Multifunctional Porphyrin-based dyes for cations detection in solution and thermoresponsive low-cost materials

Article abstract of DOI:10.1016/j.dyepig.2020.108897

Two β-pyrrolic substituted porphyrinic dyes 3 and 4, have been synthesized and fully characterized. Both compounds showed spectral alterations upon metal interaction indicating the complexation of the inner nitrogens in the porphyrin moiety by the metal i

Full text of DOI:10.1016/j.dyepig.2020.108897

Dyes and Pigments 185 (2021) 108897
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
Multifunctional Porphyrin-based dyes for cations detection in solution and
thermoresponsive low-cost materials
,
,
,
Nuno M.M. Mouraa **, Simone Valentinib d, Victoria Chepteneb d, Andrea Puccid, M. Graça P.
M.S. Neves , Jose Luis Capelo c, Carlos Lodeirob c, Elisabete Oliveirab
a
b
,
,
,c,*
´
a LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193, Aveiro, Portugal
b BIOSCOPE Group, LAQV-REQUIMTE, Nova School of Science and Technology, NOVA University Lisbon, Caparica Campus, 2829-516, Caparica, Portugal
c ProteoMass Scientific Society, Madan Parque. Rua dos Inventores, 2825-182, Caparica, Portugal
d Department of Chemistry and Industrial Chemistry, Via Giuseppe Moruzzi 13, 56124, Pisa, University of Pisa, Italy
A R T I C L E I N F O
A B S T R A C T
Keywords:
Two β-pyrrolic substituted porphyrinic dyes 3 and 4, have been synthesized and fully characterized. Both
Porphyrins
compounds showed spectral alterations upon metal interaction indicating the complexation of the inner nitro-
gens in the porphyrin moiety by the metal ion. Their sensing ability towards the divalent and monovalent, Zn2+
,
Metal ions
Low-cost polymers
Temperature
Cd2+, Co2+, Fe2þ, Ni2+, Cu2+, Ag+ and Hg2+ metal ions was evaluated in solution by absorption and fluorescence
spectroscopy. The association constants, minimal detectable and quantified amounts for all metal ions were also
determined. The results reveal the formation of mononuclear species in all cases, with the best results obtained
for the toxic Hg2+ metal ion, where the lowest detectable and quantified amounts of 0.9/5.3
μM and 1.8/3.6 μM
were observed for compounds 3 and 4, respectively. Polymeric films of poly(methylmethacrylate) (PMMA)
doped with porphyrins 3 and 4 were also prepared and tested in the presence of the aforementioned metal ions
and at different temperatures. For comparison, polymeric films of PMMA containing the non-substituted
porphyrin 5,10,15,20-tetraphenylporphyrin (TPP) and with its Zn(II) complex (ZnTPP) were also tested at
different temperatures. The temperature assays using the PMMA_3 and PMMA_4 doped polymers showed an
interesting recovery in the emission intensity of ca. 70–80% in the cooling phase (from 100 ◦C to 35 ◦C). The
results suggest the potential of these porphyrin dyes to be used in the preparation of smart plastic labels with
temperature sensing features.
1. Introduction
stokes shifts and emission bands longer than 600 nm, that minimizes the
effects of the background fluorescence [1].
During the last years, porphyrins and/or metalloporphyrins have
been the target of several investigations for the development of new
materials for multiple applications, mainly due to their unique variable
optical and physical-chemical properties, which can be modulated by
structural modifications of the tetrapyrrolic core [1]. In fact, these
macrocycles adequately functionalyzed are being successfully applied in
several areas, like catalysis, molecular sensing, artificial photosynthesis,
chemotherapy, photodynamic therapy and molecular electronics [1–7].
Porphyrin compounds show remarkable photophysical properties,
being visible excitable chromophores with characteristic absorption
bands at ca. 400 nm (Soret) and at ca. 500–700 nm (Q-bands), large
In free-base porphyrins, the NH inner groups and the size of the host
cavity allow the complexation by metal ions affording metal-
loporphyrins, with different photophysical properties from the corre-
sponding free base. The coordination of metal ions to the porphyrin
macrocycle induces the formation of Lewis acid sites that can accom-
modate several ions and basic molecules as axial ligands. The complex
formation is in general accompanied by a bathochromic shift of the Soret
which is dependent on the ligand basicity and on the metal ion. Higher
basicity of the ligands leads to higher stability constants and bath-
ochromic shifts [8].
Because of such properties, the synthesis of adequately
* Corresponding author. BIOSCOPE Group, LAQV-REQUIMTE, Nova School of Science and Technology, NOVA University Lisbon, Caparica Campus, 2829-516,
Caparica, Portugal.
** Corresponding author. LAQV-REQUIMTE, Department of Chemistry, University of Aveiro, 3810-193, Aveiro, Portugal.
E-mail addresses: nmoura@ua.pt (N.M.M. Moura), ej.oliveira@fct.unl.pt (E. Oliveira).
https://doi.org/10.1016/j.dyepig.2020.108897
Received 6 July 2020; Received in revised form 25 August 2020; Accepted 28 September 2020
Available online 2 October 2020
0143-7208/© 2020 Elsevier Ltd. All rights reserved.

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
functionalized porphyrins at meso- and β-pyrrolic positions and their
potential to act as chemical probes for metal ions have been widely
explored [1,9–11]. In this case, the porphyrin derivatives act as signal-
ling unit based on the changes in the absorption and emission spectra. In
previous works, Moura et al. reported the behaviour of a wide range of
probe and a syringe pump. Source and desolvation temperatures were
80 ◦C and 150 ◦C, respectively. Capillary voltage was 3000 V. The
spectra were acquired at a nominal resolution of 9000 and at cone
voltages of 30 V. Nebulisation and collision gases were N2 and Ar,
respectively. Porphyrin solutions in methanol were introduced at a 10
monomeric porphyrin derivatives in the presence of Ag+, Cu2+, Zn2+
,
μ
L minꢀ 1 flow rate. Mass spectra HRMS were recorded on a LTQ Orbi-
Cd2+ and Hg2+. Some of those porphyrin derivatives revealed high
selectivity to specific metal ions [12–15].
trap XL mass spectrometer (Thermo Fischer Scientific, Bremen, Ger-
many) using methanol as solvent. Column chromatography was carried
out using silica gel (Merck, 35–70 mesh). Analytical TLC was carried out
on precoated sheets with silica gel (Merck 60, 0.2 mm thick). All the
chemicals were used as supplied. Solvents were purified or dried ac-
cording to the literature procedures [23].
