Synthesis and photophysical characterization of highly luminescent silica films doped with substituted 2-hydroxyphthalamide (IAM) terbium complexes

Article abstract of DOI:10.1039/c1dt11131k

Innovative Tb³⁺ antenna complexes employing two different substituted 2-hydroxyphthalamide ligands (HxOH-IAM and bis-HxOH-IAM) acting simultaneously as coordinating sites and light collector units have been synthesized and successively anchored in silica layers by the sol-gel technique. The complexes show remarkable photoluminescence (PL) quantum yields in methanol solution, as high as 0.30 and 0.40 for (HxOH-IAM)₄?Tb ³⁺ and (bis-HxOH-IAM)₂?Tb³⁺, respectively. The grafting of the Tb³⁺ complexes in silica single layers accomplished by exploiting the terminal hydroxyl groups of the IAM chains results in highly transparent and homogeneous films displaying bright green emission and PL efficiencies of up to 0.40. The silica layers containing the (bis-HxOH-IAM)₂?Tb³⁺ show remarkable photostability even under prolonged and continuous irradiation (up to 3.5 h). The nature of the IAM ligands allows the photoexcitation of the complexes at wavelengths even longer than 350 nm, which is a spectral window suitable to develop luminescent lanthanide probes dedicated to bioanalyses and bioimaging applications.

Full text of DOI:10.1039/c1dt11131k

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PAPER
Synthesis and photophysical characterization of highly luminescent silica films
doped with substituted 2-hydroxyphthalamide (IAM) terbium complexes
Lidia Armelao,*a Gregorio Bottaro,a,b Silvio Quici,*c,d Marco Cavazzini,c,d Chiara Scalerac,d and
Gianluca Accorsi*e
Received 16th June 2011, Accepted 30th August 2011
DOI: 10.1039/c1dt11131k
Innovative Tb3+ antenna complexes employing two different substituted 2-hydroxyphthalamide ligands
(HxOH-IAM and bis-HxOH-IAM) acting simultaneously as coordinating sites and light collector units
have been synthesized and successively anchored in silica layers by the sol–gel technique. The complexes
show remarkable photoluminescence (PL) quantum yields in methanol solution, as high as 0.30 and
0.40 for (HxOH-IAM)4ÃTb3+ and (bis-HxOH-IAM)2ÃTb3+, respectively. The grafting of the Tb3+
complexes in silica single layers accomplished by exploiting the terminal hydroxyl groups of the IAM
chains results in highly transparent and homogeneous films displaying bright green emission and PL
efficiencies of up to 0.40. The silica layers containing the (bis-HxOH-IAM)2ÃTb3+ show remarkable
photostability even under prolonged and continuous irradiation (up to 3.5 h). The nature of the IAM
ligands allows the photoexcitation of the complexes at wavelengths even longer than 350 nm, which is a
spectral window suitable to develop luminescent lanthanide probes dedicated to bioanalyses and
bioimaging applications.
In the last few years, we have developed several lanthanide
Introduction
complexes, mainly Eu3+ and Tb3+, and studied their photophysical
The peculiar spectroscopic (optical and magnetic) characteri-
behaviour either in solution or incorporated into silica glassy films
stics1–4 of lanthanide ions have fuelled intense research activities
owing to a wide range of potential applications in different areas
such as biomedicine,5–7 sensing,8,9 imaging,2,10,11 lighting,2,12–21 and
telecommunications.20,22–25 The progress of many lanthanide-based
technologies26–28 may greatly benefit from the use of luminescent
complexes providing sharp, intense and long lived emission lines
upon light irradiation.
In order to design highly luminescent lanthanide complexes,
suitable metal core sensitization strategies are needed to overcome
the extremely low molar absorption coefficients (often in the range
0.1–1 M-1 cm-1) of the Laporte forbidden f –f transitions.
obtained through the sol–gel technique.29–35 In this regard, we have
proved that the coupling between very bright luminescent Eu3+ and
Tb3+ antenna complexes with the transparency and the stability of
silica is well suited for technological applications. In general, the
luminescence performances of the complexes improve in the oxide
host matrix, pointing out the effectiveness of such a strategy for
the preparation of efficient and colour tunable molecular-based
photoluminescent materials.21,29,36
Till now we have employed ligands suitably designed by syner-
gically coupling the coordination unit with properly appended
chromophores acting as sensitizers for Ln3+ emission (antenna
effect) through the induced fit approach. The mutual adaptation
of the ligand and metal cation has provided Ln3+ complexes
characterized by high emission quantum yields, remarkable ther-
modynamic stability and kinetic inertness.32,34 The use of light-
harvesting units that also contain binding sites for the metal core is
another interesting approach in the design of efficient Ln3+ ligands.
Although this method has been applied to the synthesis of a
large number of chromophores with different coordinating groups
and topologies, in most cases unstable Ln3+ complexes in aqueous
media have been obtained, thus preventing their use in biological
environment and/or showing low luminescence quantum yields.
These shortcomings have been overcome, among others,37
by Raymond and co-workers who developed new classes
of multidentate ligands containing two or more substi-
tuted 2-hydroxyphthalamide (IAM) or 1-hydroxypyridin-2-one
aIstituto 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 F. Marzolo, 1 - 35131, Padova, Italy. E-mail:
lidia.armelao@unipd.it
bIstituto di Metodologie Inorganiche e dei Plasmi (IMIP), Consiglio
Nazionale delle Ricerche (CNR) e Consorzio Interuniversitario per la
Scienza e Tecnologia dei Materiali (INSTM) - Dipartimento di Chimica -
Universita` di Bari, Via E. Orabona, 4 - 70126, Bari, Italy
cIstituto 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
dPolo Scientifico e Tecnologico (PTS), Consiglio Nazionale delle Ricerche
(CNR), via Fantoli, 16/15 - 20138, Milano, Italy
eMolecular Photoscience Group, Istituto per la Sintesi Organica e la
Fotoreattivita` (ISOF), Consiglio Nazionale delle Ricerche (CNR), Via P.
