AEE-active conjugated polymers based on di(naphthalen-2-yl)-1,2-diphenylethene for sensitive fluorescence detection of picric acid

Article abstract of DOI:10.1016/j.dyepig.2019.108041

Picric acid (PA) is an archetypal explosive material that possesses low safety coefficient and high detonation velocity. The development of reliable and effective sensors for PA using PL spectroscopy methods is practically significant. Among various PL se

Full text of DOI:10.1016/j.dyepig.2019.108041

Dyes and Pigments xxx (xxxx) xxx
Contents lists available at ScienceDirect
Dyes and Pigments
journal homepage: http://www.elsevier.com/locate/dyepig
AEE-active conjugated polymers based on di
naphthalen-2-yl)-1,2-diphenylethene for sensitive fluorescence detection
of picric acid
(
a
,
1
a
,
1
b
a
a
Yangpeng Zhuang , Jinya Yao , Zeyan Zhuang , Chunjun Ni , Hongming Yao ,
a
a
,
*
b,**
Deliang Su , Jian Zhou , Zujin Zhao
a
College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, Hangzhou, 310036, China
b
State Key Laboratory of Luminescent Materials and Devices, Key Laboratory of Luminescence from Molecular Aggregates of Guangdong Province, South China
University of Technology, Guangzhou, 510640, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Picric acid (PA) is an archetypal explosive material that possesses low safety coefficient and high detonation
velocity. The development of reliable and effective sensors for PA using PL spectroscopy methods is practically
significant. Among various PL sensors, conjugated polymers have emerged as a class of fascinating candidates for
the detection of PA due to the “molecular wire” sensing mechanism. In this work, we synthesized and charac-
terized two new di (naphthalen-2-yl)-1,2-diphenylethene (DNDPE)-based conjugated polymers with different
conjugation degree and hydrophilicity. They show good solubility, aggregation-enhanced emission (AEE) feature
and high photostability. Both polymers exhibit sensitive fluorescence quenching by PA, and the KSV at the initial
Aggregation-enhanced emission
Fluorescence detection
Conjugated polymers
di(naphthalen-2-yl)-1,2-diphenylethene
Picric acid
5
À 1
stage of the Stern–Volmer plots is up to 9.8 � 10 M . In addition, after exposure to UV light, these polymers still
could effectively detect the PA in water. They also could on-site detect PA with high sensitivity by preparing the
polymer-immobilized fluorescent test strips. The results demonstrate that these new DNDPE-based conjugated
polymeric sensors could be good candidates for the sensitive detection of PA.
1
. Introduction
The detection of explosive materials is crucial for security and sta-
anemia, anaphylaxis and potential destructions to liver, kidney and
respiratory system [3]. Therefore, the development of reliable and
effective sensors for PA using PL spectroscopy methods is practically
significant. Currently, much attention has been paid on various PL
sensors based on quantum dots [4], metal-organic frameworks [5],
conjugated polymers [6], nanosheets [7], etc. Among them, conjugated
polymers have emerged as a class of fascinating candidates for the
detection of PA due to the “molecular wire” sensing mechanism. When
the photo-induced electron in the conjugated polymers delocalizes along
the chains and transfers from electron-rich conjugated polymers to
electron-deficient PA, the fluorescence of polymers could be quenched
efficiently [8]. However, to attain ultrasensitivity and minimize the
sensor’s content for practical applications, it is still essential and
meaningful to further develop novel conjugated polymeric sensors with
special molecular structures.
bility of the society. The arising global terrorism has required the
detection methods for explosives should be sensitive and convenient. So
far, many explosives detection methods have been proposed, such as gas
chromatography, mass spectrometry, surface-enhanced Raman spec-
troscopy, ion mobility spectrometry, photoluminescence (PL) spectros-
copy and so on [1]. Among these mentioned methods, the PL
spectroscopy methods have attracted increasing attention owing to their
remarkable sensitivity, rapid response, convenience and low cost [2].
Picric acid (PA) is an archetypal kind of explosive materials that
possesses low safety coefficient and high detonation velocity. Due to the
good water solubility of PA, it is prone to contaminate soil and
groundwater when widely applied in dyes or pesticide industries. At low
concentration, it can give rise to human health disorders including
In our previous researches, we developed tailor-made naphthalene-
*
Corresponding author.
** Corresponding author.
E-mail addresses: zhoujian@hznu.edu.cn (J. Zhou), mszjzhao@scut.edu.cn (Z. Zhao).
