3-(Pyridine-4-thione)BODIPY as a chemodosimeter for detection of Hg(II) ions

Article abstract of DOI:10.1016/j.dyepig.2012.03.015

A new chemodosimeter for detection of Hg(II) ions based on boron-dipyrromethene (BODIPY) is developed. The 3-(pyridine-4-thione)BODIPY was synthesized in high yield by treating 3-(pyridine-4-one)BODIPY with Lawesson's reagent. The 3-(pyridine-4-thione) BODIPY was confirmed by HR-MS mass, ¹H, ¹H-¹H COSY, ¹⁹F and ¹¹B NMR techniques. In ¹H NMR, the protons which are adjacent to sulfur in 3-(pyridine-4-thione)BODIPY showed ～1 ppm downfield shift compared to equivalent protons in 3-(pyridine-4-one)BODIPY. The absorption and electrochemical properties experienced slight changes in 3-(pyridine-4-thione)BODIPY compared to 3-(pyridine-4-one)BODIPY. The compound 3-(pyridine-4-thione)BODIPY is very weakly fluorescent compared to 3-(pyridine-4-one)BODIPY. Furthermore, 3-(pyridine-4-thione)BODIPY can be used as an exclusive chemodosimeter for Hg²⁺ ion as it shows significant enhancement in the fluorescence intensity and a dramatic visible color change from pink to fluorescent green in the presence of Hg(II) ions.

Full text of DOI:10.1016/j.dyepig.2012.03.015

Dyes and Pigments 95 (2012) 89e95
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Dyes and Pigments
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3-(Pyridine-4-thione)BODIPY as a chemodosimeter for detection of Hg(II) ions
*
Tamanna K. Khan, M. Ravikanth
Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400076, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A new chemodosimeter for detection of Hg(II) ions based on boron-dipyrromethene (BODIPY) is
developed. The 3-(pyridine-4-thione)BODIPY was synthesized in high yield by treating 3-(pyridine-4-
one)BODIPY with Lawesson’s reagent. The 3-(pyridine-4-thione) BODIPY was confirmed by HR-MS
mass, 1H, 1He1H COSY, 19F and 11B NMR techniques. In 1H NMR, the protons which are adjacent to
sulfur in 3-(pyridine-4-thione)BODIPY showed w1 ppm downfield shift compared to equivalent protons
in 3-(pyridine-4-one)BODIPY. The absorption and electrochemical properties experienced slight changes
in 3-(pyridine-4-thione)BODIPY compared to 3-(pyridine-4-one)BODIPY. The compound 3-(pyridine-4-
thione)BODIPY is very weakly fluorescent compared to 3-(pyridine-4-one)BODIPY. Furthermore, 3-
(pyridine-4-thione)BODIPY can be used as an exclusive chemodosimeter for Hg2þ ion as it shows
significant enhancement in the fluorescence intensity and a dramatic visible color change from pink to
fluorescent green in the presence of Hg(II) ions.
Received 24 January 2012
Received in revised form
9 March 2012
Accepted 14 March 2012
Available online 23 March 2012
Keywords:
Boron-dipyrromethene
Pyridine-4-one
Pyridine-4-thione
Desulfurization
Fluorescence sensor
Chemodosimeter
Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction
derivatives have been synthesized and demonstrated their use for
Hg2þ ions detection [3,10e26]. Among the fluorophores, BODIPY
The development of chemosensors for the selective and efficient
detection of chemically and biologically important ionic species has
brought interest for many researchers in recent years [1e4]. The
design of selective and sensitive sensors for the target guests
generally requires conjugation of well established and efficient
binding site with a suitable signaling moiety as a basic requirement.