Gobetti, 101 - 40129, Bologna, Italy. E-mail: gianluca.accorsi@isof.cnr.it
11530 | Dalton Trans., 2011, 40, 11530–11538
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(1,2-HOPO) groups that cooperate in the coordination of the
Ln3+ cation thus affording kinetically inert complexes that are
stable in aqueous media.38–43 Either the IAM or the 1,2-HOPO
groups behave as bidentate ligands for Ln3+, the binding sites
being the oxygen atom of the phenolate or the deprotonated hy-
droxypyridine, and one carboxamide or ketone group, respectively.
The remaining amide group is involved in a hydrogen bond with
the phenolate in order to stabilize the overall complex structure.44
Furthermore, thanks to their electronic and structural features,
both IAM and 1,2-HOPO act as good sensitizers for most of
the Ln3+ ions emitting in the visible region (Sm3+, Eu3+, Tb3+
and Dy3+).39 While 1,2-HOPO is an excellent antenna for Eu3+,40
IAM proved to be particularly effective for Tb3+ giving rise to
structures with an overall emission quantum yield as high as 60% in
aqueous solution at physiological pH.39 According to our previous
works,29–31,33,35 we explored the possibility to prepare light emitting
silica layers using Tb3+ complexes of ligands containing the 2-
hydroxyphtalamide IAM group, that operate either as ligands
and sensitizers to Tb3+ emission, and suitably functionalized for
their covalent linking into the inorganic glass via soft solution
procedures. To this aim we report the synthesis, and chemical
and photophysical characterization of a bidentate (HxOH-IAM,
5) and a tetradentate (bis-HxOH-IAM, 8) 2-hydroxyphthalamide
ligand bearing one or two 6-hydroxyhexylamide chains in the N1
position, respectively. These ligands are used in the preparation
of Tb3+ complexes with metal to ligand stoichiometries 1 to 4 in
the case of (HxOH-IAM)4ÃTb3+ and 1 to 2 in the case of (bis-
HxOH-IAM)2ÃTb3+. Both complexes show one negative charge
that is likely saturated by a sodium cation (Fig. 1).
The two complexes are highly stable, as evidenced by the photo-
physical studies carried out in methanol solution. By exploiting the
terminal hydroxyl groups of the lateral hydroxyhexylamide chains,
both complexes have been grafted into silica films via a sol–gel
procedure. The photophysical properties of the obtained hybrid
layer materials were studied and compared with those obtained in
solution.
It is worth highlighting that these complexes can be excited
at longer wavelengths (even higher than 350 nm) with respect to
other lanthanide complexes studied and incorporated by us in
similar thin layers systems (lexc = 305 nm).30,31,33,34 This represents
one of the challenges in the design of luminescent lanthanide
probes dedicated to bioanalyses and bioimaging applications.
Indeed, the near UV or visible spectral regions are preferred
and targeted (especially when lexc ≥ 350 nm) to avoid damage
of biomolecules/biopolymers (e.g. DNA) by short- or medium-
wave UV light and tissue autofluorescence (usually below 320 nm)
interference.45
Fig. 1 Structure of ligands and of their corresponding Tb3+ complexes
evidencing the binding mode of the 2-hydroxyphthalamide (IAM) moiety.
mercaptothiazoline in CH2Cl2, in the presence of triethylamine as
base. The monomethylamide derivative 3 was selectively obtained
by adding very slowly, in 8 h via a syringe-pump, a dilute solution
of 40% aqueous methylamine in 95 : 5 (v/v) of CH2Cl2 : MeOH
into a magnetically stirred solution of 2 in CH2Cl2.43 Compound
3 was reacted with 6-hydroxyhexylamine in CH2Cl2 at room
temperature to afford 4 in an almost quantitative yield. The
methyl protecting group of this last compound was removed by
using BBr3 in CH2Cl2 to give the ligand HxOH-IAM (5) in 81%
yield after purification by column chromatography.
The Tb3+ complex was obtained by reacting four molar
equivalents of 5 with one molar equivalent of TbCl3·6H2O
in methanol as the solvent and in the presence of an excess
of pyridine as base. The pure complex (HxOH-IAM)4ÃTb3+
precipitated out as a white solid by addition of diethylether to
the reaction mixture, and the product was easily isolated by
filtration and dried under vacuum. The stoichiometry of the
complex was confirmed by high resolution ESI mass spectrom-
etry. The preparation of N1,N1¢-(pentane-1,5-diyl)bis[2-hydroxy-
N3-(6-hydroxyhexyl)isophthalamide] (bis-HxOH-IAM, 8), was
achieved starting from the bis-thiazolide derivative 2 according to
Scheme 2.
Results and discussion
Synthesis
The 2-hydroxy-N1-(hexan-6-ol)-3-(N3-methylbenzene)-1,3-dic-
arbamide (HxOH-IAM, 5) ligand was synthesized according
to Scheme 1. The bis-thiazolide derivative 2 was prepared
in 95% yield starting from the commercially available 2-
methoxyisophthalic acid 1 through the bis-acyl chloride obtained
by reacting 1 with SOCl2 in dioxane at reflux by using a few
drops of DMF as catalyst and subsequent treatment with 2-
Reaction of 2 with 6-hydroxyhexylamine was carried out under
the same conditions used in the preparation of the monomethy-
lamide derivative 3 (Scheme 1) and afforded 6 in 54% yield after
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Scheme 1 Synthesis of the bidentate ligand HxOH-IAM and of the corresponding Tb3+ complex.
purification by column chromatography. The very slow addition
of a solution of 1,5-pentandiamine in CH2Cl2 in 24 h via a syringe-
pump to a CH2Cl2 solution of 6 afforded 7 in satisfactory yield.
