A new Fe3+ fluorescent chemosensor based on aggregation-induced emission

Article abstract of DOI:10.1016/j.tetlet.2014.05.008

A new tetraphenylethylene (TPE)-based sensor M1 bearing double 2-methylpyridyl-2-methylthiophenylamino units linked with triazole moieties was reported. Both UV-vis and fluorescence spectroscopic studies demonstrated that M1 was highly sensitive and selec

Full text of DOI:10.1016/j.tetlet.2014.05.008

Accepted Manuscript
A new Fe3+ fluorescent chemosensor based on aggregation-induced emission
Jia-Hai Ye, Jian Liu, Zhenghua Wang, Yang Bai, Wenchao Zhang, Weijiang
He
PII:
S0040-4039(14)00784-9
DOI:
http://dx.doi.org/10.1016/j.tetlet.2014.05.008
TETL 44598
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
3 February 2014
22 April 2014
5 May 2014
Please cite this article as: Ye, J-H., Liu, J., Wang, Z., Bai, Y., Zhang, W., He, W., A new Fe3+ fluorescent chemosensor
based on aggregation-induced emission, Tetrahedron Letters (2014), doi: http://dx.doi.org/10.1016/j.tetlet.
2014.05.008
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Graphical Abstract
A new Fe3+ fluorescent chemosensor based
on aggregation-induced emission
Jia-Hai Ye, Jian Liu, Zhenghua Wang, Yang Bai, Wenchao Zhang, Weijiang He
N
N
S
N
Fe3+
N
N
S
N
N
N
N
N
M1

1
Tetrahedron Letters
jo u rn al h ome p ag e : w ww .e lse vie r .c om
A new Fe3+ fluorescent chemosensor based on aggregation-induced emission
Jia-Hai Ye a,∗, Jian Liu a, Zhenghua Wang a, Yang Bai b, Wenchao Zhang a, Weijiang He b,*
a School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China
b State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing, 210093, China
ARTICLE INFO
ABSTRACT
Article history:
Received
A new tetraphenylethylene(TPE)-based sensor M1 bearing double 2-methylpyridyl-2-
methylthiophenylamino units linked with triazole moieties was reported. Both UV-vis and
fluorescence spectroscopic studies demonstrated that M1 was highly sensitive and selective
towards Fe3+ over other metal ions in THF/H2O solution based on the aggregation-induced
emission quenching mechanism. The lowest detection limit of M1 for Fe3+ is 0.7 µM. The
detailed fluorescent titration study suggested that the binding stoichiometry of the M1-Fe3+
complex was 1:2, and the structure between M1 and the Fe3+ complex was confirmed by the 1H
NMR titration.
Received in revised form
Accepted
Available online
Keywords:
Fluorescence
Aggregation-induced emission
Fe3+
2014 Elsevier Ltd. All rights reserved.
Tetraphenylethylene
Chemosensor
Recently, our group found that a new TPE derivative based on
the click linkage of TPE core with multiple-pyridine substituents
exhibited good AIE property of the highly selective response to
Ag+.12 In addition, on the basis of the strong affinity between
metal ions and sulfur, some fluorescent probes bearing thiophene
groups have been designed and synthesized for the sufficient
detection of the metal ions.11b,13 The TPE core and thiophene
units were introduced into the probe structure. With these in
1. Introduction
Over recent decades, the fluorescence-based detection of
heavy and transition metal ions based on the has received
increasing attention due to the importance of metal ions in the
chemistry, biology and environment duet to its advantages of
simplicity, high sensitivity and selectivity.1 Among the heavy and
transition metal ions, Fe3+, as a physiologically important metal
ion, plays a catalytic role in chemical and biological processes
such as oxygen metabolism and electron transfer, and both its
deficiency and excess in the human body can induce a variety of
diseases.2,3,4 Up to now, only a few fluorescence probes for Fe3+
have been achieved.5
mind, we designed and synthesized
a new fluorescence
chemosensor M1. Compound M1 is composed of two moieties,
one is an AIE fluorophore, for which TPE platform is selected as
the AIE signal transducer. The other moiety is the Fe3+-response
switch, for which 2-methylpyridyl-2-methylthiophenylamino unit
is chosen as the modulator.
In 2001, Tang and co-workers first reported aggregation-
induced emission (AIE)-active molecules.6 AIE-active molecules
display no emission in solution, but an intense emission when
aggregated or in the solid state because of suppression of
nonradiative deactivation associated with restriction of
intramolecular rotations.7,8 Based on the aggregation state-
dependent fluorescence, many AIE-active molecules, such as
tetraphenylethene (TPE) derivatives, can be used as an
alternative sensing element through the introduction of functional
groups into AIE molecules which favor new development of
chemosensors for detecting metal cations, biomolecules, organic
vapors, chiral molecules, and explosives.9 Up to now, various
fluorescent probes for Fe3+ have been achieved,10 but the AIE-
based fluorescent probes were very rare.11 Herein, we report the
design and synthesis of a new fluorescent chemosensor for the
2. Results and discussion
The synthetic pathway of compound M1 is outlined in
Scheme 1. The target compound M1 was synthesized from the
substrates 1 and 2 through click reaction efficiently (Scheme
1).12,14 The chemical structure of compound M1 was
characterized by spectroscopic and elemental analysis data.
Compound M1 is soluble in common organic solvents such as
acetonitrile, chloroform, ethyl ethanoate, and tetrahydrofuran
(THF), but insoluble in water. We first investigated the AIE
behavior of compound M1 in aqueous THF with different
THF/water ratios. As shown in Figure 1, when the water fraction
in dilute THF solution is increased from 0% to 50%, compound
M1 shows almost no fluorescence as expected. However, its
suspension in a THF–water mixture with a high water fraction is
highly emissive when the water fraction is increased over 60%.
highly selective detection of
tetraphenylethene (TPE) bearing pyridine and thiophene
substituents based on click chemistry.
Fe3+ using AIE-active
———
∗ Corresponding author. Tel.: +86-25-8430-3116; fax: +86-25-8431-5857; e-mail: yejiahai@njust.edu.cn (J.-H. Ye)

