Synthesis of conjugated microporous polymer-based fluorescent “turn-off” sensor for selective detection of picric acid

Article abstract of DOI:10.1080/15421406.2019.1645471

Various structures of conjugated microporous polymers (CMPs) were synthesized via the Suzuki cross-coupling reaction to detect the explosive compound, picric acid (PA). One of the CMP, CMP-3 showed high fluorescence in the solid state because of the prese

Full text of DOI:10.1080/15421406.2019.1645471

Molecular Crystals and Liquid Crystals
ISSN: 1542-1406 (Print) 1563-5287 (Online) Journal homepage: https://www.tandfonline.com/loi/gmcl20
Synthesis of conjugated microporous polymer-
based fluorescent “turn-off” sensor for selective
detection of picric acid
Jeong Jun Lee & Taek Seung Lee
To cite this article: Jeong Jun Lee & Taek Seung Lee (2019) Synthesis of conjugated microporous
polymer-based fluorescent “turn-off” sensor for selective detection of picric acid, Molecular Crystals
and Liquid Crystals, 686:1, 1-8, DOI: 10.1080/15421406.2019.1645471
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MOLECULAR CRYSTALS AND LIQUID CRYSTALS
2019, VOL. 686, NO. 1, 1–8
https://doi.org/10.1080/15421406.2019.1645471
Synthesis of conjugated microporous polymer-based
fluorescent “turn-off” sensor for selective detection of
picric acid
Jeong Jun Lee and Taek Seung Lee
Organic and Optoelectronic Materials Laboratory, Department of Advanced Organic Materials and
Textile System Engineering, Chungnam National University, Daejeon 34134, Korea
KEYWORDS
ABSTRACT
conjugated microporous
polymers; picric acid;
fluorescence; sensors
Various structures of conjugated microporous polymers (CMPs) were
synthesized via the Suzuki cross-coupling reaction to detect the
explosive compound, picric acid (PA). One of the CMP, CMP-3
showed high fluorescence in the solid state because of the presence
of repeat unit with aggregation-induced emission (AIE). Upon expos-
ure to PA, the fluorescence of CMP-3 was noticeably quenched
(190.10-fold) compared to other CMPs. The addition of other nitroar-
omatic compounds such as 2,4-dinitrotoluene (2,4-DNT) or 2,6-dini-
trotoluene(2,6-DNT), which were similar in structure to PA, did not
affect the fluorescence of CMP-3, indicative high selectivity of the
sensing material.
1. Introduction
Since nitroaromatic explosives greatly affect security and environmental pollution, the
need for immediate and selective detection techniques for nitroaromatic explosives have
been received much attention in recent years [1,2]. One of the various nitroaromatic
explosives, PA is being used in a variety of fields, including pharmaceuticals and dye-
stuffs, as well as rocket fuel [3–5]. Because of the versatile use in various applications,
PA is regarded as an environmental pollutant, and thus interest in detection of PA is
increasing [6,7].
Most organic fluorescent molecules are highly luminescent in solution, while they are
not fluorescent in the solid state because of p-p stacking-induced, aggregation-caused
quenching (ACQ) [8,9]. In contrast to ACQ, a phenomenon of nonfluorescence in solu-
tion but strong fluorescence in the aggregated or solid state has been reported, which is
called AIE [10,11]. Typically, tetraphenylethylene (TPE) has been well-known as for an
AIE-active compound, and various investigations have been carried out to detect vari-
ous substances using TPE moiety [12,13].
Herein, we synthesized CMPs using TPE with AIE property. CMPs are amorphous
porous materials connected by long p-bonds. CMPs are used in various applications such
as gas storage [14] and catalysis [15], because they have the advantage of easy control of
their properties including pore sizes and fluorescence according to the structure design.
The fluorescence of CMPs was not quenched even in the solid state because of the TPE.
CONTACT T. S. Lee
tslee@cnu.ac.kr
ß 2019 Taylor & Francis Group, LLC

