A Novel Conjugated Polymer Consists of Benzimidazole and Benzothiadiazole: Synthesis, Photophysics Properties, and Sensing Properties for Pd2+

Article abstract of DOI:10.1002/pol.20200005

A conjugated polymer PPBIBTE based on benzimidazole and benzothiadiazole was synthesized through palladium-catalyzed sonogashira cross-coupling reaction. The chemical structures of the monomers and the polymer were indicated by ¹H NMR, and investigation of photophysics properties and sensing optical properties for metal ions were observed by ultraviolet–visible and photoluminescence spectroscopy. PPBIBTE showed remarkable selectivity for Pd²⁺ by “turn-off” fluorescence sensing progress. In addition, the Stern–Volmer and Benesi-Hildebrand plots were used to reveal the interaction between the polymer and Pd²⁺, while job's method was applied to calculate the determination of stoichiometry. The results demonstrate that PPBIBTE can utilize static quenching for Pd²⁺ by forming a 1:1 complex. And it is a potential sensing material as fluorescence chemosensor for Pd²⁺ with high selectivity and sensitivity.

Full text of DOI:10.1002/pol.20200005

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A Novel Conjugated Polymer Consists of Benzimidazole and
Benzothiadiazole: Synthesis, Photophysics Properties, and Sensing
Properties for Pd2+
Kuan Liu , Zijun Hu
College of Science, Sichuan Agricultural University, Yaan 625014, China
Correspondence to: K. Liu (E-mail: cosmicer@live.cn)
Received 2 January 2020; accepted 7 January 2020
DOI: 10.1002/pol.20200005
ABSTRACT: A conjugated polymer PPBIBTE based on benzimid-
azole and benzothiadiazole was synthesized through palladium-
catalyzed sonogashira cross-coupling reaction. The chemical
structures of the monomers and the polymer were indicated by
1H NMR, and investigation of photophysics properties and sens-
ing optical properties for metal ions were observed by ultraviolet–
visible and photoluminescence spectroscopy. PPBIBTE showed
remarkable selectivity for Pd2+ by “turn-off” fluorescence sensing
progress. In addition, the Stern–Volmer and Benesi-Hildebrand
plots were used to reveal the interaction between the polymer
and Pd2+, while job’s method was applied to calculate the deter-
mination of stoichiometry. The results demonstrate that PPBIBTE
can utilize static quenching for Pd2+ by forming a 1:1 complex.
And it is a potential sensing material as fluorescence chemo-
sensor for Pd2+ with high selectivity and sensitivity. © 2020 Wiley
Periodicals, Inc. J. Polym. Sci. 2020
KEYWORDS: benzimidazole; benzothiadiazole; conjugated polymer;
fluorescence quenching; synthesis
detection with these objects by the formation of a covalent bond
from chemical reactions and weak interactions involve coordina-
tion, hydrogen bond, electrostatic attraction and Van der waals’
force.9 For example, in metal ions detection, the oxygen atom in
the crown ether structure unit can be used as the complex site
for metal ions, especially for alkali metal ions.10 And the coordi-
nation between heteroatom and metal ions can be especially use-
ful in this type of chemosensors with specific coordination.11
INTRODUCTION In the past decades, fluorescence chemosensors
based on conjugated polymers (CPs) as sensing materials have
rapidly developed in energy, chemical engineering, environmen-
tal protection, biochemistry and medical treatment fields with
their outstanding properties.1–4 These materials have received
intensive attention because of their favorable selective and sen-
sitive ability, environment-friendly biocompatibility, and fluo-
rescence amplification effect. Compared with a small organic
compound fluorescence probe, the delocalized π-electronic con-
jugated “molecular wires” effect can help CPs display unique
signal amplification effect by the facile energy migration along
the polymer backbone upon light excitation.5 Therefore, CP-
based fluorescence chemosensors can realize an efficient fluo-
rescent quenching at very low concentrations of quencher, even
the binding of one acceptor site in polymer backbone can result
in efficient quenching of the main chain.6
In respect of fluorescence chemosensors, nitrogen heterocyclic
rings were widely used in CPs with their various photoelectric
properties and it can realize efficiently ions detection because
of the lone pair electrons in nitrogen atom. During the past
few years, 2,1,3-Benzothiadiazole (BT) has been widely used
as the acceptor in Donor–Acceptor type copolymers due to its
good electron-withdrawing ability. The planar structure of
benzothiadiazole can enhance the capacity of π-π stacking
between molecules and facilitate intramolecular charge trans-
port property which ensures the polymers have high mobility
of charge carriers. With these excellent photoelectric proper-
ties, this unit is extensively used to form materials for
photoactive layer in solar cells.12,13 Meanwhile, this unit can
be applied in fluorescence chemosensors as fluorophore
because of its strong and stable fluorescence. In previous
work, benzimidazole unit was extensively applied in small
Particularly, the fluorescence properties and sensing ability of
CP-based fluorescence chemosensors are determined by chromo-
phore and recognition site in the polymer. The design of polymer
molecule components plays a significant role in adjusting the
ability of selectivity for fluorescence chemosensors, and introduc-
ing different functional recognition sites in the polymer backbone
and side chains can realize effective detection for ions, small mol-
ecules, and biomacromolecules.7,8 The recognition site can realize
Kuan Liu and Zijun Hu contributed equally to this study.
© 2020 Wiley Periodicals, Inc.
