An anionic chromogenic sensor based on protonated Reichardt's pyridiniophenolate

Article abstract of DOI:10.1016/j.tetlet.2006.10.109

An anionic sensor based on Reichardt's betaine is described here. The dye is blue-green in chloroform but becomes colorless under protonation. Increasing amounts of different anions were added into the solution of the protonated dye. The addition of F^- and H 2 PO₄^- caused the reappearance of the original blue-green color, while the addition of I^- made the solution of the protonated dye yellow. The observations are discussed based on the fact that F^- and H 2 PO₄^- can act as bases accepting a proton from the protonated dye and also in relation to the formation of a complex between the protonated dye and iodide.

Full text of DOI:10.1016/j.tetlet.2006.10.109

Tetrahedron Letters 47 (2006) 9339–9342
An anionic chromogenic sensor based on protonated
Reichardt’s pyridiniophenolate
Dalci C. Reis, Clodoaldo Machado and Vanderlei G. Machado*
Departamento de Qu ı´ mica, Universidade Regional de Blumenau, FURB, CP 1507, Blumenau 89010-971, SC, Brazil
Received 9 August 2006; revised 19 October 2006; accepted 19 October 2006
Available online 9 November 2006
Abstract—An anionic sensor based on Reichardt’s betaine is described here. The dye is blue-green in chloroform but becomes col-
orless under protonation. Increasing amounts of different anions were added into the solution of the protonated dye. The addition of
ꢀ
ꢀ
ꢀ
2
PO4 caused the reappearance of the original blue-green color, while the addition of I made the solution of the proton-
F
and H
ated dye yellow. The observations are discussed based on the fact that F and H
protonated dye and also in relation to the formation of a complex between the protonated dye and iodide.
2006 Elsevier Ltd. All rights reserved.
ꢀ
ꢀ
PO4 can act as bases accepting a proton from the
2
ꢀ
The recognition and the detection of anionic species is a
field of increasing interest due to the fact that anions
play a fundamental role in many chemical and biological
detect only cationic species, mainly due to the interac-
tion of the cation with the negatively charged phenolate
moiety. Thus, the UV–vis spectrum of dye 1 does not
undergo alterations following the addition of anionic
species.
1
4
1
processes. Much effort is being spent in the develop-
ment of sensors not only to perform selective anion
detection visually, but which also allow the quantifica-
2
–5
tion of such species.
Although the literature reports
Recently, Hong and co-workers reported a fluoride-
selective chromogenic sensor based on the fact that
azophenols can interact with anions through hydrogen
2
,4
many examples of fluorescence anion sensors, in com-
parison, the development of simple and common colori-
metric sensors for anionic species is still limited.
3
–5
15
bonding and on the basicity of the anions. Here, we
wish to show that it is possible to ‘teach a new trick to
the old dye’: its protonation allows it to act as an anionic
sensor, as shown in Scheme 1. Compound 1 was initially
solubilized in chloroform which contained traces of
water and protonated by H CO generated by bubbling
Merocyanine dyes are heterocyclic compounds with
many applications. Reichardt’s betaine, 2,6-diphenyl-
6
4
-(2,4,6-triphenylpyridinium-1-yl)phenolate (1), is a very
7
well-known example of these dyes. Considerable work
has been devoted to this compound since its discovery
nearly four decades ago, due to its solvatochromism
2
3
1
6
CO through it. The protonation made the blue-green
2
8
solution of 1 colorless. Increasing amounts of different
7
(
its UV–vis spectrum changes if the polarity is altered).
The maximum absorption of 1 in different solvents rep-
resents the basis for the Reichardt polarity parameter
7
Ar
Ar
E (30) and this dye has been extensively used as a
F
-
or
PO
T
Ar
+N
Ar
Ar
Ar
9
-
probe, for instance in the study of mixed solvents,
H+
H
2
4
+
+
1
0
11
Ar
Ar
N
Ar
Ar
Ar
Ar
OH
+ HF
or H PO )
microheterogeneity in solution, kinetics, ionic liq-
Ar
Ar
N
Ar
Ar
(
uids,1
2
13
3
4
and solutions containing cyclodextrins.
Although this dye has also been used in the investigation
colorless
1
4
O -
1
I-
O -
of salt solutions, it is well known that it can be used to
Ar
+N
Ar
Ar
1
blue-green
blue-green
OH
Keywords: Anionic sensor; Fluoride; Pyridiniophenolate; Chromogenic
sensor.
