Tuning of the sensing properties of luminescent Eu3+ complexes towards the nitrate anion

Article abstract of DOI:10.1039/c5dt04665c

A new family of imine-based ligands containing pyridine or furan as an aromatic donating ring [N,N′-bis(2-pyridylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L1; N,N′-bis(2-furanylmethylidene)-1,2-(R,R + S,S)-cyclohexanediamine, L2 and N,N′-bis(2-thie

Full text of DOI:10.1039/c5dt04665c

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Leonzio, M. Bettinelli, M. Monari, C. Grazioli, A. Melchior and M. Tolazzi, Dalton Trans., 2016, DOI:
1
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Tuning of the sensing properties of luminescent Eu
towards the nitrate anion
D
c
OI
o
: 10
m
.10
p
39/
l
C
e
5D
x
T
e
04665C
s
[1]
[1]
[1]
[2]
Fabio Piccinelli,*
Marco Leonzio,
Marco Bettinelli , Magda Monari , Cristian
[3]
[3]
[3]
Grazioli, Andrea Melchior* and Marilena Tolazzi
[
1] Luminescent Materials Laboratory, DB, Università di Verona, and INSTM, UdR Verona, Strada
Le Grazie 15, 37134 Verona, Italy
2] Dipartimento di Chimica ‘‘G. Ciamician’’, Università di Bologna, Via Selmi 2, 40126 Bologna,
Italy
3] Dipartimento di Chimica, Fisica e Ambiente, Università di Udine, via Cotonificio 108, 33100
[
[
Udine, Italy
Abstract
A new family of imine-based ligands containing pyridine or furan as aromatic donating ring [N, N’-
bis(2-pyridylmethylidene)-
furanylmethylidene)-1,2-(R,R+S,S)-cyclohexanediamine, L2 and N, N’-bis(2-thienylmethylidene)-
,2-(R,S)-cyclohexanediamine, L3] has been prepared in high yield by means of an easy synthetic
1,2-(R,R+S,S)-cyclohexanediamine,
L1;
N,
N’-bis(2-
1
-
-
protocol. Their trifluoromethansulphonate (CF SO , OTf ) Eu(III) complexes have been employed
3
3
-
for luminescence sensing of the NO anion in anhydrous acetonitrile solution. Spectrophotometric
3
titrations have been carried out to define the speciation in solution and study the formation of
-
3
ternary species occurring with the addition of NO anions. The sensing response towards this anion
is strongly dependent on the nature of the ligand, the stoichiometry of the complexes and their
concentration.
*
Corresponding authors email: fabio.piccinelli@univr.it and andrea.melchior@uniud.it
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DOI: 10.1039/C5DT04665C
Introduction
The luminescence of metal or lanthanide complexes [Ln(III) = Sm(III), Eu(III), Tb(III), Dy(III),
1
,2
Yb(III)] has been extensively exploited in the last decades in luminescent sensors for medical
3
–5
6–8
diagnostics and optical cell imaging, contrast reagents for magnetic resonance imaging, shift
9
reagents for NMR spectroscopy, as well as in applications in fundamental and applied science
1
0
such as organic synthesis, bioorganic chemistry, catalysis.
Compared with the short emission lifetimes of most organic dyes (on the scale of nanoseconds), the
long emission lifetimes of Ln(III) ions are suitable for minimizing interference from light scattering
or autofluorescence via time-gated detection. For this reason, optical sensors based on Ln(III)
complexes are ideal candidates for the detection of specific analytes in complex microenvironments
1
1,12
such as cells, tissue or living animals.
Additional attractive optical properties of Ln(III) complexes, which make them interesting
luminescent probes, include an efficient intramolecular energy transfer from the triplet state of the
coordinated organic ligand (antenna) to the excited state of Ln(III) ion, which gives rise to a high
1
3
Ln(III) emission intensity, bypassing the Laporte-forbidden nature of the f-f transitions. Also the
large energy shift between absorbed and emitted radiations (in the case of ligand sensitization =
antenna effect) and the very narrow emission bands allow the separation between Ln(III)
1
4
luminescence and short-lived background fluorescence. Furthermore, luminescent complexes of
Eu(III) and Tb(III) are the most employed due to the low sensitivity of their excited state to
vibrational quenching effects caused by OH, NH, or CH oscillators, frequently present in solution
and imaging environments.
For these reasons, Eu(III) and Tb(III) complexes have been extensively exploited as sensors of
species both in aqueous solution and in organic solvents. Focusing our attention on anion sensing,
1
5–25
26–39
the large number of studies performed in water
importance of this field of research.
and in organic solvents
reveals the
As far as the anion sensing in aprotic organic solvent (acetonitrile, AN) is concerned, Montalti et
2
9
al. developed an Eu(III) complex with a chromophoric bis-bipyridine-phenylphosphine oxide
whose luminescence emission features are very sensitive to the nitrate anion. Several authors
showed, employing a tripodal organic ligand, the dramatic effects on the anion selectivity and
2
6,33
sensitivity, of the nature of the lanthanide center [Eu(III) or Tb(III)]
as well as of the ligand
2
6,32,33
28
chirality
. Finally, the chiral tripodal ligands presented by Masaki et al.
give rise to
complexes exhibiting anion-responsive fluorescence, luminescence, and circular dichroism spectral
2

