A new disubstituted polyacetylene bearing DDTC moieties: Postfunctional synthetic strategy, selective and sensitive chemosensor towards mercury ions

Article abstract of DOI:10.1016/j.polymer.2012.10.006

A new DDTC-functionalized disubstituted polyacetylene (P2) with strong green fluorescence was successfully synthesized by utilizing the postfunctional method. The polymer was soluble in common organic solvents, and its strong green fluorescence could be q

Full text of DOI:10.1016/j.polymer.2012.10.006

Polymer 53 (2012) 5691e5698
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Polymer
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A new disubstituted polyacetylene bearing DDTC moieties: Postfunctional
synthetic strategy, selective and sensitive chemosensor towards mercury ions
*
Daxin Ou, Jingui Qin, Zhen Li
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University, Wuhan 430072, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A new DDTC-functionalized disubstituted polyacetylene (P2) with strong green fluorescence was
successfully synthesized by utilizing the postfunctional method. The polymer was soluble in common
organic solvents, and its strong green fluorescence could be quenched by mercury ions with detection
limit down to 1.7 ꢀ 10ꢁ7 mol/L (34 ppb). Moreover, nearly no interference was observed from other
Received 18 June 2012
Received in revised form
25 September 2012
Accepted 4 October 2012
Available online 11 October 2012
metal ions, including Liþ, Naþ, Kþ, Fe3þ, Fe2þ, Ni2þ, Cu2þ, Co2þ, Mg2þ, Al3þ, Zn2þ, Mn2þ, Pb2þ, Ba2þ, Ca2þ
,
Cd2þ, Agþ and Cr3þ. Furthermore, P2 could be put into application by test strips, making P2 a practical,
sensitive and selective mercury probe.
Keywords:
Chemosensor
Ó 2012 Elsevier Ltd. All rights reserved.
Fluorescence
Functionalization of polymers
1. Introduction
As one of the widespread metal ions, mercury is highly toxic for
humans by affecting many different areas of the human body (i.e.,
As the first and well-known conjugated polymer, in recent years,
polyacetylene (PA) has attracted much attention. So far, thousands
of mono or disubstituted PAs were prepared, while the research
fields were extended from the initial traditional conductive poly-
acetylene to the modern liquid crystals, polymeric light emitting
diodes, helical polymers, gas separation membranes, organice
inorganic hybrids, nonlinear optical and magnetic materials, etc.
[1e14]. In contrast to the mono-substituted polyacetylenes, the
disubstituted ones exhibited many advantages, especially, the
stability and strong luminescence, which, interestingly, could
ensure them good candidates as polymer chemosensors [15].
However, the related reported examples were very scarce, mainly
due to the difficulty encountered in their syntheses. The monomers
bearing some acceptor groups (the receptor moieties in sensors),
such as amide, amine, hydroxyl, cyano, thio, etc., could not be
directly polymerized to yield their corresponding polyacetylenes
due to the toxicity for the catalyst [16e19]. Fortunately, through the
postfunctional polymeric reactions, we have successfully obtained
some new functional PAs, some of them were confirmed to be good
chemosensors [20e24].
brain, heart, kidney, stomach, and intestines) as well as their
associated functions [25e31], and causing a wide variety of
symptoms, including digestive, cardiovascular, and especially
neurological diseases [32e34]. In our daily life, mercury is released
into the environment through a variety of anthropogenic (i.e., coal
and gold mining, solid waste incineration, wood pulping, fossil fuel
combustion, and chemical manufacturing.) and natural sources
(i.e., forest fires, volcanic and oceanic emissions) [35e39]. Also,
some bacteria can convert inorganic mercury to methylmercury,
which is much more toxic [40e42]. Thus, nowadays, mercury
pollution has become a global problem, it is of great value to
develop a fast, highly specific mercury sensor [43e48].
Dithiocarbamate (DTC) is an effective ligand for heavy metal
ions, especially towards mercury ion [49e53]. Based on our
previous work on PA chemosensors, if DTC is linked to poly-
acetylene main chain as acceptor group, it may be a mercury sensor
with good selectivity. Herein, we introduced 2,20-(4-
hydroxyphenylazanediyl)bis(ethane-2,1-diyl)bis(dimethylcarba-
modithioate) (DDTC), as the acceptor group, to the polyacetylene
main chain through the nucleophilic substitution reactions, to yield
the corresponding functional polyacetylene, P2 (Scheme 1). The
preliminary result realized our idea: P2 can act as a mercury sensor,
with the detection limits are as low as 1.7 ꢀ 10ꢁ7 mol/L. To the best
of our knowledge, this is one of the very few polyacetylene based
mercury sensors.