I-
Ar
yellow
*
Corresponding author. Tel.: +55 47 3221 6081; fax: +55 47 3221
001; e-mail: gageiro@furb.br
6
Scheme 1.
0040-4039/$ - see front matter ꢀ 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2006.10.109

9340
D. C. Reis et al. / Tetrahedron Letters 47 (2006) 9339–9342
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
)
anions (F , Cl , Br , I , H
2
PO4 , HSO4 , and NO3
1.3
1.2
1.1
1.0
0.9
were then added to the colorless solution of the proton-
1
7
ꢀ
ated dye. It was found that only the addition of F
ꢀ
and H
2
PO4 caused the reappearance of the original
blue-green color (Fig. 1). Interestingly, the addition of
turns the solution of protonated 1 yellow, which
allows the visual discrimination of the studied anions.
ꢀ
I
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
UV–vis spectra were taken of 1 and of the protonated
dye in chloroform after the addition of each anion. It
can be observed that a solution of dye 1 displays a sol-
vatochromic band at 731 nm, which is in agreement with
7
the literature. It is well known that the addition of
hydroxylic species to dye 1 in chloroform causes a hyp-
sochromic shift in its solvatochromic band due to the
formation of a hydrogen bond between the phenolate
donor moiety in the probe and the OH group from the
400
500
600
700
800
Wavelength/nm
ꢀ
5
ꢀ3
1
3,18
ꢀ
ꢀ
Figure 2. UV–vis spectra of protonated 1 (1.7 · 10 mol dm ) in
chloroform at 25 ꢁC after the addition of increasing amounts of
tetrabutylammonium fluoride anion. The final concentration of F
was 6.7 · 10 mol dm
species added.
The addition of F and H PO4 an-
2
ions to protonated 1 leads to reappearance of the solva-
tochromic band. The similar wavelength behavior of
deprotonated 1 and protonated 1 in the presence of
the anions does not reflect the formation of hydrogen-
bonded anion: protonated 1 complexes, but rather indi-
cates that these anions are able to act as bases, accepting
protons donated by the protonated dye and generating 1
in solution. The slight hypsochromic shift observed on
comparing the position of the solvatochromic band of
ꢀ
ꢀ4
ꢀ3
.
0
.8
.7
0
0.6
0
.8
0.7
.6
0.5
.4
1
with that of the same band of 1 in the presence of
the anions is due to a halochromic behavior caused by
the increase in the medium polarity with the addition
0.5
0.4
0.3
0.2
0.1
0
1
4
of the salts.
0
0.3
0.2
0.1
Titration experiments were performed to quantify the
influence of the increase in the anion concentrations
on the solutions of the protonated pyridiniophenolate.
The sequence of UV–vis spectra shown in Figure 2 for
a titration with fluoride is similar to that obtained for
dihydrogenphosphate (not shown): the addition of the
anion leads to the appearance of the solvatochromic
band at 728 nm. A plot of the absorbance values at
0
.00
0.25
0.50
0.75
1.00
1.25
1.50
-
F /equivalents
0
.25
0.50
0.75
1.00
-
1.25
1.50
-3
1.75
2.00
2.25
-5
[
F ]/10 mol dm
Figure 3. Variation in the absorbance at 728 nm of protonated dye 1 in
chloroform with the addition of increasing amounts of tetrabutylam-
monium fluoride. The experiment was performed at 25 ꢁC and the
concentration of 1 was 1.7 · 10 mol dm
mole ratio plot for the interaction of protonated 1 with fluoride, which
7
28 nm as a function of the concentration of fluoride
ꢀ
5
ꢀ3 19
. The inset displays a
displayed a linear behavior until the dye and fluoride
concentrations were the same (Fig. 3). This provides
an important evidence for an equilibrium strongly
shifted toward the formation of HF with the participa-
tion of the dye and the anion in a 1:1 stoichiometry. A
fitting of the experimental data
stant of (7.08 ± 4.03) · 10 dm mol
clearly indicates a 1:1 stoichiometry.
1
9,20
gave a binding con-
Figure 4 shows a plot of absorbance values as a function
of dihydrogenphosphate added. From this plot a bind-
ing constant of (5.61 ± 0.14) · 10 dm mol
7
3
ꢀ1
.