1
These authors contributed equally to this work.
https://doi.org/10.1016/j.dyepig.2019.108041
Received 18 September 2019; Received in revised form 9 November 2019; Accepted 11 November 2019
Available online 14 November 2019
0
143-7208/© 2019 Elsevier Ltd. All rights reserved.
Please cite this article as: Yangpeng Zhuang, Dyes and Pigments, https://doi.org/10.1016/j.dyepig.2019.108041

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
catalyzed Suzuki coupling reaction of 13-(thiophen-3-yl)-2,5,8,11-tet-
raoxatridecane with 1,2-di (naphthalen-2-yl)-1,2-bis(4-(4,4,5,5-tetra-
methyl-1,3,2-dioxaborolan-2-yl)phenyl)ethene produced P2 in 59%
yield. The use of flexible long-chain ether groups enhances the hydro-
philicity of P2 and impels the formation of intumescent and stable ag-
w
gregates. The molecular weight (M ) values of polymers were analyzed
by the GPC experiment using polystyrene as an internal standard. The
À 1
M
w
and polydispersity (PDI) of P1 are 20669 g mol
and 1.64,
of P2 is
respectively. In the presence of long-chain ether groups, the M
w
À 1
increased to 50576 g mol and the PDI is still around 1.62. The pre-
Scheme 1. Molecular structure of DNDPE-based conjugated polymers.
pared P1 and P2 were readily soluble in common organic solvents like
3
CHCl , toluene, THF and DMSO.
Naphthalen-2-yl(p-tolyl)methanone (3): A solution of n-BuLi
(9.4 mL, 14.49 mmol) in n-hexane was added dropwise to a solution of 2-
substituted ethenes that held prominent aggregation-induced emission
AIE) property [9]. Based on the unit, di (naphthalen-2-yl)-1,2-diphe-
(
bromonaphthalene (3 g, 14.49 mmol) in dry THF (150 mL) under N
2
at
ꢀ
nylethene (DNDPE), a series of new conjugated polymers were further
synthesized [10]. In these polymers, the twisted conformation of DNDPE
constructs the “cage” structure of the polymer chains, which could
effectively capture the PA in solution, thus facilitating the
photo-induced electron transfer (PET) and strengthening the fluores-
cence quenching efficiency. With the “cage” idea in mind, we recently
developed two new DNDPE-based conjugated polymers with different
conjugation degree and hydrophilicity. Both of them show
aggregation-enhanced emission (AEE) feature and perform efficiently in
the detection of PA.
À 78 C. After stirring for 1 h, the solution was transferred to excess p-
toluoyl chloride (3.36 g, 21.73 mmol) in dry THF (30 mL). The resulting
ꢀ
mixture was kept at À 78 C for 30 min. The mixture was then warmed to
2 3
room temperature and stirred overnight. Then K CO solution (1 M,
5 mL) was added to the mixture. The mixture was extracted with ethyl
acetate by three times. The combined organic layers were washed with
4
water, and dried over anhydrous MgSO . After filtration and solvent
evaporation, the residue was purified by silica-gel column chromatog-
raphy using hexane/dichloromethane as eluent. White solid of 3 was
obtained in 26.7% yield. 1H NMR (500 MHz, CDCl
), δ (TMS, ppm): δ
.25 (s, 1H), 7.97–7.89 (m, 4H), 7.78 (d, 2H, J ¼ 8.1 Hz), 7.64–7.53 (m,
3
8
2
1
1
1
3
H), 7.32 (d, 2H, J ¼ 7.9 Hz), 2.47 (s, 3H). C NMR (126 MHz, CDCl
3
):
2
. Experimental
95.4, 142.1, 134.1, 131.2, 130.5, 129.3, 128.3, 128.0, 127.2, 127.1,
þ
26.8, 125.7, 124.8, 20.6. HRMS: m/z 247.1102 ([MþH] , calcd for
2
.1. General
18
C H15O 247.1123).
