A phenolic Schiff base for highly selective sensing of fluoride and cyanide via different channels

Article abstract of DOI:10.1016/j.tet.2011.10.105

A chemosensor based on phenolic Schiff base bearing a pyrene group (sensor 1) has been synthesized and demonstrated. Sensor 1 showed a highly selective colorimetric response to fluoride anions based on a deprotonation process and fluorescent response to c

Full text of DOI:10.1016/j.tet.2011.10.105

Tetrahedron 68 (2012) 636e641
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Tetrahedron
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A phenolic Schiff base for highly selective sensing of fluoride and cyanide via
different channels
Libin Zang, Dayan Wei, Shichao Wang, Shimei Jiang *
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699 Qianjin Avenue, Changchun 130012, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 July 2011
Received in revised form 25 October 2011
Accepted 31 October 2011
Available online 4 November 2011
A chemosensor based on phenolic Schiff base bearing a pyrene group (sensor 1) has been synthesized
and demonstrated. Sensor 1 showed a highly selective colorimetric response to fluoride anions based on
a deprotonation process and fluorescent response to cyanide anions (606-fold fluorescence quantum
yield enhancement) based on a cyclization process. Moreover, the cyclization of phenolic Schiff bases
induced by cyanide could be used as a new way to synthesize 2-substituted benzoxazoles.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Colorimetric
Fluorescence
Cyanide
Fluoride
Sensors
Benzoxazole
Schiff base
1. Introduction
decade. As the most electronegative atom, fluoride can form strong
hydrogen bonds. Thus, various chemosensors containing acidic NH
7
The design of selective molecular sensors for anions is an area of
growing interest due to their important role in a wide range of
environmental, industrial, and biological applications. Optical
and OH groups have been developed to detect fluoride. Mean-
while, the detection of cyanide has been achieved through various
approaches such as hydrogen bonding, chemodosimeters, and
1
8
9
10
sensors for anions, in which a change in color or fluorescence in-
displacement approaches.
tensity is monitored, have been researched extensively for their
simple and convenient implementation. Moreover, single molec-
In recent years, various concepts in anion sensing have been
developed to enhance the sensitivity, selectivity, and dynamic
2
11
ular optical sensors for multiple analytes have become a new re-
search focus because it could allow for simultaneous analysis of
multiple analytes, which could overcome the difficulties encoun-
working range. Several anion sensors, which could detect and
distinguish more than one anion species have been reported, but
12
most of them can only work in the same channel. Shao et al. re-
ported a simple colorimetric sensor, which displayed highly se-
lective coloration either for fluoride in aprotic solvent or for
3
tered with loading multiple indicators. It may also have potential
4
applications in the development of molecular logic gates.
13
Among various anions, fluoride and cyanide have been in-
tensively studied owing to their important roles in our life. Al-
though fluoride is a biologically important anion due to its
established role in dental care and treatment of osteoporosis, high
bisulfate in an aqueous medium. Ghosh et al. reported a series of
sensors, which could distinguish fluoride and acetate in a colori-
14
metric channel. Similarly, Peng et al. reported an ESIPT-based
fluorescence sensor with a good distinguishability for fluoride
5
7c
doses of fluoride are hazardous. On the other hand, cyanide serves
and acetate. However, analytes tend to interfere with each other
as an important raw material in various industrial processes,
however it is extremely toxic to living organisms. Therefore,
considerable effort has been devoted to the development of novel
methods for the detection of fluoride and cyanide in the past
when responding to single molecular sensors in the same channel.
Ideally, a single molecule, which can sense more than one species in
different signaling channels could efficiently eliminate this in-
terference. To the best of our knowledge, a single molecular
sensor that can selectively detect and distinguish more than one
anionic species in different channels has rarely been reported.
In this paper, we have developed a single molecular anion
sensor 1 based on a phenolic Schiff base bearing a pyrene group
6
15
*
Corresponding author. Tel.: þ86 431 85168474; fax: þ86 431 85193421; e-mail
address: smjiang@jlu.edu.cn (S. Jiang).
0
040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.10.105