* Corresponding author. Tel.: þ86 27 62254108; fax: þ86 27 68756757.
E-mail addresses: lizhen@whu.edu.cn, lichemlab@163.com (Z. Li).
0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.polymer.2012.10.006

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D. Ou et al. / Polymer 53 (2012) 5691e5698
N
N
S
S
N
K2CO3,KI
S
S
S
S
S
C
C
O
C
C
O
n
n
DMF, 75 oC
N
+
S
N
N
P1
7
P2
O
OH
(CH2)3
Br
Scheme 1. The synthetic approach of P2.
2. Experimental section
2.1. Materials and instrumentations
to 100 ꢂC and reacted for 8 h. The solution was then cooled to room
temperature, filtered and extracted with CH2Cl2. Without further
purification, brown oilyliquid crude product of 2,20-(4-
methoxyphenylazanediyl)diethanol was obtained (7.98 g).
Toluene was dried over and distilled from KeNa alloy under an
atmosphere of dry nitrogen. N,N-Dimethylformamide (DMF) was
dried over and distilled from CaH2 under an atmosphere of dry
nitrogen. Tetrahydrofuran (THF) was dried over and distilled from
KeNa alloy under an atmosphere of dry nitrogen. Dichloromethane
(CH2Cl2) was dried over and distilled from CaH2 under an atmo-
sphere of dry nitrogen. All other reagents were of analytical reagent
grade and used without further purification. Doubly distilled water
was used for all experiments. Cu(NO3)2$3H2O, LiCl, NaNO3, KNO3,
AgNO3, NiCl2$6H2O, MgCl2$6H2O, Al(NO3)3$9H2O, ZnCl2,
MnSO4$H2O, Hg(ClO4)2$3H2O, Fe(NO3)3$9H2O, FeSO4$7H2O,
Cr(NO3)3$9H2O, CoCl2$6H2O, Ba(NO3)2, Pb(NO3)2, 3CdSO4$8H2O,
CaCl2, 4-methoxybenzeneamine, 2-chloroethanol, POCl3, BBr3 were
purchased from Sinopharm Chemical Reagent Beijing Co., Ltd.
Sodium dimethylcarbamodithioate, tungsten(VI) chloride and tet-
raphenyltin were purchased from Alfa Aesar Co., Ltd. All the
experiments were conducted at room temperature.
2.2.2. Synthesis of N,N-bis(2-chloroethyl)-4-methoxyaniline (4)
4 was prepared according to the previous reported procedure
[54,55]. Into a baked 100-mL dual-port round-bottomed flask,
crude product of 3 (1.06 g) was added. POCl3 (5 mL, 53.6 mmol) was
added to the flask under the ice water bath and stirred for 10 s. Then
the stirred mixture was reacted for 1 h at 100 ꢂC. After cooling
down, 100 mL of ethyl acetate was added to the mixture, then the
resultant mixture was washed with water until pH ¼ 7. The organic
phase was separated and concentrated by a rotary evaporator. The
crude product was purified on a column chromatograph (silica gel)
with a mixture of dichloromethane and hexane (1/1) as eluent. A
yellowish-white solid was obtained (0.808 g, 64.7%). 1H NMR
(300 MHz, CDCl3), d (TMS, ppm): 6.86 (d, 2H, ArH), 6.71 (d, 2H, ArH),
3.77 (s, 3H, OCH3), 3.63 (t, 8H, CH2).
2.2.3. Synthesis of 2,20-(4-methoxyphenylazanediyl)bis(ethane-2,1-
diyl)bis(dimethylcarbamodithioate) (6)
1H and 13C NMR spectroscopy study was conducted with a Var-
ian Mercury300 spectrometer using tetramethylsilane (TMS;
6 was prepared according to the previous reported procedure
[53]. Into a 100-mL flask, 4 (0.80 g, 3.2 mmol) was dissolved in 20 mL
ethanol. Sodium dimethylcarbamodithioate (0.94 g, 6.56 mmol) and
potassium iodide of catalytic amount were added with vigorous
stirring. After the addition, the stirred mixture was reflux for 24 h,
and then the excessive inorganic salt was filtered out immediately.
After cooling down, the white solid was deposited from the filtrate.
The crude product was recrystallized from hot ethanol and then
purified on a column chromatograph (silica gel) with dichloro-
methane as eluent (0.625 g, 46.4%). 1H NMR (300 MHz, CDCl3),
d
¼ 0 ppm) as internal standard. The Fourier transform infrared
(FTIR) spectra were recorded on a PerkinElmer-2 spectrometer in
the region of 3000e400 cmꢁ1 on NaCl pellets. UVevisible spectra
were obtained using a Shimadzu UV-2550 spectrometer. Photo-
luminescence spectra were performed on a Hitachi F-4500 fluo-
rescence spectrophotometer. Gel permeation chromatography
(GPC) was used to determine the molecular weights of polymers.
