Anionic chromogenic chemosensors highly selective for fluoride or cyanide based on 4-(4-Nitrobenzylideneamine)phenol

Article abstract of DOI:10.1590/S0103-50532012005000007

4-(4-Nitrobenzylideneamine)phenol was used in two strategies allowing the highly selective detection of F- and CN-. Firstly, the compound in acetonitrile acts as a chromogenic chemosensor based on the idea that more basic anions cause its deprotonation (c

Full text of DOI:10.1590/S0103-50532012005000007

J. Braz. Chem. Soc., Vol. 23, No. 8, 1488-1500, 2012.
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Article
A
Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
Based on 4-(4-Nitrobenzylideneamine)phenol
Celso R. Nicoleti, Vanderléia G. Marini, Lizandra M. Zimmermann and Vanderlei G. Machado*
Departamento de Química, Universidade Federal de Santa Catarina (UFSC),
88040-900 Florianópolis-SC, Brazil
O composto 4-(4-nitrobenzilidenoamino)fenol foi utilizado em duas estratégias que permitem
uma detecção altamente seletiva de F– e CN–. Primeiramente, observou-se que o composto em
acetonitrila age como um quimiossensor cromogênico devido ao fato de que ânions básicos
levam ao seu desprotonamento (solução incolor), resultando numa solução colorida contendo
fenolato. A detecção seletiva de CN– sobre F– foi obtida adicionando-se 1,4% de água ao sistema:
a água solvata preferencialmente o F–, enquanto o CN– fica relativamente livre para desprotonar o
composto fenólico. Outra estratégia envolveu um ensaio de competição do fenolato e do analito pelo
calix[4]pirrol em acetonitrila, um receptor altamente seletivo para F–. O fenolato e o calix[4]pirrol
formam um complexo por ligação de hidrogênio, com mudança na cor do meio. Dentre diversos
ânions, somente o F– foi capaz de restaurar a cor original correspondendo ao fenolato, pelo fato
do ânion deslocá-lo do sítio de complexação.
4-(4-Nitrobenzylideneamine)phenol was used in two strategies allowing the highly selective
detection of F– and CN–. Firstly, the compound in acetonitrile acts as a chromogenic chemosensor
based on the idea that more basic anions cause its deprotonation (colorless solution), generating a
colored solution containing phenolate. The discrimination of CN– over F– was obtained by adding
1.4% water to acetonitrile: water preferentially solvates F–, leaving the CN– free to deprotonate
the compound. Another strategy involved an assay comprised of the competition between
phenolate dye and the analyte for calix[4]pyrrole in acetonitrile, a receptor highly selective
for F–. Phenolate and calix[4]pyrrole form a hydrogen-bonded complex, which changes the color
of the medium. On the addition of various anions, only F– was able to restore the original color
corresponding to phenolate in solution due to the fact that the anion dislodges phenolate from the
complexation site.
Keywords: anionic chromogenic chemosensor, displacement assay, naked-eye detection,
anion sensing, calix[4]pyrrole
Introduction
is CN–, which is lethal in very small amounts due to its
ability to strongly bind to the active site of cytochrome-
oxidase, leading to the inhibition of the mitochondrial
electron transport chain, and to a decrease in the oxidative
metabolism.4 CN– has many applications in metallurgy,
fishing, mining and in the fabrication of polymers.4 Some
fruit seeds and roots release CN– through hydrolysis.5
In addition, many chemical warfare compounds, such
as sarin, soman and tabun,6 deliver F– and CN– through
hydrolysis7 and this is important in relation to developing
chemosensors for the detection of these neurotoxic agents.8
For the development of anionic chemosensors, a
very simple strategy uses the design of molecules that
change color, following an alteration in their molecular
structure due to the presence of anions in the medium.
The fact that anions play a fundamental role in many
chemical and biological processes has led to increased
attention being focused on the field of the recognition and
detection of anionic species.1 Much effort is being spent on
the design of chemosensors able to perform the selective
visual detection of anions and also the quantification of
such species.1 In research concerned with anion sensing,
many studies have been directed toward the development
of chemosensors for the fluoride anion2 due to the role it
plays in many diseases, in industry and in environmental
pollution.3 Another interesting anion in terms of detection
*e-mail: vanderlei.machado@ufsc.br

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The selectivity of a chemosensor for an anionic species
is related to its differential ability to interact with the
receptor site in the chemosensor, for instance, through
intermolecular hydrogen bonding (HB). In this context,
Steiner9 considered all HB as incipient proton transfer and
that in strong HB this process can be found in a very
advanced state. Phenol groups are commonly used in the
molecular structure of these types of chemosensors.10–14
The connection of the phenol donor group to an acceptor
group through a conjugated bridge creates an interesting
feature since the deprotonation of the compound generates
a colored conjugated base. The acidity of the chemosensor
can be improved by combining the effect of the medium
polarity with modifications in the molecular structure of
the compound.12,13 A set of protonated dyes with phenol
donor groups in their molecular structures has been
studied by our group as simple anionic chromogenic
chemosensors based on acid-base strategy.11–14 One of these
dyes is 4-(4-nitrobenzylideneamine)-2,6-diphenylphenol,
compound 1a, which was recently used to selectively detect
CN– in the presence of several anionic species in acetonitrile
with very small amounts of water.14 The colorless solutions
of 1a in acetonitrile are turned blue with the addition of
basic species due to the formation of 1b. From the various
anionic species added to the solutions of 1a, only CN–,
F– and, with less intensity, CH3COO– and H2PO4– led to
colored solutions. With the addition of up to 2.4% (v/v) of
water, from all anions studied, only CN– caused a detectable
alteration in the color of the solution.14
designed by means of the integration of the calix[4]pyrrole
with different signaling subunits.24,25 An interesting
example of a colorimetric chemosensor was discovered
by Gale et al.25 and involved an efficient displacement
assay. The addition of CP made a yellow p-nitrophenolate
solution in dichloromethane colorless and the solution
changed back to yellow after adding the fluoride anion,
which is able to strongly bind at the receptor site,
displacing p-nitrophenolate.25 This finding inspired the
study of a displacement assay based on the interaction of
CP with 4-[(1-methyl-4(1H)-pyridinylidene)-ethylidene]-
2,5-cyclohexadien-1-one, a dye known as Brooker’s
merocyanine (BM), in acetonitrile.26 A solution of
BM in acetonitrile is colored and the addition of CP
changes the color in the solution due to the formation of
hydrogen-bonded CP-BM species. Finally, the original
color of the solution can be easily restored with the
addition of an adequate anion able to selectively bind to
CP and, in this manner, displacing the free dye.26
In this work, 4-(4-nitrobenzylideneamino)phenol
(compound 2a) is employed in two strategies aimed at
the detection of anions in solution. Firstly, 2a is used as
an anionic chromogenic chemosensor in acetonitrile and
in acetonitrile-water mixtures in the presence of various
anions, utilizing an acid-base approach. Another study is
performed to show the potential of 2b to detect anions in
acetonitrile by means of a displacement assay that uses CP
as an anionic receptor.
In other type of strategy, involving displacement
assays, a receptor is specially conceived to interact with a
dye (or a chromogenic reagent) forming a complex, which
causes an optical change in the system. The presence
of the analyte causes a competition scenario that can
lead to the dye being displaced from its initial complex,
causing a perceptible change in the optical signal of the
system.15 This strategy has been successfully used in the
development of assays for many types of analytes.15,16
Many receptor molecules exhibiting the capability
to recognize anions have been synthesized in recent
years and their design comprises the use of hydrogen
bonds alone or in conjugation with electrostatic forces
to generate binding sites for anionic species. Among the
most popular anion receptors, a very interesting example is
meso-octamethylporphyrinogen, more commonly known
as meso-octamethylcalix[4]pyrrole (CP). This receptor
was originally discovered by Baeyer17 in 1886 but only
recently its anion binding properties were recognized.18
The main advantages of its use in anion recognition and
sensing are its easy and quick synthesis,19–23 associated
with the fact that functionalized anion sensors can be
Scheme 1.
Experimental
General
All chemicals used were high-purity commercial
reagents. The deionized water used in the measurements

