The Role of Ligand Topology in the Decomplexation of Luminescent Lanthanide Complexes by Dipicolinic Acid

Article abstract of DOI:10.1002/cphc.201600727

In this study, we present the aqueous solution behavior of two luminescent lanthanide antenna complexes (Eu³⁺?1, Dy³⁺?9) with different ligand topologies in the presence of dipicolinic acid (DPA, pyridine-2,6-dicarboxylic acid). Macrocyclic (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic acid, DO3A, 9) and acyclic (1,4,7-triazaheptane-1,1,7,7-tetraacetic acid, DTTA, 1) ligands have been selected to form a ratiometric pair in which Dy³⁺?9 acts as a reference and Eu³⁺?1 acts as a probe for the recognition of DPA. The pair of luminescent complexes in water reveals the capability to work as a DPA luminescent sensor. The change of emission intensity of Eu³⁺ indicates the occurrence of a new sensitization path for the lanthanide cation through excitation of DPA. NMR evidence implies the presence of free 1 and mass spectrometry shows the formation of emitting [EuDPA₂]^? as a result of a ligand exchange reaction.

Full text of DOI:10.1002/cphc.201600727

DOI: 10.1002/cphc.201600727
Articles
The Role of Ligand Topology in the Decomplexation of
Luminescent Lanthanide Complexes by Dipicolinic Acid
[a]
[b]
[b]
[c]
[c]
Federica Mian, Gregorio Bottaro,* Roberta Seraglia, Marco Cavazzini, Silvio Quici,*
[a, b]
and Lidia Armelao
In this study, we present the aqueous solution behavior of two
nition of DPA. The pair of luminescent complexes in water re-
veals the capability to work as a DPA luminescent sensor. The
3
+
3+
luminescent lanthanide antenna complexes (Eu ꢁ1, Dy ꢁ9)
with different ligand topologies in the presence of dipicolinic
acid (DPA, pyridine-2,6-dicarboxylic acid). Macrocyclic (1,4,7,10-
tetraazacyclododecane-1,4,7-triacetic acid, DO3A, 9) and acyclic
3
+
change of emission intensity of Eu indicates the occurrence
of a new sensitization path for the lanthanide cation through
excitation of DPA. NMR evidence implies the presence of free
(
1,4,7-triazaheptane-1,1,7,7-tetraacetic acid, DTTA, 1) ligands
1 and mass spectrometry shows the formation of emitting
3
+
ꢀ
have been selected to form a ratiometric pair in which Dy ꢁ9
acts as a reference and Eu ꢁ1 acts as a probe for the recog-
[EuDPA ] as a result of a ligand exchange reaction.
2
3
+
1
. Introduction
The effectiveness of luminescent lanthanide complexes in ana-
Both polydentate acyclic and cyclic ligands have been con-
sidered as being able to form complexes characterized by high
thermodynamic stability and high kinetic inertness. However,
particular attention must be given to their functionalization as
the replacement of even one carboxylate arm may decrease
the values of the stability constants by several orders of mag-
[
1–2]
lytical and therapeutic applications is nothing new;
the his-
tory of this success is based on the particular electronic config-
uration of trivalent lanthanide ions, that is, shielded optically
[
3]
active 4f electrons, in combination with the high stability
and kinetic inertness of lanthanide complexes prepared from
polyaminopolycarboxylic ligands. Initially studied and em-
ployed for the development of gadolinium-based contrast
[4–5]
nitude.
The formation rates of the complexes with acyclic
polydentate ligands (DTPA, DTTA, and their derivatives) are
high, but they have a limited kinetic inertness. Spontaneous
and proton-assisted decomplexations have also been reported
[
4]
agents (CA) for magnetic resonance imaging (MRI), acyclic di-
ethylenetriaminepentaacetic acid (DTPA) and macrocyclic
[4–8]
1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)
but are considered limited at physiological pH.
Conversely,
have rapidly become cornerstones in the preparation of lumi-
nescent lanthanide complexes. Over the years, many studies
have been directed toward structural modification of the li-
gands by introduction of appropriate chromophoric groups
that are able to promote the indirect sensitization of lantha-
nide cations (“antenna effect”) and, at the same time, to pre-
vent deactivation processes induced by solvent molecules in
the first coordination sphere of the metal.
complexes formed with the macrocyclic DOTA derivatives are
[8]
kinetically extremely inert.