1
,2-Bis(4-bromophenyl)-1,2-di(naphthalen-2-yl)ethene (4): Into
THF was distilled from sodium benzophenone ketyl under dry ni-
a 250 mL two-necked round-bottom flask with a reflux condenser were
trogen immediately prior to use. Chemicals and reagents were pur-
placed 3 (1.5 g, 6.09 mmol), zinc dust (4.78 g, 79.17 mmol). The flask
chased from Aldrich and J&K Scientific Ltd., and used as received
1
13
was evacuated under vacuum and flushed with dry N three times. Dry
without further purification. H and C NMR spectra were measured on
a Bruker AV 500 spectrometer in deuterated chloroform using tetra-
methylsilane (TMS; δ ¼ 0) as internal reference. UV–vis absorption
spectra were measured on a Shimadzu UV-2600 spectrophotometer.
Photoluminescence was recorded on a PerkinElmer LS 55 spectrofluo-
rometer. Mass spectra were recorded on an Agilent 1290 Infinity LC/
2
ꢀ
THF (100 mL) was then added. The mixture was cooled to À 78 C and
4
TiCl (1.39 g, 7.31 mmol) was added dropwise by a syringe. After stirred
for 0.5 h, the reaction mixture was warmed to room temperature and
then heated to reflux for 12 h. The mixture was cooled to room tem-
perature and poured into water, and extracted with dichloromethane by
three times. The combined organic layers were washed with saturated
6
530 Q-TOF MS. The number (M
weights and polydispersity indices (PDI or M
estimated by a Waters Associates gel permeation chromatography (GPC
414) system equipped with RI and UV detectors. The ground-state
n
) and weight average (M
w
) molecular
4
brine solution and water, and dried over anhydrous MgSO . After
w
/M ) of the polymers were
n
filtration and solvent evaporation, the residue was purified by silica-gel
column chromatography using hexane/dichloromethane as eluent.
2
1
White solid of 4 was obtained in 52% yield. H NMR (500 MHz, CDCl
3
),
geometries were optimized using the density functional theory (DFT)
method with B3LYP hybrid functional at the basis set level of 6–31G(d).
All the calculations were performed using Gaussian 09 package.
δ (TMS, ppm): 7.73 (d, 1H, J ¼ 7.7 Hz), 7.65 (d, 1H, J ¼ 7.9 Hz),
7
7
3
.61–7.51 (m, 5H), 7.47 (d, 1H, J ¼ 8.5 Hz), 7.41–7.29 (m, 4H),
.22–7.16 (m, 2H), 6.99–6.92 (m, 6H), 6.85 (d, 2H, J ¼ 7.9 Hz), 2.28 (s,
H), 2.21 (s, 3H). 13C NMR (126 MHz, CDCl
): 142.0, 141.7, 141.2,
3
2
.2. Explosive detection
140.9, 140.8, 136.2, 133.3, 132.1, 131.5, 131.4, 130.5, 130.4, 129.8,
1
28.5, 128.1, 128.0, 127.5, 127.0, 125.7, 21.2. HRMS: m/z 461.2267
þ
A solution of picric acid (PA) in ethanol with a concentration of
([MþH] , calcd for C36H29 461.2269).
À 1
0
.1 mg mL was prepared by dissolving an appropriate amount of PA in
1,2-Bis(4-(bromomethyl)phenyl)-1,2-di(naphthalen-2-yl)
ethene (5): Into a 250 mL round-bottom flask with a reflux condenser
were placed 4 (1.0 g, 2.17 mmol), NBS (0.97 g, 5.43 mmol), and BPO
ethanol. Photoluminescence titration was carried out by adding aliquots
of PA solution into solutions of P1 and P2 in THF/water mixtures (P1:
f
w
¼ 90 vol%; P2: f
w
¼ 80 vol%).
(52.6 mg, 0.217 mmol) followed by addition of 30 mL of CCl . After
4
refluxed for 24 h, the mixture was cooled to room temperature and
3
. Results and discussion
filtered, the solvent was evaporated under reduced pressure. The crude
product was purified by silica-gel column chromatography using pe-
1
3
.1. Synthesis
troleum ether as eluent to give 5 as yellow solid in 53.2% yield. H NMR
(
500 MHz, CDCl
3
), δ (TMS, ppm): 7.74 (d, 1H, J ¼ 7.9 Hz), 7.67 (d, 1H,
Scheme 1 shows the molecular structures of the DNDPE-based con-
J ¼ 7.8 Hz), 7.61–7.53 (m, 3H), 7.53–7.47 (m, 3H), 7.43–7.32 (m, 4H),
1
3
jugated polymers P1 and P2. The detailed synthetic routes and charac-
terization for monomers and polymers are depicted in Scheme 2. Briefly,
the Witting reaction of 4,4’-(1,2-di (naphthalen-2-yl)ethene-1,2-diyl)
dibenzaldehyde and corresponding phosphonium salt in the presence of
potassium tert-butoxide afforded P1 in 35% yield. The palladium-
7.17–7.14 (m, 4H), 7.09–7.03 (m, 6H), 4.44 (s, 2H), 4.37 (s, 2H).
