Synthesis of tetraphenylethylene-based conjugated microporous polymers for detection of nitroaromatic explosive compounds

Article abstract of DOI:10.1039/C8RA06463F

Conjugated microporous polymers (CMPs) containing tetraphenylethylene (TPE) were synthesized via the Suzuki coupling polymerization. The tetrafunctional TPE moiety in the polymer backbone was linked with the difunctional phenylene group to exhibit a porous structure with high fluorescence in the solid state because of aggregation-induced emissive TPE. The porous polymer with a fluorescent TPE group successfully detected nitroaromatic explosive compounds that exhibited fluorescence quenching, in which the polymer shows high quenching efficiency to picric acid among nitroaromatic explosive compounds. The interaction between the electron-rich TPE group and the electron-deficient nitroaromatic compounds played a decisive role in fluorescence quenching via a photoinduced electron transfer (PET). Compared with a linear polymer containing TPE, the porous, crosslinked polymer showed better sensing performance toward nitroaromatic compounds, presumably because of the more efficient interaction between TPE and nitroaromatic compounds in the pores of TPE-based CMP (TPE-CMP).

Full text of DOI:10.1039/C8RA06463F

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Synthesis of tetraphenylethylene-based
conjugated microporous polymers for detection of
Cite this: RSC Adv., 2018, 8, 34291
nitroaromatic explosive compounds†
*
Ho Namgung, Jeong Jun Lee, Young Jin Gwon and Taek Seung Lee
Conjugated microporous polymers (CMPs) containing tetraphenylethylene (TPE) were synthesized via the
Suzuki coupling polymerization. The tetrafunctional TPE moiety in the polymer backbone was linked
with the difunctional phenylene group to exhibit a porous structure with high fluorescence in the solid
state because of aggregation-induced emissive TPE. The porous polymer with a fluorescent TPE group
successfully detected nitroaromatic explosive compounds that exhibited fluorescence quenching, in
which the polymer shows high quenching efficiency to picric acid among nitroaromatic explosive
compounds. The interaction between the electron-rich TPE group and the electron-deficient
nitroaromatic compounds played a decisive role in fluorescence quenching via a photoinduced electron
transfer (PET). Compared with a linear polymer containing TPE, the porous, crosslinked polymer showed
better sensing performance toward nitroaromatic compounds, presumably because of the more efficient
interaction between TPE and nitroaromatic compounds in the pores of TPE-based CMP (TPE-CMP).
Received 1st August 2018
Accepted 30th September 2018
DOI: 10.1039/c8ra06463f
rsc.li/rsc-advances
Conjugated microporous polymers (CMPs) are synthesized
from multifunctional organic monomers to obtain the porous
Introduction
structures, in which crosslinked, delocalized conjugated struc-
tures exhibit high uorescence, porosity, a large specic surface
area, and good physical and chemical stability.24,25 Based on
these properties, CMPs have been studied in versatile applica-
tions, including storage,26–29 catalysis,30 optoelectricity,31 and
energy harvesting.32
The development of efficient and reliable techniques for the
detection of nitroaromatic explosive compounds is becoming
an important issue because of the wide use of nitroaromatic
compounds as explosive compounds.1,2 The detection of nitro-
aromatic explosive compounds has been investigated using
a
variety of materials, including organic uorophores,3–7
quantum dots,8,9 polymer nanoparticles,10,11 metal–organic
frameworks,12–16 and conjugated polymers.17–20 In nitroaromatic
explosive compound sensing, uorescence-based sensors are
potentially useful because nitroaromatic explosive compounds
can manipulate the emission of uorophores used, resulting
from the electronic changes of the uorophore,14,19 and various
structures of uorescent probes have been used for detecting
explosive compounds.21,22 As the uorescent probe was exposed
to nitroaromatic compounds such as TNT, 2,4-dinitrotoluene
(2,4-DNT), 2,6-dinitrotoluene (2,6-DNT), and picric acid (PA),
the uorescence was quenched by a PET mechanism. Among
the nitroaromatic compounds, PA (2,4,6-trinitrophenol) is
known as a highly hazardous and powerful explosive which
causing severe human health problems when discharged into
our living environments.1,2,23
Conventional uorescent molecules generally are subjected to
decreased emission in the solid state because of the phenom-
enon of aggregation-caused quenching (ACQ). Some uo-
rophores, such as TPE, also show the opposite phenomenon of
aggregation-induced emission (AIE), exhibiting high uores-
cence in the solid state.33–37 Thus, it is expected that the intro-
duction of TPE into CMPs would provide them with uorescent
properties for versatile applications, even in the solid state. There
are several reports on specic applications of TPE-based CMPs in
color-tuning38 and supercapacitors.31 These investigations have
revealed the advantages of CMPs integrated with AIE. Thus, we
focused on the porous structure of TPE-CMP, in which the pores
would be helpful for interactions with nitroaromatic explosive
compounds and, as a result, the uorescence of TPE-based CMP
(TPE-CMP) would be effectively altered. Hence, CMP-based uo-
rescent sensors were prepared for the detection of nitroaromatic
explosives such as PA. When exposed to nitroaromatic explosive
compounds, the uorescence was immediately quenched
because of PET between the electron-decient nitroaromatic
explosives and the electron-rich TPE-CMP. The sensing efficiency
was improved over that of the linear counterpart, indicating the
importance of the porous structure.