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0
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DOI: 10.1039/C5DT04665C
characteristics as multiple anion-sensors. Kataoka et al., prepared several tripodal ligands with
mixed-donor abilities and reported their different behavior as anion sensors.
3
8,40,41
In this article, the family of tetradentate Schiff base ligands (L1, L2 in Scheme 1),
with
stereochemistry and donor ability controlled by means of a convenient synthetic protocol, is
enlarged with the new meso(cis)-pyridine-based ligand L3 (Scheme 1). The speciation of triflate
and nitrate Eu(III) complexes of L3 has been determined in AN solution by means of UV-Vis
3
8,41
spectrophotometric titrations and compared to the one previously obtained
with L1 and L2. The
-
formation of the ternary species between the Eu triflate complexes and NO anion has been also
3
studied for the ligands L1, L2 and L3. As far as the luminescent sensing of this anion is concerned,
the effect of the ligand donor ability, the ligand stereochemistry, the nature of the triflate complexes
as well as their concentration in solution have been investigated.
Material and methods.
Eu(CF SO ) (Aldrich, 98%) and Eu(NO )  5H O (Aldrich, 99.9%) have been stored under
3
3 3
3 3
2
vacuum for several days at 80°C and then transferred in a glove box. Stock solutions of the salt
were prepared by dissolving weighed amounts of them in anhydrous AN and then standardized by
4
2
complexometry with EDTA. Anhydrous AN was obtained by refluxing the solvent (Aldrich, 99%)
for 2 hours in presence of CaH . The central fraction was transferred in a MB 150 glove box and
2
stored in presence of molecular sieves (4Å). The concentration of water ( < 5 ppm) was checked by
Karl Fischer method (Metrohm). Tetraethylammoniumnitrate (C H ) NNO (Aldrich, >98%) was
2
5 4
3
stored in glove box and its stock solutions were prepared by weighing.
N, N’-bis(2-pyridylmethylidene)- 1,2-trans-(R,R+S,S)-cyclohexanediamine, (L1), N, N’-bis(2-
furanylmethylidene)- 1,2- trans-(R,R+S,S)-cyclohexanediamine (L2) and N, N’-bis(2-
thienylmethylidene)- 1,2-cis-(R,S)-cyclohexanediamine (L3) were obtained by following the same
3
8,40
synthetic protocol as reported in literature
and summarized in Scheme 1.
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DOI: 10.1039/C5DT04665C
Scheme 1. Synthetic route employed to obtain L1- L3 ligands.
Spectrophotometric titrations.
Spectrophotometric UV-Vis titrations were typically carried out by adding a solution of Eu(III) salt
to a ligand solution prepared with anhydrous AN: the concentration of the ligand was in the range 2
-5
-1
-
5  10 mol L and a ratio of the ligand to metal concentration (R = C /C ) up to 4 was reached.
c L M
The stability constants values were calculated by simultaneous fit of the absorbance values at
4
3
44,45
different wavelengths for two titrations using the HypSpec program and the Solverstat tool
statistical analysis of the results.
for
X-ray Structural Determination.
The X-ray intensity data were measured on a Bruker SMART Apex II CCD area detector
diffractometer for L3Eu(NO ) . Single crystals suitable for X-ray diffraction measurements have
3
3
been obtained by slow evaporation of an acetone solution of the complex. The cell dimensions and
the orientation matrix were initially determined from a least-squares refinement on reflections
measured in three sets of 20 exposures, collected in three different ω regions, and eventually refined
against all data. For all crystals, a full sphere of reciprocal space was scanned by 0.3°ω steps. The
4
6
software SMART was used for collecting frames of data, indexing reflections and determination
4

