Tunable Control of the Hydrophilicity and Wettability of Conjugated Polymers by a Postpolymerization Modification Approach

Article abstract of DOI:10.1002/mabi.202000087

A facile method to prepare hydrophilic polymers by a postpolymerization nucleophillic aromatic substitution reaction of fluoride on an emissive conjugated polymer (CP) backbone is reported. Quantitative functionalization by a series of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as well as with high molecular weight polyethylene glycol is demonstrated. The length of the ethylene glycol sidechains is shown to have a direct impact on the surface wettability of the polymer, as well as its solubility in polar solvents. However, the energetics and band gap of the CPs remain essentially constant. This method therefore allows an easy way to modulate the wettability and solubility of CP materials for a diverse series of applications.

Full text of DOI:10.1002/mabi.202000087

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Tunable Control of the Hydrophilicity and Wettability of
Conjugated Polymers by a Postpolymerization Modification
Approach
Shengyu Cong, Adam Creamer, Zhuping Fei, Sam A. J. Hillman, Charlotte Rapley,
Jenny Nelson, and Martin Heeney*
have also found application as the active
A facile method to prepare hydrophilic polymers by a postpolymerization nucle-
ophillic aromatic substitution reaction of fluoride on an emissive conjugated
polymer (CP) backbone is reported. Quantitative functionalization by a series
of monofunctionalized ethylene glycol oligomers, from dimer to hexamer, as
well as with high molecular weight polyethylene glycol is demonstrated. The
length of the ethylene glycol sidechains is shown to have a direct impact on
the surface wettability of the polymer, as well as its solubility in polar solvents.
However, the energetics and band gap of the CPs remain essentially constant.
This method therefore allows an easy way to modulate the wettability and
solubility of CP materials for a diverse series of applications.
material in electrochemical transistors,
where their mixed electronic and ionic
conductivity is utilized.[11] This has been
further leveraged in organic electronic ion
pumps to control drug delivery.[12] Also,
CPs have been used as biosensors such
as glucose monitors for diabetics[13] and as
implants for the restoration of physiolog-
ical functions.[14] The potential advantages
of CPs over more traditional conductors
and semiconductors in bioelectronics can
be found in their facile chemical function-
alization[15] and potential low temperature
processing.[16] In addition, CPs are soft,
flexible, and mechanically tuneable, with a better match to the
mechanical properties of biological tissue than traditional con-
ductors and inorganic semiconductors.[17,18]
1. Introduction
Conjugated polymers (CPs) are a class of organic materials that
have delocalized aromatic backbones and unique electronic and
optical properties.[1–3] They have been widely investigated for
application in a range of optoelectronic devices such as organic
light-emitting diodes and organic field-effect transistors.[4,5]
More recently the application of CPs in the bioelectronic field
has attracted much interest.[6–8] For example, CPs have been
used as electrically active tissue engineering scaffolds which
can control protein conformation and cell adhesion.[9,10] They
Most common CPs are hydrophobic and only soluble in
organic solvents. In order to function in many of bioelectronic
devices it is important that the CP can support ionic as well as
electronic transport at room temperature. Most bioelectronic
devices to date depend on well-established CPs such as doped
polypyrrole (Ppy),[19] doped polyaniline,[20] and polythiophene
derivatives such as poly(3,4-ethylenedioxythiophene) doped with
poly(styrenesulfonate) (PEDOT:PSS).[21] However these mate-
rials have some drawbacks and processability challenges.[7,16]
In order to overcome these challenges, more recent work has
shown that the incorporation of ionic or ethylene glycol based
sidechains onto the backbone of the CP can promote ionic con-
ductivity,[22,23] although the performance has been shown to be
very sensitive to relative percentage of hydrophobic/hydrophilic
groups on the backbone.[24] The addition of ethylene glycol
chains to CPs has also been shown to promote closer π-stacking
of conjugated backbones,[25,26] enhance the dielectric con-
stant,[27,28] and afford higher charge carrier mobility in transistor
devices[29,30] compared to the analogous alkylated polymers.