Another strategy that has been explored involves the immobilization
of the porphyrin derivatives in other materials in order to enhance their
reactivity to metal ions and other analytes [16–20]. Having this strategy
in mind, Moura et al. developed low-cost polymers based on poly
(methylmethacrylate) (PMMA) doped with porphyrin derivatives. These
polymers were tested towards Zn2+ and Hg2+ metal ions. Similarly to
what was observed in solution, the polymers showed a colour change
from brown to green and emission quenching in the presence of these
metal ions at room temperature (25 ◦C) [14]. Porphyrin derivatives have
also been explored for the detection of microenvironments as pH and
temperature [21,22]. Low-cost PMMA polymers were designed by
Moura et al. using a porphyrin-chalcone derivative and tested at
different temperatures (ꢀ 20 ◦C, 25 ◦C, 50 ◦C), where an enhancement of
the emission signal was visualized at ꢀ 20 ◦C and 50 ◦C [12]. The doped
polymers were further immersed in Zn2+, and Hg2+ metal ions solutions,
and similarly to what was observed in solution, an enhancement in the
emission signal was detected at room temperature (25 ◦C). The same
behaviour was observed at ꢀ 20 ◦C and 50 ◦C. These preliminary results
showed to be very promising molecules in the application of the low-cost
polymers to measure temperatures in different ranges. Based on such
results, herein we present the synthesis and characterization of two new
cationic β-pyrrolic molecules 3 and 4, and their sensing ability towards
Zn2+, Cd2+, Co2+, Fe2þ, Ni2+, Cu2+, Hg2+ and Ag+ metal ions. The
ability of these probes was also evaluated in solid state by preparing
several PMMA films doped with both ligands, 3 and 4. The resulting
doped films were explored towards metal ions and with temperature
variation studies (heating from 25 ◦C to 100 ◦C and cooling from 100 ◦C
to 35 ◦C). As reference samples, we have used the non-substituted
template 5,10,15,20-tetraphenylporphyrin (TPP) and its Zn(II) com-
plex ZnTPP.
2.3. Synthesis of ligands
2.3.1. Synthesis of 5,10,15,20-tetraphenylporphyrin (TPP) and
(5,10,15,20-tetraphenylporphyrinato)zinc(II) (ZnTPP)
TPP was prepared in 28% yield by condensation of pyrrole and
benzaldehyde following an adaptation of the well-established Adler
protocol [24–26]. ZnTPP was obtained by metalation of the free-base
TPP with the adequate metal carrier, zinc(II) acetate, according to
conventional procedures [27]. The structures of TPP and ZnTPP were
confirmed by 1H NMR, mass spectrometry and UV–vis techniques. The
experimental data are in agreement with the one described in the
literature.
5,10,15,20-Tetraphenylporphyrin (TPP) - 1H NMR (300 MHz,
CDCl3): δ 8.85 (8H, s, H-β), 8.23–8.20 (8H, m, H-o-Ph), 7.78–7.66 (12H,
m, H-m,p-Ph), ꢀ 2.77 (2H, s, N-H) ppm. MS-ESI(þ): m/z 614.2 [M]+⋅
.
UV–vis (DMF): λ
(log ε) 416 (5.27), 512 (4.09), 546 (3.75), 590
max
(3.57), 647 (3.55) nm.
(5,10,15,20-Tetraphenylporphyrinato)zinc(II) (ZnTPP)
-
1H
NMR (300 MHz, CDCl3): δ 8.95 (8H, s, H-β), 8.24–8.21 (8H, m, H-o-Ph),
7.81–7.70 (12H, m, H-m,p-Ph) ppm. MS-ESI(þ): m/z 676.3 [M]+⋅
.
UV–Vis (DMF): λmax (log ε) 424 (5.99), 559 (4.58), 597 (4.24) nm.
2.3.2. Synthesis of porphyrinic derivatives 1 and 2
The benzoporphyrin 1 and porphyrin-chalcone type 2 derivatives
were obtained through an aldol-type reaction by reaction of 2-formyl-
5,10,15,20-tetrapenhylporphyrin with the adequate 3-acetylpyridine
according to previously described procedures [28,29]. The reactions
were performed in dry toluene in the presence of catalytic amounts of
lanthanum(III) trifluoromethanesulfonate (La(OTf)3), using ammonium
acetate [28] or piperidine [29] as base.
2. Experimental section
2.1. Chemicals and starting materials
Iron(II)
perchlorate
hydrate
(>98%),
Cobalt(II)
tri-
fluoromethanesulfonate (>98%), Cadmium(II) trifluoromethanesulfonate
(>98%), Mercury(II) trifluoromethanesulfonate (>98%), Copper(II) tri-
fluoromethanesulfonate (>98%), Zinc(II) trifluoromethanesulfonate
(>98%), Silver(I) trifluoromethanesulfonate (>98%) and Nickel(II) tri-
fluoromethanesulfonate (>98%) were purchased from Solchemar. Poly
(methyl methacrylate) (PMMA, Mw = 350,000 g/mol, acid number <1 mg
KOH/g) and all solvents were from Sigma Aldrich. All chemicals and sol-
vents were used without further purification.
2.3.3. Synthesis of porphyrinic derivatives 3 and 4
To a solution of benzoporphyrin 1 or porphyrin-chalcone derivative
2 in N,N′-dimethylformamide (1 mL) was added sodium iodide and S-(4-
bromobutyl)thioacetate and the mixture was stirred for 22 h at 120 ◦C.
After cooling, the reaction mixture was washed with water and extracted
with dichloromethane. The organic phase was dried (Na2SO4) and the
solvent was evaporated under reduced pressure. The crude mixture was
submitted to column chromatography (silica gel) using dichloro-
methane/methanol (98:2) as the eluent. The fractions obtained were
fully characterized by NMR, mass and spectroscopic techniques after
crystallization in a CH2Cl2/hexane mixture.
2.2. General remarks
1H and 13C solution NMR spectra were recorded on Bruker Avance
300 (300.13 and 75.47 MHz, respectively) and Avance 500 (500.13 and
125.76 MHz, respectively) spectrometers. CDCl3 was used as solvent and
tetramethylsilane (TMS) as the internal reference; the chemical shifts
are expressed in δ (ppm) and the coupling constants (J) in Hertz (Hz).
Unequivocal 1H assignments were made using 2D COSY (1H/1H), while
13C assignments were made based on 2D HSQC (1H/13C) and HMBC
(delay for long-range JC/H couplings were optimized for 7 Hz) experi-
ments. Electrospray ionization mass spectra were acquired with a
Micromass Q-Tof 2 (Micromass, Manchester, UK), operating in the
positive ion mode, equipped with a Z-spray source, an electrospray
22-[1-(4-(acetylthio)butyl)-pyridinium-3-yl]-5,10,15,20-tetra-
phenylbenzo[b]porphyrin iodide, 3.
