Facile and efficient route to prepare luminescent terbium containing covalently anchored hybrids equipped with molecular bridge

Article abstract of DOI:10.1080/15421400500377628

A kind of monomer (abbreviated as BrBAA-APES) was prepared by modifying 2-bromophenylacetic acid (BrBAA) with a coupling agent (3-aminopropyl) triethoxysilane (APES). Then it was used to coordinate to Tb³⁺ and to react with tetraethoxysilane (T

Full text of DOI:10.1080/15421400500377628

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Facile and Efficient Route
to Prepare Luminescent
Terbium Containing Covalently
Anchored Hybrids Equipped
with Molecular Bridge
Yu-Long Sui a , Bing Yan a & Qian-Ming Wang a
a Department of Chemistry , Tongji University ,
Shanghai, China
Published online: 31 Jan 2007.
To cite this article: Yu-Long Sui , Bing Yan & Qian-Ming Wang (2006) Facile and
Efficient Route to Prepare Luminescent Terbium Containing Covalently Anchored
Hybrids Equipped with Molecular Bridge, Molecular Crystals and Liquid Crystals,
457:1, 193-201, DOI: 10.1080/15421400500377628
To link to this article: http://dx.doi.org/10.1080/15421400500377628
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Mol. Cryst. Liq. Cryst., Vol. 457, pp. 193–201, 2006
Copyright # Taylor & Francis Group, LLC
ISSN: 1542-1406 print=1563-5287 online
DOI: 10.1080/15421400500377628
Facile and Efficient Route to Prepare Luminescent
Terbium Containing Covalently Anchored Hybrids
Equipped with Molecular Bridge
Yu-Long Sui
Bing Yan
Qian-Ming Wang
Department of Chemistry, Tongji University, Shanghai, China
A kind of monomer (abbreviated as BrBAA-APES) was prepared by modifying
2-bromophenylacetic acid (BrBAA) with
a coupling agent (3-aminopropyl)
triethoxysilane (APES). Then it was used to coordinate to Tb3þ and to react with
tetraethoxysilane (TEOS) in the presence of water molecules simultaneously. Dur-
ing the reaction, cohydrolysis and cocondensation occurred at the same time as
complexation happened to Tb3þ , which is called an in situ process. After 5 days,
a kind of hybrid material was prepared. The hybrid exhibits the characteristic
luminescence of Tb3þ , and the change of the hybrid’s excitation wavelength also
testifies to the energy couple and intramolecular energy transfer between the triplet
state energy of BrBBA-APES and emissive level of Tb3þ
.
Keywords: bridge molecule; double chemical bond; luminescence; molecular hybrids;
terbium
INTRODUCTION
Rare-earth complexes are well known for their excellent luminescence
properties. Their luminescence properties are widely studied because
they can be used as visible, near-infrared radiation sources; lasers;
and optical communication devices [1]. However, they also have disad-
vantages in their mechanical properties and thermal stability. To
improve these properties, matrices are always used to form all kinds
of hybrid materials. Rare-earth complexes with ß-diketones, aromatic
carboxylic acids, and heterocyclic ligands are incorporated into the sol-
gel-derived hosts, which can provide a stable environment for central
Address correspondence to Bing Yan, Department of Chemistry, Tongji University,
Siping Road 1239, Shanghai 200092, China. E-mail: byan@tongji.edu.cn
193

