The salen based chemosensors for highly selective recognition of Zn2+ ion

Article abstract of DOI:10.1016/j.saa.2018.06.008

Two novel salen based chemosensors have been successfully synthesized. UV–vis absorption, fluorescence emission spectroscopy and cyclic voltammetry (CV) were exploited to investigate their recognition toward various metal ions, including Na⁺, K⁺, Mg²⁺, Al³⁺, Zn²⁺, Ag⁺, Pb²⁺, Co²⁺, Li⁺, Ba²⁺, Ca²⁺, Cd²⁺, La³⁺, Cu²⁺ and Mn²⁺ ions. The results indicated that the sensor L1 and L2 exhibited highly selective and sensitive recognition for Zn²⁺ ions. The binding stoichiometry ratio of L1-Zn²⁺/L2-Zn²⁺ were recognized as 4:1 by the method of Job's plot. Meanwhile, this investigation is confirmed by ¹H NMR. These results indicated that L1 and L2 can be applied as chemosensor for the detection of Zn²⁺ ion.

Full text of DOI:10.1016/j.saa.2018.06.008

Accepted Manuscript
The salen based chemosensors for highly selective recognition of
Zn2+ ion
Wenkai Zhu, LongChao Du, Wensheng Li, Jinyan Zuo, Jingrui
Shan
PII:
S1386-1425(18)30542-0
doi:10.1016/j.saa.2018.06.008
SAA 16167
DOI:
Reference:
Spectrochimica Acta Part A: Molecular and Biomolecular
Spectroscopy
To appear in:
Received date: 13 February 2018
Revised date:
Accepted
date:
13 May 2018
2
June 2018
Please cite this article as: Wenkai Zhu, LongChao Du, Wensheng Li, Jinyan Zuo, Jingrui
Shan , The salen based chemosensors for highly selective recognition of Zn2+ ion. Saa
(2017), doi:10.1016/j.saa.2018.06.008
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ACCEPTED MANUSCRIPT
The salen based chemosensors for highly selective recognition of
2
+
Zn ion

Wenkai Zhu, LongChao Du , Wensheng Li, Jinyan Zuo, and Jingrui Shan
School of Chemistry and Chemical Engineering & the Key Laboratory of
Environment-friendly Polymer Materials of Anhui Province, Anhui University, Hefei, PRC
Abstract
Two novel salen based chemosensors have been successfully synthesized. UV-vis
absorption, fluorescence emission spectroscopy and cyclic voltammetry (CV) were
+
+
exploited to investigate their recognition toward various metal ions, including Na , K ,
2
+
3+
2+
+
2+
2+
+
2+
2+
2+
3+
2+
2+
Mg , Al , Zn , Ag , Pb , Co , Li , Ba , Ca , Cd , La , Cu and Mn ions.
The results indicated that the sensor L1 and L2 exhibited highly selective and
sensitive recognition for Zn2+ ions. The binding stoichiometry ratio of
2
+
2+
L1-Zn /L2-Zn were recognized as 4:1 by the method of Job's plot. Meanwhile, this
1
investigation is confirmed by H-NMR. These results indicated that L1 and L2 can be
2
+
applied as chemosensor for the detection of Zn ion.
Keywords: Schiff base; Zinc ion; Chemosensor; Recognition
1
. Introduction
Zinc is the second most abundant essential element in the human body which plays
an extremely critical roles in human growth and development, reproductive heredity,
immunity, incretion and other important physiological processes[1-4]. However, the
2
+
[5]
normal concentration of Zn in human plasma is 6~12mM . Breaking the balance of
Zn2+ ion in the body can cause overt toxicity symptoms[6] and a variety of
neurodegenerative disorders, such as Alzheimer's disease, Parkinson's disease,
Address correspondence to Longchao Du, School of Chemistry and Chemical Engineering & the
Key Laboratory of Environment-friendly Polymer Materials of Anhui Province, Anhui University,
Hefei, PRC. E-mail address: dulongchao@sina.com

ACCEPTED MANUSCRIPT
diabetes, depression and dysplasia[7-9]. Excessive intake of Zn ion in the human body
is mainly due to environmental pollution. Accordingly, in order to maintain the
balance of nature and human health, it is necessary to develop an efficient instrument
for Zn2+ detection which possesses good selectivity, high sensitivity and simple
2+
operation[10-13]
.
In recent years, the design, synthesis and development of fluorescent chemosensors
have attracted more and more attention[14-17], because of their high sensitivity, quick
response and easy operation towards different analytes at low concentrations for the
detection of transition- and heavy-metal ions[18-23]. A variety of fluorescent
chemosensors have been reported, such as the pyrimidine[24], 2, 2’-dipicolylamine
(
DPA)[25, 26], porphyrin[27], antipyrine[28], quinoline[29, 30], etc. However, little attention
has been paid to the research of salen based chemosensors. The salen ligand have not
only better photophysical properties, can but also coordinate with some metal ions
through the O atom of OH and the N atom of C=N bond, thereby, which regulates the
selectivity and sensitivity of the fluorescent chemosensor.
In this work, we designed and synthesized a new salen ligand (L1) and its
quaternary ammonium salt (L2), and investigated their fluorescence performance and
recognition performance. They are expected to be applied to the qualitative analysis
and quantitative detection of metal ions in the environment.
2
2
. Experimental
.1. Materials
Unless otherwise explained, all manipulations were carried out in a standard glove
box under an atmosphere of dry nitrogen with the standard Schlenk techniques.
Except methanol was distilled by using sodium, all reagents (THF, CH Cl , CH CN,
cyclohexane) were dried by distillation over CaH . Hydrogen bromide was purchased
2
2
3
2
from Shanghai Richjoint Chemical Reagents Co., Ltd. The rest was used without any
further purification.
2
.2. Instrumentations
1
13
H-NMR and C-NMR (400MHz) spectra were recorded on a Varian Mercury Plus

