A new disubstituted polyacetylene bearing pyridine moieties: Convenient synthesis and sensitive chemosensor toward sulfide anion with high selectivity

Article abstract of DOI:10.1021/ma200777e

To develop sensitive and selective S^2- chemosensors, a new disubstituted polyacetylene (P2) bearing pyridine moieties in the side chains was prepared conveniently through a postfunctional method, the strong green fluorescence of which could be completely quenched by Cu²⁺ ions at the concentration as low as 2.0 × 10^-6 mol/L in diluted solutions. Based on the displacement strategy, by utilizing the much higher stability constant of the complex of S^2- and Cu²⁺, the quenched fluorescence of the solution of P2 by Cu²⁺ ions could recover upon the addition of trace S^2- anions, with the detection limit down to 5.0 × 10^-7 mol/L. Moreover, no interference were observed from other anions, including SO₃^2-, HSO ₃^-, SO₄^2-, ClO₄ ^-, I^-, Br^-, Cl^-, F^-, IO₃^-, HPO₄^2-, PO₄ ^3-, C₂O₄^2-, S₂O ₃^2-, CO₃^2-, AcO^-, CN ^-, and P₂O₇^4-, making P2 a novel, sensitive, and selective sulfide probe.

Full text of DOI:10.1021/ma200777e

ARTICLE
pubs.acs.org/Macromolecules
A New Disubstituted Polyacetylene Bearing Pyridine Moieties:
Convenient Synthesis and Sensitive Chemosensor toward Sulfide
Anion with High Selectivity
Liang Zhang, Xiaoding Lou, Yun Yu, Jingui Qin, and Zhen Li*
Department of Chemistry, Hubei Key Lab on Organic and Polymeric Opto-Electronic Materials, Wuhan University,
Wuhan 430072, China
S Supporting Information
b
ABSTRACT: To develop sensitive and selective S2ꢀ chemosensors, a new disubstituted poly-
acetylene (P2) bearing pyridine moieties in the side chains was prepared conveniently through a
postfunctional method, the strong green fluorescence of which could be completely quenched
by Cu2þ ions at the concentration as low as 2.0 ꢁ 10ꢀ6 mol/L in diluted solutions. Based on the
displacement strategy, by utilizing the much higher stability constant of the complex of S2ꢀ and
Cu2þ, the quenched fluorescence of the solution of P2 by Cu2þ ions could recover upon the addition
of trace S2ꢀ anions, with the detection limit down to 5.0 ꢁ 10ꢀ7 mol/L. Moreover, no interference
ꢀ
were observed from other anions, including SO32ꢀ, HSO3ꢀ, SO42ꢀ, ClO4ꢀ, Iꢀ, Brꢀ, Clꢀ, Fꢀ, IO3
,
HPO42ꢀ, PO43ꢀ, C2O42ꢀ, S2O32ꢀ, CO32ꢀ, AcOꢀ, CNꢀ, and P2O74ꢀ, making P2 a novel, sensitive,
and selective sulfide probe.
Chart 1
’ INTRODUCTION
Conjugated polymer-based fluorescent (CPF) chemosensors,
which could detect different analytes involved in chemical, bio-
logical, and environmental processes of particular relevance, have
attracted much attention in these years, since the “molecular
wire effect” in conjugated polymers usually greatly enhanced the
sensitivity of the polymer-based chemosensors because of the
enhanced electronic communication among them.1 Thanks to
the enthusiastic efforts of scientists, many kinds of CPF chemo-
sensors were designed to probe different analytes, including
metal ions, anions, explosives, small organic molecules, and
biomolecules. Generally, the popular design strategy for CPF
chemosensors was to introduce some acceptor groups, such as
bipyridyl, terpyridyl, and quinoline segments, to the polymer
main or side chain, which could trap or interact with the target
analytes and give out the fluorescent signal.2
As a typical type of conjugated polymers, polyacetylenes (PA),
which demonstrated various functionalities upon structural
modification on their side chains, were promising candidates, such
asliquid crystals, polymeric light-emitting diodes, helicalpolymers,
gas separation membranes, organicꢀinorganic hybrids, and non-
linear optical and magnetic materials, for the practical applica-
tions.3 However, there were very few reports concerning the
properties of disubstituted PAs as chemosensors, although their
stability and strong luminescence could ensure them good candi-
dates as polymer chemosensors.4 The reason might be partially
the difficulties encountered in their synthesis, since it was still a
big challenge to prepare disubstituted PAs containing the above-
mentioned acceptors, as well as groups such as amide, amine,
hydroxyl, cyano, thio, etc.5 Fortunately, partially based on our
previous work on functional polymers obtained through the
postfunctional polymeric reactions, we have successfully ob-
tained some new functional PAs inaccessible from their corre-
sponding monomers through the direct polymerization process,
by the utilization of polymer reactions.6 Some of which were
confirmed to be good chemosensors. For example, as shown in
Chart 1, the strong fluorescence of the imidazole-functionalized
polyacetylenes, PS1 and PS2, could be completely quenched by
copper ions selectively.6d,e Interestingly, the quenched fluores-
cence could turn on upon the addition of trace cyanide, making
them novel sensitive cyanide chemosensors. In comparison with
PS1, the structure of PS2 was much perfect, thanks to the relative
higher reactivity of bromine atoms than that of chlorine ones in
the nucleophilic substitution reactions for the preparation of PS1
Received: April 3, 2011
Revised:
May 25, 2011
Published: June 13, 2011
r
2011 American Chemical Society
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Scheme 1
and PS2. This strategy for cyanide sensors, first reported by us,6d
was further applied for the development of other cyanide sensors
by us and other scientists.7
and investigated the sensing behavior of the resultant functional
polyacetylenes. As the first attempt, the di(2-picolyl)amine
moieties were used as the acceptor group, which were success-
fully bonded to the polyacetylene main chain through the
nucleophilic substitution reactions, to yield the corresponding
functional polyacetylene, P2 (Scheme 1). The preliminary
results realized the above idea: the detection limit of P2 for
sulfide anion could be as low as 5.0 ꢁ 10ꢀ7 mol/L, nearly without
any interference from other anions. Herein, we would like to
report the synthesis, characterization, and optical properties of
P2 in detail.
