Selective and colorimetric fluoride anion chemosensor based on s-tetrazines

Article abstract of DOI:10.1039/c2dt31641b

s-Tetrazine and their derivatives are found to be a class of selective and colorimetric chemosensors for fluoride anions. The supramolecular interaction (anion-π and charge/electron transfer) between fluoride ion and π-electron deficient tetrazines receptor could facilitate the F^--tetrazines electron transfer (ET) event that generates tetrazine^- radical anion. Accordingly, the color of its solution changed from red to green. Furthermore, the recognition of F^- in DMSO is tolerant to water (DMSO-H ₂O = 9:1, v/v) and shows excellent selectivity towards other anions, especially H₂PO₄^- and OAc^-.

Full text of DOI:10.1039/c2dt31641b

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PAPER
Selective and colorimetric fluoride anion chemosensor based on s-tetrazines†
Yingjie Zhao,a Yongjun Li,*a Zhihong Qin,a,b Runsheng Jiang,a,b Huibiao Liua and Yuliang Lia
Received 23rd July 2012, Accepted 3rd September 2012
DOI: 10.1039/c2dt31641b
s-Tetrazine and their derivatives are found to be a class of selective and colorimetric chemosensors for
fluoride anions. The supramolecular interaction (anion–π and charge/electron transfer) between fluoride
ion and π-electron deficient tetrazines receptor could facilitate the F−–tetrazines electron transfer (ET)
event that generates tetrazine˙− radical anion. Accordingly, the color of its solution changed from red to
green. Furthermore, the recognition of F− in DMSO is tolerant to water (DMSO–H2O = 9 : 1, v/v) and
shows excellent selectivity towards other anions, especially H2PO4− and OAc−.
Introduction
The construction of selective chemosensors for cations has
grown very well,1 while the design of new types of anion recep-
tors are receiving increasing attention.2 Fluoride anion plays an
important diverse role from being a useful additive in the human
diet and also in industrial processes.3a–d It is also a dangerous
pollutant.3e Therefore, the sensing and recognition of fluoride
Fig. 1 Schematic illustration of the sensing mechanism of s-tetrazines
towards fluoride anion.
anion has grown into an area of great interest in supramolecular
and biological chemistry in recent years.4 Fluoride displays a
anion–π interaction would be more favorable.11 For this reason,
high hydration energy compared to other anions, which makes it
we envisioned to develop a fluoride sensor based on s-tetrazines.
less easily distinguishable from water even in aqueous mixtures
In addition, s-tetrazines generally have a deep color ranging
(organic solvent–water mixtures).5 Most of the fluoride anion
from purple to orange to red because of a weak n–π* transition
receptors commit the binding to H-bonding interactions.6
located in the visible region.9a This is an advantage in accom-
However, this interaction is nonchromogenic which means it
plishing the visible naked-eye detection.
cannot display a colorimetric response.7 Deprotonation is an
Herein we report a visible fluoride anion sensor based on
effective way to accomplish naked-eye recognition. But it can
s-tetrazines and amide. The amide can locate the fluoride anion
not differentiate F− from other strongly basic anions, such as
nearby the s-tetrazines through hydrogen-bond interaction. The
AcO− and H2PO4− 4a Recently, Saha and co-workers reported an
.
anion–π interaction between the fluoride anion and the s-tetra-
zines can induce the electron transfer from fluoride anion to the
s-tetrazines and generate an anion radical of s-tetrazine (Fig. 1).
A visible color change from red to green was observed. More-
over, this sensor has excellent selectivity towards fluoride anion
even in an aqueous environment.
example of a F− colorimetric sensor involving π-electron
deficient colorless NDI receptors.8
s-Tetrazines show some rare and useful characteristics that
make them usable for several types of applications in various
domains.9 But their applications in supramolecular chemistry are
still rare. Because of their electron-deficient character, the s-tetra-
zines can accept one electron to give a stable anion radical.10
Recently, some computational results showed that the s-tetrazines
can be used as binding units for the molecular recognition of
anions.11 Especially, the complexation with fluoride through the
Results and discussion
Synthesis
Compound 3 was obtained in three steps (Scheme 1) in 45% overall
yield. The ring closure occurs with 52% yield in high dilution con-
ditions in the presence of Et3N. Disubstitution with the sulfur could
be easily accomplished under mild conditions.12 However, the pres-
ence of sulfur makes the tetrazines non-fluorescent.9a
aBeijing National Laboratory for Molecular Sciences (BNLMS), CAS
Key Laboratory of Organic Solids, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, P. R. China.
E-mail: liyj@iccas.ac.cn; Fax: (+)86-10-82616576;
Tel: (+)86-10-62587552
Energy minimized structures of 3·F− complex
bGraduate University of Chinese Academy of Sciences, Beijing 100190,
P. R. China
†Electronic supplementary information (ESI) available: 1H NMR and
13C NMR of compound 2 and 3. See DOI: 10.1039/c2dt31641b
DFT modeling (B3LYP/6-31G*) of the complexes formed
between 3 and the halides was shown in Fig. 2. The interaction
13338 | Dalton Trans., 2012, 41, 13338–13342
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Scheme 1 Synthesis of compound 3.
Fig. 4 ESR spectra of 3 and 3 in the presence of 5 equiv. F− in DMSO
(2.0 × 10−4 M).
Fig. 2 Model (B3LYP/6-31G*) of the structures for halide complexa-
tion with 3.
Fig. 5 1H NMR titration of 3 with F− (DMSO-d6, 298 K).
instantaneous color change from red to green. The intensity of
the absorption maximum of 3 at 525 nm decreased following the
formation of a new band centered at 572 nm. The band at
427 nm increased obviously accompanied with a 20 nm blue
