Hg2+-selective ratiometric and "off-on" chemosensor based on the azadiene-pyrene derivative

Article abstract of DOI:10.1021/ol1007636

A new pyrene-based derivative bearing an azadiene group was synthesized as a ratiometric chemosensor for Hg²⁺ in aqueous acetonitrile solution. The "off-on" type signaling behavior of the fluoroionophore is due to the metal ion induced conformational changes from the weak pyrene monomer emissions to strong pyrene excimer emission.

Full text of DOI:10.1021/ol1007636

ORGANIC
LETTERS
Hg2+-Selective Ratiometric and
“Off-On” Chemosensor Based on the
Azadiene-Pyrene Derivative
2010
Vol. 12, No. 11
2566-2569
Yi Zhou,† Chao-Yun Zhu,† Xu-Sheng Gao,† Xue-Yan You,† and Cheng Yao*,†,‡
College of Science and State Key Laboratory of Materials-Oriented Chemical
Engineering, Nanjing UniVersity of Technology, Nanjing 210009, P. R. China
yaochengnjut@126.com
Received April 4, 2010
ABSTRACT
A new pyrene-based derivative bearing an azadiene group was synthesized as a ratiometric chemosensor for Hg2+ in aqueous acetonitrile
solution. The “off-on” type signaling behavior of the fluoroionophore is due to the metal ion induced conformational changes from the weak
pyrene monomer emissions to strong pyrene excimer emission.
The development of artificial receptors for the sensing and
recognition of environmentally and biologically important
species has been actively investigated in recent years.1 In
this regard, chemosensors that can highly sensitively and
selectively detect heavy and transition metal ions (HTM)
such as Hg2+, Pb2+, Cd2+, and Cu2+ are especially important.2
Mercury is considered as a prevalent toxic metal in the
environment because both elemental and ionic mercury can
be converted by bacteria in the environment to methyl
mercury, which subsequently bioaccumulates through the
food chain.3 When absorbed in the human body, mercury
causes damage to the central nervous, DNA, mitosis, and
endocrine system.4 As a result, developing new and
practical multisignaling chemosensors for Hg2+ is still a
challenge.5
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† College of Science.
‡ State Key Laboratory of Materials-Oriented Chemical Engineering.
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10.1021/ol1007636  2010 American Chemical Society
Published on Web 05/07/2010

