Discriminating detection of multiple analytes (F- and CN-) by a single probe through colorimetric and fluorescent dual channels

Article abstract of DOI:10.1016/j.dyepig.2015.07.039

Abstract Two novel Schiff base type receptors containing phenol hydroxyl group (1) or methoxy group (2) were designed and synthesized. The receptor 1 with phenol hydroxyl group exhibited different colorimetric and fluorimetric responses to fluoride and cy

Full text of DOI:10.1016/j.dyepig.2015.07.039

Accepted Manuscript
Discriminating detection of multiple analytes (F‾ and CN‾) by a single probe through
colorimetric and fluorescent dual channels
Ying Wang, Qian Zhao, Libin Zang, Chunshuang Liang, Shimei Jiang
PII:
S0143-7208(15)00307-1
10.1016/j.dyepig.2015.07.039
DYPI 4879
DOI:
Reference:
To appear in: Dyes and Pigments
Received Date: 29 April 2015
Revised Date: 27 July 2015
Accepted Date: 28 July 2015
Please cite this article as: Wang Y, Zhao Q, Zang L, Liang C, Jiang S, Discriminating detection of
multiple analytes (F‾ and CN‾) by a single probe through colorimetric and fluorescent dual channels,
Dyes and Pigments (2015), doi: 10.1016/j.dyepig.2015.07.039.
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ACCEPTED MANUSCRIPT
Discriminating detection of multiple analytes (F‾ and
CN‾) by a single probe through colorimetric and
fluorescent dual channels
a,b
a
a
a
a
Ying Wang, Qian Zhao, Libin Zang, Chunshuang Liang and Shimei Jiang*
A new “one to more” type sensor has been developed for the selective and
discriminating detection towards F‾ and CN‾ through the colorimetric and fluorescent
channels, which was based on the different mechanisms for the two anions.

ACCEPTED MANUSCRIPT
Discriminating detection of multiple analytes (F‾ and CN‾)
by a single probe through colorimetric and fluorescent dual
channels
a,b
a
a
a
a
Ying Wang, Qian Zhao, Libin Zang, Chunshuang Liang and Shimei Jiang*
a
State Key Laboratory of Supramolecular Structure and Materials, Jilin University, 2699
Qianjin Avenue, Changchun 130012, P. R. China.
b
Heilongjiang Key Laboratory of Molecular Design and Preparation of Flame Retarded
Materials, College of Science, Northeast Forestry University, Harbin 150040, P. R. China.
*
Corresponding author: Shimei Jiang
E-mail: smjiang@jlu.edu.cn
Tel: +86-431-85168474
Fax: +86-431-85193421
1

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Abstract:
Two novel Schiff base type receptors containing phenol hydroxyl group (1) or
methoxy group (2) were designed and synthesized. The receptor 1 with phenol
hydroxyl group exhibited different colorimetric and fluorimetric responses to fluoride
and cyanide ions. In the colorimetric channel, the colorless solution turned to yellow
towards F‾, but red in the presence of CN‾. In the fluorimetric channel, the
nonluminous solution showed different “turn-on” response to F‾ and CN‾,
respectively. The distinguishing responses of receptor 1 to the two anions in dual
channels were attributed to the disparate interaction mechanisms. F‾ was detected
through the formation of hydrogen bonds and the subsequent deprotonation process.
CN‾ initially formed hydrogen bonds and then it attacked the receptor via a
nucleophilic addition reaction. In contrast, the receptor 2 with methoxy group only
showed remarkable color change from colorless to yellow towards F‾. These results
proved that the phenol hydroxyl group played an important role in the discriminating
detection of F‾ and CN‾.
Keywords: colorimetric and fluorescent probe, Schiff base, fluoride, cyanide.
2

