [24–32], these TCH–Si–Ln hybrids can exhi-
bit a high thermal stability, strong fluorescent intensities and high
quantum efficiencies especially after the addition of Phen. Besides,
the hybrid materials demonstrate that a homogeneously and uni-
formly system was obtained and no phase separation could be
observed.
2.2.2. Synthesis of the precursor TCH–Si
About 2 mmol of TCH was dissolved in 10 mL of tetrahydrofu-
ran (THF), the resulting mixture was heated under reflux, and then
3-(triethoxysilyl)-propyl isocyanate (2 mmol) dissolved in 10 mL
of THF was added dropwise. The mixture was warmed at 343 K
for approximately 12 h under argon in a covered flask. After cool-
ing, the solvent was removed in vacuo, and the residue was
washed with 10 mL of hexane three times then clear oil was ob-
tained in a 94.6% yield. 1H NMR (CDC13, 500 MHz): 0.63 (t, 4H,
CH2Si); 1.22 (t, 18H, CH3CH2); 1.65 (m, 4H, CH2CH2CH2); 2.12 (s,
2H, CSNH); 3.26 (m, 2H, NHCH2); 3.83 (q, 12H, SiOCH2); 6.03 (s,
2H, CONHNH); 6.53 (s, 2H, CONHCH2); 7.52 ppm (s, 2H, CH2NH).
Elemental Anal. Calc. for C21H48N6O8SSi2: C, 41.98; H, 8.05; N,
13.99. Found: C, 41.91; H, 8.26; N, 13.88%.
2.2.3. Synthesis of the molecular hybrid materials containing
lanthanide ions (TCH–Si–Ln, TCH–Si–Ln–Bipy, TCH–Si–Ln–Phen,
Ln = Eu or Tb)
The hybrid materials were prepared by a typical procedure as
follows: the precursor TCH–Si and the second ligands (free for
TCH–Si–Ln, 2,20-bipyridyl for TCH–Si–Ln–Bipy and 1,10-phenan-
throline for TCH–Si–Ln–Phen) were dissolved in N,N-dimethyl
formamide (DMF) solution, and then an appropriate amount of
Ln(NO3)ꢀ6H2O was added. The mixture was kept under room tem-
perature for 3 h, and then TEOS and H2O were added while stirring.
The mole ratio of Ln(NO3)ꢀ6H2O/TCH–Si/Bipy (or Phen)/TEOS/H2O
was 1:3:1:6:24. When the addition of the TEOS and H2O were com-
plete, one drop of 20% hydrochloric acid was added to promote
hydrolysis. After the treatment of hydrolysis for 8 h, an appropriate
amount of hexamethylenetetramine was added to adjust the pH
value of 6–7. The resulting mixture was agitated magnetically to
achieve a single phase, after that, thermal treatment was per-
formed at 343 K in a covered Teflon beaker for about 6 days until
the sample solidified. The obtained gels were washed with ethanol
and dried at 363 K for another 3 days. The final molecular hybrid
materials were collected as monolithic bulks and were ground into
powdered materials for the further studies.
2. Experimental
2.1. Materials
2.3. Physical measurements
Starting materials were purchased from Sinopharm Chemical
Reagent Co., Ltd. (SCRC). All the reagents are analytically pure.
These solvents were purified according to the literature procedures
[33]. Other starting reagents were used as received. Europium and
terbium nitrates were obtained by dissolving Eu2O3 and Tb4O7 in
concentrated nitric acid, respectively. Tetraethoxysilane (TEOS)
was distilled and stored under a N2 atmosphere.
All measurements were completed under room temperature. 1H
NMR spectra were recorded on a Bruker AVANCE-500 spectrome-
ter with tetramethylsilane (TMS) as internal reference. FT-IR spec-
tra were measured within the 4000–400 cmꢁ1 region on a Nicolet
model 5SXC spectrophotometer with the KBr pellet technique.