L. Zang et al. / Tetrahedron 68 (2012) 636e641
637
(Scheme 1). This sensor can selectively detect fluoride and cyanide
changes could also be observed visually. The pale yellow solution of
via colorimetric and fluorescent channels, respectively. With ap-
propriate analysis, it is possible to detect fluoride and cyanide an-
ions separately in the same sample. Further studies suggest that
there are two different mechanisms for the detections of fluoride
and cyanide anions.
1 turned pink as soon as fluoride was added.
H NMR titration displayed the chemical shift changes of 1 upon
the addition of fluoride (as shown in Fig. 3). Sensor 1 showed two
single peaks at 9.162 and 9.765 ppm in DMSO, through the proton
2
exchange experiment by adding D O, we confirmed that which
1
correspond to the protons of OH and CH]N, respectively (see
Supplementary data). Upon the addition of fluoride, the signal at
9.162 ppm (OH) disappeared, while at the same time, the signal at
EtOH
9.765 ppm (CH]N) gradually shifted downfield to a stable value at
11.908 ppm after the addition of 5 equiv of fluoride (Fig. 3). It may
be due to the formation of an intramolecular hydrogen bond be-
tween the oxygen atom of hydroxyl after deprotonation and the
hydrogen atom of CH]N group (1a in Scheme 2).
O
H N
2
N
+
reflux
OH
OH
1
Scheme 1. Synthesis of sensor 1.
2
2
. Results and discussion
.1. Synthesis and characterization
Sensor 1 could be easily synthesized by condensation of
L-pyr-
enecarboxaldehyde and 2-aminophenol in refluxing absolute eth-
1
13
anol (Scheme 1) and characterized by H NMR, C NMR, MS, and
elemental analysis (see Supplementary data). A single crystal of 1
suitable for X-ray crystallography was obtained by solvent diffusion
method using methanol and THF (Fig. 1). A hydrogen bond in-
teraction between the phenolic proton and nitrogen atom was
found in the solid state of sensor 1.
Fig. 3. 1H NMR spectra of 1 (3 mM, DMSO-d
) with the addition of TBAF (DMSO-d
6
6
ꢀ
ꢀ
ꢀ
solution). (a) 1 only; (b) 1þF (0.5 equiv), (c) 1þF (1.0 equiv), (d) 1þF (2.0 equiv), (e)
ꢀ
þF (5.0 equiv), (f) 1þFꢀ (10 equiv), (g) 1þFꢀ (20 equiv).
1
Fig. 1. Crystal structure of sensor 1. Thermal ellipsoids were scaled to the 50% prob-
ability level.
2
.2. Fluoride sensing
Changes in the UVevis and fluorescence spectra of 1 were
measured upon adding various anions (tetrabutylammonium (TBA)
salts) in DMSO (10 M). The peak at 398 nm in the UVevis spectra
m
decreased gradually upon the addition of fluoride, while new bands
developed at 360 nm and 520 nm (Fig. 2), which can be attributed
16
to the deprotonation of the hydroxyl moiety. There are two iso-
sbestic points at 375 nm and 447 nm, respectively. These spectral
Scheme 2. Proposed sensing mechanism of sensor 1 for the detection of fluoride and
cyanide.
In the titration process, the protons of the phenyl shifted upfield
gradually, and it indicated the increase of the electron density on
the phenyl ring owing to the deprotonation of the OH group. A new
1
5
:2:1 triplet signal at 16.2 ppm appeared after the addition of
equiv of fluoride, which was ascribed to the FHF dimer (Fig. 3).
ꢀ
17
The existence of this new species confirmed the deprotonation of
the OH group.
2.3. Cyanide sensing
The changes of sensor 1 with cyanide were studied by UVevis
and fluorescence spectra. The sensing of 1 for cyanide displayed
a time-dependent property.
Upon the addition of cyanide, the solution of 1 became slightly
pink and gradually turned colorless, and the absorption between
405 and 600 nm decreased while the max absorption peak at
Fig. 2. Absorption spectra changes of 1 in DMSO (10
mM) upon the addition of
0e70 equiv fluoride. Inset: absorbance at 520 nm as a function of fluoride.
377 nm increased (Fig. 4a). This bleaching process could be visually

