Detection of Picric Acid by Terpy-Based Metallo-Supramolecular Fluorescent Coordination Polymers in Aqueous Media

Article abstract of DOI:10.1002/cjoc.201600885

The synthesis of two linear terpy-based metallo-supramolecular fluorescent coordination polymers through 1∶1 complexation of Zn²⁺ and Cd²⁺ ions with ditopic terpyridine ligand was reported. The dispersibility of P1 and P2 was significantly improved in organic solvent and water through the introduction of hydrophilic oligo-ethyoxy side chain. Two polymers displayed yellow light emission both in solution and the solid state due to the intra-ligand charge transfer (ILCT) between the d¹⁰ metal ions and the conjugated spacer unit. These coordination polymers were explored as fluorescent chemosensors for detecting picric acid in aqueous media, displaying high sensitivity and good selectivity. In addition, test strips were prepared from these polymers and exhibited the practical potential of detecting the NACs pollutants in the outdoor water for public safety and security.

Full text of DOI:10.1002/cjoc.201600885

FULL PAPER
DOI: 10.1002/cjoc.201600885
Detection of Picric Acid by Terpy-Based Metallo-
Supramolecular Fluorescent Coordination Polymers in
Aqueous Media
Zhongyu Ding,a Hongqing Li,a Wanqing Gao,a Yiquan Zhang,b Chunhua Liu,*,a
and Yuanyuan Zhu*,a
a School of Chemistry and Chemical Engineering, Hefei University of Technology and Anhui Key Laboratory of
Advanced Functional Materials and Devices, Hefei, Anhui 230009, China
b Jiangsu Provincial Key Laboratory for Numerical Simulation of Large Scale Complex Systems (NSLSCS), School
of Physical Science and Technology, Nanjing Normal University, Nanjing, Jiangsu 210023, China
The synthesis of two linear terpy-based metallo-supramolecular fluorescent coordination polymers through 1∶
＋
＋
1 complexation of Zn2 and Cd2 ions with ditopic terpyridine ligand was reported. The dispersibility of P1 and
P2 was significantly improved in organic solvent and water through the introduction of hydrophilic oligo-ethyoxy
side chain. Two polymers displayed yellow light emission both in solution and the solid state due to the intra-ligand
charge transfer (ILCT) between the d10 metal ions and the conjugated spacer unit. These coordination polymers
were explored as fluorescent chemosensors for detecting picric acid in aqueous media, displaying high sensitivity
and good selectivity. In addition, test strips were prepared from these polymers and exhibited the practical potential
of detecting the NACs pollutants in the outdoor water for public safety and security.
Keywords coordination polymer, fluorescent chemosensors, nitroaromatic explosives, picric acid, aqueous media
Introduction
π-electron-rich fluorescent organic polymers,[8] coordi-
nation polymers (CPs)/metal-organic frameworks
(MOFs),[9] supramolecular polymers,[10] organic mole-
cules,[11] and nanostructures[12] have been employed to
detect the presence of electron-deficient nitroaromatics.
However, most detection methods were performed in
organic solution. Therefore, the development of suitable
soluble and efficient organic fluorescent chemosensors
(FCs) with high sensitivity and selectivity for PA in
aqueous media is still a very challenging task.
Fluorescence is a simple but powerful signal trans-
duction method for the formation of chemosensory de-
vices.[1] Recently, the development of new efficient flu-
orescence quenching “turn-off” methods for the detec-
tion of nitroaromatic explosives (NACs), especially for
picric acid (PA), has received great research interest.[2]
Besides, as a powerful explosives,[3] PA is also widely
used in dye and pharmaceutical industries and in chem-
ical laboratories.[4] Because of its excellent solubility in
water, it can easily contaminate outdoor water and
groundwater when exposed. Medical research demon-
strated that intake of PA may cause skin and eye irrita-
tion and other chronic diseases.[5] Hence, for preventing
terrorist threats and environmental pollution, develop-
ment of efficient sensors to detect PA and other NACs at
very low concentrations is a very appealing and urgent
research area in scientific society.[6]
Metallo-supramolecular coordination polymers be-
long to a new type of organic/inorganic hybrid material
that combines the common features of traditional cova-
lently bound polymers with the properties imposed by
coordination metal ions.[13] Among them, d10 transition
metals (such as Zn2 and Cd2＋) based coordination
＋
polymers can display excellent photoluminescent (PL)
electroluminescent (EL) properties. In these coordina-
tion polymers, the intra-ligand charge transfer (ILCT)
between the d10 coordination site and the chromophore
of the ligand generates strong luminescence and the
wavelength and intensity of emission can be fine tuned
through the structural design of ligands. Several exam-
ples demonstrated that the mediation of emission color
Currently, various methods have been established for
the detection of NACs. Among them, fluorescence
sensing is widely investigated because of its advantages
of high sensitivity, quick response, low cost efficiency,
easy sample preparation, etc.[7] Up to date, several
*
E-mail: yyzhu@hfut.edu.cn, lchh88@hfut.edu.cn
Received December 13, 2016; accepted February 22, 2017; published online April 10, 2017.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201600885 or from the author.
