A coumarin-thiourea conjugate as a fluorescent probe for Hg(ii) in aqueous media with a broad pH range 2-12

Article abstract of DOI:10.1039/b924015b

A coumarin-thiourea conjugate (1) behaves as a highly selective fluorescent probe for Hg²⁺ in aqueous media. The probe 1 shows selective and quantitative fluorescence decrease upon Hg²⁺ addition in aqueous media with a broad pH range, 2-12. Ab initio molecular orbital calculations reveal that the fluorescence decrease of 1 upon Hg²⁺ addition is promoted by a Hg²⁺-induced desulfurization of the thiourea moiety, leading to a decrease in an intramolecular charge transfer (ICT) character of the excited-state coumarin moiety.

Full text of DOI:10.1039/b924015b

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www.rsc.org/obc | Organic & Biomolecular Chemistry
A coumarin–thiourea conjugate as a fluorescent probe for Hg(II) in aqueous
media with a broad pH range 2–12†
Yasuhiro Shiraishi,* Shigehiro Sumiya and Takayuki Hirai
Received 18th November 2009, Accepted 21st December 2009
First published as an Advance Article on the web 19th January 2010
DOI: 10.1039/b924015b
A coumarin–thiourea conjugate (1) behaves as a highly selective fluorescent probe for Hg2+ in aqueous
media. The probe 1 shows selective and quantitative fluorescence decrease upon Hg2+ addition in
aqueous media with a broad pH range, 2–12. Ab initio molecular orbital calculations reveal that the
fluorescence decrease of 1 upon Hg2+ addition is promoted by a Hg2+-induced desulfurization of the
thiourea moiety, leading to a decrease in an intramolecular charge transfer (ICT) character of the
excited-state coumarin moiety.
Introduction
Hg2+ is one of the most hazardous components in the
environment.1 The design and development of fluorescent Hg2+
probes has therefore attracted a great deal of attention.2 A vast
variety of Hg2+ probes have been proposed so far;3 however, most
of the probes act only in organic media. Hg2+ probes capable
of acting in aqueous media have also been proposed; however,
many of these probes show insufficient selectivity for Hg2+.4
Design of Hg2+ probes with high selectivity in aqueous media
is therefore of current focus. In addition, Hg2+ exists in various
places such as living organisms, soil, rivers, and seas.5 Versatile
Scheme 1 Synthesis of the probe, 1.
Hg2+ probes must therefore be applicable to the samples with high
salt concentration and a broad pH range. Several Hg2+ probes
showing high selectively in aqueous media even with high salt
rescence, but the intensity decreases quantitatively with an increase
significantly broader pH range, 2–12. The probe shows strong fluo-
in the Hg2+ amount. The Hg2+-promoted fluorescence quenching
is due to the decrease in an intramolecular charge transfer (ICT)
character of the probe, associated with the desulfurization of the
thiocarbonyl moiety.
concentration have been proposed; however, most of these probes
act only at a neutral pH,6 and only a few probes act at an acidic–
neutral pH.7
The Hg2+-induced desulfurization of a thiocarbonyl group is
one of the potential reactions for the design of selective Hg2+
probes. Recently, several Hg2+ probes have been synthesized on
this basis, where the desulfurization of a thiocarbonyl group of
the probe8 and subsequent structure rearrangement9 leads to a
drastic change in the fluorescence property of the probe. These
probes allow selective Hg2+ detection in aqueous media, and some
of them are applicable to the samples with a relatively broader pH
range, 2–9,9a 4–9,8a 4–10,9b and 5–10.9c
Results and discussion
The probe 1 was synthesized by two step reactions, as summarized
in Scheme 1 (see the Experimental section). Reaction of 7-amino-
4-methylcoumarin with 2-bromoethylamine hydrobromide in 1,4-
dioxane at 373 K affords 3 with 94% yield. Reaction of 3 with
phenyl isothiocyanate in DMF at 363 K gives rise to the probe 1
as a pale yellow solid with 12% yield. The purity of 1 was fully
confirmed by 1H, 13C NMR and LD-TOF MS analysis.
