Fluorinated Dithienylethene-Naphthalenediimide Copolymers for High-Mobility n-Channel Field-Effect Transistors

Article abstract of DOI:10.1021/acs.macromol.7b01169

We develop two donor-acceptor copolymers based on a fluorinated dithienylethene building block, namely PNFDTE1 and PNFDTE2, in which naphthalenediimide (NDI) acts as an acceptor unit. Thermogravimetric analysis displayed both copolymers having good thermal stability with high decomposition temperatures over 400 °C. Broad absorption spectra were observed in the UV-vis-NIR region, with the absorption maxima being 720 and 724 nm for PNFDTE1 and PNFDTE2, respectively. Cyclic voltammetry tests exhibited deep-lying lowest unoccupied molecular orbital energy levels of ca. -4.0 eV. Two-dimensional grazing incidence X-ray diffraction patterns showed that different packing modes for two polymers result in the variation in charge transport properties. Backbone fluorination effectively decreases electron injection barrier, thereby facilitating electron mobility. An impressive electron mobility of 3.20 cm² V^-1 s^-1 was achieved in air for PNFDTE1-based polymer field-effect transistors fabricated on the poly(ethylene terephthalate) substrate. The mobility value is almost the highest for NDI-containing polymers on the flexible substrate. This work provides a guideline for design and synthesis of fluorinated semiconductors that enables control of charge-transport polarity.

Full text of DOI:10.1021/acs.macromol.7b01169

Article
pubs.acs.org/Macromolecules
Fluorinated Dithienylethene−Naphthalenediimide Copolymers for
High-Mobility n‑Channel Field-Effect Transistors
Zhihui Chen,†,‡ Weifeng Zhang,*,† Jianyao Huang,† Dong Gao,†,‡ Congyuan Wei,† Zuzhang Lin,†,§
Liping Wang,§ and Gui Yu*,†,‡
†Organic Solids Laboratory, CAS Research/Education Center for Excellence in Molecular Sciences, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
‡University of Chinese Academy of Sciences, Beijing 100049, China
§School of Material Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
S
* Supporting Information
ABSTRACT: We develop two donor−acceptor copolymers based on a
fluorinated dithienylethene building block, namely PNFDTE1 and
PNFDTE2, in which naphthalenediimide (NDI) acts as an acceptor unit.
Thermogravimetric analysis displayed both copolymers having good
thermal stability with high decomposition temperatures over 400 °C.
Broad absorption spectra were observed in the UV−vis−NIR region, with
the absorption maxima being 720 and 724 nm for PNFDTE1 and
PNFDTE2, respectively. Cyclic voltammetry tests exhibited deep-lying
lowest unoccupied molecular orbital energy levels of ca. −4.0 eV. Two-
dimensional grazing incidence X-ray diffraction patterns showed that
different packing modes for two polymers result in the variation in charge
transport properties. Backbone fluorination effectively decreases electron
injection barrier, thereby facilitating electron mobility. An impressive
electron mobility of 3.20 cm2 V−1 s−1 was achieved in air for PNFDTE1-based polymer field-effect transistors fabricated on the
poly(ethylene terephthalate) substrate. The mobility value is almost the highest for NDI-containing polymers on the flexible
substrate. This work provides a guideline for design and synthesis of fluorinated semiconductors that enables control of charge-
transport polarity.
impart better air stability of devices by resisting the penetration
of oxygen or water.24,25 Inspired by the success of fluorination
in small-molecule semiconductors, further investigations on
fluorine-containing polymers have been a hot research topic.
Recent studies have revealed that fluorine-induced intra-
molecular interactions including S···F and H···F can promote
backbone planarity, enhance the frontier orbital overlaps, tune
the solubility, and change the packing motif of chains.1,4,23,26−30
Therefore, controlling the conjugated backbone conformations
of polymer chains with such noncovalent interactions serves as
a useful tool to better understand the charge transport
mechanism, thereby providing new rational design strategies
toward high-mobility materials.31−35 In this regard, it is
worthwhile to note that the fluorination position is one of
the key factors to conformational control. In our early studies,
we designed and synthesized two fluorinated diphenylethene-
based copolymers.26 Their optical and electrical properties
suggest that different positions of fluorine can induce diverse
conformational locks, thus predominantly affecting the
INTRODUCTION
■
Backbone fluorination has evolved as a strategic molecular
design method in organic electronics.1−16 Since the early 2000s,
many efforts have been made to develop high-mobility
semiconducting materials with fluorine substituents for organic
field-effect transistors (FETs).17−21 Diverse fluorinated materi-
als have been synthesized to realize the substituent effects on
electronic structures and crystallization behaviors. The extreme
electronegativity of fluorine atoms pulls electrons from the
backbone to stabilize frontier molecular orbitals, facilitating
electron transporting characteristics and resulting in a change in
intermolecular interactions. For example, perfluorination has
proven to be an effective way to convert a p-type semi-
conductor to an n-type or ambipolar one by greatly lowering
the lowest unoccupied molecular orbital (LUMO) energy
level.18 The quadropole moment of perfluorinated acene is
opposite in sign compared with that of acene, thus significantly
tuning the orbital energetics and intermolecular overlaps.22 In
terms of crystal engineering, the stacking of fluorinated
molecules differs from that of unsubstituted ones, which is
experimentally confirmed by single crystal characterization and
theoretically assessed by energy surface potential analysis.23
The hydrophobic nature of fluorinated compounds can also
Received: June 3, 2017
Revised: July 23, 2017
© XXXX American Chemical Society
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Scheme 1. Synthetic Routes to FDTE-Based Copolymers, PNFDTE1 and PNFDTE2
molecular aggregation in solid state. Bazan and co-workers also
systematically studied the influence of fluorine substitution on
benzothiadiazole-based polymers, indicating that different
substitution positions and regioregularities contribute greatly
to the electrical properties.28,36 In addition, the degree of
fluorination also enables the modulation of frontier orbital
energy levels and bandgaps.3 Generally, taking into account the
position and degree of fluorination is of great significance to
obtain high-performance polymer semiconductors.
