A porous tetraphenylethylene-based polymer for fast-response fluorescence sensing of Fe(III) ion and nitrobenzene

Article abstract of DOI:10.1016/j.dyepig.2019.107929

A fluorescent porous tetraphenylethylene-based organic polymer (PTOP) was constructed by the reaction of 5,5',5'',5?-(ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis(pyrimidin-2-amine) and cyanuric chloride in the presence of potassium carbonate

Full text of DOI:10.1016/j.dyepig.2019.107929

Journal Pre-proof
A porous tetraphenylethylene-based polymer for fast-response fluorescence sensing
of Fe(III) ion and nitrobenzene
Yingting Zheng, Hailong Wang, Jianzhuang Jiang
PII:
S0143-7208(19)31554-2
DOI:
https://doi.org/10.1016/j.dyepig.2019.107929
DYPI 107929
Reference:
To appear in: Dyes and Pigments
Received Date: 5 July 2019
Revised Date: 2 September 2019
Accepted Date: 23 September 2019
Please cite this article as: Zheng Y, Wang H, Jiang J, A porous tetraphenylethylene-based polymer for
fast-response fluorescence sensing of Fe(III) ion and nitrobenzene, Dyes and Pigments (2019), doi:
https://doi.org/10.1016/j.dyepig.2019.107929.
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Graphical abstract
A porous tetraphenylethylene-based polymer for fast-response
fluorescence sensing of Fe(III) ion and nitrobenzene
a
a,
a,
Yingting Zheng, Hailong Wang, * and Jianzhuang Jiang *
A fluorescent porous tetraphenylethylene-based organic polymer (PTOP) was
constructed and capable of exhibiting a fast-response and sensitive fluorescence
sensing both Fe(III) and nitrobenzene.

A porous tetraphenylethylene-based polymer for fast-response fluorescence
sensing of Fe(III) ion and nitrobenzene
a
a,
a,
Yingting Zheng, Hailong Wang, * and Jianzhuang Jiang *
a
Beijing Key Laboratory for Science and Application of Functional Molecular and
Crystalline Materials, Department of Chemistry, University of Science and Technology
Beijing, Beijing 100083, China
*
Corresponding
author.
E-mail:
hlwang@ustb.edu.cn
(H.
Wang)
and
jianzhuang@ustb.edu.cn (J. Jiang)
_
___________________________________________________________________________________________________

Abstract: A fluorescent porous tetraphenylethylene-based organic polymer (PTOP) was
constructed by the reaction of
,5',5'',5'''-(ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis(pyrimidin-2-amine) and
5
cyanuric chloride in the presence of potassium carbonate. CO sorption experiment at 195 K
2
reveals the permanent porosity of PTOP with a Brunauer-Emmett-Teller surface area of 227
2
–1
m g . In particular, the existence of rich secondary amines and pyrimidine nitrogen atoms
surrounding the 1,3,5-triazine moieties in PTOP provides abundant chelating sites for metal
ions, endowing a fast-response and sensitive fluorescence sensing Fe(III) with a low limit of
detection of 4.5 µM. In addition, this polymer also shows an excellent sensing performance
towards nitrobenzene with good selectivity and a low detection limit of 1.6 nM.
Keywords: tetraphenylethylene-based polymer; porous materials; fast-response Fe(III)
sensing; nitrobenzene sensor
2

