Environmental effects in nitroquinoline derivatives solutions: Solvatochromism, acid-base effect and ion-sensor investigation

Article abstract of DOI:10.1016/j.molstruc.2021.130260

Since the first chemical fluorescent sensor reported in 1867, it has often been sought to find intelligent organic molecules that act as chemical sensors of high selectivity and sensitivity. In this process, the colorimetric and fluorescence methods have more advantages, as they are generally very sensitive, low cost, easily executable, versatile and of quick result. Colorimetric receptors containing several chromophores with azo-compounds have been developed since the discovery of chemosensors. Quinoline derivatives attract the interest of the scientific community in this regard. Thus, the present study using a facile synthesis aimed to help in development and behavioural study of new nitroquinolinic compounds against different solvents, pHs and organic/inorganic salts to observe potential recognition of anionic or cationic species. In four compounds a good non-linear fitting between the absorption intensity and amount of acid/base were established. The structural characterization of each of the prepared compounds was carried out with different spectroscopic and spectrometric techniques. UV–Vis and fluorescence measurements were also essential to confirm the binding/interaction process of ionic species and helped to find molecules with potential use in the identification of Co²⁺ and U²⁺ species, in addition to showing the sensitivity of species to pH and different solvents. The combination of theory and experiment is able to give a better understanding of the behavior in acidic, basic and neutral medium, DFT calculations were employed for this purpose.

Full text of DOI:10.1016/j.molstruc.2021.130260

Journal of Molecular Structure 1235 (2021) 130260
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: www.elsevier.com/locate/molstr
Environmental effects in nitroquinoline derivatives solutions:
Solvatochromism, acid-base effect and ion-sensor investigation
Giovanny Carvalho dos Santos, Julia Lopes Rodrigues, Vitor Fernandes Moreno,
Luiz Carlos da Silva-Filho∗
Laboratory of Organic Synthesis and Processes (LOSP), São Paulo State University (UNESP), Department of Chemistry, School of Sciences, Bauru17033-360,
São Paulo, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Since the first chemical fluorescent sensor reported in 1867, it has often been sought to find intelligent
organic molecules that act as chemical sensors of high selectivity and sensitivity. In this process, the col-
orimetric and fluorescence methods have more advantages, as they are generally very sensitive, low cost,
easily executable, versatile and of quick result. Colorimetric receptors containing several chromophores
with azo-compounds have been developed since the discovery of chemosensors. Quinoline derivatives
attract the interest of the scientific community in this regard. Thus, the present study using a facile syn-
thesis aimed to help in development and behavioural study of new nitroquinolinic compounds against
different solvents, pHs and organic/inorganic salts to observe potential recognition of anionic or cationic
species. In four compounds a good non-linear fitting between the absorption intensity and amount of
acid/base were established. The structural characterization of each of the prepared compounds was car-
ried out with different spectroscopic and spectrometric techniques. UV–Vis and fluorescence measure-
ments were also essential to confirm the binding/interaction process of ionic species and helped to find
molecules with potential use in the identification of Co2+ and U2+ species, in addition to showing the
sensitivity of species to pH and different solvents. The combination of theory and experiment is able to
give a better understanding of the behavior in acidic, basic and neutral medium, DFT calculations were
employed for this purpose.
Received 24 October 2020
Revised 18 February 2021
Accepted 5 March 2021
Available online 10 March 2021
Keywords:
Nitroquinolines
Solvatochromism
pH-Effect
Ion-Sensor
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
manganese, copper and cobalt which are present as essential ele-
ments in biological systems [4].
The search for intelligent organic molecules that act as chemical
sensors of high selectivity and sensitivity is frequent [1]. In this
process, colorimetric and fluorescent properties of the compounds
are explored for analytical, biomedical and environmental science
applications [2].
The first sensors were the colorimetric sensors, which have the
ability to cause a shift towards blue or red within the absorption
spectrum, causing the color change in a specific species to be de-
tected. The molecular structure of these compounds usually has a
chromophore and a receptor that are linked together by a con-
jugated chain [5]. A colorimetric sensor must present the species
recognition event in a way that is easy to identify and that has
a high optical sensitivity, that is, it presents a strong change in
the absorption bands of the UV–Vis spectrum. There are several
metal ions that play a vital role in our daily physiological life, oth-
ers are toxic and cause serious health and environmental problems
[6]. The absence or excess of some of these species in the human
organism can cause damage to some organs such as kidneys and
livers, as they can also be related and help in the identification
of diseases such as Alzheimer’s, thalassemia, epilepsy, Parkinson’s,
hypoglycemia among others, so the importance to detect certain
concentrations of ions or small molecules in the body [7].