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Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
was boiled and bubbled with nitrogen and kept in a nitrogen
atmosphere to avoid the presence of carbon dioxide.
Acetonitrile (Sigma-Aldrich, HPLC grade) was dried
with calcium hydride, distilled and stored over molecular
sieves (Sigma-Aldrich, 4 Å) as previously described.27
Karl-Fischer titrations were performed with this solvent and
demonstrated the presence of water in a concentration of
7.11 × 10–3 mol L–1 (0.016%). Tetra-n-butylammonium
hydroxide was purchased from Sigma-Aldrich. All anions
(HSO4–, H2PO4–, NO3–, CN–, CH3COO–, F–, Cl–, Br– and
I–) were used as tetra-n-butylammonium salts with purity
greater than 97-99%. The anions were purchased from
Fluka (F–, > 97%; Cl–, > 98%; NO3 , > 97%; H2PO4 ,
> 97%), Vetec (Br–, > 99%; I–, > 99%; HSO4–, > 99%) and
Sigma-Aldrich (CH3COO–, > 97%) and were dried
over phosphorous pentoxide under vacuum before use.
Karl-Fischer experiments were performed for the following
tetra-n-butylammonium salts in order to determine the
content of water in each salt: CN– (0.12% water), F– (1.13%
water), H2PO4– (0.11% water) and CH3COO– (0.07% water).
to ensure the solubilization of the reactants and then two
drops of acetic acid were added. The color of the solution
changed from yellow to dark orange. The reaction mixture
was stirred at room temperature for 2 h and then left to
stand overnight. The resulting solid was filtered, washed
with cold ethanol, recrystallized from methanol and dried;
yield 52%; mp 168.8 °C (176 °C);28 1H NMR (400 MHz,
CDCl3) d 5.07 (s, 1H), 6.90 (d, 2H, J 8.4 Hz) 7.26 (d, 2H,
J 8.4 Hz), 8.06 (d, 2H, J 8.6 Hz), 8.32 (d, 2H, J 8.6 Hz),
8.57 (s, 1H); IR (KBr) nmax/cm–1 3446 (O–H), 1597 (C=N),
1508, 1338 (N=O), 1450 (C=C).
–
–
Synthesis of CP
This compound was prepared through the adaptation
of procedures described in the literature.21,22 A solution
of 3.1 mL (44.7 mmol) of freshly distilled pyrrole in
anhydrous ethanol was added to a three-necked flask
coupled to a reflux condenser. This system was placed
in an ice bath and over a period of 10 min, 0.2 mL
(1.87 mmol) of triflic acid was added dropwise to the
stirred solution. A volume of 3.3 mL (44.7 mmol) of
acetone was then added. At the end of the addition, a
white solid precipitated and the stirring was stopped.
The reaction mixture was left at room temperature for
1 h and the white solid was then crushed in ethanol and
filtered under vacuum. The product was three times
recrystallized in a 1:1 (v/v) ethanol:acetone mixture; yield
2.50 g, 47.5%; mp 298 °C (296 °C);21 1H NMR (400 MHz,
DMSO-d6) d 1.51 (s, 24H), 5.68 (d, 8H, J 2.2 Hz), 9.26
(bs, 4H, NH); IR (KBr) nmax/cm–1 3444 (N–H), 2973, 2932,
2869 (C–H), 1578, 1506, 1448, 1414 (C–C and C–N).
Instrumentation
UV-Vis experiments were carried out on a Varian Cary
Bio 50 and HP 8452A spectrophotometers equipped with
a thermostated bath and all measurements were performed
at 25 °C, employing a 1-cm quartz cuvette. The maximum
wavelengths (lmax) of the UV-Vis spectra were calculated
from the first derivative of the absorption spectrum.
Melting points were obtained on a Kofler hot stage and
were uncorrected. The nuclear magnetic resonance (NMR)
spectra were recorded on a Varian AS-400 spectrometer.
Chemical shifts were recorded in ppm with the solvent
resonance as the internal standard. Data are reported
as follows: chemical shift, multiplicity (s = singlet,
d = doublet and bs = broad singlet), integration and coupling
constants (Hz). Infrared (IR) spectra were obtained on
a Shimadzu model Prestige-21 spectrophotometer, with
KBr pellets.
pKa value for 2a in aqueous solution
A solution of 2a was prepared in a concentration
of 4 × 10–3 mol L–1 in CHCl3, and stored in glass flasks
closed with rubber stoppers to avoid the evaporation of
the solvent. An aliquot of the solution, sufficient to give
a concentration of the compound of 1 × 10–5 mol L–1,
was collected with a microsyringe and placed in a flask.
After the evaporation of the solvent, water at pH 2.22
(adjusted with 0.1 mol L–1 HCl) was added and the UV-Vis
spectrum of the compound was recorded at 25 oC. The pH
of the solution was measured and the UV-Vis spectrum
was obtained after each addition of a small amount of
0.1 mol L–1 KOH until pH 10-12.
Synthesis of the compounds
Compound 1a was synthesized and purified as described
in the literature.14
Synthesis of 2a
This compound was synthesized following a procedure
adapted from the literature.28 4-Nitrobenzaldehyde (1.51 g;
10 mmol) and 4-aminophenol (1.09 g; 10 mmol) were
stirred in ethanol (20 mL) in a round-bottomed flask. At
the beginning the reaction, the mixture was slowly heated
UV-Vis study of 2a with the anions
A solution of 2a was prepared in acetonitrile in a
concentration of 5.0 × 10–5 mol L–1. This solution was then