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1
. Introduction
Among various anions, fluoride and cyanide ions are numerous indispensable in
chemical, biological, clinical, environmental, and industrial fields [1]. Fluoride ion is
in close contact with human’s daily life such as water supply treatment, osteoporosis
treatment, dental care and even in chemical warfare agents. However, high doses of
fluoride ions is hazardous, which can led to nephrolithiasis, kidney failure and dentalor
skeletal fluorosis [2]. On the other hand, cyanide ion extraordinarily ubiquitous in
many industrial processes is one of the most prevalent toxic anions to mammals,
because a small amount of cyanide can cause vomiting, convulsions, loss of
consciousness, and even death [3]. Therefore, the development of convenient and
efficient detection techniques for fluoride and cyanide ions has drawn great attention in
recent years. Compared with the traditional sophisticated analytical techniques, optical
chemosensors that rely on fluorometric and colorimetric responses are simple,
inexpensive and rapid methods [4]. Especially, “naked-eye” optical sensors are
extremely dramatic [5].
More recently, “one to more” type sensors that can recognize two or more target
analytes with distinct responses independently, have already become a new research
focus [6]. Compared with most single-ion responsive chemosensors, they have
advantages like faster analytical processing, cost reductions and potential application
to molecular logic gates [7].
During the past years, the optical sensors for detecting either F‾ or CN‾ have been
extensively exploited [8]. As the most electronegative atom, fluoride can form strong
hydrogen bonds with some hydrogen bond donors. Thus, various chemosensors
3

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containing acidic NH and OH groups have been developed to detect fluoride [9a, 9b].
On the other hand, some fluorescent sensors containing tert-butyldimethylsilyl
(TBDMS) or tert-butyldiphenylsilyl (TBDPS) groups that are capable of detecting
fluoride ions in water have been reported. The sensing strategy resulted from the
chemical affinity between fluoride and silicon (fluoride-mediated cleavage of Si-O
bond), enabling the probe molecule unreactive to potentially interfering anions and
thus only sensitive to fluoride ions. Since such approach was first reported by Kim
and Swager, several different chemosensors or probes that can detect NaF in aqueous
media have been developed [9c, 9d]. Meanwhile, the detection of cyanide has been
achieved through various approaches such as the formation of hydrogen bond [10],
deprotonation process [11], chemodosimeters [12], and displaycement method [13].
However, owing to the similar binding affinity of F‾ and CN‾, it remains challenging
to develop new single optical sensors that can recognize both F‾ and CN‾ selectively
with distinct responses [14]. Fortunately, some novel stratagems have been applied to
improve this situation [15]. Among the previous studies of our group in molecular
recognition [16], a single molecular sensor based on phenolic Schiff base bearing a
pyrene group has been developed to detect fluoride and cyanide through different
signaling channels [16a]. This sensor showed highly selective colorimetric response to
fluoride based on a deprotonation process and exclusive fluorescent response to
cyanide based on a cyclization reaction.
To design this “one to more” type of sensors directly interacting with different
analytes, different acting sites or different interaction modes should be taken into
4

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consideration, thus increasing the design difficulty. Among various binding sites,
amide [16b, 17] and phenol hydroxyl group [16c, 18] can form typical hydrogen bonds
with F‾. In addition, CN‾ can attack the imine group [19] based on the nucleophilic
reactions. Nevertheless, the development of chemosensors using both hydrogen bonds
and addition reactions is still regarded as a challenge [20]. In view of this, two new
organic receptors containing phenol hydroxyl group (1) or methoxy group (2) were
designed and synthesized (Scheme 1). The molecular structures of 1 and 2 are similar
except for the phenol hydroxyl group which was replaced with methoxy group in the
structure of receptor 2. This small structural perturbation was expected to induce
drastic changes in the optical properties of 1 and 2 towards anions. Indeed, because
the neighboring phenol hydroxyl group could activate the C=N bond through the
intramolecular hydrogen bond and promote the nucleophilic reaction, the receptor 1
with phenol hydroxyl group could be used to sense and discriminate F‾ and CN‾
through different colorimetric and fluorimetric responses. But sensor 2 with methoxy
group could only selectively detect F‾ via colorimetric response.
2
. Experimental section
2
.1 Materials and general methods
2
.1.1 Materials
All the materials for synthesis were purchased from Aldrich and used without further
purification. All solvents used in the synthesis and spectroscopic studies are analytical
grade (Tianjin Guangfu Fine Chemical Research Institute). All anions are in the form
of tetrabutylammonium (TBA) salts, which were obtained from Alfa. And all
5