On the other hand, sulfide anion is one of the toxic anions,
which could irritate mucous membranes and even cause uncon-
sciousness and respiratory paralysis upon continuous and high
concentration exposure of sulfide anion. Once being protonated,
it becomes even more toxic. Actually, sulfide anion is widely used
in industrial settings and produced as a byproduct in a large scale.
Also, it can be formed in several ways in the biosystem, for
example, the microbial reduction of sulfate by anaerobic bacteria,
and the conversion from the sulfur-containing amino acids in
meat proteins.8 Thus, the detection of sulfide anion has become
very important from industrial, environmental, and biological
points of view. A variety of detection techniques have been devel-
oped for the determination of sulfide anion, in which fluorimetry
has received considerable attention due to its high sensitivity and
easy detection.9ꢀ12 However, the successful cases were still scarce,
and to the best of our knowledge, it seemed that there were no
reports concerning the CPF chemosensors toward sufide anions.
Considering the good chemosensors for cyanide we reported
previously,6d,e,7a,b,fꢀh according to the displacement strategy,
especially the above-mentioned polyacetylenes, PS1 and PS2,
we were wondering if it was possible to design CPF chemosen-
sors for sulfide anions. Deeply thinking about the sensing
mechanism of PS1 and PS2, the key point was that cyanide
could form very stable complex with copper ions and snatched
the copper ions from the complex of Cu2þ and its corresponding
Cu2þ chemosensor. Then, there should be some Cu2þ chemo-
sensors, which could not be utilized as cyanide chemosensors
as we did previously, due to the much higher stability constants
of their corresponding complexes formed with copper ions. If it
was the case, perhaps, other anions rather than cyanide, with
even stronger affinity toward copper ions, could snatch the
copper ions from these complexes, accompanying with the
detectable optical signals. As we knew, the solubility product
constant of CuS (Ksp= 6.3 ꢁ 10ꢀ36) is much lower than that of
cyanide one (3.2 ꢁ 10ꢀ20); thus, there might be some Cu2þ
chemosensors, which could be utilized as new sulfide chemo-
sensors according to the displacement strategy. Therefore, we
tried to link some strong ligand of copper ions to polyacetylene
’ EXPERIMENTAL SECTION
Materials and Instrumentation. N,N-Dimethylformamide (DMF)
was dried over and distilled from CaH2 under an atmosphere of dry
nitrogen. Toluene was dried over and distilled from KꢀNa alloy under an
atmosphere of dry nitrogen. Absolute ethanol was dried over and distilled
from sodium. Tetrahydrofuran (THF) was dried over and distilled from
KꢀNa alloy under an atmosphere of dry nitrogen.
1H and 13C NMR spectroscopy study was conducted with a Varian
Mercury300 spectrometer using tetramethylsilane (TMS; δ = 0 ppm) as
internal standard. The Fourier transform infrared (FTIR) spectra were
recorded on a PerkinElmer-2 spectrometer in the region of 3000ꢀ
400 cmꢀ1 on NaCl pellets. UVꢀvis spectra were obtained using a Shimadzu
UV-2550 spectrometer. Photoluminescence spectra were performed on a
Hitachi F-4500 fluorescence spectrophotometer. Gel permeation chroma-
tography (GPC) was used to determine the molecular weights of polymers.
GPC analysis was performed on an Agilent 1100 series HPLC system and a
G1362A refractive index detector. Polystyrene standards were used as
calibration standards for GPC. THF 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 °C/min in nitrogen at a
flow rate of 50 cm3/min for thermogravimetric analysis (TGA).