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molecule fluorescent probes due to its characteristic fluores-
cence in various organic solvents,14–17 and their fluorescence
properties can be affected by various factors like metal ions with
coordination. For CPs, this unit was frequently joint with fluorene,
pyridine, thiophene, and other neutral rings or rich electron rings
to form CPs used as LED materials with shorter absorption in
UV–vis spectrum and cyan or blue fluorescence.18–20 However,
there is no sufficient research for CPs which containing benzimid-
azole unit applied in fluorescence chemosensors.21
6-31 g(D) basis sets by Gaussian 09 software package. A fre-
quency calculation was performed on the optimized geome-
tries to ensure
a local minimum was found. Cyclic
voltammetry (CV) was performed on a CHI440 Electrochemi-
cal Workstation. The polymer was dissolved in degassed dic-
hloromethane (conc. 10−3 M). CH2Cl2 containing 0.1 M
tetrabutylammonium perchlorate (Bu4NClO4) was used as the
supporting electrolyte, with glassy carbon electrode as the
working electrode, Pt electrode as the counter electrode and
Ag/AgCl as the reference electrode. The sweep rate was
50 mV/s.
Palladium is one of the metal ions in the platinum group
which is largely applied as a catalyst in drug synthesis, auto-
mobile exhaust purification, and jewelry industry because of
its physicochemical properties.22,23 However, the final prod-
ucts in those industries are difficult to completely remove pal-
ladium element which comes from the catalyst. The extensive
use and abundant waste of palladium have raised plenty of
pollution to water sources and soils, and cause severe effects
and damage to plants, animals, and humans.24,25 Especially for
humans, excess palladium can get the trigger to serious aller-
gic reactions for eyes and skin in vitro, and it could destroy
DNA, coordinate with thiol-containing amino acids, bind to
vitamin B6, trigger protein denaturation, and damage several
cellular processes in vivo.26,27 Therefore, the simple and con-
venient measurement material is urgently needed to develop
and apply in palladium detection and analysis.
Materials
All of the metal salts in analytical grade were purchased from
domestic chemical companies and used without further purifi-
cation. Dimethyl-formamide (DMF) was purified by distillation
under vacuum. Other chemicals were used directly without
further purification.
Synthesis
The synthetic routes of Monomer 1, Monomer 2, and
PPBIBTE are illustrated in Scheme 1. The synthesis of com-
pound 1,2,3,4 was prepared according to references 28–30.
1,2-Bis-Octyloxy-Benzene (1)
In our work, we report the synthesis and characterization of
the 4,7-diethynyl-5,6-bis(octyloxy)benzo[c][1,2,5]thiadiazole
(Monomer 1) and 5-bromo-2-(4-bromophenyl)-1-octyl-
benzimidazole (Monomer 2), getting the ideal CP PPBIBTE.
Introduction of long alkyl groups improves the dissolving
capacity of the polymer in organic solvents. Compared with
other benzimidazole-based CPs, we can expand the absorption
range and improve the fluorescence property by using
benzothiadiazole unit. PPBIBTE can emit stable and strong
yellow-green fluorescence, this chemosensor material dis-
played a highly sensitive response toward Pd2+ over other
competitive cations.
Under argon atmosphere, to a solution of catechol (6.60 g,
0.06 mol) in dry DMF (44 mL) was added 1-bromooctane
(0.138 mol, 23.8 mL) and K2CO3 (25.05 g, 0.177 mol). The
mixture was stirred at 100 ꢀC for 2 days. After cooling to
room temperature, 100 mL water was added. The organic
layer was separated and the aqueous layer was extracted with
CH2Cl2. The combined organic phase was dried over anhy-
drous magnesium sulfate. After filtration, the solvent was
removed by rotary evaporator. The residue was chromato-
graphically purified on silica gel column eluting with CH2Cl2:
Petroleum ether (1:6, v:v) to afford 1 as a colorless oil. Yield:
17.20 g (86.0%). 1H NMR (400 MHz, CDCl3) δ 6.97–6.86 (m,
4H), 4.03 (t, J = 6.6 Hz, 4H), 1.96–1.75 (m, 4H), 1.60–1.25 (m,
20H), and 0.94 (t, J = 6.7 Hz, 6H).
EXPERIMENTAL
Measurements
1H NMR and 13C NMR measurements were carried out on a
Bruker Advance II-400 MHz spectrometer with tetra-
methylsilane as the internal reference. HRMS spectra were
recorded on a Waters Q-Tof Premier. Gel permeation chroma-
tography (GPC) was determined by Waters E2695 GPC, with
THF as mobile phase and PS as the reference in 35 ꢀC. UV–vis
spectra were performed on an Analytik Jena Specord200Plus
UV–vis spectrophotometer. Photoluminescence (PL) spectra
were recorded on a Hitachi F-4600 luminescence spectropho-
tometer. All spectra were measured at room temperature.
TGA was carried out with a TA Series SDT Q600 at a heating
rate of 20 ꢀC min−1 in a nitrogen flow. DSC was conducted
using a TA Series DSC2000 at a heating/cooling rate of 10 ꢀC
min−1 in a nitrogen flow. Molecular architecture of the mono-
mers and polymer was optimized by using RB3LYP and
1,2-Dinitro-4,5-Bis-Octyloxy-Benzene (2)
The solution of compound 1 (13.38 g, 0.04 mol) in dic-
hloromethane (110 mL) was added dropwise into 65% nitric
acid (0.26 mol, 11.76 mL). Then, sulfuric acid (0.16 mol,
8.52 mL) was added dropwise into the mixture. The mixture
was stirred for 1.5 h at room temperature. When the reaction
was completed, the mixture was poured into crushed ice, and
the aqueous layer was extracted with CH2Cl2. The combined
organic layer was dried over anhydrous magnesium sulfate.