638
L. Zang et al. / Tetrahedron 68 (2012) 636e641
Fig. 4. Time-dependent absorbance (a) and fluorescence (
l
ex¼380 nm) (b) spectra
changes in the presence of 100 equiv of cyanide to 1 (10 M, DMSO).
m
Fig. 5. (a) Partial 1H NMR spectra of 1 (3.0 mM, DMSO-d
) before (1) and after (2) the
addition of TBACN (10 equiv). (b) Partial DEPTQ C NMR spectra of 1 (3.0 mM, DMSO-
6
13
d
6
) before (1) and after (2) the addition of TBACN (10 equiv).
observed. Two fluorescence peaks at 415 nm and 438 nm appeared
after cyanide was added. The fluorescence emission intensity had
been clearly enhanced within several minutes, which could easily
be detected. The fluorescence spectrum was performed with a 774-
fold intensity enhancement within 160 min (Fig. 4b). The fluores-
Therefore, we proposed a possible mechanism that cyanide in-
duced the cyclization of sensor 1, and at the same time a new
species, 2-pyrene benzoxazole, with high fluorescence quantum
yield was obtained (structure 2 in Scheme 2). It is known that the
benzoxazole moiety is found in biologically active compounds,
18
cence quantum yield (V, in DMSO, quinine sulfate as standard
)
19
which have antitumor, antimicrobial, and antiviral properties.
reached 0.9095. There was a 606-fold enhancement when com-
pared to 1, whose fluorescence quantum yield was only 0.0015 due
to the photoinduced electron transfer (PET) between the pyrene
fluorophore and the imine group.
Changing the substituent at position 2 in benzoxazole ring is an
effective way in searching for new fluorophores due to its high
2
0
fluorescence quantum yield and photostability. Phenolic Schiff
bases have been used to synthesize 2-substituted benzoxazoles
through oxidative cyclization, and various oxidizing agents are
1
The mechanism of 1 sensing for cyanide was investigated by H
NMR and 13C NMR titration experiments. The NMR spectra of 1
21
used. Chen and Zeng prepared 2-arylbenzoxazoles from phenolic
Schiff bases in the condition of base and phototrigger, both base and
with cyanide (10 equiv) were obtained after the reaction completed
1
and the stable NMR spectrum was achieved. H NMR spectral
2
2
phototrigger played a key role in the conversion process. How-
ever, the cyclization of phenolic Schiff bases induced by cyanide has
not been reported to our knowledge. Further experiments dem-
onstrated that 1 can also be converted to 2 in the absence of light.
analysis showed the signals corresponding to the protons of OH
(
9.162 ppm) and CH]N (9.765 ppm) disappeared after adding cy-
anide (Fig. 5a). The peak at 9.221 ppm shifted downfield to
.761 ppm. The phenol proton signals of 1 around 7 ppm displayed
a significantly downfield shift, respectively, indicating the decrease
of the electron density on the phenyl ring owing to the con-
jugative effect after the cyclization reaction. C NMR spectra
Fig. 5b) showed that the carbon atom of CH]N group (158 ppm)
9
pe
p
2.4. Selectivity and interference studies
13
(
To evaluate the selectivity of sensor 1, various anions including
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
shifted downfield toward 164 ppm after adding cyanide and be-
F , Cl , Br , I , AcO , H PO , and CN contacting with 1 were in-
2 4
1
came a quaternary carbon which was consistent with the H NMR
vestigated by UVevis absorption and fluorescence spectra. Among
various anions, only the addition of fluoride displayed noticeable
color changes from pale yellow to pink (Fig. 6).
titration spectra that the proton of CH]N (9.765 ppm) dis-
1
13
appeared. The H NMR and C NMR spectra after adding cyanide
were in agreement with the structure of cyclization reaction (2 in
Scheme 2). In order to confirm this proposed mechanism, the
This highly selective detection of fluoride could be demon-
strated even in the presence of other anions (Fig. 7). The selectivity
ꢀ
ꢀ
ꢀ
product from 1 and cyanide was synthesized, separated, purified,
2
of fluoride over other anions, especially CN , AcO , and H PO is
4
1
and analyzed by elemental analysis, mass spectrometry, and
H
important because many reported fluoride sensors suffer from
deleterious interference of these anions.
Fluorescence spectra showed that sensor 1 had a high selectivity
for cyanide. Other anions almost caused no change in fluorescence
even after 160 min (Fig. 8).
NMR (see Supplementary data). The mass of m/z 319.12 was
1
assigned to the structure of 2, and the H NMR was consistent to the
previous titration experiment, it was also in agreement with the
structure of 2.