region. Both copolymers have deep-lying LUMO energy levels
of ca. −4.0 eV. PNFDTE1-based top-gate/bottom-contact
(TGBC) polymer field-effect transistors (PFETs) on the
poly(ethylene terephthalate) substrate exhibited enhanced
electron mobility of up to 3.20 cm2 V−1 s−1 in air; meanwhile,
PNFDTE2-based devices only showed lower electron mobility
of 0.266 cm2 V−1 s−1. Further thin film microstructure
characterization reveals that the difference in mobility might
originate from their different molecular aggregations in thin
films. We have also discussed how fluorination affects the
optical and electrical properties compared with unfluorinated
polymers. Our results demonstrate the great potential of FDTE
unit in constructing high-performance polymer semiconductors
and offer insights into the tuning of molecular aggregation and
field-effect property.
Vinylene unit is a versatile linker to construct highly planar
building blocks.37−39 The vinylene linker minimizes the
torsional energy and extends the π-conjugation, which
planarizes the polymer backbone and leads to enhanced
intrachain charge transport properties. A good example is
dithienylethene (DTE). DTE has been used as the electron-
donating building block in many imide- or amide-containing
high-mobility semiconducting polymers.40−42 The naphthale-
nediimide (NDI)−DTE-derived copolymers exhibited good
ambipolar charge transport behaviors with electron mobility of
1.57 cm2 V−1 s−1 and hole mobility of 0.3 cm2 V−1 s−1.37 To
obtain better electron transport properties, we herein design
and synthesize two fluorinated DTE (FDTE)-based copoly-
mers, namely PNFDTE1 and PNFDTE2, in which the FDTE
unit acts as the donor unit and NDI acts as the acceptor unit.
Fluorination of DTE at the 3,3′-positions provides three major
advantages: (1) fluorination slightly stabilizes both the highest
occupied molecular orbital (HOMO) and LUMO and
minimally changes the packing motif because of the small
radius of fluorine; (2) it could induce H···F intramolecular
interactions, thus affording enhanced planarity and backbone
rigidity; (3) it would not introduce new steric hindrance
between thiophene−NDI subunits. (If the fluorine atoms were
introduced into the 4,4′-positions of DTE unit, electrostatic
repulsion would occur between the fluorine atom and exocyclic
carbonyl group.) As expected, the two FDTE-based copolymers
show the dual-band absorption profiles in the UV−vis−NIR
RESULTS AND DISCUSSION
■
Synthesis of PNFDTE. The synthetic routes of FDTE-
based monomer and copolymers are shown in Scheme 1, and
synthetic procedures are provided in the Expermental Section.
Owing to the requirement of selective fluorination, the
synthesis of FDTE involved more steps than those of (E)-
1,2-bis(3,4-difluorothien-2-yl)ethene.43 Thiophene-2-carboxylic
acid (1) reacted with 2 equiv of n-BuLi and was then quenched
with (PhSO2)2NF (NSFI) to give 3-fluorothiophene-2-
carboxylic acid (2) in a moderate yield.44 After esterification,
reduction reaction with DIBAL-H, and oxidation reaction with
MnO2, intermediate 2 was transformed to 3-fluorothiophene-2-
carbaldehyde (5). The reductive coupling reaction of aldehyde
5 under McMurry conditions afforded the desired FDTE as a
white crystalline solid.45,46 According to a conventional
method, FDTE was converted to FDTE-based ditin monomer
(7) in 76% yield.47,48 The two FDTE-based donor−acceptor
copolymers were then readily prepared by Stille-coupling
polymerization reaction. The crude polymer materials were
purified by Soxhlet extraction and recrystallized from CHCl3/
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MeOH to give the desired copolymers in high yields. The two
FDTE-based copolymers have good solubility in warm
chlorinated solvents such as chlorobenzene, o-dichlorobenzene,
and even chloroform (ca. 10 mg mL−1). Their chemical
structures and degree of polymerization were examined by
high-temperature NMR and high-temperature gel permeation
chains should be the major contribution to electrical transport.
Therefore, a highly self-assembled crystalline structure with
tight packing could facilitate the charge transport of NDI-based
copolymers.51 The effect of fluorination can also be precisely
illustrated in frontier molecular orbital distributions. The
energy levels of frontier molecular orbitals dropped by 0.1−
0.2 eV through fluorination according to DFT calculations.
Notably, the fluorine atoms mainly give contribution to the
HOMO, thereby slightly lowering the HOMO energy level and
resulting in an increased HOMO−LUMO gap. As is consistent
from the optical bandgap, fluorination of the donor can fine-
tune the energy gaps.37
1
chromatography, respectively. The H NMR spectra of both
copolymers are depicted in the Supporting Information and
match well with their respective chemical structures. Both
polymers exhibit high degree of polymerization with the
number-average molecular weight (Mn) and polydispersity
indices (PDIs) of 38.9 kDa and 2.31 for PNFDTE1 and 38.2
kDa and 3.54 for PNFDTE2. The thermal behaviors of both
copolymers were evaluated by thermogravimetric analysis
(TGA). The resulting TGA traces are collected in Figures S1.