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experiments were carried out at room temperature.
.1.2 Spectral measurements
2
Stock solutions of the anions (0.03M) were prepared in DMSO. A stock solution of 1
or 2 (1 mM) was prepared in DMSO. The solution of 1 or 2 was then diluted to 10
µM or 50 µM with DMSO (appropriate solvent in the experiment about the effect of
solvent). In titration experiments, each time, a 3 mL solution of 1 or 2 (10 µM) was
filled in a quartz optical cell of 1 cm optical path length, and the F stock solution
was added to the quartz optical cell gradually using a micro-pipette. All spectral data
were recorded at 1 min after the addition, except the time course experiments. In
selectivity experiments, the test samples were prepared by placing appropriate
amounts of anion stock solutions into 3 mL solution of 1 or 2 (10 µM).
2
.2 Instrumentation
The UV-vis absorption spectra were taken on a Shimadzu 3100 UV-VIS-NIR
recording spectrophotometer using a 1 nm slit width. The fluorescence spectra were
1
determined with a Shimadzu RF-5301PC spectrofluorophotometer. H NMR (TMS)
was recorded on a Bruker UltraShield 500 MHz spectrometer. Mass spectra were
measured on a Thermo Scientific ITQ 1100™ GC/MSn and a Q Trap MS (Applied
Biosystems/MDS Sciex, Concord, ON, Canada) which was equipped with an
electrospray ionisation (ESI) source. Elemental analyses were carried out with a vario
MICRO cube elementar.
2
2
.3 Synthesis
.3.1 Preparation of 3
6

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In 100 ml of dry CH Cl , 1, 4-phenylene diamine (1 g, 9.24 mmol) was dissolved at
2
2
room temperature under a N atmosphere. After cooling to 0℃, distilled acryloyl
2
chloride (0.3 ml, 4.62 mmol) was added dropwise to the solution. The reaction was
stopped after 15 min of vigorous stirring. A white precipitate appeared during the
reaction process and the mixture was filtered to get a clear solution. The filtrate
solvent was concentrated under reduced pressure and purified by silica gel
chromatography using EtOAc/Hexanes as eluent. A yellow solid was obtained (yield:
1
0
8
1
.41 g, 55 %). H NMR (500 MHz, DMSO): δ 7.30 (1 H, d, J 8.6), 6.51 (1 H, d, J
.6), 6.37 (1 H, dd, J 17.0, 10.1), 6.16 (1 H, dd, J 17.0, 1.9), 5.65 (1 H, dd, J 10.1,
.9), 4.90 (1 H, s).
2
.3.2 Preparation of 1
A 0.1069 g (0.659 mmol) sample of 3 was dissolved in 20 ml of ethanol and treated
with 0.068 ml (0.65 mmol) of salicylaldehyde. Under vigorous stirring, the mixture
was refluxed for 3 h. During the reaction process, a yellow precipitate appeared. The
mixture was cooled in the ice bath to stop the reaction. The product was filtered and
washed with 15 ml of cold ethanol. Recrystallization from ethanol afforded compound
1
1
as yellow powder (0.126 g, yield 73 %). H NMR (500 MHz, DMSO): δ 13.22 (1 H,
s), 10.31 (1 H, s), 8.98 (1 H, s), 7.78 (2 H, d, J 8.7), 7.64 (1 H, dd, J 7.6, 1.2), 7.55 –
7
1
1
1
.29 (3 H, m), 6.98 (2 H, dd, J 14.5, 7.8), 6.60 – 6.36 (1 H, m), 6.29 (1 H, dd, J 17.0,
1
3
.8), 5.79 (1 H, dd, J 10.1, 1.8). C NMR (126 MHz, DMSO): δ 163.61, 162.51,
60.73, 143.60, 138.61, 133.46, 132.89, 132.26, 127.48, 122.37, 120.57, 119.56,
17.02 ppm. Elemental analysis calculated for C H N O : C, 72.16; H, 5.30; N,
16
14
2
2
7