C
NMR (126 MHz, CDCl ): 143.96, 143.81, 141.18, 140.88, 133.18,
3
132.25, 131.91, 130.64, 129.42, 128.63, 128.06, 127.56, 127.24,
þ
125.96, 125.92, 33.55. HRMS: m/z 618.0415 (M calcd for C36
H26Br
2
618.0381).
2

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
Scheme 2. Molecular structures and synthetic routes to DNDPE-based conjugated polymers P1 and P2.
3

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
À 5
Fig. 1. Absorption spectra of THF solutions of P1 (A) and P2 (B). Solution concentration: 10 M.
4
,4’-(1,2-Di(naphthalen-2-yl)ethene-1,2-diyl)dibenzaldehyde
4
saturated brine solution and water, and dried over anhydrous MgSO .
(
6): NaHCO
3
(0.27 g, 3.24 mmol) was added to a stirred solution of 5
After filtration and solvent evaporation, the residue was purified by
silica-gel column chromatography using hexane/dichloromethane as
(
ꢀ
0.5 g, 0.809 mmol) in DMSO (3.7 mL) and the mixture was stirred at 95
Cfor 4 h. The mixture was cooled back to room temperature, diluted
1
eluent. White solid of 9 was obtained in 49% yield. H NMR (500 MHz,
with water, extracted with dichloromethane by three times. The organic
layer was dried over anhydrous MgSO . After filtration and solvent
evaporation, the crude product was purified by silica-gel column chro-
matography and 6 was obtained as a yellow solid in 37.3% yield. 1
NMR (500 MHz, CDCl ), δ (TMS, ppm): 9.94 (s, 2H), 7.70 (d, 2H,
CDCl
3
), δ (TMS, ppm): 7.79 (d, 1H, J ¼ 7.7 Hz), 7.71 (d, 1H, J ¼ 7.8 Hz),
4
7.65 (dd, 2H, J ¼ 7.9, 4.7 Hz), 7.58 (d, 1H, J ¼ 7.9 Hz), 7.53 (d, 3H,
J ¼ 8.7 Hz), 7.49–7.36 (m, 4H), 7.32 (d, 2H, J ¼ 8.4 Hz), 7.24–7.15 (m,
13
H
3
4H), 7.02–6.95 (m, 4H). C NMR (126 MHz, CDCl ), δ (TMS, ppm):
3
142.58, 140.63, 140.51, 133.16, 131.23, 133.11, 130.64, 130.59,
J ¼ 8.0 Hz), 7.66 (d, 4H, J ¼ 8.3 Hz), 7.58–7.35 (m, 12H), 7.27 (s, 2H),
129.24, 128.06, 127.54, 127.51, 127.36, 126.11, 125.96, 121.00.
1
3
þ
7
1
1
4
.16 (dd, 2H, J ¼ 8.5, 1.7 Hz). C NMR (126 MHz, CDCl
3
): 191.76,
HRMS: m/z 590.0072 (M , calcd for C34
H22Br
2
590.0068).
49.73, 142.07, 139.78, 134.70, 133.12, 132.45, 132.08, 130.80,
(((1,2-Di(naphthalen-2-yl)ethene-1,2-diyl)bis(4,1-phenylene))
bis(methylene))bis(triphenylphosphonium) bromide (10): Into a
100 mL round-bottom flask with a reflux condenser were placed 5 (0.5 g,
29.36, 128.88, 128.02, 127.76, 127.63, 126.50, 126.30. HRMS: m/z
þ
88.1781 (M , calcd for C36
H
24
O
2
488.1776).