Both copolymers have good thermal stability with high
decomposition temperatures of 420 and 409 °C for PNFDTE1
and PNFDTE2, respectively.
Optical Properties. Figure 2 exhibits the UV−vis−NIR
absorption spectra of the two polymers in dilute chlorobenzene
solution and in thin film. The corresponding optical data of two
FDTE-based copolymers are listed in Table 1. Both copolymers
show typical dual-band absorption profiles. The low-energy
absorption maxima in solutions are 716 and 710 nm for
PNFDTE1 and PNFDTE2, respectively. As a result of the
planarization effect on polymer backbones in thin films, the
absorption maxima increase to 720 and 724 nm. Moreover, the
low-energy bands broaden significantly in thin film, with the
absorption edges red-shifting to ca. 850 nm. The low-energy
bands exhibit significant 0−0 and 0−1 vibronic features caused
by aggregation.51,52 It is worth noting that the oscillator
strengths of the low-energy bands and the relative strength of
0−0 and 0−1 features are different for PNFDTE1 and
PNFDTE2. The variation is related to the strength of
intermolecular coupling of aggregates, which is caused by the
different packing modes.52,53 PNFDTE1 tends to form highly
aggregated structures in both solution and thin film, as is
evident from the strong vibronic coupling peaks. The calculated
optical bandgaps are 1.41 and 1.44 eV for PNFDTE1 and
PNFDTE2, respectively. We notice that the optical bandgaps
are slightly larger compared with those of previously reported
PNVT polymers without fluorine atoms.37 The result is in good
agreement with theoretical modeling that fluorination on the
donor units has a more significant effect on the HOMO than
the LUMO. As shown in Figure S2, temperature-dependent
UV−vis−NIR absorption spectra were performed to illustrate
the aggregation tendencies. The shoulder peak becomes weak
as the temperature increases, with a slight blue-shift occurring.
Such phenomena are originated from the free rotation of
flexible single bonds and partial dissociation of aggregates. We
emphasize that there exists a certain extent of aggregation in
hot solution of PNFDTE1, revealing an intense self-aggregation
feature of this polymer that may promote solid-state packing.
Theoretical Modeling. We performed density functional
theory (DFT) calculations to illustrate the backbone planarity
and frontier molecular orbitals of PNFDTE polymers. As
shown in Figure 1, HOMO and LUMO are exclusively
Figure 1. Frontier molecular orbitals of PNFDTE trimers.
localized on the FDTE and NDI units, respectively,
demonstrating the significant charge transfer between the
FDTE and NDI units in the HOMO−LUMO transition.49,50
The dihedral angle between the NDI and FDTE is ca. 45°,
indicating the presence of steric hindrance. The extent of
frontier molecular orbital delocalization is one of the primary
parameters to judge the possibility of intramolecular charge
transport. Conformational traps within the chains hinder the
efficient charge hopping from one end of a polymer chain to
the other. Such significant π-electron localization infers that the
charge hopping between closely packed NDI units of adjacent
Figure 2. Normalized UV−vis−NIR absorption spectra of PNFDTE1 and PNFDTE2 in solution (a) and in thin film (b).
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Table 1. Optical and Electrochemical Properties of PNFDTE1 and PNFDTE2
λabs (nm)
polymer
soln
film
Egopt (eV)
ELUMO (eV)
EHOMO (eV)
PNFDTE1
PNFDTE2
398, 680, 716
394, 666, 710
401, 676, 720
402, 687, 724
1.44
1.46
−4.00
−4.00
−5.93
−5.89
Figure 3. Transfer and output characteristics of the TGBC FETs based on polymer films: (a, b) PNFDTE1 and (c, d) PNFDTE2 annealed at 120
°C.
a
Table 2. Electrical Properties of PNFDTE1 and PNFDTE2 Based Thin Film Transistors
polymer
TGBC devices
μav (cm2 V−1 s−1
)
μmax (cm2 V−1 s−1
)
Ion/Ioff
Vth (V)
PNFDTE1
n-channel
p-channel
n-channel
p-channel
2.27
3.20
103−104
101−102
102−103
101−102
42 15
4.13 × 10−3
0.255
6.42 × 10−3
0.266
−55
40
5
5
PNFDTE2
1.47 × 10−2
1.72 × 10−2
−65 15
a
The average values were obtained from 10 devices. Devices with default L = 30 μm and W = 1400 μm were used. The average thicknesses were ca.
40 and 50 nm for PNFDTE1 and PNFDTE2 thin films, respectively.
Electrochemical Properties. The electrochemical proper-
ties of PNFDTE1 and PNFDTE2 were investigated by cyclic
voltammetry (CV) (Figure S3). Both copolymers show strong
reduction and weaker oxidation during negative and positive
scans, suggesting that the two copolymers are more readily
reduced than oxidized. Ferrocene was used as the internal
standard to calibrate the energy levels by the equation E =
[Eonset(Fc/Fc+ vs Ag/AgCl) − Eonset − 4.80] eV, where 4.80 eV
is the energy level of ferrocene below the vacuum level and the
potential Eonset(Fc/Fc+ vs Ag/AgCl) is 0.37 V during the test.
On the basis of the onset reductive potentials (Eroendset) of ca. −
0.43 V versus Ag/AgCl, both LUMO energy levels were
estimated to be ca. −4.00 eV for PNFDTE1 and PNFDTE2.
Meanwhile, the HOMO energy levels of PNFDTE1 and
were calculated to be 1.93 and 1.89 eV, respectively, which were
higher than the optical band gaps (1.44 and 1.41 eV) obtained
from the UV−vis−NIR absorption spectra of their thin films.