In addition to ion conductivity, another important aspect for
many bioelectronic devices is the interface between biological
tissue and the CP at the cellular level.[31] The surface of the CP
must allow effective cell adhesion and support for cells to establish
intimate contact with living tissue.[32,33] Cell adhesion to organic
materials is affected by their surface properties such as wetta-
bility, roughness, surface charge, and chemical functionality.[34–36]
Among these properties, surface wettability has been most exten-
sively investigated, with cells found to adhere more effectively onto
surfaces with moderate wettability than hydrophobic surfaces.[37–40]
Dr S. Cong, Dr A. Creamer, C. Rapley, Prof. M. Heeney
Department of Chemistry and Centre for Processable Electronics
Imperial College London
White City Campus, London W12 0BZ, UK
E-mail: m.heeney@imperial.ac.uk
S. A. J. Hillman, Prof J. Nelson
Department of Physics and Centre for Processable Electronics
Imperial College London
South Kensington Campus, London SW7 2AZ, UK
Prof. Z. Fei
Institute of Molecular Plus
Tianjin Key Laboratory of Molecular Optoelectronic Science
Tianjin University
Tianjin 300072, P. R. China
The ORCID identification number(s) for the author(s) of this article
can be found under https://doi.org/10.1002/mabi.202000087.
© 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA,
Weinheim. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly cited.
DOI: 10.1002/mabi.202000087
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Factors such as cell types, surface functionalization, and rough-
ness are also important.[36] Thus in order to fulfil diverse cell adhe-
sion, easy control of surface wettability is required.
(THF), chloroform, and chlorobenzene at room temperature.
Molecular weight analysis by gel permeation chromatography
(GPC) indicated Mn = 38500 g mol−1 and Mw = 65100 g mol−1
(Ð = Mw/Mn: 1.69) versus polystyrene standards.
In addition to their role in bioelectronic devices, the highly
emissive nature of many CPs has led to their investigation as
fluorescent probes for various analytes and biomolecules.[41,42]
Solubility of the CP in aqueous media is again essential for
interfacing with the biomacromolecules.[43] However, current
approaches to make suitably functionalized CPs are relatively
laborious, with hydrophilic sidechains (often charged groups or
ethylene glycol oligomers) usually introduced at an early stage
of the monomer synthesis.[44] Investigating the optimal length
and nature of the hydrophilic sidechains on the CP backbone
therefore requires significant synthetic effort. Furthermore con-
trol of the percentage hydrophilic/hydrophobic groups is typi-
cally achieved by copolymerization approaches,[24,45] in which it
can be difficult to achieve similar molecular weights and disper-
sities to allow fair comparison between copolymers.
Recently we reported an approach to directly functionalize the
backbone of a CP by a nucleophilic aromatic substitution (SNAr)
reaction of an electron-deficient fluorinated co-monomer.[46] This
allowed the quantitative incorporation of a range of functional
groups onto the polymer backbone. Here we expand upon that
work to report an effective and facile approach to control the
hydrophilicity and wettability of CPs by a postpolymerization
modification protocol. We demonstrate that this method can
readily graft different lengths of ethylene glycol sidechains
(from dimers to hexamers) to the conjugated backbone, allowing
fine control of the surface wettability of thin films. A polymer
containing a branched alkyl group (2-ethylhexyloxyl) was also
prepared as a control, to examine the effect of the ether function-
alization without the presence of polar ethylene glycol groups.
Furthermore monofunctional polymers of ethyene glycol of
high molecular weight (Mn = 10000 g mol−1) were also grafted
to the conjugated backbone to afford fully water-soluble conju-
gated graft-copolymers. We believe this method opens up a way
to control wettability and hydrophilicity of a range of CPs.