Scanning electronic microscope (SEM) images were obtained with
a Philips XL-30. Elemental analyses were performed on a VarioEL
III O-Element Analyzer system. The X-ray diffraction (XRD) mea-
surements were carried out on powdered samples via a BRUKER
2.2. Synthesis
D8 diffractometer (40 mA/40 kV) using monochromated Cu Ka1
The synthesis procedure of the organic compound thiocarbo-
hydrazide (TCH) and the silylated precursor (TCH–Si) were shown
in Fig. 1A.
radiation (k = 1.54 Å) over the 2h range of 10°–70°. Differential
scanning calorimetry (DSC) and thermogravimetric analysis
(TGA) were performed on a NETZSCH STA 449C with a heating rate
of 10 K/min under a nitrogen atmosphere (flow rate: 40 mL/min).
Reflectivity spectra were recorded on a Bws003 spectrometer
equipped with a diffuse reflectance accessory. Fluorescence excita-
tion and emission spectra were obtained on a RF-5301 spectropho-
tometer with 1.5 nm excitation and 3.0 nm emission slits.
2.2.1. Synthesis of thiocarbohydrazide (TCH)
About 40 mL of 85% hydrazine hydrate was dissolved in 120 mL
of water, and then 12 mL of CS2 was added dropwise, the reaction
mixture was kept under room temperature for 1 h and then heated
to 363 K for additional 10 h. After cooling, the precipitate was fil-
tered off, washed with water and dried under room temperature.
The crude product was purified by recrystallization form water
and finally obtained as white needles, yield 12.04 g (72%). m.p.
172–173 °C. 1H NMR (DMSO, 500 MHz): d 4.48 (d, 4H, NH2);
8.69 ppm (t, 2H, NH). Elemental Anal. Calc. for CH6N4S: C, 11.31;
H, 5.70; N, 52.78. Found: C, 11.23; H, 5.82; N, 52.70%.
3. Results and discussion
3.1. Organic compound TCH and the silylated precursor TCH–Si
1H NMR spectra relative to the organic compound TCH and the
silylated precursor TCH–Si are in full agreement with their

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J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
Fig. 1. Scheme of the synthesis process of the organic component TCH, the silylated precursor TCH–Si (A) and the predicted structure of the hybrid TCH–Si–Ln (B).
proposed structures. As detailed in Section 2, 1H NMR chemical
shift observed for amino group attributed to –CONH– group, to-
gether with the disappearance of the signal of NH2 group, indicates
the accomplishment of the hydrogen-transfer reaction between
amino group and the TESPIC. Furthermore, integration of the 1H
NMR signal corresponding to ethoxy group shows that no hydroly-
sis of the precursor occurred during the grafting reaction.
absorption bands of TCH and TCH–Si are shown in Table 1. The
grafting reaction of TCH with TESPIS can be proved by the presence
of a series of strong bands at 2975, 2928, 2886 cmꢁ1, which are
originating from the vibrations of methylene –(CH2)3– group.
Moreover, the disappearance of the stretch vibration of the absorp-
tion peaks located at 2250–2275 cmꢁ1 for N@C@O of TESPIC indi-
cate that TESPIC has been completely take part in the grafting
reaction with the organic compound TCH. Besides, the stretching
vibration of Si–C located at about 1108 cmꢁ1, and the stretching
The synthesis of silylated precursor TCH–Si can also be
confirmed by the FT-IR. The assignments of the main infrared
Table 1
Assignments of the main infrared absorption bands for TCH, TCH–Si and the corresponding hybrids TCH–Si–Eu, TCH–Si–Tb, TCH–Si–Eu–Bipy, TCH–Si–Tb–Bipy, TCH–Si–Eu–Phen,
TCH–Si–Tb–Phen.
Compounds
TCH
m
(OH) (cmꢁ1
)
m
(NH2) (cmꢁ1
)
m
(CH2) (cmꢁ1
)
m
(C@O) (cmꢁ1
)
m
(Si–C) (cmꢁ1
)
m )
d(Si–O) (cmꢁ1
3271
3202
TCH–Si
TCH–Si–Eu
TCH–Si–Tb
TCH–Si–Eu–Bipy
TCH–Si–Tb–Bipy
TCH–Si–Eu–Phen
TCH–Si–Tb–Phen
2975, 2928, 2886
2966, 2932, 2883
2968, 2930, 2887
2971, 2928, 2885
2965, 2926, 2886
2963, 2930, 2884
2970, 2937, 2885
1678
1658
1660
1657
1653
1656
1652
1198
1195
1196
1198
1200
1194
1198
1108, 1072, 459
1107, 1079, 462
1109, 1072, 463
1108, 1075, 465
1106, 1071, 462
1109, 1075, 468
1110, 1073, 466
3427
3425
3422
3428
3421
3432

J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
583
vibration of Si–O at 1108 and 1072 cmꢁ1, together with the bend-
ing vibration at about 460 cmꢁ1 indicate the absorption of the
siloxane bonds.