Based on above AIE behavior of compound M1, the UV-Vis and
fluorescence experiments were carried out in THF/H2O (1:2, v:v)
aqueous solution. As shown in Figure S2 (Supplementary data),
the UV-visible absorption titration of the Fe3+ ion were carried
out using a solution of 10 µM of compound M1 in THF/H2O (1:2,
v:v), which exhibited two absorption bands at 316 nm and 238
nm. The absorption spectral increasing of compound M1 upon
the addition of Fe3+ was observed. Meanwhile, compound M1
shows two fluorescent emission peaks at 408 nm and 432 nm.
Upon addition of 4 equiv of different metal ions respectively, the
significant fluorescence decrease was detected only after the
addition of Fe3+ which disturbed the aggregation of compound
M1 in THF/H2O (1:2, v:v) solution through the coordination of
Fe3+ ion with pyridine and thiophene units to free the
intramolecular rotations of compound M1. The other two metal
ions, Ag+ and Cu2+, cause a slight fluorescence quenching of
compound M1 (Fig. 2).
respectively. A plot of ∆F (F0-Fi) versus the molar fraction of
Fe3+ ion is provided in Figure 6. It shows that the ∆F value goes
through a maximum at a molar fraction of about 0.67, exhibiting
a 2:1 stoichiometry of the Fe3+ to M1 in the complex. The
equilibrium constant K was calculated to be 7.05 × 1015,
suggesting a good binding affinity of the probe M1 to Fe3+
(Supplementary data Fig. S14).
To further elucidate the recognizing mechanism of compound
M1 for Fe3+ based on the structure of the M1-2Fe3+ complex, the
1H NMR titration experiment was performed by the addition of
FeCl3 solution (in D2O and from 0 to 0.4, 0.8, 1.6, and 2.0 mM
respectively) to the solution of M1 (0. 1 mM in THF-d8). As
shown in Figure 7, the signal of the Ha proton was gradually
upfield shifted from 8.43 ppm to 8.20 ppm with increasing the
amounts of Fe3+ (from 0 to 0.8 equiv). When the amounts of Fe3+
increased from 0.8 equiv to 2.0 equiv, the signal of the Ha proton
were slightly downfield shifted from 8.20 ppm to 8.27 ppm.
Similarly, the signal of the Hg proton was upfield shifted from
5.44 ppm to 5.34 ppm followed by downfield shifted from 5.34
ppm to 5.50 ppm. On the contrast, the signals of the Hc, Hd, and
He protons were gradually downfield shifted with increasing the
amounts of Fe3+ from 0 to 2.0 equiv. Meanwhile, the signals of
the Hb and Hf protons were downfield shifted initially, and were
then upfield shifted with the increasing the amounts of Fe3+. All
the signal variations of the protons suggested that the chemical
circumstances of them have been changed due to the
coordination between M1 and Fe3+. The facts of the changes of
the signals illustrate that the N-atom on pyridine unit and the S-
atom on thiophene unit bond to the metal ion. The chemical shift
of proton in triazole unit in our previous reported AIE-based Ag+
chemosensor exhibits downshift when the Ag+ ion concentration
was not more than 1 equiv, and the chemical shift of proton in
triazole unit exhibits upshift when the Ag+ ion concentration was
more than 1 equiv.12 In contrast, the chemical shift of proton in
triazole unit in this Fe3+ chemosensor M1 exhibits downshift
followed by slight upshift. Based on these results, the
coordination type of probe M1 with Fe3+ was proposed in
Scheme 2. Furthermore, the above AIE recognizing mechanism
was confirmed employing EDTA as a much stronger chelating
agent. As shown in Figure 8, the fluorescent intensity of M1-Fe3+
system can be recovered to the original value of M1 in a
THF/H2O (v:v) mixture when treated with EDTA. This indicates
that the fluorescence quenching of M1 induced by the chelating
Fe3+ is due to the formation of coordination complexes between