Many fluorophores have been used as signaling unit because of
their desirable characteristics in both sensitivity and ease of signal
transduction. There is a tremendous need for chemosensors which
can show high selectivity and sensitivity for heavy metal cations
such as Cd2þ, Pb2þ and Hg2þ since such metal cations can cause
severe risks for human health and the environment [5e8]. Among
the metal cations, mercury ion (Hg2þ) is considered as one of the
most toxic cation for the environment because it is widely
distributed in air, water and soil [4]. Mercury ion can accumulate in
the human body and can cause a wide variety of diseases even in
a low concentration, such as prenatal brain damage, serious
cognitive and motion disorders and minimata disease [8,9].
Because of high toxic nature of mercury ions for human health, its
detection using simple chemosensors has become main objective
for researchers in recent years [8,10]. Several small-molecule-based
Hg(II) sensors such as sensors based on rhodamine derivatives,
coumarin derivatives and boron-dipyrromethene (BODIPY)
based chemosensors have received tremendous attention in recent
years because of their advantageous characteristics, such as sharp
absorption with high intensity in visible to near NIR region, high
fluorescence quantum yields and tunable redox properties [27e30].
Interestingly, the reports on BODIPY based sensors for Hg2þ ions
detection are few and the selected examples IeV are shown in Fig.1
[10,12,14,17]. Hence, there is a scope to develop more BODIPY based
chemosensor for Hg2þ ions detection and in this paper, we report
the synthesis of a new BODIPY based chemodosimeter 3-(pyridine-
4-thione)-8-(4-methoxyphenyl)-4,4-difluoro-4-bora-3a,4a-diaza-
s-indacene(1), which is useful for the selective detection of Hg(II)
ions. The BODIPY chemodosimeter 1 is designed by keeping in
mind that Hg(II) has high affinity for sulfur and causes desulfur-
ization reaction which may reflect in the significant changes in the
color as well as fluorescence properties of BODIPY.
2. Experimental section
2.1. Chemicals
Toluene was dried over sodium benzophenone ketyl and chlo-
roform and acetonitrile dried over calcium hydride prior to use.
BF3∙OEt2 and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ)
obtained from Spectrochem (India) were used as obtained. Metal
acetates are used for the studies. All other chemicals used for the
synthesis were reagent grade unless otherwise specified. Column
* Corresponding author. Tel.: þ91 22 5767176; fax: þ91 22 5723480.
E-mail address: ravikanth@chem.iitb.ac.in (M. Ravikanth).
0143-7208/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.dyepig.2012.03.015

90
T.K. Khan, M. Ravikanth / Dyes and Pigments 95 (2012) 89e95
Fig. 1. The reported BODIPY Hg(II)-sensors IeV along with our compound 1.
chromatography was performed on silica (60e120 mesh) or
alumina.
system utilizing the three electrode configuration consisting of
a glassy carbon (working electrode), platinum wire (auxillary elec-
trode) and saturated calomel (reference electrode) electrodes. The
experiments were done in dry dichloromethane using 0.1 M tetra-
butylammonium perchlorate as supporting electrolyte. Half wave
potentials were measured using DPV and also calculated manually
by taking the average of the cathodic and anodic peak potentials. All
potentials were calibrated versus saturated calomel electrode by the
addition of ferrocene as an internal standard, taking E1/2 (Fc/
Fcþ) ¼ 0.42 V, vs SCE. High-resolution mass spectrum was obtained
from Q-TOF instrument by electron spray ionization (ESI) technique.
