Modulation of Majority Charge Carrier from Hole to Electron by Incorporation of Cyano Groups in Diketopyrrolopyrrole-Based Polymers

Article abstract of DOI:10.1021/acs.macromol.7b01524

A thienylene-vinylene-thienylene (TVT) derivative with cyano groups in the 3- and 3′-positions was synthesized as a building block of semiconducting polymers for high mobility organic field effect transistors (OFETs). (E)-1,2-Di-(3-cyanothiophen-2-yl)ethene (2CNTVT) was copolymerized with diketopyrrolopyrrole (DPP) units via Stille coupling reaction to give 2DPP-2CNTVT and 7DPP-2CNTVT. The properties of these two polymers were compared with those of the corresponding polymers without cyano groups in TVT (2DPP-TVT and 7DPP-TVT). The effects of CN groups and branched alkyl position were found to have a significant influence on the optical, electrochemical, morphological, and charge transporting properties of the polymers. The average hole mobilities of OFETs fabricated with 2DPP-TVT and 7DPP-TVT OFETs were 1.63 and 2.2 cm² V^-1 s^-1, respectively, and the average electron mobility for both 2DPP-2CNTVT and 7DPP-2CNTVT OFETs was 1.2 cm² V^-1 s^-1.

Full text of DOI:10.1021/acs.macromol.7b01524

Article
pubs.acs.org/Macromolecules
Modulation of Majority Charge Carrier from Hole to Electron by
Incorporation of Cyano Groups in Diketopyrrolopyrrole-Based
Polymers
Hee Su Kim,† Gunel Huseynova,‡ Yong-Young Noh,*,‡ and Do-Hoon Hwang*,†
†Department of Chemistry and Chemistry Institute for Functional Materials, Pusan National University, Busan 46241, Republic of
Korea
‡Department of Energy and Materials Engineering, Dongguk University, 30 Pildong-ro, 1 gil, Jung-gu, Seoul, 04620, Republic of
Korea
S
* Supporting Information
ABSTRACT: A thienylene−vinylene−thienylene (TVT) de-
rivative with cyano groups in the 3- and 3′-positions was
synthesized as a building block of semiconducting polymers for
high mobility organic field effect transistors (OFETs). (E)-1,2-
Di-(3-cyanothiophen-2-yl)ethene (2CNTVT) was copolymer-
ized with diketopyrrolopyrrole (DPP) units via Stille coupling
reaction to give 2DPP-2CNTVT and 7DPP-2CNTVT. The
properties of these two polymers were compared with those of
the corresponding polymers without cyano groups in TVT
(2DPP-TVT and 7DPP-TVT). The effects of CN groups and
branched alkyl position were found to have a significant
influence on the optical, electrochemical, morphological, and
charge transporting properties of the polymers. The average hole mobilities of OFETs fabricated with 2DPP-TVT and 7DPP-
TVT OFETs were 1.63 and 2.2 cm2 V−1 s−1, respectively, and the average electron mobility for both 2DPP-2CNTVT and 7DPP-
2CNTVT OFETs was 1.2 cm2 V−1 s−1.
Incorporation of cyano and fluorine groups in a polymer
backbone is a widely used strategy to prepare polymers with
electron transport properties as these groups have a strong
electron withdrawing ability.14,15 Such strong electron with-
drawing groups (EWGs) can enhance the n-type characteristics
by diffusing the electron density in the conjugated backbone
and lowering the frontier energy levels. Recently, Yun et al.