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1
0.52 %; Found: C, 72.32; H, 5.52; N, 10.46 %. MS: m/z 265.1.
2
.3.3 Preparation of 2
This compound was synthesized in a similar way as 1 except that
methoxybenzaldehyde (0.0885 g) was used instead of salicylaldehyde. The yield was
1
5
1
4 % (0.0984 g). H NMR (500 MHz, CDCl ): δ 8.96 (1 H, s), 8.16 (1 H, dd, J 7.7,
3
.4), 7.66 (2 H, d, J 8.3), 7.51 (1 H, s), 7.49 – 7.43 (1 H, m), 7.27 (2 H, d, J 8.7), 7.06
(1 H, t, J 7.5), 6.98 (1 H, d, J 8.3), 6.47 (1 H, d, J 16.8), 6.31 (1 H, dd, J 16.8, 10.2),
1
3
5
1
1
.80 (1 H, d, J 10.3), 3.93 (3 H, s). C NMR (126 MHz, CDCl ): δ 163.61, 159.51,
3
55.93, 148.97, 135.85, 132.68, 131.28, 127.63, 127.48, 124.75, 121.79, 120.89,
20.79, 111.14, 55.57 ppm. Elemental analysis calculated for C H N O : C, 72.84;
1
7
16
2
2
H, 5.75; N, 9.99 %; Found: C, 72.48; H, 5.77; N, 10.06 %. MS: m/z 279.9.
3
. Results and discussion
3
.1 Synthesis and characterization of 1 and 2
The synthetic routs for 1 and 2 were showed in Scheme 1. Compound 3 was
synthesized by the method which had been reported previously [21]. Compounds 1
and 2 were easily synthesized, by a Schiff base condensation reaction of reactant
amine 3 with salicylaldehyde and 2-methoxybenzaldehyde respectively in refluxing
absolute ethanol. The pure products were obtained after recrystallization twice from
1
13
appropriate solvent and characterized by H NMR, C NMR, MS, and elemental
analysis (Figs. S6-11).
3
.2 Anion binding affinity of receptor 1
8

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To evaluate the real-time detection ability of receptor 1 towards various anions such
as F‾, Cl‾, Br‾, I‾, AcO‾, H PO ‾ and CN‾, changes in the absorption and fluorescence
2
4
spectra were measured immediately upon adding various anions (tetrabutylammonium
salts) to the solutions of receptor 1.
As illustrated in Fig. 1, receptor 1 exhibited a maximum absorption band at 359
nm. The addition of F‾ led to significant decrease of this band accompanied with the
emergence of a new band at 448 nm, while CN‾ caused similar but quite weak
changes. Owing to the appearance of the new band in the visible region, the changes
could be observed by naked eye. The colorless solution of receptor 1 turned to yellow
in the presence of F‾, and very light yellow in the presence of CN‾. In contrast,
H PO ‾, AcO‾ and other halide anions didn’t induce any spectral and color changes.
2
4
On the other hand, receptor 1 represented weak fluorescence emission, due to the
efficient non-radiative deactivation through an excited-state intramolecular proton
transfer (ESIPT) from the transformation from the enol-form to the keto-form (Fig. 2)
[
22]. However a prominent fluorescence enhancement at 538 nm appeared after the
addition of F‾ to the solution of 1. Meanwhile, the change induced by CN‾ was so
little that it couldn’t be detected by naked eye. Like the colorimetric result, the
receptor also showed no response towards other anions via the fluorescence channel.
From the results, it indicated that receptor 1 could recognize F‾ selectively through
colorimetric and fluorimetric dual channels in real time. In addition, no significant
spectral changes were observed after the addition of H PO ‾, AcO‾ and other halide
2
4
anions.
9

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3
.2 F‾ sensing ability of receptor 1
The UV–vis and fluorescence titration experiments of receptor 1 were carried out to
explore the interaction between it and F‾. As showed in Fig. 3, the variation in the
UV-vis spectrum caused by the gradual addition of F‾ could be divided into two
stages. In the first stage, upon the addition of 0-30 equiv. of F‾ ions to the solution of
1
, the maximum absorption band of receptor 1 at 359 nm decreased gradually, at the
same time a new absorption band appeared at 448 nm, whose intensity increased very
slowly in this process. Neverthless, in the second stage, the intensity of the band at
4
48 nm increased rapidly when 30-40 equiv. of F‾ was added. Then the absorption
reached maxima at 60 equiv. of F‾. The two step changes indicated that the
intermolecular hydrogen bonds of N–H···F and O–H···F initially formed, and then the
deprotonation process happened. Moreover, one clear isosbestic point was observed at
3
96 nm, implying the formation of single complex species between the receptor 1 and
F‾.
As depicted in Fig. 4, the fluorescent peak at 538 nm increased progressively upon
the addition of increasing amounts of F‾. And it was performed with a 78-fold
intensity enhancement compared to 1 when the concentration of F‾ reached to 0.7
mM. Under a commercially available UV lamp illumination, the lightless solution
showed a green light in the presence of F‾. On the basis of these results, it is
hypothesized that a stable complex formed between 1 and F‾. More interestingly, the
increasing fluorescence intensity of receptor 1 had a significant linear correlation vs.
the concentration of F over a wide range of 0-100 µM (Fig. S1). Hence, 1 can
1
0

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accomplish the quantitative analysis of F

