Luminescent properties of silica and zirconia xerogels doped with europium(III) salts and europium(III) cryptate incorporating 3,3′ -biisoquinoline- 2,2′ -dioxide

Article abstract of DOI:10.1016/S0009-2614(00)01059-9

Eu⁺³ cryptate with 3,3^′-biisoquinoline-2,2^′-dioxide unit was incorporated into glass films consisting of zirconia dioxide and into hybrid ormosil films. The absorption and the emission intensities of the europium complex were compared to the intensity of the non-complexed europium ion in the films. The transition probabilities of the complex are increased as a result of the mixing of the europium orbitals with the ligand orbitals. The emission is additionally increased as the non-radiative relaxation is diminished by the cage effect.

Full text of DOI:10.1016/S0009-2614(00)01059-9

1
7 November 2000
Chemical Physics Letters 330 (2000) 515±520
www.elsevier.nl/locate/cplett
Luminescent properties of silica and zirconia xerogels doped
with europium(III) salts and europium(III) cryptate
0
0
incorporating 3; 3 -biisoquinoline-2; 2 -dioxide
b,
*
a
T. Saraidarov , R. Reisfeld , M. Pietraszkiewicz
a,1
a
Department of Inorganic and Analytical Chemistry, Hebrew University of Jerusalem, 91940 Jerusalem, Israel
b
Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01224 Warsaw, Poland
Received 6 May 2000; in ®nal form 6 September 2000
Abstract
‡3
0
0
Eu cryptate with 3; 3 -biisoquinoline-2; 2 -dioxide unit was incorporated into glass ®lms consisting of zirconia
dioxide and into hybrid ormosil ®lms. The absorption and the emission intensities of the europium complex were
compared to the intensity of the non-complexed europium ion in the ®lms. The transition probabilities of the complex
are increased as a result of the mixing of the europium orbitals with the ligand orbitals. The emission is additionally
increased as the non-radiative relaxation is diminished by the cage e€ect. Ó 2000 Elsevier Science B.V. All rights
reserved.
1. Introduction
Recently, several studies have been performed
on trivalent rare-earth ions doped in such gel
It has been shown that sol±gel method can be
used for the preparation of solid-state tunable la-
sers by incorporating laser dyes into porous glasses
2 2
matrices as SiO and ZrO via the sol±gel process
[13]. Inorganic rare-earth salts, chloride or nitrate
were used. Only a weak emission can be observed
[
1±4]. Chemical and biological sensors can also be
2 2
from rare-earth ions doped in SiO and ZrO
formed by the sol±gel method when appropriate
reagents are introduced into glass [5±7]. Also
semiconductor nanocrystals in transparent glass
prepared at moderate temperature [14] due to non-
radiative relaxation which originated from an in-
teraction of the water residue with the hydroxyl
ions. It has been shown that organic complex of
®
lms have recently received attention due to their
excellent electronic properties and promising ap-
plications in non-linear optics and optical switch-
ing [8±11].
2
rare earths doped in SiO gel had higher ¯uores-
cence properties with respect to comparable inor-
ganic salts [15±18]. The implication of this ®nding
is that rare-earth organic complexes doped in sol±
gel hosts are good candidates for phosphors, active
waveguides, optical sensors and markers of bio-
logical molecules.
*
Corresponding author.
E-mail addresses: renata@vms.huji.ac.il (R. Reisfeld), pie-
In this context, it was of interest to study the
incorporation of Eu(III) cryptate within the
trasz@ichf.edu.pl (M. Pietraszkiewicz).
1
Enrique Berman Professor of solar energy.
0009-2614/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 0 0 9 - 2 6 1 4 ( 0 0 ) 0 1 0 5 9 - 9

