Novel 2,1,3-benzothiadiazole derivatives used as selective fluorescent and colorimetric sensors for fluoride ion

Article abstract of DOI:10.1016/j.dyepig.2015.09.021

Two novel 2,1,3-benzothiadiazole derivatives, used as colorimetric and fluorescent sensors for fluoride ion were designed, synthesized and characterized. These sensors contained single hydrogen-bond functional groups, which acted as interacting sites for fluoride ion. Both sensors displayed a red color in dimethyl sulfoxide solution. Binding studies revealed that they showed noticeable colorimetric and fluorescent responses only to F^- among the eight anions tested (F^-, Cl^-, Br^-, I^-, AcO^-, H₂PO₄^-, HSO₄^- and NO₃^-). ¹H NMR titration proved that the interaction between compounds and fluoride ion: hydrogen bond at low fluoride ion concentration, deprotonation at high fluoride ion concentration. Moreover, those compounds showed high sensitivity and selectivity for fluoride ion by inhibiting the excited state intramolecular proton transfer processes, and the detective processes could be followed by direct visual observation.

Full text of DOI:10.1016/j.dyepig.2015.09.021

Accepted Manuscript
Novel 2,1,3-benzothiadiazole derivatives used as selective fluorescent and
colorimetric sensors for fluoride ion
Jiefei Wu, Guoqiao Lai, Zhifang Li, Yunxiang Lu, Taohua Leng, Yongjia Shen,
Chengyun Wang
PII:
S0143-7208(15)00368-X
10.1016/j.dyepig.2015.09.021
DYPI 4932
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 7 August 2015
Revised Date: 16 September 2015
Accepted Date: 20 September 2015
Please cite this article as: Wu J, Lai G, Li Z, Lu Y, Leng T, Shen Y, Wang C, Novel 2,1,3-
benzothiadiazole derivatives used as selective fluorescent and colorimetric sensors for fluoride ion, Dyes
and Pigments (2015), doi: 10.1016/j.dyepig.2015.09.021.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Novel 2,1,3-benzothiadiazole derivatives used as selective
fluorescent and colorimetric sensors for fluoride ion
Jiefei Wu, Guoqiao Lai, Zhifang Li, Yunxiang Lu, Taohua Leng, Yongjia Shen, and Chengyun Wang
Two Novel 2,1,3-benzothiadiazole derivatives as reactive fluorescent sensors with high sensitivity
and selectivity for fluoride ion have been fabricated.

ACCEPTED MANUSCRIPT
Novel 2,1,3-benzothiadiazole derivatives used as
selective fluorescent and colorimetric sensors for
fluoride ion
Jiefei Wua,b, Guoqiao Laib, Zhifang Lib, Yunxiang Luc, Taohua Lengd, Yongjia Shena, and
Chengyun Wang a,
aKey Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University
of Science and Technology, 130 Meilong Road, Shanghai 200237, China
bKey Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education,
Hangzhou Normal University, Hangzhou 310012, China
cKey Laboratory for Advanced Materials and Department of Chemistry, East China University of
Science and Technology, 130 Meilong Road, Shanghai 200237, China
dNational Food Quality Supervision and Inspection Center (Shanghai) / Shanghai Institute of
Quality Inspection and Technical Research, Shanghai 200233, China.
Abstract
Two novel 2,1,3-benzothiadiazole derivatives, used as colorimetric and fluorescent
sensors for fluoride ion were designed, synthesized and characterized. These sensors
contained single hydrogen-bond functional groups, which acted as interacting sites for
fluoride ion. Both sensors displayed a red color in dimethyl sulfoxide solution.
Binding studies revealed that they showed noticeable colorimetric and fluorescent
-
responses only to F- among the eight anions tested (F-, Cl-, Br-, I-, AcO-, H2PO4 ,
-
-
1
HSO4 and NO3 ). H NMR titration proved that the interaction between compounds
Corresponding author. Tel./fax: +86-021-64252967; e-mail: cywang@ecust.edu.cn; yjshen@ecust.edu.cn
1

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and fluoride ion: hydrogen bond at low fluoride ion concentration, deprotonation at
high fluoride ion concentration. Moreover, those compounds showed high sensitivity
and selectivity for fluoride ion by inhibiting the excited state intramolecular proton
transfer processes, and the detective processes could be followed by direct visual
observation.