reported the synthesis of n-type conjugated polymers with high
electron mobility (μe = 7 cm2 V−1 s−1) by introducing a cyano
group in the vinyl linkage.16 Park et al. have also reported a
high electron mobility of 2.36 cm2 V−1 s−1 in conjugated
polymers with a backbone consisting of fluorine substituents on
a phenyl unit.17
In this study, we designed and synthesized a new type of
conjugated building block, (E)-1,2-bis(3-cyanothiophene-2-
yl)ethene (2CNTVT), where cyano (CN) groups were
introduced to the 3-position of the thiophene ring in (E)-2-
(2-(thiophen-2-yl)vinyl)thiophene (TVT). It was expected that
the introduction of CN groups would enhance the n-type
characteristics because of a low-lying LUMO level and impart
good electron transporting ability. 2CNTVT was polymerized
INTRODUCTION
■
Since the first organic field effect transistor (OFET) based on
polythiophene was reported in 1986, conjugated polymers have
attracted a lot of attention from academia and industry owing
to their tremendous potential in flexible and large-area
electronic circuits, such as organic light-emitting diodes
(OLEDs), radio-frequency identification tags (RFIDs), smart
cards, and sensors.1−5 Recently, the performance of conjugated
polymer-based OFETs has been significantly improved by
proper molecular design and optimized device processing
techniques. Furthermore, several conjugated polymers have
been synthesized with diketopyrrolopyrrole (DPP) and
isoindigo (IID) derivatives, which exhibit hole mobilities (μh)
exceeding 10 cm2 V−1 s−1, surpassing those of amorphous
silicon-based transistors (μh ∼ 1 cm2 V−1 s−1).6,7 Compared to
the performance of p-type conjugated polymer-based OFETs,
the performance of the n-type analogues lags behind because of
a lack of n-type conjugated polymers with appropriate lowest
unoccupied molecular orbital (LUMO) energy level and high
electron transporting properties.8,9 Electron-transport materials
are required to have LUMO energy level below −4.0 eV for
better air stability.10−12 In addition, it is necessary to develop
both p-type and n-type conjugated polymers that exhibit high
performances for applications in complementary integrated
circuits (ICs).13
Received: July 18, 2017
Revised: September 13, 2017
© XXXX American Chemical Society
A
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a
Scheme 1
a
Reagents and conditions: (i) Zn, TiCl4, THF, 70 °C, 16 h; (ii) CuCN, DMF, 150 °C, 12 h; (iii) LDA, trimethyltin chloride, −78 °C, 12 h; (iv) 13-
(6-bromohexyl)heptacosane, K2CO3, DMF, 120 °C, overnight; (v) NBS, chloroform, 0 °C, RT.
with DPP comonomers having a long dodecylhexadecyl side
chain with two different branching points in order to enhance
the solubility and to control the morphology of the resulting
polymers. Properties of the synthesized copolymers were
compared with those of the corresponding copolymer
analogues without CN groups. The four polymers, 2DPP-
TVT, 2DPP-2CNTVT, 7DPP-TVT, and 7DPP-2CNTVT,
were evaluated by UV−vis absorption, cyclic voltammetry
(CV), theoretical calculations (density functional theory, DFT),
two-dimensional grazing incidence X-ray diffraction (2D-
GIXD), atomic force microscopy (AFM), and OFET character-
istics. The attachment of the CN group on the TVT moiety
results in lower LUMO energy levels and enhanced electron
mobility. The longer branching position of the alkyl chain
contributes to strong intermolecular π−π interactions in the
polymers, according to their optical and 2D-GIXD analysis,
because of the alleviated steric effect of the alkyl side chains on
the polymer conjugated backbone. It was found that the
fabricated devices of 2CNTVT-based polymers showed high
electron mobility, while those of corresponding TVT-based
polymers exhibited high hole mobility.
that the CN substitution in these polymer systems slightly
enhanced their thermal stability (Figure S1).