After filtration, the solvent was removed by rotary evapora-
tor. The crude product was purified by recrystallization from
ethanol to afford a yellow solid. Yield: 10.50 g (61.9%). 1H
NMR (400 MHz, CDCl3) δ 7.30 (s, 2H), 4.10 (t, J = 6.5 Hz,
4H), 1.93–1.81 (m, 4H), 1.55–1.21 (m, 20H), 0.89 (t,
J = 6.8 Hz, 6H).
2
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SCHEME 1 Synthetic routes of monomers and polymer.
5,6-Bis-Octyloxy-Benzo [1,2,5] Thiadiazole (3)
4,7-Dibromo-5,6-Bis(Octyloxy)Benze[c] [1,2,5]
Under argon atmosphere, a mixture of compound 2 (2.54 g,
6 mmol) and Sn(II)Cl2 (10.83 g, 48 mmol) in ethanol (90 mL)
and conc. HCl (36 mL) was stirred at 85 ꢀC overnight. After
cooling to room temperature, the crude product was filtered
and washed with water and methanol. Finally, the product
was dried at room temperature under a stream of argon over
a few hours and used directly. Yield: 1.15 g (44%). Subse-
Thiadiazole (4)
To a solution of compound 3 (0.39 g, 1 mmol) in dic-
hloromethane (26.3 mL) and acetic acid (11.6 mL) was slowly
added bromine (0.12 g, 7 mmol), and the final solution was
stirred in the dark for 48 h at room temperature. When the
reaction was completed, the mixture was poured into 40 mL
water. The organic layer was separated and the aqueous layer
was extracted with CH2Cl2, the combined organic phase was
dried over anhydrous sodium sulfate. After the solvent was
removed by rotary evaporator, the crude product was purified
by recrystallization from ethanol to afford a white needle-like
crystal. Yield: 0.404 g (73.4%). 1H NMR (400 MHz, CDCl3) δ
4.18 (t, J = 6.6 Hz, 4H), 1.92–1.76 (m, 4H), 1.51–1.19 (m,
20H), 0.98 (t, J = 6.9 Hz, 6H).
quently, to
a
mixture of 4,5-bis(octyloxy)benzene-1,-
2-diaminium chloride (1.15 g, 2.64 mmol) and triethylamine
(2.64 mmol, 3.72 mL) in dichloromethane (44 mL) was slowly
added a solution of thionyl chloride (5.28 mmol, 0.38 mL) in
dichloromethane (5.4 mL) under argon atmosphere. Then, the
mixture was heated to reflux for 6 h. After cooling to room
temperature, the solvent was removed by rotary evaporator
followed by trituration in water stirring for 30 min. The
product was filtered and dissolved in dichloromethane, and
the solution was dried over anhydrous magnesium sulfate.
After filtration, the solvent was removed by rotary evaporator.
The crude product was chromatographically purified on silica
gel column eluting with EtOAc: hexanes (1:9, v:v) to afford an
off-white solid. Yield: 0.684 g (66.0%). 1H NMR (400 MHz,
CDCl3) δ 7.13 (s, 2H), 4.09 (t, J = 6.6 Hz, 4H), 1.95–1.87 (m,
4H), 1.61–1.23 (m, 20H), 0.89 (t, J = 6.9 Hz, 6H).
4,7-Diethynyl-5,6-Bis(Octyloxy)Benzo[c] [1,2,5] Thiadiazole
(Monomer 1)
Under argon atmosphere, to a solution of compound 4
(0.55 g, 1 mmol) in triethylamine (14 mL) was added a cata-
lytic amount of CuI (0.0075 g, 0.039 mmol) and Pd(PPh3)2Cl2
(0.0140 g, 0.020 mmol). The mixture was carefully stirred for
30 min to deaerate and trimethyl-silylacetylene (0.42 mL,
3 mmol) was added. The final mixture was stirred at 70 ꢀC for
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24 h. After the removal of the solvent, the residue was passed
through a short silica gel eluting with CH2Cl2 to afford a crude
product which was chromatographically purified on silica gel
column eluting with CH2Cl2: Petroleum ether (1:6, v:v) to
afford the yellow oil. Subsequently, dissolved the product of
the first step in a mixture of methanol (6 mL) and dic-
hloromethane (10 mL) under an argon atmosphere, potassium
fluoride (0.22 g, 2.34 mmol) was added and the final mixture
was heated to reflux for 4 h. After cooling to room tempera-
ture, the solvent was removed by rotary evaporator, and the
residue was chromatographically purified on silica gel column
eluting with CH2Cl2: hexane(1:1, v:v) to afford monomer 1 as
a white solid. Gross yield of two-step: 0.176 g (40.0%). 1H
NMR (400 MHz, CDCl3) δ 4.34 (t, J = 6.6 Hz, 4H), 3.82 (s, 2H),
1.96–1.77 (m, 4H), 1.62–1.19 (m, 20H), 0.89 (t, J = 6.7 Hz,
6H); 13C NMR (101 MHz, CDCl3) δ 158.84, 151.81, 88.60,
75.87, 75.27, 31.83, 30.29, 29.37, 29.26, 25.92, 22.65, 14.08;
HRMS (ESI+) [M + H]+: calc’d for C26H37N2O2S: 441.2570,
found 441.2567.
white solid. Yield: 1.30 g (61.5%) 1H NMR (400 MHz, DMSO-
d6) δ 13.20 (d, J = 17.5 Hz, 1H), 8.11 (d, J = 6.9 Hz, 2H),
7.90–7.47 (m, 4H), and 7.36 (t, J = 9.4 Hz, 1H).