Synthesis of 4-(3-Bromopropyloxy)iodobenzene (3). To a 500 mL
round-bottomed flask were added 4-iodophenol (4.4 g, 20 mmol), 1,3-
dibromopropane (20.2 g, 100 mmol), potassium hydroxide (5.5 g,
98 mmol), potassium iodide (0.3 g, 1.8 mmol), and acetone (200 mL).
The mixture was refluxed for 24 h. The solid was removed by filtration,
and the solvent was evaporated. The crude product was purified on a silica
gel column using hexane as eluent. White solid of 4-(3-bromopropyloxy)-
iodobenzene was obtained (4.83 g, 73.4%). 1H NMR (300 MHz, CDCl3),
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δ (TMS, ppm): 7.57 (d, J = 9.0 Hz, 2H, ArH), 6.70 (d, J = 9.0 Hz, 2H),
4.05 (t, J = 6.0 Hz, 2H), 3.60 (t, J = 6.6 Hz, 2H), 2.31 (m, J = 6.6 Hz,
2H).
4.00ꢀ3.50 (br, CH2N), 2.80ꢀ2.42 (br, CH2). 13C NMR (75 MHz,
CDCl3), δ (TMS, ppm): 159.6, 149.3, 148.8, 137.2, 130.9, 128.5, 127.3,
126.3, 125.1, 123.0, 122.3, 108.0, 98.46, 68.8, 67.7, 60.2, 29.5, 24.2.
Preparation of Polymer Solutions. P2 (9.3 mg) was dissolved
in THF to afford the stock solution with the concentration of 2.31 ꢁ
10ꢀ3 mol Lꢀ1. This stock solution was diluted with THF and water
1-Phenyl-2-[4-(63-bromopropyloxy)phenyl]acetylene (4). To a Schlenk
tube were added copper(I) iodide (15 mg, 0.04 mmol), dichlorobis-
(triphenylphosphine)palladium (50 mg, 0.04 mmol), triphenylpho-
sphine (15 mg, 0.04 mmol), and 4-(3-bromopropyloxy)iodobenzene
(2.6 g, 7.7 mmol) in a glovebox. Triethylamine (150 mL) and phenylace-
tylene (1.0 g, 9.8 mmol) were then injected. The resultant mixture was
stirred at room temperature overnight. The solid was removed by filtration,
and the solvent was evaporated. The crude product was purified on a silica gel
column using hexane as eluent. White solid of 4 was obtained (2.8 g, 84.1%).
1H NMR (300 MHz, CDCl3), δ (TMS, ppm): 7.49 (m, J = 3.0 Hz, 4H),
7.33 (m, J = 3.9 Hz, 3H), 6.88 (d, J = 8.4 Hz, 2H), 4.05 (t, J = 5.7 Hz, 2H),
3.60 (t, J = 6.3 Hz, 2H), 2.31 (m, J = 6.0 Hz, 2H). 13C NMR (75 MHz,
CDCl3), δ (TMS, ppm): 158.6, 133.0, 131.4, 128.3, 127.9, 123.4, 115.4,
114.4, 89.2, 88.1, 67.1, 32.7, 2.5.
Poly[1-phenyl-2-[4-(3-bromopropyloxy)phenyl]acetylene] (P1). Into
a baked 20 mL Schlenk tube with a stopcock in the side arm was added 4
(1.0 g). The tube was evacuated under vacuum and then flushed with
dry nitrogen three times through the side arm. Freshly distilled toluene
(6 mL) was injected into the tube to dissolve the monomer. The catalyst
solution was prepared in another tubeby dissolving tungsten(VI) chloride
(47.7 mg) and tetraphenyltin (51.6 mg) in toluene (6 mL). The two tubes
were aged at 80 °C for 1 h, and the monomer solution was transferred to
the catalyst solution using a hypodermic syringe. The reaction mixture was
stirred at 80 °C for 48 h. The solution was then cooled to room tempera-
ture, diluted with 15 mL of chloroform, and added dropwise to 150 mL
of methanol through a cotton filter under stirring. The precipitate was
allowed tostand overnight and was thenfiltered. The polymer was washed
with methanol and dried in a vacuum oven to a constant weight (0.74 g,
74%). Mw = 18 600, Mw/Mn = 1.70. IR (thin film), υ (cmꢀ1): 3064
(ArꢀH stretching), 2954, 2870 (CH2 stretching). 1H NMR (300 MHz,
CDCl3), δ (TMS, ppm): 6.88ꢀ6.30 (br, ArH), 6.36ꢀ6.00 (br, ArH),
4.20ꢀ3.90 (br, OCH2), 3.46ꢀ3.22 (br, CH2Br), 1.75ꢀ1.42 (br, CH2).
13C NMR (75 MHz, CDCl3), δ (TMS, ppm): 149.3, 148.8, 137.2, 130.9,
128.5, 127.3, 125.1, 123.0, 122.3, 108.0, 98.46, 68.8, 60.2, 24.2.
(v/v = 1/1) to 2.31 ꢁ 10ꢀ5 mol Lꢀ1
.
