Luminescent triazine-containing bridged polysilsesquioxanes activated by lanthanide ions

Article abstract of DOI:10.1002/ejic.200800645

Luminescent lanthanide-complex-bridged polysilsesquioxanes were prepared by sol-gel processing of a monomer with a large heterocyclic bridging group in the presence of lanthanide ions under acidic conditions. SEM, XRD, FTIR, luminescence spectra and luminescence decay analysis were used to characterize the obtained materials. The ⁵D₀ quantum efficiency and the number of the water molecules coordinated to the Eu³⁺ ion were theoretically estimated on the basis of emission spectra and the lifetime of the ⁵D₀ state. Wiley-VCH Verlag GmbH & Co. KGaA, 2008.

Full text of DOI:10.1002/ejic.200800645

FULL PAPER
DOI: 10.1002/ejic.200800645
Luminescent Triazine-Containing Bridged Polysilsesquioxanes Activated by
Lanthanide Ions
[
a]
[a]
[a]
[a]
[a]
[a]
Nana Lin, Huanrong Li,* Yige Wang, Yu Feng, Dashan Qin, Quanying Gan, and
Shudong Chen[
a]
Keywords: Luminescence / Organic–inorganic hybrid composites / Lanthanides / Sol–gel processes / Nitrogen heterocycles
Luminescent lanthanide-complex-bridged polysilsesquiox-
anes were prepared by sol–gel processing of a monomer with
a large heterocyclic bridging group in the presence of lantha-
nide ions under acidic conditions. SEM, XRD, FTIR, lumines-
ciency and the number of the water molecules coordinated
to the Eu3 ion were theoretically estimated on the basis of
+
5
emission spectra and the lifetime of the D
0
state.
cence spectra and luminescence decay analysis were used to
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2008)
5
characterize the obtained materials. The
0
D quantum effi-
Introduction
state. These properties are favorable for a broad range of
applications such as fiber amplifiers and solid-state la-
Bridged polysilsesquioxanes are hybrid organic–inor-
ganic materials derived from the sol–gel processing of
[24,25]
sers.
Thus, it would be an interesting topic to develop
bridged polysilsesquioxanes containing lanthanide com-
pounds from the viewpoints of academic interest and prac-
tical applications. In this contribution, we present a novel
urea-based tris(silylated) monomer containing large organic
bridging groups of complex chemical structure, consisting
of a heterocyclic ring (triazine) integrated through flexible
alkyl chains with three triethoxysilyl groups. The synthesis
and characterization of the triazine-bridging monomer de-
rived from melamine are described; the derived multifunc-
tional monomer was afterward submitted to complexation
monomers containing a variable organic bridging group
and two or more trialkoxysilyl groups.[
1–4]
The organic
group, covalently bonded to the trialkoxysilyl groups, can
be varied in composition, length, rigidity, and functionali-
zation. Thus, the properties of the resulting hybrid solids
can be tuned by the introduction of appropriate organic
functionality into the silicate network through covalent
linkage. The easy preparation of these materials, that is, a
“one-pot” synthesis, is another tremendous advantage.
Bridged silsesquioxanes have proven to be efficient candi-
3+
with Ln ions and to a sol–gel process in order to obtain
[
5–8]
dates for applications in optical devices,
high-capacity
the luminescent bridged polysilsequioxanes. The aims of the
present work are to (i) enrich the family of bridged polysil-
sesquioxane by incorporation of triazine into the bridging
[
9–11]
[12]
absorbents,
3D information storage media, and pro-
[13,14]
ton-conducting media for fuel cells.
To date, great effort has been directed to optical applica-
3+
group and (ii) investigate the sensitization of Ln ions by
tions of bridged polysilsesquioxanes by incorporating vari-
the bridged polysilsequioxanes.
ous chromospheres in the bridging groups.[
15–18]
The use of
dye molecules as bridging groups offers a high loading of
chromospheres and avoids the leaching or phase separation
of the dye molecule. However, there are relatively few stud-
ies on the preparation of luminescent polysilsesquioxanes
Results and Discussion
Hydrolysis and condensation of monomers containing a
variable organic bridging group with two or more trifunc-
tional silyl groups under different conditions can lead to
bridged polysilsesquioxanes. For example, trifunctional ar-
ylsilanes preferentially form as oligosilsesquioxanes rather
than gels. At high monomer concentrations, however, gels
can be prepared from trifunctional derivatives by heating in
the presence of a strong base. In this work, the luminescent
bridged polysislsesquioxanes were obtained by hydrolysis
and condensation of the urea-based tris(silylated) monomer
containing large organic bridging groups under acidic con-
[19–23]
with lanthanide complexes as the bridging group.
Lan-
thanide complexes are well-known molecular luminescent
materials, which are characterized by characteristic narrow
emission bands and long radiative lifetimes of the excited
[
a] School of Chemical Engineering and Technology, Hebei Uni-
versity of Technology,
Tianjin 300130, China
Fax: +86-225264294
E-mail: lihuanrong@hebut.edu.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
[
26]
ditions.
Eur. J. Inorg. Chem. 2008, 4781–4785
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
4781