The differences in energy gaps are reasonable in consideration
of the exciton binding energy of polymer materials.54
Compared with unfluorinated NDI-based copolymers PNVTs,
the resulting HOMO energy levels were ca. 0.3 eV lower,
whereas the LUMO energy levels only reduced by ca. 0.1 eV.37
The observations are in accordance with the theoretical
modeling as aforementioned that fluorine atoms mainly give
contribution to the HOMO, thereby slightly lowering the
HOMO energy level and resulting in an increased HOMO−
LUMO gap.
Field-Effect Properties. To investigate the charge trans-
port properties of the two copolymers, thin film PFET devices
were fabricated with TGBC configuration. We fabricated the
devices on poly(ethylene terephthalate) (PET) substrate by
spin-coating a polymer solution (5 mg/mL) in chloroform.
Gold source/drain and aluminum gate electrodes were
PNFDTE2 were also estimated to be ca. −5.93 and −5.89 eV,
ox
onset
respectively, according to the onset oxidation potentials (E
)
of 1.50 and 1.46 V versus Ag/AgCl. The deep-lying LUMO
energy level would effectively enhance electron injection
behavior. Accordingly, the energy gaps of the two copolymers
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Figure 4. 2D-GIXRD patterns and pole figures of PNFDTE1 (a, b) and PNFDTE2 (c, d) before and after annealing treatment. (e) Schematic view
of packing motif for polymers. (f) The theoretical length of a repeat unit.
thermally evaporated via a shadow mask. Poly(methyl
methacrylate) (PMMA) was used as a dielectric layer and
encapsulation, leading to stable device performance under
ambient condition. We first explored the optimal annealing
temperatures (Figure S4). Annealing the film is a general
procedure to optimize the device performance. After the
annealing process, the self-assembly of side chains and the
intermolecular interactions between backbones would change.
An optimal annealing temperature provides a balance of good
crystallization behaviors and film morphologies. Both copoly-
mers showed the highest mobilities after thermal treatment at
120 °C for 5 min, meaning that both polymer thin films have
highly ordered molecular aggregations. Figure 3 presents the
typical transfer and output characteristics of PNFDTE1- and
PNFDTE2-based PFET devices. The mobilities, on/off current
ratios, and threshold voltages extracted from transfer curves in
saturation regimes are summarized in Table 2. Both copolymers
exhibited ambipolar charge transport properties with high
electron mobilities and low hole ones. For PNFDTE1, the
highest electron and hole mobilities significantly are 3.20 and
6.42 × 10−3 cm2 V−1 s−1, respectively. To the best of our
knowledge, the electron mobility is one of the highest values for
NDI-based polymer semiconductors on the flexible substrates.
PNFDTE2 only afforded lower electron mobility of 0.266 cm2
V−1 s−1, which is about a twelfth that of PNFDTE1. Both
polymer thin films exhibited moderate air stability after
exposure to the humid air (relative humidity of about 60%)
for 2 weeks with the mobilities slightly decreasing to about 80%
(Figure S8). Additionally, the difference in mobility is
sometimes observable in homologous polymer semiconductors
with similar alkyl chains. The side-chain engineering strategy
has demonstrated that such a phenomenon is attributed to the
variation in crystalline lattice and packing mode, forming
different extents of aggregation that alter the pathway of charge-
carrier transport.55 Note also that higher electron mobilities and
a lower hole ones are achieved compared with unfluorinated
PNVT polymers.37 Our results demonstrate the control over
charge-carrier injection processes by incorporation of fluorine
substitution.
Thin Film Microstructure. To further quantify the
crystallinities and molecular orientations of the thin films, we
employed two-dimensional grazing incidence X-ray diffraction
(2D-GIXRD) and tapping-mode atomic force microscopy
(AFM) measurements to study polymer packings and film
morphologies. The 2D-GIXRD patterns of both polymer thin
films show typical bimodal lamellar packing motifs (Figure 4).
The arc types of diffraction patterns indicate broad orientation
distributions.53 In the out-of-plane directions (qz vector), both
PNFDTE1 and PNFDTE2 exhibit (h00) diffraction peaks up
to the third order, indicative of moderate lamellar packing. The
d-spacings are calculated to be 23.1 and 24.7 Å for as-spun films
of PNFDTE1 and PNFDTE2, respectively. The values
marginally raise to 23.7 and 25.4 Å for annealed films. We
note that the d-spacing distances are comparable to that of
P(NDI2OD-T2) (or N2200, Polyera Activink), confirming that
the alkyl chains are highly interdigitated in the solid state.56
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Table 3. Microstructure Characterization of PNFDTE1 and PNFDTE2 Thin Films
d-spacing (Å)
coherence length (Å)
(010)
polymer
mode
(100)
(010)
(001)
NA
(100)
(001)
NA
PNFDTE1
edge-on
face-on
edge-on
face-on
23.7 (23.1)
24.3 (24.0)
25.4 (24.7)
26.5 (25.8)
3.71 (3.72)
3.77 (3.81)
3.73 (3.75)
3.89 (3.90)
147 (106)
95.7 (75.8)
66.8 (52.4)
152 (100)
23.0 (19.4)
22.6 (22.7)
27.0 (22.2)
23.0 (22.6)
16.0 (16.0)
NA
127 (107)
NA
PNFDTE2
15.9 (15.9)
105 (93)
a
The values are related to annealed thin films, and the values in parentheses are related to the as-spun ones.
significant (010) signal at qz ≈ 1.69 Å−1 was observed for
PNFDTE2, corresponding to a π−π stacking of ca. 3.73 Å.