Vol. 23, No. 8, 2012
Nicoleti et al.
1491
used to prepare the solution of each anion in a concentration
of 6.0 × 10–4 mol L–1, using 5 mL volumetric flasks.
Subsequently, these solutions were transferred to cuvettes
hermetically closed with rubber stoppers in order to minimize
the evaporation of the solvent and to avoid the entrance of
water to the system, and the UV-Vis spectra were obtained.
The experiments involving acetonitrile-water mixtures
were carried out using the following procedure. Solutions
of 2a containing each anion were prepared in acetonitrile,
as described in the previous paragraph. Small volumes of
water were then added to each system, giving a positive
response, that is, a change in the color of the acetonitrile.
The UV-Vis spectra were obtained in order to evaluate
whether this amount was sufficient to allow selective
detection with the naked eye. The absorbance values were
collected for the lmax values verified for each mixture.
methanolic solution were added. This very small amount
of added methanol did not interfere with the UV-Vis band
of 2b. This solution was used in the preparation of a stock
solution of 2.0 × 10–3 mol L–1 CP. Small aliquots of this
last solution were added to a quartz cuvette thermostated
at 25 °C, closed with a rubber septum and containing
1 mL of the solution of 2b. UV-Vis spectra were obtained
after each addition of the CP solution and the absorbance
values were registered at 456.0 nm. A similar experiment
was performed in order to study the interaction of 1b
with CP. In this case, the stock solutions contained
4.0 × 10–5 and 4.0 × 10–4 mol L–1 of 1b and CP, respectively.
All experiments were carried out in duplicate and the
reproducibility of the data was confirmed.
UV-Vis studies on the interaction of CP-2b with the anions
Titration of 2a with the anions
A solution of 2b (4.0 × 10–5 mol L–1) in acetonitrile was
prepared as described above. This solution was used in the
preparation of the CP-2b complex, with a CP concentration
of 1.2 × 10–4 mol L–1. This CP-2b solution was used to
prepare solutions of each anion with a concentration of
6.0 × 10–4 mol L–1 and UV-Vis spectra were obtained.
For the titrations, stock solutions containing F– (1.5 × 10–3
mol L–1) and HSO4– (2.0 × 10–3 mol L–1) were prepared
from the initial CP-2b solution. Small amounts of the
stock solution of each anion were added to a quartz cuvette
thermostated at 25 °C, closed with a rubber septum and
containing 1 mL of the CP-2b solution. UV-Vis spectra were
collected after each addition of the anion and the absorbance
values were registered at 547.0 nm for F– and at 372.0 nm
for HSO4–.All experiments were carried out in duplicate and
the reproducibility of the data was confirmed.
Titration experiments in acetonitrile were performed with
the preparation of the solution of 2a, as previously described.
This solution was used to prepare the stock anion solutions in
flasks that were closed with rubber stoppers and the titrations
were carried out by adding small amounts (2-50 mL) of the
salt solution with a microsyringe to closed quartz cuvettes
containing the solution of 2a. The UV-Vis spectra were
obtained after each addition and the absorbance values
were collected at 547.0 nm. Titration experiments were also
performed in acetonitrile-water systems, using the minimum
water content which allowed selective detection of the anion,
defined in the previous UV-Vis studies.
The experiment for the stoichiometry determination
was performed in acetonitrile with the method of
continuous variations (Job’s method). This plot was
obtained considering the formation of 2b produced from
2a and CN−. Thus, this study was performed at 25 °C using
stock solutions in the experiment with concentrations of
6.0 × 10–5 mol L–1 for 2a and 7.0 × 10–5 mol L–1 for CN− and
the absorbance values were read at lmax = 547.0 nm.
Titration of 2b with HSO4–
A solution of 2b was prepared in acetonitrile in a
concentration of 4.0 × 10–5 mol L–1, after which the
compound was deprotonated (see above). This solution was
used to make a 2.0 × 10–3 mol L–1 stock solution containing
the anion. Subsequently, small aliquots of the anion stock
solution were added to a quartz cuvette thermostated at
25 °C, closed with a rubber septum and containing 1 mL
of the 2b solution. UV-Vis spectra were obtained after
each addition of the anion and the absorbance values were
registered at 372.0 nm.
UV-Vis studies on the interaction of CP with 2b
The following procedure was used for the study of the
interaction of CP with 2b. A 2.94 × 10–2 mol L–1 stock
solution of 2a was prepared in anhydrous ethanol. From
this stock solution, 34 mL were transferred to a 25 mL
volumetric flask. After the evaporation of the ethanol, the
probe was dissolved in acetonitrile, resulting in a solution
with a final concentration of 4.0 × 10–5 mol L–1. In order
to fully deprotonate the compound, 20 mL of a 0.2 mol L–1
tetra-n-butylammonium hydroxide (Sigma-Aldrich)
Calculations
The binding constants were calculated using the
Origin 6.1 program.

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Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
Results and Discussion
compound in acetonitrile has a band with a maximum at
372.0 nm (emax = 1.63 × 104 L mol–1 cm–1) and this band,
under deprotonation, disappears simultaneously with the
appearance of another band with a maximum at 547.0 nm
(emax = 2.08 × 104 L mol–1 cm–1). In the same manner, as can
be observed, in the presence of the anions used in this study,
in a concentration almost 12 times greater than that of 2a,
only F–, CN–, CH3COO– and H2PO4– led to the appearance
of bands in the visible region. F– and CN– are responsible
for the appearance of the band at 547.0 nm, corresponding
to the formation of 2b. In addition, the spectrum of 2a
with F– almost coincides with that of 2b (formed by the
addition of hydroxide to the solution of 2a in acetonitrile).
CH3COO– and H2PO4– interact with the compound to form
complexes through HB, which explains the appearance of
a band at close to 400 nm. Thus, analysis of the spectral
effect of each anion gives the following decreasing order:
F– > CN– > CH3COO– > H2PO4–. All other anions used
were unable to deprotonate the compound in solution or
to cause the appearance of an intermediate band typical of
hydrogen-bonded species.
An anionic chromogenic chemosensor based on an
acid-base strategy
Figure 1 shows the behavior of solutions of 2a in
acetonitrile in the presence of various anions. Solutions
of this compound are colorless and become violet upon
deprotonation using tetra-n-butylammonium hydroxide.
When several anions (HSO4–, H2PO4–, NO3–, CN–, CH3COO–,
F–, Cl–, Br– and I–) are individually added to solutions of 2a,
only F– and CN– are responsible for the appearance of a violet
color in the solutions, typical of solutions of 2b. In addition,
CH3COO– and to a lesser extent H2PO4– cause changes in
the color of 2a solutions (to yellow), suggesting that these
less basic anions interact with the protonated compound
through HB. Figure 1 also shows that the addition of small
water volumes to the solutions of 2a changes the capability
of the anions to color the solutions: with the addition of
1.4% of water only CN– causes the deprotonation of 2a.
It has been recently demonstrated that the selectivity of
chromogenic chemosensors in solution toward CN– in the
presence of other anions can be obtained with the addition
of small amounts of water to organic solvents.12–14,29,30
This is explained by the fact that the hydration energies of
F– (−465 kJ mol−1), CH3COO– (−365 kJ mol−1) and H2PO4–
(−465 kJ mol−1) are high in comparison with that obtained
for CN– (−295 kJ mol−1).31 The strong hydration of the
anions makes them less able to act as a base. Since CN– is
less hydrated with the addition of water, this more basic
species (in comparison with the others) is consequently
more efficient in the abstraction of the proton of 2a.
Figure 2 shows the UV-Vis spectra of 2a in acetonitrile
in the absence and in the presence of added anions. This
Figure 2. UV-Vis spectra for solutions of (a) 2a, (b) 2b and 2a in the
presence of (c) F–, (d) CN–, (e) CH3COO– and (f) H2PO4– in acetonitrile and
acetonitrile with 1.0 and 1.4% (v/v) of water. For concentrations of 2a and
the anions, see Figure 1.
The pKa value for 2a in water was determined as
10.57 0.01. However, it would be of interest to estimate
the acidity of this compound in acetonitrile. A list of pKa
values for various phenols in acetonitrile was obtained
from the literature.32,33 The data were plotted as a function
of the pKa for these phenols in water (see Supplementary
Information (SI) section) and a linear correlation
was observed (r2 > 0.99). The equation obtained
(pKa (CH3CN) = 1.68 pKa (water) + 9.80) was used to
estimate (employing the pKa experimentally determined
in water) the pKa value for 2a in acetonitrile, which
is equal to 27.6. The use of the pKa for Reichardt’s
Figure 1. Solutions of (a) 2a, (b) 2b and 2a in the presence of (c) HSO4−,
(d) H2PO4−, (e) NO3−, (f) CN−, (g) CH3COO−, (h) F−, (i) Cl−, (j) Br− and
(k) I− as tetra-n-butylammonium salts in acetonitrile (A) and acetonitrile
with 1.0 (B) and 1.4% (C) of water. The concentration of each anion was
6.0 × 10–4 mol L–1 and of 2a was 5.0 × 10–5 mol L–1.