1
. Introduction
Porous organic polymers (POPs), including a wide range of subspecies like hyper
cross-linked polymers (HCPs),[1] polymers of intrinsic microporosity (PIMs),[2] covalent
organic frameworks (COFs),[3]covalent triazine frameworks (CTFs),[4] conjugated
microporous polymers (CMPs),[5,6] and porous aromatic frameworks (PAFs),[7–9] have
been attracting increasing interests due to their great potential applications in gas adsorption
and separation,[10–12] pollutes removal,[13] photoelectric conversion,[14] lithium
battery,[15] sensing,[16] and catalysis.[17,18] On the other hand, aggregation-induced
emission (AIE) molecular materials with potential applications in chemical sensing,
bioprobes, and organic light-emitting diodes have also been intensively studied since the first
discovery of AIE phenomenon by Tang and co-workers.[19–21] For the purpose of
developing new fluorescence materials with enhanced emission and multifunctional
properties, AIE luminogens (AIEgens) have been introduced/confined into various porous
crystalline materials including hydrogen‐ bonded organic frameworks (HOFs),[22,23]
COFs,[24] and metal organic frameworks (MOFs),[25,26] depending on hydrogen, covalent,
and coordination bond interactions, respectively. As one of the representative AIEgens,
tetraphenylethylene (TPE)[27,28] has also been introduced into the porous organic
polymers[29] with excellent sensing performance. Despite the relatively extensive studies in
this direction, developing new AIE-integrated POPs towards realizing their practical
applications seems to be an important and ever-lasting duty for chemists and material
scientists.
It is well-known that iron ion plays an important role in human life activities including
various metabolic processes, brain and muscle functions, and transcription and translation of
3
+
DNA and RNA.[30,31] However, excessively taking Fe would cause serious health issues
including insomnia, skin diseases, and immunity descent.[32] In addition, nitroaromatic
3

compounds (NACs) have been broadly employed in dye, firework, pharmaceutical and
leather industry.[32,33] The relevant pollution emissions severely contaminate the water and
soils and thus seriously threaten the human beings health.[33] As a consequence, great efforts
have been paid in the exploration of new fluorescence sensors for both species, resulting in
3
+
various kinds of Fe and NACs fluorescence sensing materials in the past years.[31–34]
Although some reports cover the AIE-confined fluorescence POPs for sensing small
3
+
molecules and heavy metals,[35,36] very few investigations are about the detection of Fe
and NACs. As a result, more efforts are still necessary in this regard for the real applications.
Herein, by means of newly developed TPE building block,
,5',5'',5'''-(ethene-1,1,2,2-tetrayltetrakis(benzene-4,1-diyl))tetrakis(pyrimidin-2-amine)
AP-TPE), a fluorescent porous tetraphenylethylene-based organic polymer (PTOP) was
fabricated with the help of cyanuric chloride. CO sorption measurement at 195 K reveals its
a
5
(
2
2
–1
permanent porosity nature with a Brunauer-Emmett-Teller surface area of 227 m g .
Furthermore, rich secondary amines and pyrimidine nitrogen atoms surrounding the
1
,3,5-triazine moieties of PTOP provide abundant chelating sites for metal ions, which in
combination with the porous structure and TPE AIEgen synergistically contribute to the
fast-response fluorescence sensing of Fe(III) and nitrobenzene.
Scheme 1 Synthesis and schematic structure of PTOP. (a) KOAc, Pd(dppf)Cl ·CH Cl , DME, 100°C, N ,
2
2
2
2
1
2 h; (b) K CO , Pd(dppf)Cl ·CH Cl , 1,4-dioxane, H O, 105°C, N , 12 h; (c) K CO , 1,4-dioxane, 90°C,
2 3 2 2 2 2 2 2 3
4

1
2 h. (Inset: images under sunlight and 365 nm UV-radiation of PTOP, respectively)
2
. Experimental Section
2
.1 General Directions
All chemicals were employed as received without further purification.
,1,2,2-tetrakis(4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)phenyl)ethene (TMDB-TPE),
AP-TPE, and PTOP were prepared by consulting to the reported methods.[37–39]
1
2
.2 Synthesis of PTOP
Synthesis of TMDB-TPE. A mixture of tetrakis(4-bromophenyl)ethene (0.65 g, 1.0
mmol), bis(pinacolato)diboron (1.50 g, 6.0 mmol),
,1'-bis(diphenylphosphino)ferrocene-palladium(II)dichloride dichloromethane
complex (Pd(dppf)Cl ·CH Cl ) (0.041 g, 0.05 mmol), and potassium acetate (KOAc)
1
2
2
2
(
1.60 g, 16.0 mmol) in 30 mL degassed 1,2-dimethoxyethane (DME) was heated to
00°C for 12 h under N . After cooling down, the solvent was removed in vacuum,
1
2
and the residue was subjected to chromatography on a silica gel column using
chloroform as eluent. Repeated column chromatography and the following
recrystallization from chloroform and methanol afforded the target white compound
1
TMDB-TPE (0.63 g, yield 75.0%). H NMR (400 MHz, CDCl -d): 7.50 (d, 8H, J =
3
8
.00 Hz), 6.99 (d, 8H, J = 8.00 Hz), 1.32 (s, 48H). MALDI-TOF MS: an isotopic
+
cluster peaking at m/z 836.4, Calcd. for C H B O , [
M
] 836.5. Anal. Calcd. for
50
64
4
8
C H B O ·0.4CHCl : C, 68.47; H, 7.34. Found: C, 68.43; H, 7.18.
50
64
4
8
3
Synthesis of AP-TPE. A mixture of TMDB-TPE (0.40 g, 0.5 mmol),
-bromopyrimidin-2-amine (0.44 g, 2.5 mmol), Pd(dppf)Cl ·CH Cl (0.012 g, 0.015
5
2
2
2
mmol), and potassium carbonate (K CO ) (0.69 g, 5.0 mmol) in 1,4-dioxane (20 mL)
2
3
5

