Reversible acidochromism and thermo-responsive luminescence properties of a novel Zn(II)/Eu(III) coordination polymer

Article abstract of DOI:10.1016/j.dyepig.2020.108587

A new Zn(II)/Eu(III) compound with the N,N′-ethylenebis(5-carboxysalicylideneamino) ligand (H₄Salendc) has been synthesised and its photophysical behaviour studied. This species, [Zn(Salendc)Eu(CH₃COO)(DMSO)]·xH₂O·0.5CH₃CN (x = 3.5–4), has been characterised by XRD, XEDS and SEM and HTEM microscopies. Experimental results suggest the formation of a one-dimensional polymeric compound. The compound behaves as a luminescent material in the solid state. Moreover, it exhibits acidochromic properties upon the addition of a few drops of HCl and in the presence of its vapours. The emission intensity of the bands attributed to the Eu(III) ion can be also modulated by the temperature and by the presence of water, so that this species is a good candidate for its use as a luminescent acid, temperature and water sensor.

Full text of DOI:10.1016/j.dyepig.2020.108587

Dyes and Pigments 181 (2020) 108587
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Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Reversible acidochromism and thermo-responsive luminescence properties
☆
of a novel Zn(II)/Eu(III) coordination polymer
Cristi a� n Cuervaa , Jesús Tapiador , Ana E. S a� nchez-Pel a� ez , M. Carmen Torralba ,
Angel Guti �e rrez
,
*
b
b
b
b,**
�
a
BIOSCOPE Research Group, LAQV@REQUIMTE Chemistry Department, NOVA School of Science and Technology, NOVA University Lisbon, 2829-516, Caparica,
Portugal
Departamento de Química Inorg a� nica, Facultad de Ciencias Químicas, Universidad Complutense, 28040, Madrid, Spain
b
A R T I C L E I N F O
A B S T R A C T
A new Zn(II)/Eu(III) compound with the N,N -ethylenebis(5-carboxysalicylideneamino) ligand (H Salendc) has
0
Keywords:
4
Acidochromism
Sensors
been synthesised and its photophysical behaviour studied. This species, [Zn(Salendc)Eu(CH
3
COO)(DMSO)]⋅
xH CN (x ¼ 3.5–4), has been characterised by XRD, XEDS and SEM and HTEM microscopies. Experi-
2
O⋅0.5CH
3
Salen-type compounds
Lanthanides
mental results suggest the formation of a one-dimensional polymeric compound. The compound behaves as a
luminescent material in the solid state. Moreover, it exhibits acidochromic properties upon the addition of a few
drops of HCl and in the presence of its vapours. The emission intensity of the bands attributed to the Eu(III) ion
can be also modulated by the temperature and by the presence of water, so that this species is a good candidate
for its use as a luminescent acid, temperature and water sensor.
Coordination polymers
1
. Introduction
The fascinating photoluminescence of lanthanide(III) complexes
modes. In these complexes, the salen-type ligands stabilise the metal
centres and act as an antenna for the luminescence of the lanthanide ion.
The stoichiometry and structures of the multimetallic compounds
depend on diverse factors such as the type of Schiff base used, the ionic
radius of the lanthanide, the counterion nature, the pH of the medium or
the type of synthesis [7].
make them very interesting materials with a great potential for photonic
applications, such as light-emitting diodes (LED) or amplifiers for optical
displays and sensing devices [1–5]. The luminescence properties of
polynuclear lanthanides are usually increased using chromophore li-
gands to induce an efficient emission because they transfer the absorbed
energy toward the lanthanide ion [1]. One of the main classes of chro-
mophores investigated corresponds to Schiff bases. In addition, Schiff
base ligands have developed great interest because of their multiple
applications in catalysis, in bioinorganic systems or as antitumoral drugs
The control over the stoichiometries and structures of heterometallic
polynuclear complexes containing transition metals and/or lanthanide
metal ions is a challenge because of the varied coordination numbers
and environments showed by the lanthanide cation. In addition, modi-
fications over the ligand can also influence on the formation of the
polynuclear entities affecting the final complex properties. Several d-f
polynuclear complexes have been investigated giving rise to the for-
mation of Yb(III), Nd(III), Ho(III) or Er(III) derivatives with emissions in
the range of about 900–1600 nm, close to near infrared (NIR), being
adequate for biotechnological applications and in laser systems [8].