2
J. J. LEE AND T. S. LEE
It is expected that the introduction of the AIE functionality to the porous CMP substrate
can provide more effective fluorescent-sensing abilities. The changes in fluorescence of
the CMPs could be used as a sensing signal upon exposure to nitrobenzene derivatives.
2. Experimental
2.1. Materials and Instruments
All chemicals were purchased from Sigma-Aldrich (U.S.A.) and solvents were purchased
from Samchun Chemicals (Korea). All reagents were used without further purification
1
unless otherwise noted. H NMR spectra were recorded on a Bruker Fourier-300 spec-
trometer at Korea Basic Science Institute. Solid phase 13C-NMR data were acquired on
400MHz Solid stat NMR spectrometer (AVANCE III HD) at the Korea Basic Science
Institute. FT-IR spectra were obtained from a Bruker Tensor 27 spectrometer. UV-vis
absorption spectra were recorded on a PerkinElmer Lambda 35 spectrometer.
Photoluminescence spectra were taken using a Varian Cary Eclipse spectrometer.
Scanning electron microscopy (SEM) images were obtained using a Hitachi S-4800
instrument. The Brunauer-Emmett-Teller (BET) method was used to obtain the specific
surface area of the sample (BELSORP-max).
2.2. Synthesis of 1,1,2,2-tetrakis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)
phenyl)ethene (2)
Mixture of 1 (3 g, 4.63 mmol), potassium acetate (4.56 g, 46.50 mmol), bis(pinacolato)di-
boron (5.30 g, 20.4 mmol), and anhydrous dioxane (90 mL) was put into a three-necked
flask and argon gas was bubbled for 30 min. [1,10-Bis(diphenylphosphino)ferrocene]
dichloropalladium (II) in dichloromethane (180 mg, 0.22 mmol) were added to the flask
under argon. The mixture was charged with argon gas for 30 min and then stirred at
110 ꢀC for 3 days. After cooling to room temperature, the reaction mixture was poured
into cold water. The precipitate was isolated by filtration and dried in a vacuum oven.
The residue was dissolved in chloroform and purified by column chromatography (eluent:
chloroform: ethyl acetate ¼ 1:1). The resulting white solid was washed with methanol and
1
dried (yield 3.16 g, 82%). H NMR (300MHz, CDCl3) 7.5 (8H, d), 7.0 (8H, d) ppm, 1.3
(24H, s) ppm. FT-IR (KBr pellet, cmꢁ1): 2981 (C-H), 1606 (C ¼ C), 1359 (B-O).
2.3. General procedure for the synthesis of CMPs
Monomers, N,N-dimethylformamide (DMF), 2 M K2CO3 aqueous solution were
charged in
a
three-necked flask and bubbled by argon for 30 min.
Tetrakis(triphenylphosphine)palladium(0) was added and the flask was charged with
argon gas for 30 min. The mixture was stirred at 120 ꢀC for 48 h. After cooling to room
temperature, the mixture was poured into methanol. The precipitate was isolated by fil-
tration and then washed with water. Further purification was performed by Soxhlet
extraction with THF for 2 days and the product was dried in a vacuum oven.
CMP-1: 1,2,4,5-Tetrabromobenzene (394 mg, 1 mmol), 1,4-benzene diboronic acid
bis(pinacol)ester (660 mg, 2 mmol), tetrakis(triphenylphosphine)palladium(0) (28 mg,
0.024 mmol), DMF (60 mL), and 2M K2CO3 aqueous solution (10 mL) were used.

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
3
O
B
Br
Br
Br
Br
O
+
O
B
O
n
CMP-1
Br
Br
Br
Br
O
B
O
+
O
B
O
1
n
CMP-2
O
B
O
B
Br
Br
Br
Br
O
O
O
+
O
B
B
O
O
1
2
n
CMP-3
Figure 1. Synthesis of CMP-1, CMP-2, and CMP-3.
225 mg of gray powder was obtained. 13C NMR: 139.8 (m), 128.7 (m) ppm. Anal. Calcd
for (C6H4)n: C,94.70; H, 5.30%; Found C, 84.4; H, 4.5%. FT-IR (KBr pellet, cmꢁ1): 3026
(C-H), 1606 (C ¼ C).
CMP-2: 1 (648 mg, 1 mmol), 1,4-benzene diboronic acid bis(pinacol)ester (660 mg,
2 mmol), tetrakis(triphenylphosphine)palladium(0) (28 mg, 0.024 mmol), DMF (60 mL),
and 2M K2CO3 aqueous solution (10 mL) were used. 500 mg of yellow powder was
obtained. 13C NMR: 138.4 (m), 129.5 (m), 125.2 (m) ppm. Anal. Calcd for (C36H24)n:
C,94.97; H, 5.03%; Found C, 83.3; H, 4.8%. FT-IR (KBr pellet, cmꢁ1): 3026 (C-H),
1606 (C ¼ C).
CMP-3: 1 (648 mg, 1 mmol), 2 (836 mg, 1 mmol), tetrakis(triphenylphosphine)palla-
dium(0) (28 mg, 0.024 mmol), DMF (60 mL), and 2M K2CO3 aqueous solution (10 mL)
were used. 225 mg of yellow powder was obtained. 13C NMR: 140.1 (m), 130.9 (m),
126.8 (m) ppm. Anal. Calcd for (C26H16)n: C,95.09; H, 4.91%; Found C, 80.4; H, 4.6%%.
FT-IR (KBr pellet, cmꢁ1): 3026 (C-H), 1605 (C ¼ C).
2.4. Detection of nitrobenzene derivatives
2,4-Dinitrotoluene (2,4-DNT), 2,6-Dinitrotoluene (2,6-DNT) and picric acid (PA)
were dissolved in acetonitrile at various concentrations and exposed to CMP-1,