Key words
Fluoride, fluorescent sensor, deprotonation, 2, 1, 3-benzothiadiazole
1. Introduction
Anions are known to play crucial roles in a wide range of chemical and
biological processes,[1-3] and construction of appropriate molecules for sensing
of anions is a “hot” topic [4,5]. Fluoride ion is one of the most attractive targets
owing to its smallest ionic radius, highest charge density and hard Lewis basic
nature.[6,7]So far the sensors for fluoride ion can be classified several kinds as
follows: (a) Lewis acids-based interactions;[8,9] (b)hydrogen-bond
interactions;[2,10,11] (c) reaction-based interactions;[12] (d) polymer-based
interactions;[13] (e) Quantum dots and gold nanoparticles-based
interactions;[14] (f) mesoporous silica or silica particle-based interactions.[15]
Their signaling mechanisms can be deduced as follows: photo induced electron
transfer (PET),[16] intramolecular charge transfer (ICT),[17,18] metal-to-ligand
charge transfer (MLCT) and excited-state intramolecular proton transfer
(ESIPT).[19,20]
Recently, some new methods which can quickly detect fluoride ion were
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reported.[21]In some cases, hydrogen bonds were utilized for fluorescent
sensor sites and some acidic amidic NH groups could be deprotonated by
fluoride ion for their relatively strong base.[22] At the same time,
2,1,3-benzothiadiazole (BTD) derivatives are a class of attractive fluorophores
used in many photoelectrical materials.[23] Molecules containing 2,1,3-
benzothiadiazoles usually show large molar extinction coefficients, good
photostability and intense fluorescence.[24] Wang et al.[25] reported a
benzothiadiazole derivative for fluoride ion detecting, the detective amount of
fluoride was 0-300 equiv. Furthermore, Sreenanth et al.[26] reported
bisthiocarbohydrazones for a fluoride sensor, but these sensors also possessed
response to acetate ion.
In this investigation, two compounds, 7-(9-octyl-9H-carbazol-3-yl)-
N-(4-nitrophenyl)-benzo[c][1,2,5]thiadiazol-4-amine (CBT-NH) and 7-(10-
octyl-10H-phenothiazin-3-yl)- -(4-nitrophenyl)-benzo[c][1,2,5]thiadiazol-4-
N
amine (PBT-NH) (Scheme 1), were designed and synthesized, and used as
fluorescent sensors for fluoride with the detection limits 0.86uM and 4.25uM,
respectively. And their quantum yields were 0.034 and 0.015, respectively.
CBT-NH and PBT-NH showed high selectivity for fluoride ion through the
deprotonation action and the inhibiting ESIPT processes. More importantly, the
molecules could detect fluoride ion by a fast method which was observable
with the naked eye. And they showed a faster response (within 5s) to fluoride ion
than that of some related literatures.[27-28]
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Insert Scheme 1 here
2. Experimental
All the starting materials, reagents and solvents were AR grade purchased from
1
J&K Chemical Co., which were received without further purification. H and 13C
NMR spectra were measured on a Bruker AM-400 spectrometer at room temperature
using d-chloroform or DMSO-d6 as a solvent and tetramethylsilane (TMS, δ = 0 ppm)
as an internal standard. The UV/Vis spectra were recorded on a Nicolet CARY 100
and the Fluorescence spectra were measured on a CARY Eclipse. The stock solutions
of CBT-NH and PBT-NH (5×10-5 mol/L) were prepared in DMSO solution. The
TBA salts (F-, Cl-, Br-, I-, AcO-, H2PO4 , HSO4 , and NO3 ) solutions were prepared at
a concentration of 1×10-1 mol/L in DMSO solution. Different equivalents of TBA
salts were added to the compounds and their corresponding UV-visible and
fluorescence spectra were recorded at room temperature. Compounds (CBT-NH and
PBT-NH) (6×10-3 mol/L in DMSO-d6) were titrated with fluoride anion (as
tetrabutylammonium salts) by addition of increasing equivalents of anion in
DMSO-d6 solution.
-
-
-
2.1.3-Bromo-9H-carbazole (1a)
Carbazole (1.002 g, 5.997mmol) was added into DMF solution (50 mL). Then the
mixture of N-bromosuccinimide (NBS) (1.068 g, 6.000mmol) and DMF (20 mL) was
added dropwise into reaction flask in ice-water bath. After filtration and washed by
1
water, white powder was obtained [29] (1.254 g, 85.0%)
.
Mp:199ꢀ. H NMR
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(CDCl3, 400 MHz, TMS)δ: 11.40(s, 1H), 8.33(d, J=8.4Hz, 1H), 8.12(d, J=8.4Hz, 1H),
7.43(m, 4H), 7.15(t, J=7.6Hz, 1H).
