Synthesis and coordination properties of an azamacrocyclic Zn(II) chemosensor containing pendent methylnaphthyl groups

Article abstract of DOI:10.1039/b808993k

The synthesis of a polyazamacrocycle constituted by two diethylenetriamine bridges functionalized at their central nitrogen with naphth-2-ylmethyl units and interconnected through 2,6-dimethylpyridine spacers (L1) is reported. The protonation behaviour of the new macrocycle in water and in water-ethanol 70/30 v/v mixed solvent has been examined by means of pH-metric, UV-Vis and steady-state fluorescence techniques. The fluorescence emission is slightly quenched following the deprotonation of the central tertiary amines and more deeply quenched upon deprotonation of the secondary amino groups. pH-Metric titrations show that in water-ethanol 70/30 v/v L1 forms stable mononuclear complexes with the divalent transition and post-transition metal ions Mn ²⁺, Fe²⁺, Co²⁺, Cu²⁺, Zn ²⁺, Cd²⁺, Hg²⁺ and Pb²⁺. In the case of Cu²⁺, Zn²⁺, Hg²⁺ and Pb²⁺, formation of binuclear complexes has also been observed. Steady-state fluorescence emission studies show that while interaction with Cu²⁺ leads to quenching of the fluorescence emission above pH = 2, interaction with Zn²⁺, Cd²⁺, Hg²⁺ and Pb²⁺ give rise to enhancements of the fluorescence above pH ca. 5, which is particularly noticeable in the case of Zn²⁺.

Full text of DOI:10.1039/b808993k

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www.rsc.org/dalton | Dalton Transactions
Synthesis and coordination properties of an azamacrocyclic Zn(II)
chemosensor containing pendent methylnaphthyl groups†
a
b
b
b
b
b
Javier Alarc o´ n, M. Teresa Albelda, Raquel Belda, M. Paz Clares, Estefan ´ı a Delgado-Pinar, Juan C. Fr ´ı as,
b
b
c
Enrique Garc ´ı a-Espa n˜ a, Jorge Gonz a´ lez and Conxa Soriano
Received 30th May 2008, Accepted 4th August 2008
First published as an Advance Article on the web 9th October 2008
DOI: 10.1039/b808993k
The synthesis of a polyazamacrocycle constituted by two diethylenetriamine bridges functionalized at
their central nitrogen with naphth-2-ylmethyl units and interconnected through 2,6-dimethylpyridine
spacers (L1) is reported. The protonation behaviour of the new macrocycle in water and in
water-ethanol 70/30 v/v mixed solvent has been examined by means of pH-metric, UV-Vis and
steady-state fluorescence techniques. The fluorescence emission is slightly quenched following the
deprotonation of the central tertiary amines and more deeply quenched upon deprotonation of the
secondary amino groups. pH-Metric titrations show that in water-ethanol 70/30 v/v L1 forms stable
2
+
2+
2+
mononuclear complexes with the divalent transition and post-transition metal ions Mn , Fe , Co ,
2
+
2+
2+
2+
2+
2+
2+
2+
2+
Cu , Zn , Cd , Hg and Pb . In the case of Cu , Zn , Hg and Pb , formation of binuclear
complexes has also been observed. Steady-state fluorescence emission studies show that while
2
+
interaction with Cu leads to quenching of the fluorescence emission above pH = 2, interaction with
Zn , Cd , Hg and Pb give rise to enhancements of the fluorescence above pH ca. 5, which is
2
+
2+
2+
2+
2
+
particularly noticeable in the case of Zn .
Introduction
The design of an appropriate receptor endowed with fluorescent
components is the basic requirement to make this method opera-
tive. One of the main approaches is to link covalently, through a
short spacer, a binding unit and a fluorescent moiety. Coordination
2
+
Zn is present in every living cell. Most is bound to proteins
playing catalytic roles in hydrolytic enzymes and structural roles
in transcription factors and related proteins. In addition to this
2+
of Zn to the binding unit should largely modify the fluorescence
2
+
2+
tightly bound Zn , there are pools of relatively free Zn located at
the brain and nervous system which are believed to play key roles in
relation to neurotransmission and neuromodulation. Since Zn
5
of the fluorophore.