Yield: 95%. 1H NMR (500 MHz, CDCl3): δ 9.55 (1H, d, J = 5.8 Hz, H-
2′′), 9.22 (1H, s, H-3), 8.94 (1H, d, J = 4.9 Hz, H-β), 8.91 (1H, d, J = 4.9
Hz, H-β), 8.87 (1H, d, J = 4.9 Hz, H-β), 8.75–8.71 (4H, m, H-β. H- 4′′ and
H-6′′), 8.25–8.19 (8H, m, H-o-Ph), 8.15 (1H, d, J = 8.4 Hz, H-3′), 8.10
(1H, dd, J = 6.1 and 7.9 Hz, H-5′′), 7.95 (1H, t, J = 7.6 Hz, H-p-Ph),
7.89–7.64 (12H, m, H-m,p-Ph and H-1′), 7.43 (1H, d, J = 8.4 Hz, H-4′),
5.11 (2H, t, J = 7.8 Hz, H-1′′′), 3.03 (2H, t, J = 7.6 Hz, H-4′′′), 2.35 (3H,
s, -CH3), 2.25–2.19 (2H, m, H-2′′′), 1.79 (2H, q, J = 7.6 Hz, H-3′′′), ꢀ 2.69
(2H, s, N-H) ppm. 13C NMR (125 MHz, CDCl3): δ 196.4, 159.7, 155.3,
2

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
146.4, 145.9, 145.7, 143.9, 143.6, 142.7, 142.0, 141.69, 141.66,
141.58, 140.3, 139.9, 138.9, 138.4, 138.3, 135.4, 134.53, 134.50,
134.45, 134.41, 133.7, 133.5, 129.2, 128.3, 128.2, 128.1, 128.0, 127.1,
126.91, 126.89, 126.86, 121.7, 121.6, 118.4, 118.0, 117.2, 61.2, 31.0,
30.8, 28.1, 26.4 ppm. MS-ESI(þ): m/z 873.3 [M+H]+. HRMS-ESI(+):
m/z calculated to C59H47N5OS [M+H]+ 873.3496; found 873.3344.
std = standard deviation.
Additionally, small amounts of the metal ions were added to a so-
lution of porphyrins 3 or 4 in order to determine the minimal detectable
and quantified concentration out of the LOD and LOQ values,
respectively.
UV–Vis (CH2Cl2): λmax (log ε) 430 (5.0), 525 (4.0), 600 (3.6) nm.
2.6. Preparations of thin polymeric films for sensing
2-[3-Oxo-3-(1-(4-(acetylthio)butyl)-pyridin-3-yl)propan-1-yl]-
5,10,15,20-tetraphenylporphyrin iodide, 4.
Polymeric films were obtained at room temperature and under air
ambient by dissolving 100 mg of PMMA in dichloromethane followed by
the addition of the appropriated amount of each dye, in a dye/polymer
ratio (% w/w) of 0.5, 1.5 and 3.0 for TPP and ZnTPP; 0.25, 0.5 and 1.5
for compounds 3 and 4. Solid polymeric films with an average thickness
Yield: 87%. 1H NMR (500 MHz, CDCl3): δ 10.07 (1H, d, J = 4.9 Hz,
H-2′′), 9.28–9.25 (2H, m, H-β), 8.85–8.76 (7H, m, H-β, H-4′′ and H-6′′),
8.25–8.13 (9H, m, H-o-Ph and H-5′′), 7.85–7.62 (14H, m, H-m,p-Ph H-1′
and H-2′), 5.10 (2H, t, J = 7.6 Hz, H-1′′′), 2.96 (2H, t, J = 7.6 Hz, H-4′′′),
2.21 (3H, s, -CH3), 2.18–2.15 (2H, m, H-2′′′), 1.79 (2H, q, J = 7.6 Hz, H-
3′′′), ꢀ 2.52 (2H, s, N-H) ppm. 13C NMR (125 MHz, CDCl3): δ 197.1,
183.9, 145.9, 143.8, 141.8, 141.71, 141.65, 141.5, 137.6, 134.7, 134.6,
134.5, 133.8–129.5 (C-β), 128.9, 128.8, 128.2, 128.0, 127.3, 127.0,
126.91, 126.85, 121.4, 120.6, 120.0, 61.8, 30.7, 30.1, 27.5, 26.2 ppm.
MS-ESI(þ): m/z 876.3 [M]+. HRMS-ESI(+): m/z calculated to
of 9 μm were obtained by pouring each above-mentioned solution in an
aluminium dish covered with a glass watch and keeping them under a
hood at room temperature until complete solvent evaporation (about
one day).
The obtained films were then cut in 10 × 15 mm rectangular pieces.
C
59H46N5O2S [M]+ 876.33367; found 876.3352. UV–Vis (CH2Cl2): λ
2.6.1. Chemical recognition
max
It was prepared a 10ꢀ 2 M stock solution of the corresponding cation
by the dissolution of the right amount of cation derivative powder in the
wanted volume of water. The wanted volume of such solution was then
withdrawn and opportunely diluted for the preparation of 5 vials each of
them containing a different concentration of the cation. The cation
involved were Zn2+, Hg2+, Cd2+, and the concentration of each solution
was varied in a range between 1.0 × 10ꢀ 6 and 8 × 10ꢀ 5 M.
(log ε) 420 (4.9), 530 (3.4), 600 (3.3), 675 (3.1) nm.
2.4. Spectrophotometric and spectrofluorimetric measurements
UV–Vis absorption spectra were recorded with a JASCO V-650
spectrophotometer and a fluorescence emission by a HORIBA Scientific
FLUOROMAX-4 spectrofluorimeter from the PROTEOMASS Scientific
Society facility. The linearity of the fluorescence emission vs. the con-
centration was checked out in the concentration used (10ꢀ 4 – 10ꢀ 6 M). A
correction for the absorbed light was performed when necessary.
Spectra of solid samples were recorded on the Horiba-Jobin-Yvon Flu-
oromax-4® spectrofluorimeter using a fiber-optics device connected to
the spectrofluorimeter, exciting the solid compounds at appropriated λ
(nm).
The 10 × 15 mm rectangular samples were immersed in each of the
solutions at different cation concentration previously prepared for
around 5 min and then dried for removing the excess of the solution on
the film surface. Optical changes in the absorption and emission spectra
were collected. If any changes in the optical properties of the film were
visualized, the measurement protocol were repeated three times.