and water (5 mL) was stirred at 105°C for 12 h under N . The resultant mixture was
2
then cooled to room temperature, filtrated and washed sequentially with water,
saturated ammonium chloride aqueous solution, chloroform, and methanol. The pure
1
product AP-TPE was obtained as faint yellow powder (0.25 g, yield 71.7%). H NMR
(
400 MHz, DMSO-d ): 8.55 (s, 8H), 7.47 (d, 8H, J = 8.00 Hz), 7.08 (d, 8H, J = 8.00
6
13
Hz), 6.75 (s, 8H). C NMR (400 MHz, DMSO-d ): 162.79, 155.70, 141.89, 139.70,
6
1
7
33.27, 131.51, 124.40, 121.22. MALDI-TOF MS: an isotopic cluster peaking at m/z
+
02.9,
Calcd.
for
C H N ,
[
M
]
703.2.
Anal.
Calcd.
for
4
2
32 12
C H N ·0.5CHCl ·CH OH: C, 65.59; H, 4.62; N, 21.10. Found: C, 65.55; H, 4.59;
42
32 12
3
3
N, 21.32.
Synthesis of PTOP
.
Under nitrogen gas, to a mixture of AP-TPE (0.20 g, 0.28 mmol)
and K CO (0.12 g, 0.84 mmol) in anhydrous 1,4-dioxane (15 mL), an 1,4-dioxane (2
2
3
mL) solution of CC (0.068 g, 0.37 mmol) was slowly added at room temperature. The
mixture was stirred at 90°C for 12 h. The resulting precipitate was collected by
filtration and washed with water, N,N-dimethylformamide (DMF), and methanol to
remove the unreacted starting precursors. The product was dried in vacuum at 50°C,
affording the bright yellow powder with a yield of 76.1 % (0.17 g). Anal. found: C,
6
2.42; H, 4.34; N, 21.88.
2
.3 Instrumental characterization
MALDI-TOF mass spectrum was taken on a Bruker BIFLEX III ultra-high resolution Fourier
transform ion cyclotron resonance (FT-ICR) mass spectrometer with
a-cyano-4-hydroxycinnamic acid as the matrix. NMR spectra were recorded on a Bruker
DPX 400 spectrometer in indicated solvent and referenced internally using the respective
typical residual solvent resonances relative to SiMe . Solid-state NMR spectra were collected
4
6

