An imidazole-functionalized polyacetylene: Convenient synthesis and selective chemosensor for metal ions and cyanide

Article abstract of DOI:10.1039/b717764j

A new light-emitting polyacetylene bearing imidazole moieties in the side chain (P1), was conveniently prepared through a postfunctionalization strategy, as a sensory polymer to selectively report the presence of Cu²⁺ (with a detection limit of

Full text of DOI:10.1039/b717764j

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An imidazole-functionalized polyacetylene: convenient synthesis and
selective chemosensor for metal ions and cyanidew
a
a
a
a
b
Qi Zeng, Ping Cai, Zhen Li,* Jingui Qin and Ben Zhong Tang
Received (in Cambridge, UK) 16th November 2007, Accepted 11th January 2008
First published as an Advance Article on the web 29th January 2008
DOI: 10.1039/b717764j
A new light-emitting polyacetylene bearing imidazole moieties in
the side chain (P1), was conveniently prepared through a post-
functionalization strategy, as a sensory polymer to selectively
there are very few reports concerning the properties of dis-
5
ubstituted PAs as chemosensors. The reason might be par-
tially the difficulties encountered in their synthesis. Generally,
conjugated polymer-based chemosensors should contain some
acceptor groups to trap the metal ions, such as bipyridyl,
2
+
report the presence of Cu (with a detection limit of 1.48 ppm)
based on the fluorescence ‘‘turn-off’’. Interestingly, the quenched
2
+
6
luminescence of P1 by Cu
À
could be turned on after the
terpyridyl, and quinoline segments. However, it is still a big
addition of CN , making P1 a novel, sensitive, and selective
cyanide probe.
challenge to prepare disubstituted PAs containing those
moieties, as well as groups such as amide, amine, hydroxyl,
cyano, thio, etc.
7
Polyacetylene (PA) is the archetypal conjugated polymer, and
has attracted increasing interest in recent years. Thanks to the
enthusiastic efforts of scientists, research has extended from
the initial traditional conductive polyacetylene to liquid crys-
tals, polymeric light-emitting diodes, helical polymers, gas
separation membranes, organic–inorganic hybrids, nonlinear
Very recently, partially based on our previous work on
polymer reactions, we have developed some synthetic routes to
obtain some functional disubstituted PAs inaccessible by the
direct polymerization of their monomers. Most of them con-
tain highly polar side chains, such as azo moieties, indole
8
groups and pyridine groups. These results prompted us to
1
,2
optical and magnetic materials etc. Structurally, PA is a
design disubstituted PA chemosensors by using a postfunctio-
nalization strategy. In this paper, we report the realization of
this goal, and describe an imidazole-functionalized disubsti-
tuted polyacetylene P1 (Scheme 1), which combines the ad-
vantages of the strong luminescence of disubstituted
polyacetylenes and the metal ion-coordinating ability of imi-
dazoles, to provide a novel kind of highly effective chemosen-
linear polyene chain [–(HCQCH) –]. The existence of two
n
hydrogen atoms in its repeat unit offers ample opportunity to
decorate the backbone with pendants: replacement of hydro-
gen in each repeat unit by one or two substituents yields
monosubstituted (1) and disubstituted (2) PAs, respectively.
We have been interested in functionalizing PA at the mole-
cular level and have successfully polymerized hundreds of
monosubstituted acetylenes containing a variety of functional
groups. Disubstituted PAs are generally superior to their
monosubstituted counterparts in performance: for example,
poly(1-phenyl-1-octyne), a disubstituted PA, strongly resists
thermal decomposition (no molecular weight change detect-
able after annealing in air at 120 1C for 20 h) and efficiently
emits blue light, while poly(phenylacetylene), a monosubsti-
2
+
sor. P1 can report the presence of Cu selectively based on
the fluorescence ‘‘turn-off’’. Very interestingly, it can also
À
indirectly probe the presence of trace CN both selectively
and sensitively.
As shown in Scheme 1, P1 was easily obtained from the
reaction between P0 and imidazole under basic conditions in
DMF, although this polymer could not be obtained from the
direct polymerization of its corresponding monomer, due to
the notorious problem encountered in the chemistry of dis-
ubstituted polyacetylenes, namely the deactivation of the
transition-metal catalysts for acetylene polymerization by
3
tuted PA, readily degrades and emits weakly. The stability
and strong luminescence of disubstituted PAs make them good
candidates for polymer chemosensors, since the ‘‘molecular
wire effect’’ in conjugated polymers usually greatly enhances
the sensitivity of the polymer-based chemosensors due to the
3
active protons or coordinative ligands in monomers. The
detailed synthetic procedure and spectroscopic characteriza-
tion data are presented in the ESIw. Unlike its parent polymer
P0, P1 was nearly insoluble in THF, chloroform and DMF,
but exhibited good solubility in alcohol.