N. Lin, H. Li, Y. Wang, Y. Feng, D. Qin, Q. Gan, S. Chen
FULL PAPER
The FTIR spectra of the monomer and the monomer
3
+
3+
with Eu (Eu /monomer ratio = 0.5) are shown in Fig-
ure 1. For the monomer, the ν(Si–C) and ν(Si–OEt) absorp-
tion bands characteristic of trialkoxysilyl functions appear
–
1
at 1194 and 1080 cm , respectively. The sharp bands lo-
–
1
cated at 1697 and 1534 cm corresponding to the urea
NH–CO–NH function provide strong evidence that the re-
action between melamine and 3-(triethoxysilyl)propyl isocy-
anate was successful. Furthermore, the characteristic bands
–
1
of melamine at 1616, 1480, 1360, and 816 cm are also ob-
[
27]
3+
served. Upon addition of Eu ions to the monomer, the
ν(C=O) vibration is shifted to lower frequency (∆ν =
–
1
2
2 cm ); this could be ascribed to the complexation of the
3
+
Eu ions with the oxygen atom of the carbonyl group. It Figure 2. FTIR spectra of (a) MICPTES–Hb and (b) MICPTES–
is well established that complexation of the metallic ion Eu.
with the oxygen atom of the carbonyl group can induce
the ν(C=O) vibrations to shift lower frequencies (∆ν = 20–
in other nanostructured hybrid materials by comparison
–
1 [21]
40 cm ).
Sol–gel processing of the monomer led to
[
28]
with SiO xerogels.
The pattern displayed at ca. 9–11°
2
bridged polysilsesquioxanes, which were also characterized
may be attributed to a periodic arrangement of the mono-
by infrared spectroscopy (Figure 2). In both cases, the v(Si–
[
29]
3+
mer during the sol–gel processing. The addition of Eu
–
1
C) vibration located in the1194 cm range is observed in
the IR spectra of the hybrid materials, and this is consistent
with the fact that no great (Si–C) bond cleavage occurs dur-
ing hydrolysis/condensation reactions. The broad absorp-
ions can affect the periodicity structure of the materials, as
evidenced by the decreased d value (from 9.2 to 7.9 Å) of
the first XRD pattern as indicated by the arrow in Figure 3.
–
1
tion band at 1120–1000 cm [v(Si–O–Si)] indicates the for-
mation of siloxane bonds, and residual silanol groups are
evidenced by the v(Si–OH) stretching vibration at
–
1
3+
9
16 cm .Complexation of Ln ions by the bridging or-
ganic group in the bridged polysilsesquioxane matrices can
be clearly evidenced by infrared spectroscopy. The v(C=O)
vibration is shifted to low frequency after complexation of
the carbonyl group.
Figure 3. XRD diagrams of (a) MICPTES–Hb and (b) MICPTES–
Hb–Eu.
Figure 4 shows the SEM images of MICPTES–Hb and
MICPTES–Hb–Eu. The uniform frameworks on the sur-
faces suggest that homogeneous, molecular-based materials
[
30]
were obtained. The SEM images of both samples revealed
a large particle morphology (ca.5–10 µ long) with various
sizes and shapes.
In order to investigate the sensitization of lanthanide
ions by the bridged polysilsesquioxane, the emission spec-
trum of the bridged polysilsesquioxane MICPTES–Hb was
Figure 1. FTIR spectra of (a) the monomer and (b) the monomer
with Eu3+ (Eu /monomer ratio = 0.5).
3+
X-ray powder diffraction analysis of the solids allowed studied, which was obtained by excitation at 335 nm (see
information about the nanometer scale organization to be the Supporting Information). The photoluminescence spec-
obtained. The X-ray powder diffraction diagrams of tra show the presence of two main contributions of a band
MICPTES–Hb and MICPTES–Hb–Eu are given in Fig- centered at 410 nm and another band with a peak at
ure 3. In both cases, two broad patterns at medium scat- 465 nm.
tering angle (2θ = 5–50°) exhibit two resolved Bragg diffrac-
The bridged polysilsesquioxane MICPTES–Hb–Eu
tion peaks for MICPTES–Hb at 2θ = 21.16 and 9.46° and shows red photoluminescence upon radiation with ultravio-
for MICPTES–Hb–Eu at 2θ = 21.63 and 10.51°, respec- let light (λ = 365 nm). The excitation and emission spectra
tively. For both cases, the broad pattern observed at ca. 21° are shown in Figure 5. The excitation spectrum was moni-
5
7
can be attributed to the Si–O–Si contribution, as observed tored around the more-intense line of the D Ǟ F transi-
0
2
4782
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Eur. J. Inorg. Chem. 2008, 4781–4785