on a 400 MHz Bruker Avance III spectrometer. Fourier transform infrared spectra (FT-IR)
‒
1
were recorded as KBr pellets using a Bruker Tensor 37 spectrometer with 2 cm resolution.
Elemental analysis data were carried out on Elementar Vavio El III elemental analyzer.
Thermal gravimetric analysis (TGA) datum was collected on a PerkinElmer TG-7 analyzer
‒1
with a heating rate of 5°C min in the range of 25~800°C under N atmosphere. Scanning
2
electron microscope (SEM) image was obtained using a JEOL JEM-6510A scanning electron
microscopy. Transmission electron microscopy (TEM) photo was provided by HT7700
electron microscope. Steady-state fluorescence spectroscopic studies were performed on
F4500 (Hitachi) analyzer. X-ray photoelectron spectra (XPS) data were collected from PHI
5
300 ESCA System (PerkineElmer, USA). The sorption isotherms were obtained using a
Micromeritics ASAP 2020 surface area analyzer.
2
.4 Measurement for gas adsorption
Before the gas sorption measurements, the powder sample of PTOP was degassed at 100°C
for 12 hours to drastically eliminate the volatile solvents blocked in pores. The porous nature
was examined with N and CO as adsorbate. The sorption isotherms were collected at 77,
2
2
1
95, 273, and 298 K, maintaining in a bath of liquid nitrogen, dry ice/acetone slurry, an
ice/water mixture, and water in an air-conditioned 23°C laboratory, respectively.
2
.5 Sensing experiments for metal ions and aromatic derivatives
Prior to sensing experiments, PTOP (0.25 mg) was sonicated in methanol (1.0 mL) for
0 minutes to ensure the well dispersion. For the metal ions sensing experiments, the
fluorescence detection was performed by adding the methanol solution of different
2
+
2+
3+
+
2+
3+
2+
2+
2+
2+
+
2+
2+
metal ions (Na , Mg , Al , K , Fe , Fe , Co , Ni , Cu , Zn , Ag , Cd , Ba ,
2+
2+
Hg , and Pb , respectively) into a quartz cuvette containing a suspension of PTOP
.
7

With regard to the aromatic derivatives detection experiments, aromatic solvent
toluene, paraxylene, mesitylene, benzaldehyde, chlorobenzene, benzonitrile, benzoic
(
acid, phenol, and nitrobenzene, respectively) was added into a quartz cuvette
containing a methanol solution of PTOP in well dispersed form.
3
. Results and Discussion
3
.1 Synthesis and characterization
The purity of AP-TPE has been checked by MS spectra and NMR spectra, Fig. S1.
PTOP was synthesized through the reaction between AP-TPE and CC with the
assistance of K CO , Scheme 1. As mentioned above, the existence of rich pyrimidine
2
3
nitrogen atoms and amino groups surrounding the 1,3,5-triazine segments inside the
pores of PTOP creates abundant potential chelating sites for metal ions.
13
Fig. 1a shows the solid state C CP/MAS NMR spectrum of PTOP with resonance
signals appearing at 164.4, 158.2, 144.5, 134.7, 127.2 and 125.7 ppm. With the aid of
13
C NMR spectrum of AP-TPE in DMSO-d (Fig. S1c), the signal of PTOP at
6
approximately 144.5 ppm is unambiguously assigned to the ethylene carbon atoms, the
peaks at 144.5, 134.7, and 127.2 ppm attributed to benzene carbon atoms, and those at
1
64.4, 158.2, and 125.7 ppm due to the pyrimidine carbon atoms. In addition, triazine
carbon atoms also contribute to the overlapped signal at 164.4 ppm.
Fourier transform infrared spectroscopy (FT-IR) was also used to illustrate the
successful formation of PTOP. The FT-IR spectra of AP-TPE, CC, and PTOP are
‒1
compared in Fig. 1b. As can be seen, the absorption at 3399 cm due to the -NH
2
stretching vibration for AP-TPE got significantly decreased relative after the formation
‒1
of PTOP. In addition, new bands appear at 1715 and 1426 cm after the integration of
triazine moieties with AP-TPE, confirming the formation of PTOP
.
8