2.2. 3-Bromo-9-octyl-9H-carbazole (2a)
Compound 1a (0.615 g, 2.499 mmol) and K2CO3 (1.730 g, 12.50 mmol) were
added into DMF (20 mL). Then C8H17-Br (0.580 g, 3.003 mmol) was added dropwise
o
into reaction flask, and the mixture was stirred overnight at 80 C. After removal of
the solvent, the residue was purified by column chromatography on silica (petroleum
1
ether) to give a liquid [30] (0.740 g, 82.7%). H NMR (CDCl3, 400 MHz, TMS)
δ
:
8.20 (s, 1H), 8.04 (d, J=7.6 Hz, 1H), 7.53 (d, J=8.4 Hz, 1H), 7.47 (d, J=8.4 Hz, 1H),
7.40 (d, J=8.4 Hz, 1H), 7.27 (d, J=8.4 Hz, 1H), 7.22 (d, J=7.6 Hz, 1H), 4.27 (t, J=7.2
Hz, 2H), 1.40-1.20 (m, 12H), 0.85 (t, J=7.2 Hz, 3H).
2.3.9-Octyl-3-(4, 4, 5, 5-tetramethyl-1, 3, 2-dioxaborolan-2-yl)-9H-carbazole (3a)
A solution of Compound 2a (0.716 g, 2.000 mmol) in anhydrous THF (20 mL) was
cooled to -78 oC under a nitrogen atmosphere. N-BuLi (2.5 M, 2.400 mmol) was
slowly added dropwise followed by stirred for another
1
h. Then
isopropylpinacolylborate (2.04 mL, 10.01 mmol) was added and the mixture was
o
stirred at -78 C for 1 h. After that, the mixture was cooled to room temperature and
stirred for 16 h. After removal of the solvent, the crude product was purified by
column chromatography (petroleum ether/CH2Cl2=3:1).[31] A white solid was
obtained in a yield of 63.8% (0.5170 g). 1H NMR (CDCl3, 400 MHz, TMS)
δ: 8.53 (s,
1H), 8.06 (d, J=7.6 Hz, 1H), 7.84 (d, J=7.6 Hz, 1H), 7.38 (d, J=7.2 Hz, 1H), 7.32 (d,
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J=8.4 Hz, 2H), 7.15 (d, J=7.2 Hz, 1H), 4.23 (t, J=7.2 Hz, 2H), 1.33 (s, 12H),
1.30-1.10 (m, 12H) ,0.80 (t, J=7.2 Hz, 3H).
2.4.10-Octyl-10H-phenothiazine (1b)
This compound was synthesized by the same procedure described for 2a using
1
10H-phenothiazine and C8H17-Br.[32] Yield: 75.6%. H NMR (CDCl3, 400 MHz,
TMS)δ: 7.14-7.10 (m, 4H), 6.91-6.84 (m, 4H), 3.40 (t, J=6.8 Hz, 2H), 1.30-1.10 (m,
12H), 0.82 (t, J=6.8 Hz, 3H).
2.5.3-Bromo-10H-octyl-10H-phenothiazine (2b)
The compound was synthesized by the same procedure descried for 1a using
compound 1b and N-bromosuccinimide (NBS).[33] Yield: 82.5%.1H NMR (CDCl3,
400 MHz, TMS)δ: 7.24-7.21 (m, 4H), 6.70-6.66 (m, 3H), 3.79 (t, J=7.2 Hz, 2H),
1.40-1.10 (m, 12H), 0.80 (t, J=7.2 Hz, 3H).
2.6.10-Octyl-3(3, 3, 4, 4-tetramethylborolan-1-yl)-10H-phenothiazine (3b)
The compound was synthesized by the same procedure described for 3a using
compound 2b and isopropylpinacolylborate.[31] Yield: 60.3%. 1H NMR (CDCl3, 400
MHz, TMS)δ: 7.60-7.52 (m, 2H), 7.15-7.07 (m, 2H), 6.92-6.80 (m, 3H), 3.84 (t, J=7.2
Hz, 2H), 1.35(s, 12H), 1.30-1.10 (m, 12H), 0.75 (t, J=7.2 Hz, 3H).
2.7. Benzo[c] [1, 2, 5]thiadiazole (1c)
1, 2-Diaminobenzene (3.250 g, 30.05 mmol) and triethylamine (18 mL, 130.0
mmol) were dissolved in CH2Cl2 (20 mL). A solution of SOCl2 (7.0 mL, 96.00 mmol)
in CH2Cl2 (10 mL) was added dropwise. After removal of the solvent, The combined
organic layer was concentrated under vacuum to give the product (3.956 g, 97%).[34]
6

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1
Mp
2H).
2.8. 4, 7-Dibromobenzo[c] [1, 2, 5]thiadiazole (2c)
: 43ꢀ. H NMR (CDCl3, 400 MHz, TMS)δ: 8.04-8.00 (m, 2H), 7.62-7.57 (m,
Compound 1c (1.360 g, 9.988mmol) was dissolved in the solution of HBr (35 mL,
40%), then a solution of Br2 (1.6 mL, 31.20 mmol) in HBr (35 mL, 40%) was added
dropwise under the ice-water bath. The mixture was stirred for 12 h at 120 oC. After
removal of the solvent, the residue was purified by column chromatography on silica
(petroleum ether/CH2Cl2=7:1 v/v) to give a pink solid (2.580 g).[34] Yield: 88.05%.
