Reaction-based Hg2+ signaling by excimer-monomer switching of a bis-pyrene dithioacetal

Article abstract of DOI:10.1016/j.tetlet.2013.07.109

A novel reaction-based probe for fluorescence signaling of Hg²⁺ ions was developed. Selective Hg²⁺-induced cleavage of a dithioacetal resulting in switching from pyrene excimer to monomer emission was used for the signaling. Changes in excimer and monomer emissions of pyrene were readily employed for ratiometric signaling of Hg²⁺ ions in aqueous acetonitrile. Selective signaling of Hg²⁺ ions over other common metal ions was observed with a detection limit of 9.8 × 10^-7 M.

Full text of DOI:10.1016/j.tetlet.2013.07.109

Tetrahedron Letters 54 (2013) 5341–5344
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Reaction-based Hg2+ signaling by excimer–monomer switching of a
bis-pyrene dithioacetal
⇑
⇑
Yoonha Cho, Seul Ki Lee, Jung Woo Lee, Sangdoo Ahn , Suk-Kyu Chang
Department of Chemistry, Chung-Ang University, Seoul 156-756, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
A novel reaction-based probe for fluorescence signaling of Hg2+ ions was developed. Selective Hg2+
-
Article history:
Received 3 June 2013
Revised 16 July 2013
Accepted 19 July 2013
Available online 27 July 2013
induced cleavage of a dithioacetal resulting in switching from pyrene excimer to monomer emission
was used for the signaling. Changes in excimer and monomer emissions of pyrene were readily employed
for ratiometric signaling of Hg2+ ions in aqueous acetonitrile. Selective signaling of Hg2+ ions over other
common metal ions was observed with a detection limit of 9.8 Â 10À7 M.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Hg2+ signaling
Bis-pyrene
Excimer–monomer switching
Dithioacetal
Ratiometry
Pyrene is one of the most widely used fluorophores for the
construction of a variety of smart chemosensors.1 In particular,
signaling using the characteristic monomer–excimer switching of
pyrene emission has attracted much research attention.2 An exci-
mer is a unique pyrene fluorophore emission that occurs as molec-
ular conformation changes allow two pyrene units to become close
in proximity. Intricate probes using pyrene monomer–excimer
switching have been successfully developed for the signaling of a
number of important metal ions, anions, and neutral molecules
for Hg2+ ions are a squaraine–sulfide conjugate,16 anthracene thio-
amide,17 thiocoumarins,18 rhodamine thiosemicarbazides,19 rho-
damine thioureas,20 and rhodamine thiocarbonyl-benzothiazole
derivative.21 In addition, sulfide-based probes for Hg2+ ions have
utilized the hydrolysis of a dithiolane protected coumarin to the
corresponding aldehyde,22 elimination followed by intermolecular
cyclization of dithiaspiroacenaphthene quinine with o-phenylene-
diamine,23 and conversion from a non-fluorescent dithioacetal
bearing a triphenylamine moiety to the fluorescent aldehyde.24
However, the example of reaction-based excimer–monomer
switching of pyrenes is rare and has only recently been used for
signaling of fluoride ions by fluoride-induced Si–O bond cleavage
of pyrene dimers.25
In this Letter, we devised a novel reaction-based probe for the
signaling of Hg2+ ions based on the Hg2+-assisted cleavage of a
dithioacetal moiety. The deprotection of dithio-acetal and ketals
by the selective cleavage with Hg2+ ions proceeds under mild con-
ditions and used for the protection of aldehydes and ketones in or-
ganic synthesis (Scheme 1).26 Using this deprotection process, a
compound with two pyrene moieties linked by a dithioacetal sub-
unit was designed as a Hg2+ probe. The probe structure could allow
a conformation in which two pyrene moieties would be situated in
close proximity for excimer emission. Upon dithioacetal hydroly-
sis, the pyrene subunits are no longer linked, and excimer emission
would not be possible. Using this approach, we obtained an effi-
cient pyrene excimer to monomer switching by Hg2+ ions, that is
a novel strategy for the signaling of this important but toxic
species.
such as Cu2+ 3 Zn2+ 4 Hg2+ 5 Ag+,6 fluoride,7 ATP and ADP,8 DNA,9
, , ,
and carbohydrates.10 Sensor activation is based on various supra-
molecular interactions that induce the conformation of the pyrene
groups to attain close proximity, such as guest-induced ring flips or
molecular rotation,5,11 complexation-induced dimerization,12 and
interaction with cyclodextrin.13
Hg2+ ions are toxic heavy metal ions widely used in various
industrial activities and have a serious environmental impact.14
A
number of well-designed chemosensors for the selective and
sensitive signaling of this important species have been reported.15
Among the many approaches, reaction-based chemosignaling
systems for Hg2+ ions have attracted considerable research inter-
est. In particular, sulfur derivatives that utilize the thiophilic
nature of Hg2+ ions are widely employed for this purpose. Repre-
sentative examples of thio-functionalized reaction-based probes
⇑
Corresponding authors. Tel.: +82 2 820 5230; fax: +82 2 825 4736 (S.A); tel.:
+82 2 820 5199; fax: +82 2 825 4736 (S.-K.C.).
E-mail addresses: sangdoo@cau.ac.kr (S. Ahn), skchang@cau.ac.kr (S.-K. Chang).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.07.109