As exhibited from thermal gravimetric analysis (TGA) curve of PTOP in Fig. S2,
this material experiences a gradual weight loss of 8.0% from room temperature to
2
70°C, due to the release of volatile solvent molecules embedded inside the pores of
PTOP. After that, the organic component keeps stable until 440°C and then rapidly
decomposes along with further increasing the temperature.
The morphology of PTOP was studied first by scanning electron microscope (SEM),
Fig. 1c, indicating the formation of a peony-like cluster assembled from nanosheets,
and transmission electron microscope (TEM) further revealed the layer-like PTOP
,
Fig. 1d.
1
3
Fig. 1 Various characterization of PTOP including solid state C CP/MAS NMR spectrum (a),
FT-IR spectra (b), SEM image (c), and TEM diagram (d).
3
.2 Gas adsorption studies
Towards inspecting the porous nature of PTOP, gas sorption experiments using N and CO2
2
as adsorbate, respectively, were carried out. The result indicates that PTOP does not adsorb
any nitrogen at 77 K, possibly due to the existence of narrow pores, which hinder the N gas
2
diffusion into the pores. This, however, is not true for CO because of the smaller kinetics
2
diameter of 3.3 Å of CO than that of N molecule, 3.64 Å.[40] As shown in Fig. 2, the CO
2
2
2
sorption isotherm of PTOP at 195 K shows a typical type I character, disclosing the
9

microporous nature of PTOP. The experimental Brunauer-Emmett-Teller (BET) surface area
2
‒1
amounts to 227 m g . In addition, the CO adsorption isotherms of PTOP at 273 and 298 K
2
3
‒1
were also determined, showing the uptakes at 760 mmHg of 22 and 15 cm g , respectively,
confirming the porous nature of PTOP. According to the virial equation,[41,42] the isosteric
‒
1
heat of 28.8 kJ mol was calculated by fitting the adsorption isotherms measured at 273 and
98 K. Obviously, the existence of the permanent porosity for PTOP would be favorable to
2
the diffusion of analyte in sensing.
Fig. 2 CO adsorption and desorption of PTOP at 195, 273 and 298 K, respectively.
2
3
.3 Detection of metal ions
TPE is a typical AIE chromophore. After integration of four 2-aminopyrimidin groups
with a TPE unit, the new obtained AP-TPE in DMF exhibits a very weak emission, Fig.
S3. After increasing H O content in H O/DMF solution to 30%, there suddenly
2
2
appears a strong emission at 517 nm, Fig. S3b. When the H O content was further
2
gradually increased to 70%, the emission maximum of AP-TPE undergoes the blue
shifts from 526 to 506 nm and the increased fluorescence intensity. The blue-shift at
lower water fraction is originated from the formation of a higher ordered
microstructure following the increase of water fraction.[43,44] By increasing the
1
0

content of H O to 90%, the fluorescence intensity of AP-TPE was reduced because of
2
aggregation effect and the emission maximum was gradually red-shifted back to 525
nm. These results clearly disclose the presence of aggregation-induced emission,
confirming the newly fabricated AP-TPE being AIE-active. The combination of
AP-TPE into porous organic polymers must result in an AIE-confined fluorescence
species.
As can be found in Fig. S4, the emission peak position of PTOP (0.25 mg/mL
dispersion solution) slightly depends the solvent species. In detail, the maximum
emission of this material keeps constant at ca. 507 nm in DMF,
N,N-dimethylacetamide (DMA), methanol (MeOH), ethanol, acetone, tetrahydrofuran
(
THF), ethyl acetate, chloroform, and toluene, but displays a slight blue shift in the
narrow range of 0~5 nm for water, acetonitrile, or n-hexane. However, the
fluorescence intensity of PTOP shows a solvent-dependent character associated with
the solvent polarity effect.[45] Since PTOP in methanol exhibits the strongest
emission intensity among all solvents, the subsequent sensing experiments were
carried out in methanol as detailed below.
For the purpose of studying the sensing properties of PTOP towards metal species,
its fluorescence properties were investigated by adding the methanol solution of a
+
2+
3+
+
2+
3+
2+
2+
2+
2+
series of metal ions including Na , Mg , Al , K , Fe , Fe , Co , Ni , Cu , Zn ,
+
2+
2+
2+
2+
Ag , Cd , Ba , Hg , and Pb (10 mM) into the methanol suspension of PTOP
.
+
2+
According to the experimental results, addition of Na and Mg solution induces a
slight increase in the fluorescence intensity, while addition of the other metal ions
leads to the varying fluorescence quenching phenomena depending on the metal
species, Fig. 3a and S5a. It is well known that the sensing performance of the
fluorescent sensor is closely dependent on both the selective binding of receptor to
11