6-Bromo-2-(4-Bromophenyl)-1-Octyl-1H-Benzo [d]Imidazole
(Monomer 2)
Under argon atmosphere, to a solution of compound 7
(1.76 g, 5 mmol) in DMF (20 mL) was added NaH (0.7 g,
17.5 mmol) in ice-bath and stirred for 1.5 h at room tempera-
ture. Then the mixture was heated to 40 ꢀC and was slowly
added 1-bromooctane (1.16 g, 6 mmol), the final mixture was
heated to 80 ꢀC and stirred for 12 h. After the reaction was
completed, the cooled solution was added 15 mL water and
extracted with CH2Cl2. The organic layer was washed by
water and dried over anhydrous sodium sulfate. After filtra-
tion, the mixture was concentrated under vacuum. The crude
product was chromatographically purified on silica gel column
eluting with EtOAc: petroleum ether (4:1, v:v) to afford a
1
white solid. Yield: 1.83 g (80.0%). H NMR (400 MHz, DMSO-
d6) δ 7.94 (dd, J = 37.5, 1.7 Hz, 1H), 7.82–7.69 (m, 4H), 7.66
(dd, J = 13.0, 8.6 Hz, 1H), 7.41 (ddd, J = 22.0, 8.6, 1.8 Hz, 1H),
4.31 (t, J = 7.3 Hz, 2H), 1.66–1.52 (m, 2H), 1.25–1.01 (m,
10H), 0.82 (t, J = 7.2 Hz, 3H); 13C NMR (101 MHz, DMSO-d6) δ
153.46, 144.25, 134.56, 132.05, 130.73, 129.09, 125.92,
124.58, 122.81, 115.47, 111.31, 44.91, 31.61, 29.67, 28.96,
28.86, 26.56, 22.51, 13.97; HRMS (ESI+) [M + H]+ calc’d for
5-Bromo-2-Nitroaniline (5)
To a solution of 2-nitroaniline (13.81 g, 0.10 mol) in methanol
(250 mL) and 40% hydrobromic acid (30 mL) with ice-bath.
30% hydrogen peroxide was slowly added within 15 min.
The mixture was stirred overnight at room temperature.
When the reaction was completed, the mixture was filtered
and the crude product was purified by recrystallization from
mixed solvent ethanol/water to afford the final product as an
orange needle crystal. Yield: 11.27 g (94.1%) 1H NMR
(400 MHz, CDCl3) δ 8.27 (d, J = 2.3 Hz, 1H), 7.43 (dd, J = 8.9,
2.3 Hz, 1H), 6.73 (d, J = 8.9 Hz, 1H), 6.09 (s, 2H).
C21H25Br2N2: 465.0359, found 465.0360.
Polymer (PPBIBTE)
Under argon atmosphere, monomer 1 (0.177 g, 0.40 mmol)
and monomer 2 (0.186 g, 0.40 mmol) were dissolved in tolu-
ene (4 mL). Then CuI (0.0015 g, 0.08 mmol), triphenyl-
phosphine (0.002 g, 0.008 mmol), Pd(PPh3)2Cl2 (0.0281 g,
0.04 mmol), and diisopropylamine (1 mL) were added into
the solution. The final mixture was stirred at 105 ꢀC for 24 h
and subsequently cooled to room temperature. The resulting
mixture was filtered and the filtrate was slowly added into
methanol (250 mL) and stirred for 1 h. The precipitate was
isolated by filtration followed by washing with hot methanol,
and purified through Soxhlet extraction for 24 h by using
methanol and CHCl3, respectively. The methanol fraction was
discarded while the CHCl3 fraction was collected, and the
polymer was reprecipitated by adding excess methanol into
CHCl3 fraction to afford a red-brown solid. Yield: 0.174 g
(58.4%). 1H NMR (400 MHz, CDCl3) δ 8.02–7.29 (m, 7H),
4.72–3.82 (m, 6H), 1.88 (d, 6H), 1.32 (d, 30H), 0.87 (d, 9H).
Mw: 15108, Mn: 8025, and Mw/Mn: 1.88 (GPC).
4-Bromobenzene-1,2-Diamine (6)
Under argon atmosphere, to a solution of compound 5
(6.51 g, 30 mmol) in ethanol (45 mL) was added Tin(II) chlo-
ride dehydrate (33.85 g, 150 mmol). The mixture was heated
to reflux for 15 h. The solvent was removed under vacuum,
and the pH of the residue was adjusted to 11–12 by adding
sodium hydroxide solution. Then, the mixture was extracted
with diethyl ether. The organic phase was dried over anhy-
drous magnesium sulfate overnight. After filtration, the sol-
vent was removed by rotary evaporator to afford gray pink
1
solid. Yield: 5.26 g (93.7%). H NMR (400 MHz, CDCl3) δ 6.84
(dd, J = 5.3, 2.2 Hz, 1H), 6.81 (d, J = 2.2 Hz, 1H), 6.58 (d,
J = 8.1 Hz, 1H), and 3.37 (s, 4H).
