Fluorescent Hg2+ sensors: Synthesis and evaluation of a tren-based starburst molecule containing fluorinated 1,2,4-oxadiazoles

Article abstract of DOI:10.1002/ejoc.201000763

A new tren-based starburst molecule containing fluorinated 1,2,4-oxadiazoles as fluorophores has been synthesized and its sensing behavior toward several metal cations has been investigated by V/Vis, fluorescence, ₁H NMR and ₁₉F NMR spectroscopy. Selective sensing for Hg²⁺ ions through a PETbased mechanism was evidenced, suggesting application as fluorescent sensor for Hg²⁺ of the off-on type.

Full text of DOI:10.1002/ejoc.201000763

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201000763
Fluorescent Hg2+ Sensors: Synthesis and Evaluation of a Tren-Based Starburst
Molecule Containing Fluorinated 1,2,4-Oxadiazoles
Ivana Pibiri,[a] Antonio Palumbo Piccionello,*[a] Alessandro Calabrese,[a,b]
Silvestre Buscemi,[a] Nicolò Vivona,[a] and Andrea Pace[a]
Keywords: 1,2,4-Oxadiazoles / Sensors / Electron transfer / Fluorinated ligands / Mercury
A new tren-based starburst molecule containing fluorinated
1,2,4-oxadiazoles as fluorophores has been synthesized and
its sensing behavior toward several metal cations has been
investigated by UV/Vis, fluorescence, 1H NMR and 19F NMR
spectroscopy. Selective sensing for Hg2+ ions through a PET-
based mechanism was evidenced, suggesting application as
fluorescent sensor for Hg2+ of the off-on type.
rials science,[9] as well as for their rearrangement reac-
tions[10] and photochemical reactivity.[11] Nevertheless, at
the best of our knowledge, no 1,2,4-oxadiazole-containing
fluorophore sensor has been previously reported. Starting
from these considerations, we were encouraged to synthe-
size 1,2,4-oxadiazole-containing starburst molecules as
scaffold for the design of a sensor whose fluorescence could
be turned on upon complexation of a cation. In this work
we report the synthesis and the evaluation of a tren-based
molecule where the tren moiety is responsible for metal
complexation.
Introduction
Fluorescent chemosensors for the detection of heavy
metal ions by a simple fluorescence turn-on (off-on) or
quenching (on-off) are a fundamental tool for a rapid and
nondestructive analysis of biochemical and environmental
matrices.[1] In this field, a fundamental topic is represented
by the design of selective sensors for Hg2+, which is one of
the most hazardous pollutants for the environment.[2] Many
examples of fluorescent sensors for Hg2+ have been re-
ported.[3] The majority of them is based on fluorescence
quenching (on-off response),[4] whereas others involve
fluorescence enhancement (off-on response).[5] Moreover,
due to its coordination chemistry, which takes advantage of
a tripodal structure, several sensors containing the tris(2-
aminoethyl)amine (tren) moiety have been reported.[6]
Among these, only a few tren-based sensors were selective
for Hg2+ [7]
A structural similarity with tren-based sensors
.
could be envisaged in some fluorinated 1,2,4-oxadiazole-
containing starburst molecules 1, with a triethanolamine
core, which were proposed as blue emitters in OLEDs (Fig-
ure 1).[8] For some of these derivatives, emission is very
weak or not observed at all, and this was attributed to intra-
molecular self-quenching by photoinduced electron transfer
(PET) between the tertiary amino and the oxadiazole moie-
ties. However, upon protonation of the tertiary amino nitro-
gen atom, emission of the oxadiazole fluorophore was de-
tected (Figure 1).[8] 1,2,4-Oxadiazoles are widely studied for
their applications either in medicinal chemistry or in mate-
Figure 1. pH-sensitive starburst oxadiazoles.
Results and Discussion
We previously reported that 5-(polyfluorophenyl)-1,2,4-
oxadiazoles easily undergo a classic nucleophilic aromatic
substitution of the 4Ј-fluoro moiety under mild experimen-
tal conditions.[12,13] To take advantage of such reactivity, we
synthesized 5-(pentafluorophenyl)-1,2,4-oxadiazole 3 to be
used in the functionalization of the terminal amino groups
of tren. The synthesis of oxadiazole 3 was accomplished in
a two-step procedure. Initially, amidoxime 2 was treated
with pentafluorobenzoyl chloride to yield the correspond-
ing O-(pentafluorobenzoyl)amidoxime. This product was
then heated under solvent-free conditions to give oxadi-
azole 3 (Scheme 1). Reaction of 3 with tren, in a 3:1 molar
ratio, was performed in a DMF suspension of K2CO3. Re-
[a] Dipartimento di Chimica Organica “E. Paternò”, Università
degli Studi di Palermo,
Viale delle Scienze – Parco d’Orleans II, 90128 Palermo, Italy
Fax: +39-091-596825
E-mail: apalumbo@unipa.it
[b] ENEA, Dipartimento ACS, Sezione PROT-STP,
Via Catania 2, 90141 Palermo, Italy
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201000763.
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A. Palumbo Piccionello et al.