The pyrene moiety is one of the most useful fluorophores
for the construction of fluorogenic chemosensors for a variety
of important chemical species.6 Particularly, the introduction
of two pyrene moieties can be situated closely enough to
yield face-to-face orientation excimer emission. Upon co-
ordination with a specific guest ion, the resulting compound
could be fine-tuned to yield monomer and/or excimer
emissions depending on the orientation of the two pyrene
moieties.7 Utilizing the pyrene moiety of monomer vs
excimer emission, a variety of “on-off” type HTM ion
recognition systems were successfully devised.8 However,
the pyrene excimer “off-on” type system with HTM ions
has been rarely reported until now.9 Furthermore, only a few
ratiometric fluorescent probes for Hg2+ have been found in
the literature, and most of them were working only in pure
organic solvents.10 Herein, we describe a new azadiene-
pyrene derivative chemosensor 1 (Scheme 1), which shows
Chemosensor 1 was synthesized by the condensation of
2,2′-(ethylenedioxy)dianiline with 1-formylpyrene in 42%
yield (Scheme 1). A 2,2′-(ethylenedioxy)-diimino-based
molecular framework was designed as a platform for the
construction of efficient ionophores via its ether-methylene
O and imino N atoms.11 This Shiff base was stable in neutral
acetonitrile and water-acetonitrile solutions for at least 3
days. Thermogravimetric analyses showed that chemosensor
1 was stable up to 280 °C (Figure S1, see Supporting
Information), at which point decomposition of the compo-
nents occurred. The fluorescence emission at 406 nm was
relatively unaffected by pH values between 4.13 and 9.14
(Figure S4, Supporting Information). These observations
indicate that chemosensor 1, which is thermally stable, pH-
stable, and organic solvent-stable, may be useful as a
potential chemosensor material.
The absorption spectrum of 1 shows the typical pyrene
absorption bands at 238, 283, and 326 nm,12 along with a
slightly low-energy band centered at 395 nm attributed to
the imino bridge. Upon addition of 1-1.2 equiv of Hg2+
ions to the solution of 1, the most significant changes were
broadening of the absorption bands around 350-520 nm,
and a new red-shifted band was formed at 442 nm (Figure
1). The formation of the new low-energy band may be
Scheme 1. Synthesis of Chemosensor 1
a selective, sensitive, and reversible fluorescence enhance-
ment response to the Hg2+ ion due to the two pyrene
moieties’ orientation excimer emission.
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Figure 1. UV-vis spectra of chemosensor 1 (10 µM) in the
presence of Hg2+ ions (1.0 equiv and 1.2 equiv) in CH3CN. Inset
shows the change in color of chemosensor 1 (10 µM) upon addition
of Hg2+ ions (10 µM).
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attributed to the interaction of Hg2+ ions with the imino
nitrogens leading to the intramolecular charge transfer from
the pyrene moieties to the imino groups. The new low-energy
band (47 nm red shift) is responsible for the change of color,
which is perceptible to the naked eye, from colorless to pale
yellow.
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The fluorescent properties of 1-Hg2+ were surveyed in
typical organic solvent systems including their aqueous
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Org. Lett., Vol. 12, No. 11, 2010
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solutions. 1-Hg2+ exhibited a weak monomer emission
around 395-410 nm with characteristic, but a very intense,
excimer emission of the pyrene centered at 462 nm (Figure
S3, Supporting Information). The fluorescence characteristics
of 1-Hg2+ were found to be strongly dependent on the nature
of the employed medium. 1-Hg2+ in aqueous acetonitrile
(HEPES-CH3CN, 10:90, v/v) displayed a relatively weak
excimer emission at 462 nm. The pyrene excimer region
increased significantly as the water content increased,
particularly in the vicinity of 80% water composition, and
then the changes were not so significant up to 90% aqueous
solution (Figure S2, Supporting Information). This observa-
tion implies that complexation of 1 with Hg2+ ions presum-
ably changes from its chair conformation to stacked or folded
conformation resulting in the switch of the pyrene monomer
emissions to an excimer emission.
the earlier reported alkylamine-pyrene derivative com-
pounds,13 and the fluorescence intensity increased 3.2-fold
upon added 10 µM Cu2+ ions (8.3-fold with 100 equiv of
Cu2+ ions). The discrimination between two oppositely
responding Hg2+ and Cu2+ ions could be realized ratiometri-
cally by using the large changes in Hg2+ to trigger the
monomer-excimer switching of 1. To test whether the
proposed complex could be reversed, we added Na2S (0.1
M) to the solutions of 1-Hg2+ species. The addition of 0.55
mL of Na2S could restore the initial value of free probes
due to the Kd value of 10-50 M2 for Hg2+ at a standard
condition in the form of [HgS2]2-.14 Thus, the probes could
be revived on addition of Hg2+ (Figure S5, Supporting
Information).
The titration reaction curve showed a steady and smooth
increase until a plateau was reached (15.0 µM Hg2+) with
an 18-fold increase at the plateau (Figure 3), suggesting
The fluorescence spectrum of chemosensor 1 with several
metal cations (Na+, K+, Mg2+, Ca2+, Cd2+, Mn2+, Fe3+, Ni2+,
Ag+, Pb2+, Co2+, Zn2+, Cr3+, Cu2+, and Hg2+) using their
perchlorate salts in CH3CN/HEPES (pH 7.2, 80:20, v/v) is
investigated by titration studies (Figure 2). A 15.2-fold
Figure 3. Fluorescence titration of 1 (1 µM) in HEPES buffer
(50 mM, pH ) 7.2) aqueous buffer solutions in the presence of
different amounts of Hg2+. Excitation was performed at 365 nm.
Inset: fluorescence intensity at 462 nm as a function of Hg2+
concentration.
Figure 2. Fluorescence responses of 1 µM 1 to various 10 µM
metal ions. Bars represent the final (Ff) over the initial (Fi) integrated
emission. Spectra were acquired in (HEPES-CH3CN, 80:20, v/v)
pH 7.2. The red bars represent the addition of the competing metal
ion to a 1 µM solution of 1. The blue bars represent the change of
the emission that occurs upon the subsequent addition of 10 µM
Hg2+ to the above solution. Excitation was provided at 365 nm,
with emission integrated over 380-600 nm.
the completion of the monomer-excimer switch of 1. The
chemosensor exhibited very efficient fluorescence re-
sponding, and over 65% of the total fluorescence intensity
increase was observed with 1 equiv of Hg2+ ions. The
association constant (Ka) of 1 with Hg2+ is 4.32 × 105
M-1 (error <10%), obtained by a nonlinear curve fitting
of the fluorescence titration results.15 The inset in Figure
3 exhibits the dependence of the intensity ratios of
emission at 462 nm (Ff/Fi) on Hg2+. This curve can be
enhancement of fluorescence and a continuous red shift of
the emission peak from 406 to 462 nm were observed upon
the addition of 10 equiv of Hg2+ compared to that of only 1
in CH3CN/HEPES mixture solutions. Upon interaction with
varying metal ions of alkali (Na+, K+), alkaline earth (Mg2+,
Ca2+), and transition-metal ions (Cd2+, Mn2+, Fe3+, Ni2+,
Ag+, Pb2+, Co2+, Zn2+, and Cr3+), a much weaker response
is given compared to Hg2+ at the same concentration (10
µM), and the fluorescence signal of 1-Hg2+ in the presence
or absence of these contrast ions also exhibited only a mild
difference. With Cu2+ ions, however, a slightly response of
the pyrene excimer emission was observed, consistent with
served as the calibration curve for the detection of Hg2+
.
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Org. Lett., Vol. 12, No. 11, 2010