Mp:184-185ꢀ.1H NMR (CDCl3, 400 MHz, TMS)
δ: 7.73 (s, 2H).
2.9.7-Bromo-N-(4-nitrophenyl)benzo[c][1,2,5]thiadiazol-4-amine (3c)
Compound 2c (0.600 g, 2.041 mmol), 4-nitroaniline (0.450 g, 3.258 mmol) and
K2CO3 (0.840 g, 6.000 mmol) were dissolved in toluene. Under the argon atmosphere,
Pd(pph3)4 (0.115 g, 0.100 mmol) and P(t-Bu)3 were added to the mixture and stirred
for 12 h at 120 oC. Upon cooling, the mixture was poured into water and extracted
with CH2Cl2 (20 mL×3). The combined organic layer was concentrated using a rotary
evaporator. The residue was purified by column chromatography on silica (petroleum
ether/ethyl acetate=10:1 v/v) to give an orange-red solid (0.286 g). Yield: 40%. Mp:
120-122 oCꢁ1H NMR (CDCl3, 400 MHz, TMS)
J=8.0 Hz, 1H), 7.49 (s, 1H), 7.36 (d, J=9.2 Hz, 2H), 7.34 (d, J=8.0 Hz, 1H). 13C NMR
(CDCl3, 100 MHz, TMS) :151.59, 133.28, 131.92, 129.13, 126.73, 125.33, 125.00,
δ: 8.27 (d, J=9.2 Hz, 2H), 7.79 (d,
δ
116.00, 112.33, 109.21. IR(KBrꢂv(cm-1)): 3446.32, 1611.93, 1592.01, 1558.45,
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1488.43, 1324.44, 748.15, 692.76. HRMS-ESI: [M-H]+ calcd for C12H6N4SO2Br:
348.9395, found: 348.9398.
2.10. 7-(9-Octyl-9H-carbazol-3-yl)-N-(4-nitrophenyl)-benzo[c] [1, 2, 5] thiadiazol-4-
amine (CBT-NH)
Compound 3a (0.405 g, 1.000 mmol) and Compound 3c (0.345 g, 1.000 mmol)
were dissolved in the mixed solution of 2M K2CO3 (10 mL) and THF (20 mL). Then
pd(pph3)4 (0.060 mg, 0.050mmol) was added under the argon atmosphere and the
mixture was stirred for 24 h at 80 oC. Upon cooling, the mixture was poured into
water and extracted with CH2Cl2 (20 mL×3). The organic phases were combined and
dried over Na2SO4. After the solvent was removed, the crude product was purified by
column chromatography (petroleum ether/ ethyl acetate=4:1 v/v). A red solid was
obtained (0.425 g, 77.3%). Mp: 138-140 oC. 1H NMR (DMSO-d6, 400 MHz, TMS)
δ:
9.90 (s, 1H), 8.77 (s, 1H), 8.23 (d, J=8.0 Hz, 1H), 8.18 (d, J=9.2 Hz, 2H), 8.11 (d,
J=8.4 Hz, 1H), 7.95 (d, J=8.0 Hz, 1H), 7.76 (d, J=8.0 Hz, 1H), 7.72 (d, J=8.4 Hz,
1H), 7.65 (d, J=8.0 Hz, 1H), 7.49 (t, J=7.2 Hz, 1H), 7.38 (d, J=9.2 Hz, 2H), 7.24 (t,
J=7.2 Hz, 1H), 4.46 (t, J=7.2 Hz, 2H), 1.40-1.10 (m, 12H), 0.82 (t, J=7.2 Hz, 3H). 13C
NMR (CDCl3, 100 MHz, TMS) δ: 153.2, 148.3, 146.4, 140.0, 139.9, 139.2, 129.8,
127.7, 127.2, 126.9, 125.6, 125.0, 124.9, 122.2, 121.9, 119.7, 119.5, 118.0, 115.1,
110.2, 107.9, 107.7, 42.2, 30.8, 28.4, 28.2, 28.0, 26.3, 21.6, 13.1. IR(KBrꢂv(cm-1)):
3401.98, 2923.07, 2851.57, 1592.87, 1526.59, 1486.99, 1319.21, 1107.45, 746.21.
HRMS-ESI: [M+H]+ calcd for C32H32N5O2S: 550.2277, found: 550.2290.