The deprotection of the methyl groups of 7 was carried out by
using an excess of BBr3 in CH2Cl2 at -30 ◦C for 24 h and afforded
the bis-HxOH-IAM (8) in 31% yield, as a yellow thick oil, after
purification by column chromatography. All attempts to improve
the yield of 8 failed. Indeed the use of a large excess of BBr3
or higher temperatures, in this last reaction, afforded significant
amounts of side products, derived from the substitution of the
terminal hydroxyl group by bromine which makes the purification
of 8 very difficult. The complex (bis-HxOH-IAM)2ÃTb3+ was
obtained by reacting two molar equivalents of 8 with one molar
equivalent of TbCl3·6H2O in methanol as the solvent and in the
presence of an excess of pyridine as the base. The pure complex
(bis-HxOH-IAM)2ÃTb3+ was precipitated as a white solid by
addition of diethylether to the reaction mixture, and the product
was easily isolated by filtration and dried under vacuum. The
stoichiometry of the complex was confirmed by high resolution
ESI mass spectrometry. The terbium complexes were studied by
thermogravimetric analysis (TGA) in order to investigate their
stability under heating in air. The determination of the structural
integrity upon thermal treatment is important in view of the
preparation of the doped-silica films, with the aim to define the
maximum annealing temperature for the materials. The variation
of weight loss as a function of the heating temperature for (HxOH-
IAM)4ÃTb3+ and (bis-HxOH-IAM)2ÃTb3+ is reported in Fig. 2.
For both compounds significant weight losses start at temper-
Fig.
2
Thermogravimetric curves for (HxOH-IAM)4ÃTb3+ and
(bis-HxOH-IAM)2ÃTb3+. Experiments were conducted in air at a heating
rate of 10 ◦C min-1.
mally induced decomposition processes of the terbium complexes
do not occur.
Photophysical characterization
The absorption and emission spectra of (HxOH-IAM)4ÃTb3+
complex in methanol solution are reported in Fig. 3.
As well established for Ln3+ complexes, the absorption profile is
attributed to ligand centred (LC) transitions.42,43 In the present
case (Fig. 3, left) the absorption spectrum at l ≥ 250 nm is
dominated by an intense single and broad band (e = 1.6 ¥ 104
M-1 cm-1 at l = 350 nm) associated with p–p* transitions of the
IAM chromophore.42,43
◦
atures higher than 300 C, mainly because of the decomposition
of the organic ligand cage. Based on these evidences, the hybrid
silica films (vide infra), prepared as described in the Experimental
Section, were annealed at 200 ◦C. In fact, at this temperature ther-
The photoexcitation of the ligand unit and the subsequent
photoinduced energy transfer processes result in the metal centred
emission (Fig. 3, left) which displays the typical Tb3+ bands
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CH3OD, respectively. By means of a well-established approach,46–48
we estimated the number of coordinated solvent molecules (q,
uncertainty 0.5) at the metal centre on the basis of the following
equation,
qTb = 4.2 (1/t
- 1/t
- 0.06)
(1)
CH OH
CH OD
3
3
We calculated that in the first metal ion coordination sphere, q is
approximately zero (0.17). This indicates a good ligand shielding
efficiency which prevents the occurrence of detrimental interac-
tions with the surroundings that would lead to luminescence
quenching processes via vibronic coupling with high frequency
oscillators such as OH, NH and, to a lower extent, CH.1,29
Along our previous lines,30,33 the light emitting sol–gel glasses
were obtained upon hydrolysis involving the alkoxide groups of
tetraethylorthosilicate (see Experimental Section) and the four
primary hydroxy groups of the alkyl chains appended to the IAM
or bis-IAM units of the Tb3+ complexes (see Scheme 1 and 2). This
procedure yielded highly homogeneous, transparent and green
luminescent SiO2 films, as depicted in Fig. 4.
In the silica films, the (HxOH-IAM)4ÃTb3+ complex shows
photophysical properties similar to those in solution with a PL
quantum efficiency and a lifetime of 0.19 and 1.29 ms, respectively.
The bright green luminescence of the layers irradiated at 350 nm,
clearly visible to the naked eye, was evaluated over time and was
almost unaffected by aging at ambient conditions. In fact, the
emission profile, PLQY and lifetime values acquired at increasing
ageing times (up to 6 months) were unchanged compared to those
of the freshly prepared films. Since the stability of the luminescent
hybrid materials under continuous irradiation is a key factor for
most applications such as sensing, bio-probing, lighting, etc.,49
and high resistance towards photoinduced damages is required to
improve the service life of devices, we evaluated the photostability
of the (HxOH-IAM)4ÃTb3+-based hybrids,31 under continuous ir-
radiation at l = 330 nm, i.e. at the maximum of the absorption band
7
(Fig. 3). The integrated intensity of the 5D4 → F6-3 transitions was
monitored as a function of the exposure time (Fig. 5). The emission
intensity decreases to ca. 50% of the initial value after 3.5 h,
evidencing a moderate stability toward photobleaching of the
(HxOH-IAM)4ÃTb3+ chromophore anchored in the SiO2 matrix.