Luminescent Triazine-Containing Bridged Polysilsesquioxanes
Figure 5. Excitation (left) and emission (right) spectrum of
MICPTES–Hb–Eu.
Figure 4. SEM images of materials (a, b) MICPTES–Hb and (c, d)
MICPTES–Hb–Eu.
tion at 612 nm. A large broad band between 250 and
4
3
50 nm with two main components peaking at ca. 275 and
35 nm, respectively, overlapped with relatively low-inten-
6
sity peaks ascribed to an intra-4f line can be observed. The
band at 275 nm can be attributed to π–π* transition based
on the conjugated double bonds of the heterocyclic ligand,
and the band at 335 nm is assigned to the conjugated sys-
tem between the heterocyclic and urea bonds. Moreover,
the lower relative intensity of the Eu ion strongly suggests
[19]
3
+
that the metal ions are essentially excited by a sensitized Scheme 1. Predicted structure of the bridged polysilsesquioxane
process rather than by direct population of the intra-4f6 MICPTES–Hb–Ln.
levels. The luminescence spectrum was measured with
3
35 nm as the excitation wavelength. The narrow peaks ob-
5
served in the spectrum are transitions between the D ex-
cited state and the different J levels of the ground term F
0
7
J
5
7
(J = 0–4). The presence of the forbidden D Ǟ F transi-
0 0
3
+
tion indicates that Eu ions are located in a coordination
sphere with low symmetry.[ The D Ǟ F transition cor-
31]
5
7
0
1
responds to a parity-allowed magnetic dipole transition,
[
32]
which is just about independent of the host material. The
5
7
hypersensitive D Ǟ F transition varies strongly with the
0
2
3
+
local surrounding around the Eu ions. Its intensity in-
creases when the lattice environment is distorted and con-
tains certain components of noninversion symmetry.
[
31]
5
7
Thus, the ratio (R) between the intensities of the D Ǟ F
0
2
5
7
and D Ǟ F transitions can be considered as a parameter
0
1
to probe the “asymmetry” of the Eu3+ sites. Here, the R
value is about 4, indicating that the local surrounding ob-
served in our sample is highly asymmetric. Furthermore, Figure 6. Decay curve of MICPTES–Hb–Eu.
the fine splitting of the transition lines is related to a “crys-
talline” lanthanide ion environment rather than an amorph-
[
33]
3+
5
ous one,
suggesting coordination between the Eu ion
The D quantum efficiency (q) and the number of the
0
and the oxygen atoms of the carbonyl group as well as the water molecules coordinated to the Eu3 ion can be esti-
+
nitrogen atoms from the melamine unit as shown in mated on the basis of the emission spectrum and the life-
5
Scheme 1. The luminescence decay curve of MICPTES– time of the D state by using the following equations ac-
0
[
34]
Hb–Eu was recorded with an emission at 612 nm. The fluo- cording to ref.
rescence decay profile is single exponential (Figure 6), re- radiative processes are involved in the depopulation of the
By assuming that only nonradiative and
5
vealing a lifetime value around 0.744Ϯ0.003 ms.
D state, q may be defined by Equation (1).
0
Eur. J. Inorg. Chem. 2008, 4781–4785
© 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
4783