L. Zang et al. / Tetrahedron 68 (2012) 636e641
00
639
8
6
4
2
(a)
00
00
00
0
1
+CN–
1
+ other anions
400
450
500
550
600
Wavelength (nm)
F–
Cl– Br– I– AcO–
–CN
–
(b)
1
2
H PO4
Fig. 8. Fluorescence spectra response (a) of 1 (10 mM, DMSO) upon adding various
Fig. 6. UVevisible spectra changes (a) and color changes (b) observed upon the ad-
anions (100 equiv, lex¼380 nm). Photograph (b) shows the solutions in the presence of
dition of various anions (100 equiv) to the solutions of 1 (10
mM, DMSO).
100 equiv of various anions, which was taken under a hand-held UV-lamp (365 nm).
sensor 1 could detect fluoride and cyanide, respectively, even in the
sample containing fluoride and cyanide simultaneously.
1
0
0
0
0
.3
.2
.1
.0
-
1
1
1
1
1
1
1
+ F
3. Conclusions
-
+ Cl + F
-
-
+ Br + F
-
In summary, we have developed an innovative single molec-
ular sensor 1 for fluoride and cyanide using different signaling
channels based on a phenolic Schiff base bearing a pyrene group.
This sensor showed a highly selective colorimetric response to
fluoride based on a deprotonation process and a highly selective
fluorescent response to cyanide based on a cyclization process.
Furthermore, sensor 1 could detect and distinguish fluoride and
cyanide individually even in the sample containing fluoride and
cyanide. Moreover, the cyclization of sensor 1 induced by cya-
nide, which resulted in color bleaching and fluorescence quan-
tum yield 606-fold enhancement, has not been reported to the
best of our knowledge. Thus, the cyclization of phenolic Schiff
bases could be used not only as a new approach for the design
and synthesis of cyanide probe but also could be used as a new
method for the synthesis of 2-substituted benzoxazole
derivatives.
-
+ I + F
-
-
+ AcO + F
-
-
+ H PO + F
-
2
4
-
+ CN + F
-
400
500
600
700
Wavelength (nm)
Fig. 7. Absorption spectra changes of 1 (10 mM) to fluoride (100 equiv) in the absence
and presence of 100 equiv of various anions in DMSO. The spectra were obtained after
stirring the solution containing 1 and each anion for 1 min.
4. Experimental
Again, the highly selective sensing ability of 1 for cyanide could
4.1. General
ꢀ
ꢀ
ꢀ
ꢀ
ꢀ
PO
4
be demonstrated in the presence of Cl , Br , I , AcO , and H
Fig. 9). That is to say the fluorescence response of 1 to cyanide
2
(
4.1.1. Materials and methods. All the materials for synthesis and
spectra were purchased from commercial suppliers and used
without further purification. All solvents and reagents used in the
spectroscopic studies are analytical grade. All anions are in the form
of tetrabutylammonium (TBA) salts. All anions in the form of tet-
rabutylammonium (TBA) salts were dissolved in DMSO (50 mM).
Unless otherwise noted, the concentration of sensor 1 used in the
was hardly interfered by other anions with the exception of
fluoride. The interference of fluoride was possibly due to 1 con-
verting to 1a in the presence of fluoride, in which the structure of
1
a does not favor cyclization due to intramolecular hydrogen
bonding. However, this cyclization induced by cyanide anions
could overcome the interference of fluoride in the condition of
heating.
When we added fluoride and cyanide simultaneously, the so-
lution of 1 turned to pink immediately, this phenomenon showed
the high selectivity for fluoride over cyanide (as shown in Fig. 7).
spectrum experiment was 10
out at room temperature.
mM, and all experiments were carried
4.1.2. Instrumentation. The UVevis absorption spectra were taken
on a Shimadzu 3100 UVevis-NIR recording spectrophotometer
with a 2 nm slit width. The fluorescence spectra were determined
with a Shimadzu RF-5301PC spectrofluorophotometer using 1.5 nm
But there was no obvious change in fluorescence intensity at room
ꢀ
temperature within 160 min (Fig. 9, F
1
). When we heated the
ꢁ
solution to 70 C for 160 min, the solution became colorless and
ꢀ
1
had a strong fluorescence (Fig. 9, F
2
). Thus, through observing UV
input and 1.5 nm output width (excitation at 380 nm). H NMR
13
and fluorescence channel in different time scales after heating,
(TMS) and C NMR were recorded on a Bruker UltraShield

640
L. Zang et al. / Tetrahedron 68 (2012) 636e641
Fig. 9. Fluorescent response of 1 (10
m
M) to cyanide (100 equiv) in the presence of various anions (100 equiv) in DMSO (
l
ex¼380 nm). The spectra were obtained after stirring the
ꢀ
ꢀ
solution containing 1 and each anion after 160 min. F
1
was measured in the same condition of other anions (room temperature). F
2
was measured in the condition of heating at
ꢁ
70
C.
5
00 MHz spectrometer. Mass spectra were measured on Thermo
References and notes
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8
9
9
8
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8
ꢁ
(
0.4 mmol) of TBACN, and the mixture was heated at 70 C for 10 h.
1
The solvent was removed under reduced pressure, and the residue
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1
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J¼6.5 Hz, 1H), 8.178 (t, J¼7.0 Hz, 1H), 8.3 (d, J¼9.0 Hz, 1H), 8.367 (d,
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1
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This work was supported by the National Natural Science
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Program of China (2012CB933803).
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Supplementary data
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Crystal data for sensor 1, the NMR spectra, and mass spectra for
and 2 are provided. Crystallographic data on the structure of 1
reported in this paper are available in the Cambridge Crystallo-
graphic Data Centre as deposition No. CCDC 800733. Supplemen-
tary data associated with this article can be found, in the online
version, at doi:10.1016/j.tet.2011.10.105.
1
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