Fig. 1a shows the fluorescence spectra (lex = 363 nm) of the
probe 1 (10 mM) measured in a buffered MeCN–water mixture
(1/1 v/v; HEPES 100 mM; pH 7.0) with 1 equiv. of respective
metal cations. Without cations, 1 shows a strong fluorescence
at 390–570 nm, assigned to a coumarin fluorescence, where the
fluorescence quantum yield (UF) is determined to be 0.435.12
Addition of Hg2+ leads to a significant decrease in the fluorescence
intensity (UF < 0.02). As shown in Fig. 2, a bright blue fluorescence
of 1 completely disappears upon Hg2+ addition. In contrast,
addition of other metal cations to 1 shows almost the same spectra
as that obtained without cations, indicating that 1 allows selective
fluorescence quenching against Hg2+. It must be noted that the
Coumarin is a dye used extensively due to its excellent photo-
physical properties such as high fluorescent quantum yield and
photostability.10 In particular, 7-aminocoumarin derivatives show
strong fluorescence at a broad pH range (2–12).11 In the present
work, we synthesized a 7-aminocoumarin derivative containing
a thiourea moiety (1, Scheme 1). The probe 1 allows rapid,
selective, and quantitative Hg2+ detection in aqueous media with a
Research Center for Solar Energy Chemistry, and Division of Chemical
Engineering, Graduate School of Engineering Science, Osaka Univer-
sity, Toyonaka, 560-8531, Japan. E-mail: shiraish@cheng.es.osaka-u.ac.jp;
Fax: +81 6 6850 6273; Tel: +81 6 6850 6271
† Electronic supplementary information (ESI) available: Supplementary
data (Figs. S1–S17) and Cartesian coordinates for compounds. See DOI:
10.1039/b924015b
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Fig. 1 (a) Fluorescence spectra (lex = 363 nm) of 1 (10 mM) measured in
a buffered MeCN–water mixture (1/1 v/v; HEPES 100 mM; pH 7.0) with
respective cations (1 equiv.) at 298 K, where perchlorate (Hg2+, Cu2+, Zn2+
,
Cd2+, Pb2+, Cr3+, Mn2+, Fe2+), nitrate (Ni2+, Co2+), and tetrafluoroborate
salts (Ag+) were used as the cation source. The measurements were
performed after stirring the solution for 1 min with cations. (b) The ratio
of fluorescence intensity (FI0/FI) of 1, where FI0 and FI are the intensity
measured at 445 nm without and with cations.
Fig. 3 (a) Change in fluorescence spectra (lex = 342 nm) of 1 (10 mM)
measured in a buffered MeCN–water mixture (1/1 v/v; HEPES 100 mM;
pH 7.0) upon addition of Hg2+. (b) Change in the fluorescence intensity
monitored at 445 nm.
Fig. 2 The proposed mechanism for the fluorescence quenching of 1 upon
Hg2+ addition, and fluorescence color change of the solution.
fluorescence response of 1 to Hg2+ is unaffected by other metal
cations (see ESI†, Fig. S1), indicating that 1 detects Hg2+ selectively
even in the presence of other metal cations. It must also be noted
that the fluorescence response occurs within 1 min (see ESI†, Fig.
S2), indicating that 1 allows rapid Hg2+ detection. In addition,
Hg2+ salts with different counteranions such as Hg(ClO4)2, HgCl2,
and Hg(CF3SO3)2 show almost the same spectral changes of 1
(see ESI†, Fig. S3). This suggests that the counteranion effects are
negligibly small.
