Construction and photoluminescence of monophase hybrid materials derived from a urea-based bis-silylated bipyridine

Article abstract of DOI:10.1002/ejic.200801017

A urea-based bis-silylated bipyridine ligand derived from 4,4′-diamino-2,2′-bipyridine has been prepared. Organic-inorganic hybrid materials with a high loading of lanthanide 2,2-bipyridine moieties were obtained by using the silylated bipyridine as the o

Full text of DOI:10.1002/ejic.200801017

FULL PAPER
DOI: 10.1002/ejic.200801017
Construction and Photoluminescence of Monophase Hybrid Materials Derived
from a Urea-Based Bis-Silylated Bipyridine
[a]
[a]
[a]
[a]
[a]
[b]
Huanrong Li,* Nana Lin, Yige Wang, Yu Feng, Quanying Gan, Hongjie Zhang,*
[a]
[a]
Qinglin Dong, and Yuhuan Chen
Keywords: Luminescence / Lanthanides / Hybrid material / Materials science / Energy conversion / Silylated bipyridine
A urea-based bis-silylated bipyridine ligand derived from
,4Ј-diamino-2,2Ј-bipyridine has been prepared. Organic–in-
organic hybrid materials with a high loading of lanthanide
,2-bipyridine moieties were obtained by using the silylated
bipyridine as the only siloxane network precursor in the pres-
ence of lanthanide ions (or lanthanide complexes). The in-
situ formation of lanthanide complexes from lanthanide ions
and the silylated bipyridine during the sol–gel processing
was confirmed by the luminescence behavior of the obtained
hybrid materials and that of the corresponding pure lantha-
2 3 2
nide complex [Ln(bpy) Cl ·2H O]. Tris(2-thenoyltrifluoroace-
4
tonato)europium(III) dihydrate complexes were coupled to
the hybrid materials via adduct formation with the nitrogen
atoms from bis-silylated bipyridine, and the coordinated
water molecules were expelled from the first coordination
sphere of the lanthanide(III) ion as revealed by the much
longer lifetime of the hybrid material compared with that of
the pure tris(2-thenoyltrifluoroacetonato)europium(III).
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2009)
2
Introduction
tional class II hybrid organic–inorganic luminescent materi-
als were monophasic even at a high concentration of or-
ganic chelates.
The incorporation of luminescent centers into sol–gel-
derived siloxane hybrid materials has attracted much inter-
est for photonic applications as they potentially combine
the optical quality of silica, its thermal stability, and its me-
[2,5,15–25]
With these methods, we have previously reported on red
and green light-emitting hybrid materials by grafting of lan-
[16]
thanide 2,2-bipyridine complex within silica gel.
The ri-
chanical strength, together with the optical characteristics
gid framework of 2,2-bipyridine and its high affinity toward
various cations are attractive features for the construction
of the active molecules.[
1–2]
Indeed, materials with stable
and highly luminescent properties could have many applica-
tions including fiber amplifiers, solid-state lasers, and biola-
[26]
of tunable chromophores.
2,2-Bipyridine can form che-
3+
3+
lates with lanthanide ions such as Tb and Eu . In such
cases, the organic ligand absorbs exciting radiation and
transfers the excitation energy to the lanthanide emitters.
Actually, 2,2-bipyridine ligands are antennas of near-UV
[
3–5]
beling.
The sol–gel technique represents an attractive
means of synthesizing such materials since it allows pro-
cessing into thin films, fibers, or monoliths.[
6–7]
Lanthanide complexes entrapped in sol–gel-derived host
structures have been described recently. Nevertheless, simple
incorporation of lanthanide complexes within silica gels can
radiation because of their efficient ligand-to-metal intrac-
[27–28]
omplex energy transfer.
However, the luminescent hy-
brid materials were obtained by using both tetraethyl ortho-
silicate (TEOS) and silylated 2,2-bipyridine as the siloxane
network precursors, where only a small proportion of che-
result in an inhomogeneous dispersion of both components
and leaching of dopants.[
8–14]
A successful alternative ap-
proach to obtain luminescent hybrids is to covalently bind
the complexes to a silica matrix. The as-derived multifunc-
lating ligand (only up to 20 mol-% of the total siloxane net-
[16]
work precursors) can be found.
