A reaction-based chromogenic and fluorescent chemodosimeter for fluoride anions

Article abstract of DOI:10.1039/c1cc10784d

An innovative trihexylsilylacetylene-containing BODIPY dye was designed and proved to be a highly selective, sensitive, and fast chromogenic and fluorescent chemodosimeter for fluorides.

Full text of DOI:10.1039/c1cc10784d

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COMMUNICATION
A reaction-based chromogenic and fluorescent chemodosimeter for
fluoride anionsw
Li Fu,a Feng-Lei Jiang,*a Daniel Fortin,b Pierre D. Harvey*b and Yi Liu*a
Received 10th February 2011, Accepted 14th March 2011
DOI: 10.1039/c1cc10784d
An innovative trihexylsilylacetylene-containing BODIPY dye
was designed and proved to be a highly selective, sensitive, and
fast chromogenic and fluorescent chemodosimeter for fluorides.
the desired conditions, e.g. in polar even aqueous solutions;
and (4) easily accessible and economical. Although several
reports on the different testing methods exist, there are some
examples involving reaction-based sensors for fluoride anions,
but most of them are based on the Si–O bond cleavage using
advantage of the great affinity of fluoride for silicon.10 For
instance, Akkaya et al. reported that two BODIPY derivatives
bearing phenoxy substituents sense fluoride anions, but the
visual change starts from 5-fold of fluoride anion and achieves
the strongest contrasting appearance at 50-fold.10a In these
systems, the time necessary to reach equilibrium was 40 min to
2 h,10b,e and one of the chemodosimeters needed 1400 equivalents
of FÀ to reach saturation of the signals.10g In this respect, the
design for reaction-based chemodosimeters with shorter
equilibrium time appears necessary. We previously observed
that anionic fluorides can selectively and readily break the
Si–C bond in –CRC–SiMe3.11 This reaction is quite fast
(5 min) owing to easier cleavage of Si–C than Si–O bond.
This allows one to develop a new class of sensors based on
this highly specific and efficient reaction (i.e. rapid), and
consequently this facile and reliable method can become one
of the emerging strategies in analytical chemistry.
Anions attract great attention since they play significant roles
in chemical and biological processes. Notably, fluorides are
routinely used for the prevention of dental cavities1 and the
treatment for osteoporosis.2 Because over-accumulation of
fluorides can cause fluorosis,3 the design of methods to detect
fluorides appears clearly needed. Several chemosensors have
been studied for the fluoride anion. Indeed, naphthalene-based
bidentate boranes that can chelate fluoride ions have been
reported.4 Zhao et al. prepared a series of 1,4-bis(aryl-ethylyl)-
benzenes containing dimesityl-boryls as receptors for fluoride
anions.5 Moreover, Huang et al. also reported organic boranes
as selective two-photon chemosensors in which the dimesityl-boryl
groups were used as electron acceptors.6 These chemosensors
were mainly designed on the basis that the coordination with
fluoride disrupts the pp–p* conjugation between the tri-
coordinate boron center and the attached p-conjugated
chromophore, and thereafter the change in photo-physical
properties of the chromophore is monitored. The detection
of fluoride based on a biaryl–thiourea system has also proved
to be another alternative method.7 A fluoride selective binding
receptor bearing urea groups on a naphthalene unit was also
reported by Nam et al.8 Finally, Sessler et al. developed a
receptor based on the metal coordination of fused dipyrrolyl-
quinoxaline phenanthroline derivatives.9 The reported chemo-
sensors were selective and sensitive, however, most of them
were based on complexation between the acceptor and fluoride
anion (host and guest), and many of them can only work in
chlorinated solvents.
We now wish to report an innovative reaction-based sensor,
which is highly selective and sensitive to fluoride anions,
BODIPY dye 3 (Scheme 1). This sensor drastically changes
its optical and fluorescent characteristics when fluoride anions
remove the trihexylsilyl groups.
The principle of a reliable sensor is that this sensor should
be (1) highly selective; (2) highly sensitive; (3) able to work in
a State Key Laboratory of Virology & Key Laboratory of Analytical
Chemistry for Biology and Medicine (Ministry of Education) &
College of Chemistry and Molecular Sciences, Wuhan University,
Wuhan 430072, P. R. China. E-mail: fljiang@whu.edu.cn,
prof.liuyi@263.net; Fax: +86 27 6875 4067;
Tel: +86 27 6875 6667
b
´
Departement de Chimie, Universite de Sherbrooke, PQ,
Canada J1K 2R1. E-mail: Pierre.harvery@USherbrooke.ca
´
w Electronic supplementary information (ESI) available: Experimental
details. CCDC reference number 812643. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/c1cc10784d
Scheme 1 Synthetic route for fluorescent BODIPY dye 3.