ACCEPTED MANUSCRIPT
4
00 using CDCl
3
or DMSO-d as solvent. Fourier transform infrared (FTIR) studies
6
were recorded on a Nicolet MAGNA-IR750 apparatus by using a thin film with the
ratio of sample to KBr as 1:100 by mass. The matrix-assisted laser
desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry was measured
using a Bruker Autoflex III spectrometer. UV-vis absorption spectra were recorded on
a Shimadzu UV-3600 spectrophotometer. The fluorescence profile of sensor solutions
was recorded on Perkin-ElmerS-55 Fluorescence spectrophotometer by using 1cm
path length of quartz cells.
2
.3. Synthesis
Synthesis of compound 1 2-(4-Hydroxyphenyl)ethanol (11.68g 0.08mol) and
NaOH (15g 0.375mol) were added to a flask and combined with THF (350mL). After
the mixture was stirred at room temperature for 1h, CH
subsequently added into the flask. Then, the solution was heated to 66℃ and refluxed
for 2 days. After cooled to room temperature, the solution was extracted using CH Cl
and the organic phase was collected and dried with anhydrous MgSO . Whereafter,
the solvent was removed under vacuum to obtain a pure product (12.21g, 92%).
3
I (30g 0.211mol)
2
2
,
4
-1
1
IR(KBr): 1213(Ar-O-C), 1030(C-O-C)cm . H-NMR(CDCl
3
): δ=2.80(t, J=7.51Hz,
), 3.53(t, J=7.5Hz, 2H, Ar-CH ), 3.73(s, 3H,
), 6.81(d, J=9.61Hz, 1H, Ar-H), 7.11(d, J=8.49 Hz, 1H, Ar-H).
2
H, C-CH
2
), 3.32(s, 3H, O-CH
3
2
Ar-O-CH
3
Synthesis of compound 2 Compound 1 (10g 0.06mol) was dissolved in anhydrous
dichloromethane (350mL), and then the solution was putted into a four-neck flask.
Meanwhile, AlCl
atmosphere. The reaction mixture was stirred for 24h at 0℃. Then, the solution was
treated with glacial hydrochloric acid and the organic phase was extracted by CH Cl
and the resulting solution was dried with anhydrous MgSO for 2h. After removal of
3
and acetyl chloride were weighed into the flask under a N
2
2
2
,
4
the solvent under reduced pressure, the crude product was purified by column
chromatography using silica gel. The yield was 71.4% (8.96g). IR(KBr):
-1 1
1
710(C=O)cm . H-NMR(CDCl
3
): δ=2.60(s, 3H, CH -C=O), 2.84(t, J=6.88Hz, 2H,
3
C-CH ), 3.34(s, 3H, O-CH ), 3.57(t, J=6.91Hz, 2H, Ar-CH
2
3
2
), 3,89(s, 3H, Ar-O-CH ),
3