On the other hand, Eu(III) complexes exhibit a high quantum yield
and a long-live emission, which make them suitable candidates for the
preparation of novel advanced functional materials with sensorial
abilities. It is well-known that the coordination of water, to complete the
[
6].
The Schiff base environment protects, in many cases, the Ln(III)
centre of the solvent molecules, improving the luminescence properties
of the complexes. One of the best known multidentate ligands is the
0
Schiff base H
2
salen, N,N -ethylenebis(salicylideneamino), and its de-
rivatives. These tetradentate or multidentate ligands are rather versatile
and form complexes with lanthanides providing a wide structural vari-
ety, since they are flexible ligands that can show different coordination
☆
Dedicated to Professor Jos �e Antonio Campo Santillana, in memoriam.
*
Corresponding author.
* Corresponding author.
E-mail addresses: c.alaiz@fct.unl.pt (C. Cuerva), agalonso@ucm.es (A. Gutierrez).
*
�
�
https://doi.org/10.1016/j.dyepig.2020.108587
Received 9 April 2020; Received in revised form 25 May 2020; Accepted 25 May 2020
Available online 30 May 2020
0
143-7208/© 2020 Elsevier Ltd. All rights reserved.

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
coordination sphere for example, can cause a quenching of the Eu(III)
emission bands by the antenna effect blocking [9]. As a result, only
intraligand transitions are allowed in the complex. This behaviour has
been reported in the literature as a strategy to detect water via lumi-
nescence spectroscopy in certain solvents or humid environments
for the analogous phenylene derivative [16]: to a stirred suspension of
3-formyl-4-hydroxybenzoic acid (0.668 g, 4 mmol) in 50 mL of MeOH a
solution of ethylenediamine (135 L, 2 mmol, 99%) in 10 mL of meth-
μ
anol was dropwise added. The mixture changed to yellow colour and it
was refluxed for 4 h. After cooling at room temperature, a yellow solid
was obtained that was filtered off, washed several times with cold MeOH
and dried under vacuum. Yield: 74%.
[
10–12]. However, it has also allowed to developed novel Eu(III) com-
plexes with stimuli-responsive properties. The application of an external
stimulus can induce the establishment of new intermolecular in-
teractions, and coordinated water can be replaced by other chromo-
phore ligands in order to revival the antenna effect [13–15].
Anal. Calc. (%) for C18
H
16
N
2
O
6
.0.25H
2
O: C, 59.9; H, 4.61; N, 7.76.
À 1
Found: C, 59.9; H, 4.49; N, 7.68. IR (KBr pellets, cm ): 2906 m, 2789 m,
2650 m, 2553 m, 2210w, 1635s, 1589 m, 1568 m, 1524 m, 1484 m,
1450 m, 1361s, 1316 m, 1281s, 1235w, 1215w, 1196w, 1180w, 1123w,
1112w, 1077w, 1033 m, 1012w, 981w, 932w, 868w, 844w, 819w, 789s,
In this work, a new polymeric Zn(II)/Eu(III) compound with a flex-
0
ible dicarboxylic salen-type ligand, N,N -ethylenebis(5-carbox-
ysalicylideneamino),
H
4
Salendc, has been obtained and its
685 m, 637 m, 536w, 460w, 428 m. 1H NMR (DMSO‑d
(s,4H), CH
; 6.0 (s,2H), HO–Ph; 6.83 (d,2H), Ph; 7.81 (m,2H), Ph; 8.01
(m,2H), Ph; 8.68 (s,2H) HC
6
, ppm): 3.93
photophysical behaviour studied. The salen compound used as the
ligand has been strategically functionalised with terminal carboxylic
groups to favour the formation of coordinative –COO⋅⋅⋅Eu⋅⋅⋅OOC– bonds.
Interestingly, it is possible to break and form again these short contacts
upon exposure to several external stimuli, which originates dramatic
changes in the luminescence properties of the complex. Structure/
properties relationships have been studied in deep to understand the
mechanisms of these processes; results could be of great interest for
potential application in the field of thermo and acid sensors.
2
–
–
N.
