Highly homogeneous, transparent and luminescent SiO2 glassy layers containing a covalently bound tetraazacyclododecane-triacetic acid-Eu(III)-acetophenone complex

Article abstract of DOI:10.1039/b514409d

The preparation of sol-gel glasses is described wherein highly stable Eu(III) centres are anchored to the silica glass upon acidic hydrolysis and by using tetraethylorthosilicate (TEOS) as a silica source. The complex employed is Eu·1 and is obtained from

Full text of DOI:10.1039/b514409d

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Highly homogeneous, transparent and luminescent SiO2 glassy layers
containing a covalently bound tetraazacyclododecane–triacetic
acid–Eu(III)–acetophenone complex
Silvio Quici,*a Marco Cavazzini,a Maria Concetta Raffo,a Lidia Armelao,*b Gregorio Bottaro,b
Gianluca Accorsi,c Cristiana Sabatinic and Francesco Barigelletti*c
Received 11th October 2005, Accepted 16th November 2005
First published as an Advance Article on the web 6th December 2005
DOI: 10.1039/b514409d
The preparation of sol–gel glasses is described wherein highly stable Eu(III) centres are anchored
to the silica glass upon acidic hydrolysis and by using tetraethylorthosilicate (TEOS) as a silica
source. The complex employed is Eu?1 and is obtained from bipartite ligand 1, which features a
DO3A macrocycle as a hosting unit for the Eu(III) centre, a methoxy-acetophenone unit as an
antenna chromophore, and an alkyl chain bearing one primary hydroxy group (DO3A is 1,4,7,10-
tetraazacyclododecane-1,4,7-triacetic acid). Eu?1 is linked to the forming silica glass through the
–OH groups and a uniform distribution of the luminescent centres is obtained, as indicated by
results from secondary ion mass spectrometry in-depth profiling. The luminescent efficiency of the
Eu(III) centres in the solid matrix is evaluated to be 10%, which is comparable to that of Eu?1 in
water solution and of a convenient reference complex, Eu?2 (2 is a ligand featuring the same
hosting and antenna functionalities of 1, wherein the –OH residue is replaced by an unreactive
group, a benzoylmethyl residue). A discussion is given of the photophysical properties exhibited
by the complexes in the film and in solution.
Eu(III) complexes are regarded as convenient red-emitting
= 612 nm.25–28 However, while a wealth
Introduction
em
max
phosphors, with l
Luminescent lanthanide complexes can find applications,
among others, as diagnostic tools,1–3 sensors,4 optical fibre
lasers and amplifiers,5,6 and electroluminescent materials.7
Great efforts have been devoted in order to find out chelating
agents resulting in highly stable complexes and several ligand
systems have been explored like polyaminocarboxylates,8
cryptands,9 calixarenes,10,11 podands12 and helicates.13,14
Somewhat disappointingly, approaches involving direct coor-
dination of chromophores at the 8–9 available positions of the
lanthanide centre have sometimes proved not very convenient
from the point of view of the stability of the resulting com-
plexes or of their efficiency as luminophores.9,15,16 This has
stimulated alternative approaches aimed at generating highly
stable and luminescent complexes. For instance, in carefully
designed bipartite systems17–24 one can exploit the tight coor-
dination taking place at a macrocycle unit and obtain light
absorption by an appended, not directly coordinated chromo-
phore; in several cases this approach appears to yield com-
plexes featuring both good structural and optical properties.25
of studies in solutions is available for Eu(III) complexes (and of
other lanthanides), in order to be useful for some of the target
applications, these emitters have to be included or grafted into
a solid matrix. For instance, incorporation into polymers5,29,30
or sol–gel glasses31–35 represents an appealing possibility. Yet,
within a solid matrix, the wished statistical distribution of
the chromophores is not always observed; instead changeable
degrees of aggregate formation may occur.36 Clearly, forma-
tion of aggregates can be a source of problems, because mutual
interaction between spatially close luminescent centres may
lead to self-quenching, a parasitic event against luminescence
efficiency.