ACCEPTED MANUSCRIPT
6
.90(d, J=8.45Hz, 1H, Ar-H), 7.33(d, J=8.49Hz, 1H, Ar-H), 7.59(s, 1H, Ar-H).
Synthesis of compound 3 Compound 2 (8g 0.038mol) was weighed into a
one-neck round bottom flask (100mL) in a glove box , and then, hydrogen bromide
(
6.76g 0.0836mol) was slowly added. The reaction mixture was refluxed for 2 days.
After the resulting solution was cooled to room temperature, the organic phase was
extracted by CH Cl . Afterward, the solution was dried over anhydrous MgSO , and it
2
2
4
was filtered through Celite. The solvent was removed by a rotary evaporator to give
the crude product, and it was then refined by eluting column chromatography. The
-1
1
yield was 82.5% (7.62g). IR(KBr): 3343(OH), 641(C-Br)cm . H-NMR(CDCl
δ=2.64(s, 3H, CH -C=O), 3.13 (t, J=7.30Hz, 2H, C-CH ), 3.55(t, J=7.31Hz, 2H,
Ar-CH ), 6.94(d, J=8.45Hz, 1H, Ar-H), 7.30(dd, J=2.17Hz, 1H, Ar-H), 7.57(d,
3
):
3
2
2
J=2.14Hz, 1H, Ar-H), 12.19(s, 1H, Ar-OH).
Synthesis of compound 4 To a three-neck flask containing compound 3 (7g
0
.029mol) in 200mL of CH
3
CN, NaI (4.34g 0.029mol) was added under a N
2
atmosphere in standard glove box. The reaction mixture was refluxed for 2 days, and
cooled to room temperature. Then the solid was removed by filtration and washed
three`times with CH
2
Cl . The solution was concentrated under reduced pressure, after
2
that, the resulting product was eluted with petroleum ether and ethyl acetate (v/v, 10:1)
through column chromatography with silica gel. The pure product was 6.96g (82.7%).
IR(KBr): 3462(OH)cm . 1H-NMR(CDCl
J=7.51Hz, 2H, C-CH ), 3.33(t, J=7.51Hz, 2H, Ar-CH
.33(dd, J=2.10Hz, 1H, Ar-H), 7.55(d, J=1.30Hz, 1H, Ar-H), 12.19(s, 1H, Ar-OH).
Synthesis of sensor L1 Compound 4 (6.38g 0.022mol) was placed in a three-neck
flask under a N atmosphere in a glove box, and then, anhydrous ethylenediamine
0.66g 0.011mol) and cyclohexane (150mL) were added. The reaction mixture was
-1
): δ=2.64(s, 3H, CH-C=O), 3.12(t,
3
2
2
), 6.93(d, J=8.52Hz, 1H, Ar-H),
7
2
(
stirred for 24h at 80℃. After the solution was cooled to room temperature, it was
filtered through Celite, and the filtrate was distilled by a rotary evaporator. The
resulting yellow solid was adequately pure without further purification (3.87g 58.2%).
-1 1
IR(KBr): 3481(OH), 1615(C=N)cm . H-NMR(DMSO-d
6
): δ=2.42(s, 3H, CH
3
-C=O),
3
.10(t, J=7.72Hz, 2H, C-CH ), 3.45(t, J=7.72Hz, 2H, Ar-CH
2
2
), 3.94(t, J=7.56Hz, 2H,

ACCEPTED MANUSCRIPT
N-CH
2
), 6.71(d, J=8.44Hz, 1H, Ar-H), 7.18(dd, J=2.00Hz, 1H, Ar-H), 7.58(s, 1H,
1
3
Ar-H), 12.19(s, 1H, Ar-OH). C-NMR(DMSO-d
28.61, 125.91, 117.76, 50.12, 37.29, 14.19, 8.89ppm.
Synthesis of sensor L2 L1 (0.5g 0.828mmol) was dissolved in CH
the temperature of 65℃, anhydrous triethylamine (0.419g 4.14mmol) was added under
atmosphere. The reaction mixture was heated to 81℃and stirred for 48h. After the
solution was cooled to the room temperature, CH CN was removed with a rotary
evaporator to obtain residue. Subsequently, the residue was triturated in diethyl ether.
The obtained yellow power was dissolved in CH CN (50mL), and then AgBF
0.326g 1.66mmol) was putted into the solution under a N atmosphere. After the
6
): δ=166.24, 161.52, 132.63, 129.23,
1
3
CN (50mL) at
N
2
3
3
4
(
2
solution was stirred at room temperature for 24h, it was filtered through Celite. The
solvent was removed under vacuum to give the desired sensor L2. The yield was
-1 1
7
3.6% (0.443g). IR(KBr): 3464(OH), 1645(C=N)cm . H-NMR(DMSO-d
J=7.27Hz, 18H, C-CH ), 2.42(s, 6H, CH -C=O), 3.10(q, J=7.27Hz, 12H, N-CH
.21(t, J=7.72Hz, 4H, C-CH ), 3.54(t, J=7.53Hz, 4H, Ar-CH ), 4.06(t, J=7.35Hz, 4H,
), 6.97(d, J=2.25Hz, 2H, Ar-H), 7.50(dd, J= 2.10Hz, 2H, Ar-H), 7.83(s, 2H,
6
): δ=1.17(t,
3
3
2
),
3
2
2
N-CH
2
1
3
Ar-H), 11.78(s, 2H, Ar-OH). C-NMR(DMSO-d
6
): δ=173.50, 163.95 136.33 129.79,
27.28, 126.12, 118.66, 60.30, 49.04, 45.79, 34.53, 14.63, 8.62ppm.
.4. Analysis
Stock aqueous solutions (1mM) of Na , K , Mg , Al , Zn , Ag , Pb , Co , Li ,
1
2
+
+
2+
3+
2+
+
2+
2+
+
2
+
2+
2+
3+
2+
2+
Ba , Ca , Cd , La , Cu and Mn were prepared from their nitrate salts. Stock
solutions of L1 and L2 (1 mM) were prepared in DMF. Test solutions were prepared
by placing 30 μL of the probe stock solution into cuvettes, adding an appropriate
aliquot of each ions stock, and diluting the solution to 3mL with DMF.
3
3
. Results and discussion
.1. Synthesis
The synthetic route for sensor L1 and sensor L2 is shown in Scheme 1. Firstly,
-(4-Hydroxyphenyl)ethanol was used as the raw material, which all the hydrogens of
2
hydroxyl were substituted by reacting with NaOH and CH I using THF as solvent
3