Among the wide variety of available sensitizers for lantha-
nide emission, dipicolinic acid (pyridine-2,6-dicarboxylic acid,
DPA) plays a particular role. The presence of DPA as the main
structural component of the protective outer layers of bacterial
spores, for example, Bacillus Anthracis (approximately 5–15%
of the dry mass), has aroused the interest of many researchers
in the development of fast and reliable sensor systems capable
[9–13]
of detecting this endospore biomarker.
In the past years,
several analytical approaches have been adopted for the deter-
[
a] Dr. F. Mian, Prof. L. Armelao
Dipartimento di Scienze Chimiche Universitꢀ di Padova and INSTM
Via F. Marzolo 1, 35131 Padova (Italy)
[14–16]
mination of dipicolinic acid mainly based on biochemical,
[17–18]
electrochemical,
or spectroscopic methods (photolumines-
[
b] Dr. G. Bottaro, Dr. R. Seraglia, Prof. L. Armelao
cence, Raman, and surface enhanced Raman spectrosco-
ICMATE-CNR and INSTM
[10–12,19–25]
py).
In this regard, luminescence-based techniques
Dipartimento di Scienze Chimiche Universitꢀ di Padova
Via F. Marzolo 1, 35131 Padova (Italy)
E-mail: gregorio.bottaro@cnr.it
have the edge over the others because, in general, they do
not require laborious pretreatment of the sample such as ex-
traction and/or concentration of dipicolinic acid. The most
[
c] Dr. M. Cavazzini, Dr. S. Quici
ISTM-CNR
Via C. Golgi 19, 20133 Milano (Italy)
E-mail: silvio.quici@istm.cnr.it
common approach involves the formation of Ln(DPA) (H O)
9ꢀ3x
complexes (where x=1, 2, or 3) starting from inorganic lantha-
x
2
nide salts, typically TbCl . More recently, some authors have
3
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under http://dx.doi.org/10.1002/
cphc.201600727.
suggested that the use of preformed complexes of lantha-
nides, in which the number of coordination sites available for
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DPA binding is reduced, may overcome the analytical compli-
cations owing to the presence of complexes with different sto-
and is expected to be flexible enough to allow the interaction
3
+
[19,35]
of the Ln sites with polydentate anions.
[
10–11,19–28]
ichiometry and luminescent properties.
The temporal stability of both luminescent complexes can
be easily checked by recording their emission spectra at se-
quential time intervals. This represents a practical and impor-
tant quality control for the sensor system, which would not be
possible for sensing probes in which luminescence is activated
by interaction of the lanthanide ions with the analytes.
Thus, we decided to investigate the behavior of a pair of lu-
minescent complexes with different ligand topologies
3
+
3+
(
Figure 1, Eu ꢁ1 probe and Dy ꢁ9 reference) in the presence
3
+
3+
The reaction between the Eu ꢁ1/Dy ꢁ9 ratiometric pair
and DPA has been followed by photoluminescence (PL), NMR
spectroscopy, and mass spectrometry (MS). PL measurements
of the ratiometric mixture upon addition of DPA provided evi-
3
+
dence for the modulation of the emission of Eu ꢁ1, whereas
3
+
the spectroscopic properties of Dy ꢁ9 were unaffected by
the presence of DPA in solution, demonstrating the high stabil-
ity and inertness of this complex. The calibration curve of the
sensor obtained by emission spectra allowed the quantitative
determination of DPA with very good approximation over
a range of concentrations, 10–40 mm. NMR titration experi-
ments and MS spectrometry allowed us to identify the forma-
3
+
3+
Figure 1. Structure of Eu ꢁ1 and Dy ꢁ9 complexes.
ꢀ
of a strong coordinating ligand such as DPA, and to test its ca-
pability to work as a ratiometric luminescence sensor. In both
compounds, we adopted acetophenone as the light-harvesting
unit as the favorable energy of the excited triplet state
tion of free ligand 1 and [EuDPA2] complex. The observed
changes in PL spectra are therefore due to the formation of
other emissive species rather than to the interaction of DPA
3
+
with Eu ꢁ1.