5
16
T. Saraidarov et al. / Chemical Physics Letters 330 (2000) 515±520
zirconia xerogel glass which is especially photo-
chemically stable.
2. Experimental section
Earlier works on luminescent lanthanide mac-
rocyclic and macropolycyclic complexes indicated
that the most ecient ligands were those incor-
porating 3; 3 -biisoquinoline-2; 2 -dioxide unit as
sensitiser for the lanthanide emission [19±25].
Among diverse structures, one was the most
promising, having excellent quantum yield for
emission, good luminescence lifetime in the range
of 0.6 ms, and good stability in aqueous media. Its
formula is shown in Scheme 1.
2.1. Materials
Zirconium n-propoxide (70% solution in 1-
propanol, Fluka) was used as received without
further puri®cation, n-propanol of analytical grade
(Riedel-de Haen), glacial acetic acid (analytical
grade, Frutarom, Israel), glymo (analytical grade,
Aldrich), 2-methoxyethanol of analytical grade
(Fluka) and triply distilled water were used for
solution preparations.
0
0
Very recently several articles dealing with the
luminescence of xerogels doped with Eu(III) and
Chemglass CGQ-0640-01 (USA) quartz slides
were used as substrates for the ®lm deposition.
The europium cryptate was prepared according
to the literature [19] using europium tri¯uoro-
methanesulfonate.
Tb(III)
15,25,27,28].
In the present Letter, two zirconia ®lms doped
complexes
have
been
published
[
with Eu(III) cryptate were prepared, one of them
as a hybrid material obtained by cross-condensa-
tion of zirconia tetrapropoxide and 3-glycidoxy-
propyltrimethoxysilane (glymo). The hybrid
matrix incorporating silica and organic part was
expected to bring two advantages: to bind water
via oxirane ring opening, and to provide organic
hydrophobic environment advantageous to repel
the remnant water molecules from the proximity
of the Eu(III) cryptate. The expectation was
to enhance the luminescence performance. The
Eu(III) ion was also prepared in a zirconia ®lm for
comparison.
2.2. Sample preparation
2.2.1. Preparation of zirconia stock solution (S)
First, zirconium n-propoxide (10 ml) was di-
luted in 20 ml of n-propanol and stirred for 15
min, then 3 ml of glacial acetic acid was added
and stirred further for 15 min. The solution was
hydrolysed with 3.3 ml of acetic acid±water so-
lution (1:1) and stirred for 30 min, ®ltered and
stored in a refrigerator up to 4 days, the ®nal
stock solution was obtained by diluting the con-
centrate solution with n-propanol to 72.6 ml.
Eight ml of this stock solution contains 0.00248
2
mole of ZrO .
2
±
.2.2. Preparation of zirconia ®lm doped with Eu
(1)
Eight ml of the stock solution, being precursor
2
O
3
of 0.00248 ZrO was mixed with 0.01 g of euro-
2
pium oxide (0.0114 mole per mole ZrO ), 0.5 ml of
2
nitric acid (conc.) and 4 ml of 2-methoxyethanol.
The ®lm was obtained by a dip-coating technique,
with a withdrawal speed of the quartz substrate of
2
0 cm/min, and was dried at 40°C for 1 h and
heated at 100°C for 1 h.
2
±
.2.3. Preparation of zirconia ®lm (for a reference)
(2)
The stock solution (8 ml) and 2-methoxy-
Scheme 1. Formula of the Eu(III) cryptate.
ethanol (4 ml) were mixed and used for dip-coating