Dedicated to Professor Xi-Kui Jiang on the occasion of his 90th birthday.
Chin. J. Chem. 2017, 35, 447—456
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FULL PAPER
and enhanced luminescence could be achieved through
5.5 g) in glacial acetic acid (50 mL) under stirring, bro-
mine (0.1 mol, 5.2 mL) in glacial acetic acid (5 mL) was
carefully added dropwise. After the completion of add-
ing, the solution was still stirred at ambient temperature
for 6 h. Then the resulting solution was poured in water
(100 mL) and extracted from ethyl acetate (50 mL×3).
The combined organic layer was dried over anhydrous
Na2SO4 and concentrated under reduced pressure. The
resulting solid was washed with a little glacial acetic
acid to give the product as a white solid (10.85 g, 81%).
1H NMR (600 MHz, DMSO-d6) δ: 9.84 (s, 2H), 7.04 (s,
2H).
rational ligand design.[14]
The luminescence of coordination polymers will
undergo “turn-off” changes caused by electron or ener-
gy transfer from excited-state of electron-rich fluoro-
phores to ground-state of electron-deficient nitrated or-
ganic compounds.[15] So we proposed to use soluble
luminescent coordination polymers serving as FCs for
detection of trace NACs. In previous work, we synthe-
sized a series of Zn(II) metallo-supramolecular coordi-
nation polymers based on tridentate pyridine-2,6-bis-
(oxazoline) ligands and demonstrated the fluorescence
tuning and the highly sensitive and selective detection
of trace PA both in organic solution and the solid state.
These types of polymers, however, are unstable in polar
solvents including DMF, DMSO, and H2O.[16] This dis-
advantage impedes the practical application of detection
in aqueous media. In order to solve this problem, we
propose using strong coordinated terpydine (terpy) to
replace pybox to enhance the stability of polymers in
polar solvents and introducing tetraethyleneglycol
monomethyl ether side chain into the ligand to improve
the solubility of polymers in aqueous media.[17,18] In this
contribution, we report on the synthesis of two terpy
based metallo-supramolecular fluorescent coordination
polymers and the achievement of PA detection in aque-
ous media.
Synthesis of 2[20]
10.4 g (50 mmol) of tetraethyleneglycol monomethyl
ether was taken into THF (25 mL) and cooled in an wa-
ter-ice bath. To the cooled solution, sodium hydroxide
(6.0 g, 150 mmol) in water (25 mL) was added. p-Tosyl
chloride (12.4 g, 65 mmol) in THF (15 mL) was then
added slowly (over 30 min). Then the reaction solution
was allowed to warm up to ambient temperature and
stirred overnight. After completion of reaction, the sep-
arated organic layer was collected and the aqueous layer
was extracted twice with diethyl ether (25 mL×2). The
combined organic layer was washed with 10% sodium
hydroxide solution, followed by water and saturated
brine. Then the organic phase was dried over anhydrous
Na2SO4, filtered and concentrated to obtain the pure
product as colorless viscous liquid in nearly quantitative
yield. 1H NMR (400 MHz, CDCl3) δ: 7.78 (d, J＝8.0 Hz,
2H), 7.33 (d, J＝8.0 Hz, 2H), 4.14 (t, J＝4.8 Hz, 2H),
3.73 (t, J＝6.4 Hz, 2H), 3.67 (t, J＝4.8 Hz, 2H), 3.63－
3.61 (m, 6H), 3.57 (s, 4H) , 3.54－3.52 (m, 2H), 3.36 (s,
3H), 2.43 (s, 3H).
Experimental
Materials and equipments
Melting point was uncorrected. The NMR spectra
were performed on a Bruker 400 MHz or 600 MHz
1
spectrometer. The chemical shifts (δ) in H NMR were
reported relative to tetramethylsilane (Me4Si) as internal
standard (δ 0.0) or proton resonance resulting from in-
complete deuteration of NMR solvent: CDCl3 (δ 7.26)
or DMSO-d6 (δ 2.50). Coupling constants (J) are ex-
pressed in hertz. 13C NMR spectra were recorded at 100
MHz or 125 MHz, and the chemical shifts (δ) were re-
ported relative to CDCl3 (δ 77.10) or DMSO-d6 (δ
40.50). Mass spectra were recorded on a Bruker Solarix
Fourier Transform Ion Cyclotron Resonance Mass
Spectrometer. Elemental analysis of carbon, nitrogen
and hydrogen was performed using an Elementary Vario
EL analyzer. UV-vis spectra were recorded using a
SHIMAD2U UV-2550 spectrometer. Emission spectra
were recorded using a Hitachi F-4600 fluorescence
spectrophotometer.
