A near IR di-styryl BODIPY-based ratiometric fluorescent chemosensor for Hg(II)

Article abstract of DOI:10.1016/j.tetlet.2009.12.025

A novel BODIPY-based near-IR emitting probe as a selective and sensitive fluorophore for Hg(II) is synthesized. This versatile BODIPY fluorophore is functionalized for long wavelength emission at the 3 and 5 positions via a condensation reaction in which

Full text of DOI:10.1016/j.tetlet.2009.12.025

Tetrahedron Letters 51 (2010) 892–894
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Tetrahedron Letters
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A near IR di-styrylBODIPY-based ratiometric fluorescent chemosensor for Hg(II)
a
c
Serdar Atilgan a,b, , Ilker Kutuk , Tugba Ozdemir
*
a Department of Chemistry, Middle East Technical University, Ankara TR-06531, Turkey
b Department of Chemistry, Suleyman Demirel University, Isparta 32000, Turkey
c UNAM-Institute of Materials Science and Nanotechnology and Department of Chemistry, Bilkent University, Ankara TR-06800, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel BODIPY-based near-IR emitting probe as a selective and sensitive fluorophore for Hg(II) is syn-
thesized. This versatile BODIPY fluorophore is functionalized for long wavelength emission at the 3
and 5 positions via a condensation reaction in which two dithiodioxomonoaza-based crown-containing
phenyl units are conjugated to the BODIPY core as a chelating unit. This designed fluorophore, employing
an ICT sensor can be used effectively to detect Hg(II) cations by way of a hypsochromic shift (ꢀ90 nm) in
both the absorption and emission spectra.
Received 29 August 2009
Revised 26 November 2009
Accepted 4 December 2009
Available online 11 December 2009
Ó 2009 Elsevier Ltd. All rights reserved.
Mercury is a toxic heavy metal and is a hazardous environmen-
tal contaminant. It is a considerably dangerous metal to human life
and ecology even at low concentrations.1 Problems arise in aquatic
systems in which anaerobic organisms can transform the elemen-
tal and inorganic forms of mercury into organometallic mercury,
such as methyl mercury,2 which can easily enter the food chain
and rapidly concentrate in the upper levels, especially in large edi-
ble fishes.3 Thus, it can easily transfer to the human body and can
cause serious health concerns such as brain damage,4 DNA dam-
age,5 various cognitive and motion disorders,6 and Minamata dis-
ease.7 Considering its toxicity, several analytical techniques have
been employed such as atomic absorption spectroscopy8 and
inductively coupled plasma mass spectroscopy9 to detect mercury.
However, these current techniques demand very complicated and
expensive instrumentation as well as multistep sample prepara-
tion. Therefore, emphasis has been placed on the synthesis and de-
sign of fluorescent sensors10 which offer a promising approach for
mercury ion detection in terms of sensitivity, selectivity, and low
cost.11
To date, a number of fluorescent ionophores for mercury ions
have been reported based on different fluorophores such as fluo-
rescein,12 rhodamine,13 naphthalimide,14 boradiazaindacene15
(BODIPY). However, most of these reported fluorescent ionophores
work in the visible region of the spectrum, and so far only a few
examples that have lower energy emission in the near-IR region
have been described. Recently, chemosensors that absorb or emit
in the near-IR region have become of significant interest for biolog-
ical and environmental use. In particular, the absence or significant
reduction of background absorption, fluorescence, and light scat-
tering, and the availability of low-cost sources of irradiation make
these chemosensors preferable for biological application.16 On the
other hand, coordination of mercury cations often cause a ‘turn-off’
fluorescence response because, like other heavy metal cations,
mercury is a fluroescence quencher.17 Therefore, for an ionophore
designed to select mercury cations, fluorescence enhancement
(turn-on) is preferable to fluorescence quenching (turn-off).
To date, a few near-IR emitting fluorescent chemosensors have
been presented to achieve a ratiometric analysis of mercury cat-
ions.18 Herein, we present a new chemosensor with a turn-on
near-IR optical response for ratiometric mercury detection. We
chose a promising red-emitting dye, di-styryl BODIPY,19 as the
fluorophore. So far, the versatile BODIPY dye has been utilized as
an active fluorophore in the fields of chemosensors,20 logic gates,21
light harvesting systems,22 energy transfer cassettes,23 dye-sensi-
tized solar cells,24 polymers,25 and in OLED applications26 benefit-
ting from properties such as high quantum yields (typically 0.6–
1.0), large extinction coefficients (60,000–80,000 MÀ1 cmÀ1) and
the photostability of these fluorophores. These novel intriguing
di-styryl (DS)-BODIPY fluorophores, derived from successful mod-
ification of the BODIPY core, also display several applications in di-
verse fields such as sensitizers for photodynamic therapy27 and as
a chemosensor28 for zinc cations with red shift properties in the
absorption and emission spectra.
The synthesis of our chemosensor is shown in Scheme 1. BOD-
IPY fluorophore, 1, was synthesized according to the procedure
published previously19 by condensing p-tert-butyl benzaldehyde
with 3-ethyl-2,4-dimethyl-pyrrole (Supplementary data). It is
known that Hg2+ is a soft acid and the use of soft donor atoms as
receptor units result in good selectivity for Hg2+. Thus the well-
known Hg2+ selective ligand, dithia-dioxa-aza macrocycle 215d,20d
was chosen as a receptor. We obtained compound 2 by reaction be-
tween N,N-bis(bromoethyl)aniline and 2,2-[ethane-1,2-diyl-
bis(oxy)]diethanothiol in the presence of Cs2CO3 (Supplementary
* Corresponding author. Tel.: +90 312 210 5153; fax: +90 312 210 3200.
E-mail address: atilganserdar@hotmail.com (S. Atilgan).
0040-4039/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2009.12.025