4
3
ꢀ1
is ob-
1
9
tained. The results obtained suggest that fluoride is a
stronger base than dihydrogenphosphate in chloroform
which helps to explain the observed selectivity in rela-
tion to the anions studied. It is important to observe that
the greater effect of fluoride is related to its smaller size,
higher charge density, and higher electron affinity,
important properties which make it capable of forming
a stronger interaction with the phenol group in 1. These
aspects have been pointed by other authors in order to
explain the very high selectivity to fluoride in compari-
son to other anions of phenol-based chromo and fluor-
Figure 1. Chloroform solutions of (a) dye 1, (b) protonated 1 (both
1
5,21–23
ꢀ5
ꢀ3
ꢀ
ogenic sensors.
Recently, Wang and co-workers
1
.7 · 10 mol dm ), and protonated 1 in the presence of (c) F , (d)
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
PO4 , and (i) HSO as
4
Cl , (e) Br , (f) I , (g) NO , (h) H
2
showed that conjugated polymers with phenol unities
in their structures can act as fluorescent and colorimetric
3
ꢀ3
ꢀ3
.
tetrabutylammonium salts at a concentration of 1.0 · 10 mol dm

D. C. Reis et al. / Tetrahedron Letters 47 (2006) 9339–9342
9341
addition of iodide led to the appearance of a new band
at 381 nm. It is well known that N-alkyl pyridinium cat-
ions can form charge-transfer complexes with iodide in
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
2
5
solution. Thus, pyridiniophenolate 1, once protonated
on its phenolate donor moiety, loses its characteristic
7
charge-transfer transition in the visible region, but the
acceptor pyridinium group can still form a complex with
iodide, which has a charge-transfer feature. An associa-
5
tion constant in chloroform, of (2.11 ± 0.07) · 10
3
ꢀ1 19
dm mol
,
was calculated from the data.
In summary, it is shown here that the classical pyridinio-
phenolate 1 is able, after protonation, to act as an anio-
nic chromogenic sensor which works due to two
different features: acidity/basicity and complexation
leading to an intermolecular charge-transfer nature.
An aspect common to this study and other recent stud-
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
-
-5
-3
[
H PO ]/10 mol dm
2 4
1
5,21–24,26
Figure 4. Variation in the absorbance at 728 nm of protonated dye 1 in
chloroform with the addition of increasing amounts of tetrabutylam-
monium dihydrogenphosphate. The experiment was performed at
5 ꢁC and the concentration of 1 was 1.7 · 10 mol dm
ies
is the design of chemosensors with an acidic
site for recognizing more basic anions. Since many struc-
19
tural modifications to dye 1 and related merocyanines
ꢀ
5
ꢀ3
2
.
6–8,12
can be carried out,
a change in the acidity of these
dyes would be expected to make these chromo and
fluorogenic sensors more efficient.
sensors in chloroform.23 In addition, as in our work,
they verified that the chemosensors are highly sensitive
to fluoride in relation to other anions and a low sensitiv-
ity was also observed for dihydrogenphosphate, while
other anions did not cause alterations in the fluorescence
or absorption spectra of the sensors in solution.23 In an-
other related study, Piatek and Jurczak reported an an-
ionic chromogenic sensor able to differentiate selectively
Acknowledgements
The financial support of Brazilian Conselho Nacional de
´
´
Pesquisa Cientıfica e Tecnologica (CNPq) and FURB is
gratefully acknowledged.
2
4
between fluoride and dihydrogenphosphate. Although
in these studies evidence for the formation of hydrogen-
bonded complexes formed between the sensors and an-
1
5,21–24,26
ions is presented,
the evidence provided here
References and notes
comprises the complete proton transfer from the proton-
ated dye to the anion.
1
. For reviews, see: (a) Dietrich, B. Pure Appl. Chem. 1993,
5, 1457–1464; (b) Seel, C.; Gal a´ n, A.; de Mendoza, J.
6
Titration experiments were also performed through the
addition of increasing amounts of iodide to a solution
of the protonated dye (Fig. 5). It was found that the
Top. Curr. Chem. 1995, 175, 101–132; (c) Schmidtchen, F.
P.; Berger, M. Chem. Rev. 1997, 97, 1609–1646; (d)
Bianchi, A.; Bowman-James, K.; Garcia-Espa n˜ a, E.
Supramolecular Chemistry of Anions; Wiley-VCH: New
York, 1997; (e) Antonisse, M. M. G.; Reinhoudt, D. N.
Chem. Commun. 1998, 443–448; (f) Gale, P. A. Coord.