In this respect, recently we have reported on a series of
1
2+
6
bibracchial lariat aza-crowns in which two molecules of the
1
0
possesses a closed-shell 3d electronic configuration and escapes
observation through classical techniques like electronic param-
agnetic resonance, M o¨ ssbauer spectroscopy, nuclear magnetic
resonance and electronic absorption spectroscopy, fluorescence
probes are of crucial importance for providing a sensitive and
tripodal polyamine tren functionalised with naphthyl groups had
been connected through methylene groups to benzene (L3 and
L4), pyridine (L5 and L7) and phenanthroline moieties (L6).
7–9
Such compounds showed interesting coordination properties with
2+
2+
respect to Zn , Cu and anionic species, in particular dicar-
2
+
2
selective methodology for Zn detection and quantification.
10
boxylate anions. Although the presence of donor atoms in the
2
+
Detection of Zn at low concentration has attracted increasing
interest since this ion influences DNA synthesis, microtubule poly-
merization, gene expression, apoptosis, immune system function,
and the activity of enzymes such as carbonic anhydrase and matrix
pendent arms may also be useful for attaching these compounds
to solid supports facilitating their utilization for scavenging
11
hazardous metal ions or anions, it has detrimental effects on
2+
the sensing capabilities of these systems. Indeed, although Cu
3
metalloproteinase. Moreover, zinc ion is also a contributory
2+
and Zn are known to produce CHEQ (chelation enhancement
factor in neurological disorders such as epilepsy and Alzheimer’s
of the quenching) and CHEF (chelation enhancement of the
4
diseases.
fluorescence) effects, respectively, in such compounds (L2–L5)
5,12,13
their extents were not very large.
This was attributed to the
a
Departament de Qu ´ı mica Inorg a` nica, Facultat de Qu ´ı mica, Univer-
fact that the amino groups in the dangling arms were not tightly
bound to the metals and thus the lone pairs on those atoms could
be transferred to the excited fluorophore as readily as in the free
sitat de Val e` ncia, Burjassot, Spain. E-mail: javier.alarcon@ uv.es;
Fax: +963864322; Tel: +963864879
b
Departament de Qu ´ı mica Inorg a` nica, ICMOL, Facultat de Qu ´ı mica,
14
compounds. Therefore, the behaviour of the free and complexed
receptor cannot differ markedly hampering their possible use as
Universitat de Val e` ncia, Burjassot, Spain. E-mail: enrique.garcia-es@ uv.es;
Fax: +963864322; Tel: +963864879
c
Departament de Qu ´ı mica Org a` nica, ICMOL, Facultat Farm a` cia, Universi-
14–19
metal ion chemosensors.
tat de Val e` ncia, Burjassot, Spain
In order to emphasize CHEF and CHEQ effects occurring
upon coordination to the guest species we have prepared a new
compound without donor atoms in the arms (see L1, Scheme 1)
†
Electronic supplementary information (ESI) available: L1 labelling chart,
1
pH-varied H NMR spectra, molecular model and fluorescence data
Fig. S1–S7). See DOI: 10.1039/b808993k
(
6
530 | Dalton Trans., 2008, 6530–6538
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Scheme 1 Ligand drawings. Labels used in the NMR description are specified for L1.
in which the naphthalene moieties are connected to the central
nitrogens through a single methylene group. On the other hand, the
lower number of nitrogen atoms in L1 with respect to L5 can lead
in some instances, to the formation of coordinatively unsaturated
metal centres opening up the possibility of recognising additional
substrates through the formation of mixed complexes.
Here we report on the protonation behaviour of the new ligand
2
+
as well as on its binding ability towards Zn and other divalent
This journal is © The Royal Society of Chemistry 2008
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2
+
2+
2+
2+
transition and post-transition metal ions: Mn , Fe , Co , Cu ,
the secondary amino groups closest to the pyridine unit, which
agrees with the electronic spectra (vide infra) (Fig. 2A and S2).
The protonation order of these nitrogen atoms is however much
more difficult to establish due to their close basicity, suggesting
that the protons are shared by all the secondary amino groups.
On the other hand, the greater downfield shift for the singlet peak
associated to the protons attached to the carbon atoms placed in a
position to the naphthalene ring and labeled as npH (see Scheme 1
and Fig. S1) takes place below pH = 3, the region involving the
fifth protonation step, in other words the protonation of tertiary
nitrogen atoms.
2
+
2+
2+
Cd , Hg and Pb . We also report on the photochemical
behaviour of free L1 and on how the interaction with the different
metal ions affects its luminescent properties.
