Stibonium ions for the fluorescence turn-on sensing of F- in drinking water at parts per million concentrations

Article abstract of DOI:10.1021/ja308194w

The 9-anthryltriphenylstibonium cation, [1]⁺, has been synthesized and used as a sensor for the toxic fluoride anion in water. This stibonium cation complexes fluoride ions to afford the corresponding fluorostiborane 1-F. This reaction, which occurs at fluoride concentrations in the parts per million range, is accompanied by a drastic fluorescence turn-on response. It is also highly selective and can be used in plain tap water or bottled water to test fluoridation levels.

Full text of DOI:10.1021/ja308194w

Communication
pubs.acs.org/JACS
Stibonium Ions for the Fluorescence Turn-On Sensing of F− in
Drinking Water at Parts per Million Concentrations
Iou-Sheng Ke,† Mykhaylo Myahkostupov,‡ Felix N. Castellano,*,‡ and Francois P. Gabbaï*,†
̧
†Department of Chemistry, Texas A&M University, College Station, Texas 77843-3255, United States
‡Department of Chemistry and Center for Photochemical Sciences, Bowling Green State University, Bowling Green, Ohio 43403,
United States
S
* Supporting Information
anhydrous and nonpolar media.6 A higher fluoride ion affinity is
displayed by stibonium ions such as [Ph4Sb]+ ([II]+), which, as
ABSTRACT: The 9-anthryltriphenylstibonium cation,
[1]+, has been synthesized and used as a sensor for the
documented in earlier reports, binds fluoride in biphasic water/
CCl4 mixtures (Chart 1B).7 Following a recent study in which
toxic fluoride anion in water. This stibonium cation
complexes fluoride ions to afford the corresponding
fluorostiborane 1-F. This reaction, which occurs at fluoride
concentrations in the parts per million range, is
accompanied by a drastic fluorescence turn-on response.
It is also highly selective and can be used in plain tap water
or bottled water to test fluoridation levels.
we observed fluoride binding at the antimony center of cationic
transition-metal stibine complexes,8 we decided to investigate
the use of organostibonium ions as water-compatible fluoride
sensors (Chart 1C). Another important objective of this study
was the incorporation of a fluorescent turn-on reporter.
To initiate this study, we first decided to investigate the
fluoride ion affinity of [Ph4Sb]+ in organic solvents. Addition of
tetra-n-butylammonium fluoride (TBAF) to an acetonitrile
solution of [Ph4Sb]Br resulted in a blue shift of the phenyl S2
← S0 transition, a phenomenon assigned to the formation of
the fluorostiborane Ph4SbF. Fitting of these spectral data on the
basis of a 1:1 binding isotherm afforded a fluoride binding
constant greater than 106 M−1 (see the Supporting
Information). Analogous experiments carried out with
[Ph4P]+ and [Ph4As]+ and monitored by both UV−vis and
NMR spectroscopy indicated that these two cations do not
associate with fluoride anions under these conditions. Thus, the
Lewis acidity trend observed for these tetraphenylpnictonium
ions ([Ph4P]+ ≈ [Ph4As]+ ≪ [Ph4Sb]+) is reminiscent of that
observed for halogen-bonded complexes, which become more
stable as the group is descended.9 This analogy suggests that
the increased acidity observed on going from Pn = P to Sb
originates from the increased polarizability and electropositivity
of Sb in comparison with P and As. The larger size of the
element and its ability to accommodate more ligands in its
coordination sphere may also be contributing factors. These
results were corroborated by DFT calculations (BP86/basis
sets: 6-31G(d) for H and C; 6-31+G(d) for F, and CRENBL
ECP for Pn, PCM/MeCN), which indicated that the fluoride
anion affinity of [Ph4Sb]+ exceeds those of [Ph4P]+ and Ph4As+
by 23.2 and 15.9 kcal/mol, respectively.
luoride anion salts are commonly added to drinking water
F
supplies and toothpaste because of their beneficial effects
in dental health. In the U.S., this practice has been the center of
some discussions because of the adverse effects that excessive
fluoride intake may trigger. The recent lowering of the U.S.
Department of Health and Human Services recommended
fluoride levels from 1.2 to 0.7 ppm1 has sparked a renewed
interest in Lewis acidic main-group compounds that can be
used to sense low concentrations of this anion, especially in
aqueous media.2 Examples of such Lewis acidic compounds
include cationic boron compounds3 such as [I]+ which react
with fluoride ions to form the corresponding zwitterionic
fluoroborates (Chart 1A).4 Although the fluoride ion affinity of
Chart 1
these cationic boranes enables sensing at the parts per million
level in aqueous media, the anion binding event is accompanied
by a fluorescence turn-off response, thus limiting the analytical
practicality of these derivatives.4 Some of these boranes also
show evidence of slow decomposition in aqueous media.
Because of these drawbacks, we are now searching for
alternative fluoride ion binding platforms. Inspired by the
emerging use of pnictogen-based Lewis acids in various areas of
chemistry,5 we have now turned our attention toward
pnictonium ions ([R4Pn]+, Pn = pnictogen), which react with
fluoride ions to form the corresponding fluoropnictoranes
(R4PnF). For example, tetraalkylphosphonium ions combine
with fluoride ions to form fluorophosphoranes, albeit in strictly
Following this initial survey, we considered the incorporation
of a fluorescence reporter that would turn on upon anion
binding. Inspired by the pioneering work of Yamaguchi and
Tamao, who showed a fluorescence turn-on response upon
conversion of Ant3P and Ant3SiF into Ant3PF2 and [Ant3SiF2]−
(Ant = 9-anthryl), respectively,10 we prepared the triflate salt of
9-anthryltriphenylstibonium ion, [1]OTf. This salt was
1
characterized by H and 13C NMR spectroscopy, elemental
Received: August 17, 2012
Published: September 6, 2012
© 2012 American Chemical Society
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dx.doi.org/10.1021/ja308194w | J. Am. Chem. Soc. 2012, 134, 15309−15311