Fig. 3 shows the result of fluorescence titration of 1 with Hg2+
(lex = 342 nm). The stepwise Hg2+ addition to 1 leads to a decrease
in the fluorescence intensity, where the intensity becomes almost
zero upon addition of 1 equiv. of Hg2+. As shown in Fig. 3b, a
linear relationship is observed between the fluorescence intensity
and the Hg2+ amount at the range of 0.02–1 equiv. of Hg2+, which
corresponds to 0.2–10 mM Hg2+. This clearly indicates that 1
enables quantitative Hg2+ detection at >0.2 mM Hg2+ in aqueous
media.
Fig. 4 Change in absorption spectra of 1 (10 mM) measured in a buffered
MeCN–water mixture (1/1 v/v; HEPES 100 mM; pH 7.0) upon addition
of Hg2+
.
325 nm absorption band. The spectral change almost stops upon
addition of 1 equiv. of Hg2+, which is similar to the change in
fluorescence spectra (Fig. 3). These titration data clearly suggest
that 1 associates with Hg2+ in a 1 : 1 stoichiometry. The clear
isosbestic points at 342 nm and 405 nm in the absorption spectra
(Fig. 4) indicate that the reaction of 1 with Hg2+ produces a single
component. Addition of excess amount of EDTA to the resulting
solution does not show further spectral change (see ESI†, Fig. S4),
indicating that 1 reacts with Hg2+ irreversibly.
As shown in Fig. 2, the fluorescence quenching of 1 upon
addition of Hg2+ is due to the Hg2+-promoted desulfurization of
the thiocarbonyl moiety of 1 followed by cyclization,13 leading to
the formation of an imidazoline derivative, 2. The formation of 2
is fully confirmed by 1H, 13C NMR, and FAB MS analysis of the
compound obtained by the reaction of 1 and Hg(ClO4)2 in MeCN
at room temperature (see the Experimental section).14
Fig. 4 shows the results of absorption titration of 1 with Hg2+.
Without cations, 1 shows a distinctive absorption band at 320–
420 nm with a maximum absorption at 363 nm. Hg2+ addition
leads to a decrease in this absorption, along with an increase in
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The fluorescence quenching associated with the Hg2+-promoted
transformation of 1 to 2 is due to the decrease in an intramolecular
charge transfer (ICT) character of the compound. As reported,15
the excited-state 7-aminocoumarin derivatives have an ICT char-
acter and, hence, allows strong fluorescence. The ICT character
of the excited-state 1 is confirmed by absorption spectra of 1
measured in different solvents. Fig. 5 shows the relationship
between lmax of the absorption spectra and the empirical solvent
N
16
polarity parameter, ET
, of the solvents. The lmax of 1 red-shifts
with an increase in the solvent polarity. This indicates that dipole
moment of the excited-state 1 is higher than that of the ground
state,17 and the excited-state indeed has an ICT character. The
fluorescence spectrum of 1 also red-shifts with an increase in
the solvent polarity (see ESI†, Fig. S6). This also supports the
ICT character of the excited-state 1.17d,18 In contrast, as shown in
Fig. 5, lmax of the absorption spectra of the compound 2 shows a
minor solvent-dependent shift. This indicates that the ground- and
excited-states 2 have similar dipole moments, and the excited-state
has a minor ICT character.19 These findings clearly indicate that
the decrease in ICT character of the probe molecule, associated
with the transformation from 1 to 2 leads to the fluorescence
decrease (Fig. 2).
Fig. 6 Calculated structures of (a) 1 and (b) 2 (B3LYP/6-31G*), where
the gray, blue, red, yellow, and white atoms denote C, N, O, S, and H atoms,
respectively.
Fig. 5 Relationship between lmax of the absorption spectra of 1 and 2 (10
mM) and the solvent polarity parameter, ETN, of the solvents. The solvents
are (a) DMSO, (b) DMF, (c) chloroform, and (d) ethyl acetate, respectively.
The spectra are shown in Fig. S5 (see ESI†).
Fig. 7 Change in fluorescence intensity (lex = 363 nm, lem = 445 nm) of 1
(10 mM) in a MeCN–water mixture (1/1 v/v) measured with and without
Hg2+ (1 equiv.) as a function of pH.
