A Benzodithiophene-Based Fluorescence Probe for Rapid Detection of Fluoride Ion

Article abstract of DOI:10.1002/cjoc.201600064

A novel and simple fluorescence probe was synthesized from benzo[1,2-b:4,5-b′]dithiophene (BDT) and trimethylsilylethyne via Sonogashira reaction, and showed highly selective and sensitive fluorescence decreasing response towards F^?. The probe molecule turned to a weakly fluorescent terminal alkyne moiety because its trimethylsilyl (TMS) group was cleaved by fluoride, which was proved by¹H NMR titration. Whereas no distinct fluorescent changes were observed with the addition of other anions, such as Cl^?, Br^?, I^?, AcO^?and H₂PO₄^?. Upon the addition of F^?, the maximum fluorescence emission wavelength shifted from 460 nm to 450 nm with a decrease of fluorescence intensity by 40% within 20 s. Moreover, the detection limit towards F^?was calculated to be as low as 73.5 nmol/L.

Full text of DOI:10.1002/cjoc.201600064

FULL PAPER
DOI: 10.1002/cjoc.201600064
A Benzodithiophene-Based Fluorescence Probe for Rapid
Detection of Fluoride Ion
Wenbin Tan,a Taohua Leng,b Guoqiao Lai,c Zhifang Li,c Jiefei Wu,a
Yongjia Shen,a Chengyun Wang*,a
a Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and
Technology, Shanghai 200237, China
b National Food Quality Supervision and Inspection Center (Shanghai)/Shanghai Institute of Quality Inspection and
Technical Research, Shanghai 200233, China
c Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Nor-
mal University, Hangzhou, Zhejiang 310012, China
A novel and simple fluorescence probe was synthesized from benzo[1,2-b:4,5-b']dithiophene (BDT) and trime-
thylsilylethyne via Sonogashira reaction, and showed highly selective and sensitive fluorescence decreasing re-
sponse towards F. The probe molecule turned to a weakly fluorescent terminal alkyne moiety because its trime-
1
thylsilyl (TMS) group was cleaved by fluoride, which was proved by H NMR titration. Whereas no distinct fluo-
rescent changes were observed with the addition of other anions, such as Cl, Br, I, AcO and H2PO－4 . Upon the
addition of F, the maximum fluorescence emission wavelength shifted from 460 nm to 450 nm with a decrease of
fluorescence intensity by 40% within 20 s. Moreover, the detection limit towards F was calculated to be as low as
73.5 nmol/L.
Keywords fluorescent probe, benzodithiophene, fluoride anion, trimethylsilylethynyl group
Introduction
al.[27] reported a chemodosimer based on a core-substi-
tuted naphthalene diimide for fluoride, which displayed
weak fluorescent intensity and could hardly recognize
the color changes under 365 nm UV-lamp.
The development of fluorescent probe to sense im-
portant ions has received considerable attention as these
ions play important roles in many chemical and biolog-
ical processes.[1-4] In recent years, a number of sensors
for anions have been reported,[5-7] and fluoride ion is an
important anion in the dental health and the treatment of
osteoporosis.[8-10] Fluoride ion is easy to be absorbed by
human body but difficult to be excreted. However, too
much fluoride can cause acute stomach and kidney
problems.[11-12] Therefore, many fluorescent probes have
been synthesized to detect fluoride.[13]
So far, the major features of the reported fluoride
sensors are based on three kinds of methods and listed
as follows: hydrogen bonding,[14-16] Lewis acid-base
interaction[17-19] and specific reactivity with silicon.[20,21]
Moreover, a new method, which utilizes target anions to
induce specific chemical reactions, displays high selec-
tivity when proper fluorophores and reactive groups are
introduced into the probes.[22,23] The sensors based on
Si－C bond for fluoride ion detection were report-
ed.[24,25] Rao et al.[26] reported a Boron-dipyrromethene
based specific chemodosimeter to detect F and the re-
action completed within 5 min duration. Buckland et
On the other hand, benzo[1,2-b:4,5-b']dithiophene
(BDT) has often been used as an electron-donating unit.
Its large planar π-conjugated structure promotes facile
π-π stacking and improves hole mobility.[28] At the same
time, the planar and rigid fused aromatic ring of BDT
would be helpful to strengthen interactions among pol-
ymer chains, which is favourable for charge-transporta-
tion.[29] However, the chemosensors based on BDT as
core structure have never been reported for F detection
via the fluoride-triggered cleavage of SiC. In this work,
a fluorescent probe (P1) with excellent fluorescence
quantum yields (ΦF ＝ 0.21) (See Supporting Infor-
mation, Table S1) for F detection was synthesized, in
which BDT derivative was used as fluorophore and tri-
methylsilylethynyl was used as fluoride chelating moie-
ty. Upon the addition of F into P1 solution, the fluores-
cence intensity decreased with a 10 nm blue-shift.
Meanwhile, the probe displayed high selective sensing,
lower detection limit and fast fluorescence response
towards F within 20 s (See Supporting Information,
*
E-mail: cywang@ecust.edu.cn; Tel./Fax: 0086-021-64252967
Received January 27, 2016; accepted May 16, 2016; published online XXXX, 2016.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc. 201600064 or from the author.
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1