Among the different chemosensors, the colorimetric and fluo-
rescence methods have more advantages, as they are generally very
sensitive, low cost, easily executable and versatile, offering sub-
nanometric resolution and sub-millisecond temporal resolution [3].
Today, among the different analytes, there is a special interest in
developing chemosensors for transition metal ions: they generally
represent an environmental concern when present in uncontrolled
quantities, but at the same time some of them such as iron, zinc,
∗
Corresponding author.
E-mail address: luiz.carlos@unesp.br (L.C. da Silva-Filho).
https://doi.org/10.1016/j.molstruc.2021.130260
0022-2860/© 2021 Elsevier B.V. All rights reserved.

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
Of the strategies used for the development of anionic
chemosensors, the simplest involves the design of molecules that
change color after a change in their molecular structure due to
their contact with anions. Several examples of molecules acting
as chemosensors have been reported. Examples include chemosen-
sor of organic compounds conjugated with electronic donor (D)
and acceptor (A) parts connected by single and double bonds, ex-
hibiting interesting electronic properties due to an intramolecular
charge transfer (ICT) process [8]. In this way, quinolinic deriva-
tives can also have such optical and structural characteristics, thus
showing potential application as chemosensors [9]. Nothing could
be further from the truth than to think that all the problems re-
lated to the research of chemical sensors have already been solved,
"new" sensors are still needed for analytes that are still unknown,
whether in the form of new biomarkers or traces of pollutants
in air supplies and water, as well as in biological and environ-
mental analysis in specific practical applications. So, if the prob-
lem requires new custom-made receivers or an improvement of
existing systems, we must continue to search for chemical sen-
sors to face these challenges. For this reason, here in this work
we use niobium pentachloride to synthesize new quinoline com-
pounds through an already established multicomponent reaction
methodology (MCR) [10]. The Quinoline is an aromatic azahetero-
cyclic strongly electron-accepting with high thermal and oxidative
stability, in addition to optoelectronic properties that confer ap-
plications in polymer chemistry, electronics and organic optoelec-
tronics [11]. Furthermore, this class of compounds is the target of
several biological and medicinal studies to combat various diseases
[12].
(THF). The emission was obtained using a Biotek microplate spec-
trophotometer (model Synergy H1) with 9,10-diphenylanthracene
used as standard for the quantum yields [13].
The effect of inorganic and organic salts was performed with:
chlorides - SrCl2, ZnCl2, NH4Cl, FeCl3, CuCl2 (2H2O), CoCl2, NaCl,
SnCl2, BaCl2, MnCl2, HgCl2, AlCl3; nitrates -Ag(NO3), Zn(NO3)2,
Co(NO3)2, Cu(NO3)2, Fe(NO3)2, Na(NO3), Ca(NO3)2, Cd(NO3)2 e ac-
etates - Na(ac), Co(ac)2, UO2(ac)2, NH4(ac) and Pb(ac)2.
The combination of theory and experiment is able to give a
better understanding of the behavior in acidic, basic and neu-
tral medium. All calculations were carried out in Gaussian09 [14].
The ground state geometries were optimized by Density Functional
Theory (DFT) using the Becke three-parameter Lee−Yang−Parr
exchange-correlation functional (B3LYP) and electronic structure
method (6–31+G(d,p) basis set) [15]. Conformational analysis and
frequency calculations were done to reach the absolute minimum
for all the compounds.
All procedures details for solvent analysis, acid-base effect and
effect of organic and inorganic salts are shown in the Supporting
Information (SI)
2.2. Synthesis of nitroquinoline derivatives
The synthesis of nitroquinolines followed the procedure already
used by the group [10]. A multicomponent reaction (MCR) pro-
moted by niobium pentachloride (NbCl5) was performed among p-
nitroaniline (1), benzaldehyde derivatives (2a-j) and phenylacety-
lene (3). The Scheme
1 show the synthesis of nitroquinoline
derivatives. The complete procedure of synthesis and the charac-
terization is described in the Supporting Information (SI).
2. Experimental
3. Results and discussion
2.1. Materials and reagents
3.1. Identification of active sites in molecules
All reactions were performed under air atmosphere, unless oth-
erwise specified. Acetonitrile was distilled from calcium hydride.
