towards the proton, in a DMSO solution. Such a discriminat-
ing effect can also be monitored by looking at the fluorescent
emission, which is almost completely quenched by cholate
(see Fig. 4), only slightly by glycocholate and not modified at
all by taurocholate.
Conclusions
A chemosensor is a molecular system that, following the
change of concentration of the investigated analyte, gives an
optical signal in response. Such a behaviour usually results
from the formation of a 1 : 1 complex between the receptor and
the analyte (in the present case, an anion). As the receptor–
anion complex is held together by non-covalent interactions
(hydrogen bonding, electrostatic), the topic of anion recogni-
tion and sensing rightfully belongs to the discipline of
supramolecular chemistry. In this work, we intended to
investigate a receptor suitable for recognition and sensing of
the carboxylate group and, in this sense, we chose the urea
fragment, which can donate two hydrogen bonds in a parallel
fashion and so is complementary to Y-shaped anionic
groups, such as –COO2. The urea subunit was equipped
with naphthaleneimide substituents in order to provide two
different optical signals: colour change and modification of
the fluorescent emission. However, the urea based receptor 2,
made especially acidic by the strongly electron withdrawing
substituents, does not form an H-bond complex with
carboxylates, but rather transfers a proton of one N–H
fragments to the anion. Thus, system 2 escapes from the realm
of supramolecular chemistry and enters the classic domain of
Brønsted acid–base equilibria. However, such a circumstance
is not relevant from the point of view of signalling, because
urea deprotonation induces a neat colour change and sub-
stantial quenching of the light emission in presence of most
basic carboxylates, such as acetate and cholate.
Scheme 1 Synthetic route to chemosensor 2.
N,N9-(Bis-N-butyl-1,8-naphthalenimide)-urea (2)
Into a reactor filled with argon, with 4-bromo-N-butyl-1,8-
naphthalenimide (0.200 g., 0.603 mmol) in 4 mL of dioxane
satured with argon, were added urea (0.0235 g, 0.392 mmol),
Cs2CO3 (0.275g, 0.844 mmol), Pd2(dba)3–CHCl3 (0.003 g,
0.5 mol%) and xantphos (0.0156 g, 3 mol%). The reaction
mixture was degassed by evacuation then the reactor was filled
with argon. The mixture was heated at 100 uC under magnetic
stirring. The reaction progress was monitored by TLC
(hexane–ethyl acetate, 7 : 3). On completion of the reaction
(12 h) the mixture was cooled to rt and the content of the
reactor was poured into 50 mL of saturated KCl solution.
Then the products were extracted by ethyl acetate (3 6 20 mL),
the combined extracts were washed with KCl solution and the
solid product formed in the organic phase was separated by
filtration to give a yellow solid (0.140 g, 83%) which is air
stable in the solid state, soluble in dimethyl sulfoxide and
insoluble in all other common solvents. (Found: C, 70.52; H,
5.39; N, 9.96%. C33H30N4O5 requires C, 70.45; H, 5.37; N,
9.96%). IR spectrum (nujol mull), cm21: 3292 (N–H); 1699,
1656 (CLO); 1553 (C–N). 1H-NMR (DMSO-d6, dH ppm): 9.35
(br, NH, 2H), 8.85(d, 2H), 8.55(m, 6H), 7.95(t, 2H), 4.00(t,
4H), 1.55(quintuplet, 4H), 1.35(m, 4H), 0.80(t, 6H). m/z (nega-
tive ion mode) 561.1 (M 2 H+, 100%), 597.1 (M + Cl2, 38%).
Experimental
Caesium carbonate was dried in vacuo at 150–200 uC.
Dioxane was purified and dried by standard procedures.
Pd2(dba)3–CHCl3 was prepared as described in the literature.19
4,5-Bis(diphenylphosphino)-9,9-dimethylxanthene (xantphos),
Physical measurements
n-butylamine,
4-bromo-1,8-naphthalic
anhydride
and
Spectrophotometric and spetrofluorimetric grade solvents
were used for spectroscopic measurements. UV/vis spectra
were recorded on a Varian CARY 100 spectrophotometer,
with a quartz cuvette; spectrofluorimetric measurements were
carried out on a Perkin-Elmer LS-50 luminescence spectro-
meter, using quartz cells (path length: 1 cm). ESI-MS spectra
were obtained by a Thermo Finnigan LCQ Advantage Max
spectrometer. FTIR spectra were recorbed on a Perkin-
Elmer Spectrum BX spectrophotometer. Binding constants
were calculated through non-linear least-squares fitting of
spectrophotometric titration curves by using the hyperquad
package.20
4-amino-1,8-naphthalic anhydride are commercial products.
The chemosensor 2 was prepared according to the two-step
pathway shown in Scheme 1.
4-Bromo-N-butyl-1,8-naphthalenimide
4-Bromo-1,8-naphthalic anhydride (0.24 g, 0.869 mmol) and
n-butylamine (0.103 mL, 1.048 mmol) in ethanol (40 mL)
were refluxed for 24 h. The solid product was isolated by
concentration of solution and separated by filtration. The
product was purified by column chromatography (hexane–
ethyl acetate, 7 : 3) to give an off-white solid (0.21 g, 75%). IR
spectrum (nujol mull), cm21: 1668, 1657 (CLO); 720 (C–Br).
1H-NMR (CDCl3, dH ppm): 8.70(d, 1H), 8.60(d, 1H),
8.45(d, 1H), 8,10(d, 1H), 7.90(t, 1H), 4.20(t, 2H), 1.75(quintu-
plet, 2H), 1.50(m, 2H), 1.00(t, 3H). m/z (negative ion mode),
331.4 (M 2 H+).
Acknowledgements
The financial support of the European Union (RTN Contract
HPRN-CT-2000-00029) and the Italian Ministry of University
2674 | J. Mater. Chem., 2005, 15, 2670–2675
This journal is ß The Royal Society of Chemistry 2005