Acid–base behaviour
Table 1 collects the stepwise basicity constants for the cyclic
polyamine L1 and its open-chain synthon L2 determined in either
pure water or in H
2
O-EtOH 70/30 v/v at 298.1 ± 0.1 K using
.15 mol dm-3 NaCl as an ionic strength. For comparison, the
0
8
20
The open-chain synthon L2 displays large values for the first
two protonation steps and a much lower constant for the last
step. The first two protonations, particularly the second one, are
greater than those for L1 due to the primary nature of the terminal
nitrogens. The reduction in the basicity occurring on protonation
of the middle nitrogen parallels the tendency discussed for L1.
The fluorescence emission spectra of compound L1 reported in
Fig. 1 show the spectral pattern found for naphthalene derivatives
corresponding values previously reported for L5 and for L7 in
pure water have also been included.
L1 shows both in 100% water and in H O-EtOH 70/30 v/v, five
2
stepwise basicity constants that can be classified into two groups
with the first four protonation constants much greater than the last
one. The protonation constant for the sixth and last step was too
low to be determined reliably by potentiometry in the pH range 2.0
to 11.0. On the other hand, as can be seen in Table 1, the basicity
in ethanol is slightly higher than in pure water.
The values of the first four protonation constants of L1 are
in the same range than those of the bibracchial aza-lariat ether
L5, indicating that such protonation steps occur at the secondary
nitrogens close to the pyridine spacers. The last two protonation
steps involve the tertiary nitrogens in the middle of the bridges.
These protonations are low because of electrostatic repulsion
exerted by the positive charges present in the adjacent nitrogens.
8,23
presenting interaction between two chromophores.
The first
band centered at 330 nm, is vibrationally resolved and can be
easily assigned to the emission of naphthalene (monomer). The
second band at 425 nm is much broader and can be assigned to a
naphthalene–naphthalene excimer.
21
Moreover, as is well documented, tertiary nitrogens are poorly
solvated in water which will be also contributing to the reduction
in basicity. This reduction in the basicity due to the alkylation
of the nitrogens is evidenced when L1 is compared with the
20
corresponding ligand without the methylnaphthyl groups L7.
For such pyridinophane ligands, the values of the constants
for the fifth and sixth protonation steps were 4.11 and 3.26
logarithmic units, respectively. The proposed average protonation
1
tendency is supported by H NMR spectroscopy. It is well
established that upon protonation of polyamine compounds, the
hydrogen nuclei attached to the carbon atom in a-position to
the nitrogen atom bearing the proton are those exhibiting the
22
largest downfield changes in their chemical shift. The singlet
signal of the protons located close to the pyridine ring and labeled
as pyH (see Scheme 1 and Fig. S1 in the ESI†) experiences their
largest downfield shift in the pH range 9.0–3.0, and seems to
indicate that first to fourth protonations affect preferentially to
Fig. 1 Fluorescence emission spectra of L1 (lexc = 280 nm) recorded at
-3
2
98.1 ± 0.1 K in water-ethanol 70/30 v/v 0.15 mol dm NaCl as function
5
-
of pH, [L1] = 1 ¥ 10 M (pH values: 2.11, 2.49, 2.95, 3.57, 3.96, 4.50, 5.15,
5
.59, 6.12, 6.58, 7.04, 7.40, 7.99, 8.84, 9.63, 10.36).
Table 1 Stepwise basicity constants for L1 and L2 determined in pure
water and in H O-EtOH 70/30 v/v solutions at 298.1 ± 0.1 K in
2
.15 mol dm- NaCl. Values for L5 taken from ref. 8. and values for
3
Comparison of the fluorescence behaviour of L1 and L5 is
0
L7 taken from ref. 20
displayed in Fig. 2. While first deprotonation of L5 already
8
produces a sharp decrease of the emission, the fluorescence
b
d
b
d
b
b
Reaction
L1
L1
L2
L2
L5
L7
emission intensity achieved for L1 remains relatively high also
for the tetraprotonated species when the four secondary amino
groups are protonated, suggesting that the tertiary nitrogen atoms
are somewhat sharing the protons from the neighbours. The
decrease in fluorescence intensity observed above pH = 6 can
be attributed to the well-known photoinduced electron transfer
process (PET) occurring between deprotonated amines and the
naphthalene moieties. As can be found in literature, fluorescence
emission spectra of similar ligands containing protonated pyridine
a
c
L + H ꢀ LH
9.60 (2) 9.40 (4) 9.79(1) 9.55 (4) 9.46(3) 9.25
7.95 (3) 8.60 (4) 9.15(1) 8.76 (3) 8.46(4) 8.49
LH + H ꢀ LH
2
LH
LH
LH
2
3
4
+ H ꢀ LH
3
4
5
7.5 (7)
7.04 (3) 2.34(5) 2.99 (3) 7.65(5) 7.55
+ H ꢀ LH
+ H ꢀ LH
6.76 (3) 7.02 (4)
2.83 (5) 2.94 (2)
—
—
—
—
7.25(7) 6.98
6.50(5) 4.11
a
b
c
Charges omitted for clarity. Determined in water. Values in parenthesis
d
are standard deviations in the last significant figure. Determined in H
EtOH 70/30 v/v.