ACCEPTED MANUSCRIPT
under stirring and refluxing conditions to obtain compound 1. Subsequently, under the
catalysis of AlCl
3
, the acylation reaction of compound 1 and acetyl chloride occurred
in CH Cl under N atmosphere, produced compound 2. In the next step, compound 2
2
2
2
and HBr reacted at elevated temperatures. With the breaking of the ether bond,
compound 3 was gradually formed. In the fourth step, the bromine group of
－
compound 3 was substituted by I in a refluxing solvent of CH
3
CN to give compound
4
. The vital step was the condensation of compound 4 with ethylenediamine to
synthetise the desired salen ligand (sensor L1). The sensor L2 was obtained by the
−
−. These
reaction of the ligand with triethylamine, and then I was exchanged by BF
compounds were characterized by NMR as shown in supporting information.
4
3
.2. Spectral characteristics
The cation binding behaviour of sensor L1 and L2 towards various metal ions was
initially investigated by UV-vis absorption and fluorescence emission spectroscopy.
Fig. 1 displayed the UV-vis absorption of sensors L1 (10μM) and L2 (10μM) in DMF
+
+
2+
3+
2+
+
in presence of various metal ions (1.0 equiv), such as Na , K , Mg , Al , Zn , Ag ,
2
+
2+
+
2+
2+
2+
3+
2+
2+
Pb , Co , Li , Ba , Ca , Cd , La , Cu and Mn . The maximum absorption peak
of L1 is at 331nm. Upon addition of Zn2+ to a solution of L1, the peak has a
red-shifted from 331nm to 338nm and the intensity increased significantly (Fig. 1(a)).
Whereas, addition of other metal ions to L1 produced a negligible absorption change.
2
+
Similarly, addition of tested metal ions to the L2 solution shows that only Zn ion has
obviously response, and the maximum absorption peak of L2 has a red-shifted from
3
31nm to 334nm and the intensity increased significantly (Fig. 1(b)).
The change in fluorescence intensity of L1 and L2 at wavelength of 254nm was
observed with addition of various metal ions (Fig. 2). A bright fluorescence was
clearly visible to naked eye for L1+Zn2+ and L2+Zn2+ solution. And then the
recognition ability of L1 and L2 was investigated by fluorescence emission
spectroscopy.
Likewise, the fluorescence spectra of 10μM solution of L1 and L2 were also
recorded (Fig. 3) in presence of these metal ions (1.0 equiv). The fluorescence
spectrum of L1 in DMF solution exhibited two peaks at 405 nm and 434nm. It was

ACCEPTED MANUSCRIPT
2+
found an evident enhancement of the fluorescent intensity in the presence of Zn ion.
2
+
As shown in Fig. 3(a), upon addition of Zn , the emission peak of L1 at 405nm
showed an increase, and the other emission peak has a red-shifted from 434nm to
4
54nm and the fluorescence intensity increased enormously. At addition of other
+
+
2+
3+
+
2+
2+
+
2+
2+
2+
3+
metal ions, such as Na , K , Mg , Al , Ag , Pb , Co , Li , Ba , Ca , Cd , La ,
Cu2+ and Mn , it was observed that there is no obvious change in fluorescence
intensity. These results revealed that L1 exhibited high fluorescent selectivity to Zn2+
ion more than other metal ions.
2+
The fluorescence spectrum of L2 in DMF solution exhibited two peaks at 405 nm
2
+
and 428nm. Upon addition of Zn , the emission peak of L2 at 405nm showed an
increase, and the other emission peak has a red-shifted from 428nm to 470nm and the
fluorescence intensity increased enormously (Fig. 3(b)). Alike, at addition of other
metal ions, it was observed that there is no obvious change in fluorescence intensity.
2
+
These results revealed that L2 also has highly selective recognition for Zn ion. It can
be seen from the spectral changes that the intermolecular charge transfer (ICT)
process is taking place due to the complexation. These may be that the size, charge
2
+
and electron configuration of Zn and the L1/L2 are very suitable for each other to
form metal complex compared to other metal ions. These results indicate that L1 and
2
+
L2 are an extraordinary selectivity to Zn . At the same time it can be seen that the
2
+
second emission peak of L2 and L2+Zn is red-shifted more and more obvious than
2
+
that of L1 and L1+Zn , which can be due to the quaternary ammonium salts in the
benzene ring.
2
+
In order to study the selectivity of L1 and L2 for Zn , the competitive experiments
2
+
+
+
2+
3+
were carried out in the presence of Zn with other metal ions (Na , K , Mg , Al ,
+
2+
2+
+
2+
2+
2+
3+
2+
2+
Ag , Pb , Co , Li , Ba , Ca , Cd , La , Cu and Mn ) in DMF. As shown in Fig.
and Fig. 5, it is worth noting that the miscellaneous competitive cations did not lead
to any significant spectral change by UV-vis and fluorescence methods, and in the
4
2
+
presence of competitive cations, the Zn ions still resulted in the similar absorption
2
+
2+
changes. It is worth noting that L1 and L2 for Zn haven’t been interfered by Cd
ion, though they have similar properties. This type of selectivity is probably due to the