Consequently, the con-
centration of lanthanide complexes grafted within the silica
gel is still relatively low although a stoichiometric amount
[
[
a] School of Chemical Engineering and Technology, Hebei Uni- of lanthanide ions (with respect to silylated bipyridine) can
versity of Technology,
be obtained. Therefore, it is very attractive to prepare lumi-
nescent hybrid materials by using silylated chelating ligand
as the only siloxane network precursor. Furthermore, al-
though this method is very convenient to covalently graft
lanthanide complexes to the matrix, europium bipyridine
complexes are not highly desired because the water mole-
cules can deactivate the excited state of the europium ion
via radiationless deactivation. Grafting β-diketonate com-
Tianjin 300130, P. R. China
Fax: +86-2252-64294
E-mail: lihuanrong@hebut.edu.cn
b] Key Laboratory of Rare Earth Chemistry and Physics,
Changchun Institute of Applied Chemistry, Chinese Academy
of Sciences,
Changchun 130022, P. R. China
E-mail: hongjie@ciac.jl.cn
Supporting information for this article is available on the
WWW under http://www.eurjic.org or from the author.
Eur. J. Inorg. Chem. 2009, 519–523
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
519

H. Li, H. Zhang et al.
FULL PAPER
plexes to the silica matrix via adduct formation with the drolysis and condensation of the bis-silylated bypyridine in
nitrogen atoms from bonded heterocyclic ligands can lead the presence of lanthanide ions or lanthanide complexes
to great improvements in luminescent behavior.[
22]
(Schemes 1 and 2). All the materials obtained in this work
In this paper, the synthesis and characterization of a are amorphous as revealed by XRD patterns (not shown)
urea-based bis-silylated bipyridine ligand derived from 4,4Ј- where only a broad peak at about 2θ = 22 can be observed.
diamino-2,2Ј-bipyridine are described. Michel Wong Chi
Man has described the synthesis and catalytic properties
of the urea-based bis-silylated bipyridine.[ Luminescent
29]
organic–inorganic hybrid materials were obtained by using
the silylated bipyridine as the only siloxane network precur-
sor, where lanthanide complexes were formed in situ during
the sol–gel processing as revealed by the luminescence be-
havior of the obtained materials.
Results and Discussion
The urea-based bis-silylated bipyridine precursor was
synthesized from 4,4Ј-diamino-2,2Ј-bipyridine and 3-(tri-
ethoxysilyl)propyl isocyanate. The bis-silylated bipyridine
with the capability of forming one-component hybrid or-
ganic–inorganic materials avoided the problem of phase
separation. Thus, lanthanide complexes can be anchored to
the framework of hybrid organic–inorganic materials by hy- Scheme 2. Predicted structure of Hb-Eu-TTA.
Figure 1 shows the FT-IR spectra of Hb and Hb-Eu. In
both cases, a ν(Si–C) vibration located in the 1196–
1
206 cm–1 wavelength range is observed, which is consistent
with the fact that the (Si–C) bond can survive during hy-
drolysis/condensation reactions. The broad absorption
band at 1120–1000 cm–1 [ν(Si–O–Si)] indicates the forma-
tion of siloxane bonds and residual silanol groups are evi-
–1
denced by the ν(Si–OH) stretching vibration at 904 cm .
This means the condensation reactions are not complete as
is expected when using organically modified alkoxysilanes
[
2,30]
as silica network precursors.
1
The bands at 1696 and
535 cm–1 can be ascribed to the absorption of –CONH–
groups. The bands peaking at 1592 and 1467 cm in Fig-
ure 1 (a) can be assigned to the absorption of the bpy moie-
ties. Upon the introduction of europium ions, the band
from the bpy moieties (1467 cm ) disappears and a new
–1
[
2]
–
1
Scheme 1. Synthesis of the urea-based bis-silylated bipyridine and
the predicted structure of Hb-Ln (Ln = Eu,Tb).
