Design of a modular-based fluorescent conjugated polymer for selective sensing

Article abstract of DOI:10.1002/anie.200460371

Sensitive and selective, a modular-based fluorescent polymer combines a rigid electron-conducting block (see picture; B) with a flexible binding block (A). The monopyridyl group of the coordinating module has a great affinity for Pd^II ions and

Full text of DOI:10.1002/anie.200460371

Angewandte
Chemie
Sensors
Design of a Modular-Based Fluorescent
Conjugated Polymer for Selective Sensing**
Hongmei Huang, Kemin Wang,* Weihong Tan,
Delie An, Xiaohai Yang, Shasheng Huang, Qiuge Zhai,
Leiji Zhou, and Yan Jin
We have designed and synthesized a new modular-based
fluorescent polymer 1 with a binding domain and a signaling
domain. They are coupled together through an electron-
II
conducting backbone for highly selective sensing of the Pd
[
*] Dr. H. Huang, Prof. K. Wang, Prof. W. Tan, D. An, Prof. X. Yang,
Prof. S. Huang, Q. Zhai, Dr. L. Zhou, Dr. Y. Jin
Biomedical Engineering Center
State Key Laboratory of Chemo/Biosensing andChemometrics
College of Chemistry andChemical Engineering
Hunan University, Changsha, 410082 (China)
Fax: (+86)731-8821566
E-mail: kmwang@hnu.cn
[
**] This work was supportedby the Nature Science Foundation of China
20135010), the Key Project Foundation of China (2002CB513100)
andthe Oversea Youth Scholar Co-research Foun da tion of China
20028506).
(
(
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2004, 43, 5635 –5638
DOI: 10.1002/anie.200460371
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5635