ACCEPTED MANUSCRIPT
2
+
2+
smaller radius of Zn (0.074nm) and the larger radius of Cd (0.095nm), which
2
+
3+
2+
2+
causes L1 and L2 to recognize only Zn . In addition, Fe , Fe and Hg have only a
2
+
little effect on the L1 or L2 sensing process of Zn ( in Fig. S15 (a) and (b) ).
2
+
3
.3. The binding stoichiometry of L1 and L2 with Zn ion
The fluorescent titration experiments were performed in order to investigate the
2
+
binding characteristics of L1 and L2 toward Zn ion. The fluorescent intensity was
2
+
measured during increasing of Zn ion to L1 and L2. As shown in Fig. 6(a), when the
2
+
ratio of L1 to Zn is 4:1, we can observe that the emission peak appeared a maximum
fluorescence at 447nm and the emission peak was red-shifted from 435nm to 447nm.
2
+
In the same way, the effect of Zn ion concentration on the fluorescence intensity of
L2 (Fig. 6(b)) is similar to that of L1. The results indicated that L1 and L2 showed
2
+
good selectivity and sensitivity toward Zn ion.
2
+
To determine the stoichiometry of L1 and L2 with Zn ion, we implemented Job's
plot. As shown in Fig. 7, the maximum fluorescence was observed when the molar
2
+
fraction of Zn reaches 0.20. Combined Fig. 6 and Fig. 7, it can be concluded that the
2
+
binding mode of L1/L2 and Zn ion is a 4:1 stoichiometry.
1
The H-NMR experiments were performed for more in-depth study of the complex
2
+
structure. According to Fig. 8 (a), for the L1-Zn formation of 4:1 complex, the
phenolic-OH proton (H ) signals weaken, which exhibited that oxygen is binding with
the Zn2+ ions. Aromatic H
, H and H protons become wide. H and H protons
a
b
c
d
e
h
2
+
obviously move to highfield upon the addition of Zn ion (Δδ=0.18, 0.96 ppm,
respectively). And the similar changes can be observed from Fig. 8 (b). The results
2
+
suggested that L1 and L2 may chelate Zn through interactions with the N of imine
and the O of phenolic hydroxyl group. According to these evidences, we can deduce
2
+
2+
the possible complexation mode of L1-Zn and L2-Zn (Scheme 2).
3
.4 Detection limit and binding constant studies
The detection limit was calculated based on the fluorescence titration. The limit of
2
+
detection (LOD) of L1 and L2 for Zn was determined by Stern-Volmer equation (Eq
1)).
(

ACCEPTED MANUSCRIPT
I / I 1 K [Q]
(1)
0
SV
where I
0
and I are the fluorescence emission intensities in the absence and in the
2
+
presence of Zn , respectively; KSV is the Stern-Volmer quenching constant and [Q] is
2
+
2+
the concentration of Zn . In the presence of different concentrations of Zn (5.5, 6,
2
+
6
.5, 7, 7.5, 8, 8.5, 9μM), the Stern-Volmer plot of I
0
/I versus Zn showed a linear
relationship between the fluorescence intensities of L1 and L2 with concentrations of
2
+
2+
Zn . From Fig 9, the LOD of L1 and L2 toward Zn were calculated to be 9.47μM
and 9.53μM, respectively.
2
+
To determine the binding constant of L1 and L2 for Zn , Benesi-Hildebrand
method was applied, which was shown in Fig 10. The binding constants were
calculated by Benesi-Hildebrand equation (Eq (2)).
1
1
1


(2)
I  I0 K (I  I ) I I [Q]
a
max
0
max
0
where I and I
0
represent the emission intensity of L in the presence and absence of
2
+
Zn , respectively; Imax is the saturated emission of L in the presence of an excess
2
+
2+
amount of Zn ; K
seen in Benesi-Hildebrand plots, in the presence of different concentrations of Zn2+
5.5, 6, 6.5, 7, 7.5, 8, 8.5, 9μM), the measured emission intensity [1/(I-I )] varied as a
function of 1/[Zn ] in a linear relationship. According to the calculated results, K of
a
is the binding constant and [Q] is the concentration of Zn . As
(
0
2
+
a
4
-1
4
-1
L1 and L2 was 5.18×10 M and 7.84×10 M , respectively.
3
.5 Comparison experiment
+
BF -
)
In order to study the influence of substituents (-CH
2
CH
2
I and -CH
2
CH
2
N Et
3
4
2
+
on ligand-Zn binding, the compared experiments were carried out in the absence of
these substituents, i.e with simply the diimine of hydroxyacetophenone (L3, as shown
in supporting information). Results of sensing indicated that L3 also has good
2
+
selectivity and sensitivity toward only Zn ion (Fig. S7 and S8). The competitive
experiments showed that other metal ions have no significant influences for L3-Zn2+
(Fig. S9 and S10). According to the fluorescence emission titration experiments, the
2
+
results showed a 4:1 stoichiometry of L3-Zn binding mode (Fig. S11，S12 and S14 ).