4
J. J. LEE AND T. S. LEE
Figure 2. SEM images of (a) CMP-1, (b) CMP-2, and (c) CMP-3.
CMP-2, and CMP-3 (0.375 mg/mL) dispersed in acetonitrile. The changes in the
fluorescence of CMPs were investigated after 5 min with fluorescence spectroscope.
The Stern-Volmer constant (Ksv) was evaluated using the changes in the fluorescence
intensity accoding to the concentration of the nitrobenzene compounds added.
3. Results and discussion
Monomers 1 and 2 were synthesized according to previous literature method [16]
and the chemical structures were identified with FT-IR and 1H NMR spectra.
Various structures of TPE and phenylene derivatives were used to observe the effects

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
5
1.0
0.5
0.0
300
400
500
600
700
800
Wavelength (nm)
Figure 3. Excitation (empty) fluorescence (filled) spectra of CMP-1 ( , ), CMP-2 (᭺,᭹), and CMP-3
( , ). Excitation wavelength: 332 nm.
Figure 4. Photographs of CMP-1, CMP-2, and CMP-3 dispersions in ethanol (from left to right) (a)
under ambient light; in the absence (b) and presence of PA (c) under UV light
(365 nm) [PA] ¼ 2.5 mM.
on the pore structure and fluorescence properties of CMPs (Figure 1). CMP-1,
CMP-2, and CMP-3 were synthesized via the Suzuki coupling reaction, obtained as
pale gray (CMP-1) and yellow powders (CMP-2 and 3), and were not soluble in
common organic solvents such as chloroform, THF, acetone, and methanol, indicat-
ing the crosslinked, network structure. SEM was used to investigate the morphology
of the CMPs (Figure 2). It was confirmed that CMP-1, CMP-2, and CMP-3 consti-
tuted aggregated structure of randomly-shaped beads. The average size of the beads
was 120 nm for CMP-1, 414 nm for CMP-2, and 73 nm for CMP-3. The specific sur-
face area was analyzed, obtaining the total pore volumes of CMP-1, CMP-2, and
CMP-3 were 0.3855, 0.8612, and 0.8084 cm3gꢁ1 and the average pore diameter was
2.2508, 4.1704, and 4.8049 nm, respectively. The BET surface areas of CMP-1, CMP-
2, and CMP-3 were found to be 825.99, 685.05, and 672.95 m2gꢁ1, respectively, indi-
cating high specific surface areas.
CMP-1, CMP-2, and CMP-3 exhibited fluorescence at 441, 542, and 547 nm, respect-
ively, when excited at 332 nm (Figure 3). The excitation spectra of CMPs showed that
they had 332 nm absorption in common. CMP-2 and CMP-3 showed long wavelength
absorption around 410 nm mainly because of the presence of TPE. Monomers 1 and 2
exhibited fluorescence at 481 and 476 nm, respectively, and the emission was not

6
J. J. LEE AND T. S. LEE
Figure 5. Changes in the fluorescence of (a) CMP-1, (c) CMP-2, and (e) CMP-3 (0.375 mg/mL in
MeCN) with various concentrations of PA. Arrow direction represents the PA concentration of 0,
1.56 ꢂ 10ꢁ4, 3.13 ꢂ 10ꢁ4, 6.25 ꢂ 10ꢁ4, 1.25 ꢂ 10ꢁ3, and 2.5ꢂ 10ꢁ3 M. Exposure time 5 min. Excitation
wavelength 332 nm. Stern-Volmer plots according to changes in the fluorescence of CMP-1 (b), CMP-2
(d), and CMP-3 (f). Inset plots in (b), (d) and (f) represent low PA concentration regions which had lin-
earity with fluorescence. F0 and F correspond to the fluorescence intensity of CMPs at 441 nm in the
absence and presence of PA, respectively.
observed in the monomers containing benzene. However, CMP-1, an all-phenylene
polymer, showed fluorescence in the visible region. CMP-2 and CMP-3 showed longer
wavelength emission wavelength than that of monomers. These phenomena were