PNFDTE1 thin films exhibit weak (010) signals in the out-of-
plane direction, indicative of preferential edge-on packing
modes. The calculated π−π stacking distance for PNFDTE1 is
3.71 Å, which is smaller than that of PNFDTE2, exhibiting the
tight interchain packing in the thin film. As indicated by
theoretical modeling, a small π−π stacking distance may greatly
facilitate electron transport. In the in-plane directions (qxy
vector), complex diffraction patterns are visible. The in-plane
intensity-corrected wedge cuts exhibit that the peaks can be
assigned to (h00), (010), and (00l) reflections. These
reflections are indicative of the spacing along the alkyl, π−π,
and backbone directions, respectively (Figure 4e). The
presence of (00l) diffraction peaks indicates the high extent
of crystallinity along the ordered backbone, which is generally
observed in NDI-based polymers. The (00l) diffraction peaks
are observed up to the third order, corresponding to d-spacing
of ca. 16.0 Å. The value matches well with the theoretical length
of a repeat unit (16.3 Å), as shown in Figure 4f. We further
calculated the coherence length along each direction to exhibit
the improved crystallinity upon annealing treatment. Each
interlamellar spacing slightly increases, each π-stacking distance
decreases, and the coherence lengths also increase, suggesting
that the structural and energetic disorder is reduced. For
example, after annealing treatment, the coherence length of
(001) peak corresponds to 7−8 ordered repeating units. The
crystallite orientation distributions of (100) Bragg reflections
are illustrated using pole figures.57 The areas integrated with
polar angle χ ranges of −45° to 45° correspond to the fraction
of edge-on crystallites, while the other areas indicate the ratio of
face-on crystallites. The face-on/edge-on ratio of PNFDTE1
remained unchanged after thermal annealing treatment,
whereas PNFDTE2 thin film tended to form face-on crystallites
under the same conditions. The variation in crystallites’
orientation distributions is one possible explanation for the
difference of mobility between the two copolymers.
The surface morphologies of PNFDTE1 and PNFDTE2 thin
films spin-coated on PET substrates and SiO2 substrates are
shown in Figure 5 and Figure S7, respectively. For PNFDTE1,
the thin films prepared on PET substrates and SiO2 substrates
showed typical granular features. For the as-spun PNFDTE1
films, the root-mean-square (RMS) values were 0.894 and 2.00
nm on SiO2 and PET substrates, respectively. The relative
rougher surfaces are inherited from intrinsic roughness of PET
substrates. After the PNFDTE1 thin film was thermal-annealed
at 120 °C for 5 min, larger grains and similar grain boundaries
were observed in comparison with its as-spun thin film, whereas
the annealed PNFDTE2 thin film on PET substrates displayed
similar domains and fewer grain boundaries compared with its
as-spun thin film. The change in surface morphology illustrates
that strong molecular self-aggregation occurs on both substrates
during the thermal annealing process. These results agree well
Figure 5. AFM height images of as-spun and annealed polymer thin
films of PNFDTE1 (a, b) and PNFDTE2 (c, d) on PET substrate.
with the temperature dependence of transfer characteristics.
Unlike the thin films of PNFDTE1, those of PNFDTE2
fabricated on PET substrates and SiO2 substrates both exhibit
smooth surfaces with smaller grains. It is well recognized that
large granular domains and continuous thin film morphologies
are favorable to charge transport while deep grain boundaries
result in trapping of charge carriers. Thus, the different thin film
morphologies of the two copolymers contribute to interpreting
their charge transport mobilities.
EXPERIMENTAL SECTION
■
3-Fluorothiophene-2-carboxylic Acid (2). To a solution of
thiophene-2-carboxylic acid (1) (17.0 g, 132.7 mmol) in anhydrous
THF (300 mL) was added n-BuLi (120 mL, 2.5 M in THF, 300.0
mmol) dropwise at −78 °C under an argon atmosphere. The reaction
mixture was stirred at the same temperature for 30 min. Then a
solution of N-fluorobenzenesulfonimide (50.00 g, 158.6 mmol) in
THF (300 mL) was added while keeping the temperature at −78 °C.
The mixture was stirred at the same temperature for another 3 h and
then warmed to ambient temperature and stirred for 16 h. To the
reaction mixture was added aqueous HCl (1 M, 300 mL), extracted
with ethyl acetate (600 mL × 3). The combined organic phase was
dried over Na2SO4 and concentrated to afford a solid residue, which
was subjected to silica gel column chromatography with petroleum
ether/ethyl acetate as eluent to afford the crude product (8.50 g). The
crude product was further crystallized in petroleum ether/ethyl acetate
1
to give the desired compound (4.80 g, 24.8%). H NMR (300 MHz,
d6-DMSO, δ): 13.3 (s, 1H), 7.90 (dd, J = 6.0 Hz, J = 1.5 Hz, 1H), 7.13
(d, J = 6.0 Hz, 1H). 13C NMR (75 MHz, d6-DMSO, δ): 161.6, 161.3,
157.6, 132.1, 132.0, 119.4, 119.4, 113.8, 113.5. HRMS: Calcd for
[C5H3FO2S]+: 145.9838. Found: 145.9838.
F
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Ethyl 3-Fluorothiophene-2-carboxylate (3). A mixture of
carboxylic acid, 2 (34.0 g, 232.7 mmol), potassium carbonate (64.3
g, 465.9 mmol), and ethyl iodide (43.6 g, 279.5 mmol) in DMF (400
mL) was stirred at 50 °C for 16 h. To the reaction mixture was added
water (1.5 L). The mixture was extracted with petroleum ether/ethyl
acetate (1.0 L × 3). The organic layers were dried over Na2SO4 and
concentrated to give the desired compound as a colorless oil (36.0 g,
89%). The mixture was used in next step without further purification.