194
Y.-L. Sui et al.
particles [2–13]. These studies indicate that the physical properties of
rare-earth complexes are improved, and the luminescence intensities
of rare earth are similar or superior to the complexes. In this kind of
hybrid, only weak physical interactions exist between the matrices
and the complexes. Hence, many problems such as clustering of the
emitters, inhomogeneous dispersion, and leaching of dopants occur.
So, the chemical bonding of rare-earth-based hybrids is necessary.
Based on the interaction among the different components or phases
in hybrid systems, the hybrid materials can be divided into two major
classes [14]. The first is called physically mixed with weak interactions
(hydrogen bonding, van der Waals force or weak static effects)
between the organic and inorganic phases; the second is named chemi-
cal bonded with powerful covalent bonds linking the organic and inor-
ganic parts. The powerful covalent bonds can be helpful to the
formation of the single phase [14]. In fact, the latter kind of hybrid
belongs to molecular-level materials [15–29], to which is easy to tailor
the complementary properties of novel luminescence materials. Sol-gel
technology is known as one of the most versatile methods for the
preparation of inorganic–organic hybrid materials because of its low
processing temperature [30,31]. It allows the incorporation of lumi-
nescent species into all kinds of matrices through hydrolysis and
polycondensation [32–34].
In our research, we have synthesized several sorts of compounds by
modifying amino groups, which can be applied as molecular hybrids
and which exhibit excellent luminescence properties [27–29].
Franville and coworkers modified pyridine-dicarboxylic acid and its
derivatives [18]. On the ground of investigating the coordination
environment of carbonyl groups, aromatic acid containing mono-
carboxyl group (which can also be utilized as the precursor for molecu-
lar hybrids) attracted our interest. Therefore, we attempt to use
molecular monomer (BrBAA-APES) and TEOS to form a molecular-
based hybrid by the sol-gel process. The monomer is prepared by the
acylation between carbonyl groups of BrBAA (2-bromophenylacetic
acid) and the amino group of a cross-linking ligand APES ((3-amino-
propyl)triethoxysilane).
EXPERIMENTAL
All the agents were used as received. Acids were obtained from Lance-
ster, and APES was obtained from the Nanjing ShuGuang chemistry
plant. The procedure for the preparation of BrBAA-APES bridge
ligand is shown in Fig. 1. First, 3 mmol of BrBAA was converted to acyl
chloride by refluxing in excess SOCl2 under argon at 70ꢀC in an oil

Efficient Route to Prepare Luminescent Terbium
195
FIGURE 1 Scheme of the synthesis process of BrBAA-APES and predicted
structure of hybrid materials.
bath for 4 h. After isolation, the acyl chlorides were directly reacted with
APES in ethyl ether in the presence of triethylamine. The main IR
peaks and NMR data of BrBAA-APES is shown. C17H28NO4SiBr: IR
(KBr): NꢁH (3267cmꢁ1) ꢁCONHꢁ (1638 cmꢁ1, 1554 cmꢁ1), C-Si
(1194 cmꢁ1). 1H NMR (CDCl3): d 7.17(1H,s), 7.29(1H,s), 7.37(1H,s),
7.58(1H,s), 5.62(1H,s), 1.20(9H,t), 3.75(6H,q), 3.50(2H,t), 1.82(2H,q),
2.02(2H,t), 3.72(2H, s). 13C NMR (CDCl3): d 125.4(C1), 126.3(C2),
135.4(C3), 133.6(C4), 129.6(C5), 127.0(C6), 72.5(C7), 169.4(C8), 45.8(C9),
44.2(C10), 41.9(C11), 46.8(C12), 18.3(C13).
The hybrid material was prepared as follows: BA-APES was dis-
solved in ethanol, and TEOS (tetraethoxysilane) and H2O were added
while stirring; then one drop of diluted nitric acid was dropped into it
to promote hydrolysis. After that, 1 mmol of Tb(NO3)3ꢂ6H2O was added

196
Y.-L. Sui et al.
to the final stirring mixture. The molar ratio of Tb(NO3)3ꢂ6H2O=
BrBAA-APES=TEOS=H2O was 1:3:6:24. After the treatment of
hydrolysis, 2 mL of DMF (dimethylformamide) and 1 mmL of hexam-
ethylene-tetramine were added to adjust the pH value to about 6.5.
The mixture was stirred to achieve a single phase, and thermal treat-
ment was performed at 60ꢀC until the sample solidified. Figure 1
shows the chemical procedure and the predicted structure of the
hybrid material, and Fig. 2 presents the hydrolysis and condensation
in the sol-gel process.
All measurements were completed at room temperature except for
phosphorescence spectra, which were measured at 77 K. Infrared
(IR) spectroscopy was measured on a Nexus 912 AO446 FT-IR spectro-
1
photometer using KBr pellets. H NMR and 13C NMR spectra were
recorded in CDCl3 on a Bruker Avance-500 spectrometer with tetra-
methylsilane (TMS) as internal reference. UV absorption spectra of
these powder samples (about 5 ꢃ 10ꢁ4 molꢂLꢁ1 CHCl3 solution) were
recorded with an Agilent 8453 spectrophotometer. Phosphorescence
spectra (about 5 ꢃ 10ꢁ4 molꢂLꢁ1 CHCl3 solution) and fluorescence
zexcitation and emission spectrums were obtained on a Perkin-Elmer
LS-55 spectrophotometer: excitation slit width ¼ 10 nm, emission
slit width ¼ 5 nm. The microstructure was estimated by scanning
electronic microscope (SEM, Philips XL-30).
FIGURE 2 Scheme of hydrolysis and polycondensation processes between
BrBAA-APES and TEOS.