In this paper we describe the sol–gel preparation of silica-
based glassy layers with a homogeneous distribution of the
Eu(III)-luminophores in the solid matrix.37,38 The employed
photoactive unit is a bipartite system, Eu?1, comprised of a
DO3A macrocyclic hosting unit for the lanthanide centre and a
MAP-containing chromophore (DO3A is 1,4,7,10-tetraazacy-
clododecane-1,4,7-triacetic acid, and MAP is 49-methoxy-
acetophenone).23 Chart 1 shows the schematic formula of
Eu?1 and of the reference complex Eu?2, see Experimental
section. Within Eu?1 and Eu?2, all the coordination positions
of the lanthanide centre are fulfilled by DO3A but one
(only one coordination site is available to solvent molecules,
which is indicated by q = 1),23 and these complexes are highly
aIstituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio
Nazionale delle Ricerche (CNR), Via C. Golgi 19, 20133 Milano, Italy.
E-mail: silvio.quici@istm.cnr.it
bIstituto di Scienze e Tecnologie Molecolari (ISTM), Consiglio
Nazionale delle Ricerche (CNR) e Consorzio Interuniversitario per la
Scienza e Tecnologia dei Materiali (INSTM), Dipartimento di Scienze
Chimiche, Universita` di Padova, Via Marzolo 1, 35131 Padova, Italy.
E-mail: armelao@chin.unipd.it
stable, the 1 : 1 DO3A:Ln association constant being KA
1020 M21 39
.
.
Eu?1 can be covalently linked to the forming silica
cIstituto per la Sintesi Organica e Fotoreattivita` (ISOF), Consiglio
Nazionale delle Ricerche (CNR), Via P. Gobetti 101, 40129 Bologna,
Italy. E-mail: franz@isof.cnr.it
glass through the –OH moieties, yielding highly homogeneous
layers with luminophores tightly anchored to the host medium.
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purification of the crude by column chromatography.
Treatment of 3 with CF3COOH, at room temperature,
afforded the tricarboxyl derivatives 2, in 92% yield, as a
brown solid after purification through a short Amberlite XAD
1600T column.
The Eu?1 complex was prepared in a one step procedure
starting from the benzoyl protected ligand 2 by saponification
with aqueous NaOH-2% and subsequent complexation of the
crude 1 with EuCl3?6H2O that was carried out at pH 6.5–7.0
in water at reflux. The product was purified by column
chromatography with Amberlite XAD 1600T eluting first
with water, to remove the inorganic salts, and then with
water–CH3CN 9/1 (v/v) and afforded pure complex in
quantitative yield.
Functionalized acetophenone 5 was obtained in two steps:
alkylation of the commercially available 4-hydroxyacetophe-
none with 3-bromopropyl benzoate carried out in CH3CN
and solid Na2CO3 as base and in the presence of 5%
of tetrabutylammonium bromide (TBAB) as phase transfer
Chart 1 Eu?1 and Eu?2 complexes.
catalyst; and bromination of the intermediate
6 with
N-bromosuccinimide (NBS) and para-toluensulfonic acid in
CH3CN at reflux according to a literature procedure used for
similar compounds.42
Chart 2 schematically depicts the binding of the Eu(III)
complex to the silica glass matrix.
Results and discussion
Physical and optical properties of films
Design and synthesis of ligands and complexes
The films were prepared as described in the Experimental
section. The europium distribution throughout their thickness
was investigated by secondary ion mass spectrometry (SIMS)
depth profiling on selected DO3A-silica layers. In Fig. 1 is
reported the SIMS in-depth distribution of Eu, C, and N
as well as Si and O for an as-prepared sample (Fig. 1a) and
for a film dried at 100 uC for 5 h (Fig. 1b). In both cases,
homogeneous profiles of all elements throughout the whole
film thickness are observed. Moreover, the remarkable
similarity in the shape of Eu, C and N profiles suggests a
common chemical origin of these species, in agreement with
the presence of DO3A throughout the investigated depth.
Estimates of film thickness yielded values around 40 nm
irrespective of the thermal treatment, that at our employed
conditions correspond to ca. 4.8 nm3 of film volume/
luminophore.