2.6.2. Temperature sensing
2.4.1. Photophysical characterization and titrations
The 10 × 15 mm rectangular samples were placed between two
quartz slides and then positioned on support centred below the light spot
of the optical fiber directly connected with the spectrophotometer. The
support was replaced by the black plate of a heater. The heat was pro-
vided by the heater equipped with a thermostat (±1 ◦C) capable of
maintaining the temperature stable during the optical analyses of the
samples. The reaching of the desired temperature was registered by a
digital temperature probe directly connected with the plate of the
heater. The measures were performed from 25 to 100 ◦C and from 100 to
35 ◦C. Optical changes in the emission spectra were collected.
1 mg of fluorophore powder was weighted and solubilized in 5 mL of
dichloromethane obtaining a 10ꢀ 3 fluorophore stock solution. A 3 mL
cuvette of 1 × 10ꢀ 5 M fluorophore solution in dichloromethane was then
prepared from the stock solution.
The stock solutions of the metal ions (ca. 10ꢀ 2 M) were prepared by
dissolving an appropriate amount of the corresponding cation salt de-
rivative in acetonitrile (3 mL). The volumes corresponding to the desired
cation equivalents were taken by the stock solutions using a micropi-
pette, and added step by step to the cuvette containing the fluorophore
solution in dichloromethane. The optical changes in absorption and
emission spectra were collected after each addition. The measurement
was carried until a plateau is reached in the spectra collected. All mea-
surements were performed at 298 K and the metal ions tested were:
Zn2+, Cd2+, Co2+, Fe2þ, Ni2+, Cu2+, Hg2+ and Ag+.
3. Results and discussion
3.1. Synthesis
Luminescence quantum yields were measured using a solution of
cresyl violet in ethanol [φF = 0.54] as a standard for compounds 3 and 4
[30,31]. All solvents used were of the highest purity from Merck.
The synthetic strategy to prepare the new ligands 3 and 4 is outlined
in Scheme 1 and required a previous preparation of the corresponding of
benzoporphyrin 1 and of porphyrin 2 bearing a chalcone-type unit.
These derivatives were obtained through an aldol-type condensation
involving the easily accessible 2-formyl-5,10,15,20-tetraphenylpor-
phyrin and 3-acetylpyridine in the presence of catalytic amounts of La
(OTf)3 [28,29]. The 2-formyl-5,10,15,20-tetraphenylporphyrin was
obtained by Vilsmeier-Haack formylation of 5,10,15,20-tetraphenylpor-
phyrin (TPP) [32]. Further reaction of derivatives 1 and 2 with
S-(4-bromobutyl)thioacetate in DMF at 120 ◦C (Scheme 1) afforded,
after workup and purification by column chromatography, the new
beta-functionalized mono-charged derivatives 3 (95%) and 4 (87%) in
excellent yields. The structures of all compounds were unambiguously
confirmed by 1D, and 2D NMR studies and their molecular formulas
were confirmed by HRMS (see Supporting Information, Figs. S1–S14).
The 1H NMR spectra of compounds 3 and 4 clearly shows in the
2.5. Determination of the detection and quantification limits (LOD and
LOQ)
The detection limit (LOD) and quantification limit (LOQ) are the
lowest concentration level that can be detected and quantified to be
statistically different from a blank. For the determination of LOD and
LOQ, ten different measurements of a solution containing the selected
probe were collected without the addition of any metal ion (yblank). The
LOD and LOQ were determined by the formulas:
LOD = ydl = yblank + 3std, where ydl = signal detection limit and
std = standard deviation.
LOQ = ydl = yblank + 10std, where ydl = signal detection limit and
3

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
Scheme 1. Synthesis of ligands 3 and 4.
3.2. Photophysical characterization
aliphatic region, from ca. δ 5.1 ppm to δ 1.8 ppm, the peaks due to the
resonances of the protons from the butyltioacetate unit attached to the
pyridine moiety. The singlets at δ ꢀ 2.52 ppm and δ ꢀ 2.69 ppm, cor-
responding to the resonances of inner core N-H, demonstrates that both
derivatives (3 and 4) are in their free-base form (see SI, Figs. S1 and S8).
The 13C NMR spectrum of compound 3 show a distinctive signal at δ
196.4 ppm corresponding to the carbonyl carbon of the butyltioacetate
unit, while the 13C NMR spectrum of the porphyrin-chalcone type de-
rivative 4 presents two signals at δ 197.1 ppm and δ 183.9, due to the
resonance of the carbonyl carbons of the butyltioacetate unit and chal-
cone moiety, respectively (see SI, Figs. S5 and S12).
The absorption, emission and excitation spectra of the fluorophores 3
and 4 were performed in dichloromethane solutions at 298 K and are
pesented in Fig. 1. Although the photophysical characterization of ref-
erences 5,10,15,20-tetraphenyl-porphyrin (TPP) and of its Zn(II) com-
plex ZnTPP are well described in literature [12,14,15,33], we decided,
for comparison, to present in the same figure their absorption, emission
and excitation spectra.
The absorption spectrum of TPP (Fig. 1A) shows the expected feature
of a free-base porphyrin: the highly intense peak corresponding to the
Soret or B band at 415 nm (
ε
= 1 × 106 Mꢀ 1 cmꢀ 1) and the four weak Q
bands in the range 500–700 nm [33]. Both B and Q bands in the visible
range are assigned to π–π* transitions, where the Soret band involves the
Fig. 1. Absorption, emission and excitation spectra of compounds TPP (A), ZnTPP (B), 3 (C) and 4 (D) in dichloromethane (ca.10ꢀ 5 M) at 298 K, λexcTPP = 548 nm,
λexcZnTPP = 547 nm, λexc3 = 525 and λexc4 = 530 nm.
4

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
transition from the ground state to the second excited state (S0–S2) while
the Q bands stem from a weak transition to the first excited state (S0–S1)
in agreement with the Gouterman Four-Orbital Model prevision [34].
The absorption spectrum of ZnTPP (Fig. 1B) obtained by metalation
of the free-base TPP with the adequate metal carrier, zinc(II) acetate,
according to conventional procedures [4], presents the Soret band at
around 417 nm (
ε
= 1.1 × 106 Mꢀ 1 cmꢀ 1) accompanied by only two Q
bands. This reduction in the number of bands from four to two is related
with the alteration in the symmetry of the molecule from D2h (properly
of the free-base porphyrin) to D4h with the consequent simplification of
the absorption spectra in the Q region [35].