Vol. 23, No. 8, 2012
Nicoleti et al.
1493
pyridiniophenolate in water (8.6) in the same equation
gives a value of 24.2, which is comparable to the value
experimentally obtained by Coleman and Murray33 in
acetonitrile (22.1). The pKa values for HBr, H2SO4 and
HCl in acetonitrile are 6.6, 8.7 and 10.6, respectively,34
being of very small magnitude in comparison to the
phenolate moiety of 2b. These results are consistent with
data reported in the literature for dyes with a phenolate
group in their structure.14,39
–
pKa of 2a, showing that the anions Br–, HSO4 and
Cl– are not sufficiently basic to interact with 2a. Due to
the practical difficulties associated with the titration of
acids in acetonitrile,35,36 the pKa values for compounds
such as HCN and HF are not available in the literature.
These values can be only estimated from a plot of the
pKa values for various acids in acetonitrile as a function
of the corresponding pKa values in dimethylsulfoxide
(DMSO)37 (see SI section). This approach led to values
of 25.2 for HF and 23.1 for HCN. The pKa value
experimentally determined for acetic acid in acetonitrile
of 22.338 (the value calculated using the approach shown
in the SI section is 22.5) is below the pKa values for
HF and HCN. Although these calculations can be seen
as a simplification, they match the observed order for
the effect of the anions on the UV-Vis spectrum of 2a.
In addition, the experimental data are in good agreement
with those obtained in a recent study carried out in
DMSO, in which 2,4-dinitrodiphenylamine was used as
the chemosensor, and the spectral effect of each anion
followed the decreasing order: F– > CN– > CH3COO– >
H2PO4–.30 This is the same order observed in this study and
it was also observed that the pKa values for the conjugated
acids of the anions that interact with the compound exactly
match the capability of the anions to abstract the proton
of the NH group in that chemosensor.30
Figure 2 also shows the influence of the addition of
small volumes of water on the UV-Vis spectrum of 2a in
the presence of the anions. It can be observed that with the
addition of 1.4% (v/v) of water to the system, only CN–
could fully deprotonate the compound, while practically
no effect was observed with the other anions. In addition,
the highest absorbance values for 2a in acetonitrile were
obtained in the presence of F–, but a different behavior for
this compound emerged with the addition of very small
amounts of water, with a very high selectivity for CN– being
obtained, in comparison with the other anions. Another
interesting feature observed in these experiments is that
with the addition of 1.0 and 1.4% (v/v) of water to the
solutions of 2b in acetonitrile hypsochromic shifts of 41 and
46 nm, respectively, were observed in the position of the
UV-Vis band. Therefore, 2b exhibits a significant negative
solvatochromism, and in mixtures of acetonitrile with very
small volumes of water, the dye is preferentially solvated
by water, through HB involving water molecules and the
Figure 3. (a) Influence of the addition of increasing amounts of CN− on
the UV-Vis spectra of 2a (5.0 × 10−5 mol L–1) in acetonitrile at 25 °C.
The final concentration of CN− was 4.18 × 10−4 mol L–1; (b) curve of
the variation in the absorbance at 547.0 nm of 2a with the addition of
increasing amounts of CN−.
Compound 2a was titrated in acetonitrile with CN– and
the experiment was monitored through the analysis of the
UV-Vis spectra (Figure 3a). With the addition of the anion,
the band related to compound 2a at 372.0 nm has a reduced
intensity. Simultaneously, the band with lmax at 547.0 nm
(related to 2b) has increased intensity, while an isosbestic
point occurs at 413.0 nm. A plot of the absorbance
values at 547.0 nm as a function of c(CN–) (Figure 3b)
shows a behavior typical of a 1:1 2a:CN– stoichiometry.
The experimental data were fitted with the use of
equation 1,12-14,40,41 where Abs is the absorbance value after
each addition of the anion, Abs0 is the initial absorbance
without anion added, Abs11 is the maximum absorbance
value obtained by addition of the anion considering 1:1
2a:anion stoichiometry, c(A–) is the anion concentration
in each addition, and K11 is the binding constant.
Abs = [Abso + Abs11 K11 c(A–)]/[1 + K11 c(A–)]
(1)
With the use of equation 1, a value of
K11 = (8.54 0.21) × 103 L mol–1 (standard deviation,
SD = 5.0 × 10–5) was obtained. The Job’s plot40,42 for the
system involving CN– and 2a in acetonitrile at 547.0 nm is
shown in Figure 4, which confirms the 1:1 stoichiometry.
The addition of water to the system leads to a reduction in
the magnitude of the binding constant. The UV-Vis spectra
for the titration of 2a with CN–, in acetonitrile with 1.4%
(v/v) of water added, show that on the addition of the anion
the band with lmax at 372.0 nm disappears while the band
related to the deprotonated dye (with lmax at 504.0 nm)