8

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2.11.7-(10-Octyl-10H-phenothiazin-3-yl)-N-(4-nitrophenyl)-benzo[c] [1, 2, 5]
thiadiazol-4-amine (PBT-NH)
The compound was synthesized by the same procedure described for CBT-NH
using compound 3b and compound 3c. Yield: 67.5%. Mp:143-145oC. 1H NMR
(DMSO-d6, 400 MHz, TMS) δ: 9.89 (s, 1H), 8.16 (d, J=9.2 Hz), 7.82-7.87 (m, 3H),
7.63 (d, J=7.6 Hz, 1H), 7.38 (d, J=9.2 Hz, 2H), 7.15-7.24 (m, 3H), 7.07 (d, J=7.6 Hz,
1H), 6.97 (t, J=8.0 Hz, 1H), 3.94 (t, J=7.6 Hz, 3H), 1.40-1.10 (m, 12H), 0.83 (t, J=7.6
Hz,3H). 13C NMR (CDCl3, 100 MHz, TMS)
δ: 152.6, 148.2, 146.1, 144.0, 143.8,
140.3, 130.3, 130.2, 126.7, 126.6, 126.4, 126.29, 126.2, 125.2, 125.0, 123.9, 123.2,
121.5, 115.4, 114.3, 114.2, 109.6, 46.5, 30.7, 28.7, 28.2, 26.0, 25.9, 21.6, 13.1.
IR(KBrꢂv(cm-1)): 3426.23, 2920.24, 2850.67, 1600.24, 1559.46, 1458.91, 1323.92,
1260.70, 1110.39, 800.94, 750.35. HRMS-ESI: [M-H]+calcd for C32H30N5O2S2:
580.1841, found: 580.1852.
2.12. N-ethyl-N-(4-nitrophenyl)-7-(9-octyl-9H-carbazol-3-yl)benzo[c] [1, 2, 5]
thiadiazole-4-amine (CBT-NE)
CBT-NH (0.055 g, 0.100 mmol) was dissolved in DMSO (20mL), then added
tetrabutylammonium fluoride (1M) (1mL, 1mmol) and CH3CH2I (0.032mL,
0.400 mmol). The mixture was stirred for 5h at room temperature. After that the
mixture was poured into water and extracted with CH2Cl2 (20 mL×3). The organic
phases were combined and dried over Na2SO4. After the solvent was removed, the
crude product was purified by column chromatography (petroleum ether/ ethyl
acetate=6:1 v/v). A red solid was obtained (0.0460 g , 82.6%). Mp: 147-149 oC.
9

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1H NMR (DMSO-d6, 400 MHz, TMS)
δ
: 8.82 (s, 1H), 8.24 (d, J=7.6 Hz, 1H), 8.15
(d, J=8.4 Hz, 1H), 8.05-8.02 (m, 3H), 7.84 (d, J=7.6 Hz, 1H), 7.78 (d, J=8.4 Hz, 1H),
7.66 (d, J=8.4 Hz, 1H), 7.50 (t, J=7.2 Hz, 1H), 7.25 (t, J=7.2 Hz, 1H), 6.80 (d, J=9.6
Hz, 2H), 4.46 (t, J=6.8 Hz, 2H), 4.06 (q, J=6.8 Hz, 2H), 1.35-1.15 (m, 18H), 0.82 (t,
J=7.2 Hz, 3H). 13C NMR (DMSO-d6, 100 MHz, TMS)
δ: 154.4, 153.0, 152.0, 140.5,
140.0, 137.4, 134.1, 133.6, 128.8, 127.9, 127.3, 127.1, 125.7, 122.3, 122.2, 121.2,
120.5, 119.0, 113.2, 109.5, 109.4, 56.0, 46.9, 31.2, 28.7, 28.6, 28.5, 26.5, 22.0, 13.9,
12.6. IR(KBrꢂv(cm-1)): 3105.76, 2923.45, 2860.72, 1602.37, 1560.97, 1503.43,
1302.08, 738.87. HRMS-ESI: [M+H]+ calcd for C34H36N5O2S: 578.2590, found:
578.2586.