2
O-
6
532 | Dalton Trans., 2008, 6530–6538
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-5
-3
Fig. 2 Steady-state fluorescence emission titration curves of (A) L1 (lexc = 280 nm) for a 1 ¥ 10 M solution in 0.15 mol dm NaCl at 298.1 ± 0.1 K
in water-ethanol 70/30 v/v (●) monomer emission followed at 330 nm and (B) L5 (lexc = 280 nm) for a 7.01 ¥ 10 M solution in 0.15 NaCl at
98.1 ± 0.1 K in water; (●) monomer emission followed at 323 nm. Molar fraction distribution in both cases is represented by solid lines.
-6
2
rings show bell-shaped curves with a decrease of fluorescence
emission intensity at acidic and basic pH values leading to a pH
which can be classified into three categories: i) the two pyridine
nitrogens, ii) the four secondary amino groups in the polyamine
bridges and iii) the two tertiary central nitrogen atoms of the
bridges (Scheme 1). Due to geometrical reasons the simultaneous
participation of some of these atoms in the coordination to the
same metal ion is prevented. For instance, it is unlikely that both
pyridine nitrogen atoms can simultaneously get close enough to
bind a single metal ion since they lie rather far apart (see molecular
modeling in Fig. S3). The high values of the protonation constants
displayed by the mononuclear complexes agree with the fact
that these protonation processes take place at non-coordinated
24
window. In this case, the absence of this pH window suggests
that the pyridines remain unprotonated and the small increase of
the fluorescence below pH = 3 corresponds to the protonation
of the tertiary nitrogen atoms preventing PET process from
occurring. Complementary UV-visible studies confirm the lack
of neat protonation of the pyridines throughout the pH range of
study.
Metal complexes
4
+
nitrogen atoms. Binuclear species with stoichiometries [Zn
2
L]
5
+
Table 2 gathers the cumulative and stability constants for the
[Zn HL] and [Zn L(OH) ] were found for receptor L1 (Fig. 3A).
2
2
2
2
+
interaction of L1 with the metal ion Zn determined at 298.1 ±
Emission spectra of the complexes obtained for the interaction
-
3
2+
0
.1 K in water-ethanol 70/30 v/v 0.15 mol dm NaCl solutions.
of L1 with Zn are very similar to those of the free ligand, consist-
25
For comparative purposes, stability constants previously reported
ing of a typical naphthalene band with maximum values at 262,
271, and 280 nm, and are slightly dependent on pH. Concerning
the emission curves (Fig. 4A and B) in the case of the system
8
for the formation of metal complexes with L5 are included.
Formation of mono- and binuclear complexes has been detected
through the pH studied. The mononuclear complexes display
2
+
3+
Zn –L1, the emissive complex [ZnHL1] leads to a CHEF effect.
The enhancement of the fluorescence emission is very effective for
this species either for molar ratio M:L 1:1 or 2:1 providing a wide
4
+
3+
2+
[
ZnH
2
L] , [ZnHL] and [ZnL] stoichiometries for both ligands.
Conversely to receptor L5, which contains two additional nitrogen
5
+
2+
atoms located in the dangling arms, a [ZnH
3
L] species has not
pH window for Zn detection above pH = 6. In the case of 1:1
2
+
been detected for L1. Macrocycle L1 possesses eight donor atoms
complexes, formation of [ZnL1] causes a small decrease of the
2
+
fluorescence emission since only one metal ion Zn is not able to
coordinate all nitrogen atoms in order to avoid PET processes. The
formation of the hydroxylated species also produces a decrease
of the emission intensity above pH = 9 for the system M:L in
2:1 ratio.