MOLECULAR CRYSTALS AND LIQUID CRYSTALS
7
Table 1. Stern-Volmer Constants (Mꢁ1) of CMPs
CMP-1
CMP-2
CMP-3
PA
2,4-DNT
2,6-DNT
3880
289
228
8026
583
228
14890
891
1238
200
160
5
120
2.
/F
0
80
F
40
0
CMP-3
PA
CMP-2
T
N
D
CMP-1
-
T
6
N
2,
D
-
4
2,
Figure 6. Changes in the fluorescence of CMP-1, CMP-2, and CMP-3. F0 represent fluorescence inten-
sity of CMPs in the absence of nitroaromatic compounds. F2.5. indicates the fluorescence intensity of
CMPs in the presence of nitroaromatic compounds (2.5 mM). The wavelengths of F2.5 is 478nm for
CMP-1 and 550 nm for CMP-2 and CMP-3.
probably because of extended conjugated length as a result of polymerization. Unlike
CMP-1, which contained only phenylene units, the fluorescence wavelengths of CMP-2
and CMP-3 were similar, mainly because of the presence of TPE. The fluorescence of
CMP-2 and CMP-3 was not quenched even in the aggregated forms, because of the
AIE-active TPE, which would be useful in turn-off sensory material. The CMP-1, CMP-
2 and CMP-3 were highly fluorescent so that the fluorescence was observed by the
naked-eyes under UV irradiation (365 nm) (Figure 4a and b).
To elucidate the responsiveness of CMPs toward nitroaromatic compounds, various
concentrations of nitrobenzene derivatives were exposed to CMPs dispersed in acetonitrile
and changes in the fluorescence were observed. Three CMPs were effectively quenched by
the presence of PA (Figure 5). CMP-1 showed emission shift from 441 to 478 nm as well
as fluorescence quenching upon exposure to PA. CMP-2 and CMP-3 did not show emis-
sion shift but emission quenching. The decrease in the fluorescence of CMPs resulted
from a photoinduced electron transfer. Because PA is electron-deficient, the excited elec-
trons of CMPs were transferred to PA when the CMPs were exposed to PA. The degree
of quenching (Fo/F) can be quantified using Stern-Volmer constant (Ksv):
F0=F ¼ 1 þ Ksv½Qꢃ
where F0 and F are the fluorescence intensity at 441 nm before and after exposure to
PA, respectively, Ksv is a Stern-Volmer quenching constant, and [Q] is the concentra-
tion of the quencher. Among CMPs, CMP-3 was effectively quenched by PA to have
Ksv values of 14890 Mꢁ1, in which Ksv values for 2,4-DNT and 2,6-DNT were
much lower than PA (Table 1). Moreover, CMP-1 and CMP-2 were not comparable to
CMP-3 in terms of Ksv values for nitroaromatic sensing. The Ksv was obtained using

8
J. J. LEE AND T. S. LEE
linear plot in low PA concentration region. When 2.5 mM PA was added to the CMP-3
dispersion, the fluorescence of CMP-3 was reduced by 190-fold compared to the initial
fluorescence. At the same conditions, fluorescence was decreased by 26-fold for CMP-1
and 42-fold for CMP-2 (Figure 6). The decrease in the fluorescence of CMP-3 by add-
ition of PA can be visually detected by the naked-eyes (Figure 4c). When a nitrobenzene
derivatives such as 2,4-DNT or 2,6-DNT was added at the same concentration, signifi-
cant decrease in fluorescence could not be observed with the naked-eyes (data not
shown here).
4. Conclusion
We synthesized porous, fluorescent CMPs via Suzuki polymerization to use for PA
detection. CMPs were prepared using various structures of monomers, phenylene and
TPE derivatives. It was found that all-TPE polymer, CMP-3 synthesized using only TPE
monomers, showed the high Stern-Volmer constant for PA, mainly because of higher
fluorescence compared to CMP-1 and CMP-2. In addition, the addition of 2,4-DNT
and 2,6-DNT that had structure similarity to PA resulted in a relatively small decrease
in fluorescence (3.19-fold, 4.07-fold, respectively), because of HOMO-LUMO band gap
difference. Through this, we confirmed that CMP-3 could be used as a fluorescent sen-
sor that can selectively detect PA.
Acknowledgment
Financial support from the National Research Foundation (NRF) of Korean government through
Basic Science Research Program (2018R1A2A2A14022019) and Nuclear R&D Project
(2016M2B2B1945085) is gratefully acknowledged.
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