2.13. N-ethyl-N-(4-nitrophenyl)-7-(10-octyl-10H-phenothiazin-3-yl) benzo[c] [1, 2, 5]
thiadiazol-4-amine (PBT-NE)
The compound was synthesied by the same procedure described for CBT-NE using
PBT-NH, tetrabutylammonium fluoride and CH3CH2I. A red solid was obtained
Yield: 82.6%. Mp: 150-151oC.1H NMR (DMSO-d6, 400 MHz, TMS)
δ: 8.01 (d,
J=9.2 Hz, 2H), 7.94-7.85 (m, 3H), 7.76 (d, J=7.6 Hz, 1H), 7.23 (t, J=8.0 Hz, 1H),
7.18 (d, J=7.6 Hz, 2H), 7.06 (d, J=8.4 Hz, 1H), 6.97 (t, J=8.0 Hz, 1H), 6.77 (d, J=9.6
Hz, 2H), 4.03 (q, J=6.8 Hz, 2H), 3.94 (t, J=6.8 Hz, 2H), 1.35-1.15 (m, 18H), 8.02 (t,
J=6.8 Hz, 3H). 13C NMR (DMSO-d6, 100 MHz, TMS)
δ: 153.9, 152.8, 151.8, 144.9,
144.2, 137.4, 134.7, 130.9, 130.6, 128.4, 127.7, 127.5, 127.3, 127.2, 125.6, 123.5,
123.0, 122.7, 115.9, 115.7, 113.4, 57.5, 46.9, 31.1, 28.6, 26.1, 23.0, 22.0, 19.2, 13.9,
13.5. IR(KBrꢂv(cm-1)): 3165.23, 2918.56, 2853.27, 1604.81, 1557.49, 1476.91,
10

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1323.92, 1076.92, 807.39, 757.98. HRMS-ESI: [M+H]+ calcd for C34H36N5O2S2:
610.2310, found: 610.2306.
3. Results and discussion
3.1. Synthesis
CBT-NH and PBT-NH were synthesized as shown in Scheme 2. CBT-NE
and PBT-NE, which were
N-ethyl analogue of the compounds (CBT-NH and
PBT-NH), were obtained through the reactions between compounds (CBT-NH
or PBT-NH) and tetrabutylammonium fluoride (10 equiv) in DMSO solution,
followed by the addition of CH3CH2I (4 equiv). Their structures were confirmed
by 1H and 13C NMR and HRMS spectra. (Supporting Information (SI), Fig. S3-S16)
Insert Scheme 2 here
3.2. Optical properties
The interactions between the compounds (CBT-NH or PBT-NH) and various
-
-
-
anions (F-, Cl-, Br-, I-, AcO-, H2PO4 , HSO4 and NO3 ) were investigated in DMSO
solution through colorimetric analysis. Noticeable and appreciable color changes
were observed within 5s when the compounds were treated with F-, the system
changed from red to dark green, whereas the other anions such as Cl-, Br-, I-, AcO-,
-
-
-
H2PO4 , HSO4 and NO3 did not show any visible color change (Fig. 1). Besides, there
were obvious differences in their fluorescent spectra. (Fig. 2)
Insert Fig.1 here
Insert Fig.2 here
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In order to deduce the anion sensing ability of the compounds (CBT-NH and
PBT-NH), titrations were carried out in DMSO solution and were monitored by
UV-visible spectroscopy. The experiment was performed by preparing a solution
(5×10-5 mol/L) of the sensors in DMSO and followed by the addition of
tetrabutylammonium fluoride solution (0.1 mol/L). The spectral profile was shown in
Fig. 3. CBT-NH and PBT-NH showed absorption peaks at 400 nm (CBT-NH) and
400 nm (PBT-NH), while a second absorption appeared at 470nm and 480 nm,
respectively. The molar extinction coefficient of CBT-NH(470nm) and
PBT-NH(480nm) were 1.69 × 104L/(mol·cm) and 1.47 × 104L/(mol·cm) by
calculations. Excess addition of fluoride resulted in the formation of a new peak at
628 nm (CBT-NH) and 640 nm (PBT-NH) (Fig. 3). Meanwhile, Compounds
(CBT-NH and PBT-NH) could also acted as fluorimetric sensors as demonstrated by
the fluorescence titration experiments in DMSO (Fig. 4). CBT-NH exhibited
emission at 626 nm by exciting the solution at 400 nm. The gradual addition of F-
quenched the fluorescence at 626nm, and enhanced two new bands at 460 nm and 525
nm. This phenomenon may be due to the formed hydrogen–bond interaction and the
newly formed deprotonated species. However, as shown in Fig. 4b, the PBT-NH only
experienced an enhancement of a band at 505 nm, the result may be attributed to the
influence of different electron donating abilities between carbazole and phenothiazine.
Insert Fig.3 here
Insert Fig.4 here
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Continuous variation methods were used to determine the stoichiometric ratio of
the compounds (CBT-NH and PBT-NH) to the fluoride ion guest (Fig. 5). A Job’s
plot of the CBT-NH and PBT-NH with fluoride ion in DMSO solution showed the
maxima at a molar fraction of 1. This phenomenon indicated that the CBT-NH and
PBT-NH bound with the fluoride ion in a 1:1 ratio during the initial stages of the
reaction.