Journal of the American Chemical Society
Communication
analysis, and single-crystal X-ray diffraction (see the Supporting
Information), which revealed that the stibonium cation adopts
a tetrahedral structure comparable to those reported for other
tetraarylstibonium ions. Treatment of [1]OTf with KF in
methanol afforded an essentially quantitative yield of the
corresponding fluorostiborane, 1-F, which was also fully
characterized. The presence of an Sb−F bond was confirmed
by the detection of a 19F NMR resonance at −75.8 ppm in
CDCl3, a value comparable to that for Ph4SbF (−81.4 ppm in
CDCl3). The crystal structure of 1-F was also determined and
indicated that the more sterically demanding anthryl sub-
stituent occupies an equatorial site, while the fluoride anion
adopts an axial position as in Ph4SbF (Scheme 1).11 Having
from Φ = 0.7% for [1]OTf to Φ = 9.5% for 1-F (λex = 375 nm
in CHCl3) (Figure 1). However, no shifts in the emission
spectra were observed, thus suggesting that the 0−0 transition
energy is virtually the same in the two complexes. As
summarized in Table 1, 1-F becomes a brighter emitter
Table 1. Photophysical Properties of [1]OTf, 1-F, and
Anthracene (for Comparison) Measured in CHCl3
τ (ns)
Φ (AQY)
kr (s−1
)
knr (s−1
)
[1]OTf
1-F
<0.15
3.85
0.007
0.095
0.107
4.67 × 107
2.47 × 107
5.38 × 107
6.62 × 109
2.35 × 108
4.49 × 108
Ant
1.99
Scheme 1
primarily because of a decrease of the nonradiative decay
channel (knr). These changes may result from (i) the decrease
in chromophore−solvent interactions that would accompany
the conversion of an inherently highly solvated cation ([1]+)
into a neutral compound (1-F) and (ii) the disappearance of
any intramolecular anthryl → Sb charge transfer processes in 1-
F that would otherwise quench the excited state of the anthryl
chromophore. It is also possible that changes in intramolecular
aryl−aryl interactions may be involved, as previously invoked
by Yamaguchi and Tamao.10
established the facile conversion of [1]+ into 1-F, we decided to
investigate the photophysical properties of each compound.
The low-energy part of the absorption spectrum of [1]+ in
CHCl3 is dominated by absorption bands from the anthryl
substituent, as confirmed by the characteristic vibronic
progression (Figure 1). Interestingly, this compound is only
weakly fluorescent, with an anthryl-based emission band at λfluo
= 427 nm and a quantum yield of Φ = 0.7% for [1]OTf.
Conversion of [1]OTf into 1-F by addition of TBAF induced a
blue shift of the anthryl-based absorption as well as a drastic
increase in the fluorescence intensity of the anthryl fluorophore
Initial evidence for the water compatibility of [1]+ was
provided by the observation that addition of fluoride to a
solution of [1]OTf in 9:1 (v/v) H2O/dimethyl sulfoxide
(DMSO) resulted in rapid precipitation of the fluoride
complex. To prevent the formation of this precipitate and to
study this reaction at equilibrium, we decided to use
cetyltrimethylammonium bromide (CTAB) (10 mM) as an
additive. As indicated by UV−vis spectroscopy, [1]OTf exists
as the free cation up to pH 5. Above this pH, the UV−vis
spectrum undergoes a distinct blue shift, suggesting binding of
hydroxide anion to the antimony center. To parametrize this
phenomenon, an acid−base spectrophotometric titration was
carried and afforded a pKR value4a of 7.07 0.05 (Figure 1).
+
+
This pKR value, which can be regarded as the pH at which
[1]OTf is 50% neutralized by hydroxide anions, indicates that
[1]OTf should serve as an efficient fluoride anion sensor at