GPC analysis was performed on a Waters HPLC system equipped
with a 2690D separation module and a 2410 refractive index
detector. Polystyrene standards were used as calibration standards
for GPC. DMF was used as an eluent and the flow rate was 1.0 mL/
min. Thermal analysis was performed on NETZSCH STA449C
thermal analyzer at a heating rate of 10 ꢂC/min in nitrogen at a flow
rate of 50 cm3/min for thermo gravimetric analysis (TGA).
Elemental analyses were performed by a CARLOERBA-1106 micro-
elemental analyzer.
d
(TMS, ppm): 6.92 (d, 2H, ArH), 6.87 (d, 2H, ArH), 3.77 (s, 3H, OCH3),
3.58 (t, 4H, NCH2), 3.49 (t, 4H, SCH2), 3.39 (s, 12H, NCH3).
2.2.4. Synthesis of 2,20-(4-hydroxyphenylazanediyl)bis(ethane-2,1-
diyl)bis(dimethylcarbamodithioate) (7, DDTC)
7 (DDTC) was prepared according to the previous reported
procedure [56e58]. Into a 100-mL Schlenk tube were added 6
(0.235 g, 0.56 mmol), and the tube was evacuated under vacuum
and then flushed with nitrogen three times through the sidearm.
After the addition of 10 mL of CH2Cl2, the tube was placed in
a reactor at ꢁ78 ꢂC. Stirring for 20 min, boron tribromide (2 mL,
2.15 mmol) was added dropwise to the mixture. The resultant
mixture was stirred for 2 h, and then warmed to room temperature
and stirred overnight. The reaction was hydrolyzed by shaking with
H2O (5 mL), the organic phase was separated and concentrated by
a rotary evaporator. The crude product was purified on a column
chromatograph (silica gel) with dichloromethane as eluent. A pale
yellow oilyliquid was obtained (124 mg, 54.6%). 1H NMR (300 MHz,
2.2. Synthetic approach
2.2.1. Synthesis of 2,20-(4-methoxyphenylazanediyl)diethanol (3)
3 was prepared according to the previous reported procedure
[54,55]. Into a baked 250-mL Schlenk tube with a stopcock in the
sidearmwas added 4-methoxybenzeneamine (1, 5.00 g, 40.6 mmol),
sodium carbonate (15.22 g, 143.6 mmol) and potassium iodide of
catalytic amount. The tube was evacuated under vacuum and then
flushed with dry nitrogen three times through the sidearm. 2-
Chloroethane (2, 10 mL, 149.0 mmol) and freshly distilled water
(20 mL) were injected intothe tube, the mixture solutionwas heated
CDCl3),
d (TMS, ppm): 6.87 (d, 2H, ArH), 6.79 (d, 2H, ArH), 4.36 (s,

D. Ou et al. / Polymer 53 (2012) 5691e5698
5693
3H, OH), 3.63 (t, 8H, CH2), 3.57 (t, 4H, NCH2), 3.48 (t, 4H, SCH2), 3.38
(s, 12H, NCH3). 13C NMR, (75 MHz, CDCl3),
(TMS, ppm): 196.75,
147.54, 141.47, 116.17, 114.54, 50.63, 45.36, 41.55, 34.08.
2.3.3. Reaction time of P2 with Hg2þ
d
A solution of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) was prepared in THF. A
solution of Hg2þ (1.0 ꢀ10ꢁ3 mol Lꢁ1) was prepared in distilled water.
A solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm width)
and the fluorescence spectrumwas recorded. Different amount (1, 5,
2.2.5. Synthesis of P1
P1 was prepared according to our previous reported procedure
[24]. Into a baked 25-mL Schlenk tube with a stopcock in the
sidearm was added 11 (2.008 g, 6.37 mol). The tube was evacuated
under vacuum and then flushed with dry nitrogen three times
through the sidearm. Freshly distilled toluene (6 mL) was injected
into the tube to dissolve the monomer. The catalyst solution was
prepared in another tube by dissolving tungsten(VI) chloride
(67 mg) and tetraphenyltin (64 mg) in toluene (6 mL). The two
tubes were aged at 80 ꢂC for 1 h, and the monomer solution was
transferred to the catalyst solution using a hypodermic syringe. The
reaction mixture was stirred at 80 ꢂC for 48 h. The solution was then
cooled to room temperature, diluted with 15 mL of chloroform, and
added dropwise to 150 mL of methanol through a cotton filter
under stirring. The precipitate was allowed to stand overnight and
was then filtered. The polymer was washed with methanol and
dried in a vacuum oven to a constant weight (1.078 g, 53.7%).