Following successful synthesis of F8FBT, the fluoride dis-
placement reactions were optimized based on our previous
protocol. Typically aprotic solvents perform best for SNAr reac-
tions, but F8FBT was not soluble in dimethyl sulfoxide (DMSO)
alone. Therefore, a mixture of chlorobenzene and DMSO was
utilized in the presence of KOH and excess ethylene glycol at
120 °C. The nucleophilic aromatic substitution reactions were
monitored by 19F-NMR spectroscopy of quenched aliquots. It is
worth noting that no reaction with KOH in the absence of eth-
ylene glycol was observed. The reactions were complete when
19F-NMR did not show any fluorine signal, typically after 48 h,
in good isolated yields.
After reaction, the products were precipitated into methanol,
then purified by Soxhlet washing with acetone (24 h) under
nitrogen to give the substituted polymers. However, F8BT-PEG
was soluble in methanol due to the grafted hydrophilic polyeth-
ylene glycol (PEG10K). Hence for F8BT-PEG, the reaction sol-
vent was removed under reduced pressure and the residue was
dissolved in deionized water. The excess PEG was removed by
dialysis for 2 d and pure F8BT-PEG was obtained. All substi-
tuted polymers are characterized by 1H and 19F-NMR.
Diffusion ordered NMR spectroscopy (DOSY) was used to
confirm the coupling of the polymeric PEG (Mn = 10000 g mol−1)
with the F8FBT backbone and to confirm the purity of the
graft copolymer. In DOSY each component in a mixture can
be pseudo-separated, based on its own diffusion coefficient.[50]
A physical mixture of F8FBT and PEG10K was investigated ini-
tially by DOSY (Figure S1, Supporting Information). In addi-
tion to the solvent, clear signals from PEG and F8FBT were
observed, with obviously different diffusion coefficients (values
were not calibrated). The corresponding DOSY spectra of the
graft copolymer F8BT-PEG, although nonquantitative, afforded
a single diffusion constant, indicating the successful synthesis
of F8BT-PEG (Figure S2, Supporting Information). Given the
synthetic challenge of successfully coupling polymers to one
another, this truly demonstrates the utility of the methodology.
The molecular weights of the polymers were all determined
by GPC in chlorobenzene and are summarized in Table 1. As the
substituted polymers are all from the same batch of F8FBT, we
would expect an increasing Mw/Mn as the mass of the sidechain
increases, with a similar dispersity for all polymers, although
some small variance may occur due to the work-up and purifica-
tion. The calculated degree of polymerization for each polymer is
also included in Table 1. Indeed the observed Mn and Mw gradu-
ally increases as the length of the hydrophilic chain is increased
from diethylene glycol monomethyl ether (m = 2) to tetraethyl-
eneglycol monomethyl ether (m = 4), before decreasing for pen-
taethyleneglycol monomethyl ether (m = 5) and hexaethylene
glycol (m = 6). The hydrophobic ethylhexyloxy chain exhibited a
similar profile to the starting polymer. We believe these changes
are related to differences in the hydrodynamic radius in solution
as the side chain length and polarity changes. We further note
that we could observe no signal for graft polymer F8BT-PEG
by GPC under these conditions, likely due to the adsorption of
F8BT-PEG onto the surface of GPC stationary phase.
2. Results and Discussion
2.1. Synthesis
Our starting point for the investigation was a derivative of the
well-known emissive polymer F8BT (poly(dioctylfluorene-co-
benzothiadiazole)),[47] in which the benzothiadiazole was replaced
with a fluorinated benzothiadiazole (FBT).[48] We had previously
shown that the resulting polymer F8FBT was amenable to direct
displacement of the fluoride group under nucleophillic aromatic
substitution conditions.[46] The starting materials, 2,7-dibromo-
9,9-dioctylfluorene (2) and 9,9-dioctyl-9H-fluorene-2,7-diboronic
acid bis (pinacol) ester (3) were synthesized according to
the literature.[49] Under typical Suzuki reaction conditions,
equimolar quantities of (3) were reacted with 4,7-dibromo-
5-fluorobenzo-2,1,3-thiadiazole to give F8FBT (Scheme 1). After
reaction, the polymer was purified by Soxhlet washing with meth-
anol, acetone, and hexane to remove lower molecular weight oli-
gomers and catalyst residues. The resultant F8FBT was soluble
in typical organic solvents such as toluene, tetrahydrofuran
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Scheme 1. Synthesis of monomers and substituted polymers.
