thiadiazole units that showed emission enhancements upon
coordination, through hydrogen bonding interactions, with
hydrogen carbonate in methanol.7 Very recently, Suzuki et
al. synthesized a receptor consisting of a γ-cyclodextrin
connected through a triamine linker with a pyrene residue.8
In aqueous solutions, two receptors form an association dimer
in which two pyrene subunits are in parallel conformation
inside the cyclodextrin cavity. Upon binding of HCO3- anion
with the dimer, the two pyrene units suffer a conformational
change that is reflected in the shift of the excimer fluores-
cence band from 475 to 425 nm.
Scheme 1. Structure of the Polymeric Films and the
Monomers I, II, III, IV, V, and VI Used in Their Synthesis
In spite of these encouraging examples, there are not, as
far as we know, chromogenic signaling receptors for carbon-
ate in water. To fill up this gap, we report herein the
development of a probe for the colorimetric detection of
-
HCO3 in neutral aqueous solution by using polymeric
materials containing pyrylium derivatives. Polymeric films
can be easily prepared and are ideal candidates for the
development of sensory materials for in situ sensing and rapid
“naked eye” screening applications. In fact, the combination
of certain chromogenic probes and suitable polymeric
matrices is an emerging frontier in the development of robust
and selective sensory materials for target anions.9 Polymers
not only are excellent supports but also allow fine alterations
of their hydrophilic/hydrophobic character that finally result
in a tuning of the sensing ability.
The sensing principle for the colorimetric signaling of
carbonate takes advantage of the electrophilic character of
the pyrylium ring (vide infra) due to the presence of a
positively charged oxygen atom in its structure.10 In fact
there are recent reported examples using pyrylium derivatives
as chromogenic probes,11 and we have recently reported
pyrylium-containing polymers for the colorimetric detection
of cyanide in water.9i Here, in order to incorporate the
pyrylium derivative into a polymeric matrix, several meth-
acrylate esters bearing pyrylium rings were synthesized (I-
III, see Scheme 1). Then, a series of methacrylic copolymer
films containing the pyrylium probes were prepared by
radical copolymerization of the monomer IV, 2-ethoxyethyl
methacrylate, V, 2-(2-(2-ethoxyethoxy)ethoxy)ethyl meth-
acrylate, VI, 2,3-dihydroxypropyl methacrylate, small quan-
tities of ethylene glycol dimethacrylate as cross-linking agent,
and 2,2′-azobis(isobutyronitrile) (AIBN) as radical thermal
initiator. Eleven different polymer films were prepared by
changing three parameters in order to optimize the chemical
properties of the synthesized films: (i) the concentration of
pyrylium derivatives I-III (see Scheme 1 and Table 1), (ii)
Table 1. Composition of the Polymeric Films Prepared
films
monomer (2%)a
IVa
Va
VIa
1
2
3
4
5
6
7
8
9
Ia
50
50
-
-
-
-
-
-
50
-
-
-
50
50
50
50
50
50
50
50
50
50
50
Ib
Ib
Ic
50
50
50
50
50
50
-
(7) Hennrich, G.; Sonnenschein, H.; Resch-Genger, U. Tetrahedron Lett.
2001, 42, 2805-2808.
(8) Suzuki, I.; Ui, M.; Yamauchi, A. J. Am. Chem. Soc. 2006, 128, 4498-
4499.
Ia
IIa
IIb
Id
Ie
Ie
III
(9) (a) Wu, C.-Y.; Chen, M.-S.; Lin, C.-A.; Lin, S.-C.; Sun, S.-S. Chem.s
Eur. J. 2006, 12, 2263-2269. (b) Mohr, G. J. Chem.sEur. J. 2004, 10,
1082. (c) Ho, H. A.; Leclerc, M. J. Am. Chem. Soc. 2003, 125, 4412-
4413. (d) Manesiotis, P.; Hall, A. J.; Emgenbroich, M.; Quaglia, M.; De
Lorenzi, E.; Sellergren, B. Chem. Commun. 2004, 2278-2279. (e) Aldakov,
D.; Anzenbacher, P., Jr. J. Am. Chem. Soc. 2004, 126, 4752-4753. (f) Kwak,
G.; Fujiki, M.; Masuda, T. Macromolecules 2004, 37, 2422-2426. (g) Li,
C.; Numata, M.; Takeuchi, M.; Shinkai, S. Angew. Chem., Int. Ed. 2005,
44, 6371-6374. (h) Badr, I. H. A.; Meyerhoff, M. E. J. Am. Chem. Soc.
2005, 127, 5318-5319. (i) Garc´ıa, F.; Garc´ıa, J. M.; Garc´ıa-Acosta, B.;
Mart´ınez-Ma´n˜ez, R.; Sanceno´n, F.; Soto, J. Chem. Commun. 2005, 2790-
2792.
(10) (a) Balaban, A. T.; Silhau, W. Tetrahedron 1970, 26, 743-749.
(b) Uncuta, C.; Balaban, A. T. J. Chem. Res. 2001, S170, M523.
(11) (a) Sanceno´n, F.; Descalzo, A. B.; Mart´ınez-Ma´n˜ez, R.; Miranda,
M. A.; Soto, J. Angew. Chem., Int. Ed. 2001, 40, 2640-2643. (b) Sanceno´n,
F.; Mart´ınez-Ma´n˜ez, R.; Miranda, M. A.; Segui, M.-J.; Soto, J. Angew.
Chem., Int. Ed. 2003, 42, 647-650. (c) Comes, M.; Marcos, M. D.;
Mart´ınez-Ma´n˜ez, R.; Sanceno´n, F.; Soto, J.; Villaescusa, L. A.; Amoro´s,
P.; Beltra´n, D. AdV. Mater. 2004, 16, 1783-1786. (d) Jime´nez, D.; Mart´ınez-
Ma´n˜ez, R.; Sanceno´n, F.; Ros-Lis, J. V.; Benito, A.; Soto, J. J. Am. Chem.
Soc. 2003, 125, 9000-9001. (e) Sanceno´n, F.; Mart´ınez-Ma´n˜ez, R.; Soto,
J. Chem. Commun. 2001, 2262-2263.
10
11
50
50
-
a Molar composition ratio of the monomers used for the preparation of
the films.
the hydrophobic monomer (IV or V), and (iii) the ratio of
hydrophilic (VI) to hydrophobic (IV and V) monomers.
Related to the content of the probe, the best results were
obtained with films containing 2% of the corresponding
monomer. Films containing less than 2% of pyrylium
derivative were pale yellow or colorless, and the color
changes upon interaction with the target anions were difficult
to observe. Also, the molar ratio between the hydrophobic
IV, the partially hydrophobic V, and the hydrophilic
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Org. Lett., Vol. 9, No. 13, 2007