Organic and Optoelectronic Materials Laboratory, Department of Organic Materials
Engineering, Chungnam National University, Daejeon 34134, Korea. E-mail: tslee@
cnu.ac.kr
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c8ra06463f
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Synthesis of TPE-Ph
Experimental
1,2-Bis(4-bromophenyl)-1,2-diphenylethene (0.603 g, 1.23
mmol) and 3 (0.406 g, 1.23 mmol) were dissolved in a mixture of
toluene containing aqueous 2 M potassium carbonate solution
(5 mL) under an argon atmosphere. The same procedure that
was employed for TPE-CMP was used. Aer reaction, the
mixture was cooled and added to methanol (300 mL) and the
precipitate was isolated by ltration. The precipitate was
washed with methanol and acetone. Aer drying under vacuum,
Instruments and materials
1H NMR spectra were obtained on a Bruker Fourier-300 spec-
trometer and the solid-state NMR spectrum was obtained on
a Bruker DSX-300 solid-state FT-NMR spectrometer (Korea Basic
Science Institute). Thermogravimetric analysis (TGA) was per-
formed with a Mettler-Toledo TGA/DSC analyzer. FT-IR spectra
were obtained on 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 micros-
copy (SEM) images were obtained using a Hitachi S-4800
instrument. Nitrogen sorption isotherms were obtained on
1
TPE-Ph was obtained (yield 0.33 g, 66%). H NMR (300 MHz,
CDCl3) 7.61 (4H, d), 7.41 (4H, m), 7.13 (14H, s) ppm. FT-IR (KBr
pellet, cmꢁ1): 3051–3024 (C–H), 1598–1359 (C]C), 1004 (C–Br).
Anal calcd. for C32H22: C, 94.55%; H, 5.45%. Found. C, 93.13%;
H, 6.71%.
a
micromeritics model ASAP 2010 analyzer. Before the
measurement, the polymer was degassed in a vacuum at 573 K
for 10 h. The Brunauer–Emmett–Teller (BET) method was used
to obtain the specic surface area of the sample. All chemicals
were purchased from Sigma-Aldrich (USA) and solvents were
purchased from Samchun Chemicals (Korea). All reagents were
used without further purication unless otherwise noted.
Detection of explosive compounds
TPE-CMP was suspended in methylene chloride (0.375 mg
mLꢁ1). Explosive compounds such as 2,4-DNT, 2,6-DNT, PA,
and 2,3-dinitro-2,3-dimethylethane (2,3-DN-2,3-DM) were all
dissolved in methylene chloride. Each solution of the explosive
simulant was added to the TPE-CMP suspension of and the
change in uorescence was recorded on a uorescence spec-
trometer. Fluorescence was measured with a 1 cm quartz
cuvette containing the TPE-CMPs dispersion. The widths of the
excitation and the emission silts were both 5 nm.
Synthesis of 1,1,2,2-tetrakis(4-bromophenyl)ethene (2)
1 (1 g, 3.01 mmol) was dissolved in chloroform (20 mL) at room
temperature. The temperature of the solution was cooled to 0 ꢀC
using an ice bath. Aer cooling, bromine (1.12 mL, 21.07 mmol)
was added slowly. The mixture was stirred at 0 ꢀC for 7 h. Aer
the reaction, the mixture was heated to room temperature. A
sodium bisulfate solution was added to remove the remaining
bromine. The mixture was dissolved in methylene chloride and
washed with water (100 mL) three times. The combined organic
layer obtained was dried on MgSO4. Aer the evaporation of
methylene chloride, the product was recrystallized in n-hexane,
Results and discussion
Compounds 1 and 2 were synthesized according to a previously
published method.39 The synthetic route to TPE-CMP is illus-
trated in Scheme 1, where TPE-CMP was prepared via a Suzuki
1
and a white powder was obtained (yield 1.49 g, 77%). H NMR
(300 MHz, CDCl3) 7.2 (8H, m), 6.8 (8H, m) ppm. FT-IR (KBr
pellet, cmꢁ1): 3005 (C–H), 1585–1394 (C]C), 1008 (C–Br).