Chem. Commun., 2011, 47, 5503–5505 5503
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BODIPY dye 3 was readily prepared in three steps (see
experimental details in ESIw). First, BODIPY dye 1 is obtained
through the condensation of 2,4,6-trimethylbenzaldehyde and
2,4-dimethylpyrrole under catalytic conditions of trifluoroacetic
acid (TFA), followed by oxidation with 2,3-di-chloro-5,6-
dicyanobenzoquinone (DDQ) and chelation with BF3ÁOEt2
in the presence of triethylamine. The iodination of BODIPY
dye 1 affords BODIPY dye 2. BODIPY dye 3 is obtained by a
Pd-catalyzed Sonogashira reaction of BODIPY dye 2 and
trihexylsilylacetylene (THS) in THF in the presence of
Pd(PPh3)4, CuI, and triethylamine. THS is used instead of
trimethylacetylene knowing it is more soluble rendering its
purification easier. Indeed, BODIPY dye 3 is very soluble in
common solvents and can be used in acetone to evaluate its
sensing capability. The structure of BODIPY dye 2 was
confirmed by X-ray methods (Fig. S1; data given in ESIw).
The absorption maxima of BODIPY dyes 1, 2 and 3 are
observed at 500, 530 and 555 nm, respectively, consistent with
the electron-withdrawing properties of iodide atoms in dye 2
and the expanded p-conjugation decreasing the HOMO–
LUMO energy gap in dye 3. Similarly, their fluorescence
maxima are found at 510, 546 and 571 nm, respectively. Their
quantum yields, FF, are 0.76, 0.07 and 0.71, respectively. The
lower FF value for BODIPY dye 2 reflects the heavy atom
effect of the iodide-atoms increasing spin–orbit coupling.
BODIPY dye 3 shows notable changes in the absorption
and fluorescence spectra upon the addition of FÀ ions as
tetrabutyl-ammonium salt, TBAF (Fig. 1 and 2). The addition
of FÀ to an acetone solution of BODIPY dye 3 leads to the
decrease of the absorption band at 555 nm and the appearance
of a new peak at 538 nm. It also results in the decrease of the
fluorescence intensity of the 571 nm band and an increase of
Fig. 3 Colour and fluorescence changes of BODIPY dye 3 in acetone
(1.5 Â 10À5 M) before and after addition of two equivalents of TBAF.
Left to right: BODIPY dye 3, BODIPY dye 3 + TBAF, BODIPY dye
3 (emission), and BODIPY dye 3 + TBAF (emission).
the peak intensity at 554 nm. These changes are accompanied
by a color change at room light (Fig. 3; left). Under UV light
irradiation, the orange fluorescence of BODIPY dye 3 changes
to green (Fig. 3; right). Increasing the amount of FÀ in the
chemodosimeter solution makes the fluorescence weaker but
still remains over 95% of its original intensity, then becomes
stronger and reaches saturation at two equivalents of FÀ. This
chemodosimeter has a smaller limit of detection (LOD, 3s;
67.4 nM; see ESIw) in the nM scale, so BODIPY dye 3 exhibits
an excellent sensitivity. However, preliminary data using
acetone/water (19 : 1, v/v) did not give as good results.
Conversely, there are no obvious changes in the absorption
and fluorescence peaks of BODIPY dye 3 upon addition of
À
other common anions such as ClÀÀ, BrÀ, IÀ, ClO4À, H2PO4
,
CH3COOÀ, SCNÀ, CO32À, NO3 and SO4 (Fig. 4 and 5).
The addition of only FÀ to BODIPY dye 3 clearly leads to a
remarkable change in the color, whereas no change is noted in
the presence of other anions (Fig. S2, ESIw). In addition, the
fluoride can still be detected when all the other anions exist in
the same solution (Fig. S3, ESIw). The hard nucleophile
fluoride anion can break the C–Si bond to form a strong
2À
Fig. 1 Absorption spectra of BODIPY dye 3 (8 mM) in acetone in the
presence of increasing FÀ concentrations (0, 2, 4, 6, 8, 10, 12, 14,
16 mM).
