Influence of backbone fluorination in regioregular poly(3-alkyl-4-fluoro)thiophenes

Article abstract of DOI:10.1021/jacs.5b02785

We report two strategies toward the synthesis of 3-alkyl-4-fluorothiophenes containing straight (hexyl and octyl) and branched (2-ethylhexyl) alkyl groups. We demonstrate that treatment of the dibrominated monomer with 1 equiv of alkyl Grignard reagent leads to the formation of a single regioisomer as a result of the pronounced directing effect of the fluorine group. Polymerization of the resulting species affords highly regioregular poly(3-alkyl-4-fluoro)thiophenes. Comparison of their properties to those of the analogous non-fluorinated polymers shows that backbone fluorination leads to an increase in the polymer ionization potential without a significant change in optical band gap. Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a factor of 5 for the fluorinated polymers.

Full text of DOI:10.1021/jacs.5b02785

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Article
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Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-
fluoro)thiophenes
Zhuping Fei,† Pierre Boufflet,† Sebastian Wood,‡ Jessica Wade,‡ John Moriarty,§ Eliot Gann,∥,⊥
Erin L. Ratcliff,# Christopher R. McNeill,∥ Henning Sirringhaus,§ Ji-Seon Kim,‡ and Martin Heeney*,†
†Department of Chemistry and Centre for Plastic Electronics, Imperial College London, Exhibition Rd, London SW7 2AZ, U.K.
‡Department of Physics and Centre for Plastic Electronics, Imperial College London, Exhibition Rd, London SW7 2AZ, U.K.
§Cavendish Laboratory, University of Cambridge, J J Thomson Avenue, Cambridge CB3 0HE, U.K.
∥Department of Materials Science and Engineering, Monash University, Clayton, Victoria 3800, Australia
⊥Australian Synchrotron, 800 Blackburn Road, Clayton, Victoria 3168, Australia
#Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721, United States
S
* Supporting Information
ABSTRACT: We report two strategies toward the synthesis
of 3-alkyl-4-fluorothiophenes containing straight (hexyl and
octyl) and branched (2-ethylhexyl) alkyl groups. We
demonstrate that treatment of the dibrominated monomer
with 1 equiv of alkyl Grignard reagent leads to the formation
of a single regioisomer as a result of the pronounced directing
effect of the fluorine group. Polymerization of the resulting
species affords highly regioregular poly(3-alkyl-4-fluoro)-
thiophenes. Comparison of their properties to those of the
analogous non-fluorinated polymers shows that backbone
fluorination leads to an increase in the polymer ionization
potential without a significant change in optical band gap.
Fluorination also results in an enhanced tendency to aggregate in solution, which is ascribed to a more co-planar backbone on the
basis of Raman and DFT calculations. Average charge carrier mobilities in field-effect transistors are found to increase by up to a
factor of 5 for the fluorinated polymers.
the case of FETs, the small ionization potential of P3AT, as a
result of its electron rich backbone, can lead to issues with
ambient stability, due to expeditious doping by atmospheric
oxygen and/or water.10 For solar cells, the small ionization
potential results in rather low open-circuit voltages limiting
efficiency.11 Hence, synthetic manipulations to improve the
intrinsic properties of P3AT, while maintaining the simple and
scalable polymerization chemistries, are of high potential
interest.
Most routes to modify P3AT properties have focused upon
co-polymerization with a range of co-monomers to influence
the optoelectronic properties.12 Other efforts have focused
upon manipulation of the heteroatom, for example changing
from thiophene to furan,13 selenophene14 or tellurophene15 has
been shown to significantly influence the properties of the
resulting regioregular homo-polymers. However, there has been
less focus on the manipulation of the P3AT backbone itself,
probably because the incorporation of substituents into the
vacant 4-position of the thiophene can lead to undesirable steric
interactions with adjacent monomers, and therefore twisting of
INTRODUCTION
■
Poly(3-alkyl)thiophenes (P3ATs), in particular poly(3-hexyl)-
thiophene (P3HT), are some of the most widely investigated
conjugated polymers yet reported.1 P3HT has been shown to
be a promising p-type semiconductor in field-effect transistors
(FETs)2 and exhibits a high efficiency as the donor polymer in
organic photovoltaic cells, in particular in blends with
substituted fullerene derivatives.3 The regiochemistry of the
solubilizing side chains has been demonstrated to be of
particular importance in both of these applications, as it
influences the ability of the polymer to aggregate in the solid
state.4 From a synthetic viewpoint the chemistry of P3HT and
associated P3ATs is well established,5 with readily scalable
routes reported for the synthesis of regioregular polymer.6 This
is particularly important for applications in which cost of the
semiconductor will be an important factor, such as solar cells.
Routes to carefully control and manipulate the polymer end
group are also established,7 and P3ATs have been shown to
exhibit chain growth polymerization with living character,8
enabling the synthesis of more complex block co-polymers.9
Although this combination of properties makes P3ATs
potentially attractive for many large-scale applications, there
are drawbacks in terms of performance in devices. Principally in
Received: March 17, 2015
© XXXX American Chemical Society
A
DOI: 10.1021/jacs.5b02785
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Bruker AV-400 (400 MHz), using the residual solvent resonance of
CDCl3, o-DCB-d4, or d2-1,1,2,2-tetrachloroethene, and values are given
in ppm. Number-average (Mn) and weight-average (Mw) molecular
weights were determined by using an Agilent Technologies 1200 series
Gel permeation chromatography (GPC) running in chlorobenzene at
80 °C, using two PL mixed B columns in series, and calibrated against
narrow-polydispersity polystyrene standards. Preparative Gel perme-
ation chromatography utilized a Shimadzu recycling GPC system
running in hexane at 40 °C or chlorobenzene at 80 °C, using Agilent
PLgel 10 μm 50A or MIXED-D column, DGU-20A3 degasser, LC-
20A pump, CTO-20A column oven, and SPD-20A UV detector.
Electrospray mass spectrometry was performed with a Thermo
Electron Corp. DSQII mass spectrometer. UV−vis spectra were
recorded on a UV-1601 Shimadzu UV−vis spectrometer. Flash
chromatography (FC) was performed on silica gel (Merck Kieselgel 60
F254 230−400 mesh) or using a Biotage SNAP cartridge (KP-C18-
HS, 120 g) on a Biotage Isolera instrument. Photoelectron
spectroscopy in air (PESA) measurements were recorded with a
Riken Keiki AC-2 PESA spectrometer with a power setting of 5 nW
and a power number of 0.5. Samples for PESA were prepared on glass
substrates by spin-coating. Differential scanning calorimetry (DSC)
measurements, using ∼3 mg of material, were conducted under
nitrogen at scan rate of 10 °C/min with a TA DSC-Q20 instrument.
Films for Raman, GIWAXS, and ultraviolet photoemission spectros-
copy (UPS) measurements were prepared by spin-coating from hot
(ca. 150 °C) solution in 1,2,4-trichlorobenzene (TCB, 10 mg/mL) at
3000 rpm for F-P3OT and room-temperature TCB for P3OT. Films
for F-P3EHT and P3EHT were prepared from chloroform.
Grazing-incidence wide-angle X-ray scattering (GIWAXS) measure-
ments were performed at the SAXS/WAXS beamline at the Australian
Synchrotron,26 using 9 keV photons. 2D scattering patterns were
recorded by using a Dectris Pilatus 1M detector. The sample-to-
detector distance was calibrated using a silver behenate standard.
Scattering patterns were recorded as a function of X-ray angle of
incidence, with the angle of incidence varied from 0.05° below the
critical angle of the organic film to 0.2° above the critical angle. The
images reported were just above the critical angle, as identified by the
angle with the highest scattering intensity. Data acquisition times of 3 s
were used, with three 1 s exposures taken with offset detector positions
to cover gaps in the Pilatus detector. X-ray diffraction data are
expressed as a function of the momentum transfer in the directions
perpendicular to and parallel to the plane of the sample, qz and qxy,
respectively, which have a magnitude of (4π/λ) sin(θ), where θ is half
the corresponding scattering angle and λ is the wavelength of the
incident radiation.
Raman spectra were measured using a Renishaw inVia Raman
spectrometer with 785 nm diode laser excitation. Laser power at the
sample was 130 mW, focused to a 40 μm2 area. The photo-
luminescence background was subtracted from the spectra using a
polynomial baseline, and then the spectra were normalized to the main
peak. A Linkam THMS600 hot−cold cell purged with nitrogen was
used to prevent polymer degradation as well as to control the
temperature of the sample. For room-temperature measurements, the
total laser exposure time was 25 s, whereas the exposure time for
temperature-dependent spectra was 10 s. Starting from room
temperature, the sample was heated at 10 °C/min to 300 or 410 °C
and then held for 10 min before cooling at 10 °C/min. The
temperature was held for 1 min at every 10 °C interval in order to
measure spectra.
DFT calculations were carried out using the B3LYP hybrid
functional and the 6-31G* basis set in the GAUSSIAN09 software
package.27 Alkyl chains were replaced with a methyl group to simplify
calculations and reduce computational time. Structures were first
optimized, and then a frequency analysis was used to simulate Raman
activities in order to identify the Raman-active vibrational modes.
Potential energy scans were performed upon hexamers using the
redundant coordinate editor and scanning the central dihedral angle in
36 steps of 10° increments.
the backbone out of conjugation. For example, bromination of
the polymer backbone leads to a significant blue-shift in
absorption as a result of this twisting caused by the large steric
bulk of the bromo substituent.16 We were interested to
investigate the properties of fluorinated analogues of P3AT, in
which the polymer backbone itself was fluorinated (we note
that side-chain-fluorinated P3ATs have been previously
reported17). Due to the relatively small size of fluorine, we
expected that disruptive steric interactions with adjacent 3-
alkylthiophenes would be minimized, while the strong electron-
withdrawing nature of fluorine should result in an increase in
ionization potential. In addition, fluorine’s unusual combination
of being strongly inductively withdrawing while mesomerically
donating is known to exert a significant influence on inter- and
intramolecular packing in the solid state.18 The exact role of the
F atom in solid-state morphology is complex, although intra-
and intermolecular interactions between S and F atoms have
been suggested in some instances.19
There have been relatively few examples of either oligomeric
or polymeric materials containing fluorinated thiophene. This
paucity is probably related to the synthetic complexity of
obtaining fluorinated thiophene building blocks.20 Perfluori-
nated thiophene oligomers from the trimer to the hexamer have
been reported by Suzuki and co-workers.21 Fluorination was
shown to reduce intermolecular distances within the single
crystal in comparison to non-fluorinated polymers. Poly(3-
fluorothiophene) has been synthesized electrochemically as an
insoluble material displaying a higher oxidation potential than
polythiophene,22 while more recently the electropolymerization
of a singly fluorinated terthiophene derivative has been
reported.23 Co-polymers of 3-fluorothiophene or 3,4-difluor-
othiophene in combination with solubilizing co-monomers
have also been reported as possible transistor and solar cell
polymer materials.24
In this work we report the synthesis, properties, character-
ization, and OFET performance of regioregular poly(3-alkyl-4-
fluorothiophenes) with branched and straight alkyl side chains,
and compare these with their non-fluorinated counterparts. We
observe significant differences in the physical behavior of the
polymers, with backbone fluorination leading to an increase in
melting enthalpy and temperature, and a reduction in solubility.