2.2. Instrumentation
1H NMR spectra (
d in ppm) were recorded using Bruker 400 MHz
spectrometer.13C NMR spectra were recorded on Bruker operating at
100.6 MHz 19F NMR spectra were recorded on Bruker 376.4 MHz
spectrometer. 11B NMR spectra were recorded on Bruker spectrom-
eter operating at 128.3 MHz. TMS was used as an internal reference
for recording 1H (of residual proton; 7.26) and 13C (
d d 77.0 signal) in
CDCl3. Absorption and steady-state fluorescence spectra were
obtained with PerkineElmer Lambda-35 and PC1 Photon Counting
Spectrofluorometer manufactured by ISS, USA instruments respec-
tively. Fluorescence spectra were recorded at 25 ꢀC in a 1 cm quartz
fluorescence cuvette. The fluorescence quantum yields (Ff) were
estimated from the emission and absorption spectra by comparative
method at the excitationwavelength of 488 nm using Rhodamine 6G
2.3. Details of Hg(II) ion sensing studies
All the solvents used were of analytical grade and were purified
and dried by routine procedures immediately before use. Stock
solutions of the Hg(II) (1 mM) in H2OeCH3OH mixture (1:1) and 1
(10
were performed by placing 2.5 mL solution of 1 (10
cuvette of 1 cm path length and various amounts of Hg(II) ions were
m
M) in CHCl3 were prepared. Fluorescence titration experiments
(Ff ¼ 0.88) [31]. Cyclic voltammetric (CV) and differential pulse
mM) in a quartz
voltammetric (DPV) studies were carried out with electrochemical
Scheme 1. Synthesis of 3-(Pyridine-4-thione)BODIPY 1.

T.K. Khan, M. Ravikanth / Dyes and Pigments 95 (2012) 89e95
91
added incrementally by means of a micropipette. The excitation
wavelength used was 488 nm, and emission was collected from 500
to 800 nm. The association constant has been estimated by using
the following standard BenesieHildebrand equation [34]:
intensity upon saturation with Hg(II), and Ka is the association
constant of the complex formed.
2.4. 3-(Pyridine-4-one)-8-(4-methoxyphenyl)-4,4-difluoro-4-bora-
3a,4a-diaza-s-indacene 2
1
I ꢁ I0
1
1
Â
Ã
¼
þ
ðI1 ꢁ I0ÞKa Mþ
I1 ꢁ I0
The compound 2 was synthesized by treating the 3-bromo
boron-dipyrromethene [32] 3 (100 mg, 0.26 mmol) with 4-
hydroxypyridine (38 mg, 0.39 mmol) in CH3CN (15 mL) in the
where I0 is the fluorescence intensity of 1 before addition of Hg(II),
I is the fluorescence intensity in the presence of Hg(II), I1 is the
Fig. 2. (a) Comparison of 1H NMR spectra for compounds 1 and 2. (b) 1He1H COSY NMR spectrum for compound 1.

92
T.K. Khan, M. Ravikanth / Dyes and Pigments 95 (2012) 89e95
Table 1
2 H, pyridyl), 6.65 (d, J ¼ 4.2 Hz, 1 H,
b
-py), 6.98 (d, J ¼ 4.3 Hz, 1 H,
b
-
Photophysical data of compounds 1 and 2 in different solvents.
py), 7.08e7.10 (m, 3 H, BODIPY Ar þ
b
-py), 7.55e7.57 (m, 2 H,
-py). 19
F
B
a
BODIPY Ar), 7.76 (d, J ¼ 6.24 Hz, 2 H, pyridyl), 7.98 (s, 1 H,
b
Compound
Solvent
labs (nm)
log
3
lem (nm)
FF
max
NMR (376.4 MHz, CDCl3,
NMR (128.3 MHz, CDCl3,
d
in ppm): ꢁ141.59 (q, JBꢁF ¼ 60.2 Hz). 11
1
n-Hexane
CHCl3
CH3CN
n-Hexane
CHCl3
CH3CN
526
524
509
519
517
510
3.6
4.5
3.9
4.6
4.7
4.5
548
548
548
551
551
548
0.008
0.008
0.007
0.21
0.20
0.16
d
in ppm): 0.48 (t, JBꢁF ¼ 29.5 Hz). 13C NMR
(100 MHz, CDCl3,
d in ppm): 55.76, 112.26, 113.45, 114.53, 118.57,
2
120.36, 125.55, 129.88, 131.17, 132.71, 133.27, 135.31, 139.77, 146.18,
148.30, 149.87, 162.77, 179.38. HR-MS mass calcd for C21H16BF2N3O2
392.1398 found 392.1382 [M þ H]þ
a
Rhodamine (VF ¼ 0.88) was used as a reference, and the values were based on
the peak height.