(
4-Bromophenyl) (naphthalen-2-yl)methanone (8): A solution of
0.81 mmol) and PPh
3
(0.85 g, 3.24 mmol). 30 mL of DMF was then
ꢀ
n-BuLi (9.4 mL, 14.49 mmol) in n-hexane was added dropwise to a so-
added. After heated to 100 C for 24 h, the mixture was cooled to room
temperature and poured into toluene. The precipitates were filtered and
lution of 2-bromonaphthalene (3 g, 14.49 mmol) in dry THF (150 mL)
ꢀ
1
under N
2
at À 78 C. After stirring for 1 h, the solution was transferred to
washed with toluene to give 10 as yellow solid in 53% yield. H NMR
excess 4-bromobenzoyl chloride (4.77 g, 21.73 mmol) in dry THF
(500 MHz, CDCl
3
), δ (TMS, ppm): 7.77–7.31 (m, 44H), 6.99 (d, 2H,
ꢀ
(
30 mL). The resulting mixture was kept at À 78 C for 30 min. The
J ¼ 8.0 Hz), 6.81 (s, 4H), 6.73 (s, 2H), 2.42 (s, 4H). HRMS: m/z
þ
þ
56 2
H P Br] 1061.3041).
mixture was then warmed to room temperature and stirred overnight.
Then K CO solution (1 M, 5 mL) was added to the mixture. The mixture
was extracted with ethyl acetate by three times. The combined organic
layers were washed with water, and dried over anhydrous MgSO . After
1061.3049 ([M À Br] , calcd for [C72
2
3
1,2-Di(naphthalen-2-yl)-1,2-bis(4-(4,4,5,5-tetramethyl-1,3,2-
dioxaborolan-2-yl)phenyl)ethene (11): 9 (1 g, 1.7 mmol), bis-(pina-
colato)diborane (1.22 g, 4.8 mmol), KOAc (1.17 g, 11.9 mmol), and
4
filtration and solvent evaporation, the residue was purified by silica-gel
column chromatography using hexane/dichloromethane as eluent.
White solid of 8 was obtained in 22.7% yield. 1H NMR (500 MHz,
dioxane (30 mL) were mixed together in a 100 mL flask. After degassing,
0
[Pd (dppf)Cl
2
] (0.24 g, dppf ¼ 1,1 -bis(diphenylphosphanyl)ferrocene)
ꢀ
was added. The reaction mixture was kept at 100 C for 24 h, and then
cooled to room temperature. The organic solvent was distilled out, and
the residual solid was dissolved in dichloromethane and washed with
CDCl
J ¼ 8.5 Hz), 7.67–7.58 (m, 3H), 7.57–7.53 (m, 1H). C NMR (126 MHz,
CDCl ), δ (TMS, ppm): 195.63, 136.63, 135.36, 134.42, 132.24, 131.82,
3
), δ (TMS, ppm): 8.22 (s, 1H), 7.95–7.87 (m, 4H), 7.72 (d, 2H,
13
3
4
saturated brine solution. After drying with anhydrous MgSO , the sol-
1
1
3
31.69, 131.62, 129.44, 128.53, 128.50, 127.88, 127.47, 126.97,
vent was distilled out. The crude product was purified by flash chro-
matography using dichloromethane as the eluent and then recrystallized
25.59. HRMS: m/z 311.0071 ([MþH]þ, calcd for
17
C H12BrO
1
11.0072).
in isopropanol to give 11 as white crystals (0.23 g, 20%). H NMR
1
,2-Bis(4-bromophenyl)-1,2-di(naphthalen-2-yl)ethene (9): Into
(500 MHz, CDCl
3
), δ (TMS, ppm): 7.72 (d, 2H, J ¼ 7.9 Hz), 7.59–7.53 (m,
a 250 mL two-necked round-bottom flask with a reflux condenser were
4H), 7.49 (d, 6H, J ¼ 7.7 Hz), 7.41–7.34 (m, 4H), 7.16 (dd, 2H, J ¼ 8.4,
1
3
placed 8 (1.5 g, 4.8 mmol), zinc dust (3.76 g, 57.8 mmol). The flask was
1.5 Hz), 7.09 (d, 4H, J ¼ 8.0 Hz), 1.29 (s, 24H). C NMR (126 MHz,
evacuated under vacuum and flushed with dry nitrogen three times.
CDCl ), δ (TMS, ppm): 145.68, 140.62, 140.10, 133.18, 132.10, 131.15,
3
ꢀ
1
00 mL of dry THF was then added. The mixture was cooled to À 78 C
129.91, 129.55, 128.47, 127.07, 126.46, 126.07, 124.66, 82.70, 82.65,
þ
and TiCl
4
(1.09 g, 5.76 mmol) was added dropwise by a syringe. After
23.84. HRMS: m/z 684.3579 (M , calcd for C46
H
B
46 2
O
4
684.3582).
stirred for 0.5 h, the reaction mixture was warmed to room temperature
and then heated to reflux for 12 h. The mixture was cooled to room
temperature and poured into water, and extracted with dichloro-
methane by three times. The combined organic layers were washed with
P1: To a three-necked, round-bottom flask was added 10 (50 mg,
0.044 mmol) in 3 mL of dry DMF under nitrogen in an ice-water bath.