Optical and Electrochemical Properties. The absorption
spectra of the polymers in chloroform and as thin films are
shown in Figure 1a,b. All polymers showed the typical
absorption bands from 600 to 950 nm which arise due to the
intramolecular charge transfer (ICT) in the D−A conjugated
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic routes for
preparing 2DPP-TVT, 2DPP-2CNTVT, 7DPP-TVT, and
7DPP-2CNTVT are shown in Scheme 1, and the detailed
procedures of the transformation of the monomers to the
corresponding polymers are described in the Experimental
Section. The polymers were successfully synthesized via Stille
coupling reactions. They were found to be soluble in common
organic solvents such as chloroform, toluene, tetrahydrofuran,
chlorobenzene, and 1,2-dichlorobenzene. The molecular
weights of the polymers were determined by gel permeation
chromatography (GPC) using polystyrene as the standard. The
number-average molecular weights (Mn) of 2DPP-TVT, 2DPP-
2CNTVT, 7DPP-TVT, and 7DPP-2CNTVT are 127 kg mol−1
(polydispersity index (PDI) = 2.4), 37 kg mol−1 (PDI = 1.4),
314 kg mol−1 (PDI = 2.6), and 53 kDa (PDI = 1.3),
respectively. The thermal decomposition temperatures (Td, at
5% weight loss) of 2DPP-TVT, 2DPP-2CNTVT, 7DPP-TVT,
and 7DPP-2CNTVT are 398, 401, 397, and 405 °C,
respectively, as evaluated by thermogravimetric analysis
(TGA) under a nitrogen atmosphere. These results reveal
Figure 1. Normalized absorption spectra of the polymers (a) in their
chloroform solution and (b) as thin films.
B
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Table 1. Molecular, Optical, and Electrochemical Properties of Conjugated Polymers
λmax,abs (nm)
λmax (nm)
λedge (nm)
a
b
c
c
d
e
f
polymers
Mn (kg/mol)
PDI
Td (°C)
solution
film
film
optical Egopt (eV)
HOMO (eV)
LUMO (eV)
2DPP-TVT
127
37
2.4
1.4
2.6
1.3
398.7
400.8
396.9
405.0
779
776
790
797
786
796
791
810
916
942
958
950
1.35
1.31
1.29
1.30
−5.13
−5.49
−5.05
−3.78
−4.18
−3.76
−4.10
2DPP-2CNTVT
7DPP-TVT
314
53
7DPP-2CNTVT
−5.40
a
b
Mn and PDI of the polymers were determined by gel permeation chromatography using polystyrene standards in CHCl3. Temperature at 5%
c
weight loss at a heating rate of 10 °C min−1 under nitrogen. Polymer film on a quartz plate by spin-casting from chloroform solution at 1500 rpm
d
e
for 30 s. Calculated from the absorption band edge of the copolymer films, Eg = 1240/λedge. HOMO energy levels were determined from the
f
Eox/onset of the first oxidation potential of ferrocene, −4.8 eV. LUMO = HOMO − Egopt
.
Figure 2. Transfer and output characteristics of (a) 2DPP-TVT and (b) 7DPP-TVT.
ox
backbone structure. Compared to polymer solutions, the
absorption spectra of the polymer films were red-shifted, and
two distinguished peaks were observed because of the
aggregation of the polymer main chains and interchain
interactions in the thin films.18 The absorption spectra of the
7DPP-based polymers are red-shifted compared to those of the
2DPP-based polymers. This indicates that the polymers with
alkyl side chains branched at the 7-position possessed enhanced
intermolecular π−π interactions compared to the correspond-
ing polymers with the side chains branched at the 2-position,
possibly due to favorable molecular packing. The optical band
gap energies (Egopt) of ≈1.29−1.35 eV were estimated from the
absorption edge in the absorption spectrum of the polymer
film.