All solvents were purchased from Sinopharm and
used without further purification unless otherwise de-
noted. Tetrahydrofuran, ethanol, and acetonitrile were
further dried over calcium hydride, and distilled under
high vacuum just before use. All chemicals were pur-
chased from Aladdin, Aldrich, Alfa Aesar and Sino-
pharm, and were used as received.
Synthesis of 3[20]
The solution of 1 (0.94 g, 3.5 mmol) and K2CO3
(1.93 g, 14 mmol) in acetonitrile (40 mL) was heated
under reflux in an N2 atmosphere for 30 min. The solu-
tion of 2 (2.79 g, 7.7 mmol) in acetonitrile (4 mL) was
added dropwise to the mixture, stirring and refluxing
were continued for 24 h. The reaction mixture was then
filtered and the solvent was removed in vacuo. The res-
idue was purified by column chromatography (SiO2/
Ethyl acetate) to yield 3 as a yellow liquid (1.42 g, 63%).
1H NMR (400 MHz, CDCl3) δ: 7.14 (s, 2H), 4.12 (t, J＝
4.8 Hz, 4H), 3.87 (t, J＝4.8 Hz, 4H), 3.76 (m, 4H), 3.69
－3.63 (m, 16H), 3.55－3.53 (m, 4H), 3.37 (s, 6H).
Synthesis of 4[21]
4-Formylphenylboronic acid (3.36 g, 20 mmol),
2-acetylpyridine (5.32 g, 44 mmol), and powdered
NaOH (4.80 g, 120 mmol) were added to absolute EtOH
(150 mL) in a round bottom flask. After stirring at am-
bient temperature for 5 h until 4-formylphenylboronic
acid was all consumed (monitored by TLC), the color of
solution changed from yellow to red. Then a concen-
trated aqueous NH3 solution (60 mL) was added and the
Synthesis of 1[19]
To a suspension of 1,4-dihydroxybenzene (0.05 mol,
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Chin. J. Chem. 2017, 35, 447—456

Detection of Picric Acid in Aqueous Media
Synthesis of P2
suspension was stirred at 65 ℃ for 12 h to give a dark
yellow solution with a colorless precipitate. The precip-
itate was filtered under reduced pressure and washed
with a little amount of isopropanol and chloroform to
give product as a nearly colorless solid (4.6 g, 65%). 1H
NMR (400 MHz, CD3OD) δ: 8.71－8.63 (m, 6H), 8.01
(td, J＝7.6, 2.0 Hz, 2H), 7.75 (d, J＝8.4 Hz, 2H), 7.73
(d, J＝8.4 Hz, 2H), 7.47 (ddd, J＝7.2, 4.8, 0.8 Hz, 2H).
P2 was prepared from L and Cd(ClO4)2 as a yellow
powdered solid in 91% yield according to a procedure
similar to that for P1. H NMR (600 MHz, CDCl3) δ:
1
9.11－7.98 (m, 24H), 7.60 (br, 4H), 7.28 (s, 2H), 4.27 (s,
4H), 3.77 (s, 4H), 3.60－3.30 (m, 24H), 3.12 (s, 6H);
13C NMR (100 MHz, CDCl3) δ: 150.12, 149.60, 148.01,
147.86, 147.47, 141.45, 134.26, 130.46, 129.48, 128.03,
127.88, 123.62, 121.12, 120.36, 116.18, 71.27, 70.03,
69.97, 69.91, 69.85, 69.61, 69.22, 68.77, 58.05. Anal.
calcd for C68H68CdF6N6O16S2: C 53.88, H 4.52, N 5.54;
found C 54.79, H 5.33, N 4.91.
Synthesis of L
A mixture of 3 (520 mg, 0.8 mmol), 4 (932 mg, 2.4
mmol), Pd(PPh3)2Cl2 (59 mg, 0.08 mmol), K2CO3 (884
mg, 6.4 mmol), and mixed solvent [V(toluene)/
V(t-BuOH)/V(H2O), 9∶3∶9] was placed into a 100
mL sealed container with a magnetic stirring bar and
evacuated and purged with nitrogen. Then the mixture
was heated to 95 ℃ and refluxed for 2 d. After cooling
to room temperature, the reactant was extracted with
chloroform. The combined organic layers were dried
over anhydrous Na2SO4. After the removal of solvent,
the resulting yellowish solid was recrystallized from the
mixture of methanol and chloroform to obtain the pure
product as white solid (430 mg, 64%). M.p. 183－
UV-vis titration in organic media
＋
The UV-vis titrations of ligand L with Zn2 and
＋
Cd2 ions in organic solvents were carried out by plac-
ing 2 mL chloroform solution of ligand (4.0×105
mol•L1) in a quartz cuvette of 1 cm width. At 25 ℃,
the dilute solution of Zn(ClO4)2 or Cd(ClO4)2 in metha-
nol (1.0×10-3 mol•L1) was added in an incremental
fashion. The absorption of additives was deducted
through adding the solution of additives into the cu-
vettes of sample solution and background solvent sim-
ultaneously before measurement.