shift. All these changes could be attributed to the formation of
tetrazine radical anion.10a,13 Another strong evidence for the for-
mation of the radicals 3˙− comes from the diagnostic EPR signal
which is unique to the tetrazine radical anion formed in the pres-
ence of fluoride (Fig. 4). The spin is virtually localized at the
four nitrogen centers and the four tetrazine 14N nuclei display a
typical widely spaced nonet with ca. 5.0 G hyperfine split-
ting.10e,14 During the titration, amide proton c down-field shifted
to 9.27 ppm, proton a was split into two peaks at 8.75 ppm and
8.48 ppm, proton b showed a similar trend, the proton signal of
3 gradually disappeared after more than 1 equiv. of fluoride was
added (Fig. 5), indirectly indicating the formation of the 3˙−
radical anion.8 All the results support our hypothesis that tetra-
zines–F− interactions facilitate the F−–tetrazines ET event that
generates the 3˙− radical anion.
Fig. 3 Changes in the UV-vis spectra of 3 (2 × 10−4 M in DMSO)
upon titration with n-Bu4NF from 1 to 10 equiv.
of the tetrazine and anions greatly depended on the volume of
the anions. As shown in Fig. 2, there was strong hydrogen-bond
interaction between the F− and the amide N–H. The fluoride
anion could lie in the cavity which was formed from amide N–H
and the tetrazine. F− located just above the tetrazine. The size of
Cl−, Br− and I− are too big to be inside the cavity and interact
with the tetrazine. According to the modeling of 3·F− complex,
the interaction of the F− and tetrazine is expected.
UV-vis spectral titration and mechanism study
Fig. 3 showed the absorption spectral changes of 3 with the
increase of the F− concentration in DMSO at room temperature.
The UV-vis spectrum of 3 showed two peaks at 427 nm and
525 nm. The band centered at 427 nm can be attributed to a
π–π* transition while the one at 525 nm corresponds to an n–π*
transition.9a,12 Upon addition of F−, compound 3 showed an
Selectivity study
An important feature of a new chemosensor is its high selectiv-
ity. To investigate the selectivity of 3 toward F−, other
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Fig. 7 Changes in the UV-vis spectra of 3 (2 × 10−4 M in DMSO)
upon addition of F− and subsequent excess of NOBF4.
and a smaller ionic radius (1.33 Å) compared to other anions.15
This allows F− to be captured into the cavity easily and get close
to the tetrazine π-surface and engage in efficient anion–π and
charge transfer (CT) interactions through better orbital inter-
actions with tetrazine.16 These interactions facilitate an electron
transfer from F− to the tetrazine π-system and generate the green
tetrazine˙− radical anion. Other anions, because of the larger size
and orbital mismatch, can not induce the tetrazine π-system to
generate radical anions.
Reversibility study
The 3–F− interaction is reversible by NOBF4 oxidation of the
3˙− radical anion to regenerate 3. As shown in Fig. 7, the orig-
inal UV-vis spectra reappeared, showing the maximum absorp-
tion band at 525 nm and 427 nm. And the color of the solution
changed from green to red. The result indicated that the 3–F−
interaction is noncovalent interaction, which also supports our
hypothesis that tetrazines–F− interactions induce the tetrazine
π-system to generate a radical anion.
Fig. 6 (a) Absorbance spectra change of 3 (2 × 10−4 M in DMSO)
upon addition of different anions (10 equiv.). (b) Absorbance spectra
change of 3 (2 × 10−4 M in DMSO) upon addition of the mixed anions
(mix = Cl−, Br−, I−, N3−, PF6−, AcO− and H2PO4−); each one is 10
equiv. of anions and subsequent addition of 30 equiv. of F− in DMSO.
Detection in aqueous media
As we know, fluoride is strongly solvated in water. Its hydration
free energy is about 100–110 kcal mol−1 17
All these factors
.
(c) Color change of 3 in the presence of different anions. From left to
make the detection of fluoride in aqueous media much more
difficult. Indeed, there are very few receptors which can achieve
anion binding in water developed so far. With respect to the pro-
posed mechanism we presumed that it could also be possible to
determine the fluoride content in aqueous solutions. Therefore,
we examined the detection of F− in the media containing water.
Fig. 8 showed that the sensing system tolerated up to 10% water
in the solvent. With the addition of F−, two isoabsorptive points
at 453 nm and 556 nm were observed. UV-vis spectra change is
similar to that with DMSO.
right: blank, F−, Cl−, Br−, I−, N3−, PF6−, AcO− and H2PO4
.
−
competitive anions such as Cl−, Br−, I−, N3−, PF6−, AcO− and
−
H2PO4 were chosen. As shown in Fig. 6a, under identical con-
ditions, no significant changes were observed in the UV-vis
spectra of 3 upon addition of the anions above, respectively.
Therefore, compound 3 showed good selectivity for fluoride in
DMSO, and the detection limit was obtained from three times
the standard deviation of the blank signal (3s) as 6.7 μM. Even
in the presence of 10 equiv. mixed anions, titration with F− also
caused significant changes in the UV-vis spectra (Fig. 6b). In
addition, the naked-eye detection of F− is also accomplished.
Fig. 6c showed the color change of 3 in the presence of different
anions. When F− was introduced, the color changed from red to
green. Other anions did not induce any significant color change
of 3. The unique F− induced radical anion generation process
renders the sensor excellent selectivity. The F− has a 2p orbital
Conclusion
The tetrazine derivative was first exploited for sensing F− ions.
We have synthesized a macrocyclic anion receptor with a tetra-
zine unit. The cavity is just suitable for F−. The supramolecular
tetrazine–F− (anion–π and CT) interactions promote an electron
transfer process from the F− ion to electron-deficient tetrazine
13340 | Dalton Trans., 2012, 41, 13338–13342
This journal is © The Royal Society of Chemistry 2012