Potassium tert-butoxide (14.8 mg, 0.132 mmol) was then added under
stirring. The intense orange solution was stirred at room temperature for
4

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
w 0
Fig. 2. (A and C) Fluorescence spectra of P1 and P2 in THF/water mixtures with different water fractions (f ), respectively. (B and D) Plots of I/I versus the
compositions of the aqueous mixtures. I
0
¼ fluorescence intensity in pure THF solution. E and F: photographs of P1 and P2 in THF/water mixtures with different f
M; excitation wavelength: 350 nm.
w
,
taken under the illumination of a UV lamp (365 nm). Concentration: 10
μ
3
0 min, after which a solution of 6 (21.5 mg, 0.044 mmol) in 2 mL of dry
ethanol. The residue was dissolved in THF and added into 200 mL of
1
DMF was added. The reaction mixture was stirred at room temperature
overnight. The reaction mixture was poured into ice water and yellow
precipitates were collected. The crude product was washed with cold
petroleum ether. Yellow solid of P1 was obtained in 35% yield. H NMR
(500 MHz, CDCl
3
), δ (TMS, ppm): 7.70–7.38 (m, 10H), 7.31 (s, 2H), 7.28
(s, 2H), 7.13–7.03 (m, 4H), 6.99–6.93 (m, 2H), 6.91 (s, 2H), 6.80–6.69
5

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
the density functional theory (DFT) were carried out to study their
electronic structures and energy levels. As shown in Fig. 3, the highest
occupied molecular orbitals (HOMOs) and the lowest unoccupied mo-
lecular orbitals (LUMOs) occupy all the building blocks in conjugated
fragments, demonstrating the good conjugation of the backbones. There
are also electronic clouds of HOMOs and LUMOs located on the
naphthalen-2-yl moiety in P1 and P2, indicating that the rotation of
naphthalen-2-yl groups could deactivate the excited states and result in
weak PL emissions of polymers in solutions.
3
.4. Explosive detection
To explore the potential use of P1 and P2 as fluorescent chemo-
sensors for sensitive detection of PA, the aggregates of P1 in THF/water
mixtures (f
w
¼ 90 vol%) and P2 in THF/water mixtures (f ¼ 80 vol%)
w
were used as probes for detection, respectively. As shown in Fig. 4A and
B, with the addition of PA to the aggregate suspension of P1, the fluo-
rescence intensity is gradually quenched. The detection limit of PA is
calculated to be 0.181
μ
M. At low concentrations of PA, the
Stern–Volmer plot is linear and gives a quenching constant (KSV) of
4
À 1
4
À 1
4
.7 � 10 M , which is higher than those (KSV < 1.2 � 10 M ) of the
DNDPE-based conjugated polymers reported previously [10]. Similar
quenching effect is also observed for P2. In addition, the aggregate
suspension of P2 can detect PA with extremely good sensitivity. The
Fig. 3. (A) Optimized conformations and calculated molecular orbital ampli-
tude plots of HOMOs and LUMOs for the conjugated fragments M1 and M2 for
P1 and P2, respectively. (B) Calculated energy level diagram of M1, M2
and PA.
fluorescence intensity is rapidly quenched, even at 1.4
μ
M, with an in-
crease of PA concentration. The KSV for P2 at the initial stage of the
5
À 1
Stern–Volmer plots reaches to 9.8 � 10 M (Fig. 4C and D). These
results reveal that P2 is much more sensitive to PA than P1, which is
probably owing to that the aggregates of P2 can associate with more PA
because of the existence of numerous cavities generated by the loose
packing of P2 with twisted molecular conformations and higher hy-
drophilicity. As to the quenching mechanism, the absorption spectrum
of PA hardly overlaps with the emission spectra of P1 and P2 (Fig. 5),
resulting in poor secondary inner filter effect (IFE) or fluorescence
resonance energy transfer (FRET) efficiency, though some contribution
of primary IFE cannot be ruled out [12]. Furthermore, the energy levels
of HOMOs and LUMOs of the polymer fragments and PA are calculated.