M1 and Fe3+ ions. Excess EDTA can snatch the Fe3+ ions from
the chelators of M1 and interrupt the interaction between the Fe3+
ions and the chelators of M1 to form new more stable complexes
with a much higher stability. Due to the much stronger complex
binding between EDTA and Fe3+ ions, the M1-Fe3+ complex is
destroyed, and the fluorescence of M1 switches on in the
aggregated form.
The fluorescence spectra of compound M1 upon titration with
Fe3+ ion was recorded in the next experiment for the investigation
of the AIE quenching behavior of Fe3+ ion in details. As shown in
Figure 3, The fluorescence of compound M1 (10 µM) in
THF/H2O (1:2, v:v) at 408 nm and 432 nm was dramatically
decreased with the virtually unchanged emission shift upon
addition of Fe3+. The changes of the emission intensities became
constant and 95% of the quenched fluorescence of M1 was
observed eventually when the amount of Fe3+ added reached 2.0
equiv (20 µM), and there was a good linear relationship in this
range (R2 = 0.997). Moreover, the detection limit was measured
to be 0.7 µM (Supplementary data Fig. S6).
For practical applicability, the effect of the pH on the
fluorescence of M1 was investigated in THF/H2O (1:2, v:v). As
shown in Figure 4, no change of the fluorescence emission of M1
was observed between pH 2 and 11, suggesting that the
compound M1 is stable over a wide pH range. In addition, no
obvious change of the fluorescence emission of M1 upon the
addition of 4 equiv of Fe3+ was observed in the same pH range,
which shows that the compound M1 is the potential fluorescent
sensor for the detecting of Fe3+.
Then, a competition experiment was carried out by adding
Fe3+ to the THF/H2O (1:2, v:v) solution of M1 in the presence of
other metal ions, and the results are shown in Figure 5. The
fluorescence response to the addition of these commonly
coexistent ions (Ag+, Ba2+, Ca2+, Cd2+, Co2+, Cu2+, K+, Li+, Na+,
Mg2+, Mn2+, Ni2+, Pb2+, Fe2+, Zn2+, and Hg2+) was hardly affected
by the presence of Fe3+ suggesting the exclusive sensitivity of
M1 towards Fe3+ rather than other transition-metal ions
investigated. On the other hand, the higher selectivity of M1 for
Fe3+ over Fe2+ shows that this probe has metal and redox
-
specificity. In addition, the anions such as Cl-, Br-, F-, NO3 , ClO4-
2-
and SO4 show almost no any effect on the fluorescence
quenching of compound M1 (Supplementary data Fig. S7).
3. Conclusion
Similar to other reported TPE-based fluorescent probes, the
AIE quench response of probe M1 toward Fe3+ is the result of the
intermolecular coordination of probe M1 with Fe3+ by its two
chelators bearing pyridine and thiophene units, which have been
proven to show strong affinity to Fe3+.10g-h,10k,10m,11b The
coordination will release the restriction for the intramolecular
rotations of compound M1 under aggregation state to display a
remarkable fluorescence quenching. In order to quantify the
stoichiometry of the complex of M1 and Fe3+, a Job’s plot
analysis was carried out, by keeping the sum of the initial
concentration of Fe3+ ion and 1 at 20 µM, and changing the molar
ratio of Fe3+ ion from 0.1 to 0.9. The fluorescence of M1 in the
absence (F0) or presence (Fi) of Fe3+ ion was determined
In conclusion,
a new tetraphenylethylene (TPE)-based
chemosensor M1 bearing triazole, pyridine and thiophene motifs
was designed and prepared successfully. Compound M1
demonstrates a highly sensitive and selective sensing behavior
for Fe3+ in THF/H2O solution based on the AIE quenching
mechanism. The experiment results including the lower detection
limit (0.7 µM) indicate that compound M1 can be promoted for a
lot of practical applications in chemical, environmental and
biological systems.