S. Atilgan et al. / Tetrahedron Letters 51 (2010) 892–894
893
Scheme 1. Synthesis of the near-IR emitting, double chelated DS-BODIPY-based, chemosensor 3.
data) followed by a Vilsmeier-Haack type reaction to yield com-
pound 2. To obtain chemosensor 3, compound 2 was utilized for
extended conjugation at the methyl groups at the 3, 5 positions
on the BODIPY core of 1, leading to longer wavelengths in the vis-
ible red region for the BODIPY fluorophore. Compound 3, was iso-
lated and characterized by 1H, 13C NMR, and MALDI-TOF
(Supplementary data).
absorption and emission spectra of the fluorophore (Figs. 1 and
4). The ICT process gave rise to two distinct absorption and emis-
sion wavelengths. A ratiometric analysis for Hg(II) cations is pre-
sented with the formation of these two distinct wavelengths.
Meanwhile addition of any metal ions other than Hg2+, caused no
change in the absorption maxima (Supplementary data, see
Fig. S1).
Our model consists of two chelating substituents on the BODIPY
core for mercury cation sensing. These two substituents on the
BODIPY core also enabled an advantageous ICT mechanism. The
absorption spectrum of the metal-free form of the dye 3 was ac-
quired in THF and showed a band at 720 nm. The addition of
Hg(ClO4)2 to a solution of the dye in THF caused the appearance
of a new band at 630 nm and a simultaneous disappearance of
the band at 720 nm (Fig. 1). This new band was blue-shifted by
90 nm and was observed as a color change from green to blue
(can be detected with the naked-eye). This large hypsochromic
shift with an isosbestic point at 670 nm clearly indicates that coor-
dination of one mercury cation to the fluorophore 3 kinetically fa-
vored the coordination of a second mercury cation instead of the
metal-free form of dye 3. Coordination of cations to nitrogen donor
atoms of the dithia-dioxa-aza unit block the ICT process on the
BODIPY fluorophore, observed as a spectral shift in both the
The fluorescence spectrum was also recorded with this highly
mercury selective dye 3 in THF (Fig. 2). The ICT process from the
crown unit to the BODIPY core leads to strong fluorescence
quenching. Thus, chemosensor 3, itself shows very weak emission
intensity at 740 nm (Fig. 2). The coordination of Hg2+ to the recep-
tors resulted in a reduction of the electron-donating ability of the
two amino groups conjugated to the BODIPY core, and hence the
receptor showed a 90 nm blue shift with a strong fluorescence
emission intensity (Fig. 2). Moreover, due to the selectivity of the
receptor, metal ions other than Hg2+ produced no change in emis-
sion spectra and absorption spectra.
The fluorescence response of the dye in the presence of Hg2+
(10 lM) and a competing cation (20 lM) is presented in Figure 3.
The fluorescence intensity of the dye was unaffected and the pres-
ence of a metal other than Hg2+ did not influence the coordination
of Hg2+ to the receptor moiety.
Figure 1. Absorption spectra of DS-BODIPY (1.5
increasing concentration of Hg2+ (cation concentrations; 0, 0.5, 0.75, 1.0, 1.5, 2.0,
2.5, 4.0, 5.0, 7.5, 10 M) in THF.
l
M) in the presence of an
Figure 2. Change in the emission spectrum of DS-BODIPY (1.5
l
M) in response to
different cations (cation concentration 10.0
640 nm with a slit width of 2.5 nm.
lM in THF). Excitation wavelength was
l

894
S. Atilgan et al. / Tetrahedron Letters 51 (2010) 892–894
culate binding constants and stoichiometry and digital photo-
graphs) associated with this Letter can be found, in the online
version, at doi:10.1016/j.tetlet.2009.12.025.
References and notes
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In conclusion, we have presented a highly selective and sensi-
tive BODIPY-based near-IR emitting chemosensor for Hg2+ over
other competing cations even at low concentration. This chemo-
sensor can be used as an analytical tool for qualitative monitoring,
and quantitative detection of Hg2+. Moreover a large hypsochromic
shift (approx. 90 nm) is seen with the coordination of Hg2+ in both
the absorption and emission spectrum. Analyses of the chemosen-
sor were carried out in THF to demonstrate our modeling as an
alternative chemosensor for mercury cation in the near-IR region.
It may be that this work will influence the development of new
near-IR emitting BODIPY-based chemosensors and biologically
important water soluble near-IR emitting chemosensors.
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Acknowledgment
The authors thank Professor Dr. Engin U. Akkaya for fruitful
discussions.
27. Atilgan, S.; Ekmekçi, Z.; Dogan, A. L.; Guc, D.; Akkaya, E. U. Chem. Commun.
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Supplementary data
28. Atilgan, S.; Ozdemir, T.; Akkaya, E. U. Org. Lett. 2008, 10, 4065.
29. The binding constant determination and Hill plot analysis were determined as
follows, see: Peng, X.; Du, J.; Fan, J.; Wang, J.; Wu, Y.; Zhao, J.; Sun, S.; Xu, T. J.
Am. Chem. Soc. 2007, 129, 1500.
Supplementary data (syntheses, experimental details, 1H, 13C
NMR spectra and additional spectroscopic data, figures used to cal-