2.3. Synthesis of [Zn(Salendc)Eu(CH COO)(DMSO)]⋅xH O⋅0.5CH CN
3
2
3
(x ¼ 3.5–4)
To a warm yellow suspension of H Salendc (0.250 g, 0.69 mmol) in
4
3
10 mL of DMSO, 40
μ
3
L of Et N (0.29 mmol, 0.726 g/cm , 99%) were
added, followed by a solution of Zn(CH COO) ⋅2H O (0.152 g, 0.69
mmol) in 5 mL of DMSO. After 5 min of stirring, Eu(NO
3
2
2
3
)
3
⋅5H
2
O (1.039
2
. Experimental section
g, 2.43 mmol) was added, and the mixture was refluxed for 30 min, in
order to dissolve completely the europium salt. The resulting yellow
solution was cooled to room temperature and the solvent was slowly
evaporated for two weeks, until a yellow powder is formed. The solid
was filtered off, washed several times with acetonitrile and dried in
vacuum. Yield: 57%.
2
.1. Materials and physical measurements
All chemicals were purchased from Aldrich and were used without
further purifications.
Elemental analysis (carbon, hydrogen and nitrogen) were carried out
by the Microanalytical Service of the Universidad Complutense de
Anal. Calc. (%) for C22H21EuN O SZn.3.75H O.0.5CH CN: C, 34.75;
2
9
2
3
H, 3.80; N, 4.41. Found: C, 34.74; H, 3.81; N, 4.37. IR (KBr pellets,
À 1
Madrid (UCM) using a LECO CHNS-932 analyser. FTIR spectra (4000-
cm ): 3388br, 2914w, 1641s, 1606s, 1553s, 1510 m, 1432 m, 1398vs,
À 1
6
50 cm ) of solid powder samples were recorded using a Perking Elmer
1381vs, 1314 m, 1248w, 1190w, 1130 m, 1016 m, 954w, 850w, 793 m,
745w, 689 m, 646 m, 515w.
spectrophotometer with a universal ATR accessory and FTIR KBr-
À 1
dispersion spectra (4000-400 cm ) were recorded using a THERMO
NICOLET 200 spectrophotometer. 1H NMR spectra of the ligand were
collected in the UCM Nuclear Magnetic Resonance Service using a
Burker AVIII300 (300 MHz) spectrophotometer. The thermogravimetric
data were obtained on a Perkin Elmer Pyris 1 TGA using an open pan,
3. Results and discussion
3.1. Synthesis and structural characterisation
�
À 1
which was purged with N
2
, operating at a heating rate of 5–10 Cmin
.
The ligand was obtained by condensation of ethylenediamine with 3-
formyl-4-hydroxybenzoic acid in a 1:2 ratio under refluxing conditions
in methanol, following the reported procedure for the analogous phe-
Electron microscopy studies were carried out either in a JEOL JEM-
2
100HT transmission electron microscopy (TEM) operating with an
accelerating voltage of 200 kV or in a JEOL JEM GRAND ARM300cF
operating at 60 kV with a resolution of 0.5 Å; scanning electron mi-
croscopy (SEM) was performed using a JEOL JSM 6335 FEG electron
microscope operating at 5 kV. Chemical composition analysis was also
performed on the JEOL JEM-2100HT electron microscope using energy
dispersive X-Ray spectroscopy (XEDS). X-Ray diffraction patterns were
recorded in an X’PERT-MPD diffractometer working with mono-
nylene derivative [16]. The synthesis of [Zn(Salendc)Eu(CH COO)
3
(DMSO)]⋅xH O⋅0.5CH CN (x ¼ 3.5–4) was carried out in two steps by
2
3
the “one pot synthesis” method. In a first step, the precursor [Zn
2
-
(Salendc)] was formed by reaction of the deprotonated ligand, Sale-
4
À
ndc , with Zn(CH COO) ⋅2H O. A small quantity of base (Et N) must
3
2
2
3
be present in the reaction mixture to help the carboxylate deprotona-
tion; however, the presence of the base seems to have a catalytic role
since it is not necessary the addition of a stoichiometric amount for the
reaction to proceed. The complexation of the zinc ion is proposed due to
the solubilisation of the salen ligand. In the second step, addition of Eu
(NO ) ⋅5H O to the precursor solution afforded the final compound. The
chromatic Cu K radiation at 45 kV and 40 mA.