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of lattice parameters. The collected frames were then processed for integration by the SAINT
DOI: 10.1039/C5DT04665C
4
6
47
program, and an empirical absorption correction was applied using SADABS. The structures
4
8
were solved by direct methods (SIR 97) and subsequent Fourier syntheses and refined by full-
2
49
matrix least-squares on F (SHELXTL), using anisotropic thermal parameters for the majority of
non-hydrogen atoms. The asymmetric unit contains two independent molecules and one
crystallization acetone. In one of the two molecules the cyclohexyl carbons were found to be
disordered over two positions and therefore were refined isotropically with occupation factors of 60
and 40% , respectively. One acetone molecule was also found in the asymmetric unit. All hydrogen
atoms, were added in calculated positions, included in the final stage of refinement with isotropic
thermal parameters, U(H) = 1.2 Ueq(C) and allowed to ride on their carrier carbons. Crystal data
and details of the data collection for the structure are reported in Table S1. Molecular graphics were
5
0
generated using SCHAKAL. CCDC 1436955 contains the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif.
Luminescence.
Room temperature emission and excitation spectra were measured with a Jasco fp8200
spectrofluorimeter, equipped with a Xe lamp, a double excitation monochromator, a single emission
monochromator (mod. HR320) and a photomultiplier in photon counting mode for the detection of
the emitted signal.
The anion sensing experiments have been carried out by adding sequential amounts of the target
nitrate salt to an anhydrous AN solution of triflate complex with 1:1 or 2:1 ligand (L1, L2 or L3) to
metal mole ratio [L:Eu(CF SO ) =1:1 or 2:1]. For all the ligands, each experiment has been carried
3
3 3
3
+
-4
-3
out at two different concentration of the Eu ion (≤ 10 M and 10 M) and after each addition of
the anion an emission spectrum was collected.
Results and discussion
Synthesis of the ligands
A family of ligands with tunable coordination abilities and stereochemistry has been obtained with
high yield (85% for L1, 82% for L2 and 72% for L3) in dichloromethane (DCM) as solvent,
without need of additional time-consuming purification procedures. A ring with a moiety containing
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a strong donating atom (pyridine, Gutmann Donor number , DNpyridine = 33.1) and one with a poor
DOI: 10.1039/C5DT04665C
51
donating atom (furan, DNfuran= 6) have been incorporated within the ligand backbone (ligands L1
and L2, respectively). By changing the stereochemistry of the diamine backbone (from the 1,2-
trans-(R,R+S,S) racemic isomer to the 1,2-cis-(R,S) meso one) the synthesis of the achiral isomer of
the pyridine-based ligand (L3) has been possible.
Species in solution: triflate and nitrate complexes
The speciation and stability of the complexes formed with the ligands have been determined by
spectrophotometric titrations. In Figure 1, the absorbance changes during the titration of the meso
isomer of the pyridine-based ligand (L3) with Eu(NO ) and Eu(CF SO ) in AN solution are
3
3
3
3 3
shown.
The values of the formation constants determined for L3, together with those previously obtained
3
8,41
for L1 and L2,
are reported in Table 1.
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DOI: 10.1039/C5DT04665C
(a)
(b)
7