Bipartite ligand
1 (see Scheme 1), features a 1,4,7,10-
tetraazacyclododecane-1,4,7-triacetic acid (DO3A) as the
coordination site for the lanthanide cation, bearing on the
fourth nitrogen atom a covalently bonded functionalized
acetophenone as chromophore. The choice of the chromo-
phore, besides its high efficiency in the sensitization process,
as demonstrated by Beeby, Williams and coworkers,23 is
due mainly to the easy functionalization of its 49 position
with a short alkyl chain bearing one primary hydroxyl
group. As a matter of fact, –OH groups are useful for the
anchoring of complexes to a forming silica network during a
sol-gel process.40
DO3A was selected as the Ln-chelating subunit because of
the high thermodynamic and kinetic stabilities which are
well assessed for its lanthanide complexes.39 Ligand 1 was
synthesized according to Scheme 1. Alkylation of DO3A
tris(tertbutyl) ester 4,41 with 3-[4-(2-bromoacetyl)phenoxy]pro-
pyl benzoate 5 carried out in CH3CN at reflux in the presence
of solid Na2CO3 as base afforded 3, in 91% yield, after
While ligand 1 is non-emissive upon irradiation at 305 nm,
both Eu?1 in water and the film incorporating this complex
showed a strong red emission, Table 1 and Fig. 2. Actually,
within the chromophore/luminophore associate, the MAP unit
acts as an antenna for sensitisation of the lanthanide-based
luminescence. This occurs according to the following sequence
of events:25 (i) absorption of UV light at 305 nm (e308 = 19 300
and 24 300 M21 cm21 for Eu?1 and Eu?2, respectively), (ii)
highly efficient intersystem crossing (ISC) from the populated
singlet (S) level to the triplet (T) level, lying at 25 200 cm21
(wISC = 1)43,44 and (iii) transfer of excitation energy to the
lanthanide cation. The last step, a MAP A Eu(III) energy (en)
transfer, occurs with an efficiency wen y 1,23 as expected
because the acetophenone triplet exhibits a slow decay (kT
5 6 106 s21 44,45
and the energy gap between the donor and
y
)
the acceptor levels is ca. 7300 cm21, i.e., so high as to hamper
Chart 2 The binding of the Eu(III) complex to the silica glass matrix.
742 | J. Mater. Chem., 2006, 16, 741–747
any back-transfer process.8,25
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Scheme 1 Outline of synthetic steps.
Table 1 lists the optical properties of the film as compared to
that of dilute (1 6 1025 M) air-equilibrated water solutions
of the Eu?1 parent complex and of the related Eu?2 complex.
similar complexes that were found to exhibit practically
identical luminescence properties in water solution.23 The lack
of any emission for the lanthanide-free chromophore 1 is
explained by the fact that the lowest-lying singlet level of
MAP is of acetophenone origin and is depopulated with
unit efficiency by the ISC step, leading to a non-emissive
triplet.23,43,44
Fig.
2 further illustrates the absorption and emission
behaviour of the film and of the Eu?1 model in water. The
absorption results indicate that inclusion of the lanthanide
centre causes a shift of the absorption peak of the MAP unit
from 282 (for the lanthanide-free ligand 1) to 308 for both the
Eu?1 and the Eu?2 complexes in water solution and to 310 nm
for the Eu?1 unit bound to the silica-glass. A similar
perturbation effect of the electronic properties of the MAP
ligand by the cationic Eu(III) centre was already noted for very
The luminescence spectra observed in solution and in film
are quite similar to each other and appear finely resolved. The
narrow peaks are due to the transitions between the 5D0
excited level and the various terms of the ground level, 7FJ,
J = 0–6,25,28 of which those for J = 0–4 are apparent in Fig. 2.
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Fig. 2 Room temperature absorption spectrum (left) and emission
profile (right) of the Eu?1 complex in water (a) and of the film
incorporating the Eu?1 (b); lexc = 305 nm for both cases.
Experimental
Material and methods
All commercially available solvents and reagents were used as
received. TLC was carried out on silica gel Si 60-F254. Column
chromatography was carried out on silica gel Si 60 mesh size
0.040–0.063 mm (Merk, Darmstadt, Germany). 1H and 13C
NMR spectra were recorded with a Bruker AC 300 spectro-
meter operating at 300 MHz and 75 MHz respectively.