ꢀ
1 [29–30]
(
ꢂ25000 cm )
makes it suitable for the sensitization of
4
ꢀ1
5
dysprosium ( F =21280 cm ) and europium ions ( D =
9/2
0
ꢀ
1
[31–32]
3+
1
7250 cm ).
In Dy ꢁ9, acetophenone is bonded 2. Results and Discussion
through a short and flexible spacer to the 1,4,7,10-tetraazacy-
2.1. Synthesis of Complexes
[
29,33–34]
clododecane-1,4,7-triacetic acid (DO3A),
whereas for the
3
+
3+
coordination site of the probe complex Eu ꢁ1, we employed
the 1,4,7-triazaheptane-1,1,7,7-tetraacetic acid (DTTA) unit. The
ligand produces a negatively charged complex with europium
The synthesis of Eu ꢁ1 was carried out as outlined in
Scheme 1. Compound 2, 3-benzyloxypropanol, was prepared
[36]
by simplifying a literature procedure,
whereas the corre-
3
+
Scheme 1. Synthesis protocol for Eu ꢁ1.
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[37]
sponding methanesulfonate (3) was prepared as reported.
Reaction of commercially available 4-hydroxyacetophenone
with 3-benzyloxy-1-propanol methanesulfonate (3) was carried
out in CH CN at reflux for 16 h, in the presence of solid K CO
3
3
2
as base, and afforded 4-(3-benzyloxypropoxy)acetophenone
4). Treatment of 4 with N-bromosuccinimide (NBS) and p-tol-
uenesulfonic acid monohydrate (PTSA·H O) in CH CN at reflux
(
2
3
for 2 h produced 4-(3-benzyloxypropoxy)phenacylbromide (5)
[
38]
with a 80% yield.
Alkylation of 1,4,7-triazaheptane-1,1,7,7-
[
39]
tetraacetic acid tetrakis(1,1-dimethylethyl) ester (6)
with 5
was carried out in CH CN at reflux in the presence of solid
3
Na CO as base and afforded 7 in excellent yield, after purifica-
2
3
tion of the crude by column chromatography. Treatment of 7
in anhydrous CH Cl with CF COOH at room temperature af-
2
2
3
forded the benzyloxy protected tetraacetic acid (8) in 72%
yield. The preparation of the corresponding europium complex
3
+
Eu ꢁ8 was carried out by mixing stoichiometric amounts of
EuCl ·6H O and ligand (8) at reflux for 4 h and adjusting the
3
2
pH to 6.5–7.0 with 2% aqueous NaOH. The product was puri-
fied by an Amberlite XAD 1600T column to afford 89% of
a pale-beige crystalline solid. Debenzylation of this product
was achieved by catalytic hydrogenation, at ambient pressure
and room temperature, in a solvent mixture 9:1 (v/v) MeOH/
CH COOH and in the presence of 10% Pd/C as catalyst. Con-
3
version was complete after 72 h and evaporation of the sol-
3
+
vent afforded Eu ꢁ1 as a white solid in quantitative yield.
1
3+
The H NMR spectrum of Eu ꢁ1 in D O is almost completely
2
resolved at 300 K and showed sizable downfield and upfield
shifts covering a spectral width of almost 40 ppm (Figure 2).
We observe only 20 different resonances corresponding to
23 non-equivalent protons of the complex owing to the over-
lap of two pairs of signals at ꢀ12.25 and 5.70 ppm. The signals
of the propyl and phenyl residues retain their multiplicity, with
the exception of the aromatic protons at 8.86 ppm. In analogy
with data reported in literature for europium complexes of
1
3+
Figure 2. H NMR spectra of complex Eu ꢁ1 in D
2
O at 300 K: (a) full spec-
tral width and 1.0–9.0 ppm region (inset); (b) 2D EXSY spectrum.