Fig. 4 Absorption spectra of BODIPY dye 3 (8 mM) in the presence
of FÀ, ClÀ, BrÀ, IÀ, ClO4À, H2PO4À, CH3COOÀ, SCNÀ, CO3
,
2À
2À
NO3À, SO4 (excess) in acetone.
Fig. 5 Emission spectra of BODIPY dye 3 (8 mM) upon addition of
À
Fig. 2 Emission spectra of BODIPY dye 3 (8 mM) in acetone in the
presence of increasing FÀ concentrations (0, 4, 6, 8, 10, 12, 14, 16 mM).
FÀ, ClÀ, BrÀ, IÀ, ClO4À, H2PO4À, CH3COOÀ, SCNÀ, CO32À, NO3
,
2À
SO4 (excess) in acetone.
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5504 Chem. Commun., 2011, 47, 5503–5505
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another method for designing efficient sensors. This reaction is
only observed in the presence of fluoride, thus BODIPY dye 3
can be used as a selective and sensitive chromogenic and
fluorescent chemodosimeter for fluoride anions. Moreover,
the color change from orange to green, proving the existence
of FÀ, is convenient for dynamic tracking in cellular imaging
owing to the unchanged intense fluorescence.
Scheme 2 Reaction-based chromogenic and fluorescent sensing of
fluoride.
Si–F bond. Therefore, the trihexylsilyl groups present at the
2,6-positions of BODIPY dye 3 can be easily cleaved by
fluoride ions but not by other anions (Scheme 2). The presence
of the CRCH resonance in BODIPY dye 4 is detected at
In conclusion, a clever reaction-based chemodosimeter,
BOIDPY dye 3, was prepared and proved to be an excellent
colorimetric fluoride anion sensor. It is particularly attractive
because of the necessary time to reach equilibrium of 5 min.
Moreover, with a LOD of 67.4 nM (i.e. 1.28 ppb), this
chemodosimeter is totally adequate for detecting fluorides
near and below the concentration allowed in drinking water
according to the US EPA.12 This class of chemodosimeters
should open a new research field for sensor design. The use of
hydrophilic substituents is now desired since preliminary data
with acetone/water (19 : 1, v : v) do not give as good results.
We gratefully acknowledge the National Natural Science
Foundation of China (21077081, 20921062) and the Funda-
mental Research Funds for Central Universities (1101007) for
financial support. PDH thanks the Natural Sciences and
Engineering Research Council of Canada (NSERC), Fonds
1
d = 3.3 ppm by H NMR (Fig. S4, ESIw). The proton comes
from acetone.
When one C–Si bond is broken and one –CRC–H unit is
formed, BODIPY dye 4 exhibits a lesser number of flexible
groups, here Si(C6H13)3. Consequently, FF for BODIPY
dye
3 is lower than that for BODIPY dye 4 since
FF = kF/(kF + kIC + kisc) where kF is the fluorescence rate
constant, kIC and kisc are the non-radiative rate constants for
internal conversion and intersystem crossing, respectively. The
presence of flexible groups increases kIC and decreases FF. So,
the FÀ-treated chromophore shows a stronger fluorescence
(Fig. 2).
The deprotection of the electron-donating trihexylsilyl
group leads to the blue-shift of both the absorption and
fluorescence peaks; changes that can simply be seen by naked
eyes from the color change and luminescence change upon
irradiation with UV lamp. The blue-shift of the absorption
band induced by FÀ has been corroborated by DFT and
TDDFT calculations (see the ESIw). The optimized geometries
for BODIPY dyes 3 and 4 exhibit the expected planar
geometry. The calculated absorption spectra extracted from
the 100 lowest energy electronic transitions exhibit an absorption
peak of BODIPY dye 4 blue-shifted with respect to that for
BODIPY dye 3 from 495 nm to 485 nm (Fig. 6) and the
extinction coefficient (e) is smaller for BODIPY dye 4, which
are in agreement with the experimental results. The two ethylyl
groups being conjugated with the aromatic BODIPY plane
suggest that variations of substituents on the ethylyl groups
should lead to sensitive changes in the photophysical properties
of BODIPY dyes. This is exactly what happens in these
systems. This observation is useful in the design of sensors
when the sensor recognizes a specific substrate.
Que
and Centre d’Etudes des Mate
de l’Universite de Sherbrooke for funding.
´
be
´
cois de la Recherche sur la Nature et la Technologie,
´
riaux Optiques et Photoniques
´
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Fig. 6 The calculated UV-Vis absorption spectra for BODIPY dye 3
(red) and dye 4 (blue).
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Chem. Commun., 2011, 47, 5503–5505 5505