10 m
L) of the Hg2þ solution were introduced and fluorescence
intensity changes were recorded at room temperature every 30 s
(from 30 to 180 s). The excitation wavelength was 375 nm.
2.3.4. Fluorescence titration of P2 with Hg2þ
A solution of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) was prepared in THF. A
solution of Hg2þ (1.0 ꢀ 10ꢁ3 mol Lꢁ1) was prepared in distilled
water. A solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm
width) and the fluorescence spectrum was recorded. The Hg2þ
solution was introduced in portions and fluorescence intensity
changes were recorded at room temperature each time. The exci-
tation wavelength was 375 nm.
2.3.5. Fluorescence intensity changes of the mixture of P1 and 7
with Hg2þ
A solution of the mixture of P1 and 7 (5.0 ꢀ 10ꢁ6 mol/L) was
prepared in THF. A solution of the mixture of P1 and 7 (3.0 mL) was
placed in a quartz cell (10.0 mm width) and the fluorescence spec-
trum was recorded. The Hg2þ solution (1.0 ꢀ 10ꢁ3 mol/L) was intro-
Mw ¼ 58 400, Mw/Mn ¼ 1.55. IR (thin film),
stretching), 2923, 2865 (CH2 stretching). 1H NMR (300 MHz,
CDCl3), (TMS, ppm): 6.90e6.32 (br, ArH), 6.26e5.90 (br, ArH),
4.20e3.70 (br, OCH2), 3.50e3.18 (br, CH2Br), 2.38e2.10 (br, CH2). 13
NMR (75 MHz, CDCl3), (TMS, ppm): 149.3, 148.8, 137.2, 130.9,
y
(cmꢁ1): 3050 (AreH
d
C
duced (9 mL) and fluorescence intensity changes were recorded at
d
room temperature each time. The excitationwavelength was 375 nm.
128.5, 127.3, 125.1, 123.0, 122.3, 108.0, 98.46, 68.8, 60.2, 24.2. EA (%):
calcd: C 64.78, H 4.00; found: C 64.62, H 4.24.
2.3.6. Fluorescence intensity changes of P2 with different metal ions
A solution of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) was prepared in THF. A
solution of metal ions (1.0 ꢀ 10ꢁ3 mol Lꢁ1) was prepared in distilled
water. A solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm
width) and the fluorescence spectrum was recorded. Different
metal ion solutions (9.0 mL) were introduced and the changes of the
fluorescence intensity were recorded at room temperature each
time. The excitation wavelength was 375 nm.
2.2.6. Synthesis of P2
P2 was prepared according to our previous reported procedure
[24]. Into a 50-mL Schlenk tube, P1 (61 mg), 7 (DDTC), 120 mg,
(0.30 mmol), potassium carbonate (107 mg, 0.78 mmol), potassium
iodide of catalytic amount and dry DMF (5 mL) were added. After
stirring at 80 ꢂC for 2 days under an atmosphere of dry nitrogen, the
resultant mixture was filtered, then the filtrate was added dropwise
to 150 mL of water through a cotton filter under stirring. The
precipitate was allowed to stand overnight and then filtered. The
polymer was washed with water and dried in a vacuum oven to
a constant weight to yield P2 (96 mg, 90.9%). Mw ¼ 46 100, Mw/
2.3.7. Fluorescence intensity changes of P2 with water
A solution of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) was prepared in THF. A
solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm width)
and the fluorescence spectrum was recorded. Distilled water was
introduced in portions and fluorescence intensity changes were
recorded at room temperature each time. The excitation wave-
length was 375 nm.
Mn ¼ 5.04. IR (thin film),
y
(cmꢁ1): 3136e3544 (NeH stretching),
3045 (AreH stretching), 2934, 2868 (CH2 stretching), 1607 (C]N
stretching), 1375 (CH3 stretching), 1253 (CeN stretching). 1H NMR
(300 MHz, CDCl3),
ArH), 6.42e5.80 (br, ArH), 4.12e3.90 (br, OCH2), 3.78e3.37 (br,
OCH2, NCH2), 3.37e3.10 (br, SCH2, NCH3), 2.38e1.66 (br, CH2). 13
NMR (75 MHz, CDCl3), (TMS, ppm): 195.7, 129.4, 116.3, 114.4, 89.4,
d
(TMS, ppm): 7.20e6.75 (br, ArH), 6.75e6.42 (br,
A solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm
width) and the fluorescence spectrum was recorded. 9.0 mL distilled
water was introduced and the changes of the fluorescence intensity
were recorded at room temperature. The excitation wavelength
was 375 nm.