1
185 ℃. H NMR (600 MHz, CDCl3) δ: 8.82 (s, 4H),
8.76－8.75 (m, 4H), 8.70 (d, J＝8.4 Hz, 4H), 8.00 (d,
J＝8.4 Hz, 4H), 7.90 (t d, J＝7.8, 1.8 Hz, 4H), 7.80 (d,
J＝8.4 Hz, 4H), 7.37 (ddd, J＝7.2, 4.8, 0.6 Hz, 4H),
7.12 (s, 2H), 4.16 (t, J＝4.8 Hz, 4H), 3.80 (t, J＝4.8 Hz,
4H), 3.67－3.65 (m, 4H), 3.63－3.62 (m, 8H), 3.61－
3.59 (m, 4H), 3.57－3.56 (m, 4H), 3.47－3.46 (m, 4H),
3.31 (s, 6H); 13C NMR (125 MHz, CDCl3) δ: 156.28,
155.99, 150.59, 149.90, 149.14, 138.97, 137.05, 136.84,
130.72, 130.12, 126.93, 123.82, 121.36, 118.72, 116.82,
71.86, 70.82, 70.66, 70.59, 70.55, 70.43, 69.80, 69.54,
58.93. MS (XR-FT) m/z: 1105.5 [M＋1]＋. Anal. calcd
for C66H68N6O10: C 71.72, H 6.20, N 7.60; found C
71.34, H 5.98, N 7.99.
Photoluminescence measurements
For fluorescence measurements, ligand and poly-
mers were excited at their λmax of UV absorption and
their emissions were monitored within the suitable re-
gion while keeping 5.0 nm slit width for both source
and detector at 400 V voltage.
Fluorescence quenching titration with NACs in
aqueous media
The fluorescence quenching titrations with different
NACs in aqueous media were carried out by placing 2
mL aqueous solution of two polymers P1 and P2 in a
quartz cuvette of 1 cm width. Then, the aqueous or
methanol solution of several NACs was added in an
incremental fashion at 25 ℃. Two polymers were ex-
cited at their λmax of UV absorption and their emissions
were monitored within the suitable region while keeping
10.0 nm slit width for both source and detector and
voltage as 700 V.
Synthesis of P1
An equimolar amount of the ditopic ligand L and
Zn(ClO4)2 was stirred at 85 ℃ in the mixed solvent of
acetonitrile and chloroform (V/V, 1∶1) under an N2
atmosphere for 24 h (ca. 1 mL solvent per 2 mg of the
ligand). After the reaction solution was cooled to ambi-
ent temperature, diethyl ether was added slowly to form
the yellow precipitate. The precipitated polymers were
collected by filtration under reduced pressure, washed
with diethyl ether three times, and then dried in vacuo to
give the corresponding metallo-supramolecular poly-
The fluorescence quenching efficiency (η) for each
of NACs was calculated by the following equation:
(I I)/I 100%
＝
－
×

0
0
where I0 and I are the fluorescence intensities in the ab-
sence and presence of NACs, respectively.
1
mers as a yellow powdered solid with yield of 87%. H
NMR (600 MHz, CDCl3) δ: 9.44－7.58 (m, 28H), 7.28
(s, 2H), 4.27 (s, 4H), 3.76 (s, 4H), 3.58－3.30 (m, 24H),
3.12 (s, 6H); 13C NMR (100 MHz, CDCl3) δ: 150.12,
149.60, 148.01, 147.86, 147.47, 141.45, 134.26, 130.46,
129.48, 128.03, 127.88, 123.62, 121.12, 120.36, 116.18,
71.27, 70.03, 69.97, 69.91, 69.85, 69.61, 69.22, 68.77,
58.05. Anal. calcd for C68H68F6N6O16S2Zn: C 55.61, H
4.67, N 5.72; found C 56.57, H 5.04, N 5.23.