T. Saraidarov et al. / Chemical Physics Letters 330 (2000) 515±520
517
pure zirconia ®lm which was dried and heated as
above.
2.2.4. Preparation of zirconia ®lm doped with Eu
cryptate ± (3)
The stock solution (8 ml), 0.02 g of europium
cryptate (0.0091 mole per mole ZrO ) and 2-
2
methoxyethanol (4 ml) were mixed together. The
®
lm was obtained and heated as above.
2
.2.5. Preparation of zirconia-ormosil ®lm with
glymo and doped with Eu cryptate ± (4)
.2 ml of glymo was added to solution (3) and
1
was stirred for 2 h. The ®lm was obtained and
treated as ®lm (3). The molar concentration of Eu
oxide and Eu complex in the ®lm was 0:01 Æ 0:001.
The thickness of the ®lm was 250 Æ 20 nm.
Fig. 1. Absorption spectra of zirconia and zirconia±glymo thin
®
2 3
lms doped with Eu O and Eu(III) cryptate.
tion and emission spectra of the complex in zir-
conia±glymo (2a) and zirconia thin ®lms (2b)
excited at 350 and 269 nm. The emission spectra
2.3. Measurements: UV±vis spectroscopy
5
7
Absorption spectra were measured on a Mil-
ton±Roy Spectronic M-3000 diode array spectro-
photometer.
Fluorescence spectra were measured on a JAS-
CO FP770 spectro¯uorimeter.
consist of several bands at 590 nm ꢀ D
0
! F
),
1
5
7
617 nm ꢀ D
0
! F
2
) (the most intense), 647 nm
5
5
7
7
ꢀ D
0
! F
3
) and 684 nm ꢀ D
0
! F
4
). For the
most intense band there is practically no visible
di€erence in relative intensities for zirconia cryp-
tate and zirconia±glymo-cryptate. Fig. 3 presents
the emission of quartz, zirconia oxide on quartz
and zirconia oxide doped with Eu oxide, excited at
3. Results and discussion
3
50 nm. Figs. 2 and 3 were drawn using di€erent
Absorption spectra of pure zirconia ®lm and
lm doped with Eu O were very similar. These
2 3
sensitivities. The intensities of Fig. 3 must be di-
vided by 39.4 in order to have the same scale as
Fig. 2. Hence, it is clear that the emission of Eu
oxide in zirconia heated at 100°C is negligible
compared to the complex.
Table 1 summarises the absorption and the
emission intensities obtained under excitation of
269 and 350 nm. It should be noted, however, that
because of the very low absorption of samples not
containing complexes the number of zirconia oxide
and zirconia doped with Eu oxide is insigni®cant.
These features indicated the absence of ecient
excitation of inorganic lanthanide materials, due
to the absence of sensitising ligands.
®
®
lms were transparent in the range of 350±700 nm,
showed a broad
2 3
and zirconia doped with Eu O
and weak band around 350 nm. The samples with
Eu(III) cryptate show an intense band at 269 nm
belonging to the charge transfer of Eu(III) cryptate
complex (Fig. 1).
It should be noted that the charge transfer in Eu
oxide is positioned in the same spectral range;
however, its intensity is negligible compared with
the complex prepared with the same concentration
of Eu. Both Eu(III) cryptates in zirconia and in
zirconia±glymo contain a band with a maximum at
3
50 nm characteristic of the heterocyclic ligand
Zirconia ®lms were doped with organic Eu(III)
complex that has been selected on the basis of the
best luminescence eciency among the family of
absorbance. The intensity in both cases is the same
within the experimental error. Therefore, there is
no evident in¯uence of glymo on the electronic
spectra of the cryptate. Fig. 2 presents the excita-
0
cryptates incorporating 3; 3 -biisoquinoline-2;
0
2 -dioxide. This complex, with the combination of

5
18
T. Saraidarov et al. / Chemical Physics Letters 330 (2000) 515±520
Fig. 3. Emission spectra of quartz, zirconia oxide ®lm and
zirconia doped with Eu
2
O
3
excited at 350 nm.
5
7
5
5
D ! F and F ! D (440 nm) are especially
0
0
0
2
sensitive to the chemical bond formed between
Eu(III) and its surrounding ligands. The removal
of degeneracy in low-symmetry sites of electronic
levels having J > 0 is also observed. In addition,
the non-radiative transition probabilities of
Eu(III) in aqueous solutions are increased by the
OH vibrations due to electron±phonon coupling.
The inhomogeneous broadening is typical for ions
in glasses prepared from the melt [12,26].
It is important to mention that the presence of
OH oscillators leads to the quenching of ¯uores-
cence by non-radiative multiphonon decay.
Therefore, this complex turned out to be the best
for this study.
As seen above, the excitation spectrum displays
two distinct bands with the maxima at 350 nm of
relatively weak intensity, and at 269 nm with high
intensity. There are two emission spectra for this
thin ®lm; one of low intensity when the excitation
was performed at 350 nm and the second of high
intensity with the excitation at 269 nm. This is
consistent with the expectation that excitation at
maximum of absorbance leads to high population
of the singlet excited state, therefore, radiationless
Fig. 2. (a) Excitation (617 nm) and emission spectra (excited at
69 and 350 nm) of Zr±glymo ®lm doped with Eu(III) cryptate;
2
(
ÐÐ) excitation spectra; (- - -) exc. at 269 nm; (Á Á Á) exc. at 350
nm. (b) Excitation (617 nm) and emission spectra (excited at 269
and 350 nm) zirconia ®lm doped with Eu(III) cryptate; (ÐÐ)
excitation spectra; (- - -) exc. at 269 nm; (Á Á Á) exc. at 350 nm.
the sensitising ligand and 4,13-diaza-18-crown-6,
was found to encapsulate Eu(III) cation eciently,
thus hiding it from the in¯uence of external envi-
ronment, in particular water molecules and to in-
crease the transition probabilities as a result of
mixing of the ligand orbitals with the orbitals of
europium. We have shown previously that Eu(III)
is an excellent indicator of site symmetry and
chemical bonding in glasses and biological mole-
cules. Our ®ndings were based on the fact that
Eu(III) incorporated in low-symmetry sites ex-
hibits enhanced f±f radiative transition probabili-
ties. The hypersensitive transitions such as
process of energy transfer from the singlet state via
5
triplet state to the level D
cient.
0
of the Eu(III) is e-
The excitation and emission spectra for the
Eu(III) cryptate in zirconia and Zr±glymo-cryptate
presented in Fig. 3 are very similar. This indicates