Fluorescence Titration of P2 with Cu2þ. A solution of P2 (2.31 ꢁ
10ꢀ5 mol Lꢀ1) was prepared in THF/H2O (v/v = 1:1). The solution of
Cu2þ 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 Cu2þ solution was introduced in portions, and fluorescence
intensity changes were recorded at room temperature each time (excitation
wavelength: 324 nm).
Fluorescence Intensity Changes of the Mixture of P1 and
Di(2-picolyl)amine with Cu2þ. A solution of the mixture of P1 and
di(2-picolyl)amine (2.31 ꢁ 10ꢀ5 mol Lꢀ1) was prepared in THF/H2O
(v/v = 1:1). The solutions of Cu2þ (1 ꢁ 10ꢀ3 mol/L) were prepared in
distilled water. A solution of the mixture of P1 and di(2-picolyl)amine
(3.0 mL) was placed in a quartz cell (10.0 mm width), and the fluorescence
spectrum was recorded. The ion solution was introduced (9 μL), and
fluorescence intensity changes were recorded at room temperature each
time (excitation wavelength: 324 nm).
Fluorescence Intensity Changes of P2 þ Cu2þ with S2ꢀ. A
solution of P2 (2.31 ꢁ 10ꢀ5 mol Lꢀ1) was prepared in THF/H2O
(v/v = 1:1). The solution of Na2S (1 ꢁ 10ꢀ3 mol Lꢀ1) 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
solution of Cu2þ (1 ꢁ 10ꢀ3 mol Lꢀ1, 9 μL) was added to quench the
fluorescence. Then Na2S solution was introduced in portions, and the
fluorescence intensity changes were recorded at room temperature each
time (excitation wavelength: 324 nm).
Fluorescence Titration of P2 with Other Anions. A solution
of P2 (2.31 ꢁ 10ꢀ5 mol Lꢀ1) was prepared in THF/H2O (v/v = 1:1).
The solutions of anions 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. The solution of Cu2þ (1 ꢁ
10ꢀ3 mol Lꢀ1, 9 μL) was added to quench the fluorescence. The other
anion solution was introduced in portions, and fluorescence intensity
changes were recorded at room temperature each time (excitation
wavelength: 324 nm).
Di(2-picolyl)amine (DPA). Amodified literature procedurewasused.13
To a suspension of 2-pyridinecarboxaldehyde (1.04 g, 9.67 mmol) in
absolute ethanol (20 mL) was added a solution of 2-(aminomethyl)-
pyridine (1.05 g, 9.71 mmol) in absolute ethanol (20 mL) dropwise at
0 °C. After 4 h, the solution was cooled to 0 °C and sodium borohydride
(0.726 g, 19.2 mmol) was added in small portions. After the reaction was
stirred for 12 h at room temperature, aqueous hydrochloric acid (24 mL,
0.5 M) was added slowly and stirred for 1 h. An aqueous solution of
sodium hydroxide (2 M) was then added until a pH value of 11 was
reached. The mixture was extracted with methylene dichloride (15 mL ꢁ
6), dried over sodium sulfate, and then filtered. The crude product was
purified on a silica gel column using THF/PE (1:1) as eluent. Pure di(2-
’ RESULTS AND DISCUSSION
Synthesis. The synthetic route to P2 is shown in Scheme 1.
First, monomer 4, which contained a bromine atom, was easily
prepared from the esterification reaction followed by the Sono-
gashira coupling reaction in the total yield of 61.7%, according to
the similar procedure reported earlier.6e Then, through the direct
polymerization process, this monomer was converted to the
corresponding PA, P1, in which the bromine atoms remained for
the further functionalization. During the followed nucleophilic
substitution reaction, the bromine atoms in the side chain of P1
were substituted by the di(2-picolyl)amine moieties, yielding the
expected new di(2-picolyl)amine-functionalized PA, P2.
It should be pointed out that the preparation of P2 was
through the postfunctionalization approach. Generally, if disub-
stituted 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 correspond-
ing disubstituted polyacetylenes due to the deactivation of the
transition-metal catalysts. Fortunately, by utilizing the polymer
reactions of some disubstituted polyacetylenes bearing reactive
1
picolyl)amine (DPA) was yielded as a brown oil (0.87 g, 45.1%). H
NMR (300 MHz, CDCl3), δ (TMS, ppm): 8.55 (d, J = 4.8 Hz, 2H), 7.64
(d, J = 7.8 Hz, 2H), 7.34 (d, J = 8.1 Hz, 2H), 7.16 (d, J = 5.1 Hz, 2H), 3.98
(s, 4H), 2.51 (s, 1H). 13C NMR (75 MHz, CDCl3), δ (TMS, ppm):
159.8, 149.5, 136.8, 122.6, 122.2, 54.9.