6-Bromo-2-(4-Bromophenyl)-1H-Benzo[d] Imidazole (7)
A mixture of 4-bromobenzaldehyde (1.11 g, 6 mmol), NaHSO3
(0.62 g, 6 mmol) and ethanol (25 mL) was stirred for 4 h at
room temperature. Subsequently, a solution of compound 6
(1.12 g, 6 mmol) dissolved in DMF (15 mL) was added and
the mixture was heated to reflux for 2 h. When the reaction
was complete, the solvent was removed under vacuum, EtOAc
(20 mL) was added and the mixture was washed with water.
The organic phase was dried over anhydrous sodium sulfate.
After the solvent was removed by rotary evaporator, the resi-
due was chromatographically purified on silica gel column
eluting with EtOAc: petroleum ether (4:1, v:v) to afford 7 as a
Fluorescence Sensing Analysis
The stock solutions of polymer PPBIBTE in THF was prepared
at a concentration of 5 × 10−4 M and diluted with THF before
used. The metal ions stock solutions were prepared by dis-
solving the metal salts in methanol at a concentration of
1 × 10−3 M. The emission spectra were recorded from 350 to
750 nm using 332 nm as excitation wavelength at room tem-
perature. The metal ion solution in 1 × 10−3 M was added
with a gradient of 4 μL into the PPBIBTE solution of test sam-
ple (2 mL, 2 × 10−5 M) placed in a quartz cell (1 cm width),
4
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and the fluorescence spectra were recorded by measuring the
changes of intensity after mixing for 5 min.
isomer ratio of 5-bromo-2-(4-bromophenyl)-1-octyl-benzimidazole
and 6-bromo-2-(4-bromophenyl)-1-octy-benzimidazole is about
0.80:0.20 for monomer 2 by calculating the integral areas of split-
ting peaks in protons 11, 12, and 13, respectively. Similarly, we
can calculated the isomer ratio of 5-bromo-2-(4-bromophenyl)-
1H-benzo[d]imidazole and 6-bromo-2-(4-bromophenyl)-1H- benzo
[d]imidazole for compound 7 is 0.55:0.45. The spectrum of poly-
mer always shows with sharpening wide peaks because of the dif-
ferent chain lengths and various configurations. The protons in
conjugated structures are influenced by complex deshielding
effects, and a series of peak signals in the range of 7.20–8.00 ppm
are all derived from the aromatic ring on the benzimidazole. Com-
paring these three spectra, the characteristic signal peak of the
alkynyl hydrogen in monomer 1 disappeared in the spectrum of
PPBIBTE because of the removal of the triple bond hydrogen in
the coupling reaction, which proved that the successful polymeri-
zation reaction had occurred. The 1H NMR results show that
PPBIBTE was successfully synthesized.
RESULTS AND DISCUSSION
Synthesis and Characterization
1
The H NMR results of monomer 1, monomer 2, and PPBIBTE
were listed in Figure 1. In the spectrum of monomer 1, the pro-
ton 9 of the triple bond was easy to recognize with singlet at
3.82 ppm. The proton 8 adjoins with oxygen in alkyl chain was
affected by the inductive effect which decreased rapidly with dis-
tance and shows as a triplet at 4.34 ppm. Multiplet at 1.77 ppm
and 1.62–1.99 ppm is proton 7 and proton 2–6. The proton 1 is
farthest from the heteroatom and shows with a triplet in
0.89 ppm. The law of peaks originate from alkyl chain in mono-
mer 2 is the same as monomer 1 and the signals at a range of
7.35–8.88 ppm come from aromatic protons. It can be known
from the integral area that the resonance peak of protons 9 and
10 is between 7.70 and 7.80 ppm. The relative positions of bro-
mine atoms and hydrogen atoms in the imidazole ring are differ-
ent while the ring was synthesized, which will lead to the
Thermal Analysis
occurrence of isomers in monomer
2
and PPBIBTE,
According to thermal analysis of the polymer (Figure 2), it has
high thermal stability with no weight loss up to 250 ꢀC and
5% weight loss temperature is 266 ꢀC in TGA curve. Mean-
time, as it is shown in Figure 2(b), there are no obvious endo-
thermic peaks and exothermic peaks can be observed during
the forward scan and reverse scan, indicating that there were
no obvious melting and crystallization phenomena during the
heating and cooling process. In addition, it can be found that
the polymer has a tendency to absorb heat during the heating
process, which may be related to the evaporation of the sol-
vent in the sample at high temperature. According to the mid-
point of the step in the curve during the heating process, the
Tg of the polymer was determined to be 103 ꢀC. Therefore,
the polymer PPBIBTE has a desirable thermal property for
practical applications as a fluorescence sensor.