The absorption spectrum of 3 (Fig. 1C) shows highly intense Soret
band peaked at 430 nm (
ε
= 1.2 × 105 Mꢀ 1 cmꢀ 1), i.e. 15 nm red-shifted
with respect to TPP, and flanked by only two discernible Q bands be-
tween 525 and 600 nm. The red-shift of the Soret band could be related
to the presence of the six-membered fused aromatic ring in β-position on
the porphyrin macrocycle capable of enhancing the
delocalization.
π electrons
In the case of compound 4 (Fig. 1D), the Soret band appears at 420
nm (
ε
= 1 × 105 Mꢀ 1 cmꢀ 1), with a negligible red shift compared to TPP
and with larger and partially overlapped Q bands. Moreover, the Soret
band presents a not well-defined shape with a shouldered peak red-
shifted at around 490 nm, a symptom of the possible coexistence of
aggregates. In literature, it is reported that significant changes in the
UV–Vis spectrum of porphyrins are promoted by increasing their con-
centration since aggregation phenomena can occur [36]. Notably, the
broadening of the Soret band is detectable at a concentration higher
than 5 × 10ꢀ 5 M for TPP. This phenomenon could be more likely favored
by the presence of the peripheric chain in compound 4 positively
charged. This structural feature may increase the aggregation tendency
of this porphyrin derivative in dichloromethane due to its higher
Fig. 2. Spectrophotometric (A and C) and spectrofluorimetric (B, D) titrations
of compound 3 (A, B) and 4 (C,D) with the addition of Zn2+ in dichloro-
methane. The inset represents the absorption (A, C) at 430 nm, 455 nm, 525
nm, 670 nm for (A) and 420 nm, 530 nm, 675 nm for (C); and the emission
intensity (B, D) at 662 nm (B) and 675 nm (D), as function of [Zn2+]/[3] (A, B)
and [Zn2+]/[4] (C, D). Inset Fig. 3A: Naked-eye images of 3 before and after
titration (3, 3Zn2+) and under a UV-lamp before titration (3*). ([3], [4] = 1 ×
10ꢀ 5 M, λexc3 = 525 nm, λexc4 = 530 nm, T = 298 K).
amphiphilic behaviour that probably leads to the formation of cation-
π
dimers.
As shown in Fig. 1, the emission spectrum of fluorophore 3 present
two well-defined peaks placed in the range 650–750 nm, which are
characteristic of porphyrin derivatives. The emission spectrum of com-
pound 4 presents a single enlarged band that is probably due to the
partial overlapping of the two canonical peaks.
appearance of a new band at 670–675 nm for compounds 3 and 4,
respectively. The changes observed in the absorption spectra upon
interaction with Zn2+ were also accompanied by a change of colour from
pale yellow to green (Figure. 2A). These alterations are in line with the
formation of a sitting-atop (SAT) complex in which the metal lies above
the porphyrin plane with the two inner N-H protons still present
[38–40]. Therefore, the second step involving the incorporation of the
metal into the macrocycle core accompanied by the loss of two N-H
protons was not observed under these titration conditions (see Fig. 2c for
ZnTPP).
The fluorescence quantum yield (φ) determined by using a solution
of crystal violet in methanol as a standard (φ = 0.54). The value ob-
tained for TPP (φ = 0.11) was in accordance with the data reported in
literature [14] and was higher than the ones obtained for derivatives 3
(φ = 0.012) and 4 (φ = 0.001).
Concerning the emission spectra in both compounds a quenching of
the emission signal at ca. 662–675 nm (3, 4) and at ca. 730 nm was
observed, due to the free-base porphyrin emission Q (0-0) and Q(0–1)
[41].
3.3. Metal sensing in solution
The sensorial ability of porphyrins 3 and 4 towards transition and
post-transitions metal ions (Zn2+, Cd2+, Co2+, Fe2þ, Ni2+, Cu2+
,
The stability constants were calculated for both compounds
(Table 1), and all values revealed a stoichiometry 1:1 ligand-to-metal (L:
M) in all cases. Respectively, values of Log Kass = 6.86 ± 0.02 (3) and
Log Kass = 8.11 ± 0.02 (4) were determined.
Hg2+and Ag+), was determined by titrating the respective dichloro-
methane solutions, with small amounts of the metal ion in acetonitrile.
The absorption and emission spectra were collected after every addition
at 298 K until a plateau of the optical signal was reached. The stability
constants of the interaction with all the investigated metal ions were
calculated by using the HypSpec software [37].
3.3.2. Cu2+ and Fe2+ metal ions
The titration of compounds 3 and 4 with Cu2+ metal ions led to a red
shift in the absorption bands. It was observed a decrease in the Soret
band at 430 nm and 420 nm, followed by an absorption increase at 455
nm and 450 nm, and the formation of an isosbestic point at 440 and 430
nm for 3 and 4, respectively (Fig. 3). In the Q bands, the same behaviour
was detected with a decrease in the absorption band at 525 nm for de-
rivative 3 and at 530 nm for 4 accompanied respectively by the
appearance of new bands at 670 nm (3) and 675 nm (4). As can be seen
in Fig. 4 the interaction with Cu2+ is stronger in compound 3, being
necessary a less amount of the metal ion to produce the spectral alter-
ations. This result is in agreement with the obtained association con-
stants with values of Log Kass = 7.58 ± 0.11 (3) and Log Kass = 6.25 ±
3.3.1. Zn2+ metal ion titration
The addition of Zn2+ metal ions to compounds 3 and 4 promoted
significant changes in the ground and excited states (Fig. 2). In all cases,
a red shift of the Soret band (ca. 25 nm) was observed in the absorption
spectra due to the interaction of Zn2+ metal ions with the electron lone-
pair of the inner N atoms of the porphyrin macrocycle. The red shift is
more intense in compound 3 than in 4, with the formation of a new band
at 455 nm. Interestingly, an isosbestic point is observed at 440 nm and
430 nm for 3 and 4, respectively, confirming the presence of only two
species in solution, the ligand and the metal complex. In the Q region is
observed a decrease in the absorption at 525–530 nm, with the
5

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
Table 1
Stability association constants and stoichiometry for the complexes formed with
porphyrin derivatives 3 and 4 in dichloromethane with Zn2+, Cd2+, Co2+, Fe2+
,
Ni2+, Cu2+, Hg2+, Ag+ ions (CH3CN), (
mental error).