Synthesis of P2. P1 (30 mg), DPA (70 mg, 0.35 mmol), and
potassium hydroxide (140 mg, 2.5 mmol) were added to dry DMF
(6 mL). After stirring at 80 °C for 2 days, the resultant mixture was
filtered, and 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 washedwith water and dried
in a vacuum oven to a constant weight to yield P2 (0.035 g, 89%). 1H
NMR (300 MHz, CDCl3), δ (TMS, ppm): 8.60ꢀ8.30 (br, ArH),
7.70ꢀ7.20 (br, ArH), 7.20ꢀ6.90 (br, ArH), 6.88ꢀ6.40 (br, ArH),
6.40ꢀ5.80 (br, ArH), 4.80ꢀ4.50 (br, OCH2), 4.40ꢀ4.00 (br, CH2Br),
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Figure 1. 1H NMR spectra of (A) P1 and P2 (B) in chloroform-d. The solvent peaks were marked with asterisks.
groups, such as chlorine and bromine atoms, and azido moieties,
some polar groups could be introduced to the side chains through
the Sonogashira couplingreaction, the clickchemistryreaction, the
nucleophilic substitution reaction, and the hydrolysis reaction. In
our previous case, in the nucleophilic substitution reaction, the
bromine atoms in the side chain of polydisubstituted acetylene
exhibited much higher reactivity than that of the chlorine ones.
And sometimes, the bromine atoms could be converted to another
functional group completely.6e Thus, here, the polydisubstituted
acetylene (P1) bearing bromine atoms in the side chains was
designed for the preparation of P2. This time, possibly due to the
bulk hindrance effect of di(2-picolyl)amine moieties and its rela-
tively low reactivity, 75% of the bromine atoms were converted to
the di(2-picolyl)amine moieties, and there were still unreacted
bromine atoms remained. However, this might not affect the
sensing property of the resultant P2, and we would discuss this
point later. Thus, although not so perfect, the successful prepara-
tion of P2 still confirmed that the postfunctionalization strategy
was a good alternative approach to yield disubstituted polyacety-
lenes inaccessible from their corresponding monomers through
the direct polymerization reactions. The purification procedure
was simple; we only needed to isolate the monomer 4 and di(2-
picolyl)amine by a silica gel column, and polymers P1 and P2
could be purified easily by precipitations from their solutions into
methanol or water.
Structural Characterization. P1 and P2 were characterized
by spectroscopic methods and gave satisfactory spectral data (see
Experimental Section and Supporting Information for detailed
analysis data). In the 1H NMR spectra of the polymers P1 and P2
(Figure 1), the chemical shifts were consistent with the proposed
polymer structure as demonstrated in Scheme 1. After the reac-
tion with di(2-picolyl)amine, there were some new absorption
peaks appeared at about 7.04, 7.46, and 8.48 ppm in the downfield,
which should be ascribed to the aromatic protons of moieties in
P2, indicating that the bromine atoms were converted to the di(2-
picolyl)amine moieties successfully. Also, from the integration of
these peaks, the concentration of the unit of the di(2-picolyl)-
amine moieties in P2 could be calculated to be about 75%; that is,
there were still about 25% of the bromine atoms in P1 remained
unreacted during the substitution reaction. Previously, the
bromine atoms could be converted to other functional groups
through the substitution reaction completely while reacted with
imidazole.7e Here, the bulk of the di(2-picolyl)amine moieties
was much bigger, which should hinder the substitution reaction,
directly resulting in the uncompleted conversion of the bromine
atoms in P1.
The two polymers, P1 and P2, exhibited much different solu-
bility in common organic solvents. For example, P1 was insoluble
in alcohol, and in the purification procedure, we could remove
unpolymerized monomers and some other impurities by the
precipitation from its chloroform solution into methanol; how-
ever, after most of the bromine atoms in P1 were converted to
the di(2-picolyl)amine moieties, the solubility of the resultant
polymer P2 changed dramatically: P2 could dissolve in alcohol,
in contrast to most of the reported disubstituted polyacetylenes.
This disclosed that the introduction of di(2-picolyl)amine moi-
eties to the PA backbone has brought some changes to the
resultant polyacetylene. P2 was thermally stable, and its TGA
thermograms are shown in Figure S1, with the 5% weight loss
temperature at about 285 °C.
Fluorescence Properties. As mentioned in the Introduction,
their stability and strong luminescence could ensure disubstitut-
ed polyacetylenes good candidates as polymer chemosensors. As
demonstrated in Figure 2, both of P1 and P2 emitted strong
luminescence in diluted solutions with the maximum emission
wavelength at about 535 and 507 nm, respectively. The P1ꢀ
di(2-picolyl)amine mixture nearly showed the same emission
behavior as P1, indicating that the presence of di(2-picolyl)amine
in the mixture did not affect the electronic properties of the con-
jugated polymers. However, the blue-shifted emission (28 nm)
of P2 in comparison with that of P1 disclosed the minor change
of the electronic properties of the conjugated polymers before
and after the linkage of the di(2-picolyl)amine moieties, partially
explaining the totally different sensing behavior of P2 and the
mixture of P1 and di(2-picolyl)amine discussed as follows.