Calculation of the Stern-Volmer constant
The sensitivity of two polymers toward PA was es-
timated from the Stern-Volmer constant (KSV). The
Stern-Volmer plots were plotted as a function of the PA
concentration ([C]) in the following equation:
I /I
1
－ ＝
K
[C]
×
SV
0
Chin. J. Chem. 2017, 35, 447—456
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449

Ding et al.
FULL PAPER
Thus, KSV can be calculated from the slope of the linear
Stern-Volmer plot obtained at low concentration range
of PA.
popular hybrid functional B3LYP method to optimize
these structures, and then obtain the dipole moments
and also the energy gaps between HOMO and LUMO of
the optimized structures. Triple-ζ with one polarization
function TZVP[22b,22c] basis sets was used for all atoms,
and zero order regular approximation (ZORA) was used
for the scalar relativistic effect in all calculations.
Visual detection of PA on dip-coated test strips under
UV light
A piece of filter paper (3.0 cm×1.0 cm) was im-
mersed in a chloroform solution of P1 and P2 (1×104
mol•L1) for 5 min. Then, the filter paper was removed
from the solution and dried under vacuum. Under illu-
mination with a UV lamp at 365 nm, the filter paper
emitted strong yellow fluorescence. To demonstrate its
application as a fluorescence paper sensor for the detec-
tion of PA, the test strips were dipped into aqueous so-
lution of PA at very low concentration (1.0×106
mol•L1), and no emission could be observed under 365
nm UV light. The images were obtained under UV illu-
mination at 365 nm.
Results and Discussion
Preparation of ditopic ligand and Zn(II)/Cd(II)-
based metallo-supramolecular coordination poly-
mers
Ligand L was synthesized via palladium-catalyzed
Suzuki coupling reaction of bromined substrate 3 with
boric acid linked terpy substrate 4 (Scheme 1). Subse-
quently, metallo-supramolecular polymers P1 and P2
were prepared by 1∶1 complexation of Zn(ClO4)2 or
Cd(ClO4)2 with L under reflux in the mixed solvent of
acetonitrile and chloroform (V/V, 1∶1) for 24 h
(Scheme 2). The polymers were purified by repeated
DFT calculation
Orca 3.0.3 calculations[22a] were performed with the
Scheme 1 The synthetic route of ditopic ligand L
OH
OH
TsCl, NaOH
THF/H2O
Br
Br2
O
O
OTs
O
O
O
O
OH
O
O
AcOH, r.t., 6 h
2
0°C to r.t., 12 h
Br
OH
OH
1
N
CHO
O
Br
O
O
N
N
NaOH, EtOH
25°C, 7 h
O
O
O
O
O
O
K2CO3
N
+
1 + 2
MeCN, reflux, 24 h
then NH3H2O
65 °C, 12 h
O
B(OH)2
O
Br
3
B(OH)2
O
O
O
O
4
O
O
N
N
N
N
Pd(PPh3)2Cl2, K2CO3
toluene/t-BuOH/H2O, reflux, 48 h
3 + 4
N
O
O
N
O
O
L
Scheme 2 The synthetic route of P1 and P2
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© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, 35, 447—456

Detection of Picric Acid in Aqueous Media
precipitations from acetonitrile in diethyl ether. Both
polymers exhibited good stability and solubility in polar
solvents such as DMF and DMSO. The polymers can
not be dissolved directly in other less polar solvents due
to their strong electrostatic interaction between the
polyelectrolytes and counter anions. However, after
dissolving in a little amount of DMF, the polymers can
stably dispersed in solvents including dichloromethane,
chloroform, acetone, acetonitrile, water, etc.
solutions (Figure S14). Their films show a slight
red-shift of lowest energy absorption maxima compared
to those in low polar solution. It is consistent with our
previous observation in pybox series coordination pol-
ymers,[16] which may originate from the enhanced in-
termolecular interaction between the polymer chains in
the solid state.
The metal-ion-mediated self-assembly processes
were investigated in depth by the variation of absorption
spectra. We titrated the chloroform solution of L with
Zn(ClO4)2 and Cd(ClO4)2 and monitored the processes
through UV-vis spectroscopy (Figure 2). Our measure-
ments demonstrated that the titration of L at a concen-
tration of 4.0×105 mol/L with up to 1 equiv. of
Zn(ClO4)2 caused a significant red-shift in the absorp-
tion spectra. The originally strong lowest-energy ab-
sorption of L at 287 nm was gradually red-shifted to
387 nm because of the intramolecular charge transfer
from the electron rich central terphenyl component to
the metal ion-coordinated, electron-deficient terpy moi-
ety. Upon the addition of Zn(ClO4)2, the intensity of the
absorption peak of ligand at 287 nm continuously de-
creased and the absorption peak of the polymer at 387
nm increased in a linear fashion, until a metal-to-ligand
ratio of 1∶1 was attained, implying the association
constant for the polymerization process. The appearance
Absorption spectra
The investigations of absorption spectra for ligand
and polymers were carried out in chloroform firstly. The
absorption of L displays two maxima (λmax) at 289 and
326 nm, corresponding to the -* transitions of the
conjugated core. Compared with that of ligand, the ab-
sorptions of two polymers display a significant red-shift
because of the inductive effect of the metal ions (Figure
1). In chloroform, P1 and P2 show the lowest energy
absorbance maxima (λmax) at 389 nm and 385 nm, re-
spectively [Figure 1(a)]. The solvent effects on the ab-
sorption for the ligand and polymers were tested in de-
tail. Overall, the polymers display blue-shift with in-
creasing solvent polarity [Figures 1(b) and S13].