SHORT COMMUNICATION
moval of the solvent and chromatography of the residue
produced the target compound 4 (77%) together with some
amounts of regioisomer 5 (5%), arising from SNAr on the
ortho position (Scheme 1).
–
Figure 2. Fluorescence spectra of 4 (1 µ) with addition of ClO4
salts of Hg2+, Pb2+, Cu2+, Zn2+, Ca2+, Cd2+, Co2+, Ni2+, Mg2+
,
Li+, Na+, K+, and Ag+ (10 equiv., each) in CH3CN with an exci-
tation at 313 nm.
Scheme 1. Synthesis of compound 4. (i) C6F5COCl, K2CO3, acet-
one, room temp., 1 h; (ii) 180 °C, solvent-free, 1 h; (iii) tren, K2CO3,
DMF, room temp., 2 h.
The structure of compound 4 was confirmed by means
of spectroscopic data (1H NMR, 13C NMR, 19F NMR, FT-
IR and MS), and its photophysical behavior was first evalu-
ated through UV/Vis and fluorescence spectroscopy. UV/
Vis spectra recorded in acetonitrile show a strong absorp-
tion band centered at 313 nm (ε = 79100 –1 cm–1). More-
over, compound 4 is non-fluorescent under neutral condi-
tions, whereas its emission spectra showed a distinctly en-
hanced fluorescence, centered at 430 nm, under acidic con-
ditions, by adding 2 equiv. of HCl. This suggests that fluo-
rescence of compound 4 is quenched through a PET from
the tertiary nitrogen atom of tren, as expected (see Support-
ing Information). From the UV/Vis spectra of 4 (10 µ) in
acetonitrile upon addition of 10 equiv. of various perchlo-
Figure 3. Change in fluorescence spectra (λex = 313 nm) of 4 (2 µ)
measured in CH3CN upon addition of Hg2+. Inset: change in
fluorescence intensity at 420 nm.
rate salts of metal ions, such as Hg2+, Pb2+, Cu2+, Zn2+
,
In order to assess the possibility of an Hg–fluorophore
interaction involved in the observed fluorescence enanche-
ment, compound 6 was synthesized as a model for a fluoro-
phoric group in compound 4 (Scheme 2). Indeed, fluores-
cence spectra of compound 6, were not affected by Hg2+
addition (see Supporting Information), thus excluding any
involvement of Hg–fluorophore interactions in the sensing
activity of compound 4.
Ca2+, Cd2+, Co2+, Ni2+, Mg2+, Li+, Na+, K+, and Ag+, no
spectral change could be observed. On the other hand, by
measuring fluorescence emission spectra of 4 (1 µ in
CH3CN) in the presence of 10 equiv. of metal salts, a strong
emission band, centered at 420 nm, appears in the presence
of Hg2+ ions (I/I0 = 23) (Figure 2). In contrast, other ions
lead to much smaller spectral changes (Cu2+ and Pb2+: I/I0
Ͻ 9) or almost no spectral changes (Zn2+, Ca2+, Cd2+
,
Co2+, Ni2+, Mg2+, Li+, Na+, K+, and Ag+) (Figure 2),
which indicates that 4 shows a good sensitivity toward Hg2+
and selectivity over other competitive cations. Figure 3
shows the results of fluorescence titration of 4 with Hg2+
.
The Hg2+ addition causes an increase in fluorescence, which
reaches its maximum upon addition of 1.5 equiv. of Hg2+
;
with 1 equiv. of Hg2+ an enhancement factor I/I0 = 22.5
was observed.
Scheme 2. Synthesis of reference compound 6.
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Fluorescent Hg2+ Sensors
To obtain further information about the binding mode to be affected by the presence of various metal cations (Fig-
of sensor 4, 1H NMR titration experiments were performed ure 6). Under these conditions, with 1 equiv. of Hg2+ an
in CD3CN. After addition of increasing amounts of Hg2+
,
enhancement factor I/I0 = 1.2 was observed.
the only protons affected were those of the tren moiety,
whose signals were broadened and shifted downfield,
whereas the proton signals of the ethoxycarbonyl group re-
mained unchanged (Figure 4). Similarly, 19F NMR experi-
ments showed a major downfield shift of signals relative to
fluorine atoms in ortho positions to the amino moiety (see
Supporting Information).
Figure 6. Fluorescence changes (430 nm) for 4 [2 µ in H2O/
CH3CN (5% v/v)] upon addition of metal cations Mn+ (40 µ) or
Mn+ + Hg2+ (40 µ + 40 µ).
From the titration profile at 430 nm of 4 with Hg2+ (un-
der mixed solvents conditions), the nonlinear fitting of the
titration curve (on the basis of a 1:1 stoichiometry) allowed
the determination of the association constant of 4 with the
Hg2+ ion, calculated to be 4.06Ϯ0.04ϫ104 –1 (Fig-
ure 7).[14]
1
Figure 4. H NMR spectroscopic data of 4 (10–2 ) in CD3CN in
the presence of various amounts of Hg(ClO4)2.