microstructures with some chainlike structures which appear like
the trunk of pine trees arranged quite regularly on the slippery and
clean surfaces of the hybrid materials.
Basically, the sol–gel synthesis of organic–silica hybrid materi-
als mainly involves three steps, that is, hydrolysis, condensation
and polycondensation [34]. As far as this three-step process is con-
cerned, the silylated precursor TCH–Si and TEOS are firstly hydro-
lysed to yield reactive silanol groups. After the initial hydrolysis
step, condensation reaction possibly takes place between eth-
oxy–ethoxy, silanol–silanol and ethoxy–silanol groups, leading to
the formation of siloxane chains. The further hydrolysis of the
remaining alkoxy groups and polycondensation (or gelation) be-
tween chains gives rise to a three-dimensional, crosslinked solid
network of siloxane (Si–O–Si). In fact, these three steps cannot be
totally isolated, they proceed in parallel rather than in sequence.
Their relative rates determine the final structure of the wet silica
gel [35]. During the sol–gel process, the type of solvent, pH value,
reactant ratios, temperature and the type of catalyst are major fac-
tors affecting the relative rates of hydrolysis and condensation
reactions, and thus determine the ultimate morphology of the
formed materials. In this paper, we select the similar synthesis
conditions of the sol–gel technology, such as the reaction temper-
ature, type of solvent, catalyst, and so on. We obtained the similar
microstructures for all the hybrid materials. Regarding that the
TCH–Si complex has many NH groups, the structure is probably
to form a colloidal particles chain, which can compete with the
construction of a network structure of Si–O–Si for the hydrolysis
and polycondensation process of silica. In terms of the different
steric effects and the relative rates, the trunk structure was
achieved for the dominant growth along the TCH–Si colloidal par-
ticles chain. In consideration of the discussion mentioned above,
together with the infrared spectroscopy and other references pre-
viously reported [8,36], the resulting predicted structure of the hy-
brid TCH–Si–Ln is shown in Fig. 1B as an example. However, the
structure analysis is one of the most difficult problems in studying
the organic–silica hybrid molecular materials. There is but scarce
information available about their structure and about the pro-
cesses that occur during their formation, which could be of great
importance for further investigations.
An attempt for homogeneity was made to estimate the crystal-
linity of hybrids by the powder XRD measurement. The X-ray dif-
fraction patterns of the hybrid materials TCH–Si–Eu, TCH–Si–Tb,
TCH–Si–Eu–Phen, TCH–Si–Tb–Phen, TCH–Si–Eu–Bipy and TCH–
Si–Tb–Bipy reproduced in Fig. 4 reveal that all the materials are to-
tally amorphous. The absence of any crystalline regions in these
samples is due to the presence of organic chains in the host inor-
ganic framework [37]. All the diffraction curves show similar broad
amorphous halos, with angel 2h centered around 22.6°, which is
deriving from homogeneously silica matrix [38]. The structural
unit distance, calculated using the Bragg law, is approximately
3.91 Å. This may be ascribed to the coherent diffraction of the sili-
ceous backbone of the hybrids. By comparison with all patterns, it
seems that the introducing of the second ligands (Bipy or Phen) has
no influence on the disorder structure of the siliceous skeleton. In
addition, none of the hybrid materials contains measurable
amounts of phases corresponding to the pure organic compound
or free RE nitrate, which is a further indication for the formation
of the true covalent-bonded molecular hybrid materials.