Figure 1. FT-IR spectra of hybrid materials Hb (a) and Hb-Eu (b).
520
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Eur. J. Inorg. Chem. 2009, 519–523

Monophase Hybrid Materials
band peaking at 1441 cm–1 (Figure 1, b) appears. The new can be ascribed to the fact that the relatively rigid structure
band might be caused by the absorption of bipyridine euro- of the matrix restricts the vibration of the ligand and thus
[
31–32]
[33]
pium complexes.
results in the decrease in nonradiative transition.
Replacement of europium ions with the complex
Eu(TTA) ·2H O gives birth to the highly luminescent hy-
3
2
brid material Hb-Eu-TTA. The excitation and emission
spectra are shown in Figure 3. The excitation spectrum
monitored at 612 nm displays a large broad band and one
very low-intensity peak (marked with *) ascribed to an in-
Luminescence Properties
The hybrid material Hb-Eu shows a strong red photolu-
minescence upon irradiation with ultraviolet light. The exci-
tation and emission spectra of Hb-Eu and the lanthanide
complex Eu(bpy) Cl ·2H O are shown in Figure 2. As an-
6
tra-4f line. The broad band may result from the πǞπ* elec-
tron transition of the organic ligand. The lower relative in-
tensity of the Eu3 line strongly suggests that the metal ions
are essentially excited via an efficient sensitized process
2
3
2
+
ticipated, the hybrid material Hb-Eu shows similar exci-
tation and emission spectral features as the complex
Eu(bpy) Cl ·2H O. Thus, the proposed schemes are rather
6
rather than by direct population of the intra-4f levels. The
2
3
2
luminescence was measured with 325 nm as the excitation
wavelength. The emission spectrum is similar to that of hy-
brid material Hb-Eu. Nevertheless, the intensity ratio of the
speculative and represent illustrative examples of possible
coordination (Schemes 1 and 2). The broad band in the ex-
citation spectra can be assigned to the absorption of 2,2-
bipyridine. The five prominent emission peaks at 579, 593,
5
7
5
7
D Ǟ F line to that of the D Ǟ F line for Hb-Eu-TTA
0
2
0
1
(
15) is much higher than Hb-Eu (4.0). Moreover, 86.5% of
6
12, 650, and 698 nm in the emission spectra can be attrib-
5
7
5
7
the luminescent light output is emitted via the D Ǟ F
uted to the D Ǟ F (J = 0–4) transition with red emission
0 2
0
J
line for Hb-Eu-TTA, while this value is only about 62.9%
for J = 2 as the dominant feature. There is no evidence for
emission from the triplet state of the bipyridine (no broad
band emission observed in the blue and green spectral re-
gions). This indicates that there is an efficient energy trans-
fer from the triplet states of the organic ligands to the cen-
tral europium ion. When the spectra of Hb-Eu and Eu(bpy)
Cl ·2H O are compared, it is evident that the resolution is
[
34–35]
for Hb-Eu.
This means that a higher monochromatic
purity can be obtained upon the incorporation of Eu(TTA)3·
2
H O instead of europium ions. The radiative lifetime of
2
3
+
Eu in hybrid material Hb-Eu-TTA (0.540 ms) is much
longer than in the pure complex Eu(TTA) ·2H O
3
2
(0.368 ms) and hybrid material Hb-Eu (0.459 ms). This sug-
2
3
2
gests that the tris(2-thenoyltrifluoroacetonato)europi-
um(III) dihydrate complex was bound to the 2,2-bipyridine
groups on the silica gel and the coordinated water was ex-
better in the case of Eu(bpy) Cl ·2H O, and this could be
2
3
2
[
22]
ascribed to the site-to-site inhomogeneities. The lifetimes
5
3+
of the D Eu excited level for Eu(bpy) Cl ·2H O and
0
2
3
2
[
22]
pelled.