2
+
Table 2 Stability constants for the formation of Zn complexes of the
macrocycles L1 and L5 determined in water-ethanol 70/30 v/v NaCl
-3
-3
0
.15 mol dm and in 0.15 mol dm NaCl respectively at 298.1 ± 0.1 K
a
2+
b
2+
b,c
Reaction
Zn –L1
Zn –L5
We also have investigated the interaction of L1 with the classical
M + L ꢀ ML
13.23(4)
20.33(1)
24.94(6)
—
16.81(7)
23.2(1)
—
11.75(9)
20.92(6)
28.48(3)
34.21(2)
21.14(4)
—
2
+
fluorescence quencher metal ion Cu and other transition and
post-transition metals. Tables 3 and 4 collect all the data obtained
M + H + L ꢀ MHL
M + 2H + L ꢀ MH
2
L
L
4
+
M + 3H + L ꢀ MH
3
for these studies. The mononuclear complexes display [CuH L] ,
2
2
2
2
2
M + L ꢀ M
2
L
3+
2+
[
CuHL] and [CuL] stoichiometries for ligand L1. The binuclear
M + H + L ꢀ M
M + L + H
O ꢀ M
M + L + 2H
O ꢀ M
2
HL
L(OH) + H
L(OH) + 2H
2
+
(4 + x)
Cu –L1 complexes present [Cu
2
(H
x
L1)]
stoichiometries with
2
2
12.39(8)
—
x values 0 and 1. For 2:1 M:L molar ratio these species predominate
throughout all the pH range studied. It is interesting to emphasize
that the entry of the second metal ion is characterized by a
lower stability constant than the binding of the first metal ion
2
2
2
0.62(4)
a
b
Charges omitted for clarity. Values in parenthesis are standard devia-
c
tions in the last significant figure. Taken from ref. 8.
This journal is © The Royal Society of Chemistry 2008
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2+
2+
-3
Fig. 3 Distribution diagram for the formation of (A) Zn –L1 and (B) Cu –L1 complexes determined in water-ethanol 70/30 v/v NaCl 0.15 mol dm
at 298.1 ± 0.1 K.
Fig. 4 Steady-state fluorescence emission titration curves superimposed with the molar fraction distribution curves obtained by potentiometry
2
+
2+
2+
2+
(
2
A) Zn -L1 (molar ratio M:L 1:1); (B) Zn –L1 (molar ratio M:L 2:1); (C) Cu –L1 (molar ratio M:L 1:1) and (D) Cu –L1 (molar ratio M:L
:1). Studies performed in 0.15 mol dm- NaCl at 298.1 ± 0.1 K in water-ethanol 70/30 v/v for [L1] = 1 ¥ 10 M, lexc = 280 nm. (●) Fluorescence
3
-5
emission of the free ligand and (᭢) fluorescence emission of the metal complexes.
2
+
2+
2+
26
(
log KCuL1/Cu·L1 = 18.25 (7) and log KCu2L1/CuL1·Cu = 8.75 (4),
Co < Cu > Zn (see Fig. 5). Stability constants for the
respectively). This reduction in stability has to be related to energy
consumption due to macrocycle reorganization associated with
the binding of the second metal ion. Cu
one hydroxylation step and the hydroxyl group does not behave as
bridging ligand.
As expected for a ligand with eight donor atoms, several
complexes were formed in solution with the divalent metal ions
formation of Hg(II) complexes are very similar to those obtained
for the system Cu –L1. However, these values should be taken
just as informative due to the different ionic strength employed
for the determination of the constants in each system (Cu ,
NaCl; Hg , KNO
values are a little lower than those obtained for Zn complexes,
for Cd slightly higher stability constants are found. Such larger
values might be indicating a correspondingly higher coordination
number in the Cd–L1 complex.