The LUMO energy level of PA is much lower than those of P1 and P2,
and is even close to the HOMO energy levels of P1 and P2, revealing that
excited state electron transfer from the LUMOs of P1 and P2 to the
LUMO of PA is promoted upon photoexcitation (Fig. 3), which causes
efficient PL quenching.
(
m, 2H). M
P2: To a mixture of 11 (170 mg, 0.25 mmol), 12 (75 mg, 0.25 mmol),
Pd(PPh (12 mg, 0.01 mmol) and K CO (276 mg, 2 mmol) in a 10 mL
n
¼ 12603; PDI (M
w
/M
n
) ¼ 1.64.
3
)
4
2
3
round-bottomed flask. A mixture of toluene (1 mL) and water (1 mL) was
added to the flask, and the reaction vessel was degassed. The mixture
ꢀ
was vigorously stirred at 90 C for 24 h and then cooled to room tem-
perature. After extraction with dichloromethane, the combined organic
layers were washed successively with water and then dried over anhy-
drous MgSO
4
. The polymer was filtered and precipitated into methanol,
and then dried under vacuum for 24 h to afford the neutral polymer P2
1
as a yellow solid in 59% yield. H NMR (500 MHz, CDCl
3
), δ (TMS, ppm):
.68–7.44 (m, 9H), 7.29 (s, 6H), 7.03 (s, 8H), 3.60–3.43 (m, 14H), 3.24
s, 3H), 2.95–2.66 (m, 2H). M ¼ 31220, PDI (M /M ) ¼ 1.621.
7
(
n
w
n
The photostability of P1 and P2 in THF/water mixtures was also
tested. Under continuous exposure to UV light, the fluorescence in-
tensity of P1 in THF/water mixtures declines slowly at the beginning.
After continuous radiation for 60 min, the fluorescence intensity at
3
.2. Optical property
Fig. 1 shows the absorption spectra of P1 and P2 in dilute THF
10 M). The absorption maximum of P2 is located at 379 nm, while P1
does not show obvious absorption peaks. The fluorescence spectra of P1
and P2 in dilute THF (10 M) exhibit broad and weak peaks due to the
(
μ
5
18 nm shows a moderate decrease by 43% (Fig. 6A and B). P2 displays
similar photostability within 60 min (Fig. 6C and D). Even after
μ
continuous radiation for 24 h, P2 in THF/water mixtures still retains the
random-coil configurations of the polymer, which partly restrict the
intramolecular rotations and suppress the nonradiative decay [11]. With
the gradual addition of water, a poor solvent for the polymers, into THF,
the fluorescence intensity of the polymers progressively enhances. The
strong emission peak of P1 emerges at 529 nm, whose intensity rises by
1
3% fluorescence intensity. Furthermore, to check the PA sensing
properties of polymers after exposure to UV light, fluorescence
quenching titration experiments have been further performed by addi-
tion ethanol solutions of PA into the exposure samples (60 min radia-
tion). The fluorescence quenching of ~43% for P1 aggregates is
observed upon exposure to UV light for 60 min, which is further
1
9
5
4-fold from the isolated species in THF solution to the aggregates in
9% aqueous mixture (Fig. 2A and B). P2 displays the emission peak at
23 nm in 80% aqueous mixture and decreases slightly at a higher water
immediately quenched by ~74% upon adding 10.65
μ
M PA solution
(
Fig. 7A). In the presence of the same amount of PA, however, the
w
fraction (f ) in the THF/water mixture (>80%) (Fig. 2C and D). The
fluorescence of P2 aggregates exposed to UV is quenched by ~58%,
implying the cavities in P2 aggregates are changed under UV light,
which attenuate the interaction with PA molecule (Fig. 7B).
photographs of P1 and P2 in THF/water mixtures with different water
fractions are shown in Fig. 2E and F, taken under the UV lamp illumi-
nation at 365 nm. The enhancement in fluorescence intensity is thus
caused by the aggregate formation, demonstrating the AEE feature.