As a consequence, ternary lanthanide complexes
Hb-Eu were measured at room temperature using an exci-
tation wavelength of 325 nm and monitored around the
most intense emission line at 613 nm (not shown); the data
for both are well fitted by a single exponential function,
confirming that all europium ions occupy the same average
local environment within each sample. The luminescence
lifetimes were determined to be 0.368 ms for the lanthanide
complex and 0.459 ms for Hb-Eu. The prolonged lifetime
of Hb-Eu compared with that of the lanthanide complex
bonded to the silica gel have been formed in situ during
[
22]
the sol–gel processing (Scheme 2). To further confirm the
formation of the Eu(tta)3(bpy) complex, the luminescence
features of Hb-Eu-TTA were compared with that of
Eu(TTA) (bpy) (see Supporting Information). It is evi-
3
denced that excitation and emission spectra of Hb-Eu-TTA
are very comparable to those of the pure Eu(TTA) (bpy)
3
complexes, both the peak positions and the fine structure
are the same (Supporting Information). All the experimen-
tal data support the hypothesis that Eu(TTA) (Bpy) com-
3
plexes are covalently linked to the hybrid organic–inorganic
material.
Figure 2. Excitation (dash-dot line) and emission (solid line) spec-
tra of (a) lanthanide complex Eu(bpy)
brid material hybrid-Eu.
2
Cl
3
·2H
2
O, and (b) the hy-
Figure 3. Excitation (dash-dot line) and emission (solid line) spec-
tra of hybrid material Hb-Eu-TTA.
Eur. J. Inorg. Chem. 2009, 519–523
© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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521

H. Li, H. Zhang et al.
FULL PAPER
When the europium ions in hybrid material Hb-Eu were roacetonato)lanthanide(III) dihydrate complexes were cou-
replaced by terbium ions, a hybrid material (Hb-Tb) with pled to the hybrid materials via adduct formation with the
intense green photoluminescence can be observed. The lu- nitrogen atoms from bis-silylated bipyridine.
minescence behavior of Hb-Tb is similar to that of hybrid
material Hb-Eu. As seen in Figure 4, the excitation spec-
trum monitored at 544 nm is dominated by a broad band Experimental Section
with two main components peaking at ca. 254 and 316 nm,
Starting Materials and Experimental Techniques: Starting materials
respectively, and no f-f transitions can be observed. Exci-
were purchased from Aldrich or Fluka and were used as received.
tation into the ligand at 327 nm results in a bright green
All solvents were distilled before utilization according to literature
emission, the line emissions are assigned to the transitions
[35]
procedures. Lanthanide chlorides were obtained from the corre-
5
7
from the D level to the F levels (J = 6, 5, 4, 3) at 490,
4
J
sponding oxides in HCl (37.5%). 4,4Ј-Diamino-2,2Ј-bipyridine was
5
44, 584, and 620 nm, respectively. No emission from the synthesized according to a previously reported procedure.[36] The
ligand is observed, which indicates that the surrounding li- [Eu(tta) ·2H O] and Ln(bpy) Cl ·2H O (bpy = 2,2Ј-bipyridine)
3
2
2
3
2
[
37]
gands absorb energy and transfer energy efficiently to the complexes were prepared according to the published procedure.
chelated terbium ions. The decay curve of Hb-Tb was mea-
sured under the excitation wavelength that maximizes the
Synthesis of the Precursor DPS-ICPTES: 4,4Ј-Diamino-2,2Ј-bipyr-
idine (1 mmol) was added to dry tetrahydrofuran (THF; 10 mL).
emission intensity. The decay curve is well-reproduced by To this mixture 3-(triethoxysilyl)propyl isocyanate (2.4 mmol) was
5
means of a single exponential function, yielding a D -life- slowly added and the resulting solution was refluxed for 10 h. After
4
time value of 1.12 ms, which is much longer than that of the evaporation and washing with petroleum ether, a yellow oil of DPS-
ICPTES was obtained. Yield 10%. IR (KBr): ν˜ = –CONH– 1692,
pure lanthanide complex [Tb(bpy) Cl ·2H O, τ = 0.736 ms].