2.2. Optoelectronic Properties
the parent F8BT polymer,[47] with all polymers exhibiting two
absorption bands in solution and thin films: a high energy band
corresponding to the π–π* transition and a low energy band cor-
responding to intramolecular charge transfer (ICT) to the elec-
tron-deficient benzothiadiazole monomer.[51] The parent F8FBT
exhibits peaks at 319 and 440 nm in chloroform, similar to the
reported spectrum.[48] Substitution of the electron-withdrawing
fluoride with the electron-donating ether group results in a
slight red-shift of both bands in all examples (Table 2), as well a
reduction in the relative intensity of the ICT band with respect
to the high energy band, likely as a result of the weakening of
the electron-accepting ability of the benzothiadiazole. All of
the ether functionalized polymers exhibited similar spectra,
highlighting that the length of chain has little influence on the
polymer bandgap. A similar trend is apparent in the solid state,
with all ether functionalized polymers exhibiting remarkably
similar spectra, even for the polymer with long PEG chains.
A slight weakening of the relative intensity of the ICT peak
for F8BT-PEG polymer is maybe due to the weaker interchain
interactions caused by the very long side chain.[52] Optical band
gaps, as measured by the onset of absorption, are 2.44–2.46 eV
in all cases.
The UV-Vis absorption spectra of F8FBT, F8BT-OC8, F8BT-m
(m = 2, 3, 4, 5, 6), and F8BT-PEG at room temperature in chlo-
roform solution and as thin films are shown in Figure 1 and
the data summarized in Table 2. The spectra are similar to
Table 1. Molecular weights of polymers (polystyrene as standards, chlo-
robenzene as solvent).
a)
b)
Polymer
F8FBT
Mn[KDa]
38.5
37.5
45.4
54.1
Mw[KDa]
65.1
74.2
78.3
59.3
88.1
83.9
85.4
–
DPn
71
DPw
Ðc)
1.69
1.98
1.72
1.65
1.60
1.65
1.71
–
120
113
121
129
120
110
107
–
F8BT-OC8
F8BT-2
57
70
78
75
66
62
–
F8BT-3
F8BT-4
55.2
50.8
49.9
–
F8BT-5
F8BT-6
F8BT-PEG
a)Degree of polymerization calculated from Mn; b)Degree of polymerization calcu-
lated from Mw; c)Ð calculated from Mw/Mn.
The photoluminescence spectra of the polymers in chloro-
form solution and as thin films are shown in Figure 2. The
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a Stokes shift of 104 nm and a red-shift of 14 nm compared
to the parent F8FBT. All the ether-containing polymers have
nearly identical emission profiles in chloroform solution.
Upon moving to thin films, the ether-containing polymers
again have very similar λem,
(550–556 nm), which are
max
slightly red-shifted compared to solution. The emission pro-
files of F8BT-3, -4, and -5 are slightly broader compared to
other polymers.