Tan et al.
FULL PAPER
The synthesized target compounds were characterized
Table S2).
1
by H NMR, 13C NMR, and HRMS (See Supporting
Information, Figures S1－S9).
Experimental
Material and instruments
Scheme 1 Synthesis of 2,6-bis(trimethylsilylethynyl)-4,8-di-
octyloxybenzo[1,2-b:4,5-b']-dithiophene
All the reagents were purchased from commercial
suppliers and used without further purification. And all
solvents and reagents, unless otherwise stated, were CP
or AR level. Solvents for chemical synthesis such as
THF, triethylamine were purified by dehydration and
distillation with standard methods. Pd(PPh3)2Cl2 was
synthesized according to the reference and stored in dry
O
O
S
n-BuLi
Zn, NaOH
N
n-C8H17Br
S
S
O
4 (69%)
C8H17
1
container. H NMR and 13C NMR in CDCl3 were ob-
C8H17
O
O
O
O
tained by a Bruker AV-400 spectrometer with tetrame-
thylsilane (TMS) as an internal standard at room tem-
perature. High resolution mass spectra (HRMS) were
obtained by a Waters LCT Premier XE spectrometer.
Absorption spectra were measured on a Varian Cary 500
spectrophotometer. Fluorescence spectra were recorded
on a Varian Cary Eclipse fluorescence spectrophotome-
ter (1 cm quartz cell).
S
Br2
S
Br
Br
S
S
C8H17
C8H17
3 (72%)
2 (87%)
C8H17
O
Si
S
Si
Si
Synthesis of 2,6-bis(trimethylsilylethynyl)-4,8-di-
octyloxybenzo[1,2-b:4,5-b']dithiophene (P1)
S
Pd(PPh3)2Cl2, CuI
O
C8H17
2,6-Di-bromo-4,8-dioctyloxybenzo[1,2-b:4,5-b']dithi-
ophene (1.2 g, 1.99 mmol) was dissolved into 25 mL
THF and 25 mL triethylamine in a 100 mL three-neck
flask, and degassed with Ar over 30 min. Then,
Pd(PPh3)2Cl2 (72.8 mg, 0.10 mmol) and CuI (38.1 mg,
0.20 mmol) were added into the flask under Ar atmos-
phere. Subsequently, trimethylsilylacetylene (1.56 g,
15.89 mmol) was added dropwise under Ar atmosphere.
The mixture was stirred at room temperature for 6 h.
After removing solvent, the crude product was purified
by column chromatography [V(methylene chloride)/
V(petroleum ether)＝1∶15], P1 (yield 85%) was ob-
tained as yellow solid. m.p. 99－101 ℃; 1H NMR (400
MHz, CDCl3) δ: 7.56 (s, 2H), 4.21 (t, J＝6.6 Hz, 4H),
1.87－1.80 (m, 4H), 1.53 (dd, J＝9.7, 5.1 Hz, 4H), 1.38
－1.29 (m, 16H), 0.90 (s, 6H), 0.28 (s, 18H); 13C NMR
(100 MHz, CDCl3) δ: 144.1, 132.0, 130.4, 126.2, 123.1,
101.9, 98.0, 74.4, 32.0, 30.7, 29.6, 29.5, 26.2, 22.9, 14.4,
0.22; HRMS (ESI) m/z: [M＋H]＋ calcd for C36H55O2-
S2Si2 639.3182, found 639.3182.
P1 (85%)
Response time for P1
The effect of the reaction time of P1 towards F was
evaluated by the fluorescence method. Fluorescence
spectra were recorded within 60 s in THF after F was
introduced to the solution. As shown in Figure 1, it can
be seen that there is almost no time-dependent effect in
the fluorescence sensing process, and a saturation of
fluorescence intensity appeared after 20 s. It is well
known that this reaction occurs more rapidly than that
of Si－O bond-cleavage processes.[31-35]
Results and Discussion
Synthesis
The synthetic route of P1 was shown in Scheme 1.
4,8-Dihydrobenzo[1,2-b:4,5-b']dithiophen-4,8-dione (4),
4,8-dioctyloxybenzo[1,2-b:4,5-b'] dithiophene (3), and
2,6-dibromo-4,8-dioctyloxybenzo[1,2-b:4,5-b']dithioph-
ene (2) were prepared from the starting material
N,N-diethylthiophene-3-carboxamide according to the
literature procedures.[30] Meanwhile, 2,6-bis(trimethyl-
silylethynyl)-4,8-dioctyloxybenzo[1,2-b:4,5-b'] dithio-
phene (P1) was prepared by compound 2 and trime-
thylsilylacetylene via Sonogashira reaction (85% yield).
Figure 1 Time-dependent fluorescence spectra of P1 (1×105
mol/L) in the presence of 5 equiv. fluoride.
Selectivity study
The selectivity of P1 towards F was fully investi-
2
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Chin. J. Chem. 2016, XX, 1—5