It should be noted that the degradation of lanthanide complexes
under UV irradiation involves a series of complicated processes,
often attributed to photobleaching or photon-induced chemical
damage, and it has not been yet completely elucidated.50–53
The absorption (e = 2 ¥ 104 M-1 cm-1 at l = 350 nm) and emission
spectra in methanol solutions of the corresponding (bis-HxOH-
IAM)2ÃTb3+ complex are reported in Fig. 6. The (bis-HxOH-
IAM)2ÃTb3+ complex emits very bright green light in methanol
solution when excited at l = 350 nm due to the strong intensity
7
of Tb3+ 5D4 → F6-3 transitions in the 480–650 nm range. Absolute
Scheme 2 Synthesis of the tetradentate ligand bis-HxOH-IAM and of
the corresponding Tb3+ complex.
quantum yields for these complexes are as high as 0.38 (0.49 in
CH3OD), in agreement with the value reported in the literature
for similar compounds.42,43 The corresponding PL lifetimes (Fig.
due to transitions from the 5D4 excited state. The remarkable
photoluminescence efficiencies of the complex in both deuterated
(CH3OD, f = 0.40) and non-deuterated methanol (CH3OH, f =
0.29) confirm the effectiveness of the employed ligands to act
simultaneously as light harvesting units and binding sites for
the metal cation. The corresponding luminescence lifetimes were
5
7
6, right) for the D4 → F5 transition were fitted with a single-
exponential decay yielding t values of 1.20 ms (CH3OH) and
1.33 ms (CH3OD), which are consistent with a number of solvent
molecules close to zero (q = 0.09) in the metal ion surrounding.
The slight difference between the luminescence decays recorded
for (HxOH-IAM)4ÃTb3+ and (bis-HxOH-IAM)2ÃTb3+ could be
addressed by the different arrangements of the corresponding
monitored at 544 nm (5D4
exponential decay yielding 1.35 ms and 1.56 ms in CH3OH and
→
7F5) and match with a single-
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Fig. 3 Left: Absorption profile (black) in CH3OH and emission spectra (lexc = 350 nm) in CH3OH (green) and CH3OD (blue) of (HxOH-IAM)4ÃTb3+
.
Right: Corresponding luminescence decays (lexc = 350 nm, lem = 544 nm). The number of the solvent molecules involved in the first metal coordinating
sphere has been calculated according to eqn (1) (see text).
Fig. 5 Photodecomposition profiles for (HxOH-IAM)4ÃTb3+-based hy-
brids as-prepared (blue circles) and after thermal treatment (red circles).
decreasing trend by passing from the as-prepared to the thermally
treated glass sample (1.48 ms and 1.22 ms, respectively). Upon
continuous irradiation at 330 nm, the (bis-HxOH-IAM)2ÃTb3+
Fig. 4 Schematic representation of a SiO2 film functionalized with the
green luminescent (bis-HxOH-IAM)2ÃTb3+metal complex.
doped-SiO2 films showed a modest PL intensity variation over
the time (Fig. 7) suggesting a remarkable photostability of the
light-harvesting units around the metal centre. In fact, the two
hydroxyphthalamide residues in bis-HxOH-IAM, linked by a –
(CH2)5– spacer through the nitrogen atoms of the amide groups,
may be responsible for a different topology of the resulting
complex compared to that of (HxOH-IAM)4ÃTb3+ for which such
structural constriction does not occur. The covalent incorporation
of (bis-HxOH-IAM)2ÃTb3+ into the silica thin films does not
induce any appreciable variation for both the shape and position
of the terbium emission profiles (not reported), and PLQYs up
to f = 0.40 have been detected for as-prepared layers. Instead,
the annealed thin film (200 ◦C for 5 h) revealed an emission
intensity loss down to f ~ 0.30, although the adopted thermal
treatment was not as high as to cause decomposition of the organic
ligands, as evidenced by TGA analysis on the neat complexes
(see Fig. 2 and text). Accordingly, the corresponding lifetimes,
obtained by exciting the thin films at 350 nm, followed the same
terbium complex embedded in the glassy layer. We observe that
after a initial quick intensity decrease (~20% of the initial amount
in 30 min), the integrated light output is reduced a further 10%
over the subsequent 3 h.
As already largely debated,50–53 the origin of photodecompo-
sition processes of lanthanide complexes in solid matrices has
not been yet completely clarified. Therefore, the interpretation
of the slightly different behaviour of (bis-HxOH-IAM)2ÃTb3+
with respect to (HxOH-IAM)4ÃTb3+ is not straightforward. In
this framework, the response of the hybrid luminescent materials
to continuous UV irradiation should be considered as a figure
of merit, helpful in the choice of a particular material for a
target application. As a whole, luminescent silica glassy layers
employing (bis-HxOH-IAM)2ÃTb3+ as a luminophore present
superior stability and performances (PLQY) compared to their
(HxOH-IAM)4ÃTb3+ analogues. On the other hand, the facile
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Fig. 6 Left: Absorption profile (black) in CH3OH and emission spectra (lexc = 350 nm) in CH3OH (green) and CH3OD (blue) of (bis-HxOH-IAM)2ÃTb3+
.
Right: Corresponding luminescence decays (lexc = 350 nm, lem = 544 nm). The number of the solvent molecules involved in the first metal coordinating
sphere has been calculated according to eqn (1) (see text).
The SiO2-(bis-HxOH-IAM)2ÃTb3+ films showed considerable
photostability under prolonged irradiation time (up to 3.5 h). Due
to the nature of the designed IAM ligands, the Tb3+ complexes
were efficiently excited at longer wavelengths with respect to
other lanthanide systems studied and incorporated by our team
in thin silica layers. This represents a major aspect in the design
of luminescent lanthanide probes dedicated to bioanalyses and
bioimaging applications, since photoexcitation in near UV or
visible spectral regions avoids damage of biomolecules and
interference due to tissue autofluorescence.
Experimental section
General procedures and materials
Fig. 7 Photodecomposition profiles for (bis-HxOH-IAM)2ÃTb3+-based
hybrids as-prepared (blue circles) and after thermal treatment (red circles).