T. Saraidarov et al. / Chemical Physics Letters 330 (2000) 515±520
519
Table 1
Compositions and properties of zirconia and zirconia±glymo ®lms doped with Eu oxide and Eu cryptate
Films
Absorbance
69 nm D
exc. 269/em. 617 nm
Luminescence
exc. 350/em. 617 nm
2
1
350 nm D
2
RI L
1
=D
1
2 2
Luminescence RI L =D
intensity (L
1
)
2
intensity (L )
ZrO
ZrO
ZrO
ZrO
2
2
2
2
±Eu
2
O
3
0.031
0.041
0.201
0.214
0.023
2.89
3.85
93.2
93.9
0.4
0.46
17.39
23.35
0.0197
0.0289
0.0231
±EuCpt
±glymo±EuCpt
10,728
13,168
53,373
61,532
1198
1175
41,453
50,866
that the hybrid Zr±Si ®lms do not change the
microenvironment around the Eu(III) cryptate too
much. Although the water chemical binding by
epoxy groups may lead to remarkable reduction of
residual water in organic/inorganic network, there
are still hydroxyl groups on organic chains of the
initial glymo substrate that potentially may
quench the luminescence to some degree. On the
other hand, architecture of the cryptate provides
the eight heteroatoms for the co-ordination sphere
of the Eu(III), thus the cation is well hidden from
the external environment.
When the luminescence intensity is compared
for the excitation at 350 nm for zirconia- and zir-
conia±glymo-doped Eu(III) complex, a little dif-
ference is observed for the most intense emission
band at 617 nm (Table 1). The same comparison,
when excitation is at 269 nm, shows that the hy-
brid material Zr±Si has better performance in
Eu(III) luminescence. Therefore, it seems reason-
able to say that cross-linking the zirconia gel with
glymo has some advantage to get better material.
In this way, it was possible to obtain highly lu-
minescent materials based on Eu(III) cryptate,
whose potential may still be enhanced by the ad-
dition of co-ligands complementing the co-ordi-
nation sphere of the Eu(III) cation in order to
exclude fully the residual water in the ®rst co-or-
dination sphere. An alternative can be envisioned
by adding additional photoactive ligands that will
not only complement the co-ordination sphere of
the Eu(III), but will act synergistically to stream-
line the collected light to Eu(III) (antenna, and co-
¯uorescence e€ect). In the future, the luminescence
lifetimes and the quantum yields of emission will
shed more light on the interpretation of lumines-
cence features of these new materials. Work in this
direction is in progress.
An additional feature requires attention: it is
well known [26] that the ratio between the hy-
persensitive transition picking at 617 nm
5
7
( D
picking 590 nm ( D
0
! F
2
) to the magnetic dipole transition
5
7
0
! F
1
) re¯ects the site
symmetry at which europium is situated. By
comparing these ratios in Eu cryptate±zirconia
®lms to Eu oxide in zirconia ®lm (Figs. 2 and 3,
Table 2) we conclude that the symmetry at which
the Eu is situated is much lower in the cryptate
complex than in Eu ion surrounded by water
molecules. Table 2 collects the following data:
absorbance ꢀD†, luminescence intensity ꢀL† and
relative intensity (RI) de®ned as L=D. It can be
noted that the best luminescence for cryptate was
observed in the zirconia±glymo ®lm.
The Eu oxide as well as the complexes in zir-
conia ®lm were heated to 100°C.
Table 2
Wavelength and emission intensities of Eu-doped zirconia ®lms excited at 350 nm
Films
Total emission wavelength (nm) types of transitions and intensities
The ratio between
transition intensities
5
90
617
5
647
5
684
5
5
7
7
7
7
D
0
! F
4
D
0
! F
0.401
1
D
0
! F
2
D
0
! F
3
617 nm/590 nm
ZrO
ZrO
ZrO
ZrO
2
2
2
2
±Eu
2
O
3
0.396
0.556
0.285
0.192
45.48
46.97
255
1.016
0.829
6.807
7.939
0.461
0.088
157.33
137.79
±EuCpt
±glymo±EuCpt
176
148
1198
1175

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(
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complexes into zirconia ®lms results in a dramatic
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