3
ng, L.; Yang, W.; Xu, D. -M.; Zhang, Z. -Y.; Liu, A. -F. Dyes and pi
Acknowledgments
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This project was funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions (PAPD)
Supplementary data
Supplementary material associated with this article can be
found, in the online version, at doi:
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7x105
6x105
5x105
4x105
3x105
N
3
7x105
6x105
5x105
4x105
3x105
2x105
N
1
2
+
N
S
2x105
1x105
0
N
3
Click reaction
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3+
[Fe ]/[M1]
3+
R=[Fe ]/[M1] from
N
0 to 3 eq
N
S
N
N
N
S
1x105
0
N
N
N
N
N
M1
400
450
500
550
600
Wavelength (nm)
Scheme 1. Synthesis of Sensor M1
Figure 3. Fluorescence intensity of compound M1 (10 µM) in
THF/H2O (1:2, v/v) in the presence of Fe3+ (0 to 30 µM, 0 to 3
equiv). Inset: plot of fluorescence intensity of compound M1
towards the concentration of Fe3+ (λem = 431 nm, λex = 342 nm).
10 µM M1
3+
10 µM M1+4 eq Fe
7x105
6x105
5x105
Figure 1. The aggregation-induced emission (AIE) effect (365 nm
UV lamp) of M1 in different ratios of THF and H2O (THF/H2O, v/v)
4x105
3x105
2x105
1x105
0
8x105
-1x105
-2x105
7x105
6x105
5x105
4x105
3x105
2x105
2+ 2+ 2+ 2+ + + 2+
2+
M1, Ba , Ca , Cd , Co , K , Li , Hg , Mg ,
2
4
6
8
10
12
2+
Mn , Na , Ni , Fe , Pb , Zn , Ce , Al , Cr
+
2+
2+
2+
3+ 3+
3+
2+
PH
Figure 4. Fluorescence intensity (at 431 nm) of free M1 (10 µM)
and in the presence of 4 equiv Fe3+ in KOH/HNO3 water solution
with different pH conditions (λem = 431 nm, λex = 342 nm).
+
Ag
2+
Cu
3+
Fe
1x105
0
M1+Other metal ions
400
450
500
550
600
3+
M1+Other metal ions+ Fe
Wavelength (nm)
7x105
6x105
5x105
4x105
3x105
2x105
Figure 2. The Fluorescence spectra of compound M1 (10 µM) in
THF/ H2O (1:2, v/v) upon addition of different metal ions (40 µM, 4
equiv). All metal ions were added separately (λem = 431 nm, λex
342 nm).
=
1x105
0
3+
3+
3+
Ce
+
Ag
2+
2+
Ca
2+
Cd
2+
Co
2+
+
K
+
2+
2+
+
2+
Ni
2+
2+
Zn
2+
3+
Fe
2+
Fe
Ba
Cu
Li
Mg
Mn
Na
Pb
Hg
Cr
Al
Figure 5. Fluorescence responses of M1 (10 µM) upon addition of
various metal ions (100 µM, 10 equiv) in the absence and presence
of Fe3+ (40 µM, 4 equiv) in THF/H2O (1:2, v/v) media. (Black bars:
compound M1 with other metals, red bars: compound M1 with other
metals and Fe3+ (λem = 431 nm, λex = 342 nm).