2.5. 3-(Pyridine-4-thione)-8-(4-methoxyphenyl)-4,4-difluoro-4-
bora-3a,4a-diaza-s-indacene 1
presence of Cs2CO3 (129 mg, 0.39 mmol) under nitrogen atmo-
sphere at refluxing temperature for 1 h. The progress of the reaction
was followed by TLC analysis which showed the disappearance of
spot corresponding to 3 and appearance of new spot corresponding
to the desired compound. The crude compound was subjected to
alumina gel column chromatography using petroleum ether/CH2Cl2
(5:95) and afforded pure compound 2 as orange solid in 70% yield
Compound
2
(100.0 mg, 0.25 mmol) and 2,4-bis(4-
methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-dithione (Law-
esson’s Reagent) (103 mg, 0.25 mmol) were dissolved in dry
toluene, and the reaction mixture was refluxed for 2 h. The solvent
was removed on rotary evaporator under reduced pressure, and the
crude compound was purified by chromatography on alumina
using petroleum ether/CH2Cl2 (5:95) to afford compound 1 as violet
solid in 50% yield (52 mg). M.p. 174e175 ꢀC; 1H NMR (400 MHz,
(72 mg). M.p. 212e213 ꢀC; 1H NMR (400 MHz, CDCl3,
3.93 (s, 3 H, OMe), 6.44 (d, J ¼ 4.3 Hz, 1 H, -py), 6.48 (d, J ¼ 7.8 Hz,
d in ppm):
b
Fig. 3. (a) Comparison of UVevis absorption spectra of 1 (————) and 2 (ꢁ) recorded at 10
differential pulse voltammograms of 1 and 2 in dichloromethane containing 0.1 M TBAP as supporting electrolyte recorded at 50 mV/s scan speed (c) and comparison of emission
spectra of 1 (————) and 2 (ꢁ) recorded at 10 M concentration in CHCl3 lex ¼ 488 nm).
mM concentration in CHCl3. (b) Comparison of first reduction waves along with
m
(

T.K. Khan, M. Ravikanth / Dyes and Pigments 95 (2012) 89e95
93
CDCl3,
6.68 (d, J ¼ 4.2 Hz, 1 H,
J ¼ 8.6 Hz, 2 H, BODIPY Ar), 7.12 (d, J ¼ 4.2 Hz, 1 H,
J ¼ 7.3 Hz, 2 H, pyridyl), 7.55e7.58 (m, 4 H, BODIPY Ar þ pyridyl), 8.0
d
in ppm): 3.94 (s, 3 H, OMe), 6.47 (d, J ¼ 4.2 Hz, 1 H,
-py), 6.98 (d, J ¼ 4.2 Hz, 1 H, -py), 7.09 (d,
-py) 7.46 (d,
b
-py),
BODIPY core on changing pyridine-4-one in compound 2 to pyri-
dine-4-thione in compound 1. This is also clearly reflected in
difference in their colors in CHCl3 solution (Supporting
Information). The absorption spectra recorded for compounds 1
and 2 in different solvents of varying polarity followed the same
trend like any other BODIPY (Table 1) [34]. The redox properties of
compounds 1 and 2 were investigated in CH2Cl2 by cyclic voltam-
metry and differential pulse voltammetry at different scan rates
(50e150 mV/s) using tetrabutylammonium perchlorate as the
supporting electrolyte. The comparison of first reduction wave of
b
b
b
(s, 1 H,
b
-py). 19F NMR (376.4 MHz, CDCl3,
d
in ppm): ꢁ141.09 (q,
in ppm): 0.51 (t,
d in ppm): 55.83, 112.09,
JBꢁF ¼ 60.2 Hz). 11B NMR (128.3 MHz, CDCl3,
d
JBꢁF ¼ 29.5 Hz). 13C NMR (100 MHz, CDCl3,
114.62, 120.80, 125.51, 130.85, 132.89, 133.50, 133.81, 146.98, 148.59,
162.91, 195.88. HR-MS mass calcd for C21H16BF2N3OS 408.1153
found 408.1160 [M þ H]þ.