σ is an estimate of the average experi-
Compounds (L)
Metal (M)
Association constants (LogKass),
L:M
σ
3
Zn2+
Cd2+
Co2+
Fe2+
Ni2+
Cu2+
Hg2+
Ag+
6.86 ± 0.02 (1:1)
6.52 ± 0.01 (1:1)
4.85 ± 0.01 (1:1)
6.63 ± 0.15 (1:1)
5.36 ± 0.05 (1:1)
7.58 ± 0.11 (1:1)
7.75 ± 0.02 (1:1)
2.49 ± 0.03 (1:1)
7.11 ± 0.02 (1:1)
7.40 ± 0.02 (1:1)
7.22 ± 0.05 (1:1)
6.62 ± 0.03 (1:1)
5.78 ± 0.01 (1:1)
6.25 ± 0.04 (1:1)
7.38 ± 0.04 (1:1)
5.25 ± 0.01 (1:1)
4
Zn2+
Cd2+
Co2+
Fe2+
Ni2+
Cu2+
Hg2+
Ag+
Fig. 4. Spectrophotometric (A, C) and spectrofluorimetric (B, D) titrations of
compound 3 (A, B) and 4 (C,D) with the addition of Hg2+ in dichloromethane.
The inset represents the absorption (A, C) at 430 nm, 455 nm, 525 nm, 670 nm
for (A) and 420 nm, 530 nm, 675 nm for (C); and the emission intensity (B, D)
at 662 nm (B) and 675 nm (D), as function of [Hg2+]/[3] (A, B) and [Hg2+]/[4]
(C, D). ([3], [4] = 1 × 10ꢀ 5 M, λexc3 = 525 nm, λexc4 = 530 nm, T = 298 K).
3.3.3. Hg2+ and Cd2+ metal ions
The Hg2+ ion is considered one of the most toxic metal ions because
of its inorganic form that can be converted by bacteria into the highly
toxic methylmercury. As a heavy metal ion, Hg2+ acts as a fluorescence
quencher via a spin-orbit coupling effect producing a turn-off mecha-
nism in most sensing systems [44,45]. The addition of Hg2+ induces, for
both porphyrin derivatives (3 and 4), a red shift of the Soret band from
420-430 nm to 450–455 nm, as well as the appearance of a new band at
670–675 nm. In the excited state, it was possible to observe a decrease in
the emission signal at the bands 662–675 nm Q (0-0) and ca. 730 nm Q
(0-0) (0–1). These modifications are indicative of metal complexation. In
Fig. 4 the absorption titrations of porphyrins 3 and 4 with Hg2+ are
shown. Similar behaviour was observed for Cd2+ metal (Fig. S16). The
highest association constants were obtained for Hg2+, with values of Log
Kass = 7.75 ± 0.02 (1:1) for 3 and Log Kass = 7.38 ± 0.04 (1:1) for 4, with
a stoichiometry of 1:1 (L: M).
Fig. 3. Spectrophotometric (A, C) and spectrofluorimetric (B, D) titrations of
compound 3 (A, B) and 4 (C,D) with the addition of Cu2+ in dichloromethane.
The inset represents the absorption (A, C) at 430 nm, 455 nm, 525 nm, 670 nm
for (A) and 420 nm, 530 nm, 675 nm for (C); and the emission intensity (B, D)
at 662 nm (B) and 675 nm (D), as function of [Cu2+]/[3] (A, B) and [Cu2+]/[4]
(C, D). ([3], [4] = 1 × 10ꢀ 5 M, λexc3 = 525 nm, λexc4 = 530 nm, T =
3.3.4. Co2+, Ni2+ and Ag+ metal ions
To complete the interaction of compounds 3 and 4 with the most
biological important transition metal ions, the titrations with Co2+, Ni2+
and Ag+ with compounds 3 and 4 were studied. In all cases lead to
similar spectral alterations to the ones obtained for Zn2+, Cu2+, Fe2+
,
Cd2+ and Hg2+, as it can be seen in Figs. S17–S19. All titrations showed
spectral evidence of the porphyrin inner core coordination by the metal
ions. The stability constants reveal the formation of mononuclear spe-
cies, where the sequence obtained for 3 and 4 decreases in the following
order: Ni2+>Co2+>Ag+ for 3 and Co2+>Ni2+>Ag+ for 4.
0.04 (4) for a stoichiometry of 1:1 (L:M). Both compounds exhibit a
fluorescence quenching upon the binding of Cu2+. In the case of Cu2+
,
the quenching of the emission signal is attributed to its paramagnetic
nature, which leads to non-radiative deactivation processes [42,43].
The titrations with Fe2+ induce spectral changes similar to those
observed for Cu2+. However, in the case of compound 4 a more intense
band at 450 nm was observed, suggesting a higher affinity of compound
4 for Fe2+ towards Cu2+ (see Fig. S15). Despite in the same order, the
association constant of 4 is higher for Fe2+ (6.62 ± 0.03; 1:1 (L:M)) than
for Cu2+ (6.25 ± 0.04; 1:1 (L:M)).
From the overall point of view, the final sequence of the strongest
interaction obtained was Hg2+>Cu2+>Zn2+>Fe2+>Cd2+>Ni2+
>Co2+>Ag+ for 3 and Cd2+≈Hg2+>Co2+>Zn2+>Fe2+>Cu2+>Ni2+
>Ag+ for 4.
Concerning the sensorial ability of compounds 3 and 4 towards the
studied metal ions and future application of these probes, the detection
(LOD) and quantification (LOQ) limits were also determined at 670 nm
6

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
(3) and 675 nm (4) by absorption (see Table 2). It was determined the
values of LOD = 0.025 ± 0.003/LOQ = 0.041 ± 0.05 for 3 and LOD =
0.061 ± 0.004/LOQ = 0.084 ± 0.006 for 4. The minimal detectable and
quantified amounts for all studies metal ions were also determined, and
the values are listed in Table 2. From the results obtained it was possible
to conclude that both compounds are able to detect and quantify the
emission spectra were then collected in order to evaluate the interaction
between the metal ion and the considered dye dispersed in a PMMA thin
film.
The interaction between the metal ions and the samples containing
the dye 3 do not show a significant cation sensing effect (data not
shown). However, in the case of metal ions Hg2+ and Zn2+ it was
observed a slight decrease of ca.10% of the peak’s intensity in absorp-
tion and emission response when the concentration of the metal ions is
30 ppm (data not shown). This is probably due to the fact that only very
weak interactions are established between the cations and the dye
molecules exposed on the surface of the thin sample. This result could
have been expected since PMMA is not water-soluble and therefore, only
limited interaction with the dyes distributed close to the matrix surface
can occur. Nevertheless, the segregation of dye molecules at the air/
polymer film contact surface is reasonable to occur because solvent
evaporation progressively distributes them during film formation [47].