These observations are rationalized by density functional theory
(DFT) calculations on hexameric oligomers, which suggest a
planarization of the polymer backbone upon fluorination.
Differences in the observed Raman spectra of thin films support
significant differences in backbone planarization. We also
observe notable increases in the polymer ionization potential
upon fluorination. Finally FET devices are fabricated,
demonstrating that backbone fluorination has a sizable
beneficial impact on charge transport.
EXPERIMENTAL SECTION
■
General. Reagents and chemicals were purchased from commercial
sources such as Aldrich and Acros etc. unless otherwise noted. 3-(2-
Ethylhexyl)thiophene (1c), 2,3,5-tribromo-4-hexylthiophene (2a),
2,3,5-tribromo-4-octylthiophene (2b), 3-bromo-4-hexyl-2,5-bis-
(trimethylsilyl)thiophene (3a), 3-fluoro-4-hexyl-2,5-bis(trimethylsilyl)-
thiophene (4a), 2,5-dibromo-3-fluoro-4-hexylthiophene (5a), and 2,5-
bis(trimethylsilyl)-3,4-dibromothiophene (7) were synthesized by the
reported methods.23,25
All reactions were carried out under argon using solvents and
reagents as commercially supplied, unless otherwise stated. N-
Fluorobenzenesulfonimide (NFSI) was dried overnight under high
vacuum before use. 1H, 19F, and 13C NMR spectra were recorded on a
FETs were fabricated on low-sodium glass with an interdigitated
electrode structure with a channel length and width of 20 μm and 1
B
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mm, respectively. Electrodes were fabricated from gold via photo-
lithography. Active layers were deposited via zone casting (5 mg/mL
solutions in 1,2-dichlorobenzene), solution temperature of 140 °C,
followed by thermal annealing at 140 °C. The process was performed
in nitrogen. For devices fabricated with P3HT, P3OT, and their
fluorinated variants, electronic-grade poly(methyl methacrylate)
(PMMA) dissolved in anhydrous n-butyl acetate (45 mg mL−1) was
spin-coated onto the films (PMMA thickness ≈ 550 nm). For P3EHT
and F-P3EHT, CYTOP (500 nm) was used as the dielectric layer.
Following dielectric deposition, the devices were thermally annealed
again (90 °C for 30 min). In the final step, gold contact electrodes
were evaporated through a shadow mask to complete the devices.
Devices were characterized in nitrogen using an Agilent 4155B
semiconductor parameter analyzer.
stirred for 15 min at −78 °C, NFSI (4.9 g, 17.2 mmol) in THF (50
mL) was slowly added, and the solution was stirred for 2 h at −78 °C
and then overnight with warming to room temperature. Water (100
mL) was added, and the mixture was extracted (3 × 100 mL hexane).
The combined organics were dried (MgSO4), filtered, and
concentrated under reduced pressure. The residue was purified by
silica gel chromatography (eluent: petroleum ether 40−60 °C) to
1
afford a pale yellow oil (2.7 g, yield: 52%). H NMR (400 MHz,
CDCl3) δ (ppm): 2.58−2.54 (m, 2H), 1.59−1.51 (m, 2H), 1.38−1.29
(m, 10H), 0.88 (t, J = 6.9 Hz, 3H), 0.34 (s, 9H), 0.33 (s, 9H). 13C
NMR (CDCl3, 100 MHz) δ (ppm): 163.4 (d, J = 261 Hz), 139.1 (d, J
= 24 Hz), 138.5, 122.0 (d, J = 30 Hz), 31.9, 30.8, 29.9, 29.4, 29.3, 28.2,
22.7, 14.1, −0.1, −0.5. 19F NMR (CDCl3) δ (ppm): −122.0. HRMS
(EI+) calcd for C18H35FSSi2: 358.1982; found: 358.1989.
Synthesis of 2,3,5-Tribromo-4-(2-ethylhexyl)thiophene (2c).
Br2 (12.5 mL, 244 mmol) was added dropwise to a solution of 3-(2-
ethylhexyl)thiophene (16 g, 81 mmol) in acetic acid (100 mL) in the
absence of light. After the addition of bromine, the mixture was stirred
at room temperature for 2 h and heated to 60 °C overnight. The
mixture was poured into ice water (300 mL) and neutralized with 5 M
NaOH solution. The mixture was extracted by hexane (3 × 100 mL).
The combined organics were washed by brine (2 × 200 mL) and water
(2 × 200 mL), dried (MgSO4), and filtered, and the solvent was
removed under reduced pressure. The residue was purified by silica gel
chromatography (eluent: petroleum ether 40−60 °C) to afford 2c as a
Synthesis of 3-Fluoro-4-(2-ethylhexyl)-2,5-bis-
(trimethylsilyl)thiophene (4c). A solution of n-BuLi (8.5 mL of a
2.5 M solution in hexanes, 21.3 mmol) was added dropwise to a
solution of 3c (8.1 g, 19.3 mmol) in THF (200 mL) at −78 °C. After
the solution was stirred for 15 min at −78 °C, NFSI (6.6 g, 23.2
mmol) in THF (60 mL) was slowly added, and the solution was
stirred for 2 h at −78 °C and then overnight with warming to room
temperature. Water (100 mL) was added, and the mixture was
extracted (3 × 100 mL hexane). The combined organics were dried
(MgSO4), filtered, and concentrated under reduced pressure. The
residue was purified by silica gel chromatography (eluent: petroleum
1
1
ether 40−60 °C) to afford a pale yellow oil (3.2 g, yield: 46%). H
pale yellow oil (28.6 g, yield: 81%). H NMR (400 MHz, CDCl3) δ
NMR (400 MHz, CDCl3) δ (ppm): 2.50 (d, J = 7.5 Hz, 2H), 1.65−
1.61 (m, 1H), 1.28−1.17 (m, 8H), 0.90−0.84 (m, 6H), 0.32 (s, 9H),
0.31 (s, 9H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 164.9 (d, J =
260 Hz), 139.3, 138.2 (d, J = 24 Hz), 119.3 (d, J = 30 Hz), 39.8, 32.7,
32.7, 28.8, 26.0, 23.0, 14.0, 11.0, 0.2, −0.6. 19F NMR (CDCl3) δ
(ppm): −119.5. HRMS (EI) [M + H] calcd for C18H36FSSi2:
359.2060; found: 359.2051.
(ppm): 2.57 (d, J = 7.3 Hz, 2H), 1.77−1.68 (m, 1H), 1.36−1.21 (m,
8H), 0.92−0.88 (m, 6H). 13C NMR (CDCl3, 100 MHz) δ (ppm):
140.8, 116.3, 109.6, 109.0, 38.9, 35.3, 32.5, 28.8, 25.7, 23.1, 14.2, 11.0.
HRMS (EI+) calcd for C12H17Br3S: 429.8601; found: 429.8615.
Synthesis of 3-Bromo-4-octyl-2,5-bis(trimethylsilyl)-
thiophene (3b). A solution of n-BuLi (29.1 mL of a 2.5 M solution
in hexanes, 72.7 mmol) was added dropwise to a solution of 2b (15 g,
34.6 mmol) in tetrahydrofuran (THF, 100 mL) at −78 °C. After the
solution was stirred for 15 min at −78 °C, chlorotrimethylsilane (9.7
mL, 76 mmol) was added in one portion. The cooling bath was
removed, and the reactant was allowed to warm to room temperature,
followed by stirring for 0.5 h at room temperature. Water (100 mL)
was added, and the mixture was extracted (3 × 100 mL hexane). The
combined organics were dried (MgSO4), filtered, and concentrated
under reduced pressure. The residue was purified by silica gel
chromatography (eluent: petroleum ether 40−60 °C) to afford 3b as a
Synthesis of 2,5-Dibromo-3-fluoro-4-octylthiophene (5b). A
solution of Br2 (0.79 mL, 15.3 mmol) in CH2Cl2 (10 mL) was added
dropwise to a solution of 4b (2.5 g, 7.0 mmol) in CH2Cl2 (50 mL) at 0
°C in the absence of light. The reaction was allowed to warm to room
temperature and stirred for 2 h. A saturated sodium sulfite solution (20
mL) was added. The aqueous phase was extracted with CH2Cl2 (2 ×
30 mL). The combined organic phase was dried (MgSO4), filtered,
and concentrated under reduced pressure. The residue was purified by
silica gel chromatography (eluent: petroleum ether 40−60 °C),
followed by preparative GPC in hexane to afford 5b as a colorless oil
1
pale yellow oil (12.8 g, yield: 88%). H NMR (400 MHz, CDCl3) δ
1
(ppm): 2.67−2.63 (m, 2H), 1.54−1.50 (m, 2H), 1.37−1.28 (m, 10H),
0.88 (t, J = 6.9 Hz, 3H), 0.38 (s, 9H), 0.33 (s, 9H). 13C NMR (CDCl3,
100 MHz) δ (ppm): 150.1, 139.2, 138.8, 121.7, 31.9, 31.1, 30.9, 30.1,
29.4, 29.3, 22.7, 14.2, 0.1, −0.8. HRMS (EI+) calcd for C18H35BrSSi2:
418.1181; found: 418.1181.
(2.2 g, yield: 86%). H NMR (400 MHz, CDCl3) δ (ppm): 2.52 (m,
2H), 1.59−1.44 (m, 2H), 1.30- 1.27 (m, 10H), 0.88 (t, J = 6.9 Hz,
3H). 13C NMR (CDCl3, 100 MHz) δ (ppm): 153.5 (d, J = 264 Hz),
132.3 (d, J = 23 Hz), 107.4 (d, J = 10 Hz), 89.4 (d, J = 23 Hz), 31.9,
29.3, 29.2, 29.1, 28.5, 27.1, 22.7, 14.1. 19F NMR (CDCl3) δ (ppm):
−124.2. HRMS (EI+) calcd for C12H17Br2FS: 369.9402; found:
369.9412.