detected target and the accordingly signal change of the fluorescent unit.[46,47] As a
consequence, the fluorescent sensor will exhibit selective sensing function towards
different targets. This is also true for target metal ions. However, the essential reason
+
behind the different quenching of fluorescence sensor for different metal ions such K
2+
2+
vs. Pb /Hg is still not clear at the present stage. Interestingly, the fluorescence of
3+
PTOP is completely quenched after addition of Fe . In addition to the selective
3+
2+
2+
binding between the sensor and Fe rather than other metal ions such as Pb and Co ,
3+
the present great quenching of Fe on the fluorescence sensor might be also due to the
synergistic effect of orbit and paramagnetism, which is consistent with the reported
literature.[48-50] In addition, time-dependent sensing experimental towards Fe ion
d
3+
3+
indicates the immediate emission quenching flowing the Fe addition, Fig. 3b.
Furthermore, interference experiments were carried out for PTOP by
3+
simultaneously introducing Fe ions (10 mM, 1.0 mL) and other metal ions (10 mM,
.0 mL) into the suspension (0.5 mL) of PTOP at room temperature. Even in the
1
3+
presence of various interference cations, the quenching effect of the Fe ions on the
emission of PTOP was still significant, Fig. S5b, further confirming the high-selective
3+
sensing capability of this porous organic polymer towards Fe ions.
3+
Given the fact that excess amount of Fe ion is harmful for human organisms,
3+
developing selective and sensitive sensors for Fe therefore is of significant
importance. To qualitatively investigate the quenching sensitivity of PTOP towards
3+
Fe analyte, the fluorescence spectra of PTOP were determined in different
3+
concentrations of Fe ion, Fig. 3c. According to the Stern-Volmer equation
3+
I0
/I=1+Ksv[M][49,51] (I0: the initial fluorescence intensity in the absence of Fe ; I: the
3+
fluorescence intensity in the presence of Fe ; Ksv: the quenching constant; [M]: the
3+
‒1
molar concentration of Fe analyte), quenching constant Ksv of 5.6 mM for PTOP
1
2

3+
towards Fe was deduced on the basis of experimental results, Fig. 3d. This in turn
gives the limit of detection (LOD) concentration of 4.5 µM for PTOP towards
3+
fluorescence sensing Fe according to the equation 3σ/Ksv (σ: standard deviation),
which is lower than that for most previously reported fluorescent sensing materials
3+
towards Fe ion.[52–54] Towards the practical use, the sensitivity of PTOP to detect
3+
Fe ion in aqueous solution was also tested, and the detection limit of PTOP was
calculated to be 7.0 µM by the same method, as shown in Fig. S6. In the present case,
3+
the LOD of PTOP for Fe in aqueous solution is also obviously lower than the
drinking water hygiene standard of 0.3 mg/L (5.4 mM) proposed by "drinking water
sanitary standard" (GB5749-2006) of People's Republic of China[55] as well as the
international standard water quality index and limit value (0.3 mg/L),[56] indicating
the future practical application potential of the newly developed PTOP
.
Fig. 3 (a) Quenching efficiency of PTOP immersed in MeOH solutions of different metal ions. (b) The
3
+
time-dependent emission intensity (at 507 nm) of PTOP immersed in a MeOH solution of Fe ion (inset:
the time-dependent emission spectra under the excitation wavelength of 410 nm). (c) Emission spectra by
3
+
the gradual addition of a Fe methanol solution (inset: images under sunlight and 365 nm UV-radiation of
3
+
PTOP (left) and PTOP@Fe (right), respectively). (d) K curve of PTOP immersed in MeOH solutions
sv
1
3