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Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
at 403.0 nm. In the second part of the titration (Figure 5c),
absorbance values at 420.0 nm decrease while the band
related to the appearance of 2b increases in intensity, with
the occurrence of an isosbestic point at 465.0 nm. This
behavior is typical of a situation in which the protonated
chemosensor interacts firstly by means of HB with the anion.
In the presence of an excess of the anion, the compound is
deprotonated with the formation of a [HA2]– complex.12,13
This is corroborated by the fact that the experimental data
used to obtain the titration curve could not be fitted using
equation 1, but only with equation 2, which considers both
1:1 and 1:2 2a:anion stoichiometries.
(2)
Figure 4. Job plot of 2a with CN− in acetonitrile (for details see
Experimental section).
In this equation, Abs12 is the maximum absorbance
value obtained by addition of the anion considering
1:2 stoichiometry and K12 is the respective binding
constant. The fitting of the experimental data using
equation 2 gave K11 = (4.86 1.24) × 104 L mol–1 and
K12 = (6.45 0.19) × 103 L mol–1 (SD = 1.4 × 10–5).
appears, an isosbestic point also occurring at 416.0 nm. The
titration curve for this system reveals that the stoichiometry
is also 1:1, with a value of K11 = (2.93 0.03) × 103 L mol–1
(SD = 8.0 × 10–6).
Figure 5a shows UV-Vis spectra for the titration of
2a in acetonitrile using F– as anion. The data show that
with the addition of the anion, as observed in the case
of the titration with CN– in acetonitrile, the band with
lmax = 372.0 nm decreases in intensity while for the band
with lmax = 547.0 nm the intensity increases. However, no
apparent isosbestic point is now observed: the spectra suggest
that with the addition of the anion, firstly an increase in the
absorbance values in the region between 400 and 450 nm
occurs. This is better observed in Figure 5b, which details the
first part of the titration, showing that the band with lmax at
372.0 nm decreases with an increase in the absorbance in the
range of 420.0-450.0 nm, an isosbestic point being observed
The results obtained show three possible interactions
for compound 2a in acetonitrile, as presented in Scheme 2,
depending on the basicity of the anion. If the anion is
a strong base, the proton in the phenolic moiety of the
compound is abstracted, and a violet solution is obtained.
Another possibility discussed in some publications in the
literature43 involves firstly the formation of a complex
between the anion and the protonated dye through HB,
which weakens the H–O bond in the compound. The
abstraction of the proton occurs with the addition of a
second anion equivalent, leading to the formation of a
[HA2]– complex. The formation and stability of these
species, for example [HF2]–, have been studied through
Figure 5. (a) Influence of the addition of increasing amounts of F− on the UV-Vis spectra of 2a (5.0 × 10−5 mol L–1) in acetonitrile at 25 °C. The final
concentration of F− was 9.02 × 10−4 mol L–1; (b) the first step of this titration from zero to 1.85 × 10−4 mol L–1 and (c) the last step from 3.58 × 10−4 mol L–1
to 6.11 × 10−4 mol L–1.

Vol. 23, No. 8, 2012
Nicoleti et al.
1495
Scheme 2. Possible interactions of 2a with the anions.
theoretical calculations.44 The addition of water to the
medium leads to a reduction in the magnitude of the binding
constant due to the fact that it interacts with the anion,
lowering its ability to act as a base. With less basic anions,
such CH3COO– and H2PO4–, hydrogen-bonded complexes
of 2a and the anion are formed, as indicated by the yellow
coloration obtained in solution.
The titration of 2a with the anions revealed a 1:1
2a:anion stoichiometry for CN– and both 1:1 and 1:2
stoichiometries for F–. A previous study carried out with
compound 1a in acetonitrile led to both 1:1 and 1:2
stoichiometries for CN– and 1:2 1a:anion stoichiometry for
F–.14 Recently, Reichardt’s pyridiniophenolate and Brooker’s
merocyanine, compounds with the pyridinium cation as an
acceptor center in their molecular structure, were studied
in their protonated form as anionic chemosensors in
acetonitrile and in trichloromethane.12,13 Titrations with
basic anions led to a 1:3 chemosensor:anion stoichiometry,
which was attributed to the formation of a complex between
the pyridinium group and the anion, of an electrostatic
nature, prior to the interaction of the anion with the phenol
moiety in the compound.12,13 Since the pyridinium group
is absent from the molecular structure of 1a and 2a, these
results represent important evidence to support the model
suggested for the interaction of anionic species with
protonated merocyanines.
An anionic chromogenic chemosensor based on a
displacement assay
Compound 2a deprotonated in acetonitrile gives a
violet color to the solution. If the receptor CP is added
to the solution of the dye formed (2b), the color of the
solution immediately changes to pale-yellow. This is due
to the formation of a complex between CP and 2b through
HB involving the oxygen in the phenolate group of the
dye and the NH groups of CP.25,26 Figure 6a shows a set
of UV-Vis spectra for the titration of 2b in acetonitrile
with small amounts of CP. While the intensity of the
band with lmax at 547.0 nm for the dye is reduced with
the addition of CP, other band related to the complex
formed between CP and 2b appears simultaneously with
Figure 6. (a) UV-Vis spectra of 2b (4.0 × 10–5 mol L–1) in acetonitrile at 25 ºC after addition of increasing amounts of CP. The final concentration of CP
was 1.2 × 10–4 mol L–1; (b) mole-ratio plot for the interaction of CP with 2b.

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Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
l
max at 456.0 nm. An isosbestic point occurs at 500.0 nm,
Figure 8A shows the effect of the individual addition of
different anions (F–, Cl–, Br–, I–, CN–, H2PO4–, HSO4– and
NO3 ) as tetra-n-butylammonium salts to the CP-2b
suggesting equilibrium between the species in solution.
The absorbance values were obtained at 456.0 nm for
each c(CP), giving the mol-ratio plot shown in Figure
6b, which clearly shows a 1:1 CP-2b stoichiometry.
The addition of very small amounts of water to CP-2b
in acetonitrile disrupts the complex due to the ability
of water to strongly interact with CP and 2b through
hydrogen bonding. In addition, if 1b is titrated with CP
in acetonitrile, no changes occur in the UV-Vis spectrum
of the dye presumably because the phenyl groups on the
2,6-positions are responsible for a steric blocking that
hinders the HB between the NH groups on CP and the
oxygen in the phenolate group.
–
solution. Interestingly, only F– causes the reappearance
of the violet color and in addition HSO4– makes the CP-
2b solution colorless, allowing the visual detection of
these anions, distinguishing them from between the other
species studied. Figure 8B shows the UV-Vis spectra for
the CP-2b solutions in the presence of each anion. The
spectra show two types of spectral effects: one caused by
F– (followed in a much lesser extent by Cl–), involving a
reduction in the absorbance at 456.0 nm (related to CP-2b)
with the simultaneous reappearance of the band of 2b at
547.0 nm. The data show that F– strongly binds to CP and
Figure 7a shows the treatment of the data using the
Benesi-Hildebrand equation (equation 3), which was
used in order to estimate the initial values of De11 and K11
to be subsequently used in the non-linear fit.40,45,46 In this
equation, b is the path length, c(2b) is the dye concentration,
De11 is difference in the molar absorptivity values for the
complex and free dye and K11 is the binding constant for
a 1:1 complex formation. DA is the change in absorbance
at 547.0 nm for the solution containing the dye when the
CP concentration changes from zero to c(CP). The fit
of the experimental data gives K11 = 1.85 × 104 L mol–1
(r2 = 0.999, SD = 4.4 × 10–3). Figure 7b shows the titration
curve for 2b with CP, which was used to give the binding
constant for CP-2b, applying equation 4,40,45 providing
K11 = (1.85 0.06) × 104 L mol–1 (SD = 2.0 × 10–5):
b/DA = 1/c(2b) K11 De11 c(CP) + 1/c(2b) De11
DA/b = (2b) K11 De11 c(CP)/[1 + K11 c(CP)]
(3)
(4)
Figure 8. (A) Solutions in acetonitrile, (B) UV-Vis spectra and (C) relative
absorbances of (a) 2b, (b) CP, (c) CP-2b, and CP-2b in the presence
of (d) HSO4–, (e) H2PO4–, (f) NO3–, (g) CN–, (h) F–, (i) Cl–, (j) Br– and
(k) I– (c(anion) = 6.0 × 10–4 mol L–1). The concentrations of CP and 2b
were 1.2 × 10–4 and 4.0 × 10–5 mol L–1, respectively.
Figure 7. (a) Benesi-Hildebrand treatment for the titration of 2b and
CP in acetonitrile at 25 ºC, fitted using equation 3; (b) titration curve of
2b and CP in acetonitrile at 25 ºC. Absorbance values were collected at
456.0 nm and the data were fitted using equation 2.