All available chemicals were purchased from commercial sources
and were used without any further purification unless otherwise
noted. Solvents were purified by using standard methods and
dried if necessary. Air- and moisture-sensitive reactions were
performed under a nitrogen atmosphere. Thin layer chromatog-
raphy (TLC) was conducted on plates precoated with silica gel
Si 60-F254 (Merck, Darmstadt, Germany). Column chromatog-
raphy was carried out by using silica gel Si 60, 230–400 mesh,
0.040–0.063 mm (Merck, Darmstadt, Germany). 1H and 13C
NMR spectra were recorded on a Bruker Avance 400 (400 and
100.6 MHz, respectively). Chemical shifts are indicated in parts
per million downfield from SiMe4, using the residual proton
(CHCl3 = 7.26 ppm) and carbon (CDCl3 = 77.0 ppm) solvent
resonances as internal reference. The coupling constants J are
given in Hz. Proton and carbon atom assignments were deter-
mined performing COSY and HSQC correlation experiments.
ESI mass spectra were obtained on an ICR Mass Spectrometer
APEX II & Xmann software (Bruker Daltonics) - 4.7 T Magnet
(Magnex).
synthesis of the HxOH-IAM derivative, combined with its good
luminescent performances in SiO2 films (PL efficiencies up to 0.20),
should also be considered to rationalize the chemical design and
the preparations of highly efficient light-emitting materials in the
more convenient way.
Conclusions
Efficient Tb3+ antenna complexes, employing two innovative
2-hydroxyphthalamide IAM ligands (HxOH-IAM, 2-hydroxy-
N1-(hexan-6-ol)-3-(N3-methylbenzene)-1,3-dicarbamide
and
bis-HxOH-IAM, N1,N1¢-(pentane-1,5-diyl)bis[2-hydroxy-N3-(6-
hydroxyhexyl)isophthalamide]) acting simultaneously as light
harvesting units and binding sites for the metal cation, were
synthesized. Both Tb3+ complexes, (HxOH-IAM)4ÃTb3+ and
(bis-HxOH-IAM)2ÃTb3+, were stable in methanol solution
and showed remarkable luminescence quantum yields, ca. 0.30
and 0.40, respectively. By exploiting the terminal hydroxyl
groups properly inserted in the substituted IAM ligands, the Tb3+
complexes were successively grafted in silica single layers via a sol–
gel procedure resulting in highly transparent and homogeneous
films, endowed with intense bright green photoluminescence.
(2-Methoxy-1,3-phenylene)bis[(2-thioxothiazolidin-3-
yl)methanone] (2)
To a solution of 2-methoxyisophthalic acid 1 (2 g, 10.2 mmol) in
20 ml of dry dioxane was added 2.6 ml (35.7 mmol) of thionyl
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chloride and two drops of DMF. The mixture was heated to
reflux and stirred overnight under N2. The volatiles were removed
under vacuum. The crude acid chloride was dissolved in 25 mL of
CH2Cl2, and a solution of 2-thiazoline-2-thiol (2.7 g, 22.4 mmol)
and NEt3 (2.8 mL) in 35 ml of CH2Cl2 was added dropwise
at -78 ◦C, then the reaction mixture was warmed to room
temperature and stirred overnight. The resulting yellow reaction
mixture was washed with brine (50 mL), 10% aqueous HCl
(50 mL), and 10% aqueous NaOH (2 ¥ 50 mL) successively. The
organic phase was dried over MgSO4 and evaporated to dryness
in vacuo. The residue was purified by column chromatography
(silica gel, CH2Cl2/n-hexane 8 : 2 v/v) to afford pure 2 (3.88 g,
95%) as a bright yellow solid. dH (400 MHz, CDCl3) 7.43 (2H,
d, 4-H and 6-H, J = 7.7), 7.13 (1H, t, 5-H, J = 7.7), 4.60 (4H,
t, J = 7.3 NCH2CH2S), 3.90 (3H, s, OCH3), 3.41 (4H, t, J =
7.3, NCH2CH2S). dC (100.6 MHz, CDCl3) 200.71, 155.54, 167.30,
132.11, 128.24, 123.31, 63.04, 55.70, 29.28. m/z (HRMS ESI+)
421.0 [M + Na]+ (C15H14N2O3S4 requires 397.99). Found: C, 45.28;
H, 3.55; N, 7.05. Calc. for C15H14N2O3S4: C, 45.20; H, 3.54; N,
7.03%.
155.75, 134.39, 134.31, 128.02, 127.91, 125.27, 63.44, 62.69, 39.78,
32.56, 29.60, 26.80, 26.74, 25.36. m/z (HRMS ESI+) 309.18071
[M + H]+, 331.16240 [M + Na]+(C16H24N2O4 requires 308.17).
Found: C, 62.25; H, 7.80; N, 9.07. Calc. for C16H24N2O4: C, 62.32;
H, 7.84; N, 9.08%.
2-Hydroxy-N1-(6-hydroxyhexyl)-N3-methylisophthalamide (5,
HxOH-IAM)
A sample of 2.0 mL (2 mmol) of 1 M BBr3 solution in CH2Cl2 was
added via syringe to a stirred solution of 4 (300 mg, 1 mmol) in
30 mL of CH2Cl2 cooled at -78 ◦C with an acetone/dry ice bath.