Analogous outcomes were gathered for polymer samples containing
the dye 4.
fewest amount of the Hg2+ toxic metal ion of 0.9/5.3
μM and 1.8/3.6 μM
for compound 3 and 4, respectively.
3.4. PMMA films for sensing applications
In order to increase the analytical applications of porphyrin de-
rivatives 3 and 4, several low-cost polymeric PMMA-doped samples
were prepared and studied towards Zn2+, Cd2+ and Hg2+ metal ions, and
as a function of the temperature. PMMA has been selected as the host
matrix being already investigated with good results in our research team
for similar sensing [14]. Films in PMMA with 0.25–1.5% w/w of the
dyes 3, 4 were obtained using a polymer amount of 100 mg. For the
solid-state temperature assays, PMMA films with 0.5–3.0% of TPP and
ZnTPP were also prepared for comparison. In Fig. 5 the obtained com-
posite films containing the 0.25–3.0% of the dyes are reported.
Three replicates of the same sample were obtained in order to verify
the reproducibility of the film production. The PMMA_4 films did not
show a reasonable degree of homogeneity for all range of dye concen-
tration investigated; this suggests a partial incompatibility between the
polymeric matrix and the dye that determines the migration and ag-
gregation of the dye during the evaporation of the solvent giving
differently coloured areas on the film surface.
3.4.2. Physical sensing: solid-state temperature sensing
The PMMA-based doped film with the compounds TPP, ZnTPP, 3
and 4 was cut into pieces in order to be used for verifying eventual
thermal sensing capability.
In order to determine their potential use as thermal sensors, the
prepared 10 × 10 × 0.09 mm dye-doped films were placed on a heating
plate. The emission spectra were recorded at the variation of the tem-
perature from 25 to 100 ◦C and from 100 to 35 ◦C in order to verify if the
temperature sensing was reversible.
Fig. 6 and S20 shows the emission spectra of the PMMA polymers
doped with dyes 3, 4, TPP and ZnTPP, (1.5%). Temperature increasing
(from 25 ◦C to 100 ◦C) lead to a reduction of the emission intensity for all
doped polymers. Such phenomenon is typical for porphyrins and can be
explained by the thermal quenching effect [48].
The PMMA_3 films, on the other hand, showed a more homogeneous
surface even at the highest dye content thus suggesting effective dye/
polymer interactions that, in turn, favored a good reproducibility in film
formation.
Noteworthy, in the cooling phase (from 100 ◦C to 35 ◦C), PMMA_3
and PMMA_4 showed a recovery of emission intensity of ca. 70–80%.
Similar spectra changes were observed for the other percentages of dyes
(0.25% and 0.5%) for PMMA_3 and PMMA_4. On the other hand, for
PMMA_TPP and PMMA_ZnTPP polymer films such recovery is not
observed, indicating that the temperature solicitation possibly induced a
permanent modification of the dye distribution within the PMMA ma-
trix. This phenomenon can be addressed the presence of weak TPP/
ZnTPP/polymer interactions that can be easily destroyed by tempera-
ture thus promoting the formation of poorly emissive porphyrinic ag-
gregates. Conversely, the lateral functionalization of TPP in 3 and 4
chromophores possibly increased the number of effective interactions
with the acrylate moieties of PMMA and also reducing the overall
planarity of the dye that could promote the formation of aggregated
stacked structures within the solid polymer matrix [49].
Also, TPP and Zn-TPP dyes showed a high degree of compatibility
with the PMMA host matrix in agreement with literature reports [14,
46]. Among all replicas, the samples showing the most homogeneous
surface were utilized for the successive sensing studies.
3.4.1. Sensing of metal ions: solid-state
Based on the good results obtained in solution, solid thin films doped
with different dye concentrations are prepared in order to evaluate the
possibility to replicate this sensing behaviour also in solid-state.
With this purpose, water solutions of the metal ions of interest (Hg2+
,
Cd2+ and Zn2+) were prepared at different metal concentrations (0, 2.5
and 30 ppm). The films previously obtained were immersed for 5 min in
the different solutions at growing concentrations of metal ion, and the
Table 2
Minimal detection (LOD) and quantification (LOQ) amounts (
μ
M) of studied
Among all the samples, the PMMA_3 shows the best performances as
temperature optical probe in the range 25–100 ◦C. Indeed, its optical
properties are practically full recovered after the temperature mea-
surement cycle.
metal ions (M) by compounds 3 and 4. [LOD and LOQ were measured by ab-
sorption at 670 and 675 nm for 3 and 4, respectively].
Compounds (L)
Metal (M)
LOD (
μ
M)
LOQ (μM)
These preliminarily results gathered from PMMA_3 and PMMA_4 are
quite promising for the development of low cost temperature indicators
for smart plastic labels.
3
Zn2+
Cd2+
Co2+
Fe2+
Ni2+
Cu2+
Hg2+
Ag+
2.9
5.3
5.6
2.8
2.5
5.0
0.9
11.9
4.4
5.3
2.8
2.8
2.5
6.6
1.8
11.9
8.7
13.1
53.2
8.5
12.5
8.3
4. Conclusion
5.3
110.4
5.8
Porphyrin derivatives 3 and 4 were successfully synthesized and
characterized. Porphyrin 3 showed a fluorescent quantum yield higher
than 4 (ca. 10 times), which could be related to the spacer increase of the
chain in the beta position. Such an increase could lead to a decrease in
the planarity of molecule leading then to a less emissive compound (4).
Concerning the sensorial ability of compound 3 and 4, spectral changes
in ground and excited state were observed for both compounds. The
observed spectral changes are related to the complexation of the
4
Zn2+
Cd2+
Co2+
Fe2+
Ni2+
Cu2+
Hg2+
Ag+
7.9
5.6
5.7
7.5
10.0
3.6
17.9
7

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
Fig. 5. PMMA films doped with TPP, ZnTPP, 3 and 4.
Fig. 6. Emission spectra of samples PMMA_3 (1.5%) (A) and PMMA_4 (1.5%) (C). Emission intensities at 659 nm (B) and 678 nm (D) as a function of temperature for
PMMA_3 (1.5%) (A) and PMMA_4 (1.5%), respectively.
porphyrin macrocycle by the metal ions. In the same way, upon
complexation, a colour change from pale yellow to green was detected.
The results indicate a ligand-to-metal complex stoichiometry of 1:1. In
particular, compound 3 showed the best results for Hg2+ metal ions.