Insert Fig.5 here
The binding stoichiometries of CBT-NH and PBT-NH were calculated according
to the Benesi-Hildebrand Equation (Fig. S2):
Where A0 is the absorbance of free compound, A is the absorbance with a specific
fluoride concentration, A∞ is the absorbance with excess amount of F-, K is the
association constant (M-1), and [F-]0 is the added concentration of F- (M). The plot of
1/(A-A0) against 1/[F-]0 showed a linear relationship(R=0.99), indicating that the
compounds associated with fluoride in a 1:1 stoichimetry. The association constant, K,
between CBT-NH (or PBT-NH) and F- was determined from the ratio of
intercept/slope to be 1.422×105 and 1.326×105, respectively, from which it is inferred
that CBT-NH possessed more affinity toward fluoride ion.
3.3. Theoretical calculations
In order to understand the difference between CBT-NH and PBT-NH, theoretical
calculations were carried out by using the Gaussian 09 package at the B3LYP level of
theory (Table 1 and Table 2). The calculated results revealed that the highest occupied
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molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of
CBT-NH
were
located
at
the
carbazole-benzothiadiazole
and
(4-nitrophenyl)-benzothiadiazole-4-amine units, respectively. The absorptions at
400nm and 470 nm contributed to the charge transfer transition from carbazole →
benzothiadiazole and carbazole → 4-nitroaniline groups, respectively. When
CBT-NH+F- was generated after adding fluoride ion, the HOMO and LUMO were
mainly located at the (4-nitrophenyl)benzothiadiazole-4-amine units. The charge
transfer transition from (4-nitrophenyl)-benzothiadiazol-4-amine→ alkyl chain could
be used to explain the new absorption at 628 nm. The behavior of PBT-NH was
similar to that of CBT-NH. However, the carbazole and phenothiazine moieties
influenced the electron distribution of other groups of the molecules to a large
degree.
Insert Table 1 here
In this case, as shown in Table 2, (CBT-NH+F-) was less stable than CBT-NH.
Nevertheless, (PBT-NH+F-) was more stable than PBT-NH. Thus, the CBT-NH
system tended to exist as hydrogen-bond form and the PBT-NH system was inclined
to deprotonation state.
Insert Table 2 here
3.4. 1H NMR titration
1
To further understand the interaction between the fluoride ion and receptor, H
NMR titration experiments were carried out in DMSO-d6 solution. As shown in Fig.
6a, upon addition of 1 equiv of fluoride ion, the NH peak showed severe broadening,
this result suggested that hydrogen-bond interactions were existed between target
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compounds and TBAF at low fluoride ion concentration. Upon addition of 2 equiv of
fluoride, the resonance signal corresponding to the NH proton disappeared.
Meanwhile the chemical shift values of aromatic protons continued to shift upfield
until the addition of 6 equiv of fluoride ion. These results suggested that the
compounds would be deprotonated when the fluoride ion concentration turned higher.
The behaviour of PBT-NH was similar to that of CBT-NH (As shown in Fig. 6b), the
chemical shifted values of aromatic protons shifted slightly, but their peak shapes
1
changed significantly. Consequently, the result of H NMR titration provided a
supporting proof for the proposed interaction mode between the compounds
(CBT-NH or PBT-NH) and the incoming fluoride ion.
Insert Fig.6 here
3.5. Detection mechanism
1
Both H NMR titration spectra (Fig. 6) and UV-vis absorption spectra (Fig. 3)
showed that Hydrogen-bond interactions were existed between target compounds and
TBAF at low fluoride ion concentration. Nevertheless, when the concentration of
fluoride ion turned higher, the compounds would be deprotonated (Fig. 7). To further
confirm the deprotonation mechanism, tetrabutylammonium hydroxide was used as a
1
contrast experiment. And the same H NMR spectra were obtained when enough
tetrabutylammonium fluoride and tetrabutylammonium hydroxide were added. On the
other hand, when CH3CH2I was added into the mixed system of compounds and
tetrabutylammonium fluoride, CBT(PBT)-NE was obtained, which verified the
deprotonation mechanism again. (Fig. 8)
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Insert Fig.7 here
Insert Fig.8 here
The large Stokes shifts for CBT-NH (226 nm) and PBT-NH (105 nm) (Fig. 4)
suggested appreciable structural changes between their ground and excited states.
Those parameters suggested the possibility of intramolecular proton transfer processes
1
from the Franck-Condon state. This interpretation was further supported by the H
NMR data (Fig. 6), which indicated the intramolecular hydrogen bonding between the
amino-hydrogen and the imine nitrogen in the benzothiadiazole moiety. It formed a
strain-free, five-membered ring by the hydrogen bond, and it is an ideal factor for
proton transfer processes.[35-37]
As shown in Fig. 9, the compounds (CBT-NH or PBT-NH) may exhibit an excited
state intramolecular transfer (ESIPT) process [25, 32], a similar phenomenon also has
been reported by L. Wang [25] and Y. Pang [32]. When tetrabutylammonium fluoride
was added to the system, the hydrogen bond would be destroyed by fluoride anion,
and N-H proton would be captured by fluoride anion, this result could be proved by
the addition of iodoethane. When iodoethane was added to the system, the color of
system changed from dark green to red again (Fig. 8, Fig. S1).