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DOI: 10.1039/C5DT04665C
Figure 1. Absorption spectra variations when L3 is titrated with (a) Eu(OTf) (C = 0.08 mM)
3
L3
(
C = 7.26 mM, 12 additions of 3L) final C /C = 1.7; (b) Eu(NO ) (C = 0.08 mM) (CEu=
Eu
Eu
L
3 3
L3
1
9.68 mM, 7 additions of 2L) final C /C = 1.8. Arrows indicate the direction of the spectral
Eu
L
changes. Red: spectrum of L3.
Table 1. Overall stability constants (log , j = 1, 2) for the reactions Eu + jL  Eu(L) (charges
j
j
omitted) of the triflate and nitrate complexes in AN (standard deviation in parentheses).
41
38
L1
L2
L3
Eu(NO3)3
log 1
6.87(0.07) 4.75 (0.16) 6.7(0.1)
9.03(0.09) 8.5 (0.5) 9.02(0.19)
16.5(0.6) 13.5 (0.5) 16.44(0.39)
Eu(CF SO ) log 1
3
3 3
log 2
The speciation models (Table 1) yielding the best fit of the experimental absorbances, show that for
all the ligands under investigation, two highly stable 1:1 and 1:2 triflate complexes are found in
anhydrous AN solution. The stability constants of the triflate complexes with the two pyridine-
based isomeric ligands (L1 and L3) are very similar, and higher than for the triflate complexes of
the furan-based one (L2). This is clearly related to the stronger bond between Eu(III) and the
ligands containing the pyridine ring. As far as the nitrate complexes are concerned, only a 1:1
4
1
species is observed for all the ligands. As discussed previously this is due to the high affinity of
the nitrate anion for the Eu(III) hampering the coordination of a second ligand molecule. These
results clearly show that the stereochemistry of the pyridine-based ligands does not have a
significant impact on the thermodynamic stability of their complexes with europium(III).
Species in solution in the sensing experiments
UV-Vis spectrophotometric titrations have been also employed with the aim to determine the
stability of the species formed by EuL (L = L1, L2 and L3) complexes and the nitrate anion. In our
experiments, solutions containing the ligand and the Eu(CF SO ) mixed in the desired
3
3 3
stoichiometric ratio (1:1 or 2:1) were titrated with a solution containing the nitrate salt
(C H ) NNO ]. The electronic spectra after each addition have been collected and reported in
[
2
5 4
3
figure 2a and 2b for the ligand L1.
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DOI: 10.1039/C5DT04665C
a)
b)
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DOI: 10.1039/C5DT04665C
Figure 2. Absorption spectra of: (a) solution containing 1:1 Eu:L1 molar ratio (C = 0.053 mM,V
Eu
0
=
2.1 mL), titrated with a (C H ) NNO solution (2.0 mM, 18 additions of 10 μL); final
2 5 4 3
C /C /C
= 1/1/4. (b) solution containing Eu:L1 in 1:2 molar ratio (C = 0.035 mM , V = 2.1
Eu 0
Eu
L
nitrate
mL) titrated with a (C H ) NNO solution (2 mM, 16 additions of 10 μL); final C /C /C =
2
5 4
3
Eu
L
nitrate
1
/2/4.
The addition of nitrate to the solution containing Eu(CF SO ) :L1 in 1:1 ratio results in the
3
3 3
disappearance of the maximum at  = 290nm and of the shoulder at  = 300nm and the progressive
formation of two new maxima at  = 286 and  = 296 nm (Figure 2a). In the case of titration of the
solution containing Eu(CF SO ) :L1 in 1:2 ratio (Figure 2b), the starting spectral features related to
3
3 3
the Eu(L1)2 species (maximum at  = 286 nm and shoulder at  = 298nm) disappear and two
maxima at  =288 and 296 nm are evidenced (Figure 2b). The speciation models yielding the best
fit of the experimental data for both titrations, predict the presence of only one nitrate complex. The
related speciation curves upon addition of the nitrate salt to the Eu triflate complexes are shown in
Figure 3. The ternary species formed by Eu(III), L1 and nitrate anion is rather stable, with a log =
1
3+
-
2+
7
.5(0.2) for the equilibrium EuL1 + NO  [EuL1(NO )] . Complexes with two or three bound
3
3
nitrate anions have not been detected in the experimental conditions of the spectrophotometric
-5
titration ([Eu(III)] ≤ 8∙10 M). Further, in the case of the titration of the Eu(L1) species, the nitrate
2
anion is also able to bind the Eu(III) ion replacing one ligand molecule, as suggested by the fact that
the absorbance maxima (286 and 296 nm) at the end of the titration in Figure 2b coincide with those
1
0

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obtained at the end of the titration of Eu(L1) in Figure 2a. This result is also in agreement with the
DOI: 10.1039/C5DT04665C
4
1
presence of only a 1:1 complex with L1 in the solutions containing Eu(NO ) salt (Table 1).
3
3
a)
b)
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Figure 3. Species distributions during the titrations of the Eu(OTf) :L1 solutions titrated with
3
(
C H ) NNO in the conditions reported in the caption of Figure 2: (a) Eu(L1) and (b) Eu(L1) .
2
5
4
3
2
In the case of the complexes with the ligand L2, we have not been able to clearly detect spectral
changes associated to the interaction of the complex with nitrate (Figure S1). This suggests that
either the coordination of nitrate is very weak or that the absorption spectrum of the ligand L2
bound to the Eu(III) ion is not sensitive to the coordination of this anion.
The addition of nitrate to the solution containing Eu(CF SO ) :L3 in 1:1 ratio results in the
3
3 3
disappearance of the maxima at  = 288 and 298 nm and the progressive formation of two new
maxima at  = 286 and 297 nm (Figure S2a). In the case of titration of the solution containing
Eu(CF SO ) :L3 in 1:2 ratio (Figure S2b), the starting spectral features related to the Eu(L3)
2
3
3 3
species (maximum at  = 289 nm and shoulder at  = 299 nm) disappear and two maxima at  =282
and 297 nm are formed (Figure S2b).
Again, only one nitrate is able to bind the Eu(III) complex in the experimental conditions adopted,
3
+
-
to form a ternary 1:1:1 species with a log = 8.07(0.03) relative to the equilibrium EuL3 + NO
1
3
2
+