. From the fluorescence titration spectra, the
limit of detection (LOD) of receptor 1 was calculated as 1.09 µM. Additionally, The
rapid detection of F
probes. In order to estimate the response rate of 1 towards F
fluorescence intensity change upon the addition of 100 equiv. of F
Fig. S2). It was obvious that the intensity of the fluorescence peak at 538 nm rapidly
increased at room temperature, and the increase was almost completed within 10
seconds when 100 equiv. of F was added. The low detection limit and fast response
suggested that 1 is highly sensitive toward F

is one of the most important factors in practical applications of
, the time-dependent
was monitored


(


.
As mentioned above, salicylidene Schiff bases derivatives exhibit ESIPT process
22]. Furthermore, the protons of hydroxyl group of them are easily deprotonated by
[
basic anions. Thus the fluorescence “turn-on” response towards F‾ was probably
caused by the inhibition of ESIPT process at the excited state of receptor 1. It is well
known, that ESIPT is solvent dependent. To investigate the role of ESIPT in the F
detection process, the optical properties of receptor 1 with fluoride ions in different
solvents have been examined. As shown in Fig. S3, after the addition of F to the


solution of 1 in DMSO and DMF, a new band appeared at 448 nm in the absorption
spectra and a fluorescent peak at 538 nm emerged in the fluorescence spectra. In
contrast, the addition of F
 didn’t induce any changes in the absorption and
fluorescence spectra in DCM, THF, Dioxane and CHCl3.
To further look into the nature of the interaction between 1 and fluoride, different
1
equivalents of F‾ were added to the solution of 1 and the H NMR spectral changes
1
1

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were inspected. As depicted in Fig. 5, upon the addition of 0.5 equiv. of F‾ to 1, the
signal belonged to phenolic hydroxyl proton at 13.21 ppm disappeared immediately
which indicated that the fluoride would form O–H···F hydrogen bond with the
hydroxyl group and deprotonate the phenolic proton [16c, 18]. Meanwhile, the amide
proton signal at 10.29 ppm gradually shifted downfield and broadened with increasing
fluoride concentration from 0 to 5 equivalents. The aromatic protons almost shifted to
high field which indicated the increase of the electron density on the phenyl ring and
the formation of N–H···F hydrogen bond between amide group and F‾ [16b, 17].
Additionally, after 3 equiv. of fluoride was added, there was a new signal emerged at
1
6.09 ppm, which was ascribed to the [FHF]‾ dimmer. The existence of this new
species indicated that the deprotonation process happened between compound 1 and
F‾. These results confirmed that F‾ bound to the amide and hydroxyl group of receptor
1
(Scheme 2). Receptor 1 responded to F‾ based on the formation of N–H···F and
O–H···F hydrogen bond followed by the deprotonation process between the two
groups and F‾.
3
.3 Sensing behavior of receptor 1 towards CN‾
As discussed above, receptor 1 could respond to F‾ selectively via colorimetric and
fluorimetric dual channels in real time. Meanwhile, CN‾ caused similar but quite
weak changes. However, as we all known, time consuming is a common characteristic
of the nucleophilicity addition reactions with CN‾ [12b-d]. Thus the time-dependent
UV-vis absorption and fluorescence spectral intensity changes were monitored to
investigate the sensing ability of receptor 1 towards CN‾.
1
2

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As displayed in Fig. 6a, there was a two-step absorption change during the
detection process. At first step, the spectroscopic behavior was analogous to that of
F‾. The maximum absorption band at 359 nm of 1 receded as soon as CN‾ was added,
and a new band emerged immediately at 448 nm. Similar with that of F‾, these
changes might be induced by the formation of hydrogen bonds between 1 and CN‾.
However, on the next stage, the new peak was found to be red shifted to 500 nm, and
the intensity of the absorption began to in-situ increase gradually. Accompany with
the spectrum changes, pronounced color changes could also be perceived by naked
eye (Fig. 6b). The colorless solution turned to yellowish immediately after CN‾ was
added, then to orange within 1 hour, eventually to red. The red color became deeper
over time. The distinct color changes made it feasible to distinguish CN‾ from F‾ by
naked eye. This was presumably attributed to the formation of a new compound
derived from the nucleophilicity addition reaction between CN‾ and receptor 1.
As shown in Fig. 7, a new peak at 538 nm appeared instantly upon the addition of
CN‾. It was assigned to the inhibition of ESIPT process, as discussed in the case of F‾.
Meanwhile, the intensity of the emission at 479 nm increased slightly. As time went
by, this emission was blue shifted to 470 nm with gradual increase of its intensity.
The result suggested that there were also two-step changes in the fluorescence
detection of CN‾.
The results of the time-dependent UV-vis absorption and fluorescence spectra
agreed well with each other. All of them indicated that the CN‾ sensing process might
take place in two steps. CN‾ was detected based on the hydrogen bond formation
1
3