Insert Fig. 9 here
4. Conclusions
In summary, we have synthesized two new fluorescent fluoride ion selective
sensors (CBT-NH and PBT-NH) via Suzuki cross-coupling reaction. Those two
compounds possess high sensitivity and selectivity for fluoride ion. They can detect
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fluoride ion quickly and the response can be observed directly by eye. We believe the
characteristics of a fast response, colour generation and simple observation by eye for
the systems are very important in the sensing of fluoride ion.
Acknowledgments
The authors are grateful to financial support from the National Natural Science
Foundation of China (No.20872035, 21576087), Hangzhou Normal University, and
the East China University of Science and Technology.
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Scheme captions:
Scheme 1. Molecule structures of compounds (CBT-NH, PBT-NH, CBT-NE and
PBT-NE).
Scheme 2. Synthesis routes of compounds (CBT-NH and PBT-NH).
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Scheme 1 Molecule structures of compounds (CBT-NH, PBT-NH, CBT-NE and PBT-NE).
Scheme 2 Synthesis routes of compounds (CBT-NH and PBT-NH).
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Table captions:
Table 1. Calculated surfaces of HOMO-1, HOMO, LUMO, and LUMO+1(CBT-NH,
CBT-NH+F-, PBT-NH, PBT-NH+F-).
Table 2. Calculated energy of HOMO-1, HOMO, LUMO, and LUMO+1 (CBT-NH,
CBT-NH+F-, PBT-NH, PBT-NH+F-).
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Table 1 Calculated surfaces of HOMO-1, HOMO, LUMO, and LUMO+1(CBT-NH,
CBT-NH+F-, PBT-NH, PBT-NH+F-).
LUMO-1
HOMO
LUMO
LUMO+1
CBT-NH
CBT-NH+F-
PBT-NH
PBT-NH+F-
Table 2 Calculated energy of HOMO-1, HOMO, LUMO, and LUMO+1 (CBT-NH, CBT-NH+F-,
PBT-NH, PBT-NH+F-).
HOMO-1(eV)
HOMO(eV)
LUMO(eV)
LUMO+1(eV)
HOMO-LUMO(
eV)
HOMO-(LUM
O+1)(eV)
CBT-NH
-6.055
-3.197
-6.001
-3.184
-5.495
-1.996
-5.170
-2.090
-2.946
0.194
-3.010
0.091
-2.519
0.661
-2.561
0.699
2.549
2.190
2.160
2.181
2.976
2.657
2.609
2.789
CBT-NH+F-
PBT-NH
PBT-NH+F-
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Figure captions:
Fig. 1. Visible color change spectra of CBT-NH(a) and PBT-NH(b) in the prescence
of 10 equiv of various anions (as their TBA salts)(0.1M) in DMSO (5×10-5 M).
Fig. 2. The fluorescent spectra of CBT-NH(a) and PBT-NH(b) in the prescence of 10
equiv of various anions (as their TBA salts)(0.1M) in DMSO(5×10-5 M).
Fig. 3. UV-vis absorption spectra of CBT-NH (a) and PBT-NH (b) in the presence of
TBAF(0.1 M) in DMSO (5×10-5 M).
Fig. 4. Fluorescence spectra of CBT-NH (a) (excited: 400 nm) and PBT-NH (b)
(excited: 400 nm) in the presence of TBAF (0.1 M) in DMSO (5×10-5 M).
Fig. 5. Job’s plot for complexation of CBT-NH(a)(505nm) and PBT-NH(b)(525nm)
with F- determined by fluorescent emission in DMSO, [Compounds]+[anion] =5×10-5
mol/L at 298K.
Fig. 6. 1H NMR titration spectra of CBT-NH (a) and PBT-NH (b) in DMSO-d6 in the
presence of various equivalents of TBAF.
Fig. 7. The interaction between compounds (CBT-NH and PBT-NH) and fluoride
ion.
Fig. 8 (a)The color changes and UV-vis absorption spectra (CBT-NH, CBT-NH+F-
and CBT-NE) under natural light; (b)The color changes and UV-vis absorption
spectra (PBT-NH, PBT-NH+F- and PBT-NE) under natural light (CBT-NH or
PBT-NH: 5×10-5 M in DMSO; CBT-NH+F- or PBT-NH+F-: CBT-NH (or
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PBT-NH)+ 10 equiv of F- ; CBT-NE or PBT-NE: CBT-NH (or PBT-NH) + 10
equiv of F- + 4 equiv of CH3CH2I).
Fig. 9 the interaction between the compounds (CBT-NH or PBT-NH) and
tetrabutylammonium fluoride or iodoethane.
28