A ratiometric fluorescent probe for fluoride ions with a tridentate receptor of boronic acid and imidazolium

Article abstract of DOI:10.1016/j.tetlet.2013.02.112

A new ratiometric fluorescent probe for fluoride ions was developed which complexed fluoride by a tridentate receptor of boronic acid and imidazolium. In the current study, a tridentate receptor 1 with one ortho boronic acid and two imidazolium groups was designed. The boron center can co-operate with imidazolium to bind F^-. The formation of B-F complex stabilizes the interaction between fluoride and imidazolium which induces a ratiometric fluorescence response. With the addition of F^-, a strongly increased fluorescent emission centered at 370 nm appears at the expense of the fluorescent emission centered at 445 nm.

Full text of DOI:10.1016/j.tetlet.2013.02.112

Tetrahedron Letters 54 (2013) 2755–2758
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Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
A ratiometric fluorescent probe for fluoride ions with a tridentate receptor
of boronic acid and imidazolium
Eun Jin Jun a, Zhaochao Xu b, , Minji Lee a, Juyoung Yoon a,
⇑
⇑
a Department of Chemistry and Nano Science and Department of Bioinspired Science (WCU), Ewha Womans University, Seoul 120-750, South Korea
b Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new ratiometric fluorescent probe for fluoride ions was developed which complexed fluoride by a
tridentate receptor of boronic acid and imidazolium. In the current study, a tridentate receptor 1 with
one ortho boronic acid and two imidazolium groups was designed. The boron center can co-operate with
imidazolium to bind FÀ. The formation of B–F complex stabilizes the interaction between fluoride and
imidazolium which induces a ratiometric fluorescence response. With the addition of FÀ, a strongly
increased fluorescent emission centered at 370 nm appears at the expense of the fluorescent emission
centered at 445 nm.
Received 17 December 2012
Revised 4 February 2013
Accepted 8 February 2013
Available online 3 April 2013
Keywords:
Ratiometric sensor
Fluoride sensing
Boronic acid
Ó 2013 Elsevier Ltd. All rights reserved.
Imidazolium
Fluorescent probe
The development of abiotic receptors for anionic species is cur-
rently of great interest because of their fundamental roles in a wide
range of chemical and biological processes.1 Among them, fluoride
is the smallest anion, with a high charge density and a hard Lewis
basic nature that result in unusual chemical properties. Fluoride
ions are biologically important anions because of their important
role in dental care2 and the treatment of osteoporosis,3 etc. In this
regard, the sensing of a fluoride ion has attracted growing
attention.4
tion because they can efficiently accept fluoride compared to
monoborane receptors.21 In order to increase the ability of
borane-based receptors to capture fluoride ions from water, Gabbaï
and co-workers developed a series of borane-containing bidentate
receptors, such as borane with ammonium,22 borane with phos-
phonium,23 borane with antimony24, and borane with tellurium.25
Kawachi et al. reported B/Si bidentate receptor for fluoride ions.26
These combined or hybrid receptors would illumine a new route to
devise receptors for fluoride ions which can enhance the binding
ability of fluoride ions.
In this Letter, we reported a new combined receptor of boronic
acid and imidazolium to construct a ratiometric fluorescent probe 1
(Fig. 1). The imidazolium group can interact strongly with anions
through a (C–H)+ÁÁÁXÀ type ionic hydrogen bond.27 In our system,
In a number of cases, hydrogen bonding between the N–H of
urea,5 indole,6 imidazole,7 amide,8 pyrrole9 or O–H of phenol10
and fluoride is the mechanism of recognition. Electron-deficient
Lewis acid co-ordination via orbital overlap has also received a
lot of attention and receptors containing boron,1a,4b,11 silicon,12
tin,13 aluminium14 and antimony15 have all emerged. In recent
years, receptors using anion–
p
interactions16 and specific reac-
B(OH)2
tions17,18 have been developed to sense fluoride ions. Among these
type receptors, boron–base complex is the most attractive one. The
fluorescence sensing of fluoride using boronic acid was first
reported by Tony Jams in 1998.19 Since then a lot of fluorescent
systems utilizing boron–fluoride interaction have been repor-
ted.1h,4b,11 Recently, stable triarylborons with a high Lewis acidity
were used as fluoride probes in aqueous or alcohol solvents to
overcome the competing binding of aqueous protons for fluoride
ions.20 Particularly, diborane receptors have received much atten-
Br
Br
Br
B(OH)2
B(OH)2
N
N
N
N
N
N
2
3
4
N
N
HO OH
B
N
-
N
2PF6
⇑
Corresponding authors. Fax: +86 411 84379648 (Z.X.); fax: +82 2 3277 3419
(J.Y.).
1
E-mail addresses: zcxu@dicp.ac.cn (Z. Xu), jyoon@ewha.ac.kr (J. Yoon).
Figure 1. Structures of naphthoimidazolium–boronic acid derivatives 1–4.
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.02.112