Homogenous thin films of P1 and P2 were prepared
on a quartz glass substrate by dipping their chloroform
1.5
(a)
1.5
acetone
DCE
cholorform
DMF
(b)
L
P1
P2
DMSO
water
MeCN
MeOH
1.0
0.5
0.0
1.0
0.5
0.0
250
300
350
400
450
500
300
350
400
450
500
550
Wavelength/nm
Wavelength/nm
Figure 1 (a) UV-vis absorption spectra of L, P1, and P2 in the chloroform solution (4.0×105 mol/L); (b) UV-vis absorption spectra of
P2 in different solvents (4.0×105 mol/L).
Table 1 UV-vis spectral data of L, P1, and P2 in different solvents and polymer films
Solution
Film
Solvent
λabsa/nm
P1
ε×104 b/(mol1•L•cm1)
λabsc/nm
L
P2
L
P1
P2
P1
P2
Acetone
DCE
332
329, 370
287, 324
322, 386
287
329
2.20, 1.15
2.85, 3.84, 2.23
2.76, 3.66, 2.18
3.66
1.74
1.15
253, 288, 327
253, 288, 326
288
286
7.71, 2.57, 1.90
2.86,1.71
2.26
2.57
CHCl3
DMF
321, 387
288
2.41, 1.48
1.36
287
325
369
290
382
DMSO
H2O
289
289
288
2.48
3.35
2.58
293, 355
251, 286, 327
249, 286
289
292, 350
287
0.16, 0.54
2.59, 3.43, 1.84
2.69, 3.76
1.85
1.37, 0.97
2.86
MeCN
286
3.33, 2.65, 1.73
MeOH
286
286
2.70, 2.13, 1.34
3.04
a 4.0×105 mol/L in different solvents; b Extinction coefficients at the lowest-energy absorption band. c Thin film on a quartz glass sub-
strate.
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Ding et al.
FULL PAPER
of an isosbestic point at 305 nm suggested the equilibria
between the ligand and polymer. Beyond the 1∶1 ratio,
the subsequent addition of Zn(ClO4)2 caused new spec-
tral changes (Figure 2(b)). The low-energy absorption at
387 nm was slightly blue-shifted to 376 nm after 4 equiv.
of Zn(ClO4)2 was added in total, indicating that the co-
ordination polymer partially decomposed when the met-
al-to-ligand ratio was above 1∶1. The titration with
Cd(ClO4)2 gave almost the same result (see Figures 2(c)
and 2(d)). Thus, it could be concluded that a metal-
lo-supramolecular complex formed in solution when the
metal-to-ligand ratio was 1∶1. In this type of linear
polymer, each metal ion is coordinated with two ligands
and the ditopic ligand is also coordinated with two met-
al ions through two perpy moieties. The changing trend
in absorption at two characteristic wavelengths (287 and
387 nm) for two titration experiments is shown in Fig-
ure S15, revealing that the change of the predominant
species involved in the self-assembly process at differ-
ent metal-to-ligand ratios. In this solution, the three
chromophores such as free ligand, 1∶1 metal-to-ligand,
and 1∶2 metal-to-ligand complex exist in dynamic
equilibrium so that the expected manner can be mediat-
ed by controlling the different ratios of ligand and metal
ion. Like the traditional stepwise polymerization, the
precise controlling of equal ratio of ligand and metal ion
is a key to obtain the polymers with high molecular
weight.