Communications
ion. The meta-substituted monopyridyl improves the spatial
The meta linkage of the monopyridyl can regulate the
flexibility of the backbone and may provide potential spatial
matching for interpolymer binding by using an ion as the
linker. The synthetic method to produce 1 (Ex/Em = 415 nm/
matching for selective binding, whereas the poly(p-phenylene
ethynylene)derivative provides the additional amplification
for sensing. With the increase in the number of applications
for optical sensors, the design and synthesis of new sensing
materials with high sensitivity and selectivity is a critical
4
475 nm, F = 0.34, M = 3.6 ” 10 , where Ex is the excitation
wavelength, Em is the emission wavelength, F is the quantum
yield and M is the average weight)involves the Sonogashira
coupling. To ensure the desirable flexibility of the backbone,
the reactants and their ratio were carefully considered.
H NMR spectra of 1 confirm that monopyridyl groups have
[
1,2]
challenge.
Recently, “molecular-wire effects” of conju-
gated polymers have shown great potential in signaling
[
2,3]
molecular-recognition processes,
thus making them ideal
for the development of highly sensitive sensing materials used
[
2–6]
[9]
to monitor ions, organic molecules, and even biomolecules.
been separately incorporated into the polymer. To address
Among the fluorescent conjugated polymers reported to date,
there is a diversity of properties such as conductivity, charge
transfer, redox, and energy transfer. This diversity has been
the role of the different modular domains played in the
selective metal binding, 2 (Ex/Em = 380 nm/425 nm, F = 0.12,
3
M = 4.3 ” 10 )and 3 (Ex/Em = 453 nm/475 nm, F = 0.42, M =
[
5–7]
4
accomplished by tuning the conjugated polymers,
how-
5.7 ” 10 )were also prepared by a similar procedure. Accord-
ever, selective sensing of ions by electrostatic action or ligand
ing to the experimental results, 1 with an appropriate
modular-based polymer can truly form an interchain inter-
action through palladium–pyridyl coordination that results in
dramatic fluorescence quenching and provides an excellent
[
4a–b,5,6]
coordination has remained unsatisfactory.
The chal-
lenging selection requirements of the conjugated polymer are
usually associated with properties such as special binding or
spatial matching. For spatial-matching interactions, a flexible
backbone and conformational freedom are indispensable to
build a binding pocket and to offer a high affinity for analyte,
which is common in biological macromolecules. However, for
a conjugated polymer a rigid structure is preferable for
efficient electron transfer. To resolve the conflicting architec-
tural requirements, an important goal that has not yet been
achieved satisfactorily is to develop a generic approach for
linking a flexible part of binding domain and a rigid part of the
signaling domain together in a range of polymers to be used in
chemical sensing. By this modular-based design, many
specific-binding domains with less or even no optical expres-
sion and those signaling domains that have excellent energy-
transfer properties but without selective-binding properties
can all be sufficiently used to avoid their specific flaws in each
case. Herein we report a new modular-based conjugated
II
selectivity for Pd over other heavy and transition-metal ions.
II
The Pd -ion-responsive properties of polymers and the
monomeric model 2,6-di(phenylethynyl)-pyridine (Dpp)
were monitored in THF by using fluorescence emission
spectroscopy. In the case of 1, the Stern–Volmer data (Stern–
5
À1
Volmer quenching constant K = 4.34 ” 10 Lmol )reveals
sv
II
that the best amplification quenching by Pd , which is ꢀ 56
3
À1
times greater than that of the Dpp (K = 7.72 ” 10 Lmol
sv
not shown). This result indicates that the specially designed
molecular wire (i.e., the conjugated polymer chain)signifi-
cantly amplifies the quenching. Figure 1 shows a comparison
of fluorescence quenching of polymers 1–3 with various
II
concentrations of Pd . To examine the effect of the binding
domain (A)on the fluorescence quenching properties, the
II
luminescence behavior of 3 in response to Pd was also
investigated. Although 3 has a longer effective conjugated
II
[3]
II
polymer for detecting Pd ions, the design of which exploits
length than 1, its Stern–Volmer quenching constant on Pd
3
À1
molecular architecture, the intrinsic nature of the domains,
and the properties of functional groups to yield a molecule
capable of selective and sensitive sensing.
is only 3.79 ” 10 Lmol , which is even less than that of Dpp.
This suggests that the A part of 1 functions as a necessary
II
sensitive domain for Pd binding, and only the B part of 1
In this research, we designed and synthesized a new
fluorescent polymer 1 (Scheme 1)comprising two separate
functional parts (A and B), one part to introduce specific
binding and the other to signal the response. Part B, a poly(p-
phenylene ethynylene)derivative, was chosen as the signaling
domain for its excellent photophysical and conductive
cannot bring out the amplification sensitivity. From Figure 1,
we can also learn that 2 shows far less sensitivity than 1 and its
[
2,3]
properties.
Its rigid structure is also capable of reducing
intramolecular association. Part A was chosen as the binding
domain because the monopyridyl group has a great affinity
II
[8]
for the Pd ion and should selectively bind by self-assembly.
Figure 1. Fluorescence quenching of polymer 1 (
&
lex/lem=415 nm/
4
76 nm), 2 (* lex/lem=380 nm/425 nm) and 3 (~ lex/
II
lem=453 nm/475 nm) by various concentration of Pd ions, in which
I0 and I denote the intensity of the fluorescence signal of the sensing