Synthesis of 3-Bromo-4-(2-ethylhexyl)-2,5-bis-
(trimethylsilyl)thiophene (3c). A solution of n-BuLi (37.2 mL of
a 2.5 M solution in hexanes, 93 mmol) was added dropwise to a
solution of 2c (19.2 g, 44.3 mmol) in THF (100 mL) at −78 °C. After
the solution was stirred for 15 min at −78 °C, chlorotrimethylsilane
(12.4 mL, 98 mmol) was added in one portion. The cooling bath was
removed, and the reactant was allowed to warm to room temperature,
followed by stirring for 0.5 h at room temperature. Water (100 mL)
was added, and the mixture was extracted (3 × 100 mL hexane). The
combined organics were dried (MgSO4), filtered, and concentrated
under reduced pressure. The residue was purified by silica gel
chromatography (eluent: petroleum ether 40−60 °C) to afford 3c as a
Synthesis of 2,5-Dibromo-3-fluoro-4-(2-ethylhexyl)-
thiophene (5c). A solution of Br2 (0.72 mL, 14.1 mmol) in
CH2Cl2 (10 mL) was added dropwise to a solution of 4c (2.5 g, 7.0
mmol) in CH2Cl2 (50 mL) at 0 °C in the absence of light. The
reactant was then allowed to warm to room temperature and stirred
for 2 h. A saturated sodium sulfite solution (20 mL) was added. The
aqueous phase was extracted with CH2Cl2 (2 × 30 mL). The
combined organic phase was dried (MgSO4), filtered, and concen-
trated under reduced pressure. The residue was purified by silica gel
chromatography, (eluent: petroleum ether 40−60 °C), followed by
preparative GPC in hexane to afford 5c as a colorless oil (1.98 g, yield:
1
pale yellow oil (15.3 g, yield: 82%). H NMR (400 MHz, CDCl3) δ
1
(ppm): 2.65 (d, J = 7.6 Hz, 2H), 1.82−1.76 (m, 1H), 1.27−1.21 (m,
8H), 0.89−0.83 (m, 6H), 0.39 (s, 9H), 0.35 (s, 9H). 13C NMR
(CDCl3, 100 MHz) δ (ppm): 148.9, 139.8, 139.1, 122.3, 39.7, 35.3,
32.8, 29.0, 26.1, 23.1, 14.1, 11.3, 0.7, −0.8. HRMS (EI) [M + H] calcd
for C18H36BrSSi2: 419.1260; found: 419.1270.
Synthesis of 3-Fluoro-4-octyl-2,5-bis(trimethylsilyl)-
thiophene (4b). A solution of n-BuLi (6.3 mL of a 2.5 M solution
in hexanes, 15.7 mmol) was added dropwise to a solution of of 3b (6.0
g, 14.3 mmol) in THF (200 mL) at −78 °C. After the solution was
83%). H NMR (400 MHz, CDCl3) δ (ppm): 2.43 (d, J = 7.5 Hz,
2H), 1.63−1.58 (m, 1H), 1.31−1.18 (m, 8H), 0.90−0.82 (m, 6H). 13C
NMR (CDCl3, 100 MHz) δ (ppm): 153.6 (d, J = 264 Hz), 131.8 (d, J
= 23 Hz), 107.9 (d, J = 10 Hz), 89.3 (d, J = 23 Hz), 31.9, 29.3, 29.2,
29.1, 28.5, 27.1, 22.7, 14.1. 19F NMR (CDCl3) δ (ppm): −123.2.
HRMS (EI+) calcd for C12H17Br2FS: 369.9402; found: 369.9393.
Synthesis of 2,5-Bis(trimethylsilyl)-3-bromo-4-fluorothio-
phene (8). A solution of t-BuLi (8.0 mL of a 1.7 M solution in
pentane, 13.6 mmol) was added dropwise to a solution of 7 (5.0 g, 13
C
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Journal of the American Chemical Society
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Scheme 1. Synthetic Route A to Monomers and Polymers
mmol) in diethyl ether (150 mL) at −78 °C. After the solution was
stirred for 10 min at −78 °C, NFSI (4.22 g, 14.9 mmol) in THF (7
mL) was added dropwise, and the solution was stirred for 2 h at this
temperature and then overnight with warming to room temperature.
Water (100 mL) was added, and the mixture was extracted (3 × 100
mL diethyl ether). The combined organics were dried (MgSO4),
filtered, and concentrated under reduced pressure. The residue was
purified by reverse-phase chromatography (eluent: methanol) to afford
was extracted (Soxhlet) with methanol, acetone, hexane, chloroform,
and chlorobenzene. The chlorobenzene solution was concentrated and
precipitated into methanol, and the precipitant was filtered and dried
under vacuum to afford a dark solid (167 mg, yield: 30%, rr = 93%).
1H NMR (1,2-dichlorobenzene-d4, 400 MHz, 130 °C), δ (ppm): 2.99
(broad, 2H), 1.93−1.86 (broad, 2H), 1.58−1.46 (broad, 6H), 1.04
(broad, 3H).
Synthesis of Poly[3-fluoro-4-octylthiophene-2,5-diyl] (F-
P3OT). In a three-necked flask, a solution of 2,5-dibromo-3-fluoro-4-
octylthiophene (0.82 g, 2.20 mmol) in anhydrous THF (18 mL) was
cooled to 0 °C. A solution of isopropylmagnesium chloride/lithium
chloride complex (1.65 mL of a 1.3 M solution in THF, 2.14 mmol)
was added via syringe, and the cooling bath was removed. After the
mixture was stirred for 30 min at room temperature, the reaction was
heated to reflux for 30 min. The heating bath was lowered so that the
temperature dropped just below reflux, and the catalyst Ni(dppp)Cl2
(6.0 mg, 0.011 mmol) in THF (1 mL) was added in one portion. The
reaction was refluxed for 4 h and allowed to cool to room temperature.
The reaction mixture was precipitated into a well-stirred solution of
methanol (40 mL)/25%. HCl (10 mL) and stirred for 1 h. The
resulting suspension was filtered directly into a Soxhlet funnel. This
was extracted (Soxhlet) with methanol, acetone, hexane, chloroform,
and chlorobenzene. The chlorobenzene solution was concentrated and
precipitated into methanol, and the precipitant was filtered and dried
under vacuum to afford a dark solid (210 mg, 44%, Mn = 23K, PDI =
1.8, rr = 96%).1H NMR (1,1,2,2-tetrachloroethane-d4, 400 MHz, 130
°C), δ (ppm): 2.82 (broad, 2H), 1.75−1.72 (broad, 2H), 1.44−1.37
(broad, 10H), 0.96 (broad, 3H). 19F NMR (1,1,2,2-tetrachloroethane-
d4, 400 MHz, 130 °C) δ (ppm): −123.0.
Synthesis of Poly[3-fluoro-4-(2-ethylhexyl)thiophene-2,5-
diyl] (F-P3EHT). In a three-necked flask, a solution of 2, 5-
dibromo-3-fluoro-4-(2-ethylhexyl)thiophene (0.86 g, 2.31 mmol) in
anhydrous THF (18 mL) was cooled to 0 °C. A solution of
isopropylmagnesium chloride/lithium chloride complex (1.72 mL of a
1.3 M solution in THF, 2.24 mmol) was added via syringe, and the
cooling bath was removed. After the mixture was for 30 min at room
temperature, the reaction was heated to reflux for 30 min. The heating
bath was lowered so that the temperature dropped just below reflux,
and the catalyst Ni(dppp)Cl2 (6.2 mg, 0.012 mmol) in THF (1 mL)
was added in one portion. The reaction was refluxed for 4 h and
allowed to cool to room temperature. The reaction mixture was
precipitated into a well-stirred solution of methanol (40 mL)/25%
HCl (10 mL) and stirred for 1 h. The resulting suspension was filtered
directly into a Soxhlet funnel. This was extracted (Soxhlet) with
methanol, acetone, hexane, and chloroform. The chloroform solution
was concentrated and precipitated into methanol, and the precipitant
was filtered and dried under vacuum to afford a dark solid (307 mg,
62%, Mn = 28K, PDI = 1.3, rr = 96%). 1H NMR (1,1,2,2-
tetrachloroethane-d4, 400 MHz, 130 °C), δ (ppm): 2.68 (broad,
2H), 1.67 (broad, 1H), 1.33−1.24 (broad, 8H), 0.87−0.85 (broad,
6H). 19F NMR (1,1,2,2-tetrachloroethane-d4, 400 MHz, 130 °C) δ
(ppm): −121.6.
1
a colorless oil (2.1 g, yield: 50%). H NMR (400 MHz, CDCl3) δ
(ppm): 0.39 (s, 9H), 0.33 (s, 9H). 13C NMR (CDCl3, 100 MHz) δ
(ppm): 160.1 (d, J = 260 Hz), 139.6, 120.5 (d, J = 30 Hz), 107.7 (d, J
= 30 Hz), −0.8, −1.3. 19F NMR (CDCl3) δ (ppm): −117.7. HRMS
(EI) + calcd for C10H18BrFSSi2: 323.9835; found: 323.9830.
Synthesis of 3-Fluoro-4-octyl-2,5-bis(trimethylsilyl)-
thiophene (4b) (Route B). Octylmagnesium bromide (3.33 mL of
a 2 M solution in diethyl ether) was added to a stirred solution of zinc
chloride (6.66 mL of a 1 M solution in diethyl ether) under argon. The
solution was stirred for 30 min before being added to a dry 20 mL
high-pressure microwave reactor tube containing 8 (1.67 g, 5.13
mmol) and Pd(dppf)Cl2 (0.34 g, 0.46 mmol). The tube was sealed,
flushed with Ar, and heated overnight at 80 °C (oil bath temperature).
After cooling to room temperature, the mixture was passed through a 5
× 5 × 5 cm silicon plug (eluent: hexane), and the organic solvent was
concentrated under reduced pressure. The residue was purified by
reverse-phase chromatography (eluent: methanol) to afford 4b as a
pale yellow oil (0.79 g, 43%), which exhibited NMR spectra identical
to those reported earlier.