[
40–41]
DTPA derivatives,
it is reasonable to assign the resonances
at the higher field to acetate protons and the resonances at
the lower field to ethylenic protons. The 2D exchange spec-
troscopy (EXSY) spectrum shows eight pairs of exchanging
partners corresponding to the acetate and ethylenic protons
nearby the paramagnetic nucleus, with overlap of two signals
at ꢀ12.25 ppm and no exchange behavior observed for pro-
3+
3
+
carried out as described above for ligand 8 and afforded Dy
ꢁ9 as a white solid in 85% yield after purification through an
Amberlite XAD 1600T column. The H NMR spectrum of Dy
1
3+
ꢁ9 was recorded in D O at 323 K with a Bruker Avance 400
2
1
13
equipped with a 4 mm double resonance ( H, C) gradient
high-resolution magic angle spinning (HR-MAS) probe. At spin-
13
tons at 5.70 ppm (Figure 1b). The C NMR spectrum of Eu
1 collected in D O at 297 K is complicated by coalescence of
1
3+
ꢁ
ning rate of 10 kHz, the H NMR spectrum of complex Dy ꢁ9
2
several frequencies induced by the paramagnetic center; nev-
covers a spectral width of almost 700 ppm as a result of fast
1
13
13
ertheless a H- C HSQC experiment allows us to identify the
signals related to aromatic carbons (133.9 and 113.6 ppm) and
to propyl chain carbons (65.6, 58.2, and 30.7 ppm), while no
other cross-peak can be detected with the exception of the
correlation between the carbon at 20.91 ppm and hydrogen at
electronic relaxation rates (~10 s) and the large magnetic
3
+
moment of Dy (J=15/2 and 10.5 mB) and it consists of 22
resonances, which correspond to the 27 non-equivalent pro-
tons of the complex (Figure 3). The downfield and upfield reso-
nances have the same relative intensity, with the exception of
the resonance at ꢀ31.61 ppm (relative intensity 2) and multi-
plet between ꢀ1.30 and ꢀ5.49 ppm (relative intensity 10),
probably where peaks of the methoxy group, aromatic pro-
tons, and an acetate arm overlap. Owing to the resulting low
sensitivity and the enormous spectral range, it was impractical
to obtain 2D NMR spectra of the paramagnetic dysprosium
complex.
1
.81 ppm (Figure S28 in the Supporting Information).
3
+
The reference Dy ꢁ9 was prepared by complexation of
10-[(4-methoxybenzoyl)methyl]-1,4,7,10-tetraazacyclodode-
cane-1,4,7-triacetic acid (9) bearing a 4-methoxyacetophenone
group covalently linked to the secondary nitrogen atom of the
DO3A, which has been prepared by following the procedure
[
29]
3+
reported by Beeby et al. Complexation of 9 with Dy was
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3
+
3+
Table 1. Photophysical data of Eu ꢁ1 and Dy ꢁ9.
[
b]
l
max
e
t
H2
O
t
D2
O
F
se in
O
F
se in
O
q
ꢀ
1
ꢀ1
[a]
[a]
[
nm]
[m cm
]
[ms]
[ms]
H
2
D
2
3
+
Eu ꢁ1 308
24500
24500
0.60
2.35
0.07
0.29
0.020
1.0
1.0
3
+
[c]
Dy ꢁ9 308
0.012 0.028 0.008
[
(
a] Quantum yield values obtained by using [Ru(bpy)
exc =350 nm). [b] Number of water molecules in the first coordination
Eu =1.11 (1/tH2
3 2
]Cl as standard
l
shell estimated by the Horrocks formula:
1
for the corresponding europium complex for which Horrocks equation
can be used.
q
O
ꢀ
[
43]
/tD2
O
ꢀ0.31). [c] The number of water molecules has been calculated
5
7
3+
with the D ! F transition (lꢂ580 nm) of Eu , the former is
0
0
not affected by any spectral interference and is thus adopted
as the internal reference in the ratiometric pair.
1
3+
Figure 3. H NMR spectrum with water suppression of complex Dy ꢁ9 in
D
2
O at 323 K.