4
enhanced electronic communication among them. However,
a
Department of Chemistry, Hubei Key Laboratory on Organic and
Polymeric Optoelectronic Materials, Wuhan University, Wuhan,
4
6
30072, China. E-mail: lizhen@whu.edu.cn; Fax: +86-27-6875-
757; Tel: +86-27-6225-4108
b
Department of Chemistry, The Hong Kong University of Science &
Technology, Clear Water Bay, Kowloon, Hong Kong, China
w Electronic supplementary information (ESI) available: Preparation
and characterization details for P1; PL spectra of solutions of P1 at
the presence of different ions with different concentrations; PL spectra
of films of P1 after different treatment; UV-Vis spectra of P0 and P1;
PL spectra of solutions of the mixture of P0 and imidazole at the
presence of different ions. See DOI: 10.1039/b717764j
Scheme 1 The synthetic route to imidazole-functionalized disubsti-
tuted polyacetylene P1.
1
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There was nearly no difference between the fluorescence
intensity of P1 in the presence and absence of alkali and
alkaline earth metal ions; similar phenomena were observed
absorption wavelength of P1 was nearly unchanged compared
to P0, its maximum emission wavelength was blue-shifted by
14 nm (Fig. S11w). These phenomena disclosed the minor
change of the electronic properties of the conjugated polymers
before and after the attachment of the imidazole moieties,
partially explaining the totally different sensing behaviour of
P1 and the P0–imidazole mixture.
2
+
+
2+
2+
in the cases of Zn , Ag , Mn , and Cd , due to the poor
coordination ability of the imidazole receptor with these ions.
2
Pb , Al and Cr could quench the fluorescence, but not
+
3+
3+
2
+
2+
2+
completely. However, Cu , Co , Fe , Fe
3+
2+
and Ni
2
+
could quench the fluorescence of P1 more efficiently; in
2
The detection of Cu
by P1 in the solid state was also
+
particular, Cu
could quench the fluorescence completely
investigated (in a similar manner to that reported in the
9c
at very low concentration (1.48 ppm) (Fig. 1 and Fig. S1–S9w),
3 2
literature), by dipping the film into aqueous Cu(NO )
5
with the Stern–Volmer constant determined to be 3.7 Â 10
solution (Fig. S12w); the strong fluorescence of P1 was com-
pletely quenched. However, the fluorescence recovered after
the film was immersed in ammonia solution. This reversibility
should benefit the practical application of this polyacetylene
chemosensor.
À1
M
. Thus, as demonstrated in Fig. 1, P1 could act as an
2+
2
+
2+
efficient chemosensor for the detection of Cu , Co , Fe
3
and Fe . The influence of other metal ions to the sensing of
+
2
+
Cu
was also conducted. As shown in the inset in Fig. 1,
other metal ions resulted in nearly no disturbance to the
2
Therefore, the obtained results so far have demonstrated
+
selective sensing of P1 toward Cu
.
that the strong fluorescence of P1 could be quenched com-
2
pletely and efficiently by the addition of Cu , due to the
+
The results described above are very different from those of
other conjugated polymers containing imidazole or oligopy-
ridine moieties, indicating that the selective sensing of metal
ions by using imidazole groups as receptors can be adjusted by
2
affinity of imidazole moieties toward Cu . If the interaction
+
2
+
between the Cu
ions and the imidazole moieties is then
interrupted by some other species, the fluorescence of P1
9
the choice of different conjugated backbones. This is reason-
might perhaps recover, just like the immersion of the film of
2
+
able, since the various degrees of affinity of imidazole moieties
toward metal ions should lead to different influences on the
interaction between the imidazole groups and the conjugated
polymer backbones, due to their different electronic proper-
ties. Also, we believe that if other receptors were used instead
of imidazole, different sensing behaviours should be observed
accordingly. Thus, this piece of work might be just the tip of a
huge iceberg of polyacetylene chemosensors. Further work is
currently underway in our laboratory.