Elemental analyses were carried out by the Departmental
Service of Microanalysis (University of Milan).
Fig. 1 SIMS depth profile for an as-prepared sample (a) and for a
film dried at 100 uC for 5 h (b).
Table 1 Absorption and emission dataa
Absorption
max/nm e/M21 cm21
Emission
max/nm wse
l
l
t/ms kr/s21 b
Eu?1
Eu?2
film(EuN1) 310
308
308
19 300
24 300
—
612
612
616
0.080 0.60 133
0.081 0.62 131
0.096d 0.73 132
Synthetic procedures. The synthetic paths are summarized in
c
Scheme 1.
a
b
At room temperature. kr
c
= w/t Extinction coefficient not
determined Evaluated from w = tkr,18,23,49 by taking kr = 132 s21
,
d
Synthesis of 3-(4-acetylphenoxy)propyl benzoate (6).
A
and disregarding the effect of the different refraction index for water
and film.19,48
mixture of 4-hydroxyacetophenone (2.04 g, 15 mmol),
3-bromopropyl benzoate (4.0 g, 16.5 mmol) and sodium
carbonate (4.77 g, 45 mmol) in 60 cm3 of acetonitrile was
heated at reflux and maintained under magnetic stirring for
five days. After this time the reaction mixture was cooled at
room temperature, filtered, the solvent evaporated in vacuo
and the residue was purified by column chromatography
(silica gel, CH2Cl2) to afford 6 (3.21 g, 72%) as a white
solid; dH (400 MHz, CDCl3) 2.29 (2 H, quintet, J 6.2,
COOCH2CH2CH2O), 2.55 (3 H, s, COCH3), 4.20 (2 H, t,
J 6.2, COOCH2CH2CH2O), 4.53 (2 H, t, J 6.2, COOCH2-
CH2CH2O), 6.93 (2 H, d, J 8.8, Ar(A)H2,H6), 7.43 (2 H, bt, J
7.7, Ar(B)H3,H5), 7.56 (1 H, bt, J 7.7, Ar(B)H4), 7.92 (2 H, d, J
The most intense peak occurs at 612 nm in solution (616 nm
in the film), and corresponds to the 5D0
This is the so called hypersensitive transition,23,46,47 and the
A
7F2 transition.
ratio between its intensity and that for the lower energy
7
transition (5D0 A F1) is ca. 5 and 6 for the solution and the
film, respectively. This suggests that the Eu-based lumino-
phore is embedded in a similar environment for the two cases
(solution and film), which in turn points to a high degree of
homogeneity for the physical state of the film as pointed out by
SIMS analysis.
744 | J. Mater. Chem., 2006, 16, 741–747
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8.8, Ar(A)H3,H5), 8.04 (2 H, d, J 7.4, Ar(B)H2,H6); dC
(100.6 MHz, CDCl3) 26.3 (COCH3), 28.7 (COOCH2CH2-
CH2O), 61.6 (COOCH2CH2CH2O), 64.8 (COOCH2CH2-
CH2O), 114.2 (Ar(A)C2,C6), 128.4 (Ar(B)C3,C5), 129.6
(Ar(B)C2,C6), 130.1 (Ar(B)C1), 130.5 (Ar(A)C4), 130.6
(Ar(A)C3,C5), 133.0 (Ar(B)C4), 162.7 (COOCH2CH2CH2O),
166.5 (Ar(A)C1), 196.7 (COCH3); m/z (ESI) 321.10923
(100.0%), 322.11282 (19.6%), 323.11606 (1.8%) [M + Na]+
(C18H18O4 requires 298.33).
129.9 (Ar(B)C1), 132.6 (Ar(A)C2,C6), 133.1 (Ar(B)C4),
163.1 (COOCH2CH2CH2O), 166.5 (Ar(A)C1), 172.8
(NCH2COOtBu), 197.8 (Ar(A)COCH2N10); m/z (ESI)
833.46769 (100.0%), 834.47107 (47.6%), 835.47441 (11.1%)
(M + Na+) (C44H66N4O10 requires 810.48).
