Highly selective fluoride sensing via chromogenic aggregation of a silyloxy-functionalized tetraphenylethylene (TPE) derivative

Article abstract of DOI:10.1016/j.tetlet.2013.11.059

A silyloxy-functionalized tetraphenylethylene (TPE) derivative shows a remarkable change in the absorption spectrum on deprotection with fluoride ions. The reaction process is highly selective for fluoride and the resulting charge transfer band results in a bright green solution. A simple selective visual assay of aqueous fluoride ions was also obtained by the impregnation of cellulose strips with the TPE derivative.

Full text of DOI:10.1016/j.tetlet.2013.11.059

Tetrahedron Letters 55 (2014) 456–459
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Highly selective fluoride sensing via chromogenic aggregation
of a silyloxy-functionalized tetraphenylethylene (TPE) derivative
Ilke Simsek Turan a, Fatma Pir Cakmak b, Fazli Sozmen a,
⇑
a UNAM-National Nanotechnology Research Center, Bilkent University, Ankara 06800, Turkey
b Department of Chemistry, Bilkent University, Ankara 06800, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 July 2013
Revised 24 October 2013
Accepted 14 November 2013
Available online 23 November 2013
A silyloxy-functionalized tetraphenylethylene (TPE) derivative shows a remarkable change in the absorp-
tion spectrum on deprotection with fluoride ions. The reaction process is highly selective for fluoride and
the resulting charge transfer band results in a bright green solution. A simple selective visual assay of
aqueous fluoride ions was also obtained by the impregnation of cellulose strips with the TPE derivative.
Ó 2013 Elsevier Ltd. All rights reserved.
Keywords:
Fluoride sensing
TPE
Chromogenic aggregation
Fluoride ions
Selectivity
Tetraphenylethylene (TPE) and its derivatives have attracted
remarkable attention due to their well-known aggregation-in-
duced emission (AIE)1 properties, which make this small molecule
an indispensible building block in several areas such as OLEDs,
chemosensors, and bioprobes.2 The outstanding electrochemical,
physical, and excited state properties, especially in the solid state,
The high chemical affinity between fluoride and silicon has
been widely used in previous studies to sense fluoride anions,
however, there are only a few which work in aqueous solutions.7
Herein, we report the design and synthesis of a highly selective
and sensitive chromogenic fluoride anion sensor, TPE 5. The syn-
thesis of target compound 5 started with the reaction between 4-
methoxyphenyllithium (from 1-bromo-4-methoxybenzene and
butyllithium) and 4-methoxybenzaldehyde to produce alcohol 1,
which was oxidized to compound 2 using manganese(IV) oxide
in dichloromethane. Compound 3 was then obtained through a
McMurry coupling of compound 2. This was followed by demeth-
ylation to give 4. Further reaction of compound 4 with chlorotriiso-
propylsilane in the presence of imidazole yielded the target
fluoride anion sensor 5 (Scheme 1).
Fluoride sensing of TPE 5 was accomplished by deprotection of
the silyl groups from the TPE core generating four phenoxide ions
in full conjugation with the TPE moiety, which leads to very strong
intramolecular charge transfer (ICT).8
This strong ICT resulted not only in the formation of new charge
transfer bands in the absorbance spectrum corresponding to a
bright green solution, but also led to quenching of the emission
in the fluorescence spectrum.
are a result of the extended p
-system found in TPE.3
Fluoride ion sensing is an important topic because of the biolog-
ical significance of this anion, especially in relation to dental care,
and in the treatment of osteoporosis.4 However, fluoride ion sens-
ing is a highly challenging task due to the high electronegativity of
fluorine, its strong hydrogen bonding ability and its size (smallest
anion).
In addition, real time monitoring of fluoride ions is extremely
difficult in aqueous media, especially via chromogenic chemosen-
sors due to competitive hydrogen bonding between fluoride and
water molecules.5 A reaction-based sensor could have some advan-
tages considering the simplicity of the analytical procedure. In this
respect, naked eye anion detection with chemosensors capable of
detecting anions is becoming increasingly important for rapid
on-site analysis, on a real time basis. Current chromogenic anion
sensors typically have hydrogen bonding receptor sites, some with
selectivity for fluoride, but are less likely to work in aqueous
medium.6
In the electronic absorption spectrum, a maximum was ob-
served at 340 nm for compound 5 in DMF. Next, the solution of
compound 5 (50 lM in DMF) was treated with fluoride ions in
the form of a tetrabutylammonium salt (TBAF). The absorption
spectra during titration showed new bands appearing at 452,
510, 600 and 680 nm, respectively. The band at 600 nm was the
⇑
Corresponding author. Tel.: +90 312 290 3568; fax: +90 312 266 4068.
E-mail address: sozmen@unam.bilkent.edu.tr (F. Sozmen).
0040-4039/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2013.11.059