3.2. Hybrid materials (TCH–Si–Ln, TCH–Si–Ln–Phen, TCH–Si–Ln–Bipy,
Ln = Eu or Tb)
All of the obtained hybrid materials were also characterized by
infrared spectroscopy. The IR spectra of TCH–Si–Eu (A), TCH–Si–Tb
(B), TCH–Si–Eu–Phen (C), TCH–Si–Tb–Phen (D), TCH–Si–Eu–Bipy
(E) and TCH–Si–Tb–Bipy (F) and the assignments of main infrared
absorption bands are shown in Fig. 2 and Table 1, respectively. The
presence of the
m(Si–C) absorption, which are located in 1200–
1192 cmꢁ1 wavelength range, is consistent with the fact that no
(Si–C) bond cleavage occurs [8]. While the absorption of m(Si–O–
Si) located in 1109–1060 cmꢁ1 wavelength range indicates the
formation of siloxane bonds during the hydrolysis/condensation
reactions, which is an initial indication for the formation of the true
covalent-bonded molecular hybrid materials. However, in the
spectra, the
served, which means the existence of residual silanol groups and
m
(O–H) vibration at around 3420 cmꢁ1 can also be ob-
the presence of H2O molecule. And the m(Si–OH) stretching vibra-
tion at 956 cmꢁ1 is a further evidence of the incompleteness of
condensation reactions.
Furthermore, the complexation of Ln3+ with the organic com-
pound in all of the hybrids can be clearly shown by infrared spec-
troscopy. Compared with the silylated precursor TCH–Si, the
m
(C@O) vibrations of the –CONH– groups are shifted to lower fre-
quency (from 1678 cmꢁ1 in TCH–Si to 1652–1660 cmꢁ1 in hybrids,
= 18-26 cmꢁ1). This is ascribe to the complexation of the Ln3+
D
m
ion with the oxygen atom of the C@O group in hybrids [8].
In most case, surface morphology of materials is of great impor-
tance for many technical applications requiring well-defined sur-
face or interfaces. From Fig. 3, the scanning electron micrographs
for the hybrid materials demonstrate that a homogeneous system
was obtained and no phase separation could be observed. That is,
covalent-bonding (Si–O–Si) enhanced the miscibility of the organic
compounds and the silica matrixes. They were homogeneously and
uniformly dispersed at a molecular level via a self-assemble pro-
cess during the hydrolysis/polycondensation process so that the
inorganic and the organic phases can exhibit their distinct proper-
ties together. Fig. 3 shows the selected micrographs for (A) TCH–
Si–Eu, (B) TCH–Si–Tb, (C) TCH–Si–Eu–Phen, (D) TCH–Si–Tb–Bipy.
From the figure we can see that all the samples show the similar
To examine the thermal properties of the hybrid materials, dif-
ferential scanning calorimetry (DSC) and thermogravimetric analy-
sis (TGA) were performed on all of the amorphous materials. Fig. 5
shows the DSC and TG curves of the hybrids TCH–Si–Eu as an
example. There are two main degradation steps can be obviously
observed from the TG and DSC curves. The first step of weight loss
(about 17.0%) from 200 to 350 °C was attributed to the decompo-
sition of organic ingredients, involving Si–C, C–C, and C–N bond
Fig. 2. Infrared spectra of TCH–Si–Eu, TCH–Si–Tb, TCH–Si–Eu–Phen, TCH–Si–Tb–
Phen, TCH–Si–Eu–Bipy and TCH–Si–Tb–Bipy in the 4000–400 cmꢁ1 range.

584
J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
Fig. 3. SEM images for hybrid materials, (A) TCH–Si–Eu, (B) TCH–Si–Tb, (C) TCH–Si–Eu–Phen, (D) TCH–Si–Tb–Bipy.
cleavage [39]. which is an exothermic process observed from the
DSC plot. The weight loss beyond 350 °C was ascribed to the re-
leases of water and ethanol formed from the further condensation
of silanols in the silica framework. In DSC plot, they express the
same thermal change tendency. Because there are even some Si–
OH groups on the silica surface and these groups have the ability
to undergone condensation to form Si–O–Si networks. The weight
loss beyond 450 °C became slightly with the decreasing of the Si–
OH groups. The thermal stabilities of this kind of hybrids were
largely improved due to the successful incorporation of the silica
moiety in the hybrid materials. This improvement of the thermal
stability is interpreted by the silica component inducing a protec-
tive barrier against thermal degradation for organic species.