2
+
4
+
2
L1 experiences only
2
+
2
+
2+
3
). While in the case of Pb , stability constant
2
+
2
+
2
+
studied. Mononuclear complexes with stoichiometries [ML] ,
(
i + 2)+
4+
2+
27
[MH
i
L]
(i = 1, 2), [ML(OH)
2
] and dinuclear [M
2
L] ,
5
+
(j-1)+
2+
[M
2
HL] , [M
2
L(OH)
j
]
(j = 1, 2)] have been found. The system
Regarding the fluorescence emission studies of Cu complexes,
a strong CHEQ effect can be observed at pH 3 when the first
complex species [CuH L1] is formed (Fig. 4C and D). In the
follows the expected Irving-Williams thermodynamic stability
order for the formation of high spin octahedral complexes towards
the first transition series of metal ions M : Mn < Fe
4
+
2
2
+
2+
2+
<
titrations performed at 1:1 M:L molar ratio, the quenching effect
6
534 | Dalton Trans., 2008, 6530–6538
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2
+
Table 3 Stability constants for the formation of Cu complexes of the
macrocycles L1 and L5 determined in water-ethanol 70/30 v/v NaCl
therefore have performed spectrofluorimetric titrations of L1 with
2+
2+
2+
2+
2+
2+
Mn , Fe , Co , Cd , Hg and Pb . Because the fluorescence
of this type of molecular sensors is often pH sensitive, we evaluate
its sensor capability at physiological pH. Fig. 6 shows that a
small chelation enhancement of fluorescence (CHEF effect) is only
-3
-3
0
2
.15 mol dm and in pure water 0.15 mol dm NaCl respectively at
98.1 ± 0.1 K
a
2+
b
2+
c
Reaction
Cu –L1
Cu –L5
2
+
observed in the presence of Cd and the fluorescence switching
M + L ꢀ ML
18.25(7)
25.93(4)
31.04(3)
—
27.00(4)
31.47(4)
—
20.13(9)
28.97(8)
36.09(7)
41.18(5)
31.06(9)
37.97(5)
41.3(1)
2+
“
ON” effect observed for L1 in the presence of Zn is much
M + H + L ꢀ MHL
larger. The presence of the other metal ions produces, at this pH,
a quenching of the fluorescence and consequently an “OFF” state
of the sensor. So it can also be concluded that L1 has higher
M + 2H + L ꢀ MH
2
L
L
M + 3H + L ꢀ MH
3
2
2
2
2
2
M + L ꢀ M
2
L
M + H + L ꢀ M
M + 2H + L ꢀ M
M + L + H
O ꢀ M
M + L + 2H
O ꢀ M
2
HL
2
+
selectivity for recognition of Zn at physiological pH. Taking into
account the fluorescence intensity variation with respect to the
concentration of the substrate (Fig. S4) and the minimum change
in fluorescence that can be reliably measured with the equipment
2
2
H L
2
2
L(OH) + H
L(OH) + 2H
—
—
21.2(1)
11.47(9)
2
2
2
a
b
Charges omitted for clarity. Values in parenthesis are standard devia-
2
+
c
tions in the last significant figure. Taken from ref. 8.
used, we can calculate a detection limit for Zn of 0.3 ppb at
28
physiological pH values.
Fig. 5 Plot of the first stepwise stability constants (log K; L1 + M ꢀML1)
Fig. 6 Bar diagram representation of the relative fluorescence intensity of
for M(II) ions.
-5
2+
2+
2+
2+
2+
2+
L1 ([L1] = 1 ¥ 10 M) upon addition of Cu , Mn , Fe , Cd ,Co , Pb ,
Hg and Zn in 1:1 M:L molar ratio at pH = 7. Titrations performed
in 0.15 mol dm NaCl at 298.1 ± 0.1 K in water-ethanol 70/30 v/v,
2+
2+
3
+
2+
-3
was extended throughout all species ([CuHL1] and [CuL1] )
at neutral and alkaline pH values. In the titration carried out at
l
exc = 280 nm).
2
:1 M:L molar ratio, a stronger quenching is observed for all the
4
+
5+
metal complexes presented in solution [CuH
and [Cu
Cu complexation is commonly observed in polyamine ligands
containing aromatic fluorophores and is attributed to an energy
transfer quenching of the p* emissive state through low-lying
2
L1] , [Cu
2
HL1]
Fig. S5 compares the titration curves of L1 in the absence and
4
+
2
L1] . This quenching of the fluorescence emission upon
in the presence of different transition and post-transition metals.
For 1:1 M:L molar ratio, until pH = 4, protonated forms of the
ligand are predominant in solution therefore, titration curves are
very similar except for Co which produces a small CHEF effect.
After pH = 4, Hg and Pb behave more or less the same way as
the free ligand while Co and Mn produce a quenching effect and
2
+
2
+
5,24
2+
2+
metal-centered states.