C
d
65.1, 57.9, 45.8, 42.1, 34.1, 29.3。EA (%): calcd: C 62.13, H 6.16, N
6.59; found: C 62.32, H 6.51, N 6.26.
2.3.8. Competition experiments of Hg2þ with other metal ions
A solution of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) was prepared in THF.
Different metal ions solutions (1.0 ꢀ 10ꢁ3 mol Lꢁ1) were prepared in
distilled water. A solution of P2 (3.0 mL) was placed in a quartz cell
(10.0 mm width) and the fluorescence spectrum was recorded. A
2.3. Text
2.3.1. Preparation of polymer solutions
P1 (1.58 mg) was dissolved in 5 mL THF to afford the stock
solution with the concentration of 1.0 ꢀ 10ꢁ3 mol Lꢁ1. This stock
solution was diluted to 5.0 ꢀ 10ꢁ6 mol Lꢁ1. P2 (2.30 mg) was dis-
solved in 5 mL THF to afford the stock solution with the concen-
tration of 1.0 ꢀ 10ꢁ3 mol Lꢁ1. This stock solution was diluted to
kind of metal ions (9.0
fluorescence intensity was recorded at room temperature. Then
9.0
L of Hg2þ solution was introduced and the change of the
mL) was introduced and the change of the
m
fluorescence intensity was recorded at room temperature. The
excitation wavelength was 375 nm.
5.0 ꢀ 10ꢁ6 mol Lꢁ1
.
2.3.2. Preparation of solutions of metal ions
2.3.9. Fluorescence intensity changes of P2 in different mixed
solvent with Hg2þ
A 0.1 mmol portion of each inorganic salt was dissolved in
distilled water (10 mL) to afford a 1.0 ꢀ 10ꢁ2 mol Lꢁ1 aqueous
solution. The stock solutions were diluted to the desired concen-
trations with distilled water when needed.
Solutions of P2 (5.0 ꢀ 10ꢁ6 mol Lꢁ1) were prepared in THF and
different mixed solvent (THF/H2O ¼ 98/2, 95/5, 90/10, 80/20, 50/
50). A solution of P2 (3.0 mL) was placed in a quartz cell (10.0 mm

5694
D. Ou et al. / Polymer 53 (2012) 5691e5698
OCH3
OCH3
OCH3
Na2CO3, KI
2O,reflux 8 h
POCl3
OH
+
Cl
1. 0 oC, 10 min
H
2. 100 oC, 1 h
N
N
NH2
2
1
3
4
HO
OH
Cl
Cl
OCH3
OH
S
BBr3
KI, K2CO3
N
S Na
4
+
CH2Cl2, -78 oC
EtOH, 60oC
N
N
S
S
S
S
5
N
S
S
N
N
S
S
N
7
6
Scheme 2. The synthetic route of compound 7.
width) and the fluorescence spectrum was recorded. The Hg2þ
nucleophilic substitution reaction followed by the Sonogashira
coupling reaction in the total yield of 61.7%, according to the similar
procedure reported earlier. Then, through the direct polymeriza-
tion process, this monomer was converted to the corresponding PA,
P1, in which the bromine atoms remained for further functionali-
zation. During the followed nucleophilic substitution reaction, the
bromine atoms in the side chain of P1 were substituted by DDTC
moieties, yielding the designed DDTC-functionalized PA, P2.
It should be pointed out that the preparation of P2 was through
the post functionalization approach. According to our previous
cases, if disubstituted acetylene monomers contained polar groups
or active hydrogen atoms, such as amide, amine, hydroxy, cyano,
thio, azo etc., they could not be polymerized to give the corre-
sponding disubstituted polyacetylenes, due to the deactivation of
the transition-metal catalysts [16e19]. Fortunately, by utilizing the
polymer reactions of some disubstituted polyacetylenes bearing
reactive groups, such as chlorine and bromine atoms, and azido
moieties, some polar groups could be introduced to the side chains
solution (1.0 ꢀ 10ꢁ3 and 1.0 ꢀ 10ꢁ2 mol/L) was introduced (9
mL)
and fluorescence intensity changes were recorded at room
temperature each time. The excitation wavelength was 375 nm.
3. Result and discussion
3.1. Synthesis
The synthetic route to P2 was shown in Schemes 1 and 2.