In DPA free solutions, no appreciable differences are seen
between the emission spectra (Figure 4b) obtained by exciting
the ratiometric couple at 270 nm (absorption band of DPA)
and 310 nm, where only the acetophenone moiety is able to
sensitize the lanthanide emission (Figure 4a). Upon addition of
2
.2. Optical Properties and Interaction with DPA
3
+
3+
Absorption spectra of Eu ꢁ1 and Dy ꢁ9 consist of a single
band with a maximum at 310 nm resulting from ligand cen-
tered transitions, which are overlapped with those of DPA in
the 250–290 nm region. In the absence of DPA, the absorption
and photoluminescence excitation spectra of the two com-
ꢀ
5
3+
increasing amounts of DPA to a 10 m solution of Eu ꢁ1 and
3
+
Dy ꢁ9 (1:1 molar ratio), relevant modifications in the emis-
sion spectra are observed under excitation at 270 nm (Fig-
ure 5a).
3
+
3+
plexes are superimposed. The emission spectrum of Eu ꢁ1 in
As for Dy emission, a slight intensity decrease is observed
water excited at the maximum of the excitation spectrum is
at high DPA concentrations. Indeed, owing to the overlap of
the corresponding absorption spectra, dipicolinic acid com-
petes with acetophenone at lexc <290 nm acting as an internal
3
+ 5
7
composed of several bands associated with the Eu D ! F
0
J
transitions (J=0–4); the higher the J value, the lower the tran-
[
42]
3+
sition energy (red line in Figure 4b, Figure S31). The poly-
dentate acyclic ligand is able to saturate eight of the nine eu-
ropium coordination sites. The last is occupied by a water mol-
filter (Figure 4a). Accordingly, similar modifications of the Dy
emission spectra are not found (Figure 5b) upon excitation at
310 nm. Conversely, we observed a significant variation in the
5
7
5
7
ecule, as indicated by the remarkable increase in the D ! F
relative intensity between the different D ! F multiplets of
0
J
0
J
[
43]
3+
lifetimes (Table 1) determined in D O.
Eu as a function of DPA concentration (Figure 5a). The transi-
tion J=2 is remarkably increased compared with all the others
and undergoes a moderate redshift. These changes indicate
2
3
+
Concerning the Dy complex, the macrocyclic ligand 9 is
able to saturate eight coordination sites and also in this case
one water molecule saturates the lanthanide coordination
3
+
the occurrence of structural rearrangements around the Eu
[
29]
3+
4
6
4
6
[44–46]
demand. The Dy bands F9 ! H
and F ! H
are
13/2
center.
The behavior of the ratiometric pair is particularly
/2
15/2
9/2
clearly visible in the emission spectra (blue line in Figure 4b
and Figure S33). Although the latter is partially overlapped
interesting upon excitation at 310 nm, that is, in the spectral
region where DPA is not absorbing (Figures 4a and 5b). In this
3
+
3+
Figure 4. a) Absorption spectra of dipicolinic acid (red curve) and Eu ꢁ1 (black curve; Dy ꢁ9 has the same absorption spectra). The excitation spectrum of
the lanthanide complexes is reported as a black dotted curve. The vertical dashed lines show the excitation wavelengths. b) Emission spectra of the ratiomet-
ric pair excited at 310 nm (absorption maximum of acetophenone) and at 270 nm (absorption maximum of DPA).
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Figure 5. Emission spectra of the ratiometric pair at different DPA concentrations excited at 270 (a) and 310 nm (b). The inset shows the details of the most in-
3
+
3+
5
7
tense emission bands of Dy and Eu . The arrows indicate the increase of DPA concentration. c) Excitation spectra monitored on the D
0
! F
2
europium
transition. d) Temporal response of the ratiometric sensor after addition of 10, 30, and 60 mm DPA solutions. Error bars (ꢃ15%) have been determined by per-
forming three different sets of experiments.
case, a decrease in the overall intensity of europium emission
is observed at DPA concentrations higher than 10 mm. This
trend cannot be ascribed to coordinative rearrangements in
The whole curve can be well interpolated by means of a sig-
moid function with a DPA limit of detection (LOD) of 2 mm.