The detection limit16 was also estimated from the titration
Information). These H NMR and grind reaction results
1
results and was 2 × 10-7 M.
suggest that Hg2+ coordinated with the imino of the Schiff
base forms Hg-N bonding and that O atoms of ether-
methylene forms Hg-O bonding.
1
As shown in the H NMR spectra of Figure 4a and 4b,
treatment of 1 equiv of Hg2+ resulted in a considerable
1
On the basis of the H NMR spectroscopy, the plausible
binding mechanism of 1 in the present system is schemati-
cally depicted in Figure 4c. The transformation into folded
conformation might be partly attributed to the participation
of the imino of the Schiff base and O atoms of ether-
methylene in the complex formation with the guest metal
ions, which forces the two pyrene moieties to become
relatively folded with face to face orientations. The confor-
mational changes result in the weak pyrene monomer
emission to strong pyrene excimer emission switch.
In summary, we have developed a new fluorescent sensor
for Hg2+ on the basis of the pyrene monomer vs excimer
emission with high sensitivity and selectivity. Moreover, this
simple molecule makes it possible to detect the Hg2+
ratiometrically. This chemosensor was easily prepared and
found to be stable in both alkaline and acidic solutions. The
spectral response toward Hg2+ was established to be revers-
ible. The design strategy and remarkable photophysical
properties of the two pyrene moieties would help to extend
the development of fluorescent sensors for HTM ions.
Figure 4
in DMSO-d6. (c) Proposed mechanism of binding mode of 1 with
Hg2+
.
1H NMR spectra of 1 (a) and in the presence of Hg2+ (b)
.
Acknowledgment. This work is supported by the Open
Fund of the State Key Laboratory of Materials-Oriented
Chemical Engineering (KL09-9) and the National Natural
Science Foundation of China (Grants 20971017). We also
thank Dr. Jie Song (China Pharmaceutical University) for
his kind help in NMR characterization.
downfield shift of imino protons by δ 1.22 ppm. There was
also a slight upfield shift of ether-methylene protons by δ
0.12 ppm. The corroborative evidence for the 1-Hg2+
complex was observed in their solid state reaction, which
showed that after one minute grinding in the mortar the light
yellow solid 1 turned to an obvious reddish-brown color solid
in the presence of 1 equiv of Hg2+, and other ions of our
interest showed little interference (Figure S6, Supporting
Supporting Information Available: Synthesis, experi-
mental details, and additional spectroscopic data. This
material is available free of charge via the Internet at
http://pubs.acs.org.
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1996, 68, 1414–1418.
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