ACCEPTED MANUSCRIPT
2
+
The calculated detection limits of L3 was 9.48μM for Zn , and the association
2
+
4
-1
constant (Ka) of L3-Zn was 5.23×10 M (Fig. S13). The above results displayed that
2
+
the substituents have a slight effect on ligand-Zn binding.
3
.6. Cyclic voltammetry experiments
In order to have a further understanding of sensing behavior of the L1 and L2
2
+
toward Zn ion, electrochemical investigations were performed by employing cyclic
voltammetry (CV). Initially, the electrochemical behavior of L1 was investigated in
0
.1M TBAP (tetrabutylammonium perchlorate)/CH
.1 V/s. Fig. 11 (a) showed that the sensor L1 had one oxidation peak (Epa=-0.848V)
=-0.70V, Epc =0.94V, Epc =1.075V) in the potential
range of −1.2 to 1.6 V. After L1 interacted with Zn ions, it caused a distinct behavior
3
CN solution with a scan rate of
0
and three reduction peaks (Epc
1
2
3
2
+
2
+
in the CV. The oxidation peak of L1+Zn were slightly decreased, and had only one
reduction peak (Epc=1.065V). The results confirmed the formation of L1-Zn(II)
complex. As shown in Fig. 11(b), L2 had one oxidation peak (Epa=-0.827V) and two
reduction peaks (Epc
1
=-0.68V, Epc
2
=1.00V) compared with L1, which is due to the
triethylamine group and BF −
ion were introduced. Upon the addition of Zn ion, the
2+
4
2
+
CV of L2+Zn had a lower redox current than L2, but still had one oxidation peak
Epa=-0.863V) and two reduction peaks (Epc =-0.687V, Epc =1.08V). So, we can
come to conclude that L2 can also form complex with Zn ion. All these observations
(
1
2
2
+
2
+
revealed that the sensors L1 and L2 can sensitively recognize Zn ion.
3
.7. The proposed mechanism
Based on all the above experiments, we proposed the sensing mechanism of L1 and
2
+
L2 for Zn . Originally, due to the π-π* transition of the C=N group, an entire
2
+
conjugated molecule leaded to the weak fluorescence intensity. When Zn was added
to the ligands, L1 and L2 may form complexes with Zn2+ by interacting with N of
imine and O of phenolic hydroxyl, and at the same time, the intermolecular charge
transfer (ICT) occurred between receptor and Zn2+ ions, thus enhanced the
fluorescence intensity. Meanwhile, after the formation of the complexes, the
isomerization of C=N was limited, which also contributed to the enhancement of
fluorescence intensity.

ACCEPTED MANUSCRIPT
4
. Conclusions
In summary, we have successfully synthesized two novel chemosensors L1 and L2,
and their recognition capability for various metal ions was investigated. L1 and L2
2
+
exhibited good selectivity, high sensitivity for Zn ion over other metal ions, and can
be used as fluorescence sensor. Job's plot suggested 4:1 complex formation between
Zn2+ and L1/L2. Furthermore, the binding stoichiometry was confirmed by using
1
2+
H-NMR. This study provides a new approach for the recognition of Zn ion.
Acknowledgements
The authors are thankful for financial support from the 211 Project of Anhui
University (J04100301) and Anhui Province Key Laboratory of Enviorment-friendly
Polymer Materials.
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5
OH
OCH3
OCH3
O
OH
O
OH
O
CH3I
Acetyl chloride
AlCl3
HBr
NaI
OH
OCH3
OCH3
Br
I
1
2
3
4
C
N
N
C
Anhydrous triethylamine
AgBF4
Efhylenediamine anhydrous
OH
HO
I
I
L1
C
N
N
C
OH
HO
NEt3BF4
F4BEt3N
L2
Scheme 1. The synthetic route of sensor L1 and L2.