Synthesis of 3-Fluoro-4-(2-ethylhexyl)-2,5-bis-
(trimethylsilyl)thiophene (4c) (Route B). To a dry 20 mL high-
pressure microwave reactor tube were added 8 (1.0 g, 3.1 mmol) and
Pd(dppf)Cl2 (0.11 g, 0.15 mmol). The tube was sealed and flushed
with Ar, and then dry, degassed THF (10 mL) and (2-ethylhexyl)zinc
bromide (7.4 mL of a 0.5 M solution in THF, 3.7 mmol) were added.
The solution was thoroughly degassed under argon, and the reaction
was heated overnight at 80 °C (oil bath temperature). After cooling to
room temperature, the mixture was passed through a 5 × 5 × 5 cm
silicon plug (eluent: hexane), and the organic solvent was
concentrated under reduced pressure. The residue was purified by
reverse-phase chromatography (eluent: methanol) to afford 4c as a
pale yellow oil (0.34 g, 31%), which exhibited NMR spectra identical
to those reported earlier.
Synthesis of Poly[3-fluoro-4-hexylthiophene-2,5-diyl] (F-
P3HT). In a three-necked flask, a solution of 2,5-dibromo-3-fluoro-4-
hexylthiophene (1.02 g, 2.96 mmol) in anhydrous THF (20 mL) was
cooled to 0 °C. A solution of isopropylmagnesium chloride/lithium
chloride complex (2.21 mL of a 1.3 M solution in THF, 2.88 mmol)
was added via syringe, and the cooling bath was removed. After the
mixture was stirred for 30 min at room temperature, the reaction was
heated to reflux for 30 min. The heating bath was lowered so that the
temperature dropped just below reflux, and the catalyst Ni(dppp)Cl2
(8.0 mg, 0.015 mmol) in THF (1 mL) was added in one portion. The
reaction was refluxed for 4 h and allowed to cool to room temperature.
The reaction mixture was precipitated into a well-stirred solution of
methanol (40 mL)/25% HCl (10 mL) and stirred for 1 h. The
resulting suspension was filtered directly into a Soxhlet thimble. This
D
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tetrabromothiophene in two steps and could be separated from
the non-fluorinated byproduct by reverse-phase chromatog-
raphy in reasonable quantities. Surprisingly, 8 was unreactive to
the standard Kumada coupling conditions used for 3-
bromothiophene,28 so alkyl chains were incorporated by
Negishi cross-coupling with octyl or 2-ethylhexyl zinc bromide
in the presence of Pd(dppf)Cl2 as catalyst. Alkylation yields of
the branched side chain were slightly lower than the straight
chain analogue (31% and 43%, respectively), presumably due to
the larger steric demand of the (2-ethylhexyl)zinc reagent,
although a limited number of catalytic systems were screened,
so these yields could reasonably be increased.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of Monomers and
Polymers. We decided to focus upon the Grignard metathesis
(GRIM) route to synthesize the fluorinated polymers, because
of the well-known robustness and good control that this route
offers for P3HT.1b The critical building block was therefore the
synthesis of 2,5-dibromo-4-fluoro-3-alkythiophene. Fluorina-
tion of electron-rich aromatic thiophenes in the 3,4-positions is
non-trivial but has been achieved by the electrophilic
fluorination of lithiated thiophenes.21a Due to the propensity
of the 3- or 4-lithiated thiophenes to rearrange to the more
thermodynamically stable 2,5-positions, these positions must be
protected prior to reaction. Our initial synthesis route to 2,5-
dibromo-3-fluoro-4-alkylthiophene monomers is shown in
Scheme 1. A similar synthetic route to 3a has recently been
reported.23 We investigated two straight-chain derivatives, hexyl
and octyl, as well as a branched 2-ethylhexyl derivative.
3-Alkylthiophene was exhaustively brominated by treatment
with bromine to afford 2,3,5-tribromo-4-alkylthiophene (2a−
2c) in yields of 81−88%. Subsequent treatment with n-BuLi at
low temperature, followed by quenching with chlorotrimethyl-
silane afforded 3-bromo-4-alkyl-2,5-bis(trimethylsilyl)thiophene
(3a−3c) in typical yields of 82−88%. Then 3-fluoro-4-
alkylthiophene (4a−4c) was prepared by lithiation of 3a−3c,
followed by addition of N-fluorobenzenesulfonimide in yields
of 46−52%. The low yields for fluorination reactions are
principally attributed to the losses of material during
purification. Indeed, the major byproduct of these reactions
was the non-fluorinated analogue, which has only a slight
difference in retention factor from the product, rendering
purification through normal-phase chromatography particularly
time-consuming. For that reason, preparative GPC or reverse-
phase chromatography was used, the former being particularly
effective for separating 5 from 2 (byproduct from the
bromination of 4). Hence, mixtures of 4 containing small
amounts of the non-fluorinated byproduct were typically
brominated to give a mixture of 5 and 2, which could be
more readily separated. This route affords the monomers in a
moderate overall yield of 20−30%.
Polymerization was achieved by following a similar method
to the GRIM route developed for thiophene polymers.29
Interestingly the presence of the fluorine substituent has a
beneficial impact of the selectivity of the metathesis step.
Whereas treatment of 2,5-dibromo-3-octylthiophene with 0.97
equiv of isopropylmagnesium bromide/lithium chloride affords
an approximate 4:1 mixture of Grignard isomers in agreement
with the literature,30 reaction of 2,5-dibromo-3-fluoro-4-
octylthiophene afforded essentially 100% of a single Grignard
isomer (as observed by GCMS of a quenched sample,
Supporting Information, Figure S1.). A doublet peak at 6.62
1
(J = 1.2 Hz) ppm and at −125.9 ppm in H and 19F NMR,
respectively (Figures S2 and S3), suggested that metathesis had
occurred exclusively with the Br which is adjacent to the F atom
(as shown in Scheme 1). 2D HOESY NMR (1H and 19F NMR,
Figure S4) further confirmed the close spatial proximity of the
H and F. No quenched products arising from bis-Grignard
formation were detected. The formation of a single isomer of
the thienyl Grignard has previously been shown to be beneficial
in improving the control of the polymerization,31 although for
the non-fluorinated monomer this was achieved by differ-
entiation of the 2 and 5 positions of the thiophene ring with
different halogen substituents.
Addition of Ni(dppp)Cl2 to the hot Grignard solution
afforded rapid formation of the polymer F-P3OT. The same
procedure was applied to synthesize F-P3HT and F-P3EHT.
The crude polymers were purified by a combination of
precipitation and Soxhlet extraction to remove low-weight
oligomers and catalyst impurities. After removal of impurities
the polymers were extracted into chlorobenzene to isolate
soluble fractions. The resulting polymers had substantially
different solubility compared to their non-fluorinated ana-
logues, and F-P3HT and F-P3OT could only be dissolved in
chlorinated aromatic solvents at high temperature. The
branched F-P3EHT exhibited improved solubility and could
be dissolved in hot chloroform as well as chlorobenzene. The
yields of F-P3HT, F-P3OT, and F-P3EHT were 30%, 44%, and
62%, respectively. The low yields of F-P3HT and F-P3OT may
be attributed to precipitation of polymer chain from solvent
during polymerization progress.31
One difficulty with this initial route was the early-stage
introduction of the alkyl side chain, which meant that all four
steps needed to be repeated to change the side chain. Due to
tedious nature of the separation of the fluorinated monomer
from the non-fluorinated byproduct, we designed an alternate
synthesis route in which the alkyl side chains are introduced
after the fluorination step (Scheme 2). Here, the fluorinated
intermediate 8 was made from commercially available 2,3,4,5-
Scheme 2. Improved Synthetic Route B to Intermediates 4b
and 4c
F-P3OT and F-P3EHT had Mn = 23 kDa with Đ = 1.8 and
Mn = 28 kDa with Đ = 1.3, respectively (Table 1), as measured
by GPC in hot chlorobenzene (80 °C) against polystyrene
standards. However, the molecular weight of F-P3HT could not
be measured due to its poor solubility. In order to allow for a
direct comparison to the fluorinated polymers, similar
molecular weight P3OT and P3EHT were prepared by an
identical polymerization procedure and fractionated by
preparative GPC (chlorobenzene) to obtain comparable
molecular weights. The percentage of side-chain regioregularity
(rr) was measured by integration of the methylene signals in 1H
E
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interactions in the fluorinated polymer are increased compared
to those in the non-fluorinated polymer. Similar to F-P3HT, F-
P3OT also showed significantly higher melting temperatures
and crystallization enthalpy compared to P3OT. The melting
temperature and crystallization enthalpy of F-P3OT are both
decreased in comparison to those of F-P3HT as a result of the
increased side-chain length. Interestingly, both of the
fluorinated polymers display double endotherms upon melting,
which may indicate the presence of different polymorphs within
the sample.
Table 1. Molecular Weight, Polydispersity, and Percentage
Regioregularity of P3HT, F-P3HT, P3OT, F-P3OT, P3EHT,
and F-P3EHT
a
a
b
polymer
Mn (kDa)
Mw (kDa)
Đ
rr (%)
P3HT
39
55
1.4
97
93
95
96
97
96
c
c
c
F-P3HT
P3OT
−
−
−
19
23
29
28
26
41
44
37
1.4
1.8
1.5
1.3
F-P3OT
P3EHT
F-P3EHT
The difference in the thermal behavior of the branched EH
containing polymers is perhaps most striking. As has been
previously reported P3EHT exhibits two melting peaks in the
initial heating cycle, but forms a glass upon relatively rapid
cooling in the DSC.32 The glass slowly crystallizes over time as
a thin film, or by sustained (1 h) thermal annealing close to the
melting point.32 The slow crystallization kinetics have been
related to the irregular geometry of the side chains due to the
stereocenter in the EH group.33 However, upon fluorination,
the thermal behavior changed significantly, and F-P3EHT
showed a clear melting and crystallization peak, with an
increase in melting point of around 50 °C compared to that of
P3EHT. The thermal transitions of F-P3EHT are fully
reversible and reproducible in subsequent cycles, in contrast
to P3EHT.
Optical and Electronic Properties. The UV−visible
absorption spectra of the polymers in dilute 1,2-dichloroben-
zene (DCB) and hot DCB, and as spin-coated films from DCB,
are shown in Figure 2, and the data are summarized in Table 2.