3
+
3+
Eu ꢁ1 as the shape of the Eu emission bands is not modi-
fied by the presence of DPA. A similar behavior has been also
3
+
3+
found for each of the Eu ꢁ1 and Dy ꢁ9 complexes (Fig-
ures S34 and S35). Taken together, all these observations sug-
3
+
gest that DPA molecules are able to coordinate Eu ions. Evi-
dence of this mechanism is also supported by the excitation
spectra of the complexes (Figure 5c), where a new excitation
pathway via dipicolinic acid is clearly discernible by increasing
the DPA concentration in solution. The response toward the
molecular recognition event is displayed in Figure 5d for three
DPA concentrations (10, 30, and 60 mm). Although the presence
of DPA is revealed instantaneously, a longer time is required to
reach equilibrium. In fact, the ratiometric parameter shows
a continuous variation until reaching a constant value, which
occurs approximately 60 min after DPA addition.
The calibration curve of the ratiometric sensor (Figure 6) is
determined from the emission spectra recorded 60 min after
DPA addition, by evaluating the integrated intensity ratio be-
Figure 6. Calibration curve of the sensor (lexc =270 nm): experimental points
and curve obtained from fitting by means of a sigmoid function. Error bars
(ꢃ15%) have been determined by performing three different sets of experi-
ments.
3
+
5
7
3+
4
6
tween Eu ( D ! F ) and Dy ( F ! H15/2) transitions ex-
0
1-4
9/2
cited at 270 nm.
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3
+
2
.3. NMR Titration and Mass Spectrometry
entirely for [DPA]/[Eu ꢁ1] ratios greater than 2.13. All these
0
observations point to a slow exchange regime between two
species. The second species is identifiable with free ligand 1, in
analogy with the spectrum of ligand 8; also the signals at 3.36
and 3.05 ppm are compatible with the protons of ethylenic
groups of the free ligand. We assumed that the lack of stability
NMR titration and mass spectrometry experiments were per-
formed to get detailed information about the structure of the
3
+
chemical species formed in the reaction of DPA with Eu ꢁ1.
Proton NMR titrations were performed by adding aliquots of
a D O solution of DPA to a D O solution of the europium com-
3
+
of the complex Eu ꢁ1 could be mainly due to the chelating
ability of DPA towards the Eu cation.
2
2
3
+
plex. We observed the interconversion of the signals of the
3
+
3+
pristine complex Eu ꢁ1 into a new pattern of signals shifted
To identify the species formed in water with Eu ꢁ1 after
in frequency (Figure 7).
the addition of DPA, electrospray mass spectrometry (ESI/MS)
3
+
ꢀ5
experiments have been performed on pure Eu ꢁ1 (10 m)
and in the presence of DPA (60 mm). The negative ion ESI mass
3
+
spectrum of the Eu complex, reported in Figure 8, shows
only one peak resulting from an anion corresponding to
ꢀ
[
MꢀNa] , with an isotopic cluster in agreement with the pres-
ence of one Eu atom.
3
+
Figure 8. Negative ion ESI mass spectrum of water solution of Eu ꢁ1
10 m).
ꢀ
5
(
After the addition of DPA, ESI mass spectra were recorded at
different times and the results of these experiments are sum-
marized in Figure 9.
1
3+
Figure 7. H NMR titration with water suppression for complex Eu ꢁ1
2.8ꢁ10 m, D O) with aliquots of DPA (0.047m in D O): a) full spectral
width and b) expanded region 1.0–9.5 ppm.
ꢀ
3
(
2
2
3
+
In particular, the AX spin system of Eu ꢁ1 (8.75 and
.25 ppm) gradually disappears, while a new AX spin system at
.82 and 6.94 ppm becomes visible for increasing amounts of
7
7
DPA. A similar situation occurs with respect to the signals of
the propyl chain at 4.29, 3.69, and 2.11 ppm, which are re-
placed by new signals at 4.08, 3.63, and 1.89 ppm, respectively,
the latter being superimposed on another signal not well iden-
tified. Even low-field and high-field frequencies gradually de-
crease for increasing amounts of DPA in solution, disappearing
3
+
ꢀ5
Figure 9. ESI mass spectra of Eu ꢁ1 (10 m in water) after the addition of
DPA (60 mm) recorded at different times: a) t=2 min, b) t=35 min,
c) t=60 min, and d) t=180 min.