A Benzodithiophene-Based Fluorescence Probe for Rapid Detection of Fluoride Ion
gated. Various anions including F, Cl, Br, I, AcO
and H2PO－4 were also investigated by UV-vis absorp-
tion and fluorescence spectra. As shown in Figure 2a,
only upon the addition of F (5 equiv.), the absorbance
at 401 nm decreased gradually with a blue shift (Δλabs＝
9 nm), and a new peak at 392 nm appeared. In compar-
ison, only slight changes were observed in the spectra
even upon the addition of 10 equiv. of other anions.
These results suggest that desilylation of TMS can be
caused by fluoride, rather than other anions. Based on a
literature survey, it is also clear that the change is due to
desilylation of the TMS moieties.[36] And P1 exhibits
strong fluorescence emission at 460 nm in the THF so-
lution (Figure 2b). It was found that only F induced an
obvious blue shift in the fluorescence maximum from
460 nm to 450 nm, while the other anions (10 equiv. of
each) did not cause significant fluorescence spectral
changes. The fluorescence color change can be observed
under 365 nm UV-lamp (Figure 2c). Furthermore, as
shown in Figure 3, competing experiment indicated that
the detection could hardly be interfered by other anions
(See Supporting Information, Figure S10).
2.0
1.5
F
1.0
F
0.5
0.0
F-
Cl-
Br-
I-
AcO- H2PO4-
Blank
Figure 3 The selectivity of P1 (1×105 mol/L). The black bars
represent the emission intensity of P1 in the presence of various
anions (10 equiv.). The red bars represent the emission intensity
that occurs upon the subsequent addition of F (5 equiv.) to the
above solution with excitation at 401 nm.
(a)
probe
0.25
-
F
-
0.20
0.15
0.10
0.05
0.00
Cl
-
Br
-
I
-
AcO
Titration experiment
-
H PO
2
4
The absorption and fluorescence titration experi-
ments of P1 at different F concentrations were per-
formed in THF at room temperature (Figure 4). In ab-
sorption titration spectra (Figure 4a), the absorption
peak of 401 nm faded away with the appearance of new
blue-shifted peak of 392 nm. There was a linearity of
the absorption ratio (A392nm/A401nm) versus the F con-
centration (See Supporting Information, Figure S11).
The results of the fluorescence titrations for P1 towards
various amount of Fat room temperature are shown in
Figure 4b. Upon the addition of F to the solution, the
fluorescence intensity at 460 nm gradually decreased,
and 40% of the fluorescence intensity was quenched.
And there was also a good linearity of the fluorescence
intensity at 460 nm versus the F concentration (See
Supporting Information, Figure S12). At the same time,
the fluorescence color also changed, which maybe stem
from the deprotection of electron-donating TMS group
by F.[37,38] Moreover, the Si－C and Si－F bond disso-
ciation energies were different (69 and 141 kcal/mol,[1]
respectively), it is likely that the Si－C bond was splited
and the short wavelength fluorescent product was re-
leased with the reaction between compound 1 and F,
which is responsible for the fluorescence decreasing and
blue shifting. In addition, the near-linear correlation
curves of P1 at 460 nm demonstrated the potential utili-
ty of P1 for quantitative determination of F. The detec-
tion limit of P1 for F was about 73.5 nmol/L (calculat-
ed from Figure 5).
360
380
400
420
440
Wavelength/nm
(b)
probe
-
600
400
200
0
F
-
Cl
-
Br
-
I
-
AcO
-
H PO
2
4
450
500
550
600
Wavelength/nm
Figure 2 Absorption (a) and fluorescent spectra (b) of P1 (1×
105 mol/L) in the presence of F (5 equiv.), Cl, Br, I, AcO,
and H2PO－4 (10 equiv., respectively). (c) Color changes ob-
served upon the addition of various anions (10 equiv.) to the solu-
tions of P1 (1×105 mol/L) under 365 nm UV-lamp.
1H NMR titration experiment
To further describe the mechanism of detection, the
Chin. J. Chem. 2016, XX, 1—5
© 2016 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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3