In dilute DCB solution at room temperature, P3HT, P3OT,
P3EHT, and F-P3EHT all exhibit a single absorption peak
indicative of good dissolution, whereas the fluorinated straight-
chain polymers F-P3HT and F-P3OT show clear signs of
aggregation, with spectra that resemble those of the solid-state
films. Upon heating, the absorption maxima of P3HT, P3OT,
P3EHT, and F-P3EHT all blue shift slightly, indicating there is
not a strong aggregation between polymer backbones in dilute
solution. F-P3HT and F-P3OT show pronounced blue-shifts
(121 and 118 nm) upon heating, as the interaction between
polymer backbones is reduced, presumably due to increased
torsional twisting. F-P3HT still shows signs of aggregation even
in hot DCB solution, with small vibronic peaks observable at
539 and 584 nm. In addition the λmax of the branched polymers
are more blue-shifted in hot solution than either of their
straight chain analogues, suggesting the branched EH group
causes more perturbation to the conjugated backbone than the
straight chain thereby disrupting conjugation.
a
Measured by GPC in chlorobenzene at 80 °C relative to polystyrene
b
1
standards. Measured by integration of the methylene region by H
NMR. Molecular weight of F-P3HT could not be measured due to
low solubility.
c
NMR spectra (Figures S5−S7, S10−12) as shown in Table 1.
F-P3HT showed a relatively low rr degree of 93% likely due to
the low molecular weight and resulting end-group effects, while
F-P3OT and F-P3EHT exhibited high rr’s greater than 96%
(and comparable to those of the non-fluorinated polymers)
The 19F NMR spectrum (Figures S8 and S9) of F-P3OT and F-
P3EHT exhibited a single resonance, providing additional
evidence for the high degree of uniformity of the backbone of
these polymers.
Polymer Thermal Properties. The thermal properties of
the polymers were evaluated by DSC under nitrogen. The first
(P3EHT) and second (P3HT, F-P3HT, P3OT, F-P3OT, and
F-P3EHT) heating and cooling DSC curves are shown in
Figure 1. The detailed thermal data including melting and
The spectral shape of all four of the straight chain polymers is
very similar upon film formation, with the appearance of clearly
defined vibronic shoulders to the red of the maximum
absorption peak, which are indicative of intermolecular
interactions.34 For the fluorinated straight chain polymers,
film formation from hot DCB was difficult to control, and spin
coating from hot TCB was found to afford better
reproducibility. There was little difference in the spectra of
films obtained from DCB or TCB for either P3OT or F-P3OT
(Figure S13). Annealing of the films up to 125 °C did not result
in significant changes to the spectra, although there were some
subtle changes in the relative intensity of the long wavelength
shoulder peaks (Figures S14 and S15). The peaks for the
fluorinated polymers are slightly blue-shifted in comparison to
those for the non-fluorinated polymers, with an increased
optical band gap of 1.98 eV for the fluorinated straight chain
Figure 1. DSC heating and cooling traces of P3EHT (first cycle) and
P3HT, F-P3HT, P3OT, F-P3OT, and F-P3EHT (second cycle) at a
scanning speed of 10 °C/min under N2 (endo up). Note: traces offset
for clarity.
cooling peaks, as well as the corresponding enthalpy values are
presented in Table 2. F-P3HT exhibits two endothermic peaks
at 267 and 279 °C and a single exothermic peak upon cooling
at 245 °C, which are higher than those of P3HT (233 and 194
°C). In addition to raising the melting point, introducing the F
atom to the backbone has a clear influence on the crystallization
enthalpy, with F-P3HT exhibiting significantly higher values
than P3HT (ca. 32 J/g vs ca. 19 J/g). Although care needs to be
taken in the interpretation of polymer enthalpy values, this
significant increase suggests that attractive intermolecular
F
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Table 2. Physical Properties of P3HT, F-P3HT, P3OT, F-P3OT, P3EHT, and F-P3EHT
UV−vis (nm)
a
a
c
d
polymer
Tm (°C)
Tc (°C)
ΔH(Tc) (J/g)
RT DCB
hot DCB
452
film
Eg, opt (eV)
IP (eV)
P3HT
233
194
245
152
212
−
19
32
17
28
−
457
524, 558, 602
509, 538, 584
527, 559, 604
511, 539, 586
490, 526, 567
503, 539, 584
1.91
1.98
1.91
1.98
2.03
−4.64
−5.10
−4.70
−4.99
−4.95
−5.20
F-P3HT
P3OT
267, 279
184, 191
240, 252
507, 540, 584
419, 539, 584
455
449
416
432
F-P3OT
P3EHT
F-P3EHT
431, 507, 538, 583
b
56, 72
443
416
123
81
20
410
1.98
a
b
c
Melting temperature (Tm) and crystallization temperature (Tc) determined by DSC. Data from first cycle of DSC trace. Optical band gap
estimated from the low-energy band edge in the optical spectrum. Ionization potential level in thin film measured from PESA data (error 0.05
d
eV).
Figure 2. UV−vis absorption spectra of P3HT, F-P3HT, P3OT, F-P3OT, P3EHT, and F-P3EHT (a) in dilute dichlorobenzene solution at room
temperature and (b) in dichlorobenzene solution at 90 °C, (c) of as-spun films of P3HT, F-P3HT, P3OT, and F-P3OT, and (d) of as-spun films of
P3EHT and F-P3EHT.
derivatives in comparison to 1.91 eV for the non-fluorinated.
Again substantial differences are observed in the case of the
branched EH polymers. The spectrum of the as-cast P3EHT is
very similar to that reported previously,33,35 with a maximum
around 490 nm and two weak shoulders at higher wavelengths.
The band gap is also substantially wider than the straight chain
derivatives, at 2.03 eV, suggesting that conjugation is disrupted
in the solid state, most likely as a result of the increased size and
mixed stereoisomers of the side chain. In contrast, the as-cast F-
P3EHT spectrum is substantially different and displays a sharp
and strong vibronic shoulder of almost equal intensity to the
main absorption band. The red-shift between solution and solid
λmax at 123 nm is also substantially increased compared to the
47 nm observed for the non-fluorinated polymer, suggesting
that the fluorinated polymer is better able to planarize in the
solid state. The intensity of this vibronic shoulder versus the
main absorption peak has been related to the degree of
intermolecular ordering and aggregation in the thin film for
P3HT, according to the weakly interacting H-aggregation
model of Spano and co-workers.36 Although this may not be
directly applicable for the branched-side-chain polymer, the
strong vibronic peaks certainly suggests pronounced coupling
between the optical dipoles in the solid state and along with the
pronounced red-shift, suggesting that backbone fluorination
partly overcomes the propensity of the branched EH group to
perturb backbone planarity.
The energetics of the polymers in thin films were
investigated by a combination of PESA and UPS measurements
in ultra-high vacuum. The PESA measurements show a
consistent increase in ionization potential upon backbone
fluorination of around 0.3 eV, which we ascribe primarily due to
the strong electron-withdrawing influence of the F atom. The
effect of increasing side-chain length or branching on both of
the P3AT series is to increase ionization potential of the film. In
the case of F-P3HT, we believe the low molecular weight and
reduced rr may give an anomalously high ionization potential.
The branching of the side chain clearly causes an increase in
ionization potential for both the fluorinated and the non-
fluorinated analogues, although the magnitude is smaller for the
fluorinated polymer, in agreement with the idea that the
branched EH side chain increases backbone perturbation.
G
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In order to further investigate the energetics of the polymers
with and without fluorination, UPS studies were performed on
the P3OT and F-P3OT, with the spectra shown in Figure 3 and
lowest energy conformations to ensure the geometry was not
the result of a local minima. The minimum-energy
conformations of both hexamers are shown in Figure 4.
Figure 4. Top view of energy-minimized structures of a methyl-
substituted hexameric (a) P3AT and (b) F-P3AT. Side view of energy-
minimized structure of (c) a methyl-substituted P3AT and (d) F-
P3AT. HOMO (e) and LUMO (f) distributions for the energy-
minimized structure of F-P3AT. All calculated using DFT at the
B3LYP/6.31G(d,p) level.
Figure 3. Ultraviolet photoemission spectroscopy of spin-coated films
of P3OT and F-P3OT.
the energy band diagrams in Figure S16. In Figure 3a the UPS
spectra show the onset of the secondary edge, which is used to
determine the work function of the materials, as defined by the
energy difference between the Fermi level and the local surface
vacuum level. In Figure 3b,c the observed spectra are
representative of the local density of states associated with
the two polymer films, with the corresponding energies
reported with respect to the Fermi level. In Figure 3c, three
relevant energies have been identified: i.) the low-energy onset
of the easiest ionized, highest occupied states (∼0.5 eV below
the Fermi) associated with tailing intragap states and typically
used to define the polymer ionization potential; ii.) the onset of
the main HOMO-like feature (∼2.5 eV below Fermi),
attributed to the sp2-hybridized orbitals; and iii.) the local
maximum of the HOMO-like feature (∼3.5−3.6 eV below the
Fermi). We detect two observable differences upon fluorination
of the thiophene backbone. The first is the increase in the
polymer work function, from 3.8 to 4.2 eV (Figure 3a). In
Figure 3b, there is a change in the density of states for the two
peak-like features, one at ∼10 eV and the other at ∼7 eV below
the Fermi level; both of these regions are consistent with
fluorination (F 2p atomic orbitals). However, there is very little
observed difference in the valence band feature shape or
energies with respect to the Fermi level (Figure 3c), which is
suggestive of minimal change to the HOMO-like features of the
polymers. This is not surprising, given that the HOMO of
P3HT is well-known to be highly delocalized. When combining
the differences in the work function with the observed valence
features into a band diagram (Figure S16), we readily observe
that the major change in the ionization potential is due to the
change in the surface vacuum level, as manifested in the
reported change in the work function. However, it is important
to note that there were some subtle changes in the core levels,
as observed by XPS, which are given in the Supporting
Information (Figure S17).
Consistent with other reports,37 the calculations clearly show
that P3AT has a slightly twisted backbone conformation, with a
torsion angle (θ) of 14° between the central two thiophene
rings and slightly larger twist angles toward the terminal
thiophene rings. In contrast F-P3AT possesses a completely
planar backbone conformation, with all thiophene rings co-
planar to one another. Upon closer inspection of the Mulliken
̈
partial charges on the atoms in the model, we believe the origin
of this planarization could be electrostatic in nature, as the
positively charged sulfur atom (+0.308) and negatively charged
fluorine atom (−0.281) are in close proximity in F-P3AT. In
P3AT on the other hand, while the sulfur is still positively
charged (+0.264), the hydrogen is also positively charged
(+0.148), causing a slightly repulsive interaction and hence a
backbone twist.