As shown in Figure 2, after the addition of a little amount of
Cu2þ, the things became different. Similar to that of P1, the emis-
sion spectrum of the P1ꢀdi(2-picolyl)amine mixture remained
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Figure 2. Fluorescence spectra of P1 (A), the mixture of P1 and di(2-picolyl)amine (DPA) (B), and P2 (C) in THF/water (1:1) before and after the
addition of Cu2þ. The polymer concentration was 2.31 ꢁ 10ꢀ5 mol/L. Excitation wavelength (nm): 324.
Figure 4. Fluorescence eꢀm5ission response profiles of P2. The polymer
concentration: 2.31 ꢁ 10 mol/L. The concentration of copper ions:
2 ꢁ 10ꢀ6 mol/L, while 5 ꢁ 10ꢀ5 mol/L for other metal ions. Excitation
Zn2þ (C), P2 þ Kþ (D), P2 þ Mg2þ (E), P2 þ Mn2þ (F), P2 þ Ca2þ
(G), P2 þ Cd2þ (H), P2 þ Ba2þ (I), P2 þ Naþ (J), P2 þ Liþ (K), P2 þ
Pb2þ (L), P2 þ Agþ (M), P2 þ Fe2þ (N), P2 þ Fe3þ (O), P2 þ Co2þ
(P), P2 þ Ni2þ (Q), P2 þ Hg2þ (R), P2 þ Cr3þ (S), P2 þ Al3þ (T).
Figure 3. Emission quenching of the solution of P2 by Cu2þ in THF/
wavelength (nm): 324. Inset photo: polymer (A); P2 þ Cu2þ (B), P2 þ
H2O (1:1). Polymer concentration: 2.31 ꢁ 10ꢀ5 mol/L. Excitation
wavelength (nm): 324. The pH value was 6.32.
almost unchanged. As to P2, the strong luminescent emission
was completely quenched by the added Cu2þ ions, although its
absorption curve only changed a little upon the addition of cop-
per ions (Figure S2a). These results indicated that after being
linked to the polymer main chain, the di(2-picolyl)amine moieties
could efficiently transfer the energy from the conjugated polymer
backbone to the copper 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 efficiently transfer the energy from the conjugated backbone to
the metal ions, similar to those observed previously.6e
photoluminescence was observed at very low level of Cu2þ
(1.0 ꢁ 10ꢀ7 mol/L), and the fluorescent intensity decreased
rapidly upon the increase of the added concentration of Cu2þ
.
While the concentration of Cu2þ was 2.0 ꢁ 10ꢀ6 mol/L, nearly
no luminescence could be seen. The quenching efficiency was
nearly fit to the SternꢀVolmer equation, I0/I = Ksv[A] þ 1,
which related to the fluorescence intensity, I, at different con-
centrations of analyte quencher, [A], where I0 was the intensity at
[A] = 0, and Ksv was the SternꢀVolmer constant. According to
the fluorescence titration of P2 in THF solutions with Cu2þ, Ksv
was determined to be 8.2 ꢁ 106 Mꢀ1. As the pyridine group is apt
to act as a hydrogen acceptor, we thought that perhaps P2 could
be a hydrogen acceptor and might give out some detectable
signals upon the addition of acids. As shown in Figure S2b, really,
To study the quenching behavior of Cu2þ ions in detail, we
investigated the decrease in fluorescence intensity by adding
successive aliquots of aqueous stock solutions of Cu2þ ions to
the diluted solution of P2. As shown in Figure 3, quenching of
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Chart 2. Schematic Representation of Cu2þ and S2ꢀ Sensors Based on the Fluorescence “Turn-Off” and “Turn-On” of P2
the fluorescence of P2 could be quenched by the addition of hy-
drochloric acid, and nearly no luminescence could be observed
while the pH value of the resultant solution decreased to 0.94.
This phenomenon indicated that the di(2-picolyl)amine moi-
eties acted as hydrogen acceptor, and the energy could be trans-
ferred from the conjugated backbone to the hydrogen ions, similar
as to the copper ions. Interestingly, the quenched fluorescence
could be recovered after the addition of some base (Figure S2c), as
observed in our previous case.7b The above titration experiments
of copper ions were further conducted at different pH values
(Figure S2d,e), and the obtained results demonstrated that the
optimized pH value was 6.32, as utilized in the experiment for
Figure 3. Thus, all the following fluorescent experiments were
conducted at the pH value of 6.32.
To evaluate the copper ion-selective nature of P2, the influ-
ence of other metal ions was investigated. As shown in Figure 4
and Figures S3ꢀS21 (in Supporting Information), other metal
ions nearly gave no disturbance to the selectively sensing of cop-
Figure 5. Fluorescence emission spectra of P2 before and after the
per ions, except very little response from Co2þ. Thus, the selec-
tivity for copper ions over other metal ions was relatively high.