The electronic properties of the substituted polymers
were investigated using cyclic voltammetry (CV). Samples
were prepared by drop-casting solution onto the surface of
a Pt rod. Samples were measured in anhydrous, degassed
solutions of acetonitrile with tetrabutylammonium hex-
afluorophosphate (0.1 m) electrolyte using an Ag/Ag+ as
reference electrode. The CV of all polymers are shown in
Figure S3 (Supporting Information) and summarized in
Table 2. The highest occupied molecular orbital (HOMO)
level of each polymer was estimated from the onset of oxida-
tion, assuming that the ferrocene/ferrocenium (FOC) refer-
ence redox system is 4.8 eV below the vacuum level.[53] The
HOMO level of F8FBT was −6.09 eV, which is very similar to
the reported value (−6.01 eV).[48] Substitution of the electron-
deficient fluorine with the electron-donating ether group
results an increase in the HOMO level of all functional-
ized polymers, to values between −5.77 and −5.67 eV. These
values approximately identical, within the error of the meas-
urement ( 0.1 eV),[54] and similar to the optical results show
that changing the length of the sidechain does not have a
significant impact on the electronic properties. We note that
the thin-film CV of F8BT-PEG could not be measured, as it
was soluble in acetonitrile.
2.3. Thermal Properties
Differential scanning calorimetry (DSC) has been widely
applied to explore the crystallinity and thermal behavior of
polymers. The thermal behavior of F8FBT, F8BT-OC8, F8BT-m
(m = 2, 3, 4, 5, 6), and F8BT-PEG was measured using DSC
on powder samples under nitrogen (Figure S4, Supporting
Information). For the parent F8FBT, we observe two overlap-
ping endotherms on initial heating, with an onset at 226 °C
and peaks at 243 °C and 262 °C. Upon cooling, an exothermic
Figure 1. UV-Vis absorption spectra of polymers in chloroform solution
a) and thin film b).
emission maxima (λem, max) of the polymer solutions and
thin films are summarized in Table 2. F8FBT apart, all poly-
mers show broad emission profiles with maxima in the green
region of the spectrum (λem, max of 546 nm), corresponding to
Table 2. Optical and electronic properties of polymers.
Polymer
F8FBT
Eg(opt) [eV]
2.45
HOMO [eV] (CV)
λ
abs, max (sol)[nm]
319a), 440b)
331a), 445b)
331a), 442b)
331a), 442b)
331a), 442b)
331a), 442b)
331a), 442b)
331a), 442b)
λ
abs, max (film) [nm]
319a),452b)
334a),454b)
335a),454b)
335a),455b)
336a),455b)
335a),456b)
336a),455b)
335a),454b)
λ
em, max (sol) [nm]
λ
em, max (film) [nm]
532
548
546
546
546
546
546
546
538
554
556
556
555
556
554
550
−6.07
−5.77
−5.73
−5.73
−5.72
−5.69
−5.67
-
F8BT-OC8
F8BT-2
2.44
2.44
F8BT-3
2.45
F8BT-4
2.44
F8BT-5
2.45
F8BT-6
2.44
F8BT-PEG
2.46
a)λmax of high energy band; b)
λmax of low energy band.
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Figure 3. Photographs showing F8BT-PEG solubility in polar solvents
(1 mg mL−1).
2.4. Solubility and Wettability
The alkoxy-substituted F8BT-OC8 and F8BT-m (m = 2, 3, 4, 5,
6) are all soluble in organic solvents, such as chloroform, THF,
and chlorobenzene, but insoluble in polar solvents, such as
methanol, acetonitrile, or water. However, the grafted F8BT-
PEG is soluble in chloroform and THF, but also soluble in
polar solvents, such as methanol, DMSO, or even pure water
(Figure 3). The change of solubility is due to dominant effect of
the grafted PEG10K chains.
In order to investigate the hydrophilicity of the substituted
polymers more quantitatively, water was sequentially added to
THF solutions of the polymers (2.5 mg mL−1) at room tem-
perature until a set transmittance was reached (65% of original
transmittance) at 550 nm. This is a slight modification of a
typical cloud point experiment, because although solutions of
F8FBT, F8BT-OC8, and F8BT-m (m = 2, 3, 4, 5, 6) became visibly
cloudy as water was added, F8BT-6 did not, regardless of the
amount of water added. Therefore the reduction in transmis-
sion at 550 nm due to scattering was taken to demonstrate the
onset of precipitation. The data shown in Table 3, shows a sys-
tematic trend in which more water is required to cause precipi-
tation as the length of the ethylene glycol sidechains increases,
in agreement with an increase in polymer hydrophilicity. We
note that F8BT-PEG is both water and THF soluble, and the
transmittance of solution at 550 nm did not change when water
added.