the complex of P1 and Cu
in ammonia solution. Thus,
possibly, we could probe other species based on the fluores-
2
+
cence turn-on of the complex of P1 and Cu . According to
this idea, also considering that the new added species should
2
coordinate more easily with Cu than imidazole moieties, we
+
À
thought that CN might be suitable, due to: (1) the much
2+
À
higher stability constant of the complex of CN and Cu
1
0
(eqn (S1) and (S2)w); (2) the importance of the detection of
CN , since it is extremely toxic to mammals, and is used
À
To test the chemosensing ability of polyacetylene in the
presence of the receptors, we measured the fluorescence beha-
viour of a mixture of P0 and imidazole in the presence of
various metal ions (Fig. S10w). It is easily seen that no metal
ions, including Cu , could quench the strong fluorescence of
P0. This indicated that the receptor needed to be linked to the
polyacetylene backbone to efficiently transfer the energy from
the conjugated backbone to the metal ions. We note that, after
attaching the imidazole moieties, although the maximum
industrially in gold mining, electroplating and metallurgy, and
1,12
could therefore be found in food and plants.
1
The qualitative analysis verified the above thought: addition
À
of CN turned on the fluorescence of P1, as shown in Scheme 2.
2
+
Encouraged by this result, we studied the recovery behaviour of
2+
the fluorescence of the complex of P1 and Cu in detail. As
shown in the inset in Fig. 2, the completely quenched fluores-
À
2+
cence of P1 by Cu was turned on after the addition of CN ,
À5
at concentrations as low as 7.0 Â 10 M. Further increasing the
À
concentration of CN led to stronger fluorescence; however, the
results were not much better. For example, at a concentration of
À4
2
.0 Â 10 M, the fluorescence intensity was only 1.23 times that
À5
when the concentration was 7.0 Â 10 M (Fig. S13w). However,
À5
the fluorescence intensity at 7.0 Â 10 M was strong enough to
Fig. 1 Fluorescence emission response profiles of P1. Inset: Emission
2+
quenching of the solution of P1 in ethanol by Cu (1.7 ppm) with and
À5
2
+
À
without other metal ions (2.67 Â 10 M). The polymer concentration
Scheme 2 Schematic representation of Cu and CN sensors based
on the fluorescence ‘‘turn-off’’ and ‘‘turn-on’’ of the polyacetylene.
À4
was 1.06 Â 10 M. Excitation wavelength: 335 nm.
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Notes and references
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+
Fig. 2 Fluorescence emission response profiles of P1 + Cu . Inset:
2
+
Fluorescence emission spectra of P1 in ethanol after added Cu , and
À
after ‘‘turning on’’ by CN . The polymer concentration was
.528 Â 10À4 M. Excitation wavelength: 335 nm.
0
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J. W. Y. Lam and B. Z. Tang, Acc. Chem. Res., 2005, 38, 745.
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2
turn-off–turn-on cycle, P1 was both a selective chemosensor for
À
2+
Cu and a sensitive chemosensor for CN .
(
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and Fig. S14w, other anions gave nearly no disturbance to the
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3À
2À
À
5 Y. Liu, R. C. Mills, J. M. Boncella and K. S. Schanze, Langmuir,
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HPO4 and PO4 . Thus, the selectivity for CN over other
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6
À
The detection of CN by P1 in the solid state was also
1
998, 178–180, 1211.
3 2
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À
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solution to quench the fluorescence, then into aqueous CN
solution to recover the fluorescence (Fig. S15–16w). Although
not so sensitive as the solution, the CN concentrations as low
À
À3
as 1.4 Â 10 M could reverse the quenched fluorescence of
À2
P1. When the concentration increased to 1.0 Â 10 M, the
fluorescence recovered to nearly the same as that of P1.
In summary, we have successfully prepared a new imida-
zole-functionalized disubstituted polyacetylene by utilizing a
postfunctional strategy, and studied its ability to sense metal
ions and anions by using its fluorescence spectra. These
preliminary results show that:
9
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(
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solved, many new polyacetylene-based chemosensors can be
conveniently synthesised. The results presented here are just
one such example, and thus, a new avenue for promising
fluorescent polyacetylene chemosensors might now open up.
1
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2
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We acknowledge the financial support from the National
Natural Science Foundation of China (Project Nos. 20402011
and 20674059), the National Basic Research ‘‘973’’ Program, the
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1
096 | Chem. Commun., 2008, 1094–1096
This journal is ꢀc The Royal Society of Chemistry 2008