Chem. Rev. 2000, 199, 181–233; (g) Beer, P. D.; Gale, P. A.
Angew. Chem., Int. Ed. 2001, 40, 487–516; (h) Gale, P. A.
Coord. Chem. Rev. 2001, 213, 79–128; (i) Special issue on
anionic recognition: Coord. Chem. Rev. 2003, 240, 1–226;
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
(
j) Kubik, S.; Reyheller, C.; St u¨ we, S. J. Inclusion Phenom.
Macrocycl. Chem. 2005, 52, 137–187.
2
3
. (a) de Silva, A. P.; Gunaratne, H. Q. N.; Gunnlaugsson,
T.; Huxley, A. J. M.; McCoy, C. P.; Rademacher, J. T.;
Rice, T. E. Chem. Rev. 1997, 97, 1515–1566; (b) Callan, J.
F.; de Silva, A. P.; Magri, D. C. Tetrahedron 2005, 61,
8
551–8588.
. Wiskur, S. L.; Ait-Haddou, H.; Lavigne, J. J.; Anslyn,
E. V. Acc. Chem. Res. 2001, 34, 963–972.
4. Mart ´ı nez-Ma n˜ ez, R.; Sancen o´ n, F. Chem. Rev. 2003, 103,
419–4476.
. (a) Suksai, C.; Tuntulani, T. Chem. Soc. Rev. 2003, 32,
4
5
2
4
6
-
8
10
12
14
-6
-3
1
2
92–202; (b) Suksai, C.; Tuntulani, T. Top. Curr. Chem.
005, 255, 163–198.
[
I ]/10 mol dm
Figure 5. Variation in the absorbance at 381 nm of protonated dye 1 in
6. Mishra, A.; Behera, R. K.; Behera, P. K.; Mishra, B. K.;
Behera, G. B. Chem. Rev. 2000, 100, 1973–2011.
7. (a) Reichardt, C. Chem. Rev. 1994, 94, 2319–2358; (b)
Reichardt, C. Pure Appl. Chem. 2004, 76, 1903–1919.
chloroform with the addition of increasing amounts of tetrabutylam-
1
9
monium iodide. The experiment was performed at 25 ꢁC and the
ꢀ5
ꢀ3
.
concentration of 1 was 1.7 · 10 mol dm

9342
D. C. Reis et al. / Tetrahedron Letters 47 (2006) 9339–9342
8
9
. Dimroth, K.; Reichardt, C.; Siepmann, T.; Bohlmann, F. microsyringe to closed quartz cuvettes containing the
J. Liebigs Ann. Chem. 1963, 661, 1–37.
. (a) Dimroth, K.; Reichardt, C. Z. Anal. Chem. 1966, 215,
solution of protonated 1. These solution transfer experi-
ments were carried out using flasks and the cuvettes were
hermetically closed with rubber stoppers in order to
minimize problems with the evaporation of the chloro-
form and all experiments were carried out at 25 ꢁC.
344–350; (b) Skwierkzynski, R. D.; Connors, K. A. J.
Chem. Soc., Perkin Trans. 2 1994, 467–472; (c) Ros e´ s, M.;
R a` fols, C.; Ortega, J.; Bosch, E. J. Chem. Soc., Perkin
Trans. 2 1995, 1607–1615; (d) Leit a˜ o, R. E.; Martins, F.;
Ventura, M. C.; Nunes, N. J. Phys. Org. Chem. 2002, 15,
18. (a) Ortega, J.; R a` fols, C.; Bosch, E.; Ros e´ s, M. J. Chem.
Soc., Perkin Trans. 2 1996, 1497–1503; (b) Dawber, J. G.;
Williams, R. A. J. Chem. Soc., Faraday Trans. 1 1986, 82,
3097–3112; (c) Dawber, J. G.; Ward, J.; Williams, R. A. J.
Chem. Soc., Faraday Trans. 1 1988, 84, 713–727.
6
23–630.
0. (a) Tada, E. B.; Novaki, L. P.; El Seoud, O. A. Langmuir
001, 17, 652–658; (b) Klijn, J. E.; Engberts, J. B. F. N. J.