To further investigate the conformational preference of the
fluorinated and non-fluorinated rings, we performed relaxed
potential energy scans (PES) for two model hexamers of head-
to-tail 3-methylthiophene, both fluorinated and non-fluori-
nated. We chose hexamers since they have been shown to be
much more representative of a polymer chain than a simple
dimer.37 Here the inter-ring C−C bond between the central
two thiophenes was fixed at a set angle (θ), and all other
degrees of freedom were allowed to relax to their potential
energy minima at a hybrid DFT B3LYP/6-31G* level of
calculation. The total energy for each conformer as a function
of interring bond angle is shown in Figure 5a, where 0°
corresponds to the cisoid configuration and 180° to the
transoid configuration. The PES profile for the non-fluorinated
hexamer is very similar to that previously reported by DFT for a
fully hexyl-substituted hexamer,37 and shows two energy
minima at a dihedral angles of 170° and 30° from the
cisoid co-planar configuration. The profile exhibits global
maxima at 90° when the thiophenes are fully twisted out of
conjugation, as well as a local maxima at 0° and 180° for the
fully co-planar cis or transoid conformations. The cisoid
conformation is higher in energy than the transoid due to steric
interaction between the methyl group and the 3-hydrogen of
the thiophene ring. Upon moving to the fluorinated hexamer,
we now find that the energy minima are at 180°, i.e. fully co-
Computational Studies. To help explain the influence of
fluorination on both the polymer energy levels and the physical
structure, DFT calculations of hexamers of P3AT and F-P3AT
were modeled using Gaussian at the B3LYP/6-31G* level. The
side chains were modified to methyl groups in order to simplify
the calculations. For each hexamer an all trans configuration
with respect to adjacent thiophenes was allowed to relax to its
energy minima. Frequency calculations were performed on the
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Figure 5. a) Potential energy distribution of P3AT and F-P3AT, where A = methyl. A dihedral angle of 0° corresponds to the cisoid conformation;
b) Relative probability densities for populating the energy distribution with a Boltzmann energy distribution at room temperature. A dihedral angel
of 0° corresponds to the transoid configuration.
Figure 6. (a) DFT-calculated Raman spectra of P3AT and F-P3AT, where A = methyl. (b) Experimental Raman spectra at room temperature of
P3OT and F-P3OT.
planar in the transoid conformation, and at 40°. These results
are in agreement with calculations upon 3,4-difluorothiophene
dimers, which also predict a fully co-planar transoid
conformation.38 The profile now exhibits three maxima, at
90° identical to the non-fluorinated hexamer as the rings twist
fully out of conjugation. Significantly, however, the fully co-
planar cisoid conformation (0°) now becomes a high-energy
conformation due the steric interaction between the methyl
group and the fluorine substituent. The energy barrier to rotate
through this conformation becomes significantly larger for the
case of the fluorinated hexamer versus the non-fluorinated (ca.
10 kJ/mol).
Another important difference is that fluorination significantly
steepens the potential energy surface in the regions around the
energy minima. This is shown in the transoid region in Figure
5b, where the degrees of freedom are populated with a
Boltzmann distribution at room temperature. This suggests that
the polymer backbone is significantly more co-planar and stiffer
upon fluorination.
Taken together, these data suggest, therefore, that fluorina-
tion of the thiophene backbone will lead to an increase in co-
planarity of the polymer backbone for sections where the
polymer is able to adopt an all trans configuration leading to a
more rigid backbone. A more rigid backbone would be in
agreement with the higher tendency to aggregate observed
experimentally, as well as the increased melting temperatures.
However, when the fluorinated polymer does adopt the cisoid
configuration of adjacent thiophenes, the barrier to reorganiza-
tion by rotation of the backbone is significantly larger, which
may hinder solid-state reorganization and crystallization. In
addition, the higher energy of the fully co-planar cisoid
conformation may prevent full planarization even in the solid
state when solid-state packing forces are known to promote
planarization of the thiophene oligomers.
Similar to P3AT (Figure S18), the HOMOs and LUMOs of
F-P3AT were predicted to be delocalized over the entire
conjugated backbones (Figure 4e,f). The HOMO is predom-
inantly aromatic, and the LUMO is mainly of quinoidal
character. The difference between P3AT and F-P3AT is that
the F atom contributes to both HOMO and LUMO, which
means that the F atom can affect both the HOMO and LUMO
levels. The calculated HOMO and LUMO of P3AT and F-
P3AT are −4.64 and −2.16 eV, and −4.78 and −2.40 eV,
respectively, which are in good agreement with the
experimentally observed trends by PESA and UPS.
Raman Investigations. The Raman spectra of polythio-
phene and related analogues have been extensively studied and
provide useful insight into the solid-state ordering of the
polymer.39 Figure 6 compares the simulated and measured
Raman spectra for P3OT and F-P3OT. The main Raman peaks
lie in the range 1300−1550 cm−1, which corresponds with
carbon bond stretching modes in conjugated structures. The
spectra of P3OT is very similar to that observed for P3HT, and
so the two strong peaks measured for P3OT are readily
identified as the collective backbone stretching modes of the
C−C (1381 cm−1) and CC (1445 cm−1) bonds. The peak
positions, relative intensities, and shapes of these modes are
strongly associated with the molecular conformation (primarily
co-planarity) of the conjugated polymer backbone.39
The fluorinated polymer also has two strong Raman peaks
centered at 1417 and 1492 cm−1. The natures of these
vibrational modes are ascertained by comparison with the DFT
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Figure 7. Raman spectra measured during heating from (a) 30 to 300 °C for P3OT and (b) 30 to 410 °C for F−P3OT. Normalized Raman spectra
for P3OT during (c) heating and (d) cooling. Normalized Raman spectra for F−P3OT during (e) heating and (f) cooling.
calculations, which show similar values. The calculated P3AT
has two peaks centered at 1357 and 1461 cm−1, where the latter
is stronger and also shows sub-peaks on the lower frequency
side, leading to a skewed peak shapethese characteristics
match well with the measured spectrum for P3OT. The
calculated spectrum for F-P3AT has a strong peak at 1389 cm−1
and two weaker peaks at 1454 and 1483 cm−1, where the 1454
cm−1 mode corresponds most closely with the shoulders on the
main mode observed for the P3AT spectrum, and the 1483
cm−1 mode is the main CC collective stretch shifted to
higher frequency. The 1454 cm−1 mode in the DFT spectrum
for F-P3AT is not represented in the experimental Raman
spectrum, which may be attributed to the fact that the
calculated Raman activity does not correspond directly to
experimentally measured Raman intensities. The modes
calculated at 1357 and 1389 cm−1 for P3AT and F-P3AT,
respectively, both correspond to a collective C−C bond stretch.
The effect of fluorination on the polythiophene Raman modes
can be summarized as a shift in both the C−C and CC
collective stretching modes to higher frequencies, and an
increase in the intensity of the C−C mode relative to the CC
mode. These effects are visible in both the simulated and
experimental spectra.
In previous studies an increase in the intensity of the C−C
mode relative to the CC mode has been associated with an
increased π electron density in the C−C bond resulted from
the increased co-planarity of the conjugated polymer back-
bone,39 providing further evidence that fluorination enhances
backbone planarity. The shift of the C−C mode to higher
frequency has also been associated with an increase in effective
conjugation length, but this is typically accompanied by a shift
of the CC mode to lower frequency. In this case the shift of
the CC mode from 1445 to 1492 cm−1 upon fluorination
suggests that the fluorine substitution results in an increase in
force constant for the CC bonds. This is consistent with
other theoretical studies showing that fluorination leads to
reduced lengths of the C−C and CC bonds.40 In addition to
this, the narrowing in FWHM of the CC mode peak from
26.0 cm−1 in P3OT to 13.1 cm−1 in F-P3OT suggests that the
fluorinated polymer sample contains much narrower distribu-
tion of molecular conformations, which is similarly consistent
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with the fluorinated polymer having a highly planar and rigid
polymer backbone.
Temperature-Dependent Raman Spectroscopy. Figure
7 shows the changes in the Raman spectra of P3OT and F-
P3OT films during heating from 30 to 300 °C (P3OT) or 410
°C (F-P3OT) under nitrogen. In both cases we observe an
overall reduction in the Raman scattering intensity and shifts in
the peak positions, this effect is significantly more pronounced
for P3OT than F-P3OT.
A clearer comparison is achieved by normalizing the spectra.
Figure 7c,d shows the normalized spectra for both heating and
cooling for P3OT. In this case we find a gradual reduction in
the relative intensity of the C−C mode and a shift to lower
frequency at high temperatures, which is consistent with
reduced molecular planarity at elevated temperature as a result
of thermally induced backbone twisting, similar to that
observed in other thiophene-based polymers.41 In addition,
we observe a pronounced shift in the position of the CC
peak from 1445 to 1469 cm−1 occurring between 260 and 270
°C, which reverses more gradually upon cooling (270−240
°C). This transition is indicative of a significant conformational
change and may represent the temperature at which the
energetic rotational barrier for the inter-thiophene bond is fully
overcome to allow full rotational freedom of the backbone. We
note the transition occurs at a higher temperature than the melt
transitions identified using DSC. The Raman spectrum of F-
P3OT shows a similar temperature dependence, as shown by
the normalized spectra in Figure 7e,f. In this case the C−C
mode again shifts to lower energy at high temperatures and
decreases in relative intensity, though now the CC peak
shifts toward lower energy (1492 to 1485 cm−1) at temper-
atures up to 300 °C before a sudden loss of intensity and a shift
to around 1505 cm−1 between 300 and 310 °C. This sharp
transition reverses more gradually upon cooling at 290−280
°C. This transition at 300−310 °C for F-P3OT shows the same
characteristics as that observed at 260−270 °C for P3OT, and
the increase in transition temperature for F-P3OT supports the
suggestion that the fluorinated polythiophene has a more planar
and rigid backbone with a greater inter-unit rotational barrier.