Vol. 23, No. 8, 2012
Nicoleti et al.
1497
displaces 2b from the receptor site, making the dye free to
signal the presence of the analyte. HSO4– and to a minor
extent H2PO4– are responsible for another spectral effect that
is a reduction in the intensity of the band related to CP-2b
simultaneously with the appearance of a band with lmax at
372.0 nm. The latter coincides with the band observed for
the solutions of 2a in acetonitrile, meaning that HSO4– is
sufficiently acidic to protonate 2b, which is responsible for
breaking the CP-2b complex.
constant for CP and F– to be estimated 1.09 × 105 L mol–1,
which is a value comparable with that previously obtained
by Schmidtchen48 using calorimetric measurements.
–
The effect of the addition of HSO4 on the UV-Vis
spectrum for CP-2b in acetonitrile at 25 ºC can be seen in
Figure 10a, which shows a decrease in the band related to
CP-2b at 456.0 nm with the simultaneous occurrence of the
band related to the formation of the protonated dye (2a), with
lmax at 372.0 nm. Figure 10b represents the corresponding
Figure 9a shows UV-Vis spectra corresponding to the
titration of CP-2b in acetonitrile with increasing amounts
of F–. On addition of F–, a reduction in the intensity of the
CP-2b band at 456.0 nm occurs with the simultaneous
appearance of the band for 2b at lmax = 547.0 nm, which
indicates that 2b is displaced from the binding site of
CP by F–. The titration curve in Figure 9b was obtained
using the absorbance values at 547.0 nm as a function of
c(F–). The sigmoidal profile of the curve is typical of that
occurring in competition assays, revealing the competition
of the anion for free CP and CP-2b in the solution. The
treatment of the experimental data40,47 allowed the binding
titration curve obtained from the absorbance values at 372.0
nm for each addition of the anion. The data could not be fitted
using equations 2 and 3 and, in addition, no isosbestic point
occurs in the set of UV-Vis spectra. Data suggest that HSO4–
is sufficiently acid to protonate the dye. This finding was
verified by carrying out the titration of the dye (2b) with the
anion. Figure 11a shows the UV-Vis spectra for the titration
of 2b by HSO4– in acetonitrile at 25 ºC, and a decrease in the
band related to 2b with the simultaneous appearance of the
band related to the formation of 2a was observed. Figure 11b
shows the corresponding titration curve, which was fitted
using equation 3, giving K11 = (5.32 0.09) × 103 L mol–1
Figure 9. (a) UV-Vis spectra of CP-2b in acetonitrile at 25 ºC after addition of increasing amounts of F–; (b) titration curve of CP-2b with F– in acetonitrile.
Absorbance values were collected at 547.0 nm, c(CP) = 1.2 × 10–4 mol L–1 and c(2b) = 4.0 × 10–5 mol L–1.
Figure 10. (a) UV-Vis spectra of CP-2b in acetonitrile at 25 ºC after addition of increasing amounts of HSO4-; (b) titration curve of CP-2b with HSO4– in
acetonitrile. Absorbance values were collected at 372.0 nm, c(CP) = 1.2 × 10–4 mol L–1 and c(2b) = 4.0 × 10–5 mol L–1.

1498
Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
Figure 11. (a) UV-Vis spectra of 2b (4.0 × 10–5 mol L–1) in acetonitrile at 25 ºC after addition of increasing amounts of HSO4–; (b) titration curve of 2b
with HSO4– in acetonitrile, with the absorbance values collected at 372.0 nm.
Scheme 3. A chromogenic chemosensor based on a displacement assay.
(SD = 3.95 × 10–6). The absence of an isosbestic point in
Figure 10a and the fact that the corresponding titration curve
could not be mathematically fitted indicate that the anion
acts not only by protonating 2b but is also able to interact
with CP and CP-2b.
of a strong hydrogen-bonded complex comprising CP and
the anion. The change in the color of the solution, from
orange to violet, which accompanies the addition of F–,
should be used for the visual detection of the anion.
The displacement assay studied is summarized in the
Scheme 3. The first step involves the formation of a 1:1 HB
complex, formed by CP and 2b. The addition of F– to this
complex causes the displacement of 2b due to the formation
Conclusions
It was demonstrated herein that compound 2a can be
used in solution for the highly selective detection of F– or