equation: HOMO (eV) = −(Eonset + 4.71). The HOMO
energy levels of 2DPP-TVT, 2DPP-2CNTVT, 7DPP-TVT, and
7DPP-2CNTVT were −5.13, −5.49, −5.05, and −5.40 eV,
respectively (Figure S2). These results demonstrated that the
introduction of a CN group in the thiophene moiety effectively
lowered the HOMO energy level of the polymers because of its
strong electron withdrawing ability. The lowest unoccupied
molecular orbital (LUMO) energy levels of the polymers were
determined from the HOMO energy levels obtained from the
CV experiments and the Egopt values obtained from the onset
point of the UV−vis absorption spectra of the films. Thus, the
LUMO energy levels of 2DPP-TVT, 2DPP-2CNTVT, 7DPP-
TVT, and 7DPP-2CNTVT were evaluated as −3.78, −4.18,
−3.76, and −4.10 eV, respectively. These results imply that the
2CNTVT-based polymers would likely show better electron
injection property from the Au electrode compared to the
TVT-based polymers because of their low-lying LUMO energy
levels. The UV−vis absorption measurements, Egopt values, and
The highest occupied molecular orbital (HOMO) energy
levels of the polymers in thin films were characterized by CV.
These energy levels of the polymers were determined by
measuring the oxidation onset potentials (Eox) of the polymer
films by cyclic voltammetry and calculated using the following
C
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Figure 3. Transfer and output characteristics of (a) 2DPP-2CNTVT and (b) 7DPP-2CNTVT.
electrochemical properties of the polymers are summarized in
Table 1.
were also calculated. It was found that DPP-TVT has a higher
dipole moment (1.507 D) than DPP-2CNTVT (1.3635 D),
which is expected to enhance molecular ordering as described
in a previous report.21 Interestingly, the direction of the dipole
moment in DPP-2CNTVT was opposite to that of DPP-TVT,
which indicated that the 2CNTVT unit is a stronger electron
withdrawing moiety than the DPP unit. It is assumed that the
conjugated backbone changes from donor−acceptor type to
acceptor−acceptor type. These results are shown in Figures S4
and S5.
OFET Characteristics. In order to study the structure−
activity relationships, top-gate bottom-contact (TG/BC)
OFETs were fabricated with a very thin gold source and
drain electrodes (13 nm Au on 3 nm Ni pads) prepatterned by
conventional photolithography. Figures 2 and 3 illustrate the
typical transfer and output characteristics of the OFETs
fabricated with 2DPP-TVT, 7DPP-TVT, 2DPP-2CNTVT,
and 7DPP-2CNTVT. A dramatic shift of the nature of the
majority carriers from holes to electrons was observed in these
fabricated devices. The OFETs fabricated using 2DPP-TVT
and 7DPP-TVT exhibited typical p-channel characteristics,
while those fabricated using 2DPP-2CNTVT and 7DPP-
2CNTVT showed n-channel characteristics. The OFETs
fabricated using 2DPP-TVT and 7DPP-TVT showed fairly
good average hole field-effect mobilities (μFET) of 1.63 and 2.2
cm2 V−1 s−1, respectively. On the other hand, the OFETs
fabricated using 2DPP-2CNTVT and 7DPP-2CNTVT showed
average electron μFET of 1.2 cm2 V−1 s−1 after thermal annealing
at 200 and 250 °C, respectively. The values of μFET of the
polymer OFETs depend on the annealing temperatures of the
active polymer films. The optimum annealing temperatures of
the polymer films are in the range 200−250 °C. The detailed
Theoretical Calculations. DFT calculations were per-
formed by using the B3LYP functional and 6-31G(d) basis set
in order to understand the effect of CN substitution on the
frontier molecular orbitals, dihedral angles, dipole moments,
and the charge distributions along the conjugated backbone.
