Chemistry Reference
In-Depth Information
1
Introduction
Conjugated polymers (CPs) are unsaturated rigid-rod macromolecules with sp
1
-or
sp
2
-hybridized backbones. CPs in their neutral states are organic semiconductors
that exhibit efficient absorption and emission. Their band gaps can be desirably
fine-tuned by backbone structures, allowing emission ranging from ultraviolet to
near-infrared. During the past decades, CPs have been extensively investigated for
applications in optoelectronic devices, such as light-emitting diodes [
1
], photovol-
taic cells [
2
], organic field-effect transistors [
3
], and organic lasers [
4
]. Recently,
CPs have been proven useful for trace detection of analytes in a variety of
environments [
5
], which benefit from their easily perturbed properties, including
charge transport [
6
], conductivity [
7
], emission [
8
,
9
], and absorption [
10
]. In
particular, their delocalized electronic structures impart efficient electronic cou-
pling among optoelectronic segments, leading to rapid intramolecular and intermo-
lecular energy/charge transfer [
11
]. As compared to small-molecule fluorophores,
CPs have larger absorption cross-section and unique collective optical response,
making them superior in the transduction of optical signals.
Water solubility, a prerequisite for fluorescent materials to interrogate biomole-
cules in physiological environments, necessitates the development of water-soluble
CPs. Conjugated polyelectrolytes (CPEs) are CPs with water-soluble ionic side
chains [
12
]. These polymers combine optoelectronic properties of CPs with charge-
mediated behaviors of polyelectrolytes, providing a unique platform for the con-
struction of chemical and biological sensors [
13
]. According to the charge sign,
water-soluble CPEs can be simply divided into two categories, cationic CPs (CCPs)
and anionic CPs (ACPs). The chemical structures of some typical CPEs are
summarized in Scheme
1
. Cationic groups of CCPs are usually quaternary ammo-
nium (CCPs 1-4), while anionic groups of ACPs are carboxylate (7), phosphonate
(6) or sulfonate (5 and 8). The water-solubility of CPEs is not only dependent on the
N(Et)
3
Br
N(Et)
3
Br
S
N
N
O
O
CCP:
n
n
n
n
S
O
N
(Et)
3
B
r
X
(Me)
3
N
1I:
X = I
1Br:
4
N
(Me)
3
X
B
r(Me)
3
N
N
(Me)
3
B
r
3
2
X = Br
SO
3
Na
PO
3
Na
2
COO Na
O
O
O
ACP:
n
n
n
O
O
O
n
N
a
O
OC
P
O
3
N
a
2
N
a
O
3
S
S
O
3
N
a
5
6
7
8
Scheme 1 Chemical structures of some typical CPEs
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