[EuL3(NO )] . The log value is close to that found to the formation of the complex with L1,
3
1
thus the speciation curves are very similar to those calculated previously (Figures 3 and S3).
In conclusion, Eu(III) complexes with the pyridine-imine isomers are both able to form a stable 1:1
adduct with nitrate which can be detected by absorbance changes, on the contrary, the absorbance
spectra of complexes with L2 are insensitive to the presence of the anion in the coordination sphere
1
2

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of the metal. The stereochemistry of the ligand [R, R (+ S, S) or R, S] does not show a remarkable
DOI: 10.1039/C5DT04665C
impact on the stability of the complex formed.
Single crystal X-ray structural determination
EuL3(NO3)3
Complex L3Eu(NO ) crystallizes in the centric triclinic space group P-1. In the asymmetric unit
3
3
of the crystal two independent molecules of [L3Eu(NO ) ] and four fifths of one crystallization
3
3
acetone molecule (occupation factor (OF) 0.80) are present. This is due to a partial loss of
solvent by the crystal, which however does not deteriorate the quality of the structural data. In both
molecules the Eu(III) ion has a 10-fold coordination, being coordinated by one tetradentate ligand
L3 and three bidentate nitrates. Even if the two complexes exhibit a similar arrangement of the
ligands (nitrate anions and L3) around Eu(III), they are not superimposable as they differ slightly by
the Eu-O bond distances, the N-Eu-N bond angles (table S2) and by the orientation of the
cyclohexyl rings. Actually, one of the two complexes shows a disorder over two position of the
cyclohexyl ring (figure 4).
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DOI: 10.1039/C5DT04665C
Figure 4. View of one of the two independent molecules present in the asymmetric unit of the
crystal. Conformer 1: purple cyclohexyl moiety (occupation factor 0.60). Conformer 2: grey
cyclohexyl moiety (occupation factor 0.40). The H atoms of the cyclohexyl unit are omitted for
clarity.
The main orientation of the cyclohexyl moiety (purple carbon atoms, OF 0.60) is converted into the
second one (grey carbon atoms) by the well-known chair-boat-chair conformational
interconversion, typical of the 6-membered cyclohexane rings. As a consequence of this process,
the equatorial N(8) substituent in the dominant conformational isomer (conformer 1, OF 0.60) is
converted into the axial substituent in the conformer 2 (OF 0.40). At the same time, the axial N(9)
atoms (conformer 1) is in the equatorial position in the conformer 2 (fig. 4). The small imbalance of
the occupation factors of the two conformers (0.6/0.4 instead of the expected 0.5/0.5) should be
ascribed to small packing effects. From a close inspection of Table 2, reporting the selected bond
lengths and angles of the L1Eu(NO ) and L3Eu(NO ) crystal structures, we can conclude that the
3
3
3 3
stereochemistry of the ligand [R, R (+ S, S) (L1) or R, S (L3)] does not seem to play a relevant role
on its coordination geometry.
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Table2. Comparison of selected bond lengths (Å) and angles (°) between complex EuL1(NO )
3
3
4
0
(
taken from ref. ) and EuL3(NO ) . For the sake of simplicity only the bond parameters of one
3 3
EuL3(NO ) complex present in the asymmetric unit of the crystal (figure 4) are reported. The close
3
3
similarity between the bond parameters of the two EuL3(NO ) molecules in the asymmetric unit is
3
3
documented in table S2.
Bond lengths
EuL3(NO3)3
2.561(9) Eu(2)-N (8)
EuL1(NO3)3
Eu(1)-N (1)
2.544(4)
2.570(4)
2.598(5)
2.593(4)
2.475 (4)
2.460(4)
2.520(4)
2.523(4)
2.529(4)
2.504(4)
im