The reaction mixture was then warmed to room temperature and
stirred for a further 24 h. The solvent was removed under vacuum
and the residue was purified by column chromatography (silica
gel, 5% MeOH in CH2Cl2) to afford pure 5 (285 mg, 81%) as
a yellow oil. dH (400 MHz, CD3OD) 7.96 (1H, dd, 4-H or 6-
H, J = 7.8, J = 1.7), 7.93 (1H, dd, 4-H or 6-H, J = 7.8, J =
1.7), 6.92 (1H, t, 5-H, J = 7.8), 3.53 (2H, t, CH2OH, J = 6.6),
3.38 (2H, t, CON1HCH2, J = 7.1), 2.92 (3H, s, CON3HCH3),
1.61 (2H, br quint, N1HCH2CH2, J = 7.1), 1.53 (2H, br quint,
CH2CH2OH, J = 6.6), 1.37–1.42 (4H, m, CH2). dC (100.6 MHz,
CD3OD) 168.68, 167.90, 160.00, 132.60, 132.39, 118.10, 118.02,
117.70, 61.51, 39.43, 32.18, 29.00, 26.55, 25.41, 25.28. m/z (HRMS
ESI+) 317.14679 [M + Na]+ (C15H22N2O4 requires 294.16). Found:
C, 61.32; H, 7.54; N, 9.54. Calc. for C15H22N2O4: C, 61.21; H, 7.53;
N, 9.52%.
2-Methoxy-N-methyl-3-(2-thioxothiazolidine-3-
carbonyl)benzamide (3)
A solution of methylamine (34 mg, 1.1 mmol, 85 ml of a 40%
aqueous solution) in 95 : 5 CH2Cl2/MeOH (20 mL) was added
via syringe-pump in 8 h to a solution of 2 (860 mg, 2.2 mmol) in
2 mL of CH2Cl2 maintained under magnetic stirring and at room
temperature. The solvents were removed under reduced pressure.
The residue was redissolved in 100 mL of CH2Cl2 and washed
with 50 mL of 1 M KOH aqueous solution. The solvent was
evaporated to dryness and the residue purified by silica gel column
chromatography eluting with pure CH2Cl2 to recover unreacted
starting material, and then CH2Cl2/EtOAc 8 : 2 v/v to obtain the
desired product 3 (320 mg, 94%) as a thick yellow oil. dH (400 MHz,
CDCl3) 8.13 (1H, dd, 4-H or 6-H, J = 7.6, J 1.8), 7.41 (2H, br s
and dd overlapping, 4-H or 6-H and CONHCH3, J = 7.6, J =
1.8), 7.23 (1H, t, 5-H, J = 7.6), 4.67 (2H, t, NCH2CH2S, J =
7.3), 3.86 (3H, s, OCH3), 3.44 (2H, t, NCH2CH2S, J = 7.3), 3.01
(3H, d, CONHCH3, J = 4.8). dC (100.6 MHz, CDCl3) 201.22,
167.42, 165.38, 155.58, 134.27, 131.91, 129.24, 127.35, 124.49,
63.13, 55.62, 29.03, 26.69. m/z (HRMS ESI+) 333.10 [M + Na]+
(C13H14N2O3S2 requires 310.04). Found: C, 50.28; H, 4.56; N, 9.05.
Calc. for C13H14N2O3S2: C, 50.30; H, 4.55; N, 9.03%.
N-(6-Hydroxyhexyl)-2-methoxy-3-(2-thioxothiazolidine-3-
carbonyl)benzamide (6)
To a solution of 2 (1 g, 2.5 mmol) in 3 ml of CH2Cl2 was added
6-amino-1-hexanol (294 mg, 2.5 mmol) in 95 : 5 CH2Cl2/MeOH
(20 ml total) via syringe-pump in 8 h. The solvents were removed
under vacuum and the reaction mixture was dissolved in 100 ml
of CH2Cl2 and washed with 50 ml of 1 M KOH. The reaction
mixture was evaporated to dryness and applied to a silica column.
Unreacted starting material was eluted with 5% AcOEt in CH2Cl2,
and the desired product 6 (540 mg, 54%), a thick yellow oil, was
eluted with 20% EtOAc in CH2Cl2. dH (400 MHz, CDCl3) 8.12
(1H, dd, 4-H or 6-H, J = 8.0, J = 1.8), 7.41 (2H, br s and dd
overlapping, 4-H or 6-H and CONHCH2, J = 8.0, J = 1.8), 7.22
(1H, t, 5-H, J = 8.0), 4.68 (2H, t, NCH2CH2S, J = 7.3), 3.87
(3H, s, OCH3), 3.65 (2H, t, CH2OH, J = 6.5), 3.43–3.49 (4H,
m, NCH2CH2S and CONHCH2), 1.56–1.65 (4H, m, -CH2CH2-),
1.41–1.45 (4H, m, -CH2CH2-). dC (100.6 MHz, CDCl3) 201.23,
167.45, 164.71, 155.57, 134.35, 131.85, 129.27, 127.61, 124.53,
63.13, 62.70, 55.63, 39.71, 32.57, 29.52, 29.01, 26.69, 25.31. m/z
(HRMS ESI+) 419.10654 [M + Na]+ (C18H24N2O4S2 requires
396.12). Found: C, 54.46; H, 6.11; N, 7.08. Calc. for C18H24N2O4S2:
C, 54.52; H, 6.10; N, 7.06%.