3
+
of Fe ion with different concentrations (insert: the changed I /I curve with the increased concentration of
0
3
+
Fe ion).
3
+
Towards understanding the fluorescence quenching mechanism of PTOP by Fe , X-ray
3
+
photoelectron spectroscopic (XPS) studies were conducted on the Fe -tested material
3
+
(
denoted as PTOP@Fe ). As can be seen from Fig. 4a, besides C and N elements, XPS
3
+
3+
spectrum of PTOP@Fe also displays the Fe element signal at the binding energy of 711.0
3
+
eV,[57] indicating the inclusion of Fe within the pores of PTOP. Nevertheless, the peak due
2
3
to the combination of sp (-C=N) and sp (-NH) nitrogen atoms at the binding energy of
98.4 eV in the high-resolution N 1s XPS spectrum of PTOP takes a slight up-shift to 398.6
3
3
+
3+
eV for PTOP@Fe , Fig. 4b, suggesting the existence of interaction between Fe cations and
3
+
imine group basic sites in PTOP@Fe .
3
+
Fig.4 XPS spectra (a) and N 1s XPS spectra (b) for PTOP (purple) and PTOP@Fe (orange).
3
.4 Detection of aromatic derivatives
As mentioned above, the wide range of industrial and biochemical application of
various nitroaromatic compounds not only contaminates the environment but also
seriously threats human health.[58] It is therefore important to develop the effective
fluorescent sensors for such kind of organic compounds. As a consequence, the
fluorescence properties of PTOP were also investigated in the presence of a series of
aromatic derivatives. As shown in Fig. 5a and b, the emission of PTOP was not
1
4

influenced by adding the aromatic analytes including toluene, paraxylene, mesitylene,
benzaldehyde, chlorobenzene, benzonitrile, benzoic acid, and phenol into the methanol
of PTOP. In contrast, addition of nitrobenzene (NB), the exemplary and harmful
explosive, leads to a sharp fluorescence quenching of PTOP due probably to the
electron transfer from the polymer to analyte,[59,60] indicating the high selectivity of
PTOP toward NB over other aromatic compounds. The quenching process was also
monitored by the time-dependent emission intensity at the maximum peak, Fig. S7,
revealing the occurrence of immediate fluorescence quenching. Studies over the
change of emission intensity as a response of the added NB concentration revealed that
addition of NB even with the concentration as low as 14.7 nM induces a serious
fluorescence quenching of PTOP, Fig. 5c. When increasing the concentration of NB
introduced to 0.6
µM, the fluorescence signal got almost completely disappeared.
Aiming for identifying the quenching constant of PTOP towards NB, the
Stern-Volmer equation was employed again. As shown in Fig. 5d, from the linear
‒1
fitting, the quenching constant Ksv of 16.3 µM between PTOP and NB was deduced,
with the LOD of PTOP being determined to be 1.6 nM according to 3σ/Ksv. This value
again is much lower than the excellent fluorescent materials for nitrobenzene detection
reported previously,[61–65] indicating the good selectivity and high sensitivity of
PTOP to NB.
1
5

Fig. 5 Emission spectra (a) and quenching efficiency (b) of PTOP immersed into the MeOH solutions of
different aromatic compounds. (c) Emission spectra by the gradual addition of a NB methanol solution
(inset: images under sunlight and 365 nm UV-radiation of PTOP (left) and PTOP@NB (right),
respectively). (d) K curve of PTOP immersed in MeOH solutions of NB with different concentrations
sv
(
insert: the changed I /I curve with the increased concentration of NB).
0
4
. Conclusion
Briefly summarizing above, a new TPE-based porous organic polymer has been
fabricated and characterized. The permanent porosity, TPE AIEgen, and rich chelating
sites synergistically contribute to the immediate fluorescence sensing of Fe(III) and
nitrobenzene with high selectivity and sensitivity. In particular, the limit of detection
3+
concentration of PTOP for fluorescent sensing Fe is much lower than the
international standard water quality index, suggesting its good future practical
applications.
Acknowledgements
Financial support from the Natural Science Foundation of China (Nos. 21631003 and
1
6

2
1805005), the Fundamental Research Funds for the Central Universities (No.
FRF-BD-17-016A and FRF-BR-18-009B), and University of Science and Technology
Beijing is gratefully acknowledged.
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2
4

Highlights
ò
ò
ò
A fluorescent porous tetraphenylethylene-based organic polymer (PTOP) was
constructed.
2
CO sorption experiment reveals that the BET surface area of PTOP is 227 m
2
–
1
g .
PTOP endows a fast-response and sensitive fluorescence sensing Fe(III) and
nitrobenzene with a low limit of detection of 4.5 µM and 1.6 nM, respectively.