Considering the practical application in explosives detection, the
instant surface sensing of PA by preparing fluorescent test strips was
performed. Several pieces of Whatman filter paper were impreganted
À 1
3
.3. Electronic structure and energy level
with a THF solution of P1 (or P2) at a concentration of 0.1 mg mL and
then dried. A solvent spot was also applied as a blank. Under the irra-
diation of 365 nm UV light, the discernible dark spots on the filter paper
Theory calculations of the conjugated fragments of P1 and P2 using
6

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 4. (A and C) Fluorescence spectra of P1 and P2 in THF/water mixtures (f
w
¼ 90 vol%) containing different amounts of PA. (B and D) Plots of I
0
/I - 1 value versus
PA concentration in THF/water mixtures (P1: f
w
¼ 90 vol%; P2: f ¼ 80 vol%). Inset: photographs of P1 and P2 in THF/water mixtures with different PA concen-
w
tration (P1: 0 and 65.01
wavelength: 350 nm.
μ
M; P2: 0 and 56.9
μ
M), taken under the illumination of a UV lamp (365 nm). Concentration: 0.53
μ
M (P1), 0.21 M (P2); excitation
μ
7

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
were observed by adding 1
μ
L of different concentrations of PA, indi-
cating the fluorescence was quenched in varying degrees by PA (Fig. 8).
The minimum amount of PA that can be detected by naked eye using P1-
À 10
immobilized filter paper was found to be as low as 22.9 fg (10
M),
À 9
while the detection limit of 229 fg (10 M) was obtained for PA using
P2-immobilized filter paper. To confirm the reusability of the fluores-
cent test strips, the filter papers having dark spots were rinsed with
ethanol and added PA solution again. As a result, the dark spots on the
filter paper faded away and then reappeared. After the third rinse, the
dark spots on the filter paper could still be observed by adding PA so-
lution at a low or high concentration (Fig. 9). These results demonstrate
that the polymer-immobilized fluorescent test strips can provide a
convenient and portable fluorescent platform for on-site PA detection
with proper reusability and high sensitivity.
4
. Conclusions
In summary, two new DNDPE-based conjugated polymers P1 and P2
are synthesized and characterized. They possess good solubility, AEE
feature and high photostability. Both polymers exhibit sensitive fluo-
rescence quenching by PA. In comparison with P1, P2 displays higher
Fig. 5. Normalized absorption spectra of PA in ethanol solution and fluores-
cence spectra of P1 and P2 in THF/water mixtures (f
w
¼ 90 vol%).
Fig. 6. (A and C) Fluorescence spectra of P1 and P2 in THF/water mixtures (P1: f
w
¼ 90 vol%; P2: f ¼ 80 vol%) under UV light at different exposure times. (B and D)
w
Plots of I
0
/I value versus different exposure time. I
0
¼ fluorescence intensity at 0 min exposure time. Concentration: 0.65
μ
M (P1), 0.26 M (P2); excitation wave-
μ
length: 350 nm.
8

Y. Zhuang et al.
Dyes and Pigments xxx (xxxx) xxx
Fig. 7. (A) Fluorescence spectra of P1 in THF/water mixtures (f
w
¼ 90 vol%) at 0 min exposure time without and with PA, and at 60 min exposure time with PA, (B)
Fluorescence spectra of P2 in THF/water mixtures (f
w
¼ 80 vol%) at 0 min exposure time without and with PA, and at 60 min exposure time with PA. Concentration:
0
.65 M (P1), 0.26 M (P2), 10.65 M (PA); excitation wavelength: 350 nm.
μ
μ
μ
Fig. 9. Photographs of the fluorescent test strips upon the addition of PA so-
lution at high or low concentrations under 365 nm UV light before and after
rinse (P1: A, B, C; P2: D, E, F).
Acknowledgments
Fig. 8. Photographs of the fluorescent test strips upon the addition of PA so-
We acknowledge the financial support from the National Natural
Science Foundation of China (21788102 and 21404029), National Stu-
dents’ Platform for Innovation and Entrepreneurship Training Program
lution at various concentrations under 365 nm UV light (A: P1, B: P2).
(
201810346010), the Natural Science Foundation of Guangdong Prov-
sensitive to PA, and the KSV at the initial stage of the Stern–Volmer plots
ince (2019B030301003).
5
À 1
is up to 9.8 � 10 M . The sensitive detection of PA can be attributed to
the existence of numerous cavities generated by the loose packing of
polymer with twisted molecular conformations and high hydrophilicity.
These sizeable cavities can effectively capture the PA molecules and
ameliorate the PET process between polymers and PA. In addition, after
exposure to UV light, these polymers still could effectively detect the PA
in water. The results presented in this study hold great potential for the
development of new DNDPE-based conjugated polymeric sensors for the
sensitive detection of PA.
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0