All commercially available reagents were used without further pu-
rification. The NbCl5 used was supplied by Companhia Brasileira de
Metalurgia e Mineração (CBMM). Thin-layer chromatography was
performed on 0.2 mm Merck 60F254 silica gel aluminum sheets,
which were visualized with UV-365 nm irradiation. Bruker DRX
400 spectrometer was employed for the NMR spectra (CDCl3 and
DMSO–d) using tetramethylsilane as internal reference for 1H and
CDCl3 as an internal reference for 13C. The melting points were ac-
quired using the Digital Apparatus of Melting Point, model MQAPF-
302 of “Microquímica Equipamentos LTDA”.
After the computational chemical calculations give the best op-
timized structures and show the locations of the molecular or-
bitals, and based on other studies of molecules with similar groups
and azaheterocycles [16], we can then predict the possible active
sites of the synthesized molecules. Fig. 1 shows the possible in-
teraction sites and the type of interaction that can occur in the
different environments that the studies were carried out.
In Fig. 1, we observe the structure of the synthesized com-
pounds, and we can divide the structure into three parts, an ac-
ceptor group (NO2), the quinolinic ring which also as an accep-
tor and the other end of the molecule with donor groups (ex-
cept for pyridine that is an acceptor type of substituent. Thus, we
have some hetero atoms sites that can serve as complexation / in-
We obtained the UV–Vis absorption spectra in a Cary 8454
spectrophotometer of Agilent Technologies, using a 1.0 cm light
path quartz cuvette at room temperature. We performed the ab-
sorptions measurements in three conditions (Acidic, neutral and
basic). For the acidic condition we control the pH with sulfuric
acid. The basic condition was obtained by addiction of ammo-
nium hydroxide. The quantity of each acid/base addicted in the
solution is ranging from 0.5 to 10000 μL of acid/base in 3 mL
of ethanol solution with ~10−4 M of each compound. The ab-
sorption intensity for the limit detection and linearity calculations
were measured at the maximum absorption wavelength on which
the acid or base had an effect. For acid medium: 4d = 510 nm,
4f = 406 nm, 4g = 423 nm and 4h = 441 nm. For basic medium:
4d = 431 nm, 4f = 445 nm, 4g = 454 nm and 4h = 495 nm.
Measurements with different solvents also were performed. Here,
we used ethylacetate (EA), dimethylsulfoxide (DMSO), dimethyl-
formamide (DMF), ethanol (EtOH), methanol (MeOH), 1-butanol,
hexane, toluene, dioxane, acetonitrile (ACN), chloroform (CHCl3),
Dichloroethane (DCE), Dichloromethane (DCM), acetone, diethyl
ether (Et2O), water (H2O), ethylene glycol and tetrahydrofurane
–
–
–
teraction sites (eg O O, N N, N-S, N O) with other species [16].
The phenyl substituents at positions 2 and 4 of the quinolinic ring
play an important role in the solubility and hydrophobic interac-
tions of these compounds. The OH and NH groups are passive to
be (de)protonated, thus being important in the interaction of the
compounds in an acidic/basic medium [16].
3.2. Computational calculations of nitroquinoline derivatives
3.2.1. Structural optimization
It is important to know the molecular structure of a potential
sensor. Through the molecular structure here we can observe the
active sites for complexation and later identification of the ana-
lyte, for example the rotation possibilities in the rings (Fig. 2a).
Therefore, structural optimization via computational chemistry ap-
pears as an important tool to predict the shape of the optimized
structure, showing the most energetically stable structure and the
2

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
Scheme 1. Synthesis of nitroquinoline derivatives.
Fig. 1. Compounds studied and possible interaction sites in quinoline derivatives.
Fig. 2. a) Dihedral angle in the vacuum-optimized structure b) HOMO and LUMO topologies as calculated by DFT (B3LYP) in vacuum and electronic analysis for the S0→S1
transition.