compounds 1 and 2 is shown in Fig. 3b. The compounds 1 and 2
red
3. Results and discussions
showed one reversible (E
¼ ꢁ0.66 and ꢁ0.60 V respectively) and
1=2
red
one irreversible reduction (E
¼ ꢁ1.70 and ꢁ1.47 V respectively)
1=2
To synthesize 3-(pyridine-4-thione) BODIPY 1, we need an
access to the key precursor 3-(pyridine-4-one) BODIPY 2. We
showed earlier that 3,5-dibromo BODIPY on reaction with 2- and 4-
hydroxypyridines forms 3,5-bis(pyridone)-BODIPYs and with 3-
hydroxypridine, it forms 3,5-bis(oxypyridine)BODIPY [33]. We
adopted similar synthetic strategy to prepare 3-(pyridine-4-one)
BODIPY 2 by using 3-bromo BODIPY [32] as a precursor. Treatment
of 3-bromo BODIPY with 4-hydroxypyridine using Cs2CO3 as a base
under reflux conditions for 1 h followed by standard work-up and
column chromatographic purification afforded compound 2 in 70%
yield (Scheme 1). The target 3-pyridine-(4-thione) BODIPY 1 was
synthesized by treating 2 with one equivalent of Lawesson’s
reagent in toluene under reflux conditions for 2 h (Scheme 1). The
crude compound was subjected to silica gel column chromato-
graphic purification and afforded pure compound 1 as green solid
in 50% yield. The BODIPYs 1 and 2 are readily soluble in common
organic solvents such as CHCl3, CH2Cl2, CH3OH, toluene etc and
confirmed the identities by molecular ion peaks in HR-MS mass
spectra (Supporting Information). The compounds 1 and 2 were
characterized further by 1D & 2D NMR, absorption, fluorescence
and electrochemical techniques. 1H, 13C, 19F and 11B NMR were used
to characterize the compounds 1 and 2 and the comparison of 1H
NMR spectra of compounds 1 with 2 is presented in Fig. 2a. We
used 1He1H COSY NMR to identify all signals of BODIPY 1 and
spectrum is presented in Fig. 2b. In compounds 1 and 2, because of
unsymmetrical substitution, the five pyrrole protons of BODIPY
core (represented as a, b, c, d and e) appeared as five sets of signals.
The protons of pyridine-4-one in compound 2 and pyridine-4-
thione in compound 1 represented as f and g appeared as two
sets of signals and the four meso-aryl protons in compounds 1 and 2
also appeared as two sets of signals. As clear from the Fig. 2a, there
is no significant shifts in the pyrrole protons of BODIPY core (aee
protons) when pyridine-4-one in compound 2 was changed to
pyridine-4-thione in compound 1. However, the pyridine-4-thione
protons represented as f and g in compound 1 showed shifts as
compared to pyridine-4-one protons in compound 2. The proton ‘f’
experienced w0.2 ppm upfield shift whereas proton ‘g’ experi-
enced nearly 1 ppm downfield shift in compound 1 compared to
the same protons in compound 2. The compounds 1 and 2 showed
a triplet in 11B NMR at w0.51 ppm and a single quartet in 19F NMR
spectra because of coupling with 11B (I ¼ 3/2, J ¼ 32 Hz) and appears
at w ꢁ141 ppm like other reported BODIPYs [34].