Tan et al.
FULL PAPER
presence of F. Upon the addition of 1 equiv. of fluoride
ion, a new peak at δ＝3.30 appeared, and a saturated
height was observed when 2 equiv. of fluoride were
added into the solution. This may be due to the for-
mation of terminal acetylene, which was originated
from the cleaved reaction of TMS group in the presence
of fluoride. In addition, the SiCH3 signal shifted upfield
from δ 0.28 to δ 0.0, which suggested that the free TMS
moieties were generated. Moreover, no significant
1
changes in the H NMR signals or chemical shift were
observed for other anions.
460 nm
(b)
600
F-
400
450 nm
450
200
0
500
550
Wavelength/nm
Figure 4 Spectra properties of P1 (1×105 mol/L) upon the
addition of F (0－2 equiv.) in THF: (a) absorption spectra, (b)
fluorescence spectra.
Figure 6 1H NMR titration spectra of P1 in CDCl3 in the pres-
ence of various equivalents of TBAF.
700
Y = -176.43X + 707.609
R2 = 0.985
Conclusions
600
In summary, a novel and simple fluorescent probe
for F based on benzodithiophene derivative was syn-
thesized. The fluorescent probe performs high selectivi-
ty, short response time and excellent sensitivity toward
F over other anions (Cl, Br, I, AcO, H2PO－4 ) in the
THF solution with small Stokes shift and excellent flu-
orescence quantum yield. Upon the addition of fluoride
anions, the color change was observed under 365 nm
UV-lamp, which is attributed to the elimination of the
TMS substituents through a strong interaction between
the fluoride anion and the silicon atoms. All these ex-
perimental results indicate that P1 could be used to de-
tect F as an excellent fluorescent probe. In addition, the
detection limit is calculated to be as low as 73.5 nmol/L.
500
400
0.0
0.5
1.0
1.5
2.0
Concentration of F-/(10-5 mol L )
-1
.
Figure 5 Linear region of fluorescence intensity of P1 (1×105
mol/L) in THF upon the addition of F (0－2 equiv.) in THF. The
fluorescence intensities of five blank measurements for P1 were
258.012, 258.201, 258.782, 258.946, and 259.143, respectively.
σ＝0.432302 μmol/L; S＝176.43. The detection limit was calcu-
lated to be 73.5 nmol/L (3σ/S).
Acknowledgement
1
This work is sponsored by Natural Science Founda-
tion of Shanghai (16ZR1408000), and the National
Natural Science Foundation of China (No. 21576087).
detection process of P1 was studied by H NMR titra-
tion in CDCl3. As shown in Figure 6, it depicts the
changes in the chemical shifts of P1 in the absence and
4
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A Benzodithiophene-Based Fluorescence Probe for Rapid Detection of Fluoride Ion
[21] Fu, L.; Tian, F. F.; Lai, L.; Liu, Y.; Harvey, P. D.; Jiang, F. L. Sens.
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