5342
Y. Cho et al. / Tetrahedron Letters 54 (2013) 5341–5344
Hg2+
of probe 1 (vide infra). Other metal ions resulted in virtually no
S R
S R
O
H
changes in the fluorescence profile. Meanwhile, the UV–vis spec-
trum of 1 was characterized by strong pyrene absorptions at
326–342 nm (Fig. S1, Supplementary data). However, changes in-
duced by Hg2+ ions were not observed because the electronic prop-
erties of the pyrene moiety were not significantly affected by
dithioacetal cleavage.
Scheme 1. Deprotection of dithioacetal with Hg2+ ions.
Dithioacetal probe 1 was prepared by the three-step reaction
sequence shown in Scheme 2. Reaction of 4-ethoxybenzaldehyde
with ethyl thioglycolate afforded diester 2 (I2, CH2Cl2, 94%), which
was hydrolyzed to dicarboxylic acid 3 (NaOH, aqueous MeOH,
77%). Esterification of 3 with 1-pyrenebutanol (EDC, HOBt, DMF)
yielded dithioacetal 1 in moderate yield (60%) (Scheme 2).27
Signaling behavior of 1 by excimer–monomer switching was
optimized in a series of mixed aqueous solutions of varying water
content. A strong pyrene excimer was observed in mixtures up to
50% aqueous acetonitrile, while higher proportion of water led to
unstable fluorescence due to limited solubility of 1. However, the
signaling profile of 1, including the concentration dependent emis-
sion behavior and relative intensities of monomer and excimer,
was most favorable in 10% aqueous acetonitrile. Therefore, all sub-
sequent signaling measurements of probe 1 were carried out in
10% aqueous acetonitrile.
The Hg2+-selective signaling behavior of 1 was more clearly
visualized by ratiometric analysis of changes in fluorescence spec-
tra induced by various metal ions (Fig. 1). Changes in the fluores-
cence intensity ratio of excimer and monomer at 480 and
395 nm (I480/I395) clearly showed the Hg2+ selectivity of 1. Upon
treatment with Hg2+ ions, the intensity ratio significantly de-
creased from 6.1 to 0.03 (Fig. S2, Supplementary data). Other metal
ions gave a nearly constant ratio (I480/I395 = 6.0 for Cu2+ and 6.3 for
Fe3+).
Signaling from excimer–monomer switching is due to Hg2+-in-
duced cleavage of the dithioacetal moiety of 1 to the corresponding
precursor compounds.26 Cleavage of the dithioacetal group of 1 by
Hg2+ ions removes the distance restriction between pyrene groups,
which drastically reduced the tendency for excimer formation
(Scheme 3). From this transformation, as shown in Figure 1, the
signaling strategy using excimer–monomer conversion of dithio-
acetal 1 by Hg2+ ions performed well. The signaling process could
be followed by 1H NMR and mass spectral measurements. The 1H
NMR spectrum obtained for the purified reaction product of probe
1 in the presence of 5 equiv of Hg2+ ions revealed 1-pyrenebutanol
(Fig. 2). In addition, when the 1H NMR spectrum was obtained di-
rected from the reaction mixture, resonances for 4-ethoxybenzal-
dehyde, thioglycolic acid, as well as 1-pyrenebutanol were
observed (Fig. S3, Supplementary data). The resonance for an alde-
hyde proton of 4-ethoxybenzaldehyde at 9.83 ppm appeared while
resonance for the same methine proton at 5.15 ppm of the dithio-
acetal moiety disappeared. Under the measuring conditions,
thioglycolic acid forms a stable complex with Hg2+ ions to form
Hg(SCH2CO2H)2 (logKassoc = 43.82).28 The mass spectral measure-
ment of the 1-Hg2+ system also revealed a peak for 1-pyrenebuta-
The fluorescence spectrum of 1 in 10% aqueous acetonitrile was
characterized by a strong excimer emission at 480 nm along with
weak monomer emissions at 375–395 nm. Upon treatment with
Hg2+ ions, a prominent emission at 375–415 nm appeared, while
the pyrene excimer emission disappeared (Fig. 1). This excimer
to monomer switching is due to cleavage of the dithioacetal moiety
O
OR
I2
H
O
O
S
S
+
EtO
OEt
EtO
CH2Cl2,
rt, 4 h
HS
OR
O
2
3
: R = Et
: R = H
1) NaOH/MeOH
nol at m/z = 274.12. These observations suggest that the Hg2+
-
2) 1M HCl
induced dithioacetal cleavage product was further hydrolyzed to
produce 1-pyrenebutanol and Hg2+-complexed thioglycolic acid
under the signaling conditions. The signaling of Hg2+ ions by 1
was fast, allowing analysis within 10 min after sample preparation
(Fig. S4, Supplementary data).
To assess the potential for practical applications of the probe,
competitive signaling of Hg2+ ions by 1 was measured. In the pres-
ence of 10 equiv of a variety of common metal ions as background,
Hg2+-selective signaling was not significantly affected (Fig. 3). Only
Fe3+ ions gave significant interference. The interaction of triply
charged Fe3+ with the ester carbonyls of probe 1 appears to hinder
the approach of Hg2+ ions and the subsequent cleavage reaction.
O
O
4
4
S
S
1-Pyrenebutanol
EtO
3
EDC, HOBt / DMF,
rt, 4 h
O
O
1
Scheme 2. Preparation of dithioacetal probe 1.
8
7
6
5
4
3
2
1
0
only
1
1
+ Li(I), Na(I), K(I), Mg(II),
Ca(II), Ba(II), Fe(II), Fe(III),
Co(II), Ni(II), Cu(II), Zn(II),
Ag(I), Cd(II), Pb(II)
+ Hg (II)
1
O
O
Hg2+
S
S
4
4
EtO
Acetate buffer / Acetonitrile
(1:9, v/v) pH = 4.8
O
O
1
Pyrene Excimer
OH
OH
+
HS
4
O
350
400
450
500
550
600
650
Wavelength (nm)
+
CHO
EtO
Pyrene Monomer
Figure 1. Fluorescence spectra of probe 1 in the presence of various metal ions.
[1] = 5.0 Â 10–6 M, [Hg2+] = [Pb2+] = [Fe3+] = 5.0 Â 10–5 M, [Mn+] = 1.0 Â 10–4 M for
other metal ions. In a mixture of acetate buffer (pH 4.8) and acetonitrile (1:9 v/v).
[Acetate buffer] = 10 mM. kex = 340 nm.
Scheme 3. Hg2+ signaling by pyrene excimer to monomer conversion via cleavage
of dithioacetal 1.