I. Simsek Turan et al. / Tetrahedron Letters 55 (2014) 456–459
457
Figure 2. The digital photographs show the appearance of the solutions under a
hand-held UV lamp (360 nm) (top) and under ambient light (bottom). Probe
concentrations were 50
all in DMF.
lM and the anions were added at 0.4 mM concentrations,
Scheme 1. Synthesis of fluoride sensor 5.
of 5. Clearly, no color change was observed upon addition of the
other anions; only fluoride ions led to a color change.
The emission spectra of compound 5 (50 lM in DMF) showed
most intense, resulting in a bright green color of the solution
(Fig. 1).
drastic quenching upon addition of fluoride anions in the form of
TBA salts (Fig. 3). The strong charge transfer character of the
formed phenoxide moieties leads to this quenching. As expected,
no emission quenching was observed upon addition of other ions
as TBA salts as shown in Figure 4 (top). The spectacular difference
between fluoride anions and the other anions in terms of emission
intensity is clearly shown in the bar graphs (Fig. 4, bottom). Fur-
thermore, selective quenching of fluoride is shown in the digital
photographs of DMF solutions under the indicated conditions
(Fig. 2, top).
The electronic absorption spectra of 5 were also investigated
upon the addition of eight equivalents of the TBA salts of HSOÀ4 ,
H2PO4À, AcOÀ, NO3À, CNÀ, IÀ, BrÀ, and ClÀ to DMF solutions of 5.
The addition of these anions did not result in any changes in the
absorption spectrum (Fig. S1), or the emission spectrum.
Figure 2 (bottom) shows the color change after the addition of
eight equivalents of the anions as TBA salts to the DMF solution
Figure 1. Absorbance spectra of compound 5 + FÀ in the presence of increasing FÀ
concentrations. The inset is a digital photograph of the probe (left) and probe
Figure 3. Emission spectra of compound 5 + FÀ in the presence of increasing FÀ
following the addition of 8 equivalents of FÀ (right). Probe concentration is 50
DMF.
lM in
concentrations. Probe concentration is 50
340 nm.
lM in DMF. Excitation wavelength is

458
I. Simsek Turan et al. / Tetrahedron Letters 55 (2014) 456–459
Figure 5. The color changes of the treated cellulose strips prepared for the
detection of fluoride anions in aqueous environments, on exposure to different
fluoride concentrations.
Acknowledgment
The authors thank Professor Dr. Engin U. Akkaya for fruitful dis-
cussions. I.S.T. acknowledges support from TUBITAK-BIDEB in the
form of a scholarship.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2013
11.059.
References and notes
Figure 4. EmissÀion spectra of compound 5 upon the addition of 8 equivalents of
ClÀ, BrÀ, IÀ, HSO4 , AcOÀ, NOÀ3 , H2PO4À, and CNÀ. The FÀ and probe concentrations are
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In order to explore whether compound 5 could respond to fluo-
ride anions in solid state via chemical reactions, we decided to pre-
pare a test paper for detecting fluoride in an aqueous environment
by dipping a cellulose strip (3.0 Â 0.5 cm2) into a tetrahydrofuran
solution of 5 (4.0 mM). After drying the cellulose strip under ambi-
ent conditions, it was immersed in an aqueous fluoride solution
(water/tetrahydrofuran, 2:8) for several seconds followed by a
short heat treatment.
The color of the cellulose strip was clearly and reproducibly
changed from white to green (Fig. 5). However the color change
was not uniform throughout the test strip. The underlying reason
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reaction taking place on the cellulose fibers, introducing some het-
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silyl groups results in four donor phenoxide groups causing strong
intramolecular charge transfer. This strong ICT also induces consid-
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