Calculations were performed on two repeating units (D−A−
D−A) with simplified alkyl chains of the TVT (DPP-TVT)-
and 2CNTVT (DPP-2CNTVT)-based polymers for reducing
the calculation time. These calculations were carried out using
the Gaussian 09 software package. The HOMO and LUMO
energy levels calculated by the simulation are consistent with
the experimental results, and it was possible to confirm that the
strong electron withdrawing property of the CN group lowered
the energy levels. Furthermore, both the molecular orbital
distributions in the HOMO and LUMO levels are spread over
the entire conjugated backbone (Figure S3). These isosurfaces
differ from the general D−A conjugated polymers in which the
LUMOs are localized on the electron-withdrawing units.19,20
Furthermore, it was found that the HOMO in the DPP-
2CNTVT copolymer was slightly localized on the DPP units
and the LUMO was more delocalized along the conjugated
backbone as compared to DPP-TVT. Therefore, it can be
expected that the DPP-2CNTVT polymer would have better
electron transporting properties than DPP-TVT. In order to
investigate the effects of CN substitution on the conjugated
backbone geometries, the dihedral angles A1, A2, A3, and A4
were calculated (Table S1). Both DPP-TVT and DPP-
2CNTVT exhibit highly planar molecular backbone structure
with very small dihedral angles (≈1°). Furthermore, the dipole
moment and direction of the repeating unit of the polymers
D
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Table 2. Calculated Device Parameters
semiconductor
2DPP-TVT
spin-coating rate
1000 rpm/30 s
annealing temp (°C)
av mobility (cm2/(V s))
threshold voltage (V)
Rc (Ω·cm)
Ion/off
100
150
200
250
300
250
0.20
0.32
0.85
1.54
0.15
1.63
1.12
0.10
0.20
0.50
0.34
0.53
1.20
0.52
0.40
1.24
2.20
1.20
1.55
1.44
1.50
0.10
0.09
0.30
1.20
0.62
1.11
1.13
−47.7
−21.8
−35.8
−40.5
−29.9
−36.5
−37.1
56.4
7.4 × 104
2.3 × 103
1.7 × 103
7.7 × 104
3.8 × 103
4.0 × 103
9.5 × 103
5.5 × 107
3.9 × 105
9.2 × 104
6.3 × 105
1.1 × 106
3.3 × 104
3.4 × 104
7.8 × 104
5.7 × 102
7.1 × 103
3.1 × 103
2.2 × 104
3.8 × 103
2.0 × 102
8.2 × 109
1.2 × 108
7.4 × 107
2.3 × 104
1.5 × 105
1.7 × 104
2.0 × 104
103
104
103
103
∼102
∼103
∼103
∼103
103
1500 rpm/60 s
2000 rpm/60 s
1000 rpm/30 s
2DPP-2CNTVT
100
150
200
250
300
200
27.8
28.6
103
36.1
103
43.2
103
1500 rpm/60 s
2000 rpm/60 s
1000 rpm/30 s
30.0
∼104
22.3
103
7DPP-TVT
100
150
200
250
300
200
−13.9
−25.2
−38.7
−40.3
−45.0
−22.4
−18.7
−1.0
34.9
104
103
103
∼103
∼103
∼103
∼103
∼103
102
1500 rpm/60 s
2000 rpm/60 s
1000 rpm/30 s
7DPP-2CNTVT
100
150
200
250
300
250
33.0
102
35.7
103
36.0
103
1500 rpm/60 s
2000 rpm/60 s
30.6
103
18.9
103
Figure 4. Two-dimensional grazing-incidence X-ray diffraction (2D-GIXD) images (a−d) and (f) out-of-plane and (e) in-plane XRD diffractogram
profiles of the polymer thin films (200 °C for 7DPP-TVT and 2DPP-2CNTVT, 250 °C for 2DPP-TVT and 7DPP-2CNTVT).
device characteristics of the fabricated OFETs including their
annealing temperatures are summarized in Table 2. The
fabricated OFETs exhibited reasonably low contact resistances
(10−3−10−4 Ω·cm) for either hole or electron injection from
the Au electrode.
The operational stabilities of OFETs were also tested. The
transfer characteristics of the four OFETs under gate-bias stress
and long-term storage in air are given in Figure S6. All OFETs
showed good operational stability even after application of
continuous bias for 6 h. Electrical properties of p-type 2DPP-
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TVT and 7DPP-TVT OFETs slightly degraded even after 7
days storage in the ambient environment. For 2DPP-2CNTVT
and 7DPP-2CNTVT OFETs, the degradation of electron
transport was much noticeable, although the devices still
operated (Figure S7). The poor air stability of n-type 2DPP-
2CNTVT and 7DPP-2CNTVT OFETs is likely to the
generation of trap sites by reaction with hydrogenated
oxygen.22
The data obtained from AFM measurements is in good
agreement with the 2D-GIXD results and the observed charge
transporting properties.