ACCEPTED MANUSCRIPT
C
N
N
C
R
O
O
R
R
R
C
N
C
N
O
O
O
O
Zn
N
C
N
C
R
R
R
O
O
R
C
N
N
C
R=-CH2CH2I
-
CH2CH2N Et3BF4-
+
2
+
Scheme 2. The binding mode of L1/L2 and Zn .
a
0.5
0.4
0.3
0.2
0.1
0.0
+
L1, Ag , Al , Ba ,
3+
2+
2
Ca , Cd , Cu , Co ,
+
2+
2+
2+
+
3+
+
2+
K , La , Li , Mg ,
2
Mn , Na , Pb .
+
+
2+
2
Zn
+
300
400
500
600
Wavelength (nm)

ACCEPTED MANUSCRIPT
b
0.25
0.20
0.15
0.10
0.05
0.00
+
L2, Ag , Al , Ba ,
3+
2+
2
+
2+
2+
2+
Ca , Cd , Cu , Co ,
+
K , La , Li , Mg ,
3+
+
2+
2
+
+
2+
Mn , Na , Pb .
2
Zn
+
-0.05
300
400
500
600
Wavelength (nm)
Fig. 1. UV-vis absorption response of (a) L1 (10μM) and (b) L2 (10μM) upon
addition of various metal ions (1.0 equiv) in DMF solution.
(a)
(b)
Fig. 2. The photograph of (a) L1 (10μM) and L1 upon addition of various metal ions
1.0 equiv) and (b) L2 (10μM) and L2 upon addition of various metal ions (1.0 equiv)
in DMF solution excited with UV lamp at wavelength of 254nm.
(

ACCEPTED MANUSCRIPT
a
800
600
400
200
0
2
Zn
+
+
3+
2+
L1, Ag , Al , Ba ,
2
+
2+
2+
2+
Ca , Cd , Cu , Co ,
+
3+
+
2+
K , La , Li , Mg ,
2
+
+
2+
Mn , Na , Pb .
400
450
500
550
600
650
Wavelength (nm)
b
1400
1200
1000
2+
Zn
+
3+
2+
L2, Ag , Al , Ba ,
800
600
400
200
0
2+
2+
2+
2+
Ca , Cd , Cu , Co ,
+
3+
+
2+
K , La , Li , Mg ,
2
+
+
2+
Mn , Na , Pb .
400
450
500
550
600
650
Wavelength (nm)
Fig. 3. Fluorescence spectra of (a) L1 (10μM) and (b) L2 (10μM) upon addition of
various metal ions (1.0 equiv) in DMF solution.

ACCEPTED MANUSCRIPT
a
2+
L1+Zn
2+
+
L1+Zn +Ag
2+
3+
L1+Zn +Al
2+
2+
2+
L1+Zn +Ba
2+
0.4
0.2
0.0
L1+Zn +Ca
2+
2+
2+
2+
L1+Zn +Cd
2+
L1+Zn +Co
2+
L1+Zn +Cu
2+
+
L1+Zn +K
2+
3+
L1+Zn +La
2+
+
L1+Zn +Li
2+
2+
2+
L1+Zn +Mg
2+
L1+Zn +Mn
2+
+
L1+Zn +Na
2+
2+
L1+Zn +Pb
300
350
400
450
500
550
600
Wavelength (nm)
2
+
b
L2+Zn
2+
+
0.30
0.25
0.20
0.15
0.10
0.05
0.00
L2+Zn +Ag
2+
3+
L2+Zn +Al
2+
2+
L2+Zn +Ba
2+
2+
L2+Zn +Ca
2+
2+
L2+Zn +Cd
2+
2+
L2+Zn +Co
2+
2+
L2+Zn +Cu
2+
+
L2+Zn +K
2+
3+
L2+Zn +La
2+
+
L2+Zn +Li
2+
2+
2+
L2+Zn +Mg
2+
L2+Zn +Mn
2+
+
L2+Zn +Na
2+
2+
L2+Zn +Pb
-0.05
300
350
400
450
500
550
600
Wavelength (nm)
Fig. 4. UV-vis absorption response of (a) L1 (10μM) and (b) L2 (10μM) containing
2
+
Zn (1.0 equiv) and other competitive metal ions (1.0 equiv) in DMF solution.

ACCEPTED MANUSCRIPT
2
2
+
+
L1+Zn
a
+
L1+Zn +Ag
800
600
400
200
0
2+
3+
L1+Zn +Al
2
+
2+
L1+Zn +Ba
2
+
2+
L1+Zn +Ca
2
+
2+
L1+Zn +Cd
2
+
2+
2+
L1+Zn +Co
2
+
L1+Zn +Cu
2
+
+
L1+Zn +K
2+
3+
L1+Zn +La
2
+
+
L1+Zn +Li
2
+
2+
2+
L1+Zn +Mg
2
+
L1+Zn +Mn
2
+
+
L1+Zn +Na
2
+
2+
L1+Zn +Pb
400
450
500
550
600
650
700
Wavelength (nm)
2
2
+
+
b
L2+Zn
+
L2+Zn +Ag
2
+
3+
L2+Zn +Al
1
1
1
400
200
000
2
+
2+
2+
2+
L2+Zn +Ba
2
+
L2+Zn +Ca
2
+
L2+Zn +Cd
2
+
2+
2+
L2+Zn +Co
2
+
L2+Zn +Cu
8
6
4
2
00
00
00
00
0
2+
+
L2+Zn +K
2
+
3+
L2+Zn +La
2+
+
L2+Zn +Li
2
+
2+
2+
L2+Zn +Mg
2
+
L2+Zn +Mn
2
+
+
L2+Zn +Na
2
+
2+
L1+Zn +Pb
400
450
500
550
600
650
700
Wavelength (nm)
Fig. 5 Fluorescence spectra of (a) L1 (10μM) and (b) L2 (10μM) containing Zn2+
(1.0 equiv) and other competitive metal ions (1.0 equiv) in DMF solution.