The competition experiments using solutions containing copper
ions and all the other metal ions demonstrated that P2 could still
report the presence of copper ions in the presence of other com-
mon interfering metal ions (Figure S22). As the sensing sensi-
tivity of P2 toward Cu2þ was also very high, we wondered if it was
possible to detect this metal ion visually at very low concentration
with the aid of a normal UV lamp, but not a fluorescence spectro-
photometer. As shown in the photographs of the solutions of P1
(Figure 4), after the addition of the copper ions, the strong green
fluorescence of P2 was completely quenched and the corre-
sponding bottle became dark, under UV illumination. However,
upon the addition of other metal ions, no changes could be ob-
served. Thus, the difference was very apparent and could be seen
visually, even the concentration of Cu2þ was only 2.0 ꢁ
10ꢀ6 mol/L (0.13 ppm). The above obtained results were very
different from those of small fluorophors bearing di(2-picolyl)-
amine moieties,14 which could bind with Zn2þ and produce fluo-
rescence changes, indicating that the selectively sensing of metal
ions by using di(2-picolyl)amine groups as receptor could be
adjusted after being linked to the conjugated backbones. This is
reasonable, since the various degrees of affinity of di(2-picolyl)-
amine groups toward metal ions should lead to different influ-
ence to the interaction between the di(2-picolyl)amine groups
and the conjugated polymer backbone or other fluorophors
addition of Cu2þ and turned on by S2ꢀ. The polymer concentration is
fixed at 2.31 ꢁ 10ꢀ5 mol/L.
repoted in the literature, due to their different electronic
properties.
From the obtained experimental results, we could conclude
that P2 could report the presence of trace Cu2þ ions based on the
fluorescence “turn-off”, since the di(2-picolyl)amine moieties
could efficiently transfer the energy from the conjugated back-
bone to the copper ions, leading to the quenching of the strong
luminescence of the polymer. Previously, according to the
displacement strategy, we have developed some good chemo-
sensors for cyanide,6d,e,7a,b,fꢀh since cyanide could snatch the
copper ions from the complex of copper ions and the corre-
sponding chemosensors. So, here, if some species, which could
snatch the Cu2þ ions from the di(2-picolyl)amine moieties in P2
to form more stable new complexes, were added in the P2/Cu2þ
complex, the quenched luminescence of P2 might recover. As
discussed in the Introduction, the solubility product constant of
CuS (Ksp = 6.3 ꢁ 10ꢀ36 mol2/L2) was very low; thus, the P2/
Cu2þ complex might give some response to the S2ꢀ anions.
Really, the qualitative analysis demonstrated that the addition
of sulfide anion to the P2/Cu2þ complex turned on the fluo-
rescence of P2 (Chart 2), verifying the above idea. The recover-
ing behavior of the fluorescence of the complex of P2 and Cu2þ
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Figure 6. Fluorescence emission response profiles of P2 þ Cu2þ in
THF/H2O (v/v = 1/1) and turned on by S2ꢀ (4.5 ꢁ 10ꢀ6 mol/L). The
concentrations of different anions were 2.0 ꢁ 10ꢀ5 mol/L. Excitation
wavelength (nm): 324. Photograph: P2 (A), P2 þ Cu2þ (B), P2 þ
Cu2þ þ S2ꢀ (C), P2 þ Cu2þ þ Acꢀ (D), P2 þ Cu2þ þ Brꢀ (E), P2 þ
Cu2þ þ C2O42ꢀ (F),ꢀP2 þ Cu2þ þ Clꢀ (G), P2ꢀþ Cu2þ þ CNꢀ (H),
Figure 7. Fluorescence emission spectra of P2 and Cu2þ (3ꢁ 10ꢀ6 mol/L)
in H2OꢀTHF (1:1, v/v) in the presence of different amounts
of anions. The concentration of S2ꢀ was 6 ꢁ 10ꢀ6 mol/L. Excitation
wavelength (nm): 324.
As shown in Figure 7, the complex P2ꢀCu2þ could still report
the presence of S2ꢀ. Thus, this sulfide selective sensor was not
affected by the presence of other common interfering anions,
indicating its possible practical application for the probe of S2ꢀ in
the real water samples.
P2 þ Cu2þ þ ClO3 (I), P2 þ Cu2þ þ ClO4 (J), P2 þ Cu2þ
þ
CO32ꢀ (K), P2 þ Cu2þ þ Cr2O7 (L), P2 þ Cu2þ þ Fꢀ(M), P2 þ
2ꢀ
ꢀ
ꢀ
Cu2þ þ H2PO4 (N), P2 þ Cu2þ þ HCO3 (a), P2 þ Cu2þ
þ
HPO4ꢀ (b), P2 þ Cu2þ þ HSO3ꢀ (c), P2 þ Cu2þ þ HSO4ꢀ (d), P2 þ
Cu2þ þ Iꢀ (e), P2 þ Cu2þ þ IO3ꢀ (f), P2 þ Cu2þ þ P2O74ꢀ (g), P2
3ꢀ
þ Cu2þ þ NO2ꢀ (h), P2 þ Cu2þ þ NO3ꢀ (i), P2 þ Cu2þ þ PO4
(j), P2 þ Cu2þ þ S2O32ꢀ (k), P2 þ Cu2þ þ SCNꢀ (l), P2 þ Cu2þ þ
SO32ꢀ (m), P2 þ Cu2þ þ SO42ꢀ (n).
