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in visible region responsible for their yellow–orange
colour. The intense band in the ultraviolet zone is
due to electronic transitions on the benzene rings,
whereas the visible band is attributed to charge trans-
fer between the donor and the acceptor part of the
molecule.10 When L3 interacts with Hg2+ the visible
band suffers a bathochromic shift changing the colour
of the solution from yellow–orange to red. When L1,
L2 or L3 react with Fe3+ the visible band disappears
and the colour of the solution changes to pale yellow.
Fig. 1 shows the UV–vis spectra of L3 and L3 in the
presence of Hg2+, Ba2+ and Fe3+ in dioxane.
Sensing receptors for heavy toxic metal ions are of
interest in areas such as environmental chemistry,
where the development of highly selective analytical
tools is of importance. Metal-ion detection by coordi-
nation with molecules containing electrochemical or
fluorescent signalling subunits have recently been
developed but the use of those receptors would need
large and expensive analytical equipment.5 An alter-
native is to design molecules able to selectively cap-
ture target-metal cations combined with colour
changes.6 Colour reactions are popular criteria for the
identification and quantitative determination of sub-
stances and are the basis for some easy-to-use appli-
cations as DIP-sticks assays or suitable for optical
fibre applications.
It is noticeable that only Hg2+ from near 20 metal
ions is able to produce a bathochromic shift on L3
indicating that the colour variation is highly specific
for Hg2+. Hg2+ only modifies the colour of L3 but not
that of L1 or L2 showing the importance of the size
Here we report the synthesis of dyes bearing binding
sites that show high metal ion specific colour changes.
The synthesis of L1, L2 and L3 began with the con-
densation of N-phenyldiethanolamine in the presence
of sodium hydride with the corresponding dimesylated
polyehyleneglycol in acetonitrile as solvent.7 The
obtained benzo polyazaoxaalkane compounds were
diazoated with p-nitroaniline and sodium nitrite in
aqueous 5% hydrochloric acid8 (Scheme 1). The lig-
ands were obtained as orange solids in ca. 70% yield.
1H, 13C NMR and mass spectra were in agreement
with the proposed structures.9
1
of the aza–oxa open chain. H NMR studies on L3 in
the presence of Hg2+ showed a remarkable shift of
signals of both the aza–oxa binding site and the aro-
matic ring attached to the aza–oxa open chain. Free
receptor shows signals for the protons of the aza–oxa
group in the 3.5–3.8 ppm range and three groups of
resonances centred at
l 6.9, 7.9 and 8.3 ppm
attributed to the aromatic protons which are respec-
tively nearer to the amino, the azo and the nitro
groups. In the presence of Hg2+ the protons of the
aza–oxa binding site are in the 3.5–4.1 range and the
signal at 6.9 ppm has been shifted to l 7.4. These
shifts suggest a strong interaction of the Hg2+ cation
with the aza–oxa chain. This contrasts with the 1H
NMR of L3 in the presence of other metal ions such
as Zn2+ for which only a slight shift of the signals of
the protons of the aza–oxa binding site was found,
suggesting a lower degree of metal–ligand interaction.
Fe3+ also produces a considerable alteration of the
absorption spectrum of the ligands; its presence
induces the disappearance of the visible band (see
Fig. 1). This effect is the same for L1, L2 and L3 and
this total elimination of the charge transfer visible
band that is independent of the aza–oxa binding site
chain might suggest a strong coordination of the
metal ion with the azo group or may be produced as
a result of a redox reaction.
The nitrate or perchlorate salts of the cations Li+,
Na+, K+, Ca2+, Mg2+, Sr2+, Ba2+, Cr3+, Mn2+, Fe3+,
Co2+, Ni2+, Cu2+, Zn2+, Cd2+, Hg2+, Pb2+, Al3+, Sb3+
were added to solutions of L1, L2 and L3, in trying to
find selective colour changes. Solutions of receptors in
dioxane were prepared in a 10−4 M concentration. All
measurements were carried out using ligand:metal 1:1
molar ratios.
The UV–vis spectra of L1, L2 and L3 show intense
bands in the UV zone and a band centred at 460 nm
In conclusion, the chromogenic aza–oxa derivative L3
shows an extraordinary selectivity for Hg2+ over
nearly 20 other common metal ions. Additionally
Fe3+ also produces a selective elimination of the visi-
ble absorption band. These two features can provide
the occasion of using these or related ligands as new
chromogenic reagents for the sensing of target toxic
heavy metal ions.
Acknowledgements
We should like to thank the DGICYT (proyecto
PB98-1430-C02-02 and 1FD97-0508-C03-01). F.S.
should like to thank the Ministerio de Educacio´n y
Cultura for research fellowship (MT99-252676011).
Figure 1. UV–vis spectrum of L3 and L3 in the presence of
Hg2+, Ba2+ and Fe3+ in dioxane.
Sancenón, Felix