α
Variable-temperature magnetic susceptibility measurements in the
temperature range of 2–300 K were performed on a Quantum Design
MPMSXL SQUID magnetometer using a constant magnetic field of 0.5 T.
All susceptibility data were corrected for the diamagnetic contribution
of the sample holder, while the molar diamagnetic corrections from the
sample were calculated using the Pascal constants.
3
3
2
solid was obtained by slow evaporation of the solvent. The obtained
derivative presents very low solubility in common organic solvents,
suggesting that a coordination polymer has been formed (Scheme 1).
The thermogravimetric study of this compound shows successive and
The excitation and emission spectra were recorded on a Horiba
Jobin-Yvon Fluoromax-4 spectrofluorimeter using a fibre-optics device
connected to the spectrofluorimeter, exciting the solid compounds at the
appropriated wavelength. The emission spectra at variable temperature
were obtained by heating the samples over a hotplate with temperature
control.
�
ill-defined weight losses on heating between 50 and 760 C (Fig. S1). A
�
first step between 50 and 220 C corresponds to a weight loss of 12%
that can be attributed to the removal of the crystallisation solvent
molecules. This result, along with the analytical data, allows an esti-
mation of the solvent content corresponding to an average of 3.5–4
water molecules and half acetonitrile per metal complex (11% weight
loss). Upon further heating, the metal complex decomposes with a 59%
weight loss (expected 58% loss) and the final products are identified by
2
.2. Synthesis of H
4 2
Salendc.0.25H O
This ligand has been synthesised following the reported procedure
2 4 2
X-ray diffraction as Eu (SO )O and ZnO in an approximate 1:1 metal
2

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
Scheme 1. Proposed coordination environments for the metal ions in the coordination complex.
ratio. The sulphate ion comes from the oxidation of part of the DMSO
that is not easily removed due to its high boiling point. In a parallel
experiment, when the dinuclear complex is washed with MeOH, the
coordinated DMSO molecule is displaced and the thermogravimetric
characteristic of the salen-type ligands [17].
When the metal complex is formed, the spectrum is modified in
consonance with the coordination of the metal ions. A broad
(O–H)
ν
band, attributed to either crystallisation or coordinated water, is
À 1
–
À 1
residue of this new sample corresponds to a mixture of Eu
2
O
3
and ZnO.
observed at 3388 cm . The
ν
(C O) band at 1641 cm along with the
–
À 1
The IR spectrum of H
4
Salendc exhibits several OH stretching vibra-
ν
(C–O) band at 1381 cm can be attributed to the salen carboxylate
group, whose frequencies are slightly shifted to higher values due to the
coordination to the europium ion. The other pair of carboxylate
À 1
tion bands in the 3100–2500 cm region, which is typical of H-bonded
–
–
À 1
carboxylic acids, along with a
ν(C
O) band at 1635 cm and a
ν(C–O)
À 1
À 1
band at 1361 cm , also characteristic of the dimerised carboxylic
stretching bands, at 1553 and 1398 cm , can be respectively assigned
À 1
group. Other bands at 1589, 1281 and 789 cm can be respectively
to asymmetric and symmetric stretching modes of the acetate group,
which is probably coordinated in a chelate fashion to the europium ion.
–
–
attributed to the
ν(C
N),
ν(C–Ophenoxo) and δ(CH)ring and they are
Fig. 1. (a,b,c) SEM images of the Zn(II)/Eu(III) complex; (d) TEM images of the Zn(II)/Eu(III) complex. (e) Filtered TEM image; (f) Plot of interplanar spacing along
the yellow line in the previous image.
3

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
–
The
ν(C
–
N) and
ν
(C–Ophenoxo) stretching bands of the salen ligand are
COO–Eu–OOC–… in the crystal, being the Eu(III)–Eu(III) distance the
responsible for the 17.1 Å spacing observed in the electron microscopy
images. The formation of a polymeric species would also agree with the
extremely low solubility of this compound.
À 1
shifted to higher values, 1606 and 1314 cm , respectively. This shift is
indicative of the metal coordination in the complex cavity. Finally, there
À 1
is one broad and medium intensity band at 1016 cm that can be
–
assigned to the
ν
(S
–
O) of a DMSO molecule coordinated to the euro-
The magnetic susceptibility of the complex has been measured in the
temperature range of 2–300 K, and results are shown in Fig. 3. The
pium ion (Fig. S2) [18].