im
Eu(1)-N (2)
2.53(1)
2.63(1)
Eu(2)-N (9)
im
im
Eu(1)-N (3)
Eu(2)-N (10)
py
py
Eu(1)-N (4)
2.622(9) Eu(2)-N (11)
py
py
Eu(1)-O(1)
Eu(1)-O(3)
Eu(1)-O(4)
Eu(1)-O(5)
Eu(1)-O(7)
Eu(1)-O(8)
2.549(8) Eu(2)-O(10)
2.539(9) Eu(2)-O(11)
2.488(8) Eu(2)-O(13)
2.496(8) Eu(2)-O(14)
2.485(8) Eu(2)-O(16)
2.465(8) Eu(2)-O(17)
Bond angles
N(1)-Eu(1)-N(2) 63.4(3)
N(8)-Eu(2)-N(9)
63.6(1)
124.8(1)
63.5(1)
63.2(1)
125.5(1)
N(1)-Eu(1)-N(3) 123.2(3) N(8)-Eu(2)-N(10)
N(1)-Eu(1)-N(4) 63.3(3)
N(2)-Eu(1)-N(3) 64.2(3)
N(8)-Eu(2)-N(11)
N(9)-Eu(2)-N(10)
N(2)-Eu(1)-N(4) 126.7(3) N(9)-Eu(2)-N(11)
N(3)-Eu(1)-N(4) 158.5(3) N(10)-Eu(2)-N(11) 152.3(1)
Very similar Eu-N bond distances and N-Eu-N bond angles are detected and a square planar
geometry of the nitrogen ligands is observed. The N atoms of both tetradentate ligand slightly
deviate from the ideal square planar geometry [N(imine): +0.20(1) Å and -0.20(1) Å, N(py):
+
0.102(5) Å, and -0.101(6) Å for L1Eu(NO ) . N(imine): +0.180(2) Å and -0.184(2) Å, N(py):
3 3
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+
0.098(1) Å, and -0.094(1) Å for L3Eu(NO ) ], and the distance of the Eu ion from the N4 least
3 3
DOI: 10.1039/C5DT04665C
square plane of the coordinating N atoms is 0.41(1) Å and 0.525(3) Å for L1Eu(NO ) and
3
3
L3Eu(NO ) , respectively. Finally, very similar Eu-O bond distances are detected for both
3
3
complexes (table 2). The main structural difference between the Eu(III) complexes of L1 and L3
ligands is the relative orientation of the cyclohexyl unit and the coordinating N4 square plane. In the
case of the L1 racemic(trans) ligand, the cyclohexyl and the N4 units lie on the same plane; on the
contrary, in the case of L3 meso(cis) ligand, the N4 plane and the cyclohexyl unit are perpendicular
to each other (Fig S4). The conclusions drawn from the crystal structure determination, in particular
the almost identical coordination geometry of the ligands in L1Eu(NO ) and L3Eu(NO )
3
3
3 3
complexes, can be strongly related to the similar values of their stability constants calculated in
acetonitrile solution (see the above sections).
Luminescence sensing experiments
The impact of the ligand donor ability, the complex stoichiometry and the complex stereochemistry
In order to evaluate the role of the ligand donor ability, the complex stoichiometry and the complex
stereochemistry on the luminescence sensing response towards nitrate anion, several luminescence
emission spectra have been recorded in different conditions upon addition of the nitrate salt
[
(C H ) NNO ] to an anhydrous AN solution of the desired triflate complexes. In particular, two
2 5 4 3
solutions of L:Eu(CF SO ) = 1:1 and L:Eu(CF SO ) = 2:1 (with L = L1, L2 and L3), have been
3
3 3
3
3 3
prepared at the same concentrations employed during the spectrophotometric titration experiments.
The related luminescence emission spectra are presented in the SI (Figures S5-S10). The change of
the Eu(III) emission features during the sensing experiments are usually strongly connected to the
alterations of its inner coordination sphere, in particular to the degree of asymmetry of its geometric
5
2,53
environment. The asymmetry ratio
:
5
7
I( D  F )
0
2
R =
5
7
I( D  F )
0
1
is indicative of the degree of asymmetry of the coordination polyhedron around the Eu(III) ion and
3
8
therefore is broadly exploited in the sensing experiments involving Eu(III) ion. The sensitivity of
the Eu(III) luminescence response towards the nitrate anion is easily estimable by plotting the R vs.
the number of added nitrate equivalents. These diagrams are reported in Figures 5a and 5b,
respectively, for the EuL and EuL (L =L1, L2 and L3) triflate complexes.
2
1
6