3

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
Fig. 3. UV–Vis absorption spectra of nitroquinoline derivatives in different solvents (Left). Solutions of the compounds in different solvents under UV-Lamp 365 nm irradia-
tion (Right).
possibilities of interactions with different species [17]. In support-
ing information we observe the structures of the 10 compounds
with fundamental state geometries fully optimized by the Den-
sity Functional Theory (DFT), using the three-parameter Becke ex-
change correlation function Lee-Yang-Parr (B3LYP) and the func-
tions of 6–31 G base set (d) in vacuum [15]. In Fig. 2a, we show
the first rotation (A) of the ring (r1) of position 4 of the quino-
linic nucleus and the other rotation (B) of the substituent R1 (po-
sition 2 of the quinolinic ring). It is observed that in the case of
all compounds, regardless of substitution, we have in rotation A a
tional can describe geometric optimization, however for the TD
part, the functional Offset-corrected M06HF, presents a better cor-
relation with the experimental absorption spectrum, especially for
nitroquinolines with donor-π-acceptor structure [19]. Nevertheless,
for this work B3LYP is quite sufficient.
3.3. Solvatochromic effect on quinolinic derivatives
An important property that we must know is the change in the
structural behavior of quinolinic derivatives in relation to various
solvents. The behavior of the compound in different solvents show
an important characteristic for the molecule to act as a solvent
sensor, with that we can also know structural modifications of the
solvated molecule [20]. Through this study we can also know some
structural characteristics and stabilization of the compound by sol-
vent. In Fig. 3, we show the UV–Vis absorption spectra of 3 com-
pounds (4f, 4d and 4h) that showed a great variation in the max-
imum absorption wavelength, in addition to fluorescence emission
that can also be observed with the naked eye. The remaining 6
molecules showed no visible changes. The studies were carried out
with 18 different solvents.
dihedral angle of approximately 54°, varying from
1° However,
in the other position, rotation B, there is a great variation depend-
ing on the substituent, where we see that the thiophene ring (4j)
and the furan ring (4i) are only 0.5° outside the planarity. The tor-
sion is greater when we see compound 4c and 4 g, both have an
ortho-substitution that causes the ring to rotate 36.49° and 25.68°,
respectively.
3.2.2. Molecular orbitals
In Fig. 2b and the other figures on SI, we see the molecular
orbitals HOMO and LUMO. We can see that HOMO is located at
the end coming from the aldehyde part (R1), and LUMO is located
mainly towards the nitro group. This configuration leads to say
that the molecules have a charge transfer character from HOMO
to LUMO, thus creating a donor-acceptor system [18].
In Table 1, we show the maximum absorption wavelength val-
ues (λabs) of the compounds 4a-4j.
Through the absorption spectra in UV–Vis and Table 1 with the
maximum absorption data, we can observe that a great variation in
values occurs. The dielectric constant is an important parameter to
predict some direct behavior of comparison with the λabs values
[21]. However, other factors are also important, such as the sol-
vent’s refractive index, polarity, among others. Therefore, a study
using equation as Lippert-Mataga type to observe the correlation
of these parameters cannot be done considering that in many sol-
vents the fluorescence was not observed [22].
In all compounds it was possible to observe that the main tran-
sition responsible for the maximum absorption wavelength band
(from 352 nm to 452 nm) is HOMO-LUMO type. Thus, it is pos-
sible to observe that despite the delocalization of LUMO MOs in
the central structure of the quinolinic ring (r2 and r3), the ni-
tro group is the main responsible for this topology. In HOMO, on
the other hand, it can be seen the great influence of donor sub-
stituents on R1 (-N(CH3)2, -OH, -CH3OH, thiophene and furan) that
locate the MOs. In most of the studied compounds, it is observed
that the ring r1 does not contribute significantly to the topolo-
gies of the orbitals. However, for 4b, 4c and 4e a contribution
of r1 can be seen in HOMO, these molecules contain electron-
withdrawing substituents such as pyridine (4b and 4c) and car-
boxylic acid (4e). Here the purpose of computational calculations
was mainly to observe geometric optimization and the trend in
nitroquinoline transitions using B3LYP. We know that this func-
It is known that quinoline derivatives and many other conju-
gated organic compounds containing the nitro group do not show
fluorescence in solution [10,23]. However, here we can see that
some compounds showed fluorescence emission in some specific
solvents. The spectroscopic data of these compounds can be seen
in Table 2.
Here we can see that five of the study compounds showed
fluorescence in some solvents. Mostly the quantum fluorescence
yield (QY) does not exceed 2.0%, however the behavior of com-
4

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
Table 1
Maximum absorption wavelength of nitroquinoline in different solvents.