CONCLUSION
■
In summary, we successfully synthesized novel n-type
conjugated polymers consisting of DPP and TVT monomers
with cyano groups, and the effect of the cyano group on the
frontier orbital energy levels, crystal orientation, and charge
transporting properties of the synthesized polymers was also
investigated. The CNTVT-based polymers showed lower
HOMO/LUMO energy levels compared to the TVT-based
polymers, which was more favorable for electron injection from
the Au electrode. Moreover, the polymers exhibited bimodal
orientation with good crystallinity such that the fabricated
OFETs using CNTVT-based polymers showed high electron
mobility. This work demonstrates that the newly synthesized
2CNTVT unit could be used as a good electron accepting
building block for high performance n-type conjugated
polymers for OFETs.
Thin-Film Microstructural Analyses. In order to clarify
the crystallinity and molecular packing of the polymers, 2D-
GIXD was performed on the polymer thin films at their
optimum annealing temperature (200 °C for 7DPP-TVT and
2DPP-2CNTVT, 250 °C for 2DPP-TVT and 7DPP-
2CNTVT). Figure 4 shows the 2D-GIXD images and the
corresponding diffractogram profiles of the polymer thin films.
The bare TVT-based polymers (2DPP-TVT and 7DPP-TVT)
exhibited an edge-on dominant orientation with distinct
lamellar peaks up to the fourth order, which is consistent
with the previous reports on the crystal structures of DPP-
TVT-based polymers.7 Interestingly, unlike the known TVT-
based polymers, the 2CNTVT-based polymers (2DPP-
2CNTVT and 7DPP-2CNTVT) exhibited a bimodal orienta-
tion in which the edge-on and face-on orientations coexist. In
order to gain a deeper insight into this orientation, we
performed azimuthal angle scans of the (100) reflection (Figure
S8). The integrated intensity with azimuthal angle (χ) in the
range 0−45° (or 0 to −45°) was defined as Az, and that ranging
from 45° to 90° (or −45° to −90°) was designated as Axy,
corresponding to the fractions of edge-on and face-on
orientations, respectively. The area ratio of Axy to Az (Axy/Az)
for 2DPP-TVT, 7DPP-TVT, 2-DPP-2CNTVT, and 7DPP-
2CNTVT was found to be 0.054, 0.048, 0.102, and 0.219,
respectively. The face-on portions of the 2CNTVT-based
polymers are more than those of the corresponding TVT-based
polymers. These face-on portions of the 2CNTVT based
polymers can form 3D charge-conduction channels which
would be beneficial for electron charge transport.6,23 The
lamellar and π−π spacings of the 2DPP-TVT, 2DPP-2CNTVT,
7DPP-TVT, and 7DPP-2CNTVT thin films were 23.2, 24.9,
30.5, 30.5 Å and 3.748, 3.627, 3.605, 3.590 Å, respectively.
Although the lamellar spacing of the 7DPP-based polymers
increased because of the change in the bridging point of the
alkyl spacer, the π−π spacing decreased, leading to increased
mobility due to a better orbital overlap between the
neighboring polymer main chains. 7DPP-2CNTVT and
2DPP-2CNTVT showed comparable π−π spacing and thus
possessed similar electron mobilities. Further details of the
crystal structure of the polymers are summarized in Table S2.