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(a)
(b)
Figure 5. Asymmetry ratio (R) vs. nitrate concentration plots for the (a) EuL and (b) EuL triflate
2
complexes. [EuL1] = 0.04 mM, [EuL2] = 0.02 mM, [EuL3] = 0.08 mM, [Eu(L1) ] = 0.04 mM,
2
[
Eu(L2) ] = 0.02 mM and [Eu(L3) ] = 0.04 mM
2 2
1
7

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From inspection of Figure 5, several observations can be made: i) the sensitivity towards the nitrate
DOI: 10.1039/C5DT04665C
a
anion is strongly dependent on the ligand donor ability. The poorer donating ligand L2 shows the
best sensitivity. This is true for both triflate complexes differing by the ligand to metal
stoichiometric ratio (EuL and EuL ). ii) The luminescence sensing response is affected by the
2
nature and stoichiometry of the triflate complex. In particular, as far as the curve profile is
concerned for the complexes of the L1 and L3 ligands, a sigmoidal trend is observed in the cases of
EuL1 and EuL3 whereas a logarithmic-like trend is observed in the case of Eu(L1) and Eu(L3) .
2
2
For the complexes of L2 the trend is logarithmic for EuL2 and sigmoidal for Eu(L2) . iii) The plot
2
of R vs. nitrate concentration is little dependent on the stereochemistry of the pyridine-based ligand.
In fact, the curves are very similar for EuL1 and EuL3 and for Eu(L1) and Eu(L3) . In conclusion,
2
2
the stereochemistry of the ligand does not seem to play a crucial role on the luminescence sensing
response towards nitrate anion. This observation is an obvious consequence of the similar speciation
during the titration of the triflate complexes with the nitrate anion for the isomeric ligands L1 and
L3 (Figures 3 and S3). Furthermore, also the close similarity of the stability constants of the triflate
and nitrate complexes discussed above and the similar coordination geometry of the L1 and L3
ligands observed in the solid state, support the same conclusion.
From a close inspection of Figures 3 and 5, for the ligand L1 (the same applies for the ligand L3)
the R value increases up to the addition of 1 eq of anion for Eu(L1) (CF SO ) and up to the
2
3
3 3
addition of 2 eq. for EuL1(CF SO ) (Figure 5). In the case of EuL1(CF SO ) , further additions of
3
3 3
3
3 3
nitrate do not change the R value (a constant number is reached; R≈4.2). On the contrary, in the
case of Eu(L1) (CF SO ) the R value continues to increase (even if very slowly) after the addition
2
3
3 3
of 1 eq of nitrate and a constant value is not reached upon the addition of 4 eq. of anion. This
behavior is in agreement with the calculated speciation during the spectrophotometric titration of
the triflate complexes with nitrate anion (figure 3). In fact, in the case of EuL1(CF SO3)3 the
3
relative amount of the luminescent species [EuL1 and EuL1(NO )], and consequently the R value
3
of the mixture, changes significantly only up to the addition of 2 eq. of anion. On the other hand, in
the case of Eu(L1) (CF SO ) , the concentration of the luminescent species changes more
2
3
3 3
drastically at the beginning of the titration (up to the addition of 1-1.5 eq of nitrate) and changes
only slightly, after 1.5 eq.
a
The different charge transfer in the complexes with L1 and L2 can be also estimated from the partial charges
calculated by DFT and reported in the Supplementary Information.
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8

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The impact of the concentration of the complexes.
54
It is well known that the nitrate anion shows a high affinity for the lanthanide ions. As discussed
5
5
by Bünzli et al., Tb(NO ) behaves as non-electrolyte in anhydrous acetonitrile if its concentration
3
3
is above 1 mM. On the other hand, it starts to behave as electrolyte when the concentration is
decreased to 0.2 mM. As far as the triflate anion is concerned, it shows a low affinity for Ln(III) and
56
its lanthanide salts are completely dissociated at concentration below 0.05 mM. In order to
observe an equilibrium between the undissociated Ln(CF SO3)3 and the 1:1 electrolyte
3
+
57
(
[Ln(CF SO ) ] ) the concentration of the salt must be 33 mM. In conclusion, the number of
3 3 2
triflate or nitrate anions in the inner coordination sphere of the Ln(III) and the related value of the
asymmetry ratio (R), is strongly dependent on the concentration of the complex. As far as the nitrate
complex with the ligand L1 is concerned, from infra-red absorption spectroscopy the coordination
of three nitrate anions to the Eu(III) has been demonstrated when the concentration of the complex
4
1
is higher than 1 mM. On the other hand, as reported before, only one nitrate anion is present in
the first coordination sphere of the metal ion when the concentration is decreased to 0.04-0.08 mM
(Figure 3). From these data, a concentration-dependent behavior of the sensing towards nitrate
anion is expected. In fact, as reported in Figure 6, the plots of R values vs. nitrate concentration are
different at different EuL1 concentrations.
Figure 6. Asymmetry ratio (R) vs nitrate concentration plot for the EuL1 at two different
concentration (i.e. 0.04 and 1 mM) of Eu(OTf) salt
3
.
In particular, at 1mM of complex concentration, the luminescence response is poorly sensitive
towards nitrate concentration up to the addition of 2 eq. of anion. The addition of the third
1
9