3
À 1
Due to the impossibility of getting good quality crystals to solve the
structure of this complex, an electron microscopy study of the sample
has been performed. The SEM images (Fig. 1a–c) show homogeneous
needle shape crystals that are formed by thin sheets stacked together.
These crystals are unstable under the electron beam and decompose
after a short period of time. It is possible, however, to get some TEM
images (Fig. 1d) before decomposition has taken place. A filtering of the
image allows measuring an interplanar spacing of 17.1 Å (Fig. 1e and f),
observed T values steadily decrease from 0.88 cm Kmol at 300 K to a
χ
3
À 1
minimum value of 0.008 cm Kmol at 2 K. Since the Zn(II) ion is
diamagnetic, the observed susceptibility should be attributed to the
europium ion. The usual behaviour of the Eu(III) is to populate only the
6
7
ground level (4f , J ¼ 0, S ¼ 3, L ¼ 3,
0
F ), leading to a diamagnetic
ground state. The observed behaviour must correspond to a thermal
population of the low level excited states; these levels will depopulate on
cooling leading to a final diamagnetic state when the temperature ap-
proaches the zero.
which is consistent with twice the d-spacing of the most intense peak
�
(
10.3 , 8.59 Å) found in the X-ray diffractogram (Fig. S3).
The crystals have been analysed under the microscope using the
XEDS technique in several areas of the sample (Fig. 2). The data indicate
that, on average, the Eu/Zn ratio is 1:1. There is also a small signal
corresponding to the presence of sulfur, but in a Eu/S ratio of 4:1,
suggesting that the DMSO coordinated to the rare earth ion is being lost
under the electron beam, a fact that can correspond to the low sample
stability under these conditions. By contrast, the SEM analysis of the
sample surface shows a higher proportion of sulfur atoms with an
approximate ratio S:Zn:Eu of 3:1:1, indicative of the DMSO would be
migrating to the surface prior to the crystal decomposition under the
electron beam. Additionally, the XEDS of a sample washed with meth-
anol does not show the signal corresponding to sulfur atoms, supporting
the previous observation of the DMSO removal by the methanol from the
coordination sphere of the europium ion.
The electron microscopy data can bring some light on the structure of
the complex when compared to the known structures of the similar
dicarboxylate salen-type ligands in the CSD database. These data show a
distance of 17–19 Å between metal atoms coordinated to the carboxylic
groups in both ends of the ligand [19–21]. This distance is dependent on
the size of the metal ion linked to the carboxylate and on the flexibility of
the central diimide portion of the salen ligand, being longer with rigid
moieties, such as phenylene groups. This similitude allows us to propose
the formation of one-dimensional chains … –Eu–OOC–(ZnSalendc)–
Fig. 3. Temperature dependence of
χ
T for the Zn(II)/Eu(III) complex. The solid
line represents the best fit using the parameters described in the text.
Fig. 2. XEDS spectrum of the Zn(II)/Eu(III) complex using Cu standard.
4

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
The analytical expression to fit the experimental data takes into ac-
attributed to singlet metal-to-ligand charge transfers between the Zn
7
7
7
7
7
7
7
count the seven first energy levels F
, F
0 1
2
, F , F
3
, F
4
, F
5
, F
6
[22].
atom and the salen-type ligand appears centred at 400 nm, and the
Nβ 24 þ ð13:5w À 1:5Þe þ ð67:5w À 2:5ÞeÀ 3w þ ð189w À 3:5Þe
2
À w
À 6w
À 10w
À 15w
þ ð1228:5w À 6:5ÞeÀ 21w
þ ð405w À 4:5Þe
þ ð742; 5w À 5:5Þe
χ
¼
À w
À 3w
À 6w
À 10w
À 15w
À 21w
3
λ
1 þ 3e þ 5e
þ 7e
þ 9e
þ 11e
þ 13e
corresponding emission band is located at around 443 nm. In addition,
5
7
where w ¼ λ/kT and β and k have their usual meanings. The fitting
the typical sharp band of the Eu(III) ion associated with the
0 2
D - F
À 1
parameter λ ¼ 391 cm indicates the energy difference between the
transition is clearly detected at 614 nm, as well as the remaining ones
5
7
ground and first excited states and its value is comparable to that found
originated from the D
0
- F
J
(J ¼ 0, 1, 3 and 4) transitions, which appear
between the 5
D
→
7
F
0
(578 nm) and 5
D
→
7
F
(590 nm) transitions
at 578, 590, 652 and 700 nm, respectively [23]. As a result of these
features, the complex emits red light under exposure to UV light
(Fig. 4b). Unfortunately, the new complex forms a poorly soluble co-
ordination polymer and thus, further characterisation in solution cannot
be conducted.