2756
E. J. Jun et al. / Tetrahedron Letters 54 (2013) 2755–2758
HO
OH
O
O
O
O
B
B
B
OH
HO
NBS/AIBN
CCl4/reflux
Br
Br
benznene/reflux
5
7
6
N
N
1) CH3CN/reflux
2) KPF6
8
N
N
N
HO OH
B
N
-
2PF6
1
Scheme 1. Synthesis of compound 1.
the naphthoimidazolium group was utilized for anion binding and
also as a fluorescent reporter. In our previous study, we have
proved that only the ortho phenylboronic acid 2 other than meta,
and para ones (3 and 4) can co-operate with imidazolium as a hy-
brid receptor for fluoride (Fig. 1).28 In order to increase the affinity
with fluoride, a reasonable way is to introduce more binding sites
to the receptor. In the current study, a tridentate receptor 1 with
one ortho boronic acid and two imidazolium groups was designed.
2-(2,6-Dimethylphenyl)-1,3,2-dioxaborolane 6 and bis(bromo-
methyl) adduct 7 were synthesized by modifying the reported pro-
cedures from 2,6-dimethylphenylboronic acid 5 (Scheme 1).29
Then, treatment of the bis(bromomethyl) adduct 7 with 1-methyl-
naphthoimidazole 830 in acetonitrile followed by the anion ex-
change with KPF6 gave the desired product 1 in 47% yield. The
detailed synthetic procedure as well as the characterization data
Figure 2. Fluorescent emission spectra of compound 1 (10 lM) in the presence of
different concentrations of FÀ. Inset: emission observed from solution of 1 and 1/FÀ.
Excitation at 325 nm.
1
including H NMR, 13C NMR, and ESI mass spectra are reported in
the Supplementary data.
The emission spectra and fluorescence titration experiments
with FÀ were first recorded in acetonitrile solution (Fig. 2). The
emission spectrum of free 1 displays a broad band with a maxi-
mum at 445 nm. When FÀ was added progressively to the solution
of 1, a significant decrease of the 445 nm emission and a blue-
shifted emission band centered at 370 nm were seen, which were
attributed to the formation of 1/FÀ complex and increase in inten-
sity. The (C–H)+ÁÁÁFÀ interaction blue-shifts the fluorescence emis-
sion. We have found the electrostatic interaction between the
imidazolium cation and FÀ can only quench the fluorescence with-
out a shift in emission. The emission shift is due to the hydrogen
bonding between imidazolium C2–H and FÀ.30 This indicates the
key role of the C2–H in the interaction with fluoride. Therefore,
the d of C2–H and the blue-shift in the fluorescence emission can
be used to probe the hydrogen bond between C2–H and FÀ. Then
1 is a ratiometric fluorescent probe for FÀ which involves the
observation of changes in the ratio of the intensities of the emis-
sion at two wavelengths. Ratiometric fluorescent probes have the
important feature in that they permit signal rationing and thus in-
crease the dynamic range and provide built-in correction for envi-
ronmental effects. Based on the variation of the fluorescence, the
binding constant of FÀ with imidazolium moiety is
2.3( 0.2) Â 106 MÀ1, which is much larger than the one of biden-
tate receptors.28 Addition of FÀ to the solution of 1 sufficiently blue
shifts the emission from 445 to 370 nm that makes a significant
color change. This color change allows 1/FÀ to be readily distin-
guished by the naked eye (Fig. 2, inset), and probe 1 thus combines
the sensitivity of fluorescence with the convenience and esthetic
appeal of a colorimetric assay.
Figure 3. Fluorescence responses of
Excitation at 325 nm.
1 (10 lM) to various anions (45 equiv).
tion of ClÀ, IÀ and HSO4 produced a nominal change in the
À
fluorescence spectra of 1 due to their low affinity with probe 1.
Other tested anions including BrÀ, CNÀ, CH3CO2,À and H2PO4
À
quench the fluorescence without a shift in emission, while the ex-
tent of quenching depends on the nature of the anion. These results
indicate these anions except FÀ interact with the imidazolium part
of 1 only by electrostatic attraction. Then probe 1 has a very high
selectivity for FÀ. The fluorescence titrations with other anions
and fluorescence changes of FÀ in the presence of other anions
are explained in the Supplementary data.
In our previous work,28 based on fluorescence and NMR studies,
we confirmed that the addition of fluoride to boron center can oc-
cur prior to the (C–H)+—FÀ type ionic hydrogen bond from the imi-
dazolium moiety. Only after binding 2 equiv FÀ by the boron
center, the imidazolium part can interact with the third fluoride
ion.28 In this Letter, 1H NMR and 19F NMR of 1 with FÀ were also
investigated to clarify the binding process. Tetrabutylammonium
fluoride hydrate is used as the fluoride source. As shown in Figure
4, with the addition of first equivalent fluoride, the peak of C2–H in
1 shifted downfield slightly from 9.11 to 9.29 ppm. This means, un-
like our previous bidentate B/Imidazolium receptor,28 the triden-
tate one has a better architecture, that is the boron center can
The fluorescence titration of 1 with various anions was con-
ducted to examine the selectivity. As shown in Figure 3, the addi-

E. J. Jun et al. / Tetrahedron Letters 54 (2013) 2755–2758
2757
Figure 4. 1H NMR spectra of 1 with fluoride in CD3CN at room temperature.
co-operate with imidazolium to bind FÀ. In 19F NMR studies
(Fig. S5), with the addition of first equivalent FÀ, the peak for
(C2–H)+—FÀ was also observed. Unfortunately, due to the poor sol-
ubility of both the 1-PF6 and 1-BrÀ, the solution of 1 became an
emulsoid with excess fluoride ion.
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In conclusion, we reported a tridente receptor 1 for FÀ contain-
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Acknowledgments
This work was supported by the Converging Research Center
Program through the Ministry of Education, Science and Technol-
ogy (2012K001512). This work was also supported by the Ewha
Global Top 5 Grant 2011 of the Ewha Womans University. Z.X.
thanks the Dalian Institute of Chemical Physics, Chinese Academy
of Sciences for financial support of this work.
Supplementary data
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