The 1H NMR spectra of ligand 1 recorded in CD3CN upon
the addition of increasing concentrations of Al3+ showed
significant spectral changes (see ESIw). The most important
shifts of the signals of 1 were observed in the xanthene moiety,
especially the a protons to the carbonyl group at 6.25 and
6.39 and the ortho-protons to the lateral chain at 7.19 and
6.95 ppm, which underwent a significant downfield shift upon
the addition of Al3+. On the other hand, the singlet signal at
4.67 ppm displayed a similar behaviour, with a downfield shift
of 0.3 ppm. There were no appreciable changes in the peak
positions of the ethylene chain next to the amide group. These
data strongly suggest the direct involvement of the xanthene
moiety and the lateral chain for Al3+ coordination. In order
to suggest a complexation model for the 1 : 1 complexes,
complementary NMR studies with Ru3+, which showed to
form 1 : 1 complexes with ligand 1, were carried out (see ESIw).
The similarity of the spectra modifications, with those observed
with Al3+, suggests that the same groups are involved in both
kinds of complexes.
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In summary, we report herein the synthesis and characterisation
of a new probe for the fluorogenic detection of trivalent cations
Fe3+, Cr3+ and Al3+. The choice of the fluorescein group allowed
the optical detection of these ions, whereas the probe remained
silent in the presence of monovalent and divalent cations such as
Li+, Cu2+, Cd2+, Zn2+, Co2+, Ni2+, Fe2+ and Hg2+. The
acetonitrile solutions of 1 in the presence of Fe3+, Cr3+ and Al3+
resulted in an associated ‘‘turn-on’’ response via the formation of
the corresponding metal complexes. This chelation-enhanced
fluorescence might be attributed to the change in polarity of the
dye by increase of the donor–acceptor electronic delocalization
after complexation. Fe3+ and Cr3+ were found to form 1 : 1
ligand-to-metal complexes with 1, whereas the formation of 2 : 1
complexes was observed for Al3+. A discrimination based on the
fluorescence response according to the amount of water in solution
can be envisaged.
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nez-Manez, F. Sancenon, A. M. Costero,
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AECID (A/026355109), DGICYT (projects MAT2009-
14564-C01 and MAT 2009-14564-C04-3) and Generalitat
Valenciana (project PROMETEO/2009/016) are gratefully
acknowledged. A.B.B. thanks MICINN for a pre-doctoral
FPI fellowship.
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27 (a) A. M. Costero, J. V. Colomer, S. Gil and M. Parra, Eur. J. Org.
Chem., 2009, 3673; (b) A. M. Costero, M. Parra, S. Gil,
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J. Soto, A. Maquieira and P. Amoro
49, 7281; (b) M. Comes, E. Aznar, M. Moragues, M. D. Marcos,
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3002 Chem. Commun., 2012, 48, 3000–3002
This journal is The Royal Society of Chemistry 2012