Angewandte
Communications
Chemie
N-type conjugated polymers have been extensively devel-
oped and investigated in OFETs and PSCs over the past
decades, but less so for electrochemical transistors, mainly
due to their low performance, and the lack of robust design
rules to achieve high mixed mobility and stability in aqueous
conditions. The majority of current n-type semiconducting
polymers are donor(D)-acceptor(A) alternating co-polymers,
synthesized via a transition metal-mediated cross coupling
polymerization. The alternation of electron-rich and electron-
deficient units of the polymers is facilitated with single bonds
along the backbone, and promotes molecular orbital hybrid-
mance with a maximum dimensionally normalized trans-
conductance reaching 0.212 Scm and the product of charge
À1
carrier mobility m and volumetric C* (mC*) of 0.662 Æ
À1 À1 À1
0.113 Fcm
V s , which are higher than previously
reported glycolated polymers, and comparable with the best
[11c,16]
performing BBL polymer.
Additionally, as prepared
through a metal-free aldol condensation polymerization,
these polymers should have a low concentration of toxic
residues and be good candidates for bioelectronics. To the
best of our knowledge, these are the first reported rigid n-type
conjugated polymers with OEG side chains for OECT
applications.
[
8]
ization. Rationally, rotational torsions around these single
bonds are allowed which will give rise to conformational
disorder, and consequently lead to a negative effect on the
charge carrier mobility of the polymers. Meanwhile, the
highly localized distribution of the LUMO frontier orbitals of
D-A polymers further hinders intermolecular charge hopping
The chemical structures of the polymers, PgNaN and
PgNgN, are shown in Figure 1. Both polymers comprise of
fused electron-deficient lactam rings with OEG (and also
alkyl in the case of PgNaN) side chains. To investigate the
effect of OEG side chains on ion transport, the two polymers
were designed to have a differing ratio of OEG groups, with
PgNgN having a higher OEG chain density than that of
PgNaN. As shown in Figure S1, the bis-isatin monomer was
synthesized by a substitution reaction with bis-isatin and the
iodine end-capped glycol chain. The most challenging step is
to carry out the purification processes of glycolated bis-isatin
monomer due to its large polarity and sticky physical nature.
Finally, the pure monomer was obtained through recycling gel
permeation chromatography (GPC) following earlier purifi-
cation procedures of precipitation and silica gel column
chromatography. Both polymers were prepared via acid-
catalyzed aldol polymerization in toluene, which is more
sustainable and benign to the environment compared with
transition-metal-catalyzed polymerizations.
[
9]
and subsequently the charge carrier mobility. Typically, the
electron withdrawing functionality along these polymers is
either insufficiently strong or prevalent to ensure that the
polymer has a high enough electron affinity (EA) required to
both facilitate electron injection, reduce the influence of
shallow trapping and ensure the oxidative stability of the n-
doped polymers, in operation in the presence of oxygen and/
[10]
or in aqueous environment. In addition, residual metallic
species arising from polymerization, such as organostan-
nannes as well as the transition metal catalysts, present
a limitation for the polymers to be applied in bioelectronics.
Thus, it is desirable to design polymers that comprise only of
electron-deficient repeat units into the polymer backbone,
have a delocalized LUMO energy level distribution, and
eliminate torsional twists along the backbone. A series of
conjugated n-type polymers with oligo(ethylene glycol)
(
OEG) side chains, are herein reported, with low-lying
LUMO energy levels, polymerized via a metal-free aldol
condensation, where all the repeat units are electron-deficient
and locked in-plane by double bond linkers for use in
electrochemical transistors.
OEG chains have been previously used in semiconducting
[3b,5b,6a,11]
polymer side chains for OECT applications
owing to
their hydrophilicity and solubility. It has been demonstrated
that replacing alkyl chains by glycol analogues endow the
resulting semiconducting polymers with a decreased p-p
[
12]
[12a,13]
stacking distance,
higher dielectric constant,
larger
It has
Figure 1. The typical structure of an OECT device (left) and the
chemical structures (right) of PgNaN and PgNgN.
[
1h,11c,14]
volumetric capacitance and higher ion uptake.
also been reported oxygen atoms along OEG chains can
coordinate with the ions facilitating penetration into the
polymer morphology from the electrolyte, in a manner similar
It was observed that as the density of OEG chains get
higher, the solubility of the resulting polymers significantly
reduces during polymerization and the reaction solution
becomes a gel at room temperature. This contributes to the
lower molecular weight of PgNgN compared to PgNaN
(Table 1). The high PDI of PgNaN measured by GPC may be
ascribed to its strong aggregation behavior resulting from the
rigid polymer backbone. Both PgNaN and PgNgN show good
solubility only in chloroform, with limited solubility in other
common solvents, for example, chlorobenzene (CB), o-
dichlorobenzene (o-DCB), 1,1,2,2-tetrachloroethane and
dimethylformamide (DMF). Thermogravimetric analysis
[
15]
to crown ethers, and enhance ion transport. Herein, we
report novel n-type semiconducting polymers that simulta-
neously incorporate a rigid p-conjugated backbone and OEG
side chains. Owing to the high rigidity and planarity of the
polymer backbone and the lack of donor-acceptor character,
the LUMO frontier orbitals are highly delocalized which
À3
2
À1 À1
leads to a high electron mobility of 10 cm V
s
range,
measured in an OECT. Meanwhile, both polymers exhibited
a deep-lying LUMO energy level lower than À4.0 eV due to
an all electron-deficient polymer backbone. While applied in
OECTs, the polymers demonstrated a high device perfor-
Angew. Chem. Int. Ed. 2021, 60, 9368 –9373
ꢀ 2020 Wiley-VCH GmbH