Vol. 23, No. 8, 2012
Nicoleti et al.
1499
CN–, depending on the strategy used for anion sensing.
In its protonated form, this compound behaves as an
efficient anionic chromogenic chemosensor exploiting the
differences in the basicity of the anions. A high selectivity
of CN– in relation to the other anions studied was obtained
on addition of very small amounts of water to solutions
of 2a in acetonitrile. The preferential solvation through
Moragues, M. E.; Martínez-Mañez, R.; Sancenón, F.; Chem.
Soc. Rev. 2011, 40, 2593;Wenzel, M.; Hiscock, J. R.; Gale, P.A.;
Chem. Soc. Rev. 2012, 41, 480.
2. For reviews, see for instance: Cametti, M.; Rissanen, K.; Chem.
Commun. 2009, 2809; Wade, C. R.; Broomsgrove, A. E. J.;
Aldridge, S.; Gabbai, F. P.; Chem. Rev. 2010, 110, 3958.
3. Faibish, D.; Ott, S. M.; Boskey, A. L.; Clin. Orthop. Relat.
Res. 2006, 28; Ayoob, S.; Gupta, A. K.; Crit. Rev. Environ. Sci.
Technol. 2006, 36, 433; Miretzky, P.; Cirelli, A. F.; J. Fluorine
Chem. 2011, 132, 231.
–
HB of H2PO4 , CH3COO– and F– occurs by water or
hydrogen-bonded acetonitrile-water complexes, leaving
the CN– free to interact with the chemosensor.
It was also shown that if the compound is used in its
deprotonated form it can be recognized by CP, which
results in a change in the color of the system. Since F– has
a very high binding constant for CP in comparison with
that of the dye, the anion is able to displace 2b from the
complexation site, which leaves the dye free to restore
the original color of the solution. Another interesting
4. Lv, J.; Zhang, Z. J.; Li, J. D.; Luo, L. R.; Forensic Sci. Int. 2005,
148, 15 ; Nelson, L. ; J. Emerg. Nurs. 2006, 32, S8 ; Zelder,
F. H.; Männel-Croisé, C.; Chimia 2009, 63, 58.
5. Agatemor, C.; EJEAFChe. 2009, 8, 189.
6. Dale, T. J.; Rebek, J.; J. Am. Chem. Soc. 2006, 128, 4500;
Royo, S.; Martínez-Máñez, R.; Sancenón, F.; Costero, A. M.;
Parra, M.; Gil, S.; Chem. Commun. 2007, 4839; Burnworth, M.;
Rowan, S. J.; Weder, C.; Chem. Eur. J. 2007, 13, 7828.
7. Yang,Y.-C.;Bakerand,J.A.;Ward,J.R.;Chem.Rev.1992,92,1729.
8. See for instance: He, S.; Iacono, S. T.; Budy, S. M.; Dennis,A. E.;
Smith Jr., D. W.; Smith, R. C.; J. Mater. Chem. 2008, 18, 1970;
Dale, T. J.; Rebek, J.; Angew. Chem., Int. Ed. 2009, 48, 7850;
Gotor, R.; Costero,A. M.; Gil, S.; Parra, M.; Martínez-Mañez, R.;
Sancenón, F.; Chem. Eur. J. 2011, 17, 11994; Royo, S.; Costero,
A. M.; Parra, M.; Gil, S.; Martínez–Mañez, R.; Sancenón, F.;
Chem. Eur. J. 2011, 17, 6931.
–
feature is that with the addition of HSO4 , the solution
becomes colorless because it protonates the dye. This
allows, in principle, the use of 2b in organic solvents for
–
the selective detection of HSO4 in the presence of other
anionic species.
Supplementary Information
The UV-Vis spectra, titration curves, association
constant determined by displacement titration, results
of studies on the stability of 2a in acetonitrile, pKa
determination for 2a and pKa estimates in acetonitrile are
available free of charge at http://jbcs.sbq.org.br as PDF file.
9. Steiner, T.; Angew. Chem. 2002, 114, 50; Angew. Chem., Int.
Ed. 2002, 41, 48.
10. Lee, D. H.; Lee, K. H.; Hong, J.-I.; Org. Lett. 2001, 3, 5;
Tong, H.; Zhou, G.; Wang, L.; Jing, X.; Wang, F.; Zhang, J.;
Tetrahedron Lett. 2003, 44, 131; Zhang, X.; Guo, L.; Wu, F.-Y.;
Jiang,Y.-B.; Org. Lett. 2003, 5, 2667; Zhang, X.; Shiraishi,Y.;
Hirai, T. Tetrahedron Lett. 2007, 48, 8803.
Acknowledgements
The financial support of the Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq) and
UFSC are gratefully acknowledged.
11. Reis, D. C.; Machado, C.; Machado, V. G.; Tetrahedron Lett.
2006, 47, 9339.
12. Zimmermann-Dimer, L. M.; Machado,V. G.; Dyes Pigm. 2009,
82, 187.
References
13. Zimmermann-Dimer, L. M.; Reis, D. C.; Machado, C.;
Machado, V. G.; Tetrahedron 2009, 65, 4239.
1. For recent reviews, see for instance: Martínez-Mañez, R.;
Sancenón, F.; Chem. Rev. 2003, 103, 4419; Suksai, C.;
Tuntulani, T.; Chem. Soc. Rev. 2003, 32, 192; Suksai, C.;
Tuntulani, T.; Top. Curr. Chem. 2005, 255, 163; Kubik, S.;
Reyheller, C.; Stüwe, S.; J. Inclusion Phenom. Macrocyclic
Chem. 2005, 52, 137; Callan, J. F.; de Silva,A. P.; Magri, D. C.;
Tetrahedron 2005, 61, 8551; Gunnlaugsson, T.; Glynn, M.;
Tocci, G. M.; Kruger, P. E.; Pfeffer, F. M.; Coord. Chem. Rev.
2006, 250, 3094; Anslyn, E. V.; J. Org. Chem. 2007, 72, 687;
Zimmermann-Dimer, L. M.; Machado, V. G.; Quim. Nova
2008, 31, 2134; Cho, D. G.; Sessler, J. L.; Chem. Soc. Rev.
2009, 38,1647; Gale, P.A.; Chem. Soc. Rev. 2010, 39, 3746;
14. Marini, V. G.; Zimmermann, L. M.; Torri, E.; Machado, V. G.;
Arkivoc 2010, xi, 146.
15. For reviews, see: Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.;
Anslyn, E. V.; Acc. Chem. Res. 2001, 34, 963; Nguyen, B. T.;
Anslyn, E. V.; Coord. Chem. Rev. 2006, 250, 3118; Liu, K.;
Su, X.; Huo, J.; Mini-Rev. Org. Chem. 2012, 9, 118.
16. For recent papers, see for instance: Comes, M.; Aznar, E.;
Moraques, M.;Marcos, M. D.;Martínez-Máñez, R.;Sancenón, F.;
Soto, J.; Villaescusa, L. A.; Gil, L.; Amorós, P.; Chem. Eur. J.
2009, 15, 9024; Mullen, K. M.; Davis, J. J.; Beer, P. D.; New
J. Chem. 2009, 33, 769; Carolan, J. V.; Butler, S. J.; Jolliffe,
K. A.; J. Org. Chem. 2009, 74, 2992; Kim, S. Y.; Hong, J. I.;