First, compound
3 was obtained by the reaction of 4-
methoxybenzeneamine and 2-chloroethane, and then directly
chlorinated with POCl3 to yield compound 4. Through the nucleo-
philic substitution reaction with sodium dimethylcarbamodi-
thioate, compound 4 was converted to compound 5, and then
treated with BBr3 in CH2Cl2, the demethylated compound 7 (DDTC)
could be obtained in the total yield of 15.3%. Monomer 11, which
contained
a
bromine atom, was easily prepared from the
P1
∗
∗
P2
∗
∗
9
8
7
6
5
4
3
2
1
0
-1
δ
(ppm)
Fig. 1. 1H NMR spectra of P1 and P2 in chloroform-d.

D. Ou et al. / Polymer 53 (2012) 5691e5698
5695
1.00
0.75
0.50
0.25
0.00
1000
P1
P2
P1
P2
750
500
250
0
300
400
Wavelength (nm)
500
600
400
480
560
640
720
Wavelength (nm)
Fig. 2. Absorption spectra (left) and fluorescence emission spectra (right) of P1 (5.0
m
M, THF, 20 ꢂC) and a solution of P2 (5.0
m
M, THF, 20 ꢂC).
through the Sonogashira coupling reaction, the click chemistry
reaction, the nucleophilic substitution reaction and the hydrolysis
reaction [20e24]. Based on our experience, in the nucleophilic
substitution reaction, the bromine atoms in the side chain of
disubstituted polyacetylene exhibited much higher reactivity than
that of the chlorine ones. And sometimes, the bromine atoms could
be converted to another functional group completely. Thus, here,
the disubstituted polyacetylene (P1) bearing bromine atoms in the
side chains, was designed for the preparation of P2. In accordance
with the elemental analysis result, almost all the bromine atoms in
the side chain were converted to DDTC moieties. The successful
preparation of P2 once again, confirmed that the post functionali-
zation strategy was a good alternative approach to yield disubsti-
tuted polyacetylenes inaccessible from their corresponding
monomers through the direct polymerization reactions. The
purification procedure was simple, we only needed to isolate
compound 4, compound 6, compound 7 and monomer 11 by a silica
gel column, and polymers P1 and P2 could be purified easily by
precipitations from their solutions into methanol or water.
3.2. Structural characterization
P1 and P2 were characterized by spectroscopic methods, and all
gave satisfactory spectral data. The detailed analysis data was
presented in Experimental section and Supporting information. In
the 1H NMR spectra of the polymers P1 and P2 (Fig.1), the chemical
shifts were consistent with the proposed polymer structure as
demonstrated in Scheme 1. After the reaction with DDTC, there
were some new absorption peaks appeared at about 6.86 ppm in
the downfield and 3.35, 3.54 ppm in the high field, which should be
1000
1000
P1
P1
+
P2
800
DDTC
2+
P1+Hg
750
P1+DDTC
2+
+Hg
600
500
250
400
200
0
2+
P2+Hg
0
720
400
480
560
640 720 400
480
560
640 720 400
480
560
640
Wavelength (nm)
Fig. 3. Fluorescence spectra of P1 in THF before and after the addition of Hg2þ (left), the mixture of P1 and DDTC in THF before and after the addition of Hg2þ (middle), P2 in THF
before and after the addition of Hg2þ (right).

5696
D. Ou et al. / Polymer 53 (2012) 5691e5698
A
B 1600
2+ -6
[Hg ] (10 M)
2+
P2+Hg
1.00
0.75
0.50
0.25
0.00
0
0.17
0.33
0.50
0.67
0.83
1.07
1.33
1.67
2.00
2.33
2.67
3.00
3.33
4.00
5.00
P2
1200
-7
P2+3.33×10 M Hg
2+
-6
P2+1.67×10 M Hg
2+
-6
P2+3.33×10 M Hg
2+
800
400
0
0
50
100
Time (s)
150
200
400
480
560
Wavelength (nm)
640
720
Fig. 4. (A) Time course of the fluorescence response of P2 (5.0
Polymer concentration: 5.0 M. The excitation wavelength was 375 nm.
m
M, THF) upon addition of various concentration of Hg2þ. (B) Emission quenching of the solution of P2 by Hg2þ in THF.
m
ascribed to the protons of DDTC moieties in P2, indicating that the
bromine atoms were converted to the DDTC successfully. The
elemental analysis results and IR spectra (Figure S1) indicated the
same conclusion.
As a result of the introduction of DDTC moieties to the PA
backbone, P2 exhibited different stability from P1. Their TGA
thermograms were shown in Figure S2, it was obvious that P2 was
a little more thermally stable than P1. The 5% weight loss temper-
ature of P2 is about 255 ꢂC while that of P1 is about 246 ꢂC.