Thin-Film Morphology. Figure 8 shows the 2D GIWAXS
patterns of the fluorinated and non-fluorinated P3OT and
P3EHT films prepared by spin-coating. P3OT shows a series of
alkyl stacking reflections along qz corresponding to edge-on
oriented crystallites. F-P3OT also shows a series of alkyl
stacking reflections along qz however these peaks are broader
with an increased orientational distribution. The alkyl stacking
distance is determined to be 19.57 Å for P3OT with a slightly
smaller value of 19.07 Å for F-P3OT (see Table 3). Scherrer
analysis of the first alkyl stacking peak yields a coherence length
of 12.9 nm for P3OT, which is larger than that of F-P3OT (9.8
nm). Taken together, these observations indicate that
fluorination of P3OT frustrates crystallization with reduced
crystalline order, at least along the alkyl stacking direction. The
2D GIWAXS pattern of P3EHT, Figure 8c, is consistent with
previous observations with additional mixed-index peaks in
addition to the alkyl stacking peaks that are observed along qz.33
The scattering pattern of P3EHT has been indexed to a triclinic
unit cell rather than an orthorhombic cell typically observed for
poly(3-alkythiophenes).33 Similar to the case for P3OT,
fluorination of F-P3EHT leads to reduced crystalline order
with only weak scattering observed. The alkyl stacking distance
for F-P3EHT of 13.67 Å is similarly reduced compared to that
of P3EHT (14.33 Å). A dramatic decrease in the alkyl stacking
Figure 8. Two-dimensional GIWAXS images of spun-cast films of (a)
P3OT, (b) F-P3OT, (c), P3EHT, and (d) F-P3EHT. Films were
prepared from hot 1,2,4-trichlorobenzene (a,b) or hot chloroform
(c,d).
Table 3. Summary of GIWAXS Peak Fitting Results
alkyl-stacking
coherence
π-stacking
coherence
polymer
d-spacing (Å)
length (Å)
d-spacing (Å)
length (Å)
P3OT
19.57 0.01
19.07 0.02
14.33 0.01
129
98
2
2
2
2
3.88 0.002
3.91 0.002
3.54 0.002
3.82 0.002
68
55
28
1
2
3
F-P3OT
P3EHT
188
67
F-P3EHT 13.67 0.02
87 28
coherence length is also seen dropping from 18.8 nm for
P3EHT to 6.7 nm for F-P3EHT. Weak π-stacking peaks are
also observed in plane (along qxy) with π-stacking distances of
3.88 Å for P3OT and a slightly larger value of 3.91 Å for F-
P3OT. Coherence lengths of 6.8 nm are found for P3OT along
the π-stacking direction compared to 5.5 nm for F-P3OT. π-
stacking peaks are also observed for P3EHT and F-P3EHT,
although these are likely to be associated with the liquid
crystalline (rather than crystalline) component of the film.
Nevertheless, a π-stacking distance and corresponding coher-
ence length of 3.54 Å and 2.8 nm respectively are observed for
P3EHT, and 3.82 Å and 8.7 nm respectively for F-P3EHT.
The suppressed degree of thin-film crystalline order evident
upon fluorination could be the result of the reduced rotational
freedom of the fluorinated polythiophene backbone discussed
earlier. Thus, while the enhanced planarity of the all-trans
sections of fluorinated polymer might be expected to lead to
the signs of aggregation and short-range order apparent by
thermal and spectroscopic techniques, the higher rotational
barrier of the fluorinated polymers in a cis arrangement may
lead to “trapping” of the polymer is this conformation and
therefore reduced long-range order.
Charge Transport Behavior in Field Effect Transistors.
The charge carrier transport behavior was investigated in FETs
in a bottom contact, top gate configuration using gold source−
drain electrodes. Due to the poor solubility of the straight chain
fluorinated polymers, films were formed in all cases by zone-
casting from hot 1,2-dichlorobenzene under identical con-
ditions. The zone-casting technique gives excellent control over
the film formation, since both solvent temperature and
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substrate temperature can be readily controlled.42 Following
the semiconductor film deposition, the dielectric (PMMA) was
deposited by spin coating, followed by thermal evaporation of
the gate electrode. In the case of the P3EHT, the high solubility
of the polymer in the solvent used to deposit PMMA (n-
butylacetate) necessitated the use of a fluorinated dielectric,
Cytop, which could be deposited from a non-solvent for the
polymer. To allow for a consistent comparison, Cytop was
utilized for both P3EHT and F-P3EHT. The results of the
transistor measurements are summarized in Table 4, and the
individual transfer and output plots are shown in Figure 9 and
Figures S19−S22.
to differences in the molecular weight of the fluorinated and
non-fluorinated polymers. It is interesting that despite the
reduced crystalline order upon fluorination evident from the
GIWAXS data, fluorination clearly has a positive effect on
charge carrier mobility. The increased mobility is in agreement
with the evidence suggesting a more planar backbone
arrangement for the fluorinated materials, as a higher degree
of planarity can improve both intra- and intermolecular charge
transport. These results further confirm recent studies
suggesting that backbone rigidity and co-planarity are
important design features for high-performance transistor
materials.43
CONCLUSIONS
Table 4. Summary of Average and Peak FET Mobility
(Channel Length and Channel Width 20 μm and 1 mm,
Respectively)
■
We have reported two strategies toward the synthesis of the
useful building block 2,5-dibromo-3-alkyl-4-fluorothiophene.
The presence of the fluorine substituent on the thiophene was
shown to have a pronounced directing effect on the Grignard
metathesis reaction, leading to almost quantitative formation of
2-bromo-3-alkyl-4-fluoro-5-thienyl magnesium bromide, as
opposed to a roughly 4:1 mixture of the 2-bromo and 5-
bromo isomers in the absence of the fluorine group.
Polymerization of the Grignard monomer by the addition of
a Ni(II)dpppCl2 catalyst afforded highly regioregular polymers
of good molecular weight.
Comparison of the fluorinated polymers with their analogous
non-fluorinated counterparts demonstrates that fluorination
results in a considerable increase in polymer ionization
potential. The fluorinated and non-fluorinated polymers
exhibited very similar optical band gaps, indicating that
fluorination leads to a lowering of both the HOMO and
LUMO energy levels. Significant differences were observed in
the physical behavior of the polymers, with fluorination leading
to a reduction in polymer solubility, as well as substantial
increases in melting temperature and crystallization enthalpy.
These changes are consistent with a more planar, rigid polymer
μlin (cm2 V−1 s−1
)
μsat (cm2 V−1 s−1
)
material
average
maximum
average
maximum
P3HT
0.088
0.14
0.13
0.26
0.16
0.22
0.49
0.47
F-P3HT
P3OT
0.14
0.18
0.13
0.18
F-P3OT
0.70
0.81
0.44
0.53
a
P3EHT
4.35 × 10−4
2.46 × 10−3
1.48 × 10−3
6.45 × 10−3
6.91 × 10−4
3.09 × 10−3
1.84 × 10−3
6.66 × 10−3
a
F-P3EHT
a
Cytop was used as gate dielectric; all other examples utilized PMMA.
All transistors exhibited p-type behavior with negligible
operating hysteresis, good drain current saturation, and on/off
current ratios in the range of 104−105. From the transistor
measurements it is clear that fluorination generally results in a
significant increase in average charge carrier mobility for both
the linear and saturated mobility. This is most pronounced in
the comparison of the F-P3OT and F-P3EHT with their non-
fluorinated counterparts. In the case of the hexyl-substituted
polymers, the difference is more modest, which may be related
Figure 9. P3HT (top) and F-P3HT (bottom) transfer and output characteristics (channel length and width of 20 μm and 1 mm, respectively;
PMMA dielectric).
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(13) Politis, J. K.; Nemes, J. C. J. Am. Chem. Soc. 2001, 123, 2537.
(14) Heeney, M.; Zhang, W.; Crouch, D. J.; Chabinyc, M. L.;
Gordeyev, S.; Hamilton, R.; Higgins, S. J.; McCulloch, I.; Skabara, P. J.;
Sparrowe, D.; Tierney, S. Chem. Commun. 2007, 5061.
(15) Jahnke, A. A.; Djukic, B.; McCormick, T. M.; Buchaca Domingo,
E.; Hellmann, C.; Lee, Y.; Seferos, D. S. J. Am. Chem. Soc. 2013, 135,
951.
(16) (a) Li, Y.; Vamvounis, G.; Holdcroft, S. Macromolecules 2002,
35, 6900. (b) Koo, B.; Sletten, E. M.; Swager, T. M. Macromolecules
2015, 48, 229.
(17) (a) Li, L.; Counts, K. E.; Kurosawa, S.; Teja, A. S.; Collard, D.
M. Adv. Mater. 2004, 16, 180. (b) Li, L.; Collard, D. M. Macromolecules
2005, 38, 372. (c) Robitaille, L.; Leclerc, M. Macromolecules 1994, 27,
1847. (d) Hong, X. Y.; Tyson, J. C.; Middlecoff, J. S.; Collard, D. M.
Macromolecules 1999, 32, 4232.
(18) (a) Babudri, F.; Farinola, G. M.; Naso, F.; Ragni, R. Chem.
Commun. 2007, 1003. (b) Losurdo, M.; Giangregorio, M. M.;
Capezzuto, P.; Cardone, A.; Martinelli, C.; Farinola, G. M.; Babudri,
F.; Naso, F.; Buechel, M.; Bruno, G. Adv. Mater. 2009, 21, 1115.
(19) (a) Kim, B.-G.; Jeong, E. J.; Chung, J. W.; Seo, S.; Koo, B.; Kim,
J. Nat. Mater. 2013, 12, 659. (b) Gundlach, D. J.; Royer, J. E.; Park, S.
K.; Subramanian, S.; Jurchescu, O. D.; Hamadani, B. H.; Moad, A. J.;
Kline, R. J.; Teague, L. C.; Kirillov, O.; Richter, C. A.; Kushmerick, J.
G.; Richter, L. J.; Parkin, S. R.; Jackson, T. N.; Anthony, J. E. Nat.
Mater. 2008, 7, 216. (c) Crouch, D. J.; Skabara, P. J.; Lohr, J. E.;
McDouall, J. J. W.; Heeney, M.; McCulloch, I.; Sparrowe, D.;
Shkunov, M.; Coles, S. J.; Horton, P. N.; Hursthouse, M. B. Chem.
Mater. 2005, 17, 6567. (d) Subramanian, S.; Park, S. K.; Parkin, S. R.;
Podzorov, V.; Jackson, T. N.; Anthony, J. E. J. Am. Chem. Soc. 2008,
130, 2706.
backbone, which leads to enhanced aggregation in the solid
state. However, this enhanced aggregation does not lead to any
increase in thin-film crystalline order compared to that of the
non-fluorinated polymer, likely as a result of reduced rotational
freedom upon fluorination. Fluorination also results in a
significant increase in charge carrier mobility in transistor
devices, with average mobilities increasing 5-fold upon
fluorination to values around 0.7 cm2/(V s). We believe these
results demonstrate that backbone fluorination of polythio-
phene and its analogues is a useful strategy to optimize the
solid-state packing and performance in a range of optoelec-
tronic devices.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional figures as mentioned in the text, including GCMS,
monomer and polymer NMR, DFT calculations, UV−vis
spectra of annealed films, and FET plots. The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/jacs.5b02785.