Dieletric Constant
Maximum wavelength of compounds (nm)
Solvent
(20 °C)
4a
4b
4c
4d
4e
4f
4g
4h
4i
4j
Hexane
Dioxane
Toluene
Et2O
1.9
333
337
340
335
340
336
338
339
340
336
337
335
334
341
336
338
343
344
315
322
325
319
325
323
323
324
325
322
329
320
320
327
322
323
328
330
320
329
330
324
331
330
330
332
332
327
330
326
326
327
328
329
336
339
410
424
426
423
436
425
431
435
434
427
429
431
425
438
427
434
443
437
332
338
338
334
336
334
336
336
338
334
335
333
331
341
334
337
342
342
364
365
368
369
364
363
368
364
366
369
366
367
366
374
362
369
376
372
395
377
389
383
384
378
376
377
379
390
380
386
383
391
378
386
393
392
377
382
380
383
378
381
388
377
378
384
383
384
378
391
377
387
394
388
371
366
368
367
370
364
367
369
369
367
366
368
367
371
363
369
372
369
354
366
368
362
369
363
368
367
369
365
366
367
363
369
363
369
368
357
2.2
2.4
4.3
CHCl3
4.8
EA
6
THF
7.6
DCM
9.1
DCE
10.4
17.8
20.6
22.4
32.6
36.7
37.5
37.7
46.6
80.4
1-Butanol
Acetone
EtOH
MeOH
DMF
ACN
Ethylene glycol
DMSO
H2O
Table 2
Spectroscopic data of quinoline derivatives.
Compound 4d
Dieletric Constant (20ᵒC)
λabs (nm)
λemissão (nm)
ꢀStk (10−3 cm−1
)
ꢁ (%)
f
Toluene
2.4
2.2
1.9
6.0
426
424
410
425
569
5.9
5.1
4.2
7.4
13.04
3.06
1.41
0.20
Dioxane
541
Hexane
495
Ethyl Acetate (EA)
Compound 4f
620
λemissão (nm)
ꢀ
(%)
f
Acetone
20.6
37.5
4.8
366
362
364
366
364
536
8.6
9.9
8.9
8.8
9.2
0.28
0.25
0.38
0.44
0.52
Acetonitrile (ACN)
Chloroform (CHCl3)
Dichloroethane (DCE)
Dichloromethane (DCM)
Compound 4h
566
539
10.4
9.1
541
549
λemissão (nm)
ꢀ
(%)
(%)
(%)
f
Ethyl Acetate (EA)
Dioxane
6.0
2.2
2.4
381
382
380
551
8.1
7.4
6.7
0.30
1.75
0.42
532
Toluene
510
Compound 4i
λemissão (nm)
518
ꢀ
f
Acetonitrile (ACN)
Dimethylformamide (DMF)
Dimethylsulfoxide (DMSO)
Compound 4j
37.5
36.7
46.6
363
371
372
8.2
7.7
8.1
0.13
0.19
0.10
521
532
λemissão (nm)
ꢀ
f
Dimethylformamide (DMF)
Dimethylsulfoxide (DMSO)
36.7
46.6
369
368
504
508
7.2
7.5
0.14
0.12
∗All other solvents showed ꢁf (%) less than 0.05%, only compounds with ꢁf (%) greater than or equal to 0.10% are shown.
pound 4d in toluene is noteworthy, showing 13.04% QY (Observed
dergo protonation or deprotonation depending on the medium.
Thus, 3 of these compounds showed colorimetric changes in ba-
sic medium (4f, 4g, 4h). As for the behavior in basic medium, all
compounds showed a slight color change, however these changes
were more expressive in 4 of them (4f, 4g, 4h, 4d). The highlighted
compounds are shown in Fig. 5. The quantity of each acid/base ad-
dicted in the solution is ranging from 1–1250 μL for the base, and
1–500 μL for the acid. For the compound 4h, 4f and 4g after the
addition of 50 μL, the spectra do not change for further acid addi-
tion.
in Figs. 3 and 4).
In a solvathochromic behavior in emission measurements we
can see the relationship between the dielectric constant and the
emission of the compounds. It can be observed that compounds
4d, 4f and 4h showed fluorescence emission, even with a low
quantum yield, in solvents with a low dielectric constant (eg
toluene, dioxane, hexane, ethyl acetate, CHCl3, DCM). Compounds
4i and 4j showed emission in solvents with high dielectric con-
stant (ACN, DMF, DMSO). In addition, it is possible to highlight the
emission of compound 4f in all the chlorinated solvents examined
(CHCl3, DCM, DCE). Nevertheless, considering all compounds that
presented emission, a linearity between the dielectric constant and
the maximum emission wavelength could not be found. This shows
the importance caused by different substituents at position 2 of
the nitroquinoline ring.