The absorption spectra and fluorescence emission spectra were
shown in Fig. 2. Obviously, owing to the introduction of DDTC
moieties, a new absorption peak appeared at 248 nm in the
absorption spectra of P2 in comparison with that of P1.
Since the optical signal changes relied on the interaction
between P2 and Hg2þ, thus, the reaction rate might affect the
experimental results. From this point, we investigated the influence
of the reaction time on the sensing results, with the results
demonstrated in Fig. 4. It was obvious that a plateau of the emission
intensity changes could be achieved after 90 s with various
concentration of Hg2þ. Therefore, we fixed the detection time at
90 s after the addition of Hg2þ solution.
To study the quenching behavior of Hg2þ ions in detail, we
investigated the decrease in the fluorescence intensity by adding
successive aliquots of aqueous stock solutions of Hg2þ ions to the
diluted solution of P2. As shown in Fig. 4B, the quenching of pho-
toluminescence was observed at very low level of Hg2þ
(1.7 ꢀ 10ꢁ7 mol/L), and the fluorescent intensity decreased rapidly
3.3. Fluorescence properties
As mentioned in the introduction part, their stability and strong
luminescence could assure disubstituted polyacetylenes good
candidates as conjugated polymer chemosensors. It was easily seen
that, both of P1 and P2 emitted strong luminescence in diluted
solutions with the maximum emission wavelength at about 510
and 508 nm, respectively (Fig. 2). The mixture of P1 and DDTC
nearly showed the same emission behavior as P1, indicating that
the presence of DDTC in the mixture did not affect the electronic
properties of the conjugated polymer. However, the blue-shifted
emission (2 nm) of P2 in comparison with that of P1, perhaps,
disclosed the minor change of the electronic properties of the
conjugated polymers before and after the linkage of the DDTC
moieties, partially explaining the totally different sensing behavior
of P2 and the mixture of P1 and DDTC discussed as following.
28
21
14
7
As shown in Fig. 3, after the addition of a little amount of Hg2þ
,
the things became different. Similar to that of P1, the emission
spectrum of the P1eDDTC mixture remained nearly unchanged,
while the strong luminescent emission of P2 was completely
quenched by the added Hg2þ ions. These results indicated that after
being linked to the polymer main chain, the DDTC moieties could
efficiently transfer the energy from the conjugated polymer back-
bone to the mercury ions, leading to the quenching of the strong
luminescence of the polymer. Thus, P2 could be utilized as polymer
chemosensor for metal ions. Also, the results confirmed that the
receptor should be linked to the polyacetylene backbone to effi-
ciently transfer the energy from the conjugated backbone to the
metal ions, similar to those observed in our previous cases [22e24].
0
Fig. 5. Fluorescence emission response profiles of P2. The polymer concentration was
5.0 mM, while the concentration of metal ions was 3.0 mM. The excitation wavelength
was 375 nm. Inset: Fluorescence photograph of P2 solution (10.0 mM, THF) to various
metal ions, 6.0
m
M. (A) P2; (B) P2 þ Hg2þ; (C) P2 þ Agþ; (D) P2 þ Naþ; (E) P2 þ Pb2þ
;
(F) P2 þ Mn2þ; (G) P2 þ Fe3þ; (H) P2 þ Fe2þ; (I) P2 þ Zn2þ; (J) P2 þ Mg2þ; (K)
P2 þ Cu2þ; (L) P2 þ Ca2þ; (M) P2 þ Ni2þ; (N) P2 þ Liþ; (O) P2 þ Kþ; (P) P2 þ Ba2þ; (Q)
P2 þ Cr3þ; (R) P2 þ Cd2þ; (S) P2 þ Co2þ; (T) P2 þ Al3þ. The excitation wavelength was
365 nm.

D. Ou et al. / Polymer 53 (2012) 5691e5698
5697
40
30
20
10
0
visually at very low concentration with the aid of a normal UV lamp,
but not a fluorescence spectrophotometer. As shown in the
photographs of the solutions of P2 (Fig. 5), after the addition of the
mercury ions, the strong green fluorescence of P2 was completely
quenched and the corresponding bottle became dark, under UV
illumination. However, upon the addition of other metal ions, no
changes could be observed, except very little response from Agþ.
Thus, the difference was very apparent and could be seen visually,
even the concentrations of Hg2þ ions were only 3 ꢀ 10ꢁ6 mol/L
(0.6 ppm).
P2+other cations
P2+other cations+Hg
2+
We also took photos of the resultant solutions at different
concentrations of the Hg2þ. Figure S26 demonstrated the change
process from strong fluorescence to week ones, by adjusting the
concentration of the added Hg2þ ions from 0 to 7.0
mM. It was easily
seen that the strong fluorescence was quenched step by step,
accompanying with an increase in the concentration of Hg2þ ions.