N. Lin, H. Li, Y. Wang, Y. Feng, D. Qin, Q. Gan, S. Chen
FULL PAPER
(
1)
where k and k are the radiative and nonradiative proba-
r
nr
bilities, respectively. The radiative contribution may be cal-
5
7
culated from the relative intensities of the D Ǟ F (J =
0
J
0–4) transitions and can be expressed by Equation (2).
(
2)
where A0–1 is Einstein’s coefficient of spontaneous emission
5
7
between the D and F level, usually considered to be
0
1
–
1
equal to 50 s , and E
and S
are the energy and the
0
–J
0–J
5
7
integrated intensity of the D Ǟ F transitions, respectively.
0
J
The number of water molecules (n ) coordinated to the
w
Eu3+ ions in the bridged polysilsesquioxanes can be esti-
Figure 7. Excitation (left) and emission (right) spectra of
mated by modification of Horrock’s equation,[ and it is MICPTES–Hb–Tb.
35]
given in Equation (3).
n
w
= 1.1(kexp – k
r
– 0.31)
(3)
5
where kexp is the reciprocal value of the D lifetime. The
0
data obtained are summarized in Table 1.
5
Table 1. Experimental D
0
lifetime, calculated radiative and nonra-
diative 5
5
0 0
D decay rate, D quantum efficiency value, and the
number of water molecules coordinated to the Eu
3+
ions.
nr /ms–1 q / %
0.94 30
Sample
τ / ms
k
r
/ ms–1
0.404
k
n
w
MICPTES–Hb–Eu
0.744
0.6
The relatively longer lifetime and lower number of coor-
[
19]
dinated water molecules relative to the published data
3
+
show that the Eu ions are well protected from the envi-
ronment. It is well established that the chelating moieties
are in their enolized form.[ Thus, it is reasonable to pre-
dict the structure (Scheme 1) of the obtained bridged poly-
silsequioxanes.
36]
Figure 8. Decay curve of MICPTES–Hb–Tb.
The europium(III) ion can be replaced in this system by
terbium(III) ions. The obtained material, MICPTES–Hb–
Tb, shows strong green photoluminescence when irradiated Conclusions
under a UV lamp. The excitation spectrum was monitored
In summary, we synthesized a monomer containing large
at 544 nm, and it is similar to that of MICPTES–Hb–Eu,
that is, a large broad band with two main components
peaking at ca. 275 and 335 nm are observed (Figure 7).
Nevertheless, no f–f transition could be detected. This me-
ans that very effective energy transfer from the hybrid mate-
rials to the terbium(III) ions occurs. The luminescence spec-
trum was measured with 335 nm as the excitation wave-
length. The emission spectrum of MICPTES–Hb–Tb ob-
tained by excitation at 335 nm presents four emissions lines
organic bridging groups of complex chemical structure,
consisting of a heterocyclic ring integrated through flexible
alkyl chains with three trimethoxysilyl groups. Luminescent
bridged polysilsesquioxanes possessing the luminescence
characteristics of Eu3 and Tb ions were obtained by hy-
+
3+
drolysis and condensation of the new monomer in the pres-
ence of Eu3 and Tb ions, respectively. The predicted
structure of the materials was given based on FTIR analy-
sis, luminescence spectra, and data from the literature.
+
3+
3
+
5
7
for the Tb
ions, peaking at 489 ( D Ǟ F ), 545
4 6
5
7
5
7
5
7
(
D Ǟ F ), 586 ( D Ǟ F ), and 620 nm ( D Ǟ F ). The
4 5 4 4 4 3
5
7
D Ǟ F green emission is the most prominent peak, and
4
5
Experimental Section
no emission arising from the host material can be detected,
suggesting efficient energy transfer from the host material
Materials: Melamine (Acros) and 3-(triethoxysilyl)propyl isocyan-
ate (Aldrich) were used as received. EuCl ·6H O and TbCl ·6H O
3
+
5
to the Tb ions. The luminescence decay of the D level
0
3
2
3
2
can be well fitted by one exponential as evidenced in Fig- were obtained by dissolving Eu O and Tb O into HCl solution.
2
3
4
7
ure 8. The fitted value for the lifetime is 1.31Ϯ0.006 ms.
(37.5%).
4784
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Eur. J. Inorg. Chem. 2008, 4781–4785

Luminescent Triazine-Containing Bridged Polysilsesquioxanes
Preparation of Monomer: To a solution of melamine (1 mmol) dis-
solved in DMSO (5 mL) was added 3-(triethoxysilyl)propyl isocy-
anate (0.75 mL, 3 mmol), and the mixture was heated at reflux at
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[
Preparation of MICPTES–Hb: To a solution of the monomer
Mater. Chem. 2005, 15, 3598–3627.
(
(
1 mmol) dissolved in DMSO (5 mL) was dropwise added ethanol
7.5 mL) and a solution of HCl (0.2 , 1.25 mL). The mixture was
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0% of its original volume. The solution was then poured into a
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(
1 mmol) dissolved in DMSO (5 mL) was dropwise added a solu-
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[
[
[
[
[
[
[
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for 12 h, and the mixture was the concentrated to 30% of its origi-
nal size. The solution was then poured into a flat-bottomed plastic
box and dried at 80 °C for 4 d. The transparent orange monolith
was obtained. It is named as MICPTES–Hb–Eu and MICPTES–
Hb–Tb for Eu3+ and Tb ions, respectively.
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Received: June 26, 2008
Published Online: September 10, 2008
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