1
2
Am. Chem. Soc. 2003, 125, 1825–1833; (c) Vodolazkaya,
N. A.; Mchedlov-Petrossyan, N. O.; Heckenkemper, G.;
Reichardt, C. J. Mol. Liquids 2003, 107, 221–234; (d)
Fuguet, E.; R a` fols, C.; Bosch, E.; Ros e´ s, M. Langmuir
19. Data related to the fluoride experiments were fitted with
2
0
the use of the following equation:
Abs = Abs
0
+
+
(Abs
ꢀ Abs )/2C
{C + C + 1/K ꢀ [(C + C
max
2
0
1
1
Aꢀ
1
Aꢀ
1/2
1/K) ꢀ 4C
1
C
Aꢀ
]
}, while the experiments with dihydro-
2003, 19, 55–62.
genphosphate and iodide were fitted by means of the
2
0
1
1. (a) Linert, W.; Strauss, B.; Herlinger, E.; Reichardt, C. J.
Phys. Org. Chem. 1992, 5, 275–284; (b) Mancini, P. M. E.;
Terenzani, A.; Adam, C.; Vottero, L. R. J. Phys. Org.
Chem. 1999, 12, 430–440.
equation Abs = (Abs
these equations, Abs is the absorbance value after each
addition of the anion, Abs is the initial absorbance
0
+ Absmax KCAꢀ)/(1 + KCAꢀ). In
0
without anion added, Absmax is the maximal absorbance
1
2. Reichardt, C. Green Chem. 2005, 7, 339–351.
value obtained by the addition of the anion, C1 is the
1
3. Venturini, C. de G.; Andreaus, J.; Machado, V. G.;
Machado, C. Org. Biomol. Chem. 2005, 3, 1751–1756.
4. (a) Gageiro, V.; Aill o´ n, M.; Rezende, M. C. J. Chem. Soc.,
Faraday Trans. 1992, 88, 201–204; (b) Reichardt, C.;
Asharin-Fard, S.; Sch a¨ fer, G. Chem. Ber. 1993, 126, 143–
concentration of 1, C is the concentration of the anion
Aꢀ
in each addition and K is the binding constant. The values
2
ꢀ5
ꢀ
ꢀ5
ꢀ
1
of v for the plots were 5 · 10 (F ), 7 · 10 ðH2PO Þ,
4
ꢀ5
ꢀ
and 1 · 10
(I ). A discussion on the use of these
equations, as well their deductions, can be found in Ref.
20.
147; (c) Machado, C.; Nascimento, M. G.; Rezende, M.
C.; Beezer, A. E. Thermochim. Acta 1999, 328, 155–159.
5. Lee, K. H.; Lee, H.-Y.; Lee, D. H.; Hong, J.-I. Tetra-
hedron Lett. 2001, 42, 5447–5449.
20. Valeur, B.; Pouget, J.; Bourson, J.; Kaschke, M.; Ernsting,
N. P. J. Phys. Chem. 1992, 96, 6545–6549.
21. Lee, D. H.; Lee, K. H.; Hong, J. Org. Lett. 2001, 3, 5–8.
22. Zhang, X.; Guo, L.; Wu, F.; Jiang, Y. Org. Lett. 2003, 5,
2667–2670.
1
1
6. Karl–Fischer measurements revealed that the chloroform
employed contained water in
3
a
concentration of
.56 · 10 mol dm , which was sufficient to interact
with the CO and, consequently, to acidify the medium.
7. The experiments were performed as follows: a stock
ꢀ
3
ꢀ3
23. Zhou, G.; Cheng, Y.; Wang, L.; Jing, X.; Wang, F.
Macromolecules 2005, 38, 2148–2153.
2
1
24. Piatek, P.; Jurczak, J. Chem. Commun. 2002, 2450–2451.
25. (a) Kosower, E. M. J. Am. Chem. Soc. 1958, 80, 3253–
3260; (b) Mackay, R. A.; Poziomek, E. J. J. Am. Chem.
Soc. 1970, 92, 2432–2439; (c) Medda, K.; Pal, M.; Bagchi,
S. J. Chem. Soc., Faraday Trans. 1 1988, 84, 1501–1507.
26. Mizuno, T.; Wei, W.-H.; Eller, L. R.; Sessler, J. L. J. Am.
Chem. Soc. 2002, 124, 1134–1135.
ꢀ5
ꢀ3
solution of 1 (1.7 · 10 mol dm ) was prepared in
anhydrous chloroform. After that, CO was bubbled into
2
this solution with a syringe in order to protonate the dye.
This colorless solution was then used to prepare the stock
solution of the anion. Titration experiments were per-
formed by adding small amounts of this solution with a