N1-(6-Hydroxyhexyl)-2-methoxy-N3-methylisophthalamide (4)
A solution of 6-amino-1-hexanol (154 mg, 1.3 mmol) in 20 mL
of CH2Cl2was added dropwise over 10 min to a solution of 3
(340 mg, 1 mmol) in 10 mL of CH2Cl2 and the resulting reaction
mixture was stirred overnight at room temperature. The solvent
was removed in vacuo and the residue was purified by column
chromatography (silica gel, 5% MeOH in CH2Cl2) to afford pure
4 (300 mg, 97%) as a white solid. dH (400 MHz, CDCl3) 8.07 (2H,
d, 4-H and 6-H, J = 7.7), 7.31 (3H, br s and t overlapping, 5-H,
CON1HCH2 and CON3HCH2, J = 7.7), 3.86 (3H, s, OCH3), 3.66
(2H, t, CH2OH, J = 6.4), 3.49 (2H, dt, CON1HCH2, J = 7.0, J =
5.8), 3.05 (3H, d, CON3HCH3, J = 4.9), 1.58–1.66 (4H, m, CH2),
1.43–1.47 (4H, m, CH2). dC (100.6 MHz, CDCl3) 165.72, 164.99,
Synthesis of (HxOH-IAM)4ÃTb3+ complex
To a solution of ligand 5 (HxOH-IAM) (420 mg, 1.43 mmol) in
30 ml of MeOH was added 133 mg (0.36 mmol) of TbCl3·6H2O. A
sample of 290 mL of pyridine was added and the reaction mixture
was stirred at reflux for a further 24 h. The solvents were removed
in vacuo, the resulting off-white solid residue was redissolved in a
minimal amount of MeOH and the product was precipitated by
11536 | Dalton Trans., 2011, 40, 11530–11538
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addition of Et2O. The precipitate was separated by filtration and
dried to afford the pure complex (HxOH-IAM)4ÃTb3+ (370 mg,
77%) as a white solid. m/z (HRMS ESI-) (C60H84N8NaO16Tb re-
quires 1354.52) 1331.53125 (100%), 1332.53348 (68.2), 1333.53674
(22.7), 1334.53473 (9.1) [M - Na]-. Found: C 47.10, H 6.50, N
7.20; Calcd for C60H84N8NaO16Tb·NaCl·6H2O·CH3OH: C 47.15,
H 6.49, N 7.21%.
dried to afford pure (bis-HxOH-IAM)2ÃTb3+ (140 mg, 95%) as
a white solid. m/z (HRMS ESI-) (C66H92N8NaO16Tb requires
1434.5782) 1411.58924 (100%), 1412.59354 (74.7), 1413.59680
(28.7), 1414.60410 (9.6) [M - Na]-. Found: C 47.97, H 6.88, N
6.57. Calc. for C66H92N8NaO16Tb·NaCl·8H2O·2CH3OH: C 47.99,
H 6.87, N 6.58%.
Thermogravimetric analysis
N1,N1¢-(Pentane-1,5-diyl)bis[N3-(6-hydroxyhexyl)-2-
Thermogravimetric analysis (TGA) of the terbium complexes was
performed in air on SDT 2960 TA Instruments equipment in the
20–800 ◦C temperature range. The pow◦der samples were placed in
an alumina crucible and heated at 10 C min-1 under a 110 cm3
min-1 flow rate.
methoxyisophthalamide] (7)
To a solution of 2 (1 g, 2.5 mmol) in 20 ml of CH2Cl2 was added 1,5-
diaminopentane (129 mg, 1.3 mmol) in 15 ml of CH2Cl2 dropwise
via syringe-pump in 24 h. The solution was stirred overnight. The
solvent was removed under vacuum and the resulting residue was
applied to a silica gel column. The product, a white solid (710 mg,
83%) was eluted with 2% MeOH in CH2Cl2. dH (400 MHz, CDCl3)
8.01 (2H, dd, 4-H or 6-H, J = 7.7, J = 1.9), 7.96 (2H, dd, 4-H
or 6-H, J = 7.7, J = 1.9), 7.44 (2H, br t, CONHCH2, J = 5.7),
7.33 (2H, br t, CONHCH2Hex, J = 5.7), 7.25 (2H, t, 5-H, J =
7.7), 3.83 (6H, s, OCH3), 3.62 (4H, t, CH2OH, J = 6.4), 3.44–
3.52 (8H, m, CONHCH2), 1.89 (2H, s, CH2OH), 1.51–1.73 (14H,
m, -CH2CH2-), 1.41–1.44 (8H, m, -CH2CH2-). dC (100.6 MHz,
CDCl3) 165.25, 165.07, 155.68, 134.22, 134.05, 128.24, 127.98,
125.17, 63.39, 62.56, 39.79, 39.64, 32.54, 29.52, 29.19, 26.73, 25.38,
24.29. m/z (HRMS ESI+) 679.36593 [M + Na]+ (C35H52N4O8
requires 656.38). Found: C, 64.02; H, 8.00; N, 8.51. Calc. for
C35H52N4O8: C, 64.00; H, 7.98; N, 8.53%.
Preparation of luminescent silica films
Silica single layers embedding (HxOH-IAM)4ÃTb3+ or (bis-
HxOH-IAM)2ÃTb3+ complexes were obtained by sol–gel dip-
R
coating using TEOS [Si(OC2H5)4], Sigma Aldrichꢀ, 99.999%] as
the silica source. The terbium complexes were first dissolved in
ethanol (Fluka, ≥99.8%) at room temperature and subsequently
TEOS and deionized water were added (the molar composition of
the solution employed was 1 TEOS : 15 EtOH : 3 H2O) while the
Si/Tb molar ratio was 440. Before deposition, sol–gel solutions
were aged at room temperature under vigorous stirring for 72
R
h. Herasilꢀ silica slides (2 ¥ 1 cm2) were used as the substrates.
Film deposition was carried out in air at room temperature
with a controlled withdrawal speed of about 10 cm min-1. The
obtained layers were used in the luminescence experiments both
as-prepared and after thermal treatment at 200 ◦C for 5 h. Higher
treatment temperatures were not considered in order to avoid
complex degradation, as evidenced from the thermal analysis. All
as-prepared and annealed samples were homogeneous and well
adherent to the substrates, transparent, and crack free.
N1, N1¢-(Pentane-1,5-diyl)bis[2-hydroxy-N3-(6-
hydroxyhexyl)isophthalamide] (8)
To a solution of 7 (400 mg, 0.61 mmol) in 50 ml of CH2Cl2
dry cooled in an acetone/dry ice bath (-30 C) was added 3 ml
(3 mmol, 5 mol. equiv.) of BBr3 in 1 M CH2Cl2 solution with
◦
a syringe while stirring under N2. The reaction mixture was
◦
preserved at -30 C and was stirred for 24 h. When the reaction
Spectroscopic measurements
was complete, it was quenched by 10 ml of MeOH, diluted with
30 ml of H2O and heated at reflux for 4 h. The organic solution was
separated, dried over MgSO4 and applied to a silica gel column.