’ CONCLUSIONS
In summary, we have successfully prepared a new di(2-
picolyl)amine-functionalized disubstituted polyacetylene (P2)
by utilizing the postfunctional strategy and studied its ability to
sense metal ions and anions by the fluorescence spectra. P2 could
reportthe presenceoftrace S2ꢀ anionsaslow as5.0 ꢁ 10ꢀ7 mol/L
selectively by an indirect strategy, making it good candidate
for potential application of human safeguard. The preliminary
results demonstrated that:
1. By utilization of the postfunctional strategy, the di(2-
picolyl)amine moieties could be conveniently introduced
to the PA backbone, offering the resultant polymer the
sensing ability to report the presence of trace copper ions.
This, once again, confirmed that disubstituted polyacety-
lenes could be promising candidates as CPF chemosensors.
2. It is a good and alternative approach to develop sensitive
and selective CPF anion probes by applying a “turn-offꢀ
turn-on” circle with the usage of CPF metal ion chemo-
sensors. Or in other words, it is really possible to find many
good CPF anion probes even in the present CPF metal ion
chemosensors. Thus, the large amount of the reported CPF
metal ion chemosensors could be possibly utilized as “new”
CPF anion probes with good performance, especially those
Cu2þ chemosensors. Specially, we could utilize the differ-
ent coordination ability of different anions toward coppers
to develop different chemosensors for the probe of different
anions, by using the displacement strategy.
was studied in detail (Figure 5). The completely quenched fluo-
rescence of P2 by Cu2þ turned on after the addition of sulfide
anion, even at a very low concentration (2.0 ꢁ 10ꢀ7 mol/L).
Further increasing the concentration of sulfide anion led to
stronger fluorescence. When the added concentration of S2ꢀ
was 4.5 ꢁ 10ꢀ6 mol/L (0.144 ppm), the fluorescent intensity
could recover to more than 80% of the original intensity of P2
without Cu2þ, and this intensity was strong enough to be used as
“turn-on” to probe trace S2ꢀ. Thus, by applying a “turn-offꢀturn-
on” circle, P2 was both selective chemosensor for Cu2þ and
sensitive one for S2ꢀ. To exclude the possibility of the fluorescent
signal response of P2 toward S2ꢀ directly without the presence of
Cu2þ, another control experiment was conducted: successive
aliquots of aqueous solutions of S2ꢀ were added to the diluted
solution of P2, and the changes of the fluorescence spectra were
recorded. As shown in Figure S23, no changes could be observed
before and after the addition of the S2ꢀ solutions; even the
concentration of S2ꢀ was 1.0 ꢁ 10ꢀ5 mol/L, much higher than
those used in the above experiment (Figure 5). Thus, the obtained
results confirmed the speculated mechanism (Chart 2) for the
sensing of S2ꢀ again.
To evaluate the S2ꢀ-selective nature of P2, the influence of
other anions toward the complex of P2 and Cu2þ was studied
(Figure 6). Nearly nonluminescence could be observed upon the
addition of most of anions at the concentration of 2.0 ꢁ 10ꢀ5 mol/L,
such as Fꢀ, Clꢀ, Brꢀ, ClO4ꢀ, NO2ꢀ, and SO4ꢀ. Thus, other
anions gave nearly no disturbance to the selective sensing of
sulfide anion, indicating that the selectivity for S2ꢀ was rela-
tively high. Furthermore, the competition experiments using
solutions containing S2ꢀ and all the other anions was conducted.
’ ASSOCIATED CONTENT
S
Supporting Information. TGA and IR spectra and PL
b
spectra of solutions of P2 in the presence of different metal ions.
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This material is available free of charge via the Internet at http://
pubs.acs.org.
41, 7433–7439. (c) Chung, S.-Y.; Nam, S.-W.; Lim, J.; Park, S.; Yoon, J.
Chem. Commun. 2009, 45, 2866–2868. (d) Ballesteros-Garrido, R.;
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’ AUTHOR INFORMATION
K. P.; Sreejith, S.; Balakrishna, B.; Jayamurthy, P.; Aneesa, P.;
Ajayaghosh, A. Chem. Commun. 2010, 46, 6069–6071. (f) Lou, X.; Li,
Z.; Qin, J. Sci. China, Ser. B: Chem. 2009, 52, 802–808. (g) Lou, X.; Qin,
J.; Li, Z. Analyst 2009, 134, 2071–2075. (h) Lou, X.; Qiang, L.; Qin, J.; Li,
Z. ACS Appl. Mater. Interfaces 2009, 1, 2529–2535.
Corresponding Author
*Phone: 86-27-62254108; Fax: 86-27-68756757; e-mail: lizhen@
whu.edu.cn.
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We are grateful to the National Science Foundation of China
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