Synthesis of TPE-CMP
A molar ratio of 1 : 2 (2 : 3) is depicted here. 2 (0.80 g, 1.23
mmol) and 1,4-benzenediboronic acid bis(pinacol)ester (3)
(0.812 g, 2.46 mmol) were dissolved in a mixture of toluene
containing an aqueous 2 M potassium carbonate solution (5
mL) under argon atmosphere. Aer addition of tetrakis-
(triphenylphosphine)palladium(0) (0.071 g, 0.062 mmol), the
ꢀ
reaction mixture was stirred at 100 C for 24 h. Aer reaction,
the mixture was cooled and added to methanol (300 mL) and
the precipitate was isolated by ltration. The precipitate was
washed with water and acetone. The product was puried with
tetrahydrofuran (THF), methylene chloride, and chloroform for
each 1 day in a Soxhlet apparatus. Aer drying under vacuum,
TPE-CMP was obtained (yield 0.37 g, 74%). Solid 13C NMR
(ppm): 210–190 (m), 138–125 (m), 79–55 (m), 22.53 (s) ppm. FT-
IR (KBr pellet, cmꢁ1): 3024 (C–H), 1598–1388 (C]C), 1003 (C–
Br). Anal calcd. for C38H24: C, 94.97%; H, 5.03%. Found. C,
94.54%; H, 5.32%.
Scheme 1 Synthesis of polymers.
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coupling reaction between 2 and 3 using feed ratios of 1 : 1, absorption at 330 nm and strong uorescence at 480 nm
1 : 2, and 1 : 3. The TPE-CMPs were insoluble in organic (Fig. 3a). TPE-CMP containing 2 exhibited a strong emission at
solvents, such as acetone, methanol, ethanol, THF, DMF, and 550 nm and excitation was observed at 330 nm, indicating that
chloroform, indicative of the formation of a crosslinked struc- the absorption was dependent on the TPE unit. The absolute
ture. The chemical structure of TPE-CMP was conrmed by quantum yield of TPE-CMP was 32%. Such a red shi in the
solid-state 13C NMR, FT-IR, and elemental analysis. The uorescence of TPE-CMP compared with 2 resulted from the
porosity of the TPE-CMPs was analyzed by nitrogen adsorption extended conjugation of the polymer backbone structure. The
isotherms, and the specic surface area (as,BET), total pore blue emission of 2 and the yellow uorescence of TPE-CMP
volume, and average pore size of the polymers were calculated could be observed by the naked eye (Fig. 3b). The UV-vis
using the BET equation (Table 1). The TPE-CMP prepared with absorption spectrum of TPE-CMP in solution could not be ob-
the 1 : 2 monomer ratio exhibited the highest specic surface tained because of the difficulty in sample preparation (the
area and pore volume and, thus the TPE-CMP with the molar compounds are not soluble in any solvent and a transparent
ratio of 1 : 2 was used for subsequent experiments. The nitrogen solid sample is not easy to prepare).
gas adsorption isotherms of the TPE-CMP with 1 : 2 ratio
The detection of explosive compounds, namely 2,4-DNT, 2,6-
showed type I physisorption isotherms with a steep increase in DNT, PA, and 2,3-DN-2,3-DM was investigated in terms of
gas uptake in the lower P/Po region (<0.05), indicative of uorescence change in TPE-CMP, in which the nitroaromatics
a permanent microporous structure with a pore diameter acted as uorescence quenchers via the PET mechanism
between 2 and 50 nm (Fig. 1).40,41 The TGA thermogram shows (Scheme 2). Upon exposure of the TPE-CMP dispersion to the
that the TPE-CMP did not signicantly decompose up to 400 ꢀC explosive compounds, an excited electron of TPE-CMP was
ꢀ
and 86% of the compound remained even at 500 C (Fig. 2a). transferred to the explosive compounds. The uorescence was
TPE-CMP exhibited an irregularly shaped morphology in SEM then quenched immediately through PET between the TPE
images (Fig. 2b).
moieties and the explosive compounds (inset photographs in
TPE is well known for its AIE, and thus monomer 2 showed Scheme 2). Among the explosive compounds investigated, the
strong uorescence in the solid state. Solid 2 showed a UV-vis nitroaromatic compounds including 2,4-DNT, 2,6-DNT, and PA
efficiently quenched the uorescence of TPE-CMP, whereas the
non-nitroaromatic 2,3-DN-2,3-DM showed a negligible effect on
this property (Fig. 4). As soon as 2,4-DNT, 2,6-DNT, and PA were
Table 1 BET data of TPE-CMPs
exposed to TPE-CMP, the uorescence was quenched. Such
Molar ratio (2 : 3) in feed
1 : 1
1 : 2
1 : 3
a decrease in the uorescence was caused by the aromatic
structures of 2,4-DNT, 2,6-DNT, and PA, which promoted
absorption of these explosives through the p–p interaction.