These pieces of evidence confirm that only the tren por-
tion is involved in the Hg2+ complexation, suggesting the
formation of a 1:1 complex. As a consequence of metal
binding, which inhibits the PET from the tertiary nitrogen
atom of tren to the oxadiazole ring, the fluorescence of the
oxadiazole luminophore is restored (Figure 5).
In order to evaluate a practical applicability of the Hg
sensor in aqueous media, the sensing ability of compound
4 was tested in H2O/CH3CN mixtures in the presence of
Hg2+, by adding increasing amounts of water. As the water
content increases, a decrease of the fluorescence was ob-
served. In this context a mixture containing 5% H2O (v/v)
was selected as a good compromise for further investi-
gations. Under these conditions, compound 4 continues to
Figure 7. Change in fluorescence intensity at 430 nm (λex = 313 nm)
of 4 [2 µ in H2O/CH3CN (5% v/v)] upon addition of Hg2+
.
The 1:1 stoichiometry was also confirmed by means of
show a higher selectivity toward Hg2+, and competitive the Benesi–Hildebrand method;[15] a plot of 1/(I – I0) vs.
binding experiments reveal that the Hg sensing seems not 1/[Hg2+]0 shows a linear relationship, and also the associa-
Figure 5. Rationale for the sensing mechanism of compound 4.
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A. Palumbo Piccionello et al.
SHORT COMMUNICATION
tion constant of 4 with the Hg2+ ion (calculated to be
4.07Ϯ0.05ϫ104 –1) is in agreement with the value found
from non-linear fitting (see Supporting Information).
gram. Financial support through the University of Palermo is
gratefully acknowledged.
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Conclusions
A new tren-based sensor with good selectivity toward
Hg2+ has been designed and synthesized in few steps and
high yields. The sensing mechanism of this new fluorogenic
chemosensor, containing for the first time an 1,2,4-oxadia-
zole as luminophore, is based on the inhibition of a photo-
induced electron transfer from tren to oxadiazole, after co-
ordination of the metal ion with a 1:1 stoichiometry. The
activity of this sensor has been investigated also in mixed
aqueous/organic solvents. Moreover, the coordinating abi-
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be affected by the link with the electron-withdrawing fluo-
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new sensors based on the easily obtainable fluoroarylated
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Experimental Section
Synthesis of Compound 4: To a stirred solution of tris(2-amino-
ethyl)amine (44.8 µL, 0.3 mmol) in dry DMF (3 mL), K2CO3
(0.138 g, 1 mmol) and compound 3 (0.277 g, 0.9 mmol) were
added, and the suspension was maintained at room temperature
under efficient stirring for 2 h. The reaction mixture was then di-
luted with water (50 mL) and extracted with EtOAc (200 mL). The
organic phase was dried with Na2SO4, filtered, and concentrated
in vacuo. The obtained residue was chromatographed to give unre-
acted 3 (28 mg, 10%), compound 4 (233 mg, 77%) and compound
5 (15 mg, 5%).
1
Compound 4: M.p. 161–163 °C (EtOH). H NMR (300 MHz, [D3]-
acetonitrile): δ = 1.39 (t, J = 7.2 Hz, 3 H, CH2CH3), 2.84 (t, J =
6.0 Hz, 2 H, NCH2), 3.57–3.63 (m, 2 H, NHCH2), 4.46 (q, J =
7.2 Hz, 2 H, OCH2) 5.46 (br. s, 1 H, exchangable with D2O) ppm.
13C NMR (1H-decoupled, 75 MHz, [D3]acetonitrile, ): δ = 14.2,
43.1, 54.1, 63.8, 133.9, 134.9 (d, JC,F = 128.5 Hz), 146.8 (d, JC,F
=
261.8 Hz), 158.3, 163.0, 170.7 ppm. 19F NMR (283 MHz, [D3]-
acetonitrile): δ = –140.4, –162.7 ppm. UV (CH3CN) λmax = 313 nm
(ε = 79100 –1 cm–1). FT-IR (Nujol): ν = 3379, 1751, 1734,
˜
1655 cm–1. GC–MS: m/z (%) = 1010 (100) [M+]. C39H30F12N10O9
(1010.70): calcd. C 46.35, H 2.99, N 13.86; found C 46.40, H 2.90,
N 13.90.
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Spectroscopic Material and Methods: Stock solutions (0.01 ) of
the metal perchlorate salts were prepared in CH3CN or H2O/
CH3CN. Stock solutions of 4 and 6 were prepared in CH3CN or
H2O/CH3CN. For all fluorescent tests, the excitation wavelength
was 313 nm with excitation and emission slit widths of 3 nm.
Supporting Information (see footnote on the first page of this arti-
cle): Experimental procedures, NMR spectra of compounds 3–6,
and additional spectra and titration data.
Acknowledgments
Prof. Y. Kuroda, Kyoto Institute of Technology, is deeply acknowl-
edged for useful discussions and for the use of the SPANA pro-
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Fluorescent Hg2+ Sensors
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[14] The association constant was calculated with the computer
program SPANA: Yasuhisa Kuroda, SPANA, Department of
Polymer Science Kyoto Institute of Technology Matsugasaki,
Sakyo, Kyoto, Japan, 2005.
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Received: May 27, 2010
Published Online: July 16, 2010
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