5
N
N
N
N
N
S
N
S
7x 1 0 5
6x 1 0 5
5x 1 0 5
4x 1 0 5
3x 1 0 5
2x 1 0 5
N
N
N
N
M1
Fe3+
1.0 equiv.
N
N
N
S
N
1x 1 0 5
N
n
N
Fe
0
N
-1x 1 0 5
S
N
0 .0
0 .2
0 .4
3+
0 .6
3+
]+ [M 1]
0 .8
1.0
N
N
[Fe
]/[Fe
M1-1Fe3+
Figure 6. Job plot of compound M1 with Fe3+ ([M1] + [Fe3+] = 20
µM) in THF/H2O (1:2, v/v). The ordinate △F = F0-Fi represents the
change of the Fluorescence intensity, while the abscissa represents
the ratio of [Fe3+] in [Fe3+]+[M1] which represents the total
concentration. (λex = 342 nm, λem = 431 nm)
Fe3+ 2.0 equiv.
1.0 equiv.
N
N
N
N
Fe
S
N
N
H
b
S
Fe
N
H
N
a
N
S
N
N
N
N
N
H
N
S
M1-2Fe3+
d
H
i
N
N
N
N
N
Scheme 2. Proposed binding mode of M1 with Fe3+
H
c
H
M1
e
H
g
H
f
7x105
M1
6x105
5x105
4x105
3x105
2x105
1x105
0
3+
M1+Fe +EDTA
3+
M1+Fe
400
450
500
550
600
Wavelength (nm)
Figure 8. Fluorescence spectra of M1 (10 µM), M1-Fe3+ mixture
(Fe3+, 40 µM, 4 equiv), and M1-Fe3+ mixture with addition of EDTA
(EDTA, 60 µM, 4 equiv) in THF/H2O (1:2, v/v) at room temperature.
(λex = 342 nm, λem = 431 nm)
Figure 7. Partial 1H NMR spectra (500 MHz) of compound M1 (0.1
mM) in THF-d8 in the presence of increasing amounts of Fe3+ (in
D2O): 0, 0.4, 0.8, 1.6, 2.0 equiv [from (1) to (5)].

Graphical Abstract
A new Fe3+ fluorescent chemosensor based
on aggregation-induced emission
Jia-Hai Ye, Jian Liu, Zhenghua Wang, Yang Bai, Wenchao Zhang, Weijiang He
N
N
S
N
Fe3+
N
N
S
N
N
N
N
N
M1

7
Highlights
(1) A new fluorescent chemosensor for Fe3+
was developed.
(2) Detection of Fe3+ in aqueous solution
(THF/H2O)
successfully based
on
aggregation-induced emission mechanism.
(3) High selectivity and sensitivity with low
detection limitation (0.7 µM).
(4) The
recognizing
mechanism
was
investigated through fluorescence titration
and 1H NMR titration.