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equivalent gives rise to a drastic increase of the R value. Unlike the speciation at low concentration
DOI: 10.1039/C5DT04665C
(
0.04 mM), we can propose the presence of several nitrate complexes [EuL1(NO ) ; x = 1-3] during
3 x
the titration with nitrate at the 1 mM concentration. In this context, it is worth to point out that the
luminescence emission spectrum of the complex upon the addition of 3 eq. of nitrate is completely
superimposable with the one of the nitrate complex EuL1(NO ) (figure S11) at 1mM, containing
3
3
4
1
three anion in the inner coordination sphere, as mentioned before .
The concentration-dependent sensing towards nitrate anion is also observed for the L2-based triflate
complexes (Figure 7).
Figure 7. Asymmetry ratio (R) vs. nitrate concentration plot for the EuL2 at two different
concentration (i.e. 0.02 and 1 mM).
At 0.02 mM the logarithmic-like curve approaches an asymptote after the addition of 5 eq. of
nitrate. On the other hand, at 1mM the curve profile is almost linear and a change of the slope is
observed around 2 eq. of anion. The curve approaches an asymptote after the addition of 4 eq. of
58
nitrate. As shown by Bünzli et al. , the maximum of nitrate ions bound to a Ln(III) ion [Tb(III)] is
2
-
5
([Tb(NO ) ] ) at 2mM. This conclusion is confirmed also for Eu(III) ion, since upon addition of
3 5
the nitrate anion to a 1mM solution of Eu(CF SO ) the value of the asymmetry ratio increases from
3
3 3
almost 1 to 5 up to the addition of 5 eq. of anion. After 5 eq the value of R, as well as the
luminescence emission spectrum, does not change thereafter (figure 7, red squares) and therefore
the Eu(III) complex at the end of the titration should have 5 nitrate anions bound to the metal ion.
With a similar reasoning, we can propose the presence of 4 nitrate anions bound to the Eu(III) at the
end of the titration of the triflate complex EuL2 with nitrate (Figure 7 , open circles). To confirm
2
0

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this assumption, the luminescence emission spectra of solution 1, 2 and 3 are superimposed in
DOI: 10.1039/C5DT04665C
figure S12. Solution 1) is composed of EuL2 (1 mM) + 4.5 eqs. of nitrate (a composition at the end
of the titration, figure 7), solution 2) of EuL2(NO ) and solution 3) of EuL2(NO ) upon addition
3
3
3 3
of one eq of nitrate anion.
The perfect match between the luminescence emission spectra of the solutions 1 and 3, leads us to
confirm the aforementioned assumption.
Also in the case of the triflate complexes with 2 to 1 ligand to metal mole ratio (EuL ) the sensing
2
response towards nitrate is dependent on the concentration of the complex (Figures S13 and S14 for
Eu(L1) and Eu(L2) , respectively). At 1mM, in the case of Eu(L1) the presence in solution of the
2
2
2
luminescent Eu(L1) , EuL1NO , EuL1(NO ) and EuL1(NO ) species is expected. Their
2
3
3 2
3 3
percentage of formation is dependent on the concentration of the nitrate anion. As for the titration of
the triflate complex Eu(L1) with nitrate at 0.04 mM (Figure 5b), a constant value of R is not
2
reached even after the addition of 4 eq of nitrate, also at 1 mM (Figure S13).
Finally, as far as the meso ligand L3 is concerned, the effect of the concentration of the complex on
the luminescence sensing towards nitrate is quite coincident with the one observed for the ligand
L1 (data not shown).
Conclusions
In the present paper a family of isomeric (rac-L1 and meso-L3) pyridine-based and rac- L2 furan-
based ligands, of easy synthetic approach, are presented. The luminescence sensing towards nitrate
anion in anhydrous acetonitrile solution has been studied employing their Eu(III) triflate complexes.
The crucial role of the donor ability of the ligand on the sensing properties is demonstrated: when a
poorly donating furan aromatic ring is present in the ligand backbone (L2), the best luminescence
sensitivity towards nitrate anion is obtained. On the contrary, the absorption spectrum of the ligand
3
+
L2 when bound to the Eu ion, is not sensitive to the coordination of the nitrate. This is probably
due to the weaker bond between the ligand and the metal ion reducing the sensitivity of the
spectrophotometric absorption to the alterations of the inner coordination sphere. The stoichiometry
of the triflate complexes (EuL or EuL ) plays also an important role on the luminescence sensing
2
properties. On the contrary, the stereochemistry of the ligands (rac or meso) does not strongly affect
the luminescence sensing towards nitrate. Finally, also the concentration of the complexes modifies
the sensing response, since during the titration of the triflate complexes with nitrate anion, the
number of the nitrate or triflate anions bound to the metal ion, which influences its luminescence, is
dependent on the concentration of the species.
2
1

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Acknowledgments
The authors gratefully thank Erica Viviani for the expert technical assistance.
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The dependence of the luminescence sensing towards nitrate on ligand donor ability, ligand
stereochemistry, complex stoichiometry and concentration is shown

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