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3
+
As can be seen, the addition of DPA to Eu ꢁ1 leads to the
formation of other ionic species, detectable after just 2 min. In
particular, the signals at m/z=526 are related to the free
ligand 1 and those at m/z=481, characterized by the isotopic
ing and presaturation experiments were performed to attribute
spurious signals that were due to rotation. Chemical shifts are indi-
cated in parts per million downfield from SiMe , using the residual
4
proton (CDCl =7.26 ppm, (CD ) SO=2.50 ppm, HOD=4.80 ppm)
3
3 2
and carbon (CDCl =77.0 ppm, (CD ) SO=40.45 ppm) solvent reso-
3
3 2
cluster of europium, are in agreement with the presence of
nances as internal references. Coupling constants values J are
ꢀ
[
Eu(DPA)2] species. This indicates that DPA is able to replace
given in Hz. Protons and carbon assignments were achieved by
1
in the europium coordination sphere, thus forming a new
13
1
1
1
13
C-APT, H- H COSY, H- C heteronuclear single quantum coher-
3
+
stable complex. Signals ascribable to Eu ꢁ1–DPA adducts
ence (HSQC) and heteronuclear multiple-bond correlation (HMBC)
experiments. 2D exchange spectroscopy (EXSY) experiments were
collected by using the standard 908–t –908–t -90 (noesyph) pulse
have not been detected.
1
m
sequence and a mixing time (d8) of 10 ms. High-resolution mass
spectra were obtained with an electrospray ion-trap mass spec-
trometer ICR-FTMS APEX II (Bruker Daltonics) by the Centro Interdi-
partimentale Grandi Apparecchiature (C.I.G.A.) of the University of
Milano.
3
. Conclusions
We have studied the behavior of two luminescent lanthanide
3
+
3+
complexes, Eu ꢁ1 and Dy ꢁ9, in water in the presence of
dipicolinic acid and their suitability to work as DPA ratiometric
luminescent sensor. Both antenna complexes bear acetophe-
none as a common sensitization unit for lanthanide emission.
General Procedure for NMR Titration Experiments
3
+
The reference complex (Dy ꢁ9) contains a rigid DO3A macro-
cyclic ligand, which prevents the bonding of DPA to the Dy
3+
A solution of Eu ꢁ1 was prepared by weighing a solid sample of
3
+
the complex (1.5–1.9 mg) in a 5 mm NMR sample tube and by dis-
3
+
centers. In this way, the luminescence of Dy ꢁ9 is not affect-
ed by the presence of DPA, thus demonstrating the inertness
of the macrocyclic structure that, being constrained to its
cyclic form, allows less conformational freedom. On the other
hand, recognition of the DPA target molecule was possible be-
solving it in D
O. A reference spectrum with water suppression
2
(zgpr pulse program) was collected at 297 K. Then, aliquots of 10–
2
0 ml of a solution of DPA were added; after each addition, the
sample was manually stirred for about one minute prior to record
the spectrum.
3
+
cause of the properties of the coordination unit in Eu ꢁ1. In
fact, the use of a flexible coordination site such as DTTA allows
Photoluminescence Sensing Experiments
3
+
3+
the binding of DPA to Eu centers, but the Eu –DPA interac-
tions are strong enough to induce a ligand exchange reaction
ꢀ
5
Sensing experiments were performed with 10 m water solutions
3
+
3+
ꢀ
of Eu ꢁ1, Dy ꢁ9 and with their 1:1 molar mixtures by adding
with formation of free 1 and [EuDPA2] as evident from NMR
and mass spectrometry. The observed modulation of the lumi-
nescence signal is therefore due to the decomplexation of
ꢀ2
into the cuvette successive amounts (3 mL each) of 10 m DPA sol-
ꢂ
utions with a Gilson micropipette, up to a final DPA concentration
of 70 mm. In this way, dilution effects can be neglected. As a ratio-
metric parameter (R), we have adopted the ratio between the inte-
3
+
Eu ꢁ1 and the formation of new emitting species. The ratio-
metric pair allows the quantitative determination of DPA with
very good approximation in the 10–40 mm concentration
range. These results show the importance of the topology of
the ligands for the stability and inertness of the complexes. In
particular, the employment of macrocyclic ligands that have
a preformed cavity in which the metal ions can be hosted in-
sures a better stability over acyclic ligands having the same
number and typology of coordination sites. This different be-
havior can be usefully exploited in analytical applications.