Y. Cho et al. / Tetrahedron Letters 54 (2013) 5341–5344
5343
the fluorescence intensities of two characteristic emissions at 395
and 480 nm. In particular, the ratio changed significantly with up
to 1.0 Â 10–5 M of Hg2+ ions and could be used as a calibration plot
for Hg2+ quantification. The detection limit of 1 for the determina-
tion of Hg2+ ions was 9.8 Â 10–7 M.29
In summary, a new reaction-based probe for the signaling of
Hg2+ ions using excimer to monomer switching of pyrene emission
was investigated. Cleavage of the dithioacetal linkage of bis-pyrene
compound by Hg2+ ions resulted in pronounced fluorescence sig-
naling. The signaling was readily analyzed by a ratiometric ap-
proach using the changes in excimer and monomer pyrene
emissions. The designed reaction-based strategy using excimer to
monomer switching of suitably designed bis-pyrene compounds
could be useful for the development of other smart molecular
devices.
Figure 2. Partial 1H NMR spectra of 1, the purified reaction product of 1 with Hg2+
ions, and 1-pyrenebutanol in CDCl3. [1] = [1-pyrenebutanol] = 2.7 Â 10–2 M.
Acknowledgment
7
6
5
4
3
2
1
0
This research was supported by a Chung-Ang University Re-
search Scholarship Grant in 2013 (Y.C.).
Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.tet-
let.2013.07.109. These data include MOL files and InChiKeys of
the most important compounds described in this article.
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Figure 3. Competitive signaling as expressed by fluorescence intensity ratio (I480
/
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[Hg2+
]
2
0
5
4
3
2
1
0
0
5
10
Equivof Hg(II)
350
400
450
500
550
600
650
Wavelength (nm)
Figure 4. Changes in the fluorescence spectrum of
1 as a
function of Hg2+
concentration. [1] = 5.0 Â 10–6 M, [Hg2+] = 0–5.0 Â 10–5 M. In a mixture of acetate
buffer (pH 4.8) and acetonitrile (1:9 v/v). [Acetate buffer] = 10 mM. kex = 340 nm.
Inset: Changes in fluorescence ratio I480/I395 as a function of Hg2+ concentration.
27. Preparation of 1: 1-pyrenebutanol (0.19 g, 0.70 mmol) was added to a solution
of
3 (0.10 g, 0.32 mmol), EDC-HCl (0.18 g, 0.96 mmol), HOBt (0.043 g,