1H NMR (300 MHz, CDCl3, δ): 7.42 (d, J = 9.0 Hz, 1H), 6.86 (d, J =
6.0 Hz, 1H), 4.79 (q, J = 7.5 Hz, 2H), 1.40 (t, J = 7.5 Hz, 1H). 13C
NMR (75 MHz, CDCl3, δ): 161.9, 160.7, 158.3, 129.9, 129.7, 118.6,
118.3, 61.2, 14.3. HRMS: Calcd for [C7H7FO2S]+: 174.0151. Found:
174.0150.
2H), 6.70 (s, 2H), 0.22 (s, 18H). 13C NMR (75 MHz, CDCl3, δ):
158.6, 154.9, 136.7, 125.9, 125.7, 125.2, 124.9, 115.9, 115.8, −8.65.
HRMS: Calcd for [C16H22F2S2Sn2]+: 551.9170, 553.9169, 555.9201.
Found: 551.9162, 553.9167, 555.9194.
General Procedures for Polymer Synthesis and Purification.
To a Schlenk tube were added (E)-1,2-bis(3-fluoro-5-(trimethyl-
stannyl)thiophen-2-yl)ethene, 7 (0.20 mmol), and NDI-based
monomer, 8a or 8b (0.20 mmol), a catalytic amount of Pd2(dba)3
(6.0 mg), P(o-tol)3 (17.0 mg), and chlorobenzene (5.0 mL). The tube
was charged with argon through a freeze−pump−thaw cycle. The
reaction mixture was heated to 115 °C and stirred for 3 days. The
resulting gel was cooled to room temperature and then precipitated in
200 mL of methanol containing 5 mL of HCl(aq) (6 M). The
precipitate was purified using Soxhlet extraction with methanol,
acetone, and hexane for 24 h. The residue was extracted with o-
dichlorobenzene and precipitated in methanol to afford the desired
polymer materials.
(3-Fluorothiophen-2-yl)methanol (4). To a solution of
intermediate 3 (33.0 g, 189.4 mmol) in DCM (500 mL) was slowly
added a solution of DIBAL-H (1 M in toluene, 480 mL, 480 mmol) at
−78 °C under an argon atmosphere. The reaction mixture was stirred
at −78 °C for another 2 h at the same temperature, and then a
solution of NaOH (10%, 300 mL) was added dropwise. The resulting
mixture was extracted with DCM (500 mL × 3). The combined
organic layers were dried over Na2SO4 and concentrated to give crude
product (20.06 g). The crude product was further purified by silica gel
column chromatography with petroleum ether/ethyl acetate as eluent
PNFDTE1 (179 mg, 85%). 1H NMR (300 MHz, C2D2Cl4, δ): 8.84
(s, 2H), 7.25 (s, 2H), 7.16 (s, 2H), 4.20 (br, 4H), 2.08 (br, 2H), 1.46−
1.33 (br, 64H), 0.93 (m, 12H). GPC: Mn = 38.9 kDa, Mw = 90.3 kDa,
PDI = 2.31. Anal. Calcd for C64H90F2N2O4S2: C, 72.96; H, 8.61; N,
2.66. Found: C 72.94, H 8.52, N 2.58.
PNFDTE2 (191 mg, 82%). 1H NMR (300 MHz, C2D2Cl4, δ): 8.83
(s, 2H), 7.22 (s, 2H), 7.18 (s, 2H), 4.19 (br, 4H), 2.09 (br, 2H), 1.44−
1.33 (br, 80H), 0.93 (m, 12H). GPC: Mn = 38.2 kDa, Mw = 135.5 kDa,
PDI = 3.54. Anal. Calcd for C72H106F2N2O4S2: C, 74.18; H, 9.17; N,
2.40. Found: C 74.23, H 9.04, N 2.32.
1
to afford the desired compound as a colorless oil (14.0 g, 56%). H
NMR (300 MHz, CDCl3, δ): 7.14 (t, J = 6.0 Hz, 1H), 6.78 (d, J = 6.0
Hz, 1H), 4.76 (s, 1H), 1.74 (m, 1H). 13C NMR (75 MHz, CDCl3, δ):
156.6, 153.1, 123.6, 123.5, 120.6, 120.4, 117.4, 117.1, 55.1. HRMS:
Calcd for [C5H5FOS]+: 132.0045. Found: 132.0044.
General Procedures for Thin Film PFETs Fabrication. Top-
gate bottom-contact (TGBC) PFET devices were fabricated on PET
substrates, and gold source−drain electrodes were prepared by thermal
evaporation by a shadow mask with the channel length/width of 30/
1400 μm. The substrate surfaces were cleaned by ultrasonication in
acetone, deionized water, and ethanol. Then the polymer solution in
chloroform (concentration: 5 mg/mL) was spin-coated onto the PET
interfaces at the speed of 2000 rpm for 1 min. Polymer films were
annealed in nitrogen at 120 °C for 5 min. PMMA (Mw = 1000 kDa)
dissolved in anhydrous n-butyl acetate (Aldrich, concentration: 65 mg/
mL) was heated to 80 °C and stirred for 3 h. The PMMA solution was
spin-coated to form a dielectric film on the semiconductor layer with a
thickness of about 900 nm. An 80 nm aluminum gate electrode were
then evaporated on top of the samples dried at 90 °C for 20 min. The
gate leakage currents were on the order of 1−10 nA (Figure S9).
Generally, the relatively low gate leakage current allows the accurate
estimation of carrier mobilities from the transfer curves.