The fluorescence emission response of P2 to different amount of
distilled water was also investigated. As shown in Figure S24 and
S25, the introduction of distilled water (even 50 mL) caused just
a little decrease of the fluorescence intensity of P2 solution.
From the obtained experimental results, we could conclude that
P2 could report the presence of trace Hg2þ ions based on the
fluorescence “turn-off”. The above obtained results were similar to
those of small fluorophores bearing a pair of DTC moieties [49e53].
In comparison with those small fluorophores, the possible mech-
anism for P2 of detecting mercury could be described as Chart 1. As
mentioned in the Introduction part, DTC is an effective ligand for
heavy metal ions, especially towards mercury ion. The DDTC
moieties consist of a pair of DTC, thus, when mercury ions are
added to the solution of P2, the four sulfur atoms on the side chain
of P2 can coordinate with mercury to form the metal complex. With
the aid of DDTC moieties, the energy could be efficiently transferred
from the conjugated backbone to the mercury ions, leading to the
quenching of the strong luminescence of the polymer.
To investigate the practical application of chemosensor P2, test
strips were prepared by immersing filter paper into the THF solu-
tion of P2 (10.0 mM) and then air-dried. The test strips containing
P2 were utilized to sense mercury ions. For the solutions of
mercury ions, different test strips were immersed for 5 min and
then the obvious color change was observed: the higher the
concentration of mercury ions, the more apparent the emission
intensity change from strong green fluorescence to week fluores-
cence. As depicted in Fig. 7, the discernible concentration of
Fig. 6. Competition experiments of Hg2þ with other metal ions. Red bar: fluorescence
intensity of P2 (5.0 M) with the addition of the respective competing cations (3.0 M).
Green bar: fluorescence intensity of P2 (5.0 M) with the addition of the respective
competing cations (3.0 M). The excitation wavelength was
M) and the Hg2þ (3.0
375 nm. (For interpretation of the references to color in this figure legend, the reader is
referred to the web version of this article.)
m
m
m
m
m
upon the increase of the added concentration of Hg2þ. When the
concentration of Hg2þ was 3 ꢀ 10ꢁ6 mol/L, nearly no luminescence
could be observed. The quenching efficiency was nearly fit to the
SterneVolmer equation, I0/I ¼ Ksv [A] þ 1, which was related to the
fluorescence intensity, I, at different concentrations of analyte
quencher, [A], where I0 was the intensity at [A] ¼ 0, and Ksv was the
SterneVolmer constant. According to the fluorescence titration of
P2 in THF solutions with Hg2þ
, Ksv was calculated to be
7.2 ꢀ 106
M
ꢁ1, respectively.
To evaluate the selectivity of P2, the influence of other metal
ions was investigated. As shown in Figs. 5 and 6 and Figure S4e23
(in Supporting information), other metal ions nearly gave no
disturbance to the selective sensing of mercury ions, except little
response from Agþ. Thus, the selectivity for mercury ions over other
metal ions was relatively high. The competition experiments using
solutions containing mercury ions and one of the other metal ions
demonstrated that P2 could still report the presence of mercury
ions in the presence of other common interfering metal ions
(Fig. 6). As the sensing sensitivity of P2 towards Hg2þ ions was very
high, we wondered if it was possible to detect the mercury ions
mercury ions could be as low as 10
mM, and the strong green
fluorescence of the test strips almost completely quenched. Thus,
these strips could be easily handled by people at any moment for
the detection of mercury ions.
"On"
"Off"
hv'
N
S
N
Hg2+
S
S
S
C
C
O
C
C
O
n
n
Hg2+
S
N
N
S
N
N
S
S
O
O
hv
hv
Chart 1. The possible mechanism for P2 of detecting Hg2þ
.

5698
D. Ou et al. / Polymer 53 (2012) 5691e5698
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In summary, we have successfully prepared a new DDTC-
functionalized disubstituted polyacetylene (P2) by utilizing the
postfunctional strategy, and studied its sensing behavior towards
metal ions by the fluorescence spectra. P2 could report the pres-
ence of trace Hg2þ ions as low as 1.7 ꢀ 10ꢁ7 mol/L (34 ppb), making
it a good candidate for potential application of human safeguard. In
addition, the chemosensor P2 could also be put into practical
application by test strips. The discernible concentration of mercury
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Acknowledgment
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We are grateful to the National Natural Science Foundation of
China (20974084) for financial support.
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Appendix A. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.polymer.2012.10.006.
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