Efficient Route to Prepare Luminescent Terbium
197
RESULTS AND DISCUSSION
Figure 3 shows the IR spectra of BrBAA (a) and the final material (b).
The occurrence of the amidation reaction was supported by the band
located at 1650 cmꢁ1, due to the characteristic absorption of amide
groups (ꢁCOꢁNHꢁ), suggesting that APES has been successfully
grafted onto 2-bromophenylacetic acid. Then the presence of the
stretching vibration (mNH, 3387 cmꢁ1) and bending vibration (dNH
,
1548 cmꢁ1) further proved the formation of amide groups. The broad
absorption bands locate in 1074 cmꢁ1 (mSi-O-Si) and 800 cmꢁ1 (dSi-O-Si
)
originate from the formation of the siloxane network.
Figure 4 shows the UV absorption spectra of (a) BrBAA, (b) BrBAA-
APES, and (c) BrBAA-APES-Tb3þ . Comparing the absorption spec-
trum of BrBAA-APES (b) with that of BrBAA (a), we can see a major
absorption peak located in 265 nm disappears after modification, and
the peak (241 nm) derived from the major p–pꢄ electronic transitions
of phenyl scarcely change (from 241 nm to 243 nm). So as far as both
(b) and (c) are concerned, we still can observe a broad absorption band
by a sharp peak due to the introduction of terbium ions, indicating
that the coordination behavior functionalizes the carbonyl groups
and central metal ions.
FIGURE 3 IR spectra for BrBAA (a) and hybrids (Tb-BrBAA-APES) (b).

198
Y.-L. Sui et al.
FIGURE 4 Ultraviolet absorption spectra for (a) BrBAA, (b) BrBAA-APES, (c)
Tb-BrBAA-APES.
Figure 5 shows the phosphorescence spectra of BrBAA (a) and
BrBAA-APES (b). There is an obvious blue shift from 408 nm
to 444 nm, which we attribute to the modification of BrBAA.
FIGURE 5 Phosphorescence spectra of BrBAA (a) and BrBAA-APES (b).

Efficient Route to Prepare Luminescent Terbium
199
FIGURE 6 Excitation spectrum of hybrids Tb-BrBAA-APES.
Phosphorescence spectrum attributes to the excitation state character-
istic of the organic molecular ligands and different phosphorescence
bands correspond to different ligand molecules. According to the
intramolecular energy transfer mechanism [35,36], the corresponding
intramolecular transfer efficiency from the BrBAA-APES to Tb3þ
mainly depends on the energy match between the triplet state energy
of BrBAA-APES (corresponding to the phosphorescence band) and the
resonant emissive energy level of the central Tb3þ . Therefore, it can be
FIGURE 7 Emission spectra of hybrids Tb-BrBAA-APES.

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Y.-L. Sui et al.
predicted that BrBAA-APES shows an energy match and sensitizes
the luminescence of Tb3þ
.
Figures 6 and 7 show the excitation and emission spectra of
the resulting hybrid material. The excitation spectrum monitored
at 544 nm is dominated by two broad bands centered at 326 nm and
392 nm. The characteristic excitation spectrum located at about 250 nm
arising from the efficient transition based on the aromatic ligand does
not appear because there is no conjugation effect between phenyl and
acyl. Upon excitation at 326 nm, only the characteristic emission peaks
7
of Tb3þ 5D4! Fj (j ¼ 6, 5, 4, 3) transitions at 490, 544, and 587 nm
7
(5D4! F3 is covered by the double frequency of exciting wavelength)
are observed, and no emission from the ligand is detected, suggesting
energy transfer from the ligand to Tb3þ . Upon excitation at 392 nm, no
emission is observed. As is shown in Fig. 6, the hybrid finally exhibits
characteristic emission under a unique stable chemical environment of
rigid molecular network formed by the ꢁSiꢁOꢁSiꢁ backbone.
CONCLUSIONS
In conclusion, the modification of 2-bromophenylacetic acid with APES
results in the formation of a new hybrid compound, which can act as
both the ligand of Tb3þ and the sol-gel precursor. The final hybrid
shows the characteristic emission of Tb3þ by the intramolecular
energy transfer from the triplet state energy of BrBAA-APES to the
resonant emissive energy level of the central Tb3þ . To our knowledge,
it is the first example of luminescent molecular-based hybrid material
on the basis of the modification of 2-bromophenylacetic acid.
ACKNOWLEDGMENTS
This work was supported by the National Natural Science Foundation
of China (20301013).
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