Diffuse reflectance experiments were performed on powdered
materials for all the materials. Fig. 6 shows the corresponding
absorption spectra of (A) TCH–Si–Eu, TCH–Si–Eu–Phen, TCH–Si–
Eu–Bipy and (B) TCH–Si–Tb, TCH–Si–Tb–Phen, TCH–Si–Tb–Bipy.
All of the spectra exhibit the similar broad absorption bands in
Fig. 4. The X-ray diffraction (XRD) graphs of the hybrids TCH–Si–Eu, TCH–Si–Tb,
TCH–Si–Eu–Phen, TCH–Si–Tb–Phen, TCH–Si–Eu–Bipy and TCH–Si–Tb–Bipy.
Fig. 5. The selected TG and DSC curves of the hybrids TCH–Si–Eu.

J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
585
determined in the order: TCH–Si–Tb > TCH–Si–Tb–Phen > TCH–
Si–Tb–Bipy, the fluorescence emission intensity is not improved
after the addition of the second ligands, this can also be proved
by the fluorescence spectra in Fig. 7.
Emission of the RE complexes usually results from the so-called
‘‘antenna effect”, which is defined as a light conversion process via
an energy absorption, energy-transfer and energy emission se-
quence. In such a process, the emission of RE chelates proceeds
through the following steps: (a) energy absorption via a ground
singlet-excited singlet transition. (b) non-radiative intersystem
crossing from the excited singlet to the triplet states. (c) energy-
transfer from the ligands to lanthanide ions, and (d) lanthanide
ions emission from the excited states. Fig. 7 shows the excitation
and emission spectra of the europium hybrid materials (A) TCH–
Si–Eu, TCH–Si–Eu–Phen, TCH–Si–Eu–Bipy and the terbium hybrid
materials (B) TCH–Si–Tb, TCH–Si–Tb–Phen, TCH–Si–Tb–Bipy. The
excitation spectra were obtained by monitoring the emission of
Eu3+ or Tb3+ at 614 or 545 nm. For Eu3+ hybrids, all the systems
have similar excitation spectra that were dominated by a broad
band from 296 to 365 nm with the maximum peak at about
350 nm. As a result, the emission lines of the hybrid materials were
assigned to the characteristic 5D0 ? 7F1 and 5D0 ? 7F2 transitions
at 590 and 614 nm, respectively, while the emission lines of
5D0 ? 7F3 and 5D0 ? 7F4 are too weak to be observed. The
5D0 ? 7F2 emission around 614 nm is the most predominant
transition, which agrees with the amorphous characters of the hy-
brid materials. From the excitation spectra, we can see that the
Fig. 6. The ultraviolet–visible diffuse reflection absorption spectra of (A) TCH–Si–
Eu, TCH–Si–Eu–Phen, TCH–Si–Eu–Bipy and (B) TCH–Si–Tb, TCH–Si–Tb–Phen, TCH–
Si–Tb–Bipy.
the UV–Vis range (200–450 nm), which partially overlap with the
fluorescence excitation spectra (wide bands at 280–370 nm in (A)
and 250–380 nm in (B)) in Fig. 6. This absorption band may be as-
cribed to the transition from the ground state of to the first excited
state (S0 ? S1) of the organic ligand. According to Dexter’s ex-
change energy-transfer theory: the matching degree between the
ligand’s triplet state energy and lanthanide iron’s emission energy
can largely influence the luminescence intensity of hybrid mate-
rial. From the absorption spectra we can primarily predicted that
the energy levels matching degree between TCH–Si and RE ions
is suited and appropriate so that the organic ligand can absorb
abundant energy in ultraviolet–visible extent and to transfer the
energy to the corresponding hybrid materials via an intramolecular
energy-transfer process. Then the final hybrid materials can be ex-
pected to have strong luminescence, which can be proved by the
intension of the peaks in 450–700 nm ranges. The signals of
614 nm in Fig. 6A and 545 nm in Fig. 6B were assigned to the tran-
sitions from the 5D0 ? 7F2 transitions for Eu3+ ions and the
5D4 ? 7F5 transitions for Tb3+ ions, respectively. For europium hy-
brid systems, after the addition of the second ligand 1,10-phenan-
throline, the fluorescence emission intensity is largely improved,
even the transitions from the 5D0 ? 7F1 transitions at 590 nm can
be clearly seen from the spectra. It is indicate that the addition
of the second ligand 1,10-phenanthroline can largely sensitize
the emission of Eu3+ ions. However, for terbium systems, the
fluorescence emission intensities of these kinds of materials are
Fig. 7. The excitation and emission spectra of the europium hybrid materials (A)
TCH–Si–Eu, TCH–Si–Eu–Phen, TCH–Si–Eu–Bipy and the terbium hybrid materials
(B) TCH–Si–Tb, TCH–Si–Tb–Phen, TCH–Si–Tb–Bipy.