5344
Y. Cho et al. / Tetrahedron Letters 54 (2013) 5341–5344
0.32 mmol), and diisopropylethylamine (0.27 mL, 1.6 mmol) in DMF (5.0 mL).
4H), 3.82 (q, J = 7.0 Hz, 2H), 3.38 (d, J = 15.0 Hz, 2H), 3.32 (t, J = 7.8 Hz, 4H), 3.14
(d, J = 15.0 Hz, 2H), 1.88 (dd, J = 15.1, 7.9 Hz, 4H), 1.77 (dd, J = 14.9, 6.7 Hz, 4H),
1.32 (t, J = 7.0 Hz, 3H): 13C NMR (151 MHz, CDCl3) d 170.0, 158.9, 136.2, 131.4,
130.9, 130.0, 129.8, 129.1, 128.6, 127.5, 127.3, 127.2, 126.6, 125.8, 125.1, 125.0,
124.9, 124.8, 124.7, 123.2, 114.5, 65.2, 63.3, 53.1, 33.9, 32.9, 28.5, 28.0, 14.7;
MS (ESI+); m/z calcd for C53H48NaO5S2 [M+Na]+: 851.3, found 851.3.
28. Stricks, W.; Kolthoff, I. M.; Heyndrickx, A. J. Am. Chem. Soc. 1954, 76, 1515–
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The mixture was stirred for 4 h at room temperature. The resulting mixture
was partitioned between water and CH2Cl2, and the organic layer was
separated and washed with 0.1 N HCl, 0.1 N K2CO3 solution, and water. The
organic solution was dried over anhydrous MgSO4 and filtered and evaporated.
The crude product was purified by silica gel chromatography using CH2Cl2/
MeOH (19:1 v/v) and hexane/ethyl acetate (5:1 v/v) as eluent to give
compound
1 as a
brown syrup (0.16 g). Yield, 60%; 1H NMR (600 MHz,
CDCl3) d 8.21 (d, J = 9.2 Hz, 2H), 8.19–7.93 (m, 14H), 7.81 (d, J = 7.7 Hz, 2H),
7.29 (d, J = 8.6 Hz, 2H), 6.69 (d, J = 8.6 Hz, 2H), 5.29 (s, 1H), 4.13 (t, J = 6.8 Hz,
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