Synthesis of 10-{4-[3-(benzoyloxy)propyloxy]benzoylmethyl}-
1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid (2). A solu-
tion of 3 (1.67 g, 2.06 mmol) in 30 cm3 of trifluoroacetic acid
was stirred at room temperature for 4 h. The solvent was
evaporated and the viscous oil residue was heated with 30 cm3
of diethyl ether to give 2.32 g of a brown clear solid. This
product was dissolved in 5 cm3 of water and adsorbed on a
short Amberlite XAD 1600T column which was eluted with
water until the pH was neutral and then with water–
acetonitrile 1/1 (v/v) to recover 2 (1.21 g, 92%) as a brown
solid; dH (400 MHz, CDCl3) 2.27 (2 H, quintet, J 6.2,
COOCH2CH2CH2O), 3.09 (8 H, bs, (NCH2)H2,H3,H5,H6),
3.45 (8 H, bs, (NCH2)H8,H9,H11,H12), 3.69 (2 H, s, (N4CH2-
COOH)), 3.77 (4 H, s, (N1CH2COOH, N7CH2COOH)), 4.23
(2 H, t, J 6.2, COOCH2CH2CH2O), 4.51 (2 H, t, J 6.2,
COOCH2CH2CH2O), 6.99 (2 H, d, J 8.8, Ar(A)H2, H6), 7.48
(2 H, bt, J 7.7, Ar(B) H3,H5), 7.60 (1 H, bt, J 7.7, Ar(B)H4),
7.95 (2 H, d, J 8.8, Ar(A)H3,H5), 8.02 (2 H, d, J 7.4,
Ar(B)H2,H6); dC(100.6 MHz, CDCl3) 28.3 (COOCH2-
CH2CH2O), 48.5 and 48.8 ((NCH2)C2,C3,C5,C6), 50.8 and
50.9 ((NCH2)C8,C9,C11,C12), 53.5 (N4CH2COOH), 56.2
(N1CH2COOH, N7CH2COOH)), 61.6 (COOCH2CH2CH2O),
64.8 (COOCH2CH2CH2O), 114.0 (Ar(A)C3,C5), 128.4
(Ar(B)C3,C5), 128.6 (Ar(A)C4). 129.1 (Ar(B)C2,C6), 130.0
(Ar(B)C1), 130.2 (Ar(A)C2,C6), 132.9 (Ar(B)C4), 163.3
(COOCH2CH2CH2O), 166.6 (Ar(A)C1), 169.2 (NCH2-
COOH), 172.9 (Ar(A)COCH2N10); m/z (ESI) 665.27906
(100.0%), 666.28282 (34.6%), 667.28755 (5.9%), [M + Na]+
(C32H42N4O10 requires 642.29).
Synthesis of 3-[4-(2-bromoacetyl)phenoxy]propyl beanzoate
(5). N-Bromosuccinimide (0.89 g, 5 mmol) was added in two
portions to a solution of 6 (1.49 g, 5 mmol) and p-toluensul-
fonic acid monohydrate (1.71 g, 9 mmol) in 100 cm3 of
acetonitrile. The reaction mixture was then heated at reflux
and stirred for one hour. The solvent was evaporated in vacuo
and the residue was dissolved in 150 cm3 of CH2Cl2 and
washed with 100 cm3 of H2O. The organic phase was
separated, dried over MgSO4 and the solvent evaporated to
afford a white solid residue which was purified by column
chromatography (silica gel, CH2Cl2) to give 5 (1.56 g, 83%)
as a white solid; dH (400 MHz, CDCl3) 2.30 (2 H, quintet,
J 6.2, COOCH2CH2CH2O), 4.21 (2 H, t, J 6.2, COOCH2CH2-
CH2O), 4.39 (2 H, s, COCH2Br), 4.53 (2 H, t, J 6.2,
COOCH2CH2CH2O), 6.93 (2 H, d, J 8.9, Ar(A)H3,H5), 7.43
(2 H, bt, J 7.6, Ar(B)H3,H5), 7.56 (1 H, bt, J 7.3, Ar(B)H4),
7.95 (2 H, d, J 8.9, Ar(A)H2,H6), 8.03 (2 H, d, J 7.4,
Ar(B)H2,H6); dC (100.6 MHz, CDCl3) 28.7 (COOCH2CH2-
CH2O), 30.7 (COCH3), 61.5 (COOCH2CH2CH2O), 64.9
(COOCH2CH2CH2O), 114.5 (Ar(A)C3,C5), 127.0 (Ar(A)C4),
128.4 (Ar(B)C3,C5), 129.6 (Ar(B)C2,C6), 130.1 (Ar(B)C1),
131.4 (Ar(A)C2,C6), 133.0 (Ar(B)C4), 163.3 (COOCH2-
CH2CH2O), 166.5 (Ar(A)C1), 189.9 (COCH2Br); m/z (ESI)
399.01968 (100.0%), 400.02372 (19.5%), 401.01783 (97.3%),
402.02167 (18.9%) [M + Na]+ (C18H17BrO4 requires 376.0).