0
0
1
À 1
observed in the electronic spectrum of this compound (λ ¼ 352 cm
see the luminesce behaviour section).
)
(
3
.1.1. Luminescence behaviour
Fig. 4a displays the excitation and emission spectra of the Zn(II)/Eu
III) complex in the solid state. As observed, the excitation band
Temperature-dependent studies were performed in order to check
the fluorescence response by the effect of this external stimulus. As ex-
(
�
pected, the increase of temperature from 25 to 90 C causes a progres-
sive quenching of the emission bands, in agreement with the presence of
thermally-activated non-radiative processes (Fig. 5a) [24]. The
quenching effect is similar for all the emission bands, regardless their
nature. Otherwise, since the reached temperature is not enough for
thermal decomposition to occur (see TGA experiments), the initial red
emission can be practically recovered by cooling back the sample to
room temperature (Fig. 5b). The intensities at the emission maximum of
the bands centred at 443 and 614 nm as a function of temperature in
both heating and cooling cycles are plotted in Fig. 6. Note that, upon
5
7
cooling, the emission intensity of the band associated with the
0 2
D - F
transition of the Eu(III) ion (λem ¼ 614 nm) is quickly recovered at each
temperature, whereas the blue emission from the Zn(II) ion (λem ¼ 443
nm) is slower than that recorded on heating, especially at lower
temperatures.
Although suitable single-crystals for XRD studies could not be ob-
tained, all data seem to indicate that the Eu(III) ion is coordinated via the
carboxylate groups of two neighbouring molecules, generating a poly-
mer complex in which the coordination sphere around the Eu(III) ion
presents labile coordinated ligand such as DMSO. To demonstrate this
fact, two experiments were performed taking into account the lumi-
nescence behaviour of the compound. On the one hand, some few drops
of water were added over the solid sample and then, the emission
spectrum was recorded. Note that the emission intensity of the bands
associated with the 5
D
- F
7
transitions of the Eu(III) ion remarkably
0
J
decreases, and the red emission of the compound is quenched (Fig. 7).
To understand this fact, we have to keep in mind that non-radiative
transitions are also vibrational-activated processes, so that the typical
emission of the Eu(III) complexes strongly depends on the coordination
environment. Water molecules have easy access to the coordination
sphere of Eu(III) in this compound by displacement of coordinated
DMSO and, consequently, energy is transferred in form of O–H vibra-
tions, originating non-radiative processes that result in the luminescence
quenching effect observed [25]. Alternatively, as it has been previously
commented, repeated washing with methanol displaces the DMSO from
the europium coordination sphere, quenching the luminescence
behaviour of the complex.
On the other hand, we also explore the luminescence response of the
complex after treatment the solid with a concentrated solution of HCl
(
12 M). As shown in Fig. 8a, the addition of just only two drops of this
acidic solution again produces a total quenching of the red emission
attributed to the Eu(III) ion. Most likely, protonation of the carboxylate
groups causes the rupture of the –COO⋅⋅⋅Eu⋅⋅⋅OOC– bonds, and water
molecules are coordinated to Eu(III). Concomitantly, the band involving
the Zn(II) centre and the salen-type ligand is red-shifted from 443 nm to
Fig. 4. (a) Normalised excitation and emission spectra of the Zn(II)/Eu(III)
complex recorded in the solid state at room temperature. (b) Magnification of
5
7
the emission spectrum showing the
D
0
- F
J
(J ¼ 0–4) transitions from Eu(III)
ion. Inset displays images of the complex in the solid state taken with the naked
eye and upon UV light (λexc ¼ 365 nm).
5

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
Fig. 5. (a) Normalised emission spectra recorded in the solid state at variable temperature. (b) Normalised emission spectra at selected temperatures during the
heating and cooling processes.