AUTHOR INFORMATION
Corresponding Author
*m.heeney@imperial.ac.uk
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(20) Serdyuk, O.; Abaev, V.; Butin, A.; Nenajdenko, V. In Fluorine in
Heterocyclic Chemistry, Vol. 1; Nenajdenko, V., Ed.; Springer Interna-
tional Publishing: Berlin, 2014; p 233.
(21) (a) Sakamoto, Y.; Komatsu, S.; Suzuki, T. J. Am. Chem. Soc.
2001, 123, 4643. (b) Osuna, R. M.; Ortiz, R. P.; Ruiz Delgado, M. C.;
■
This work was supported by EPSRC grants EP/G060738/1
and EP/K029843/1, a KAUST Competitive Research grant
under agreement no. CRG-1-2012-THO-015, and the Austral-
ian Research Council (FT100100275, DP130102616). We
thank Dr. Scott E. Watkins (CSIRO Melbourne) for the PESA
measurements. Part of this research was undertaken on the
SAXS/WAXS beamline at the Australian Synchrotron, Victoria,
Australia.
Sakamoto, Y.; Suzuki, T.; Hernan
Chem. B 2005, 109, 20737.
́ ́
dez, V.; Lopez Navarrete, J. T. J. Phys.
(22) El Kassmi, A.; Fache, F.; Lemaire, M. J. Electroanal. Chem. 1994,
373, 241.
(23) Gohier, F.; Frere, P.; Roncali, J. J. Org. Chem. 2013, 78, 1497.
̀
(24) (a) Crouch, D. J.; Sparrowe, D.; Heeney, M.; McCulloch, I.;
Skabara, P. J. Macromol. Chem. Phys. 2010, 211, 2642. (b) Huang, L.;
Yang, D.; Gao, Q.; Liu, Y.; Lu, S.; Zhang, J.; Li, C. Chin. J. Chem. 2013,
31, 1385. (c) Fei, Z. P.; Shahid, M.; Yaacobi-Gross, N.; Rossbauer, S.;
Zhong, H. L.; Watkins, S. E.; Anthopoulos, T. D.; Heeney, M. Chem.
Commun. 2012, 48, 11130. (d) Jo, J. W.; Jung, J. W.; Wang, H.-W.;
Kim, P.; Russell, T. P.; Jo, W. H. Chem. Mater. 2014, 26, 4214.
(25) Heeney, M.; Farrand, L.; Giles, M.; Thompson, M.; Tierney, S.;
Shkunov, M.; Sparrowe, D.; McCulloch, I. U.S. Patent 6676857 B2,
2004.
REFERENCES
■
(1) (a) Mishra, A.; Ma, C. Q.; Bauerle, P. Chem. Rev. 2009, 109,
1141. (b) Osaka, I.; McCullough, R. D. Acc. Chem. Res. 2008, 41, 1202.
(c) Zhang, L.; Colella, N. S.; Cherniawski, B. P.; Mannsfeld, S. C. B.;
Briseno, A. L. ACS Appl. Mater. Interfaces 2014, 6, 5327.
(2) Nielsen, C. B.; McCulloch, I. Prog. Polym. Sci. 2013, 38, 2053.
(3) Dang, M. T.; Hirsch, L.; Wantz, G. Adv. Mater. 2011, 23, 3597.
(4) McCullough, R. Adv. Mater. 1998, 10, 93.
(5) Okamoto, K.; Luscombe, C. K. Polym. Chem. 2011, 2, 2424.
(6) Bannock, J. H.; Krishnadasan, S. H.; Nightingale, A. M.; Yau, C.
P.; Khaw, K.; Burkitt, D.; Halls, J. J. M.; Heeney, M.; de Mello, J. C.
Adv. Funct. Mater. 2013, 23, 2123.
(26) Kirby, N. M.; Mudie, S. T.; Hawley, A. M.; Cookson, D. J.;
Mertens, H. D. T.; Cowieson, N.; Samardzic-Boban, V. J. Appl.
Crystallogr. 2013, 46, 1670.
(27) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Frisch, M. J.;
Trucks, G.; Schlegel, H. B.; Scuseria, G.; Robb, M.; Cheeseman, J.;
Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.; et al. Gaussian
09; Gaussian, Inc.: Wallingford, CT, 2009.
(28) Pham, C. V.; Mark, H. B.; Zimmer, H. Synth. Commun. 1986,
16, 689.
(7) Liu, J.; McCullough, R. D. Macromolecules 2002, 35, 9882.
(8) (a) Sheina, E. E.; Liu, J.; Iovu, M. C.; Laird, D. W.; McCullough,
R. D. Macromolecules 2004, 37, 3526. (b) Yokoyama, A.; Miyakoshi,
R.; Yokozawa, T. Macromolecules 2004, 37, 1169.
(9) (a) Bhatt, M. P.; Magurudeniya, H. D.; Rainbolt, E. A.; Huang,
P.; Dissanayake, D. S.; Biewer, M. C.; Stefan, M. C. J. Nanosci.
Nanotechnol. 2014, 14, 1033. (b) Bryan, Z. J.; McNeil, A. J.
Macromolecules 2013, 46, 8395.
(29) Loewe, R. S.; Khersonsky, S. M.; McCullough, R. D. Adv. Mater.
1999, 11, 250.
(30) Loewe, R. S.; Ewbank, P. C.; Liu, J.; Zhai, L.; McCullough, R. D.
Macromolecules 2001, 34, 4324.
(31) Boyd, S. D.; Jen, A. K. Y.; Luscombe, C. K. Macromolecules
2009, 42, 9387.
(10) (a) McCulloch, I.; Bailey, C.; Giles, M.; Heeney, M.; Love, I.;
Shkunov, M.; Sparrowe, D.; Tierney, S. Chem. Mater. 2005, 17, 1381.
(b) Abdou, M. S. A.; Orfino, F. P.; Son, Y.; Holdcroft, S. J. Am. Chem.
Soc. 1997, 119, 4518.
(32) Ho, V.; Boudouris, B. W.; Segalman, R. A. Macromolecules 2010,
(11) Marrocchi, A.; Lanari, D.; Facchetti, A.; Vaccaro, L. Energy
Environ. Sci. 2012, 5, 8457.
43, 7895.
(12) Guo, X.; Baumgarten, M.; Mullen, K. Prog. Polym. Sci. 2013, 38,
1832.
(33) Boudouris, B. W.; Ho, V.; Jimison, L. H.; Toney, M. F.; Salleo,
A.; Segalman, R. A. Macromolecules 2011, 44, 6653.
̈
M
DOI: 10.1021/jacs.5b02785
J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
(34) (a) Brown, P. J.; Thomas, D. S.; Kohler, A.; Wilson, J. S.; Kim,
̈
J.-S.; Ramsdale, C. M.; Sirringhaus, H.; Friend, R. H. Phys. Rev. B 2003,
67, No. 064203. (b) Hu, Z. J.; Adachi, T.; Haws, R.; Shuang, B.; Ono,
R. J.; Bielawski, C. W.; Landes, C. F.; Rossky, P. J.; Vanden Bout, D. A.
J. Am. Chem. Soc. 2014, 136, 16023.
(35) Burkhart, B.; Khlyabich, P. P.; Thompson, B. C. Macromolecules
2012, 45, 3740.
(36) (a) Clark, J.; Silva, C.; Friend, R. H.; Spano, F. C. Phys. Rev. Lett.
2007, 98, No. 206406. (b) Clark, J.; Chang, J.-F.; Spano, F. C.; Friend,
R. H.; Silva, C. Appl. Phys. Lett. 2009, 94, No. 163306.
(37) Darling, S. B.; Sternberg, M. J. Phys. Chem. B 2009, 113, 6215.
(38) Salzner, U. J. Phys. Chem. A 2010, 114, 5397.
(39) (a) Tsoi, W. C.; James, D. T.; Kim, J. S.; Nicholson, P. G.;
Murphy, C. E.; Bradley, D. D. C.; Nelson, J.; Kim, J.-S. J. Am. Chem.
Soc. 2011, 133, 9834. (b) Tsoi, W. C.; James, D. T.; Domingo, E. B.;
Kim, J. S.; Al-Hashimi, M.; Murphy, C. E.; Stingelin, N.; Heeney, M.;
Kim, J.-S. ACS Nano 2012, 6, 9646.
(40) Raya, A.; Mora, M. A. Polymer 2004, 45, 6391.
(41) Garreau, S.; Leclerc, M.; Errien, N.; Louarn, G. Macromolecules
2003, 36, 692.
(42) (a) Pisula, W.; Menon, A.; Stepputat, M.; Lieberwirth, I.; Kolb,
U.; Tracz, A.; Sirringhaus, H.; Pakula, T.; Mullen, K. Adv. Mater. 2005,
17, 684. (b) Duffy, C. M.; Andreasen, J. W.; Breiby, D. W.; Nielsen, M.
M.; Ando, M.; Minakata, T.; Sirringhaus, H. Chem. Mater. 2008, 20,
7252. (c) James, D. T.; Frost, J. M.; Wade, J.; Nelson, J.; Kim, J. S. ACS
Nano 2013, 7, 7983.
(43) (a) Zhang, X.; Bronstein, H.; Kronemeijer, A. J.; Smith, J.; Kim,
Y.; Kline, R. J.; Richter, L. J.; Anthopoulos, T. D.; Sirringhaus, H.;
Song, K.; Heeney, M.; Zhang, W.; McCulloch, I.; DeLongchamp, D.
M. Nat. Commun. 2013, 4, 2238. (b) Fei, Z.; Pattanasattayavong, P.;
Han, Y.; Schroeder, B. C.; Yan, F.; Kline, R. J.; Anthopoulos, T. D.;
Heeney, M. J. Am. Chem. Soc. 2014, 136, 15154. (c) Noriega, R.;
Rivnay, J.; Vandewal, K.; Koch, F. P. V.; Stingelin, N.; Smith, P.;
Toney, M. F.; Salleo, A. Nat. Mater. 2013, 12, 1038. (d) Venkateshvar-
an, D.; Nikolka, M.; Sadhanala, A.; Lemaur, V.; Zelazny, M.; Kepa, M.;
Hurhangee, M.; Kronemeijer, A. J.; Pecunia, V.; Nasrallah, I.;
Romanov, I.; Broch, K.; McCulloch, I.; Emin, D.; Olivier, Y.; Cornil,
J.; Beljonne, D.; Sirringhaus, H. Nature 2014, 515, 384.
N
DOI: 10.1021/jacs.5b02785
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