3
+ 5
7
3+ 4
grated area of Eu D ! F J=1–4 (583–715 nm) and Dy F9/2
!
0
J
6
H15/2 (456–505 nm).
The temporal response of the sensor was studied for 10, 30, and
ꢀ2
6
0 mm DPA solutions. The proper amount of DPA (10 m) was
added directly into the cuvette. Subsequently, the emission spectra
(lexc =270 and 310 nm) were recorded at regular time intervals,
every 7 min up to 3 h. Kinetic curves were determined by plotting
R vs. time for each concentration.
Optical Properties
Experimental Section
Absorption spectra in the UV/Vis region were performed with
water solutions by using a double-beam CARY5E spectrophotome-
ter with a spectral bandwidth of 1 nm. The luminescence spectra
Chemicals and Characterization
All available chemicals and solvents were purchased from commer-
cial sources and were used without any further purification. Thin
layer chromatography (TLC) was conducted on plates precoated
with silica gel Si 60-F254 (Merck, Darmstadt, Germany). Column
chromatography was carried out by using silica gel Si 60, 230–400
were recorded in water (or D O) solutions at room temperature
2
with
a Fluorolog-3 (Horiba Jobin Yvon) spectrofluorimeter
equipped with a double-grating monochromator in both the exci-
tation and emission sides coupled to a R928P Hamamatsu photo-
multiplier, and a 450 W Xe arc lamp as the excitation source. The
emission spectra were corrected for detection and optical spectral
response of the spectrofluorimeter through a calibration curve sup-
plied by the manufacturer. The excitation spectra were corrected
for the spectral distribution of the lamp intensity by using a photo-
diode reference detector. The luminescence lifetimes in the micro-
second–millisecond scales were measured by a pulsed Xe lamp
with variable repetition rate and elaborated with standard software
1
mesh, 0.040–0.063 mm (Merck, Darmstadt, Germany). H and
1
3
C NMR spectra were recorded with a Bruker Avance 400 spec-
trometer (400 and 100.6 MHz, respectively) equipped with a 5 mm
1
3
1
1
3+
C- H dual probe; the H NMR spectrum of Dy ꢁ9 was recorded
with a Bruker Avance 400 spectrometer (400 MHz) equipped with
1
13
a 4 mm double resonance ( H, C) gradient HR-MAS probe; spin-
ning rates of 8 kHz and 10 kHz were used to reduce line broaden-
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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These are not the final page numbers! ÞÞ

Articles
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Keywords: decomplexation · dipicolinic acid · lanthanide
complexes · luminescence · ratiometric sensor
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127, 4484–4489.
Final Article published: && &&, 2016
&
ChemPhysChem 2016, 17, 1 – 9
www.chemphyschem.org
8
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ÝÝ These are not the final page numbers!

ARTICLES
Topological influence: The behavior of
two luminescent lanthanide complexes
F. Mian, G. Bottaro,* R. Seraglia,
M. Cavazzini, S. Quici,* L. Armelao
3
+
3+
(
Eu ꢁ1, Dy ꢁ9) in water in the pres-
&& – &&
ence of dipicolinic acid (DPA) is strongly
influenced by their different topologies.
The Role of Ligand Topology in the
Decomplexation of Luminescent
Lanthanide Complexes by Dipicolinic
Acid
3
+
Whereas the luminescence of Dy ꢁ9 is
not affected by the presence of DPA,
thus providing evidence for the inert-
ness of the macrocyclic structure, the
recognition of the DPA was possible be-
cause of the flexible acyclic coordination
3
+
3+
site in Eu ꢁ1. The Eu –DPA interac-
tions are strong enough to induce
a ligand exchange reaction.
ChemPhysChem 2016, 17, 1 – 9
www.chemphyschem.org
9
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
&
These are not the final page numbers! ÞÞ