This can be more clearly observed in the case of PA, in which
almost all the uorescence was quenched based on the relative
uorescence intensity ratios. Among the three explosives, PA
exhibited effective uorescence quenching (the quenching
Specic surface area, as,BET [m2 gꢁ1
]
20.40
0.026
5.15
680.57
0.403
2.37
599.64
0.468
3.12
Total pore volume [cm3 gꢁ1
]
Average pore diameter [nm]
Fig. 3 (a) Absorption (-) and fluorescence spectra (,) of solid 2 and
fluorescence spectrum (B) of TPE-CMP, excitation wavelength
330 nm. (b) Photographs of 2 and TPE-CMP under ambient and UV
lights (365 nm).
Fig. 1 Nitrogen gas adsorption isotherm of TPE-CMP at 77 K.
Scheme 2 Fluorescence quenching of TPE-CMP in the presence of
nitroaromatic compounds via PET.
Fig. 2 (a) TGA thermogram and (b) SEM image of TPE-CMP.
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efficiency improved by a factor of more than 10), presumably
because of the lower energy of the lowest unoccupied molecular
orbital (LUMO) of PA than other explosive resulting in facile
electron transfer (Fig. 5).14
A less efficient quenching of TPE-CMP than PA was observed
upon exposure to 2,4-DNT and 2,6-DNT. The uorescence of
TPE-CMP was gradually decreased with increase in the
concentration of nitroaromatic compounds (Fig. 6), resulting in
the maximum quenching with approximately 2.5 ꢂ 10ꢁ3 M of
each nitroaromatic compound. This can be explained by the
Stern–Volmer equation:20
Fo/F ¼ 1 + KSV[Q]
The equation provides a quantitative relationship between
the changes in uorescence intensity (Fo/F) and the concentra-
tion of added nitroaromatic compounds ([Q]). The slope of the
plot is the Stern–Volmer constant (KSV). The KSV values for 2,4-
DNT, 2,6-DNT, and PA were estimated as 1.84 ꢂ 103 Mꢁ1, 1.70 ꢂ
103 Mꢁ1, and 1.19 ꢂ 104 Mꢁ1, indicating that the PA could be
most sensitively detected, in accordance with the result from
Fig. 5 For comparison, monomer 2 in methylene chloride
showed ꢃ5.0 ꢂ 102 Mꢁ1 of KSV toward PA, indicative of a rela-
tively low sensitivity.
Fig. 4 Changes in the relative fluorescence intensities (Ix/Ii) of 1 : 2
TPE-CMP (0.375 mg mLꢁ1 in CH2Cl2) upon various exposure time in
the presence of (a) 2,4-DNT, (b) 2,6-DNT, (c) PA, and (d) 2,3-DN-2,3-
DM. [Explosives] ¼ 2.5 ꢂ 10ꢁ3 M. Excitation wavelength 330 nm. Ii and
Ix represent fluorescence intensity at 550 nm in the absence and
presence of explosives, respectively.
For further elucidation of the uptake of PA by porous TPE-
CMP, FT-IR spectroscopy was used to investigate whether the
PA was present in TPE-CMP; the FT-IR sample was obtained
aer separation of TPE-CMP from the PA solution by centrifu-
gation at 12 000 rpm (Fig. 7). The characteristic bands
Fig. 6 Changes in the fluorescence of TPE-CMP (0.375 mg mLꢁ1 in
CH2Cl2) with various concentrations of 2,4-DNT (a); 2,6-DNT (b); PA
Fig.
5
Relative changes in fluorescence intensity of TPE-CMP (c). Exposure time 5 min. [Explosives] ¼ 0; 1.56 ꢂ 10ꢁ4; 3.13 ꢂ 10ꢁ4
;
(0.375 mg mLꢁ1 in CH2Cl2) upon exposure to 2,4-DNT, 2,6-DNT, PA, 6.25 ꢂ 10ꢁ4; 1.25 ꢂ 10ꢁ3; 2.5 ꢂ 10ꢁ3 M. Excitation wavelength 330 nm.
and 2,3-DN-2,3-DM. [Explosives] ¼ 2.5 ꢂ 10ꢁ3 M. Excitation wave- Right figures: Stern–Volmer plots of the fluorimetric titration. Fo and F
length 330 nm. I0 and I represent fluorescence intensity at 550 nm represent fluorescent intensity at 550 nm before and after exposure to
before and after exposure to explosives, respectively.
explosives, respectively.
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Acknowledgements
Financial support from the National Research Foundation
(NRF) of Korean government through Basic Science Research
Program (2018R1A2A2A14022019) is gratefully acknowledged.
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