respectively,20 and the introduction of alkyl chain will further
affect the generation of peak signals. Therefore, the signals of aro-
matic protons 11, 12, and 13 in monomer 2 could appear with
splitting. The peak signal of the proton 12 which is far away from
the imidazole ring shown in high filed at 7.36–7.45 ppm, it is less
affected by the deshielding effect of imidazole compared with pro-
tons 11 and 13. Since the introduction of alkyl group increases
the effect of electron-donating group of the nitrogen atom and
leads to the increase of electron density of proton 13, which
result in the chemical shift of proton moves to a high field. There-
fore, the peaks at 7.62–7.69 ppm belong to 13 and the peaks at
7.98 and 7.89 ppm are the peak signals of the proton 11. In the
spectrum of PPBIBTE, the peaks between 0.75–1.90 and
4.30–4.60 ppm are signals of hydrogens in the alkyl chain. The
FIGURE 1 1H NMR spectra of Monomer 1, Monomer 2, and PPBIBTE. [Color figure can be viewed at wileyonlinelibrary.com]
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maxima are listed in Table 1. PPBIBTE show two broad
absorption peaks at the range of 300–370 nm and
380–480 nm, respectively. The shorter wavelength peak origi-
nates from localized π-π* electron transition of the polymer
backbone, while the longer wavelength peak can be attributed
to intramolecular charge transfer (ICT) effect between mono-
mer 1 and monomer 2. Meanwhile, the longer wavelength
absorption peak intensity is weaker than short one which
may indicate the ICT effect is weak in PPBIBTE.31 The absorp-
tion and emission wavelengths of the thin film were listed in
Table 1. Compared with solution, a large red shift for emission
wavelength was observed due to the strong intermolecular
interaction in solid state. There is no apparent difference
between absorption and emission maxima among different
solvents. The absorption peak shifts only 4 nm in wavelength
from the low-polar solvent (Toluene) to strong polar solvent
(DMF) with increasing Dimroth-Reichardt solvent polarities
(from ET(30)
=
33.9 kcal/ mol for toluene to
ET(30) = 43.2 kcal/ mol for DMF, ET (30) is polarity index:
data from ref. 32), indicating that the electronic and structural
nature of the ground and the Franck-Condon excited states
does not change much.33 But the emission intensity in DMF is
much less than other solvents which may come from
π-stacking of polymer in solution caused by the high permit-
tivity of DMF. Meantime, a bathochromic shift caused by intra-
molecular charge transfer interaction results in the emission
peak shifts 6 nm in wavelength from the Toluene to DMF.
Considering the soluble capacity of metal ions and the Stokes
shift of polymer in these solvents, THF can be a favorable sol-
vent for sensing properties experiment below.
Sensing Properties of PPBIBTE
FIGURE 2 (a) TGA and (b) DSC curves of the conjugated
polymer PPBIBTE. [Color figure can be viewed at
wileyonlinelibrary.com]
To investigate the response performance of PPBIBTE in THF
at the concentration of 2 × 10−5 M with different metal ions
(including Pd2+, Fe3+, Mg2+, Sn2+, Co2+, Zn2+, Cu2+, Mn2+, Ni2+
,
Cr3+, Fe2+, Hg2+, and Cd2+) at the same concentration which is
the fluorescence-quenching extreme limit of Pd2+, the selective
behavior is shown in Figure 4(a). Upon addition of these
metal ions, it can be observably found that Pd2+ can cause
instantly and mostly quenching of the polymer while Cu2+, Fe3
+, and Co2+ only lead to a slight quenching of the polymer and
Photophysical Properties of Polymers
The UV–vis absorption spectra and fluorescence emission
spectra of PPBIBTE in different solvents at the same concen-
tration (1 × 10−5 M for absorption spectra and 2 × 10−5 M for
emission spectra, calculated by repeating unit) is shown in
Figure 3, and the parameters of absorption and emission
other common metal ions (Mg2+, Sn2+, Zn2+, Mn2+, Ni2+, Cr3+
Fe2+, Hg2+, and Cd2+) only exhibit negligible effect to the
,
TABLE 1 Photophysical Parameters of PPBIBTE in Thin Film and Different Solvents
Thin film
Absorption
max(nm)
Emission
max(nm)
Absorption
max(nm)
Excitation
ex(nm)
Emission
max(nm)
Stokes
shift λ(nm)
ET(30)
kcal/mol
λ
λ
Solvent
λ
λ
λ
454
602
DMF
434
436
433
433
433
437
438
328
332
328
331
331
334
334
518
517
509
519
520
513
512
84
81
76
86
87
76
74
43.2
37.4
38.1
40.7
39.1
34.3
33.9
THF
EtOAc
CH2Cl2
CHCl3
Benzene
Toluene
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FIGURE 3 UV–vis spectra and fluorescence spectra of PPBIBTE
in different solvents. [Color figure can be viewed at
wileyonlinelibrary.com]
fluorescence intensity. To further study the interaction
between palladium ion and the polymer, the fluorescence
spectra of PPBIBTE in THF with different concentrations of
Pd2+ is shown in Figure 4(b).
When the concentration of Pd2+ in the solution increases from
0
to 2 × 10−5 M with a gradient of 2 × 10−6 M, the fluores-
cence intensity of the polymer decreased obviously and regu-
larly at a significant extent. Subsequently, there is no visible
fluorescence can be observed with the Pd2+ at the concentra-
tion of 2 × 10−5 M, and the maximum intensity of fluores-
cence emission in the fluorescence spectra reduced from 6992
to only 177 and almost 97 percent of fluorescence is
quenched. Meanwhile, the fluorescence spectra did not change
regularly and there is no significant effect on the fluorescence
intensity of polymer with other metal ions in the same
experiment.