Synthesis of 10-{4-[3-(benzoyloxy)propyloxy]benzoylmethyl}-
1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid tris(1,1-
dimethylethyl) ester (3). A suspension of 5 (1.13 g, 3 mmol),
4HCl (1.65 g, 3 mmol) and sodium carbonate (3.18 g, 30 mmol)
in 50 cm3 of acetonitrile was refluxed for 30 h under magnetic
stirring. The mixture was cooled at room temperature
and filtered over a fritted glass and the filtrated evaporated
to afford a viscous yellow oil. Purification by column
chromatography (silica gel, CH2Cl2–MeOH 95/5) afforded
3 (2.22 g, 91%) as a white solid; dH (400 MHz, CDCl3)
1.46 (27 H, s, C(CH3)3), 2.29 (2 H, quintet, J 6.2, COOCH2-
CH2CH2O), 2.09–3.90 (22 H, m, (NCH2)H2,H3,H5,H6),
Synthesis of 10-[4-(3-hydroxypropoxy)benzoylmethyl]-
1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid euro-
pium(III) complex (Eu?1). A solution of 2 (321 mg, 0.5 mmol)
and sodium hydroxide (200 mg, 5 mmol) in 50 cm3 of ethanol–
water 95/5 was heated at reflux and stirred for two hours. The
solvent was evaporated in vacuo; the residue was taken up in
50 cm3 of water and the pH was adjusted at 6.5 by addition of
5% aqueous chlorydric acid. A solution of EuCl3?6H2O
(201 mg, 0.55 mmol) in 5 cm3 of water was slowly added to
the previously prepared solution containing the ligand keeping
the pH at 6.5–7.0 by contemporary addition of 5% aqueous
sodium hydroxide. The reaction mixture was then heated at
reflux for four hours, cooled at room temperature and
concentrated to 5 cm3 by evaporation of the solvent under
reduced pressure. The product was purified by column
chromatography with Amberlite XAD 1600T, eluiting first
with water to remove all inorganic salts and then with water–
acetonitrile 9/1 to afford Eu?1 (365 mg, quantitative yield) as
a light brown solid (Found: C, 39.13; H, 5.87; N, 6.92.
C25H35EuN4O9?5H2O requires C, 38.60; H, 5.84; N, 7.20);
m/z (ESI) 711.15193 (100.0%), 709.14954 (89.7%), 712.15509
(28.0%), 710.15331 (25.9%), [M+Na]+ (C25H35EuN4O9
requires 688.16).