The compounds that showed change in basic medium have in
common a hydroxyl group substituted in the ring R1. In the case
of the acid effect, the same compounds that showed changes in
basic medium were those that showed greater shift in the max-
imum absorption wavelength. In addition, the 4d derivative also
showed a strong change in the absorption spectrum, this is due to
the fact that the N(CH3)2 group is a strong donor, thus causing a
greater electronic density in the N of the quinolinic ring, which fa-
cilitates its protonation and intensifies the push-pull effect of this
molecule. These results were confirmed by the theoretical absorp-
tion spectra of TD-DFT calculations in a previous work [19]. There-
3.4. Acid-base effect on quinolinic derivatives
Here, we verify the effect of acidity and basicity in the com-
pounds, as they have as previously mentioned, sites that can un-
5

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
Fig. 4. Fluorescence spectra (left) and quantum yield of quinoline 4d, 4f and 4h in different solvents (right).
Fig. 5. a) (Left) Acid-effect in quinoline derivatives ranging from 1–500 μL of sulfuric acid b) (Right) Base-effect in quinoline derivatives ranging from 1–1250 μL of ammo-
nium hydroxide.
6

G.C. dos Santos, J.L. Rodrigues, V.F. Moreno et al.
Journal of Molecular Structure 1235 (2021) 130260
4. Conclusion
Regarding the solvatochromism studies, in the tests carried out
on 18 different solvents it was observed that certain compounds
(4f, 4j, 4d, 4i, 4h) showed fluorescence emission in some sol-
vents, even with the compound presenting the nitro group, which
is a known suppressor fluorescence. It could also be observed that
changing the acidity/basicity of the medium is capable of caus-
ing a shift in the absorption spectrum, thus changing the visi-
ble color of the solutions, an important characteristic of indica-
tor compounds. Finally, a weak interaction with the organic salts
of uranyl acetate and cobalt acetate with compounds 4f, 4g and
4h was seen. The study showed that these nitroquinolinic deriva-
tives have a potential as a sensor for solvents, pH and chemical
species. The types of solvents, pH and metal ions all can cause sim-
ilar changes in the UV–Vis absorption spectra of compounds (yel-
low to the naked-eye). Therefore, the three detection objects have
the problem of mutual interference, which can affects the practical
application of the sensor. However, simple structural changes (eg,
the simple reduction of the NO2 group to NH2), could enhance the
sensory effect of these compounds, knowing that aminoquinolines
have a high fluorescence emission, thus being more sensitive to
changes in the medium. Ongoing studies are seeking to carry out
these structural modifications to understand the effect caused by
changes in the environment.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
Fig. 6. Assay with different salts in solution with quinolinic derivatives.
Ackowledgments
fore, except for compound 4d in acidic conditions that showed an
intense color change yellow-red-colorless, the others compounds
only showed soft “naked-eye” color change. The values of spectral
changes in the maximum absorption wavelength, limits of detec-
tion, and linearity of the absorption and pH can be seen in the SI
(Fig. S5, S6, S7). The compounds 4d, 4f, 4 g, and 4h exhibited a
good non-linear relationship with a correlation coefficient (R2) su-
perior to 0.946. When plotting the fluorescence intensity of these
four compounds versus pH, it is clear that no good correlation can
be drawn. This can happen because the compounds have different
interaction sites within the same molecule, thus the protonation
and deprotonation mechanism is different for each compound.
The authors would like to thank Fundação de Amparo
à
Pesquisa do Estado de São Paulo (FAPESP) (Procs. 2015/00615–0,
2016/01599–1 and 2018/14506–7), Conselho Nacional de Desen-
volvimento Científico e Tecnológico (CNPq) (Proc. 302769/2018–
8) and Pró-Reitoria de Pesquisa (PROPe-UNESP) for their finan-
cial support. We would also like to thank CBMM – Companhia
Brasileira de Mineralogia e Mineração for the NbCl5 samples, and
we thank Juan Carlos Roldao for the suggestion and help in the
Quantum Chemical studies.
Supplementary materials
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.molstruc.2021.130260.
3.5. Ion-sensor on quinolinic derivatives
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