FIGURE 4 (a) The fluorescence spectra of polymer solution with
different metal ions. (b) The fluorescence spectra of polymer
solution with Pd2+ in different concentrations from 0 to 2 × 10−5 M.
[Color figure can be viewed at wileyonlinelibrary.com]
To further study the recognition ability of PPBIBTE to Pd2+
,
the values of I0/I1 for each metal ions are calculated and com-
pared in Figure 5(a), where I0 and I1 are the fluorescence
intensities of PPBIBTE in the absence and the saturated con-
centration of metal ions, respectively. All the concentrations of
these metal ions are 2.0 × 10−5 M which is the fluorescence
quenching extreme limit of Pd2+. As shown in Figure 5(a), the
I0/I1 value of Pd2+ is almost 40 while the I0/I1 values of the
remaining ions are all approximately equal to 1. Meanwhile,
the competition experiment was carried out in Figure 5(b) to
evaluate the interference from other metal ions. To the solu-
costing, and a timely sensing method, the solution of PPBIBTE
and a mixed solution of the polymer with different metal ions
were prepared and placed under the daylight and the ultravio-
let light in Figure 6. Both the concentration of polymer and
metal ions are 2.0 × 10−5 M. In this photo, these solutions do
not have any observable difference from each other under the
daylight. Under the ultraviolet light, the blank polymer solu-
tion on the far left presents a strong yellow-green color, and it
is noticeable that the fluorescence of PPBIBTE disappeared
with the concentration of Pd2+ in 2.0 × 10−5 M while other
metal ions showed no visible fluorescence change. The result
is coinciding with the recognition experiment above which is
further confirmed that PPBIBTE have strong specificity for
Pd2+ recognition.
tion of PPBIBTE/metal system was subsequently added Pd2+
,
no significant interference was observed and the fluorescence
was still extremely and rapidly quenched, illustrating the
favorable anti-interference ability toward Pd2+. The results
indicate the polymer has no ability to recognize these metal
ions except Pd2+
.
The common mechanism of fluorescence quenching can be
divided into dynamic quenching and static quenching, and it
has been reported that benzimidazoles derivatives can form
complexes with Pd2+. Therefore, it is speculated that the
In the processing of the test, the fluorescence change of each
mixed solution can be observed preliminarily and obviously
with naked eyes. For developing a convenient, effective, low-
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imidazole of the polymer and Pd2+ 34,35
.
The color and UV–vis
spectra of the mixed solution will change because the ground
state complex generated during the static quenching process,
comparing the UV–vis spectra of polymer solution
(2 × 10−5 M) containing Pd2+ in the solution increasing from
0
to 2 × 10−5 M with a gradient of 5 × 10−6 M in Figure 7.
The right bottle which contains both polymer and Pd2+ in
2 × 10−5 M is slightly darker than the polymer solution in the
illustration. As shown in Figure 7, with the increase of Pd2+
concentration in the solution, both absorption peaks intensity
of the mixed solution decrease and show a hypochromatic
shift. While the short-wavelength absorption peak exhibited a
blueshift from 331 to 327 nm, and the long-wavelength
absorption peak moved from 435 to 427 nm. The formation
of an MD-A complex through metal ion (M) binds with a
Donor–Acceptor structure molecule at the donor site, which
will decrease the donor strength and restrain ICT. On the other
hand, the formation of a D-AM complex by metal-acceptor bind-
ing will facilitate ICT.36,37 Therefore, the colorimetric result
shows that there is an interaction between the donor unit and
Pd2+ with the quenching type “static quenching.”
In order to further investigate the interaction between poly-
mer and metal ions, Job’s method which is often used in ana-
lytical chemistry to determine the stoichiometry of the
combination of substances was adopted to study the binding
properties by using eq 1.
F
job = ð1−xÞF0 – F
ð1Þ
where x is the molar fraction of Pd2+, F0 and F are the fluo-
rescence intensities of polymer solution in the absence and
presence of Pd2+, respectively. In this method, the total vol-
ume molar concentration of the two components should be
kept at a certain value while the molar fraction of any compo-
nent is variable. The total concentration by the addition of the
polymer and Pd2+ was 2 × 10−5 M, and the molar fraction of
Pd2+ in the mixture was 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8,
0.9, and 1.0, respectively. As shown in Figure 8, the Job’s plot
reached a turning point when the molar fraction of Pd2+ was
about 0.5, which indicated that the complex binding ratio
between PPBIBTE and Pd2+ is 1:1.
FIGURE 5 (a) Comparison of I0/I1 of PPBIBTE with different
metal ions in THF (Metal ions concentration = 2.0 × 10−5 M). (b)
Fluorescence spectra histogram of competition experiments for
PPBIBTE in THF. (Black bar: free PPBIBTE or PPBIBTE + other
metal ions; Red bar: PPBIBTE
+ other metal ions + ;
Pd2+
PPBIBTE or Metal ions concentration = 2.0 × 10−5 M). [Color
figure can be viewed at wileyonlinelibrary.com]
fluorescence quenching between the polymer and palladium
ion may be caused by the static quenching of the polymer-
metal ion complexes formed by the binding of N atoms in
FIGURE 6 The color comparison of PPBIBTE with different metal ions under the daylight and the ultraviolet light. [Color figure can be
viewed at wileyonlinelibrary.com]
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FIGURE 8 Job’s plot of a 1:1 complex formed between PPBIBTE
and Pd2+. [Color figure can be viewed at wileyonlinelibrary.com]
FIGURE 7 The UV–vis spectra of polymer solution containing
Pd2+ in different concentrations and the color comparison of the
polymer solution and polymer solution containing Pd2+ at the
concentration of 2 × 10−5 M. [Color figure can be viewed at
wileyonlinelibrary.com]
The Stern–Volmer analysis is commonly used to explore the
quenching or enhancement process of fluorescence sensors; it
has specific equations for both dynamic and static
mechanisms,38 for static quenching between PPBIBTE and Pd2
+ satisfy the following eq 2:
F0
ln = eK ½Qꢁ −1
ð2Þ
SV
F
where F0 and F are the fluorescence intensities of polymer
solution with the absence and presence of Pd2+, respectively.