ACCEPTED MANUSCRIPT
a
1
9
8
7
6
5
4
3
2
1
0:0
:1
:2
:3
:4
:5
:6
:7
:8
:9
4000
3000
2000
1000
0
2
+
(
L1:Zn )
400
450
500
550
600
Wavelength (nm)
b
1
9
8
7
6
5
4
3
2
1
0:0
5
4
3
2
1
000
:1
:2
:3
:4
:5
:6
:7
:8
:9
000
000
000
000
0
2
+
(
L2:Zn )
400
450
500
550
600
Wavelength (nm)
Fig. 6. Stepwise changes in fluorescence emission spectra of (a) L1 (10 μM) and (b)
2
+
L2 (10 μM) with increasing concentrations of Zn .

ACCEPTED MANUSCRIPT
a
4000
3000
2000
1000
0
0.0
0.2
0.4
0.6
0.8
1.0
2
Zn ]/[L1]+[Zn ]
+
2+
[
b
6
000
000
000
000
000
000
0
5
4
3
2
1
0.0
0.2
0.4
0.6
0.8
1.0
2
Zn ]/[L2]+[Zn ]
+
2+
[
2
+
Fig.7. Job's plot for (a) L1 and (b) L2 upon addition of Zn ion
.

ACCEPTED MANUSCRIPT
a
h
e
C
N
N
C
b
f
OH
HO
g I
a
d c
I
2
+
L1+Zn
L1
a
g
b c d
e
f
h
12
10
8
6
4
2
ppm
b
i
e
C
N
N
C
b
f
OH
HO
g NEt3BF4d c
a
F4BEt3N
h j
2
+
L2+Zn
h
i
j
L2
b c d
e f
g
a
12
10
8
6
4
2
ppm
1
2+
Fig. 8. H-NMR spectral changes of (a) L1 and (b) L2 upon addition of Zn ion
DMSO-d6).
(

ACCEPTED MANUSCRIPT
a
0.6
0.5
0.4
0.3
0.2
0.1
0.0
y = -0.1457x + 1.3799
2
R = 0.9934
5.0
5.5
6.0
6.5
7.0
[
7.5
8.0
8.5
9.0
9.5
2+
Zn ]
b
0
0
0
0
0
0
0
0
0
.8
.7
.6
.5
.4
.3
.2
.1
.0
y = -0.178x + 1.6979
2
R = 0.9927
5.0
5.5
6.0
6.5
7.0
7.5
8.0
8.5
9.0
9.5
2
+
[
Zn ]
Fig. 9 Stem-Volmeere pots of (a) L1 (λex=416nm) and (b) L2 (λex=441nm) for
estimation of enhancement constants at different concentrations of Zn2+

ACCEPTED MANUSCRIPT
a
0
0
0
0
0
0
0
.0045
.0040
.0035
.0030
.0025
.0020
.0015
y = 0.0328x - 0.0017
2
R = 0.9932
0.10
0.12
0.14
0.16
0.18
0.20
2
+
1
/[Zn ]
b
0.007
0.006
0.005
0.004
0.003
0.002
y = 0.0676x - 0.0053
2
R = 0.9921
0.10
0.12
0.14
0.16
0.18
2
+
1
/[Zn ]
Fig. 10. Benesi-Hildebrand plots to calculate the association constant of (a) L1
2
+
(λex=416nm) and (b) L2 (λex=441nm) towards Zn .

ACCEPTED MANUSCRIPT
a
L1
2
+
L1+Zn
0.00008
0.00004
0.00000
-
0.00004
0.00008
-
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential(V)
b
L2
0.00010
0.00005
0.00000
2
+
L2+Zn
-0.00005
-0.00010
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
2.0
Potential(V)
2
+
Fig. 11. CV profile of (a) L1 and (b) L2 after complexation with Zn ion.
.

ACCEPTED MANUSCRIPT
Graphical abstract
C
N
N
C
R
O
O
R
R
R
C
N
C
N
O
O
O
O
Zn
N
C
N
C
R
R
R
O
O
R
C
N
N
C
R=-CH2CH2I
-
CH2CH2N Et3BF4-
+
2
+
2+
The possible complexation mode of L1-Zn and L2-Zn

ACCEPTED MANUSCRIPT
Highlight
Two salen based chemosensors for highly selective recognition of Zn ion
2
+