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process and a subsequent nucleophilic addition reaction between receptor 1 and CN‾.
Moreover, because of the second step in the CN‾ sensing process, the discriminate
and selective detection of the two anions was realized.
1
In order to get insight into the cyanide sensing mechanism of 1, the H NMR
spectrum was investigated after adding different equivalents of TBACN to the
solution of 1 in real time (Fig. 8). The signal corresponding to the proton of OH
(13.21 ppm) disappeared as soon as cyanide anions were added to 1, which might be
attributed to the formation of the O–H···CN hydrogen bond between the hydroxyl
group and CN‾. Meanwhile, the proton signal of NH at 10.29 ppm shifted to 10.61
ppm and broadened with growing CN‾ concentration. The aromatic protons almost
shifted to high field. These results indicated the hydrogen bonding complex was
obtained between the receptor 1 and CN‾.
On the other hand, the signal of the proton of CH=N (H ) at 8.98 ppm became
g
weaker gradually, while at the same time a new peak (H ) at 5.65 nm grew up
g’
distinctly. This phenomenon didn’t exist in the case of F‾. Thus, it evidently indicated
that a nucleophilicity addition reaction happened between the cyanide anion and the
imine group [4e, 19, 23]. A new species was obtained after the reaction, which also
contained a NH proton (H ) in its structure with the signal at 10.01 ppm. Moreover,
d’
in comparison with the relevant protons of the hydrogen bonding complex, the
aromatic protons of the product of addition reaction shifted to higher field (Fig. 8b). It
suggested that the distribution of electric charge was influenced by the addition
reaction of CN‾, which brought an increase of the electron density on the phenzene
1
4

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ring. From the results, it could be deduced that two species, the hydrogen bonding
complex and the product of the nucleophilicity addition reaction between 1 and CN‾
were obtained after the addition of CN‾. The protons signals corresponding to the
hydrogen bonding complex progressively became weaker upon the addition of
increasing amounts of CN‾, while the relative ones belonging to the product of the
addition reaction became stronger gradually. According to the results of the integral
of every peak’s area, when 2.0 equiv. of CN‾ was added, the molar ratio between the
hydrogen bonding complex and the product of the nucleophilicity addition reaction
was 1: 1.5. High resolution ESI-MS spectral analysis further proved the presence of
the nucleophilicity addition reaction product at m/z 317.5, which was assignable to
[
1-CN + CN‾] (Fig. S4).
Therefore, based on the analysis of UV–vis spectra, fluorescence spectra, H NMR
1
spectra and ESI-mass spectromet-ry, we proposed a possible two-step mechanism for
the interaction between receptor 1 and CN‾ in the optical detection process (Scheme
3
). In the first step, the intermolecular hydrogen bonds (O–H···CN and N–H···CN)
formed between CN‾ and 1, just like the case of F‾. In the second step, cyanide ions
were believed to attack the CH=N bond based on a nucleophilicity addition reaction,
which was promoted by the intramolecular hydrogen bond between the neighboring
phenol hydroxyl group and imine group.
3
.4 Anions recognition capability of receptor 2
In order to figure out the specific structural contribution of the hydroxyl group to the
detection of F‾ and CN‾, the hydroxyl group was replaced with methoxy group in the
1
5