As reported,20 the ICT character of 7-aminocoumarin deriva-
tives depends strongly on conformation of the amine moiety; the
amine moiety planar to the benzopyran plane creates an ICT
character, while the distortion of the moiety leads to a decrease in
ICT character. To clarify the conformation of the amine moiety,
ab initio molecular orbital calculations were performed with the
Gaussian 03 program.21 Fig. 6a shows the optimized geometry of
the compound 1. The dihedral angle of the C1-N1-H1 plane of
the amine moiety to the benzopyran plane is determined to be
17◦. In contrast, as shown in Fig. 6b, the C1¢-N1¢-C2¢ plane of
the compound 2 has a larger angle (30◦), indicating that 2 has
a more distorted amine moiety as compared to 1. The results
clearly suggest that the Hg2+-promoted desulfurization of 1 leads
to a distortion of the amine moiety; this causes a decrease in ICT
character, resulting in fluorescence quenching.
at the same region. This indicates that 1 successfully reacts with
Hg2+ and allows Hg2+ detection at a pH range 2–12, which is the
broadest among the previously reported Hg2+ probes.6–9
The sensitivity of 1 is not affected by water content of the
sample; as shown in Fig. S7 (see ESI†), Hg2+ addition to 1 creates
successful fluorescence quenching even in the presence of 20–80%
water. In addition, as shown in Fig. S8 (see ESI†), the fluorescence
quenching of 1 by Hg2+ occurs successfully even in the presence of
excess amount of NaCl (100 equiv.). This indicates that 1 allows
Hg2+ detection for samples with high salt concentration.
Conclusions
We found that a coumarin–thiourea conjugate (1) behaves as a
fluorescent probe for selective Hg2+ detection in aqueous media
with a broad pH range 2–12. The selective fluorescence response of
1 is due to the Hg2+-promoted desulfurization of the thiocarbonyl
moiety, leading to a decrease in ICT character of the excited
state coumarin moiety. The molecular design presented here may
A notable feature of the probe 1 is that it is able to detect Hg2+
at a broad pH range. Fig. 7 shows the fluorescence intensity of
1 obtained without and with Hg2+ as a function of pH. Without
Hg2+, 1 shows a strong fluorescence at a broad pH range 2–12.
Upon addition of Hg2+, the intensity decreases to almost zero
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contribute to the development of more efficient and more useful
Hg2+ probes in aqueous media.
The eluent was dried in vacuo, affording 2 as a white solid (48.7 mg,
66%). 1H NMR (CD3CN, 270 MHz, TMS): d (ppm) = 7.87 (d, J =
8.41 Hz, 1H), 7.37–7.53 (m, 5H), 7.28–7.32 (m, 2H), 6.34 (q, J =
1.21 Hz,1H), 4.21–4.27 (m, 2H), 3.75–3.82 (m, 2H), 2.46 (d, J =
1.32 Hz, 3H). 13C NMR (DMSO-d6, 68 MHz, TMS): d (ppm) =
159.1, 154.8, 153.1, 152.4, 139.4, 135.5, 129.3, 126.7, 126.4, 124.3,
120.1, 118.1, 114.0, 112.4, 50.8, 40.9, 18.0. FAB MS: Calcd for
C19H17N3O2: 319.4, found: m/z 320.3 (M+H+). HRMS (FAB+)
m/z calcd for C19H18N3O2 [M+H+] 320.1399, found 320.1381. 1H,
13C NMR and FAB MS charts are shown in Figs. S15–S17 (see
ESI†).
Experimental
General
Apparatus and instruments were described elsewhere.22 The spec-
tral measurements were carried out with a 10-mm path length
quartz cell under aerated conditions. LD-TOF MS chart was
obtained with a Shimadzu/Kratos AXIMA-CFR spectrometer.23
Synthesis
Computational details
Compound 3. 7-Amino-4-methylcoumarin (0.18 g, 1.0 mmol)
and 2-bromoethylamine hydrobromide (0.11 g, 0.54 mmol) were
refluxed in 1,4-dioxane (3 mL) for 12 h. The resultant was
concentrated by evaporation. Ethyl acetate was added to the
residue, and the solution was extracted with water (200 mL ¥ 2).