586
J. Liu, B. Yan / Journal of Organometallic Chemistry 695 (2010) 580–587
Table 2
The luminescence efficiencies and lifetimes for the europium hybrid materials TCH–Si–Eu, TCH–Si–Eu–Bipy and TCH–Si–Eu–Phen.
a
a
Hybrids
m01 (cmꢁ1
)
m02 (cmꢁ1
)
A0J (sꢁ1
)
Aexp (sꢁ1
)
Arad (sꢁ1
)
Anrad (sꢁ1
)
N (l
s)b
g (%)
TCH–Si–Eu
TCH–Si–Eu–Bipy
TCH–Si–Eu–Phen
16 978
16 978
16 978
16 313
16 313
16 313
50 250
50 290
50 421
2457
1709
1340
300
340
471
2157
1369
869
407
585
746
12.2
20.0
35.1
a
The energies of the 5D0
? m0J).
7FJ transitions (
b
For 5D0
? .
7F2 transition of Eu3+
fluorescence emission intensities of these kinds of materials are
determined in the order: TCH–Si–Eu–Phen > TCH–Si–Eu–Bi-
py > TCH–Si–Eu, which indicate that the second ligands 1,10-phe-
nanthroline and 2,20-bipyridyl can efficiently sensitize the
luminescence of Eu3+ ions. For Tb3+ hybrids, a broad band centered
at around 347 nm was observed in the excitation spectra and as a
result, the emission lines were assigned to the 5D4 ? 7FJ transitions
located at 490, 544, 587 and 622 nm, for J = 6, 5, 4 and 3, respec-
tively. The most striking green fluorescence (5D4 ? 7F5) was ob-
served due to the fact that this emission is the most intense one.
Corresponding to the emission spectra of Eu3+ hybrids, the fluores-
cent intensities of Tb3+ hybrids change with the sequence: TCH–Si–
Tb > TCH–Si–Tb–Phen > TCH–Si–Tb–Bipy. The enhancement of the
fluorescence intensities after the addition of the second ligands
was not observed in the terbium systems. To further investigate
the luminescence efficiency of these covalent hybrids, we selec-
tively determined the emission quantum efficiencies of the 5D0 ex-
cited state of europium ion for Eu3+ hybrids on the basis of the
emission spectra and lifetimes of the 5D0 emitting level. The de-
tailed principles and methods were adopted from Refs. [40–48]
and the quantum efficiencies of the europium hybrid materials
intensities than the binary hybrids, indicating that the introduc-
tion of the second ligands can sensitize the luminescence emis-
sions of the europium hybrid systems. However, this
sensitization effect was not observed in the terbium hybrid sys-
tems. This may be ascribe to the energy match between the li-
gands triplet state and the resonant emissive energy level of
the central Eu3+ ions not as proper as for Tb3+ ions. As the syn-
thesis process can be easily applied to other organic ligands and
to different alkoxysilanes, we may expect to obtain stable and
efficient hybrid materials in optical or electronic areas because
the desired properties can be tailored by an appropriate choice
of the precursors. However, the structure of the hybrid materials
and the processes that occur during the sol–gel technology need
further fundamental investigations.
Acknowledgements
This work was supported by the National Natural Science Foun-
dation of China (20971100) and Program for New Century Excel-
lent Talents in University (NCET-08-0398).
are shown in Table 2, the value
g mainly depends on the values
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