(NCH2)H8,H9,H11,H12,
N4CH2COOH,
N1CH2COOH,
N7CH2COOH, 4.21 (2 H, t, J 6.2, COOCH2CH2CH2O), 4.54
(2 H, t, J 6.2, COOCH2CH2CH2O), 6.94 (2 H, d, J 8.8, Ar(A)H2,
H6), 7.45 (2 H, bt, J 7.7, Ar(B) H3,H5), 7.58 (1 H, bt, J 7.7,
Ar(B)H4), 7.88 (2 H, d, J 8.8, Ar(A)H3,H5), 8.05 (2 H, d, J 7.4,
Ar(B)H2,H6); dC (100.6 MHz, CDCl3) 28.0 (C(CH3)3), 28.6
(COOCH2CH2CH2O), 55.6 and 55.8 ((NCH2)C2,C3,C5,C6,
(NCH2)C8,C9,C11,C12), 59.9 (N4CH2COOtBu, N1CH2-
COOtBu, N7CH2COOtBu), 61.6 (COOCH2CH2CH2O), 64.8
(COOCH2CH2CH2O), 81.9 (C(CH3)3), 114.3 (Ar(A)C3,C5),
128.4 (Ar(B)C3,C5), 128.9 (Ar(A)C4), 129.5 (Ar(B)C2,C6),
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10-{4-[3-(benzoyloxy)propyloxy]benzoylmethyl}-1,4,7,10-tet-
raazacyclododecane-1,4,7-triacetic acid europium(III) complex
(Eu?2). The pH of a solution of 2 (321 mg, 0.5 mmol) in water
(50 cm3) was adjusted at 6.5 by addition of 2% aqueous sodium
hydroxide. A solution of EuCl3?6H2O (201 mg, 0.55 mmol) in
5 cm3 of water was slowly added to the previously prepared
solution containing the ligand keeping the pH at 6.5–7.0 by
contemporary addition of 2% aqueous sodium hydroxide. The
reaction mixture was then heated at reflux for three hours,
cooled at room temperature and concentrated to 5 cm3 by
evaporation of the solvent under reduced pressure. The
product was purified by column chromatography with
Amberlite XAD 1600T, eluting first with water to remove all
inorganic salts and then with water–acetonitrile 8/2 to afford
Eu?2 (313 mg, 79%) as a light brown solid (Found: C, 45.39; H,
5.30; N, 6.59. C32H39EuN4O10?3H2O requires C, 45.45; H,
5.36; N, 6.63); m/z (ESI) 815.17798 (100.0%), 813.17613
(91.6%), 816.18020 (34.6%), 814.17914 (31.7%), [M + Na]+
(C32H39EuN4O10 requires 792.19).
Absorption spectra of water solutions of Eu?1, Eu?2 (see
Chart 1) and of films on glass substrate containing Eu?1 were
measured at room temperature with a Perkin-Elmer Lambda 5
UV/Vis spectrophotometer. The absorption profile for both
solutions and films is ascribed to the methoxy-acetophenone
unit,23 with subsequent energy transfer to the lanthanide center
resulting in the observed luminescence, Fig. 2. For the steady
state luminescence experiments performed at room tempera-
ture, slightly different geometries were employed for the cases
of solutions or films. For the water solutions, 1 cm cuvettes
were used with a 90u arrangement between the excitation light
and the detector. For the films, the glass plane was at 30u to
the incident light in order to ensure detection of luminescence
and avoid scattered light from the source. The selected
excitation wavelength was 305 nm in all cases, which is close
to the peak maximum of the lowest-energy absorption band,
vide infra. For both solution and film cases, the absorbance of
the samples was lower than 0.05, thus ensuring transmittance
of more than 90% of the incident light. The luminescence
spectra were registered with a Spex Fluorolog II spectro-
fluorimeter equipped with Hamamatsu R928 phototube or
with an Edinburgh FLS920 spectrometer equipped with
Hamamatsu R5509-72 supercooled photomultiplier tube
(193 K), and a TM300 emission monochromator with NIR
grating blazed at 1000 nm; 150 and 450 W Xenon arc lamps
were used, respectively as light sources. Corrected lumines-
cence spectra in the range 500–820 nm were then obtained by
taking care of the known wavelength dependence of the
phototube response. The excitation spectra taken at 612 or
616 nm, for solution and film cases, respectively, were found to
correspond to the absorption spectra of the complexes.