The combined aqueous solution was washed with ethyl acetate,
concentrated by evaporation, and dried in vacuo, affording 3 as
Preliminary geometry optimization was performed using the Win-
MOPAC version 3.0 software (Fujitsu Inc.) at the semiempirical
PM3 level.24 The obtained structures were fully refined at the DFT
level with the Gaussian 03 package,21 using the B3LYP/6-31+G*
basis set for all atoms. Cartesian coordinates for 1 and 2 are shown
in ESI†.
1
a pale orange solid (108.2 mg, 92%). H NMR (D2O, 270 MHz,
TMS): d (ppm) = 7.45 (d, J = 8.90 Hz, 1H), 6.70 (d, J = 8.90 Hz,
1H), 6.48 (s, 1H), 5.97 (s, 1H), 3.57 (t, J = 6.02 Hz, 2H), 3.30 (t,
J = 6.02 Hz, 2H), 2.32 (s, 3H). 13C NMR (DMSO-d6, 68 MHz,
TMS): d (ppm) = 160.2, 155.2, 153.3, 151.4, 125.8, 110.3, 109.1,
107.8, 96.5, 37.6, 28.9, 17.9. EI MS: Calcd for C12H14N2O2: 218.3,
found: m/z 218.1 (M+). HRMS (EI+) m/z calcd for C12H14N2O2
[M+] 218.1055, found 218.1067. 1H, 13C NMR and EI MS charts
are shown in Figs. S9–S11 (see ESI†).
Acknowledgements
This work was supported by Grant-in-Aids for Scientific Research
(No. 19760536) from the Ministry of Education, Culture, Sports,
Science and Technology, Japan (MEXT). We thank Prof. Yoshito
Tobe and Dr Rui Umeda (Osaka University) for LD-TOF MS
analysis.
Notes and references
Compound 1. 3 (0.25 g, 1.1 mmol) and phenyl isothiocyanate
(0.50 mL, 4.2 mmol) were stirred in DMF (6 mL) at 363 K
for 24 h. The resultant was concentrated by evaporation. Water
was added to the residue, and the resulting aqueous phase
was extracted with ethyl acetate (200 mL ¥ 2). The combined
organic layer was washed with water, dried over Na2SO4, and
concentrated by evaporation. The crude product was purified by
silica gel column chromatography with CH2Cl2–ethyl acetate (20/1
v/v) and CH2Cl2–ethyl acetate (1/1 v/v). The latter eluent was
concentrated by evaporation and dried in vacuo, affording 1 as
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1
a pale yellow solid (47.8 mg, 12%). H NMR (CD3CN, 270 Mz,
TMS): d (ppm) = 8.11 (br, 1H), 7.42 (d, J = 8.57 Hz, 1H), 7.33–
7.39 (m, 2H), 7.20–7.24 (m, 3H), 6.71 (br, 1H), 6.60 (dd, J =
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1.15 Hz, 1H), 5.41 (br, 1H), 3.77 (q, J = 6.15 Hz, 2H), 3.40 (q,
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d6, 68 MHz, TMS): d (ppm) = 180.1, 160.3, 155.4, 153.3, 152.1,
138.5, 128.4, 125.6, 124.1, 123.1, 110.0, 108.6, 107.3, 96.2, 42.7,
41.2, 17.9. MS (LD-TOF): Calcd for C19H19N3O2S: 353.4, found:
m/z 375.8 (M+ + Na). Elemental anal.: Calcd for C19H19N3O2S: C,
64.57; H, 5.42; N, 11.89; Found: C, 64.34; H, 5.22; N, 11.69. 1H,
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1314 | Org. Biomol. Chem., 2010, 8, 1310–1314
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