A Keithley 4200 SCS semiconductor parameter analyzer was
utilized to demonstrate FET parameters. TGBC FETs were measured
under ambient conditions. The mobilities were extracted according to
the equation IDS = (W/2L)Ciμ(VGS − Vth)2 in saturated regions, where
IDS is source−drain saturation current, L the channel length, W the
channel width, Vth the threshold voltage, Ci the capacitance per unit
area of the insulator, VGS the gate voltage, and μ the mobility.
3-Fluorothiophene-2-carbaldehyde (5). To a solution of
intermediate 4 (14.0 g, 105.9 mmol) in DCE (500 mL) was added
MnO2 (47.50 g, 546.0 mmol). The mixture was refluxed for 16 h until
the disappearance of intermediate 4 monitored by TLC. The mixture
was cooled to room temperature and filtered. The filtrate was
concentrated to afford a residue (12.50 g). The residue was purified by
silica gel column chromatography with petroleum ether/ethyl acetate
as eluent to give the desired compound as a colorless oil (8.0 g, 58%).
1H NMR (300 MHz, CDCl3, δ): 10.03 (s, 1H), 7.66 (t, J = 4.5 Hz,
1H), 6.91 (t, J = 1.5 Hz, 1H). 13C NMR (75 MHz, CDCl3, δ): 179.9,
152.7, 149.9, 134.2, 134.1, 130.9, 128.8, 118.3, 117.9. HRMS: Calcd
for [C5H3FOS]+: 129.9889. Found: no visible.
(E)-1,2-Bis(3-fluorothiophen-2-yl)ethene (6). To a stirred
suspension of zinc powder (10.2 g, 156.1 mmol) in dry THF (500
mL), TiCl4 (9.02 mL, 97.60 mmol) was added slowly at 0 °C under an
argon atmosphere. The resulting mixture was heated to reflux and
stirred for 2 h under Ar. Then pyridine (6.6 mL, 182 mmol) and
intermediate, 5 (8.00 g, 61.47 mmol) were added successively. The
reaction system was heated to reflux and stirred for 16 h. After the
mixture was cooled to room temperature, water (500 mL) was added.
The resulting mixture was extracted by DCM (500 mL × 3), dried
over Na2SO4, and concentrated to give crude product. Further
purification with silica gel column chromatography with petroleum
ether/ethyl acetate as eluent and recrystallization with methanol
1
CONCLUSION
afforded the desired compound as a white solid (4.71 g, 67.3%). H
■
NMR (300 MHz, CD2Cl2, δ): 7.00 (dd, J = 6 Hz, J = 0.3 Hz, 2H), 6.81
(s, 2H), 6.89 (d, J = 6 Hz, 2H). 13C NMR (75 MHz, CD2Cl2, δ):
157.1, 153.6, 122.6, 122.4, 120.5, 120.3, 118.1, 117.7, 116.08, 116.05,
116.02, 115.99. HRMS: Calcd for [C10H6F2S2]+: 227.9879. Found:
227.9876.
(E)-1,2-Bis(3-fluoro-5-(trimethylstannyl)thiophen-2-yl)-
ethene (7). To a solution of intermediate 6 (1.14 g, 5.0 mmol) in
THF (50 mL) was added dropwise 2.5 M solution of n-BuLi in hexane
(4.8 mL, 12.0 mmol) at −78 °C. The mixture was stirred for 1 h at the
temperature, then slowly warmed to −40 °C, and stirred for 30 min.
After the suspension was cooled to −78 °C again, a 1.0 M solution of
Me3SnCl in THF (12.0 mL, 12.0 mmol) was added. The reaction
solution was stirred overnight at room temperature, followed by
quenching with H2O and extraction with diethyl ether (60 mL × 3).
After the removal of the solvent in vacuo, the crude solid was
recrystallized from hexane twice to give the desired compound as a
colorless crystal (2.1 g, 76%). 1H NMR (300 MHz, CDCl3, δ): 6.72 (s,
In this contribution, we have synthesized two fluorinated
dithienylethene (FDTE)-based copolymers, PNFDTE1 and
PNFDTE2, and investigated their charge transport properties
by fabricating polymer field-effect transistors on PET substrates
with TGBC configuration. Both copolymers have good thermal
stability, a broad absorption in UV−vis−NIR region, and deep-
lying LUMO energy levels. The highest electron mobility in air
for PNFDTE1-based PFET devices is 3.20 cm2 V−1 s−1, which
is among the highest for NDI-containing polymers. By contrast,
a lower electron mobility of 0.266 cm2 V−1 s−1 was afforded for
PNFDTE2-based counterpart. Thin film microstructure of the
two copolymers demonstrates that PNFDTE1 thin films exhibit
crystalline granular intercalating networks and preferential
edge-on crystallites, whereas PNFDTE2 thin films have smooth
surfaces and smaller crystallites with mainly face-on packing
G
DOI: 10.1021/acs.macromol.7b01169
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
Influence of Backbone Fluorination in Regioregular Poly(3-alkyl-4-
fluoro)thiophenes. J. Am. Chem. Soc. 2015, 137, 6866−6879.
(5) Zhang, A.; Xiao, C.; Wu, Y.; Li, C.; Ji, Y.; Li, L.; Hu, W.; Wang,
Z.; Ma, W.; Li, W. Effect of Fluorination on Molecular Orientation of
Conjugated Polymers in High Performance Field-Effect Transistors.