vents, so the solution of ligand and polymers in different
solvents was measured to test the solvatochromism ef-
fect. The PL properties in different solvents were exam-
ined by exciting the lowest-energy absorption maxima
bond. It was found that the solvent polarity imposed
strong influence on the wavelength and intensity of flu-
orescence emission (Figures 3, 4 and S16). For ligand L,
the fluorescence peaks emerge around 430 nm in low
polarity solvents like chloroform, dichloroethane, and
THF. The peaks, however, exhibit a significant red-shift
in polarity solvents (Table 2). The emission of L is
largely quenched in solvents such as H2O, methanol,
and acetone due to the poor solubility. The fluorescence
emissions of polymers show stronger dependence of
solvent polarity than L and the light-emitting color and
intensity differ significantly in different solvents. They
show yellow light emission in solvents such as chloro-
form, dichloroethane, H2O, and acetonitrile with emis-
sion peaks above 520 nm. However, the light-emitting
shows apparent blue-shift in DMF and DMSO so that
the emission colors changed to cyan-blue. Under the
same concentration, the fluorescence intensity of P1 in
chloroform is the strongest but the emission is largely
quenched in acetone, acetonitrile, and methanol. In
aqueous media, the emission of P1 is quenched but still
visible clearly by naked eyes. As for P2, it shows the
strongest emission in DMSO and also displays the rela-
tively weak fluorescence emission in aqueous media.
The PL spectra of the spin-coated thin films of P1 and
P2 were investigated as well, they both exhibited lumi-
nescence emission similar to the situation in low polari-
ty solvents (Figure S17).
Emission spectra
It is well known that the photoluminescence (PL)
behavior of compounds is highly dependent on the sol-
Figure 2 UV-vis spectra acquired upon titration of L in chloroform (4.0×105 mol/L) with Zn(ClO4)2 and Cd(ClO4)2, respectively.
Shown are spectra at selected M2＋: L ratios ranging from 0 to 1 (a, c) and above 1 (b, d).
452
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Detection of Picric Acid in Aqueous Media
80
(a)
(b)
acetone
DCE
cholorform
DMF
DMSO
water
MeCN
MeOH
acetone
DCE
cholorform
DMF
DMSO
water
MeCN
MeOH
150
60
40
20
0
100
50
0
450
500
550
600
650
700
750
450
500
550
600
650
700
750
Wavelength/nm
Wavelength/nm
Figure 3 PL spectra of P1 (a) and P2 (b) in different solvents under the excitation at 390 nm (4.0×105 mol/L).
Table 2 Emission data of L, P1, and P2 in different solvents and polymer films
Solution
Film
Stokes shift/cm−1
Solvent
maxa/nm
P1
Stokes shift/cm−1
P1
λmaxb/nm
L
P2
L
P2
P1
P2
P1
P2
Acetone
DCE
446
442
426
452
460
462
450
389
602
494
537
514
454
463
558
557
438
7699
7956
7201
12598
12863
6524
8359
9258
10416
12414
6713
10152
16343
6385
542
CHCl3
DMF
521
456
12913
12957
12696
13124
10650
16890
12134
536
526
8444
7167
DMSO
H2O
462
551
16453
18045
MeCN
591
MeOH
440
12238
a 4.0×105 mol/L in different solvents; b Thin film on a quartz glass substrate.
aqueous solution of polymers, the fluorescence quench-
ing caused an obvious color change that could be easily
observed by naked eye. Two polymers both exhibited
very high quenching efficiency. The fluorescence emis-
sion of P1 and P2 was quenched by 85% and 99% upon
the addition of 1 equiv. of PA, resulting in the almost
disappearance of the yellow light emission (Figures 5
and S18). The Stern-Volmer plot was used to study the
response to PA of two polymers quantitatively (Figures
5(b) and S18(b)). Under low concentration, the
Stern-Volmer constants were determined to be 5.08×
105 mol1•L for P1 and 1.77×106 mol1•L for P2, re-
spectively.
Fluorescence quenching methods are dominant in
fluorescence based explosives sensors.[23] Up to date,
researchers have developed numerous mechanisms re-
sponsible for fluorescence quenching process that in-
clude photo-induced electron transfer (PET), resonance
energy transfer (RET), electron exchange (EE), inter-
molecular charge transfer (ICT), inner filter effect (IFE),
etc.[15,24] Because there is little spectral overlap between
the emission of polymers and the absorption of PA, the
RET approach is unfavorable (Figure S19). Electron and
charge transfer processes, such as PET, ICT, and EE, are
common approaches in fluorescence quenching. How-
ever, no new absorption peak but decreased absorption
intensity could be observed with the increasing concen-
tration of PA (Figure S20), so the possible mechanisms
Figure 4 The emission colors of L, P1, and P2 in different
solvents (4.0×105 mol/L) under UV illumination at 365 nm.
Detection of nitroaromatic explosives
The visible emission of polymers in aqueous media
prompted us to explore the possibility of their potential
application as FCs for the detection of NACs. First, pic-
ric acid (PA) was selected as a model compound to test
the fluorescence variation towards P1 and P2. As ex-
pected, two polymers both displayed the sensing be-
havior towards PA. After the addition of PA to the
Chin. J. Chem. 2017, 35, 447—456
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FULL PAPER
Figure 5 (a) Changes in fluorescence spectra of P2 (4.0×105 mol•L1) with the addition of PA in water. Inset: change in the emission
color after the addition of 1 equiv. of PA (under UV illumination at 365 nm); (b) Stern-Volmer plot in response to PA. Inset: Linear
Stern-Volmer plot obtained at lower concentration range of PA.