The change in hydrophility is further supported by con-
tact angle measurements on polymer thin-films (Figure 4,
Figure S5, Supporting Information, and Table 3). The contact
angle of the parent F8FBT was 101.4°, similar to that of typical
alkylated CPs.[57] Adding a hydrophobic alkyl sidechain resulted
in little change (100.8° for F8BT-OC8). The addition of the short
ethylene glycol dimer did not significantly change the contact
angle (F8BT-2), probably because the surface is still dominated
by the octyl sidechains on the fluorene co-monomer. However,
increasing the ethylene glycol sidechain length resulted in a
systematic decrease in contact angle, from hydrophobic films
to ambiphilic for F8BT-6 (85.6°). Furthermore, the contact
angle for F8BT-PEG dropped dramatically to 31.0° (Figure 4).
The contact angle of a pure PEG film is reported to be around
Figure 2. Emission spectra of polymers in chloroform solution a) and
thin film b).
transition onset is seen at 154 °C with a peak at 138 °C. These
transitions are similar to those observed in the unfluorinated
F8BT polymer, in which the endotherms are ascribed to a tran-
sition to a liquid crystalline and isotropic phase.[47,55] After dis-
placement of the fluoride from the conjugated backbone, the
thermal properties of substituted polymers become very dif-
ferent, with no transitions observed between 30 °C and 300 °C,
for F8BT-OC8 and F8BT-m (m = 2–6), suggestive of amor-
phous materials. This is likely due to the additional steric bulk
of the sidechains disrupting the packing of the conjugated
backbones.
However, for F8BT-PEG we observed a strong endothermic
transition on heating, which has an onset at 53 °C and peaks
at 60 °C. The corresponding exotherm on cooling has an onset
at 43 °C and peaks at 38 °C. These transitions are very close to
the melting point and crystallization point for PEG10K (66 °C
and 40 °C respectively).[56] Clearly, the PEG side chain plays
a dominant role in the crystallinity and thermal behavior of
F8BT-PEG.
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Table 3. Hydrophilicity and contact angle.
CP materials. More importantly, this convenient approach may
open up a way to synthesize diverse CPs that can be used for
bioelectronic applications.
Polymer
F8FBT
Precipitation volumea) v:v [%]
Contact angleb)
101.4 1.3°
100.8 1.1°
99.7 1.3°
2.6
4.6
F8BT-OC8
F8BT-2
4. Experimental Section
12.9
15.3
28.6
41.9
45.3
-
CV Measurement: CV was carried out with a standard three-electrode
cell in a 0.1 m tetrabutylammonium hexafluorophosphate solution
((TBA)PF6) in acetonitrile at room temperature with a scanning rate of
0.1 V s−1. A Pt rod working electrode (1.6 mm diameter), a Pt wire counter
electrode, and an Ag/AgCl reference electrode were used. The oxidation
potentials were calibrated with a standard FOC redox system as the
standard (assuming the energy level of FOC was 4.8 e V below vacuum)
for estimating the HOMO energy level of polymers. Samples were
prepared by drop-casting solution on the Pt rod surface.
F8BT-3
92.9 1.6°
92.0 1.6°
90.9 1.4°
85.6 1.0°
31.0 1.2°
F8BT-4
F8BT-5
F8BT-6
F8BT-PEG
a)Volume of water added to THF solution to produce 65% of original transmittance
at 550 nm; b)Average of five measurements
Solubility Measurement: The transmittance of
a THF solution
(2.5 mg mL−1) of the polymer was measured at 550 nm in a UV-1800
Shimadzu UV–vis spectrometer. Water was added portionwise until the
transmittance dropped to 65% of the original value. The precipitation
percentage was then calculated as volume H2O/total volume
(THF+H2O).