Preliminarily results gathered from PMMA films doped with 3 and 4,
indicated promising results as temperature optical probes when
compared with the non-substituted derivatives TPP and ZnTPP.
PMMA_3 revels to be the best candidate showing reversible optical
properties in function of temperature, with the recovery of ca. 80% in
the cooling phase.
8

N.M.M. Moura et al.
Dyes and Pigments 185 (2021) 108897
photodynamic inactivation efficiency against Escherichia coli. Dyes Pigments
2020;178:108330. https://doi.org/10.1016/j.dyepig.2020.108330.
[6] Dias CJ, Sardo I, Moura NMM, Felgueiras J, Neves MGPMS, Fardilha M, et al. An
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biological assessment in prostate cancer cells. Dyes Pigments 2020;173:107996.
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CRediT authorship contribution statement
Nuno M.M. Moura: Conceptualization, Methodology, Validation,
Investigation, Resources, Data curation, Writing - original draft, Writing
- review & editing, Visualization, Supervision, Project administration.
Simone Valentini: Validation, Investigation, Data curation, Visualiza-
tion. Victoria Cheptene: Validation, Data curation, Investigation.
Andrea Pucci: Conceptualization, Methodology, Resources, Supervi-
sion, Writing - original draft, Writing - review & editing, Funding
acquisition. M. Graça P.M.S. Neves: Conceptualization, Methodology,
Resources, Supervision, Writing - original draft, Writing - review &
[7] Moura NMM, Esteves M, Vieira C, Rocha GMSRO, Faustino MAF, Almeida A, et al.
Novel β-functionalized mono-charged porphyrinic derivatives: synthesis and
photoinactivation of Escherichia coli. Dyes Pigments 2019;160:361–71. https://
doi.org/10.1016/j.dyepig.2018.06.048.
[8] Beletskaya I, Tyurin VS, Tsivadze AY, Guilard R, Stern C. Supramolecular
Chemistry of metalloporphyrins. Chem Rev 2009;109:1659–713. https://doi.org/
10.1021/cr800247a.
[9] Liu B-W, Chen Y, Song B-E, Liu Y. Amphiphilic porphyrin assembly as a highly
selective chemosensor for organic mercury in water. Chem Commun 2011;47:
4418. https://doi.org/10.1039/c0cc05413e.
´
editing, Funding acquisition. Jose Luis Capelo: Resources, Writing -
original draft, Writing - review & editing, Funding acquisition. Carlos
Lodeiro: Conceptualization, Methodology, Resources, Writing - original
draft, Writing - review & editing, Funding acquisition, Supervision.
Elisabete Oliveira: Conceptualization, Methodology, Validation,
Investigation, Resources, Data curation, Writing - original draft, Writing
- review & editing, Visualization, Supervision, Project administration.
[10] Zhu M, Zhou Y, Yang L, Li L, Qi D, Bai M, et al. Synergistic coupling of fluorescent
“turn-off” with spectral overlap modulated FRET for ratiometric Ag + sensor. Inorg
Chem 2014;53:12186–90. https://doi.org/10.1021/ic502141q.
[11] Choi JK, Sargsyan G, Olive AM, Balaz M. Highly sensitive and selective
spectroscopic detection of mercury(II) in water by using pyridylporphyrin-DNA
conjugates. Chem Eur J 2013;19:2515–22. https://doi.org/10.1002/
chem.201202461.
[12] Moura NMM, Nunez C, Faustino MAF, Cavaleiro JAS, Neves MGPMS, Capelo JL,
et al. Preparation and ion recognition features of porphyrin-chalcone type
compounds as efficient red-fluorescent materials. J Mater Chem C 2014;2:
4772–83. https://doi.org/10.1039/C3TC32496F.
Declaration of competing interest
˜
[13] Moura NMM, Núnez C, Santos SM, Faustino MAF, Cavaleiro JAS, Neves MGPMS,
et al. Synthesis, spectroscopy studies, and theoretical calculations of new
fluorescent probes based on pyrazole containing porphyrins for Zn(II), Cd(II), and
Hg(II) optical detection. Inorg Chem 2014;53:6149–58. https://doi.org/10.1021/
ic500634y.
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.
˜
[14] Moura NMM, Nunez C, Santos SM, Faustino MAF, Cavaleiro JAS, Neves MGPMS,
et al. Functionalized porphyrins as red fluorescent probes for metal cations:
spectroscopic, MALDI-TOF spectrometry, and doped-polymer studies.
Chempluschem 2013;78:1230–43. https://doi.org/10.1002/cplu.201300216.
Acknowledgments
˜
[15] Moura NMM, Núnez C, Santos SM, Faustino MAF, Cavaleiro JAS, Almeida Paz FA,
EO and CL thank the financial support by Associate Laboratory for
Green Chemistry - LAQV which is financed by national funds from FCT/
MCTES (UIDB/50006/2020), as well as the Scientific Society PRO-
TEOMASS (Portugal) for funding support (General Funding Grants 2018
and 2019). E.O thanks FCT/MEC (Portugal) for the individual contract,
CEECIND/00648/2017.
et al. A new 3,5-bisporphyrinylpyridine derivative as a fluorescent ratiometric
probe for zinc ions. Chem Eur J 2014;20:6684–92. https://doi.org/10.1002/
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mercury ions based on a porphyrin dimer. Anal Chem 2002;74:821–5. https://doi.
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fluorescence quenching of tetra(p-dimethylaminophenyl)porphyrin. Anal Chim
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colorimetric Hg 2+ sensor based on the porphyrin-functionalized polyacrylonitrile
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reaction: fluorescence “turn-on” sensing of Cu(II) ion. Chem Mater 2016;28:
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The authors are grateful to University of Aveiro and FCT/MCT for the
financial support for QOPNA research Unit (FCT UID/QUI/00062/
2019) and the LAQV (UIDB/50006/2020) through national funds and,
where applicable, co-financed by the FEDER, within the PT2020 Part-
nership Agreement, and to the Portuguese NMR Network. The research
contract of N.M.M. Moura (REF.-048-88-ARH/2018) is funded by na-
˜
ˆ
tional funds (OE), through FCT – Fundaçao para a Ciencia e a Tecno-
logia, I.P., in the scope of the framework contract foreseen in the
numbers 4, 5 and 6 of the article 23, of the Decree-Law 57/2016, of
August 29, changed by Law 57/2017, of July 19.
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SV and VC would like to acknowledge the Erasmus exchange pro-
gram for the financial support in their exchange programme between the
University of Pisa (Italy) and the NOVA University Lisbon (Portugal).
Appendix A. Supplementary data
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