NACs, PA possesses the strongest acidity and lowest
HOMO-LUMO gap, which may account for the high
sensitivity and good selectivity. The good detection to-
ward PA further demonstrates that IFE is one major
process in the fluorescence quenching. The anti-inter-
ference ability of this type of polymers was investigated
as well. It was found that, in the presence of 1 equiv. of
other NACs, the fluorescence responses of P2 to PA
were unaffected in efficiency (Figure S23). These re-
sults further verified the high sensitivity and selectivity
of this type coordination polymer for PA detection. The
emerge of free ligand's emission peak indicates that the
polymers may decompose when large amounts of NACs
were added.
Finally, we explored the potential applications of this
type of FCs so that the facile PA detection outdoors
could be carried out. Test strips were prepared through
dip-coating the acetonitrile solution of P1 and P2 into
filter paper and then drying under vacuum. The test
stripts both showed strong yellow light emission in the
solid state. After dropping the test strips into PA dilute
aqueous solution, no emission could be observed by
naked eye under 365 nm UV light (Figure 7). So these
type FCs based test strips display the potential facile
applications in detecting the NACs pollution for the
outdoor water.
such as PET, ICT, or EE can not be verified at this stage
which need in-depth study by time-resolved transient
spectrum in the future. Based on the strong IFE of PA
over that of other NACs, the IFE should be considered
as one of the major processes in the fluorescence
quenching. The Stern-Volmer plots show non-linearity
at higher concentration range, meaning the process is
complicated which may be combined with diverse
mechanisms.
Subsequently, in order to inspect the selectivity of
these polymers towards PA, several other representative
nitro-containing aromatic explosives (NACs), including
2-methyl-1,3,5-trinitrobenzene (TNT), 3,5-dinitrophenol
(m-DNP), 4-nitrophenol (p-NP), 1-chloro-2,4-dinitro-
benzene (CDNB), 2,6-dimethyl-4-nitrophenol (DMNP),
1,3-dinitrobenzene (DNB), 4-nitrobenzoic acid (NBA),
and 4-nitrobenzaldehyde (NBD) were tested as well.
Compared with PA, all other NACs displayed insignifi-
cant efficiency of the emission quenching (Figures S21
and S22). The fluorescence quenching efficiencies ()
upon adding 1 equiv. of different NACs towards P1 and
P2 are presented in Figure 6. Although the fluorescence
emission was almost totally quenched by PA, the
quenching efficiencies for other NACs were relatively
low and the same tendency was also observed in the
measurement of KSV constant (Table S1). Among these
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
P1
P2
Figure 7 Images (under UV illumination at 365 nm) of P1 and
P2 on test strips before and after diping into the pure water and
PA diluted aqueous solution (1.0×105 mol•L1).
Figure 6 Fluorescence quenching efficiencies towards P1 and
P2 (4.0×105 mol•L1) in water after the addition of 1 equiv. of
various NACs.
Conclusions
In summary, we synthesized one ditopic terpy-de-
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© 2017 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Chin. J. Chem. 2017, 35, 447—456

Detection of Picric Acid in Aqueous Media
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rived conjugated ligand containing hydrophilic oligo-
ethyoxy side chain via Suzuki coupling reaction. The
＋
＋
ligand could coordinate with Zn2 and Cd2 ions to
construct linear metallo-supramolecular coordination
polylmers whose dispersibilities were significantly im-
proved in organic solvent and water through the intro-
duction of tetraethyleneglycol monomethyl ether side
chain. Due to the large Stokes shift caused by the con-
jugated structure of the ligand and the inductive effect
of the d10 transition metal ions, the polymers showed
yellow light emission in chloroform and water and ob-
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and DMSO. This type of coordination polymers dis-
played good selectivity and high sensitivity in the detec-
tion of PA in aqueous media. These visual FCs could be
prepared into solution-coated test strips to achieve the
facile detection outdoors, exhibiting the practical appli-
cations for public safety and security in the future.
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Acknowledgement
This work is supported by the National Natural Sci-
ence Foundation of China (Nos. 21371043 and
21302035), and the Fundamental Research Funds for
the Central Universities (No. 2014HGCH0009). Y.-Y.Z.
expresses the most sincere respect to Prof. Xi-Kui Jiang
(Shanghai Institute of Organic Chemistry) and Prof.
Zhan-Ting Li (Fudan University) for sustained encour-
agement and support for his research.
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