25°.[58] Clearly, the dramatic decrease of contact angle was due
to the presence of the polymeric PEG sidechains.
Overall the solubility and contact angle data show that the
hydrophilicity and wettability of the CPs can be modulated in
a systematic way by nucleophilic aromatic substitution with
ethylene glycol side chains. Importantly, such substitution does
not affect the optoelectronic properties of the CP backbone.
Contact Angle Measurement: A Krüss DSA100E goniometer was used
to carry out contact angle measurements using the static sessile drop
method. All measurements took place at 25 °C at a humidity of 65%.
Polymer solutions (5 mg mL−1) were spin-coated onto quartz substrates
at 2000 rpm. Prior to spin-coating, quartz substrates were washed
sequentially by soap water, acetone, and isopropanol. Quartz substrates
were then dried by flushing with nitrogen. For each measurement,
a 5 µL drop of deionized water was suspended at the end of a needle
and slowly brought into contact with the sample. Contact angles were
measured 30 s after the withdrawal of the needle tip from the droplet to
allow the drop time to equilibrate on the sample surface. Drop shapes
were then fitted using the polynomial method. Each quoted contact
angle was calculated from an average of five measurements made on
the same sample.
Synthesis—F8FBT: A mixture of 9, 9-dioctyl-9H-fluorene-2, 7-diboronic
acid bis (pinacol) ester (975.47 mg, 1.518 mmol), 4,7-dibromo-5-fluoro-2,
1,3-benzothiadiazole (473.42 mg, 1.518 mmol), tetrabutylammonium
iodide (48 mg, 0.13 mmol), and Pd(PPh3)4 (35.1 mg, 0.0304 mmol)
was added to a high pressure microwave vial. The vial was sealed with
a septum and degassed with argon before toluene (10 mL) and K2CO3
(2 M, 4 mL) solution was added. The mixture was heated at 120 °C for
72 h. The reaction was cooled to room temperature and precipitated into
methanol, stirred for 30 min, and filtered through a Soxhlet thimble. The
precipitates were further purified by washing with methanol, acetone,
and hexane under argon for 24 h, respectively. The remaining polymer
3. Conclusion
A series of emissive CPs with increasingly hydrophilic side
chains have been synthesized by nucleophilic aromatic substi-
tution reactions on FBT monomer units within a CP backbone.
Quantitative substitution of all fluoride was demonstrated with
a series of monofunctionalized ethylene glycol oligomers, as
well as with high molecular weight polyethylene glycol to afford
a graft copolymer. The optical, electrical, and thermal properties
of the resultant polymers were investigated. The length of the
ethylene glycol sidechains was shown to have a direct impact
on the surface wettability of the polymer, as well as its solubility
in polar solvents. However, the energetics and band gap of the
CPs remained essentially constant. This facile method therefore
allows an easy way to modulate the wettability and solubility of
Figure 4. Water contact angle pictures of F8BT-OC8 and F8BT-PEG.
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in the thimble was then dissolved in hot chloroform and precipitated
into methanol to give F8FBT as a fibrous yellow solid (591 mg, 72%).
Mn: 38.5 kDa, Mw: 65.1 kDa, Mw/Mn (Ð): 1.69. 1H-NMR (400 MHz,
CDCl3, δ): 8.15–7.79 (m, 7H, ArH), 2.16 (m, 4H, CH2), 1.33–1.08 (m, 20H,
CH2), 0.99 (m, 4H, CH2), 0.84 (m, 6H, CH3).19F-NMR (400 MHz, CDCl3)
δ -114.5 ppm.
was very weak compared to the dominate PEG10K peaks (see Figure S6,
Supporting Information) making accurate integrations difficult.