Preparation of the film. The sol–gel starting solution was
prepared using tetraethylorthosilicate Si(OC2H5)4 (TEOS), as
silica source. The europium complex Eu?1 was first dissolved in
ethanol at room temperature and subsequently TEOS, water
and hydrochloric acid were added. The molar composition of
the resulting solution was: 1TEOS : 15EtOH : 0.1HCl : 3H2O
while the Si/DO3A molar ratio was 440. Before film deposi-
tion, the transparent and clear solution was aged at room
temperature under stirring for 24 h.
Silica based thin films were obtained by the dip-
coating procedure using Herasil1 silica slides (Heraeus,
Luminescence efficiencies for lanthanide-based sensitized
emission (wse) were evaluated either by comparison of
relative intensities,48 or by exploiting known photophysical
parameters for the Eu(III)-based emission.49 In the former
case, one compares the wavelength integrated area of the
lanthanide luminescence with that of isoabsorbing samples of
[Ru(bpy)3]Cl2 as a luminescence standard (w = 0.028 in air-
equilibrated water50). The latter approach is based on the fact
that the luminescence lifetime of an emitter is governed by
radiative (r) and non-radiative (nr) processes, t = 1/(kr + knr),
and that for a Eu(III) centre the intrinsic radiative rate
Quarzschmelze, Hanau, Germany) as
a substrate. Film
deposition was carried out in air at room temperature with a
controlled withdrawal speed of about 10 cm min21. Coatings
were obtained by means of a multi-dipping process, up to
3 depositions, without any thermal treatment between them.
The as-prepared samples containing the Eu?1 luminescent
component resulted homogeneous, well-adherent to the sub-
strates, transparent, and crack free.
The obtained layers were subsequently used in the lumines-
cence experiments both as-prepared and dried at 100 and
200 uC for 5 h. Higher treatment temperatures were dis-
regarded in order to avoid the degradation of the europium
complex occurring at T . 250 uC, as evidenced from thermal
analysis performed in the same conditions (in air, 10 uC min21
heating rate) adopted for film annealing.
constant is moderately affected by the environment, kr
=
90/170 s
.
21 19,23,49 This allows evaluation of kr and knr from the
observed value of t, and the luminescence efficiency is in turn
evaluated on the basis of w = kr/(kr + knr).23 Luminescence
lifetimes on the ms time scale were obtained with a Perkin-
Elmer LS50B luminescence spectrometer; all decays were
single exponentials, both for solutions (1 cm cuvettes, 90u
between the excitation light and the detector) and films (30u
between the excitation light and the plane of the film). The
experimental uncertainty on the absorption and luminescence
maxima is 2 and 1 nm, respectively, that for the w and t values
is 20 and 10%, respectively.
Equipment. Secondary ion mass spectrometry (SIMS) depth
profiles were obtained by an IMS 4f mass spectrometer
using a 14.5 keV Cs+ primary beam (10 nA) and negative
secondary ion detection. Beam blanking mode was used to
improve the depth resolution. The charge compensation was
achieved by means of an electron gun. For each sample the
erosion speed was evaluated at various depths measuring
the corresponding crater by means of a Tencor Alpha Step
profiler. The dependence of the sputtering rate on the material
composition was therefore taken into account in the film
thickness determination.
Acknowledgements
Financial support is acknowledged by MIUR through FIRB
projects (RBNE019H9K, entitled ‘‘Nanometric machines
746 | J. Mater. Chem., 2006, 16, 741–747
This journal is ß The Royal Society of Chemistry 2006

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20 F. C. J. M. Van Veggel, G. A. Hebbink, S. I. Klink, L. Grave and
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21 M. H. V. Werts, J. W. Verhoeven and J. W. Hofstraat, J. Chem.
Soc., Perkin Trans. 2, 2000, 433.
through molecular manipulation’’ and RBNE033KMA,
entitled ‘‘Molecular compounds and hybrid nanostructured
materials with resonant and non resonant optical properties
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for photonic devices’’) and
a
FISR project (entitled
la
‘‘Nanotecnologie molecolari per l’immagazzinamento
e
trasmissione delle informazioni’’). The authors are grateful to
Dr Cinzia Sada (INFM and Department of Physics, Padova
University) for SIMS measurements and to Gennaro Ferro
(Department of Chemistry, Padova University) for useful help
in the experimental work.
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