1500
Anionic Chromogenic Chemosensors Highly Selective for Fluoride or Cyanide
J. Braz. Chem. Soc.
Tetrahedron Lett. 2009, 50, 1951; Saeed, M. A.; Powell, D. R.;
Hossain, M.A.; Tetrahedron Lett. 2010, 51, 4904; Gao, J.; Riis-
Johannessen, T.; Scopelliti, R.; Qian, X. H.; Severin, K.; Dalton
Trans. 2010, 39, 7114; Watchasit, S.; Kaowliew,A.; Suksai, C.;
Tuntulani, T.; Ngeontae, W.; Pakawatchai, C.; Tetrahedron
Lett. 2010, 51, 3398; He, X. A.; Zhang, Q.; Wang, W. T.; Lin,
L. L.; Liu, X. H.; Feng, X. M.; Org. Lett. 2011, 13, 804; Kim,
S. H.; Hwang, I. J.; Gwon, S.Y.; Burkinshaw, S. M.; Son,Y.A.;
Dyes Pigm. 2011, 88, 84; Tang, L. J.; Liu, M. H.; Li, F. F.;
Nandhakumar, R.; J. Fluoresc. 2011, 21, 701.
32. Jover, J.; Bosque, R.; Sales; J. QSAR Comb. Sci. 2007, 26, 385.
33. Coleman, C. A.; Murray, C. J; J. Org. Chem. 1992, 57, 3578.
34. Eckert, F.; Leito, I.; Kaljurand, I.; Kütt, A.; Klamt, A.;
Diedenhofen, M.; J. Comput. Chem. 2009, 30, 799.
35. Coetzee, J. F.; Prog. Phys. Org. Chem. 1967, 4, 45.
36. Kütt, A.; Rodima, T.; Saame, J.; Raamat, E.; Mäemets, V.;
Kaljurand, I.; Koppel, I.A.; Garlyauskayte, R. Y.;Yagupolskii,
Y. L.; Yagupolskii, L. M.; Bernhardt, E.; Willner, H.; Leito, I.;
J. Org. Chem. 2011, 76, 391.
37. Bordwell, F. G.; Acc. Chem. Res. 1988, 21, 456.
38. Kolthoff, I. M.; Chantooni Jr., M. K.; Bhowmik, S.; J. Am. Chem.
Soc. 1968, 90, 23; Kolthoff, I. M.; Chantooni Jr., M. K.; J. Am.
Chem. Soc. 1975, 97, 1376.
17. Baeyer, A.; Ber. Dtsch. Chem. Ges. 1886, 19, 2184.
18. Gale, P.A.; Sessler, J. L.; Král,V.; Lynch,V.; J. Am. Chem. Soc.
1996, 118, 5140.
19. Dennstedt, M.; Zimmermann, J.; Chem. Ber. 1887, 20, 850.
20. Dennstedt, M.; Ber. Dtsch. Chem. Ges. 1890, 23, 1370.
21. Rothemund, P.; Gage, C. L.; J. Am. Chem. Soc. 1955, 77, 3340.
22. Brown, W. H.; Hutchinson, B. J.; MacKinnon, M. H.; Can. J.
Chem. 1971, 49, 4017.
39. Ortega, J.; Ràfols, C.; Bosch, E.; Rosés, M.; J. Chem. Soc.,
Perkin Trans. 2 1996, 1497; Bosch, E.; Rosés, M.; Herodes, K.;
Koppel, I.; Leito, I.; Koppel, I.; Taal, V.; J. Phys. Org. Chem.
1996, 9, 403; da Silva, D. C.; Ricken, I.; Silva, M. A. R.;
Machado, V. G.; J. Phys. Org. Chem. 2002, 15, 420.
40. Connors, K. A.; Binding Constants: the Measurement of
Molecular Complex Stability; Wiley Interscience: New York,
1987;Valeur, B.; Pouget, J.; Bourson, J.; Kaschke, M.; Ernsting,
N. P.; J. Phys. Chem. 1992, 96, 6545.
23. Sobral, A. J. F. N.; J. Chem. Educ. 2005, 82, 618; Shriver,
J. A.; Westphal, S. G.; J. Chem. Educ. 2006, 83, 1330.
24. Sessler, J. L.; Gale, P. A.; Genge, J. W.; Chem. Eur. J. 1998, 4,
1095; Miyaji, H.; Anzenbacher Jr., P.; Sessler, J. L.; Bleasdale,
E. R.; Gale, P. A.; Chem. Commun. 1999, 1723; Miyaji, H.;
Sato, W.; Sessler, J. L.; Lynch, V. M.; Tetrahedron Lett. 2000,
41, 1369; Gale, P. A.; Hursthouse, M. B.; Light, M. E.; Sessler,
J. L.; Warriner, C. N.; Zimmerman, R. S.; Tetrahedron Lett.
2001, 42, 6759.
41. Chen,Y.-L.; Xu, T.-K.; Shen, X.-H.; Gao, H.-C.; J. Photochem.
Photobiol., A 2005, 173, 42.
42. Job, A.; Liebigs Ann. Chem. 1928, 9, 113.
43. Boiocchi, M.; Del Boca, L.; Gómez, D. E.; Fabbrizzi, L.;
Licchelli, M.; Monzani, E.; J. Am. Chem. Soc. 2004, 126,
16507;Amendola,V.; Boiocchi, M.; Fabbrizzi, L.; Palchetti,A.;
Chem. Eur. J. 2005, 11, 120; Boiocchi, M.; Del Boca, L.;
Esteban-Gómez, D.; Fabbrizzi, L.; Licchelli, M.; Monzani, E.;
Chem. Eur. J. 2005, 11, 3097; Wu,Y.; Peng, X.; Fan, J.; Gao, S.;
Tian, M.; Zhao, J.; Sun, S.; J. Org. Chem. 2007, 72, 62; Gómez,
D. E.; Fabbrizzi, L.; Licchelli, M.; Monzani, E.; Org. Biomol.
Chem. 2005, 3, 1495.
25. Gale, P. A.; Twyman, L. J.; Handlin, C. I.; Sessler, J. L.; Chem.
Commun. 1999, 1851.
26. Linn, M. M.; Poncio, D. C.; Machado, V. G.; Tetrahedron Lett.
2007, 48, 4547.
27. Furniss, B. S.; Hannaford, A. J.; Smith, P. W. G.; Tatchell,
A. R.; Vogel’s Textbook of Practical Organic Chemistry, 5th ed.;
Longman: London, 1989, p. 410.
28. Kaya, I.; Culhaoglu, S.; Iran. Polym. J. 2006, 15, 487.
29. Ros-Lis, J. V.; Martínez-Máñez, R.; Soto, J.; Chem. Commun.
2002, 2248; Tomasulo, M.; Raymo, F. M.; Org. Lett. 2005,
7, 4633; Ros-Lis, J. V.; Martínez-Máñez, R.; Soto, J.; Chem.
Commun. 2005, 5260; Sun,Y.; Wang, G.; Guo, W.; Tetrahedron
2009, 65, 3480.
44. Gronert, S.; J. Am. Chem. Soc. 1993, 115, 10258.
45. Venturini, C. de G.;Andreaus, J.; Machado,V. G.; Machado, C.;
Org. Biomol. Chem. 2005, 3, 1751.
46. Benesi, H. A.; Hildebrand, J. H.; J. Am. Chem. Soc. 1949, 71,
2703.
47. Zhong, Z. L.;Anlyn, E. V.; J. Am. Chem. Soc. 2002, 124, 9014;
Kim, S. Y.; Hong, J.-I.; Tetrahedron Lett. 2009, 50, 1951.
48. Schmidtchen, F. P.; Org. Lett. 2002, 4, 431.
30. Marini, V. G.; Zimmermann, L. M.; Machado, V. G.;
Spectrochim. Acta, Part A 2010, 75, 799.
31. Marcus, Y. J. Chem. Soc., Faraday Trans. 1991, 87, 2995;
Dickins, R. S.; Parker, D. In Gloe, K., ed.; Macrocyclic
Chemistry: Current Trends and Future Perspectives; Springer:
Dordrecht, 2005, p. 121.
Submitted: April 16, 2012
Published online: July 12, 2012