2
+
2+
2+
To evaluate the feasibility of L1 as a fluorescent sensor for Zn ,
2
+
the selectivity to various metal ions is one of the key elements. We
Cd is the only metal that causes an increase in the fluorescence
2+
2+
2+
2+
2+
2+
Table 4 Stability constants for the formation of Mn , Fe , Co , Cd , Hg and Pb complexes of the macrocycle L1 determined in water-ethanol
7
0/30 v/v NaCl 0.15 mol dm-3 at 298.1 ± 0.1 K
a
2+ b
2+
2+
2+
2+
c
2+
Reaction
Mn
Fe
Co
Cd
Hg
Pb
M + L ꢀ ML
7.86(2)
15.13(1)
—
—
—
—
—
—
—
10.74(2)
17.41(4)
23.02(8)
1.86(4)
-6.7(2)
—
—
9.07(3)
—
12.72(4)
20.39(2)
25.26(4)
—
—
—
—
—
-1.62 (4)
14.15(3)
20.62(1)
25.19(3)
—
—
—
—
—
—
18.36(9)
25.08(4)
—
—
—
26.17(2)
29.91(2)
—
11.58(2)
18.74(1)
24.58(1)
—
—
15.56(3)
—
M + H + L ꢀ MHL
M + 2H + L ꢀ MH
2
L
M + L + H
M + L + 2H
2
2
2
2
2
O ꢀ ML(OH) + H
O ꢀ ML(OH) + 2H
M + L ꢀ M
M + L + H ꢀ M
M + L +H
O ꢀ M
M + L +2H
O ꢀ M
2
2
2
L
2
HL
L(OH) + H
L(OH) + 2H
2
2
8.01(2)
—
2
2
2
—
a
Charges omitted for clarity. Values in parenthesis are standard deviations in the last significant figure. Determined in 0.15 mol dm-3 KNO
b
c
3
.
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Dalton Trans., 2008, 6530–6538 | 6535

View Article Online
emission. However, if operatingatpH 4, the maximumeffect would
6,20-Bis(naphthalene-2-ylmethyl)-3,6,9,17,20,23,29,30-octaaza-
2
+
11,15
be produced by addition of Cu that will give a decrease of the
tricyclo[23.3.1.1
]
triaconta-1(29),11,13,15(30),25,27-hexane
2
+
fluorescence (see Fig. S6). Addition of a ten-fold excess of Cu and
(L1·6HCl). 2,6-Pyridine-dicarboxaldehyde (0.51 g, 3.78 mmol)
dissolved in 100 mL of EtOH was added to a solution of 4-(2-
naphthylmethyl)-1,4,7-triazaheptane (L2) (0.92 g, 3.78 mmol) in
2
+
2+
Fe with respect to the detection limit of Zn at pH = 4 increases
to 0.6 ppb the amount of Zn required to restore emission of
2
+
+
+
the systems (see Fig. S6). However, even large excesses of Na . K ;
ethanol and was stirred for 4 h. NaBH (0.56 g, 15 mmol) was
4
2
+
2+
Ca and Mg , which are the most biologically relevant potentially
competing mobile ions do not perturb the fluorescence of L1. At
this pH, the extent of formation of the complexes of the other metal
ions is very limited and therefore the changes in fluorescence are
meaningless.
then added portionwise. The mixture was stirred for 2 h at room
temperature and the solvent was vacuum evaporated to dryness.
The residue was treated with 50 mL of water and extracted
with CH
2
2
Cl (3 ¥ 40 mL). The organic phase was dried with
anhydrous sodium sulfate and the solvent evaporated to dryness.