Supporting Information
General Procedure for Substituted Polymers: A mixture of F8FBT
(54 mg, 0.1 mmol) and KOH (112 mg, 2.0 mmol) was added to a
high pressure microwave vial. The vial was sealed with a septum and
degassed with argon before anhydrous chlorobenzene and DMSO (3:1,
v:v, 8 ml total) and the desired alcohol (0.2 mmol) was added. The
solution was heated at 120 °C for 48 h. During this time the reaction
was monitored by NMR spectroscopy (1H and 19F) of quenched aliquots
(quenched by precipitation into methanol, centrifugation and washing
with acetone). After reaction, the solution was precipitated into methanol
dropwise, stirred for 30 min, and filtered through a Soxhlet thimble. The
precipitates were further purified by washing with acetone under argon
for 24 h and the polymer was extracted with chloroform. The chloroform
was removed under reduced pressure to afford the product.
Supporting Information is available from the Wiley Online Library or
from the author.
Acknowledgements
The authors thank the China Scholarship Council (CSC) via the CSC
Imperial Scholarship, the European Research Council (Action no.
742708), the Royal Society and the Wolfson Foundation (for Royal Society
Wolfson Fellowship) and EPSRC (EP/L016702/1) for financial support.
F8BT-OC8: Mn: 37.5 kDa, Mw: 74.2 kDa, Mw/Mn (Ð): 1.98 1H-NMR
(400 MHz, CDCl3, δ): 8.17–7.74 (m, 7H, ArH), 4.22 (m, 2H, CH2), 2.17
(m, 4H, CH2), 1.81 (m, 2H, CH2), 1.44 (m, 2H, CH2), 1.32 (m, 8H, CH,
CH2, CH3), 1.19 (m, 20H, CH2), 1.01 (m, 4H, CH2), 0.92 (t, J = 6.0 Hz,
3H, CH3), 0.83 (t, J = 6.0 Hz, 6H, CH3). 19F-NMR (400 MHz, CDCl3) no
fluorine signal.
Conflict of Interest
The authors declare no conflict of interest.
F8BT-2: Mn: 45.4 kDa, Mw: 78.3 kDa, Mw/Mn (Ð): 1.72 1H-NMR
(400 MHz, CDCl3, δ): 8.23–7.72 (m, 7H, ArH), 4.39 (m, 2H, CH2), 3.85
(m, 2H, CH2), 3.68 (t, J = 4.8 Hz, 2H, CH2), 3.57 (t, J = 4.4 Hz, 2H, CH2),
3.39 (s, 3H, CH3), 2.14 (m, 4H, CH2), 1.19 (m, 20H, CH2), 1.09–0.91 (m,
4H, CH2), 0.83 (t, J = 6.0 Hz, 6H, CH3). 19F-NMR (400 MHz, CDCl3) no
fluorine signal.
Keywords
conjugated polymers, hydrophilicity, postpolymerization, wettability
Received: March 20, 2020
Revised: March 28, 2020
Published online:
F8BT-3: Mn: 54.1 kDa, Mw: 89.3 kDa, Mw/Mn (Ð): 1.65 1H-NMR
(400 MHz, CDCl3, δ): 8.33–7.68 (m, 7H, ArH), 4.38 (m, 2H, CH2), 3.86
(m, 2H, CH2), 3.67 (m, 6H, CH2), 3.54 (t, J = 4.4 Hz, 2H, CH2), 3.38 (s,
3H, CH3), 2.18 (m, 4H, CH2), 1.19 (m, 20H, CH2), 0.99 (m, 4H, CH2),
0.84 (t, J = 6.2 Hz, 6H, CH3). 19F-NMR (400 MHz, CDCl3) no fluorine
signal.
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F8BT-PEG: A mixture of F8FBT (10.8 mg, 0.02 mmol), poly(ethylene
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