but no oxidation supporting the electron deficient nature of BOD-
IPY compounds 1 and 2. The first reduction potential of compounds
1 and 2 are almost in the same range indicating that the compounds
1 and 2 are equally electron deficient. The fluorescence properties
of compounds 1 and 2 were studied in different solvents (Table 1)
and the comparison of fluorescence spectra of compounds 1 and 2
recorded in CHCl3 under identical experimental conditions is
shown in Fig. 3c. As is clear from the Fig. 3c that compound 2 is
decently fluorescent with emission peak maxima of 551 nm and
quantum yield of 0.20 whereas the compound 1 is very weakly
fluorescent with emission peak maxima at 548 nm, and quantum
yield of 0.008. The low fluorescence behavior of 1 was tentatively
The absorption spectra of compounds 1 and 2 were recorded in
CHCl3 and the data is given in Table 1. The comparison of normal-
ized absorption spectra of compounds 1 and 2 is shown in Fig. 3a. In
general BODIPYs exhibit a strong S0 / S1 transition at w500 nm
with a vibronic transition on the higher energy side as a shoulder
and an ill-defined weak band corresponding to the S0 / S2 tran-
sition at w400 nm [34]. The compounds 1 and 2 showed typical
absorption features like any other BODIPY. However, the compound
1 showed w8 nm red shift in S0 / S1 transition compared to
compound 2 indicating the alteration in electronic properties of
Fig. 4. (a) Fluorescence emission changes (lex ¼ 488 nm) of compound 1 (10
addition of different concentrations of Hg2þ in CHCl3. The Hg2þ concentration were
varied from 0 to 100 M. The inset shows the plot of I/Io vs. different concentrations of
Hg2þ ions. (b) Emission spectra of 1 (10 M of Hg2þ ions and
M) in the presence of 60
100
M of each Cd2þ, Co2þ, Cu2þ, Kþ, Mg2þ, Naþ, Ni2þ, Pb2þ, and Zn2þ ions.
mΜ) upon
m
m
m
m

94
T.K. Khan, M. Ravikanth / Dyes and Pigments 95 (2012) 89e95
[1 þ H]þ. After the addition of one equivalent of Hg(OAc)2 to the
solution of 1, the peak at m/z 408.11 disappeared with the
appearance of a new peak at m/z 392.13 corresponds to the
compound [2 þ H]þ.
4. Conclusion
We synthesized 3-(Pyridine-4-thione)-8-(4-methoxyphenyl)-4,
4-difluoro-4-bora-3a, 4a-diaza-s-indacene(1) in high yield and
used it as a chemodosimeter for the detection of Hg(II) ions. In
addition, the dye 1 can be used as colorimetric sensor for Hg(II)
ions, as detection of Hg(II) ions induces clear visible color for
naked eye. The high selectivity of 1 for Hg2þ ions was attributed
tentatively to Hg2þ-induced desulfurization mechanism. It is
anticipated that 1 could contribute to the development of mercury
ion sensors.
Fig. 5. Color change of 1 (10 m
M) in the presence of different metal ions (Hg2þ in 0.6
equivalents and other metal ions in excess). From left to right: Pb2þ, Mg2þ, Zn2þ, Co2þ
,
K
þ, Hg2þ, Naþ, Cd2þ, Ni2þ and Cu2þ
.
ascribed to the internal heavy atom effect of sulfur. The empty
d-orbitals of sulfur atom with an appropriate symmetry interact
with the p-system of BODIPY resulting in the low quantum yield of
1. Thus, the study clearly indicates that the conversion of pyridine-
4-one in compound 2 to pyridine-4-thione in compound 1 reduces
the fluorescence significantly.
Acknowledgments
The 3-(pyridine-4-thione)BODIPY 1 can be used as an exclusive
chemodosimeter for Hg2þ ions which we investigated by emission
studies. Since compound 1 is completely insoluble in water and
very sparingly soluble in methanol, the studies were carried out by
dissolving compound 1 in CHCl3 and titrated with Hg(II) ion in
water/methanol mixture (1:1). The fluorescence spectral response
of compound 1 with the increasing addition of Hg(II) ions is shown
in Fig. 4a. Upon addition of Hg(II), a gradual increase in the intensity
of fluorescence was observed. The fluorescence titration profile of 1
versus concentration of Hg(II) ions revealed that the gradual
enhancement of the emission intensity was obtained with the
MR thanks BRNS for financial support. TKK thanks Indian
Institute of Technology, Bombay for fellowship.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.dyepig.2012.03.015.
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