Macromolecules 2016, 49, 6431−6438.
mode. Such results match well with their different charge
transport properties. Our work highlights the promising
prospect of FDTE unit in developing high-performance
polymer semiconductors and the important influences of
intramolecular interaction on conformational control of organic
semiconductors. The efforts toward the use of high-mobility
fluorinated polymers for flexible electronics applications are
currently underway in our lab.
́ ́ ̂
(6) Leclerc, N.; Chavez, P.; Ibraikulov, O.; Heiser, T.; Leveque, P.
Impact of Backbone Fluorination on π-Conjugated Polymers in
Organic Photovoltaic Devices: A Review. Polymers 2016, 8, 11.
(7) Wood, S.; Kim, J.-H.; Hwang, D.-H.; Kim, J.-S. Effects of
Fluorination and Side Chain Branching on Molecular Conformation
and Photovoltaic Performance of Donor−Acceptor Copolymers.
Chem. Mater. 2015, 27, 4196−4204.
(8) Bronstein, H.; Frost, J. M.; Hadipour, A.; Kim, Y.; Nielsen, C. B.;
Ashraf, R. S.; Rand, B. P.; Watkins, S.; McCulloch, I. Effect of
Fluorination on the Properties of a Donor−Acceptor Copolymer for
Use in Photovoltaic Cells and Transistors. Chem. Mater. 2013, 25,
277−285.
(9) Wang, C.; Mueller, C. J.; Gann, E.; Liu, A. C. Y.; Thelakkat, M.;
McNeill, C. R. Influence of Fluorination on The Microstructure and
Performance of Diketopyrrolopyrrole-Based Polymer Solar Cells. J.
Polym. Sci., Part B: Polym. Phys. 2017, 55, 49−59.
(10) Fan, Q.; Liu, Y.; Jiang, H.; Su, W.; Duan, L.; Tan, H.; Li, Y.;
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Increase The Photovoltaic Performance of Indacenodithiophene-alt-
Quinoxaline Based Wide-Bandgap Copolymers. Org. Electron. 2016,
33, 128−134.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b01169.
■
S
TGA curves of the polymers, temperature-dependent
absorption spectra, CV, GIXRD data, NMR spectra,
AFM images, and other device characteristics and data
(PDF)
AUTHOR INFORMATION
Corresponding Authors
*E-mail: zhangwf@iccas.ac.cn (W.Z.).
*E-mail: yugui@iccas.ac.cn (G.Y.).
■
ORCID
(11) Jo, J. W.; Kim, J. H.; Jung, J. W. Isoindigo-Based Fluorinated
Low Band Gap Polymers for Environmentally Stable Field Effect
Transistor. Dyes Pigm. 2016, 133, 333−338.
(12) Qiao, X.; Wu, Q.; Wu, H.; Zhang, J.; Li, H. Bithienopyrrole-
dione-Based Copolymers, Versatile Semiconductors for Balanced
Ambipolar Thin-Film Transistors and Organic Solar Cells with Voc
> 1 V. Adv. Funct. Mater. 2017, 27, 1604286.
Jianyao Huang: 0000-0003-4177-6393
Gui Yu: 0000-0001-8324-397X
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
(13) Yi, Z.; Wang, S.; Liu, Y. Design of High-Mobility
Diketopyrrolopyrrole-Based π-Conjugated Copolymers for Organic
Thin-Film Transistors. Adv. Mater. 2015, 27, 3589−3606.
(14) Li, P.; Xu, L.; Shen, H.; Duan, X.; Zhang, J.; Wei, Z.; Yi, Z.; Di,
C. A.; Wang, S. D-A1-D-A2 Copolymer Based on Pyridine-Capped
Diketopyrrolopyrrole with Fluorinated Benzothiadiazole for High-
Performance Ambipolar Organic Thin-Film Transistors. ACS Appl.
Mater. Interfaces 2016, 8, 8620−8626.
(15) Uddin, M. A.; Lee, T. H.; Xu, S.; Park, S. Y.; Kim, T.; Song, S.;
Nguyen, T. L.; Ko, S.-j.; Hwang, S.; Kim, J. Y.; Woo, H. Y. Interplay of
Intramolecular Noncovalent Coulomb Interactions for Semicrystalline
Photovoltaic Polymers. Chem. Mater. 2015, 27, 5997−6007.
(16) Zhang, X.; Wang, Z.; Chen, S.; Zhao, Z.; Yuan, W.; Wang, H.;
Gao, X. Tuning the Charge Transport Property of Naphthalene
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Atom on Molecular Backbone. Chin. J. Chem. 2014, 32, 1057−1064.
(17) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.;
Ratner, M. A.; Marks, T. J. Building Blocks for N-Type Molecular and
Polymeric Electronics. Perfluoroalkyl- versus Alkyl-Functionalized
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(18) Sakamoto, Y.; Suzuki, T.; Kobayashi, M.; Gao, Y.; Fukai, Y.;
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The authors acknowledge the financial support from the
National Natural Science Foundation of China (Grants
21474116, 51473021, 21673258, and 51233006), the National
Key Research and Development Program of China
(2016YFB0401100), and the Strategic Priority Research
Program of the Chinese Academy of Sciences
(XDB12030100). The high temperature GPC measurements
are supported by Open Research Fund of State Key Laboratory
of Polymer Physics and Chemistry, Changchun Institute of
Applied Chemistry, Chinese Academy of Sciences. The GIXRD
analyses were performed at the BL14B1 Station of Shanghai
Synchrotron Radiation Facility (SSRF), 23A1 Station of
National Synchrotron Radiation Research Centre (NSRRC,
Taiwan), and 1W1A station of Beijing Synchrotron Radiation
Facility (BSRF). The authors are very grateful for the assistance
of scientists from the stations during the experiments.
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