4
84 nm and the emission intensity is also increased, so that the complex
displays now an intense blue emission (Fig. 8b). Similar features have
been previously reported for other complexes with -conjugated ligands,
π
associating this effect to energy changes in the frontier orbitals as a
result of protonation [26]. Moreover, this luminescence response is not
only produced by the action of concentrated HCl in solution, but also by
the presence of its vapours (Fig. S4). Although experiments were per-
formed in the solid state and mobility is reduced, proto-
nation/deprotonation can easily occur in the surface of the compound
by adsorption of HCl molecules [27,28]. Additional experiments were
performed in several acidic media by exposure of the sample to sulfuric,
nitric and acetic acids, but no remarkable changes in the fluorescence
emission were detected.
From the point of view of the potential application of this material as
a sensor of HCl in highly acid media, we were also interested in
exploring if deprotonation can again favour the establishment of con-
tacts between the Eu(III) ion and the carboxylate groups because it could
allow to recover the initial emission and reuse the material. For this
purpose, several drops of an ammonia solution (25%) were added over
the solid. Interestingly, the emission spectrum shows a similar pattern
than that of the complex as prepared (Fig. 8a), and the resulting solid
emits an orange light as demonstrated in Fig. 8b, which confirms the
regeneration of the –COO⋅⋅⋅Eu⋅⋅⋅OOC– bonds and the recovery of the
polymer complex. The permutable ability of these contacts has been
evaluated by submitting the complex to successive cycles of treatment
Fig. 6. Emission intensity as a function of temperature read at 443 and 614 nm
during the heating (circles) and cooling (crosses) cycles.
3
with HCl and NH solutions. Results show good reversibility, and no
degradation is observed during at least 6 cycles. In fact, the emission
maximum at 484 nm is progressively blue-shifted after each treatment
with NH
Fig. 8c). Similar results were obtained by exposure of the complex to
ammonia vapours (Fig. S5).
3
, reaching values near to those found in the initial solid
(
4
. Conclusions
A new derivative containing Zn(II) and Eu(III) ions coordinated to a
dicarboxilate salen-type ligand has been obtained and characterised. The
observed data suggests that the zinc ion would coordinate to the N
2 2
O
central cavity of the salen ligand while the europium ion is bonded to the
carboxylate groups of several Salendc4 ligands, leading to the forma-
tion of a polymeric structure. The europium coordination is completed
by the presence of acetate anions and dimethylsulfoxide molecules. The
latter seem to be essential for the luminescence behaviour of the euro-
pium ion, since the treatment of the complex with water or excess
methanol quenches the emission by displacement of the DMSO from the
europium coordinative sphere.
À
Fig. 7. Emission spectra of the complex as prepared and after the addition of
some drops of water.
6

C. Cuerva et al.
Dyes and Pigments 181 (2020) 108587
Fig. 8. (a) Emission spectra of the Zn(II)/Eu(III) complex recorded as prepared, after adding some drops of a concentrated HCl solution (12 M), and upon treatment
with ammonia solution (25% in water). (b) Images showing the acidochromic behaviour of the complex, taken with the naked eye and under UV light (λexc ¼ 365
3
nm). (c) Wavelength at the emission maximum after successive cycles of treatment with HCl and NH solutions.
The new complex behaves as a luminescent material, emitting a red
review & editing, Funding acquisition.
5
7
0 J
light in the solid state as a result of the D - F transitions from the Eu
(
III) ion. The activation of non-radiative processes by the effect of
Acknowledgements
temperature causes a quenching of the emission, which can be recovered
after back to initial conditions. The complex also exhibits acidochromic
properties because of the rupture of the –COO⋅⋅⋅Eu⋅⋅⋅OOC– bonds by
protonation of the terminal carboxylate groups located in the ligand.
The chromic behaviour can be reverted without causing degradation of
the sample, this feature demonstrating the usefulness of the material for
application as sensor of HCl.
This work was supported by the Associate Laboratory for Green
Chemistry – LAQV, which is financed by national funds from FCT/
MCTES (UIDB/50006/2020), and the PROTEOMASS Scientific Society.
C. Cuerva acknowledges the Spanish Foundation Alfonso Martín Escu-
dero for his postdoctoral fellowship.
Appendix A. Supplementary data
Declaration of competing interest
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.dyepig.2020.108587.
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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