Ksv is the Stern–Volmer quenching constant and [Q] is the
concentration of Pd2+. The plots and non-linear fitting results
obtained in this experiment according to eq 2 are shown in
Figure 9(a). It can be investigated that the plots in these con-
centrations of Pd2+ conform to the Perrin model of static
quenching,39 which further indicate that the quenching type
between the polymer and palladium ions is static quenching.
Metal ions will continuously cooperate with polymer mole-
cules and form polymer-metal complexes with increasing con-
centration. Until the concentration of Pd2+ reaches
2 × 10−5 M, the metal ions and polymer molecules are
completely combined to form 1:1 complex. The KSV quenching
constant values about 7.80 × 104 M−1, R2 > 0.99 of the non-
linear fitting result proves that the fitting equation coincides
with static quenching type between PPBIBTE and Pd2+
.
Accordingly, the above researches prove that polymer mole-
cules can form a 1:1 complex with Pd2+, and the association
constant
K
is determined by the following Benesi–
Hilderbrand eq 3:
FIGURE 9 (a) The plot of ln (F0/F) versus the concentrations of
Pd2+; (b) The Benesi–Hildebrand plot of PPBIBTE with Pd2+
[Color figure can be viewed at wileyonlinelibrary.com]
.
1
1
1
=
+
ð3Þ
n
F0 −F K × ðF0 −FminÞ × ½Qꢁ
F0 −Fmin
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FIGURE 10 Two primary repeating units configuration of PPBIBTE and energy-level diagram which showing band gap and
topographical representation of monomers and different polymer repeat units. [Color figure can be viewed at wileyonlinelibrary.com]
where F0 and F are the fluorescence intensities in the absence
and presence of Pd2+, and Fmin is the minimum fluorescence
intensity in the presence of Pd2+, [Q] is the concentration of
Pd2+ and n is the stoichiometric ratio of fluorescence mole-
cules to quencher which is 1 between PPBIBTE and Pd2+. The
this situation compared with monomer 2. Meanwhile, the
HOMO of both repeat units are roughly distributed through-
out the molecular chain, while the HOMO of P-repeat unit-2
are more uniform, which indicates that the intramolecular
charge is transferred from the benzimidazole unit to the
benzothiadiazole unit.42
association constant
K
was evaluated graphically in
Figure 9 (b) by plotting 1/(F0 − F) against 1/[Pd2+] and
obtained from the slope and intercept of the line, and the
value is about 1.78 × 104 M−1. Both the association constant
and quenching constant are higher than what has been
reported,40,41 and the polymer has realized the specificity
detection of palladium ions. These results indicate that poly-
Electrochemical Properties
In order to investigate the electronic properties of PPBIBTE
and make comparisons of HOMO, LUMO, and Eg levels
between practical and theoretical results, the CV experiment
which is an important electrochemical method for measuring
the oxidation and reduction potentials for CPs was performed.
All the potentials are reported versus Ag/Ag+ with the ferro-
cene/ferrocenium couple as an internal standard. The onset
potentials of CV plot can be used to determine the HOMO,
LUMO, and band gap energy levels. Onset potentials were
defined as the potential at which 10% or 20% of the current
value at the peak potential was reached.43 As it shown in
Figure 11, the oxidation potential (Eoxonset) is 0.67 V versus
Ag/Ag+ while the onset reduction potential (Eredonset) is
mer PPBIBTE has a good recognition for Pd2+
.
Theoretic Calculation
To provide an insight into the molecular architecture of the
monomers and model compound of polymer, the molecular
simulation was carried out for these molecules with a chain
length of n = 1 using density functional theory at the
RB3LYP/6-31g(D) level with the Gaussian 09 program
package. The asymmetric structure of the benzimidazole unit
can result in different repeat units in the polymer main
chain, and two primary atomic arrangements are listed in
Figure 10. Using the theoretic calculation to get the molecu-
lar orbital configuration and band gap of monomers and the
polymer repeat units. It is obvious to observe that the LUMO
of P-repeat unit-1 and P-repeat unit-2 were mainly located
in the benzothiadiazole unit, and the orbital morphology was
consistent with the LUMO of monomer 1, suggesting that
the LUMO formation of the polymer was determined by
benzothiadiazole unit and the band gaps can be reduced in
−0.60 V versus Ag/Ag+. From the Eox
polymers, the HOMO and LUMO energy levels of the PPBIBTE
can be calculated from the following eqs
(Figure 11):44,45
and Ered
of the
onset
onset
4
and
5
onset
EHOMO = −e ð4:71 + Eox
ELUMO = −e ð4:71 + Ered
Þ
ð4Þ
ð5Þ
onset
Þ
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