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analysis calcd (%) for C14H13N7O4S2: C 41.27, H 3.22,
N 24.06; found: C 41.19, H 3.28, N 24.01.
Acknowledgements
We are grateful for financial support from the National Nature
Science Foundation of China (21031006, 90922017 and
20971127); the National Basic Research 973 Program of China
(2011CB932302 and 2012CB932900) and NSFC-DFG joint
fund (TRR 61).
Notes and references
Fig. 8 Changes in the UV-vis spectra of 3 (2 × 10−4 M) upon titration
by n-Bu4NF from 5 to 20 equiv. in a DMSO–water solution (9 : 1, v/v).
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and generates the tetrazine˙− radical anion accompanied with the
color changes from red to green. A class of selective and colori-
metric chemosensors for fluoride anion was developed based on
this compound. This sensor showed excellent selectivity and can
be used in aqueous mixtures. The applications of tetrazines as
supramolecular and functional organic materials were developed.
Experimental
Synthesis of 2
A solution of 5-nitroisophthaloyl dichloride 1 (2.5 g, 10 mmol)
and Et3N (2.5 g, 25 mmol) in CH2Cl2 (50 mL) was cooled to
0 °C. 2.9 g 2-Aminoethanethiol hydrochloride (25 mmol) in
20 mL CH2Cl2 was added dropwise and the solution was stirred
at room temperature (R.T.) for 5 h. The solution was concen-
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(50 mL × 3) and dried in vacuum to yield compound 2 (3.0 g,
1
yield = 90%). H NMR (600 MHz, DMSO-d6) δ 9.14 (s, 2H),
8.81 (s, 2H), 8.77 (s, 1H), 3.48 (dd, J = 13.0, 6.5 Hz, 4H), 2.70
(dd, J = 14.6, 7.6 Hz, 4H); 13C NMR (150 MHz, DMSO-d6) δ
164.2, 148.3, 136.5, 132.8, 124.7, 43.5, 23.6; MS (EI): m/z
Calcd for C12H15N3O4S2 (M) 329.05; Found 329 (M); elemental
analysis calcd (%) for C12H15N3O4S2: C 43.76, H 4.59, N
12.76; found: C 43.70, H 4.66, N 12.85.
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Synthesis of 3
In a 500 mL three-necked round bottom flask, 300 mL of aceto-
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(2.5 mmol) of triethylamine dissolved in 30 mL acetonitrile and
300 mg (2 mmol) of dichloro-s-tetrazine dissolved in 30 mL of
acetonitrile were added dropwise at the same speed through two
separate addition funnels. After completion of the addition, the
solution was evaporated and the residue was purified by column
chromatography (CH2Cl2–CH3OH = 50 : 1 as eluent) to yield 3
as a red solid (215 mg, yield = 52%). 1H NMR (400 MHz,
DMSO-d6) δ 8.43 (s, 2H), 8.33 (s, 2H), 7.69 (s, 1H), 3.85 (s,
4H), 3.73 (s, 4H); 13C NMR (150 MHz, DMSO-d6) δ 173.2,
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Calcd for C14H13N7O4S2 (M) 407.05; Found 407 (M); elemental
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This journal is © The Royal Society of Chemistry 2012