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structure of receptor 2. Its anions detection property was also studied.
Changes in the absorption and fluorescence spectra were measured upon adding
various anions to the solution of receptor 2. As illustrated in Fig. 9, receptor 2 showed
a maximum absorption band at 349 nm. The intensity of this band decreased as soon
as F‾ was added, while a new absorption band appeared at 426 nm, accompanied with
the naked eye color change from colorless to yellow. It might be induced by the
formation of H-bond and the followed deprotonation interaction between amide group
and F‾. But, other anions including CN‾ didn’t induce any significant color and
absorbance spectral changes. On the other side, receptor 2 displayed nearly no
fluorescence response to any anions including F‾. It suggested that because of losing
the proton of the hydroxyl group, there was no ESIPT phenomenon of receptor 2.
Therefore, it failed to respond to F‾ through the fluorescence channel. From the
results, it could be concluded that due to the binding site amide group, receptor 2
presented naked eye detectable colorimetric changes towards F‾ with high selectivity.
The binding behavior of receptor 2 towards F‾ was studied by UV–vis titration
experiments. Upon addition of F‾ ions to the solution of 2, the absorption band at 349
nm gradually decreased, while a new absorption band at 426 nm significantly
increased and reached maxima at 60 equiv. of F‾. The obvious isosbestic point at 378
nm indicated the formation of single complex species between receptor 2 and F‾ (Fig.
S5). The change of the intensity of the new band at 426 nm also underwent a two-step
process, which was similar as receptor 1. This phenomenon could also be explained
by the formation of hydrogen bond between amide NH and F‾ followed by the
1
6

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deprotonation process. But the intensity of the new band increased more slowly than
that of receptor 1 at first step, which might be attributed to the lack of the hydroxyl
group in the structure of receptor 2.
Besides that, nearly no absorption and fluorescence changes were observed after
adding CN‾ to the solution of receptor 2, even the solution was placed for several
hours. It indicated that the neighboring phenol hydroxyl group of Salicylidene Schiff
base derivatives indeed made a great contribution to the detection of CN‾. It not only
forms intermolecular hydrogen bond with CN‾, but also activates the C=N bond
through the intramolecular hydrogen bond and promotes the nucleophilicity addition
reaction.
Overall, the molecular structures of 1 and 2 are similar except for the phenol
hydroxyl group which is replaced with methoxy group in the structure of receptor 2.
This small structural perturbation induced drastic changes in the optical properties of
1
and 2 towards anions. Comparing the detection behavior of receptor 1 and 2, it can
be concluded that the neighboring phenol hydroxyl group of receptor 1 plays a key
role in the detection of F‾ and CN‾. Firstly, the phenol hydroxyl group can form
hydrogen bonds with anions. Secondly, it causes ESIPT phenomenon in receptor 1
and consequentially responds to anions in fluorescence channel. Thirdly, it can
activate the C=N bond through the intramolecular hydrogen bond and promote the
nucleophilicity addition reaction of CN‾.
4
. Conclusions
Two new Schiff base type receptors containing phenol hydroxyl group (1) or
1
7

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methoxy group (2) were designed and synthesized. The receptor 1 with phenol
hydroxyl group can be used to detect and discriminate F‾ and CN‾ through different
colorimetric and fluorimetric responses. F‾ was detected through the formation of
N–H···F and O–H···F hydrogen bonds followed by the deprotonation process. The
colorless solution of receptor 1 turned to yellow in the presence of F‾. Due to the
inhibition of excited-state intramolecular proton transfer (ESIPT) process, it also
induced fluorescence “turn-on” response at 538 nm. CN‾ initially formed hydrogen
bonds with OH and NH groups and then it attacked the CH=N bond based on a
nucleophilicity addition reaction. The specific reaction resulted in a color change from
colorless to red and a prominent fluorescence enhancement at 470 nm. This two-step
mechanism for the interaction between receptor 1 and CN‾ have been confirmed by
1
UV–vis spectra, fluorescence spectra, H NMR spectra and ESI-mass spectrometry
analysis. The vivid color and fluorescence changes towards F‾ and CN‾ could be
observed by naked eye. In addition, H PO ‾, AcO‾ and other halide anions did not
2
4
induce any significant color and spectral changes. In contrast, receptor 2 with
methoxy group only showed remarkable color change from colorless to yellow
towards F‾. It might be attributed to the formation of N–H···F hydrogen bond
followed by the deprotonation process. But, other anions including CN‾ didn’t induce
any significant color and absorbance spectral changes. It can be concluded that the
neighboring phenol hydroxyl group makes a great contribution to the highly selective
detection of fluoride and cyanide via dual channels. These results provide a strategy to
design novel salicylidene Schiff bases derivatives for discriminating detection of
1
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anions, although it could only work in organic solvent. Besides of that, due to the
C=C bond which could be used as polymerizable section, receptor 1 and 2 may also
have potential use in many other aspects, like polymer material, polymer probe, and
so on. Based on that, some works about this aspect have been done in our group
recently.
Acknowledgements
This work was supported by the National Basic Research Program of China
(2012CB933800) and the National Natural Science Foundation of China (21374036).
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