slightly acidic pH. Indeed, a fluoride titration experiment
carried out in 9:1 (v/v) H2O/DMSO (CTAB, 10 mM) at pH
4.8 (pyridine buffer, 10 mM) indicated that [1]OTf binds
fluoride anion with a binding constant of 12000 1100 M−1,
making this compound compatible for the detection of fluoride
in the parts per million range (Figure 1). Gratifyingly and as
observed in CHCl3, an increase in the fluorescence intensity
was also observed on going from [1]OTf to 1-F, as indicated by
Φ = 2.2% for [1]OTf and Φ = 14.1% for 1-F (Figure 2) in
aqueous solution. This assay is remarkably selective and does
not produce any response to other anions such as Cl−, Br−, I−,
−
−
−
−
NO3 , N3 , HCO3 , and HSO4 . Because of this lack of
interference, this compound can be used with untreated tap
water. For example, this assay returned a value of 1.04 0.01
ppm for a tap water sample (College Station, TX) and a value
Figure 1. Top left: absorption spectra for [1]OTf (blue) and 1-F (red)
in CHCl3. Top right: emission spectra of [1]OTf (blue) and 1-F (red)
in CHCl3. Bottom left: spectrophotometric acid−base titration curve
for [1]OTf in a 9:1 (v/v) H2O/DMSO solution containing CTAB (10
mM) and sodium phosphate (10 mM). The absorbance was measured
of 0.14
0.01 ppm for bottled water (Evian, France). To
confirm the accuracy of these measurements, the two samples
were also tested independently using a fluoride-selective
electrode, which returned comparable values (1.07 and 0.16
ppm for the tap and bottled water samples, respectively). Last
but not least, the fluorescence turn-on response can be detected
with the naked eye in under 1 min for concentrations of at least
1 ppm (Figure 2).
at 381 nm and fitted to KR = [1-OH][H+]/[[1]+] with ε([1]OTf) =
+
8130 M−1 cm−1, ε(1-OH) = 4460 M−1 cm−1, and pKR = 7.07 0.05.
+
Bottom right: fluoride titration of [1]OTf in a 9:1 (v/v) H2O/DMSO
solution containing CTAB (10 mM) and pyridine (10 mM) at pH 4.8.
The absorbance was monitored at 381 nm and fitted to a 1:1 binding
isotherm with K = 12000 1100 M−1, ε([1]OTf) = 8130 M−1 cm−1,
and ε(1-F) = 4850 M−1 cm−1.
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Journal of the American Chemical Society
Communication
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Figure 2. Fluorescence spectra of [1]OTf (5.0 μM) in 9/1 (v/v)
H2O/DMSO at pH 4.8 (10 mM CTAB/pyridine buffer) before and
after addition of fluoride. The inset shows the visible fluorescence
changes (under a hand-held UV lamp) accompanying the addition of
1.9 ppm F−.
In conclusion, we have described an original approach to the
detection of aqueous fluoride anions in water at the parts per
million level. This approach is based on the discovery that
stibonium ions are sufficiently fluorophilic to bind fluoride
anions in highly competing solvents such as water. The stability,
selectivity, and optical turn-on response of this new sensor
should make it especially useful for analytical applications.
ASSOCIATED CONTENT
* Supporting Information
Additional experimental and computational details and
crystallographic data (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
AUTHOR INFORMATION
Corresponding Author
francois@tamu.edu; castell@bgsu.edu
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Science Foundation
(CHE-0952912), the Welch Foundation (A-1423), and Texas
A&M University (Davidson Professorship). The work at BGSU
was funded by the Air Force Research Laboratory Space
Vehicles Directorate (AF9453-08-C-0172) and the BGSU
Research Enhancement Initiative.
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