2
3
2
1
1
2
2 3
534; aromatic: 1590, 1467; SiOCH CH : 1166, 956; Si–C: 1198–
–
1
1
3,3Ј
6
205 cm . H NMR ([D ]DMSO): δ = 8.13 (d, 2 H, H ), 7.90 (s,
6
,6Ј
4,4Ј
H, H ), 7.46 (d, 2 H, H ), 7.10 (s, 2 H, NH), 6.5 (brs, 2 H,
), 3.21 (t , 4 H, NCH), 1.55 (quint, 4 H,
(OEt)], 0.55 (t, 4 H, SiCH ) ppm.
NH), 3.73 (q, 12 H, OCH
CH ), 1.16 [t, 18 H, CH
2
2
2
3
2
Synthesis of Hybrid Material Hb-Ln (Ln = Eu, Tb): A solution of
LnCl in DMF (2 mL) and deionized H O (0.5 mL) (acidified to
.2  HCl) was added to a DPS-ICPTES solution in THF. The
3
2
0
mixture was vigorously stirred at room temperature for 12 h and
the volume of solvent was evaporated to ca. 30% of its original
size. Then the solution was poured into a flat-bottomed plastic box
and was dried at 80 °C for 3 d. A transparent orange solid was
3
+
obtained. The molar ratio of Ln ion to DPS-ICPTES was 1:2.
Synthesis of Hybrid Material Hb-Eu-TTA: Hb-Eu-TTA was pre-
Figure 4. Excitation (dash-dot line) and emission (solid line) spec-
3 2
pared similarly except that Eu(tta) ·2H O was added. The molar
tra of (a) lanthanide complex Tb(bpy)
brid material hybrid-Tb.
2 3 2
Cl ·2H O, and (b) the hy-
ratio of Eu(tta) ·2H O to DPS-ICPTES was 1:1.
3
2
Characterization: Infrared spectra were obtained with a Bruker
–
1
Vector 22 spectrometer using KBr pellets, from 400–4000 cm at
In order to further confirm the formation of lanthanide
complexes between Tb3+ ions with 2,2-bipyridine within the
–
1
a resolution of 4 cm (16 scans collected). About 2 mg of each
compound was mixed with potassium bromide (Merck, spectro-
scopic grade), finely ground, and pressed into pellets. X-ray diffrac-
tion measurements were carried out on powdered samples with a
hybrid material, we compared the luminescence properties
of Hb-Tb with those of the corresponding pure lanthanide
complex [Tb(bpy) Cl ·2H O]. The Hb-Tb material shows a
2
3
2
Bruker D8 diffractometer using monochromatic Cu-K
α
radiation
similar excitation spectral feature as the complex Tb(bpy)2·
H O and no obvious shift in wavelength of emission line
(
λ = 1.540 Å).
2
2
The steady-state luminescence spectra and the lifetime measure-
ments were measured with an Edinburgh Instruments FS920P
near-infrared spectrometer, with a 450-W xenon lamp as the
steady-state excitation source, a double-excitation monochromator
peaks exists. The similarity in luminescence properties fur-
ther indicates the formation of lanthanide complexes within
the sol–gel-derived matrix.
–
1
–1
(
1800 lines mm ), an emission monochromator (600 lines mm ),
and a semiconductor-cooled Hamamatsu RMP928 photomultiplier
tube.
Conclusions
In summary, new luminescent materials covalently Supporting Information (see also the footnote on the first page of
this article): Excitation and emission spectra of Eu(TTA)3(bpy)
and hybrid material Hb-Eu-TTA.
bonded with lanthanide complexes were prepared via an in-
situ method by hydrolysis and polycondensation of a urea-
based bis-silylated bipyridine precursor in the presence of
lanthanide ions or lanthanide complexes. The high concen-
tration of lanthanide complexes grafted to the hybrid silica
Acknowledgments
was anticipated because the silylated bipyridine was used as This work was financially supported by Hebei University of Tech-
the only siloxane network precursor. Tris(2-thenoyltrifluo- nology, the Key Project of Chinese Ministry of Education (208016),
522
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© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2009, 519–523

Monophase Hybrid Materials
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Received: October 16, 2008
2001, 13, 4542–4549.
Published Online: December 19, 2008
Eur. J. Inorg. Chem. 2009, 519–523
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