The product, a yellow thick oil, was eluted with 5% MeOH in
AcOEt (120 mg, 31%). dH (400 MHz, CD3OD) 7.95 (4H, d, 4-
H and 6-H, J = 7.8), 6.92 (2H, t, 5-H, J = 7.8), 3.54 (4H, t,
CH2OH, J = 6.5), 3.39–3.43 (8H, m, CONHCH2), 1.48–1.70 (14H,
m, -CH2CH2-), 1.38–1.41 (8H, m, -CH2CH2-). dC (100.6 MHz,
CD3OD) 168.09, 167.92, 160.04, 132.48, 118.09, 117.94, 61.49,
39.29, 39.07, 32.17, 28.99, 28.66, 26.53, 25.25, 23.89. m/z (HRMS
ESI-) 627.33784 [M - H]-, 649.32081 [M - 2H + Na]-, 313.16593
[M - 2H]2- (C33H48N4O8 requires 628.35). Found: C, 63.02; H,
7.70; N, 8.93. Calc. for C33H48N4O8: C, 63.04; H, 7.69; N, 8.91%.
Absorption spectra were recorded with
a Perkin-Elmer
Lambda950 spectrophotometer. For luminescence experiments,
the samples were placed in fluorimetric 1 cm path cuvettes and,
when necessary, purged from oxygen by bubbling with argon.
Uncorrected emission spectra were obtained with an Edinburgh
FLS920 spectrometer equipped with a Peltier-cooled Hamamatsu
R928 photomultiplier tube (185–850 nm). Corrected spectra were
obtained via a calibration curve supplied with the instrument.
Luminescence quantum yields (fem) in solution obtained from
spectra on a wavelength scale (nm) were measured according to the
approach described by Demas and Crosby54 using air-equilibrated
[Ru(bpy)3]Cl2 in water solution [fem = 0.028]55 as standard. The
luminescence lifetimes in the microsecond–millisecond scales were
measured by using a Perkin-Elmer LS-50 spectrofluorimeter
equipped with a pulsed xenon lamp with variable repetition rate
and elaborated with standard software fitting procedures. The
Synthesis of (bis-HxOH-IAM)2ÃTb3+ complex
A solution of TbCl3·6H2O (187 mg, 0.5 mmol) in 2 ml of
MeOH was added to a stirred solution of ligand 8 (126 mg,
0.2 mmol) in 40 ml of MeOH. Sym-collidine (42 ml) were added
while stirring and the reaction mixture was heated to reflux
for 24 h. The solvents were removed under reduced pressure
and the solid residue was redissolved in a minimal amount of
MeOH. The product was precipitated with Et2O, separated and
2
quality parameters (c ) for all the fittings were ~1. For solid
samples, fem have been calculated by corrected emission spectra
obtained from an apparatus consisting of a barium sulphate
coated integrating sphere (4 inches), a 450 W Xe lamp (lexc
tunable by a monochromator supplied with the instrument) as
the light source and a R928 photomultiplayer tube as the signal
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Dalton Trans., 2011, 40, 11530–11538 | 11537
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detector, following the procedure described by De Mello et al.56
Experimental uncertainties are estimated to be 8% for lifetime
determinations, 20% for emission quantum yields, 2 nm and 5
nm for absorption and emission peaks, respectively. Photostability
tests were performed on a home built apparatus employing a 100 W
Hg lamp as light source. The samples were kept under continuous
irradiation, for up to 5 h, with a constant power of ª100 mW cm-2
at 330 nm by means of light intensity attenuators.
26 A. Aebischer, F. Gumy and J.-C. G. Bunzli, Phys. Chem. Chem. Phys.,
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27 B. Francis, D. B. A. Raj and M. L. P. Reddy, Dalton Trans., 2010, 39,
8084.
28 G. Jia, G.-L. Law, K.-L. Wong, P. A. Tanner and W.-T. Wong, Inorg.
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29 L. Armelao, S. Quici, F. Barigelletti, G. Accorsi, G. Bottaro, M.
Cavazzini and E. Tondello, Coord. Chem. Rev., 2010, 254, 487.
30 L. Armelao, G. Bottaro, S. Quici, M. Cavazzini, M. C. Raffo, F.
Barigelletti and G. Accorsi, Chem. Commun., 2007, 2911.
31 L. Armelao, G. Bottaro, S. Quici, C. Scalera, M. Cavazzini, G. Accorsi
and M. Bolognesi, ChemPhysChem, 2010, 11, 2499.
32 S. Quici, M. Cavazzini, G. Marzanni, G. Accorsi, N. Armaroli, B.
Ventura and F. Barigelletti, Inorg. Chem., 2005, 44, 529.
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34 S. Quici, G. Marzanni, M. Cavazzini, P. L. Anelli, M. Botta, E.
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35 S. Quici, C. Scalera, M. Cavazzini, G. Accorsi, M. Bolognesi, L.
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41 S. Petoud, G. Muller, E. G. Moore, J. D. Xu, J. Sokolnicki, J. P. Riehl,
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Acknowledgements
The research was funded by MIUR (PRIN 2009, PRIN 2008,
FIRB RBPR05JH2P, FIRB RBAP114AMK), MSE (Industria
2015 ALADIN), University of Padova (Progetto Strategico HE-
LIOS) and CNR PM.P04.010 MACOL. We are very grateful to
Prof. E. Tondello, University of Padova, for helpful discussion.
Mr. A. Ravazzolo (ISTM CNR) and Dr R. Saini (University of
Padova) are gratefully acknowledged for technical assistance.
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©