The resulting oil was dissolved in EtOH and L1 was precipitated
as its hexahydrochloride salt with aqueous HCl (0.82 g, 22%). Mp
Conclusions
◦
1
1
3
69–171 C. H NMR (D
2
O): d (ppm), 3.23 (t, 8H, J = 6 Hz),
.45 (t, 8H, J = 6 Hz), 4.04 (s, 4H), 4.18 (s, 8H), 7.12 (d, 4H,
We have described the synthesis, protonation and metal coordi-
nation properties of a new fluorescent chemosensor. The simple
and straightforward synthetic methodology combined with a
high overall yield provides a new approach for the design of
J = 8 Hz) 7.52–7.53 (m, 6H), 7.72 (t, 2H, J = 8 Hz), 7.79 (d, 2H,
O):
1
3
J = 8 Hz), 7.82–7.85 (m, 4H), 7.96 (b. s., 2H). C NMR (D
2
d (ppm), 44.1, 50.3, 50.5, 58.3, 123.1, 127.5, 127.6, 127.8, 128.1,
2
+
2+
128.2, 129.2, 130.4, 131.1, 133.1, 133.2, 139.6, 144.8, 150.2. Anal
selective Zn sensors. The binding of Zn linked to a 2:2 polyaza
pyridinophane macrocycle in which one of the amino groups of
each polyamine chain is attached to a fluorophore through a
single methylene carbon has profound enhancement effects on the
fluorescence of the system. This CHEF effect is much more marked
than that observed in the case of other full d-shell metal ions like
Calcd. For C44
H
66
N
8
6
Cl : C, 53.8, H, 6.8, N, 11.4. Found: C, 53.6,
H, 6.8, N, 11.7%.
Electromotive force measurements. Potentiometric measurements
2
+
2+
Cd allowing for selective detection of Zn in the presence of
such metal ions within a physiological pH window opening the
The potentiometric titrations were carried out either in water or
in the mixed solvent H O-EtOH 70/30 v/v at 298.1 ± 0.1 K
2
2
+
-3
possibility of Zn sensing in biological tissues. No perturbation of
the fluorescence is observed by other biologically relevant mobile
ions like Na , K , Mg and Ca . We are currently exploring the
use of this amplified fluorescence in the presence of Zn for the
detection of additional substrates.
using NaCl 0.15 mol dm as supporting electrolyte. NaCl was
chosen as inert electrolyte because of both the higher solubility
of the receptor in this medium and the high content of this
salt in extracellular fluids. The experimental procedure (burette,
potentiometer, cell, stirrer, microcomputer, etc.) has been fully
+
+
2+
2+
2
+
29
described elsewhere. The acquisition of the emf data was
30
performed with the computer program PASAT. The reference
electrode was an Ag/AgCl electrode in saturated KCl solution.
The glass electrode was calibrated as a hydrogen-ion concentration
probe by titration of previously standardized amounts of HCl with
Experimental
Materials
All chemicals and solvents were obtained from commercial
CO -free NaOH solutions and determining the equivalent point
by the Gran’s method, which gives the standard potential, E ¢,
2
31
◦
sources and used without further purification. L2 was prepared
24b
as described previously and isolated as its hydrochloride salt of
and the ionic product obtained were 13.73(1) in pure water and
32
formula L2·3HCl (Scheme 2).
pK
w
= 14.16(1) in H
2
O-EtOH 70/30 v/v.
Scheme 2
6
536 | Dalton Trans., 2008, 6530–6538
This journal is © The Royal Society of Chemistry 2008

View Article Online
The computer program HYPERQUAD was used to calculate
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33
the protonation and stability constants. The pH range investi-
+
gated (pH = -log [H ]) was 2.0–11.0. The different titration curves
4
for each ligand were treated as separated curves without significant
variations in the values of the stability constants. Finally, the sets
of data were merged together and treated simultaneously to give
the final stability constants.
1
515.
6
7
8
9
(a) G. W. Gokel, Chem. Soc. Rev., 1992, 21, 39; (b) G. W. Gokel, L. J.
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Spectrofluorimetric measurements
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The solvents used were of spectroscopic or equivalent grade. Water
was twice distilled and passed through a Millipore apparatus. All
-
3
solutions were prepared in H
NaCl. The pH values were measured with a Mettler-Toledo MP-
20 pH-meter and adjustments of the hydrogen ion concentration
2
O-EtOH 70/30 v/v 0.15 mol dm
J. Pina, J. Seixas de Melo, F. Pina, C. Lodeiro, J. C. Lima, A. J. Parola, C.
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1
of the solutions were made with diluted HCl and NaOH solu-
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spectroscopy system. The emission spectra were recorded with a
PTI MO-5020 spectrofluorimeter in the 300–500 nm range with
excitation wavelength of 280 nm.
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NMR measurements
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13
The H and C NMR spectra were recorded on Bruker Avance
1
DPX 300 MHz spectrometer operating at 299.95 MHz for H and
1
1
3
13
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1
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1
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4
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1
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538 | Dalton Trans., 2008, 6530–6538
This journal is © The Royal Society of Chemistry 2008