materials in the absence andpresence of the analyte, respectively (the
scale andlegendof the x-axis of the inset is similar to those of the
figure).
Scheme 1. The structures of polymers 1–3.
5
636
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4
À1
Stern–Volmer quenching constant (K = 2.18 ” 10 Lmol )is
corresponding excitation band is red-shifted from 415 nm to
sv
only 2.8 times greater than that of monomeric model Dpp,
although it provides slightly more efficient quenching than 3.
The great disparity in the response efficiency of 1 and 2
demonstrates that the B domain in 1 provides a remarkable
444 nm, whereas the emission band is only slightly red-shifted
(ꢀ 3 nm). Consistent with the excitation variation, the
absorption band at 415 nm of 1, which is attributed to the
conjugated backbone, is also red-shifted by ꢀ 30 nm upon
II
II
amplification, that is, the binding of Pd with the A domain
adding an appropriate quantity of Pd ions (Figure 3). The
can lead to a significant quenching of the B domain. Despite
the attraction of special binding of the A domain, the
excessive distortion in the conjugated polymer of 2 decreases
the quantum yield, the average weight, thus the sensitivity,
and even the selectivity are seriously reduced. From these
results, we believe that neither specific-binding domain, A,
nor the signaling domain, B, could achieve the desired
sensitivity. A combination of them together, however, leads
to high sensitivity.
II
In contrast to those of 2 and 3, the selectivity of 1 for Pd
is excellent. Figure 2 shows that many transitional-metal and
III
III
II
II
II
III
II
main-group ions such as Ru , Rh , Co , Ni , Zn , Fe , Sn ,
À6
II
II
II
I
I
II
II
Figure 3. Absorption spectra of 1 in THF (5.0”10 m) as a function of
Pb , Cu , Hg , Ag , Na , Mg , and Mn have little effect on
II
different concentrations of Pd ions.
absorptive bands of 2 and 3 display no obvious changes. These
spectroscopic results clearly show that there is a difference in
II
the energy state of 1 in the presence and absence of Pd ions.
The red-shift absorption of 1 suggests that the species formed
is on average conjugated over a greater length, which may be
II
ascribed to the interpolymer interaction by Pd -pyridyl
binding (I in Scheme 2). As for the excessive distortion
backbone of 2, its lower absorption shift results indicate that
intrachain linkage may take place (II in Scheme 2)instead of
interchain linkage. For 3, without the binding domain A, the
Figure 2. The relative fluorescence quenching andanisotropy changes
of 1 with different metal ions.
II
interchain linkage by the association between Pd and pyridyl
can not produce at all (III in Scheme 2)and which is
confirmed by the no changes of absorption. The almost
unchanged emission spectra of 1 demonstrate the facile
energy transfer that can occur along the different segments of
the backbone and even through the interchain system.
[
5,6]
the fluorescence of 1. Compared with previous reports, the
modular-based conjugated polymer 1 provides better selec-
II
tivity for Pd ions. That means proper flexibility of the
backbone mainly contributes to the selective response. Other
elements such as the varying coordination ability or mode
À
Stronger ligands than pyridine, such as CN ions, were
II
(
mono- or multi-dentate)between the ion and pyridyl group
added into a solution of 1 to complex the Pd ions.
may also influence the selectivity. The trend of fluorescence
quenching of 1 by ions is similar to that of Dpp, but 1 enlarges
the discrimination. Compound 1 thus exhibits high selectivity
Consequently, the fluorescence of the conjugated polymer
almost fully recovered, thus indicating the reversibility of the
sensing property of this molecule.
II
towards Pd ions and provides an excellent avenue for
designing conjugated fluorescent polymers for sensing appli-
cations. We also investigated fluorescence quenching with
IV
IV
Pt ions. The experimental results suggest that with Pt ions
with compound 1 give a similar effect although its quenching
is about 58.1% and fluorescence anisotropy changes about
II
5
À1
6
8.4% of that of Pd at the concentration of 1.5 ” 10 molL .
This suggests that Pt has a much stronger response than
most of the other ions tested.
IV
To further clarify the relationship between the structural
arrangement of a conjugated polymer and its specific
response, detailed investigations of the mechanism were
carried out by using absorption measurements and fluores-
cence anisotropy. At room temperature, the significant
II
spectroscopic variations that occur as Pd ions interact with
1
can be observed within a few minutes by progressively
Scheme 2. Schematic interaction of the different polymer structures (1,
2, and 3) with Pd ions.
II
II
adding a solution containing Pd ions into a solution of 1. The
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Fluorescence anisotropy was used to identify the link
applications. This new design will provide an excellent
approach to efficiently and simply regulate the optical and
electronic properties of conjugated polymers, which is essen-
tial to make them useful for electrical devices and bio/
chemical sensors.
joining the chains of polymer 1 as this method has been used
for studying molecular interactions, particularly in cases in
which there is a significant change in molecular weight upon
[
11]
binding or interaction. We used it to study the aggregation
II
of 1 due to the binding of Pd ions through the pyridyl groups.
II
Figure 4 shows the effects of varying concentrations of Pd
Received: April 20, 2004
Keywords: coordination modes · palladium · polymers · self-
.
assembly · sensors
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