In order to gain further insight into the polymer structures,
film morphologies of the polymers were investigated by AFM,
using tapping mode under ambient conditions. For the
roughness measurements, the polymer films (2DPP-TVT,
2DPP-2CNTVT, 7DPP-TVT, and 7DPP-2CNTVT) were
annealed at temperatures for their best device performances
(200 °C for 7DPP-TVT and 2DPP-2CNTVT, 250 °C for
2DPP-TVT and 7DPP-2CNTVT) for 30 min (Figure S9). As
revealed in the images, the 2DPP-TVT, 2DPP-2CNTVT,
7DPP-TVT, and 7DPP-2CNTVT films have a crystalline
morphology and exhibit smooth surfaces with root-mean-
square (rms) roughness of 0.802, 0.644, 1.12, and 0.913 nm,
respectively. Compared to 2DPP-based polymers, the 7DPP-
based polymers exhibit more developed fibrillar networks with
higher rms roughness values because of their high crystallinity.
EXPERIMENTAL SECTION
■
Materials. All reagents were purchased from Aldrich, Alfa Aesar, or
TCI Korea and used without further purification. Tris(dibenzylidene-
acetone)dipalladium(0) and tri(o-tolyl)phosphine were purchased
from Strem. Solvents were dried and purified by fractional distillation
over sodium/benzophenone and handled in a moisture-free
atmosphere. Column chromatography was performed using silica gel
(Electronic Materials Index, SL-60-60A). 3-Bromothiophene-2-carbal-
dehyde, (E)-1,2-bis(5-(trimethylstannyl)thiophen-2-yl)ethene, 3,6-bis-
(5-bromothiophen-2-yl)-2,5-bis(2-dodecylhexadecyl)pyrrolo[3,4-c]-
pyrrole-1,4-(2H,5H)-dione, 13-(6-bromohexyl)heptacosane were syn-
thesized following previously reported procedures.24−26
1
Measurements. H and 13C NMR spectra were measured on a
Varian Mercury Plus 300 MHz spectrometer. High-resolution mass
spectra (HRMS) were acquired on high-resolution mass spectrom-
eters: Q-TOF (ionization mode: ESI) and matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry (MALDI-
TOF) was performed on Voyager DE-STR (Applied Biosystems).
Thermal analyses were carried out on a TA Instruments Q600 (PH407
PUSAN, Korea Basic Science Institute (KBSI)) under an inert N2
atmosphere, with heating and cooling rates of 10 °C min−1. The UV−
vis absorption spectra were measured on a JASCO JP/V-570 model
spectrophotometer. The number-average (Mn) and weight-average
(Mw) molecular weights were determined by gel permeation
chromatography (GPC) (Waters model M590) using polystyrene as
a standard. Cyclic voltammetry experiments were performed with a
CH Instruments electrochemical analyzer in acetonitrile solutions
containing 0.1 M tetrabutylammonium tetrafluoroborate (Bu4NBF4)
as the supporting electrolyte, with Ag/AgNO3 as the reference
electrode, a platinum wire as the counter electrode, and a platinum
working electrode. Density functional theory (DFT) calculations were
performed using the Gaussian 09W package with the Becke three-
parameter Lee−Yang−Parr (B3LYP) function and the 6-31G(d) basis
set to elucidate the HOMO and LUMO energy levels and backbone
planarity. 2D-GIXD experiments were performed at the 3C beamlines
of the Pohang Accelerator Laboratory (PAL), Korea. X-rays with a
wavelength of 1.2096 Å (9.9899 keV) were used. The incidence angle
(0.13°) was chosen to allow for complete penetration of the X-rays
into the polymer film. The surface morphologies of the polymer thin
films were characterized by using an AFM (SPM System) operated in
the tapping mode. The polymers were spin-coated on the SiO2/Si
substrates and postannealed at the optimum temperatures for the 2D
GIXD measurements and AFM analysis.
Device Fabrication and Characterization. Top-gate bottom-
contact configuration was adopted for device fabrication. Glass
substrates with prepatterned Ni/Au (3 nm/13 nm) source and drain
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Scheme 2. Synthesis and Chemical Structures of the Polymers
electrodes were cleaned ultrasonically in deionized water, acetone, and
isopropyl alcohol for 10 min each, blown dry by nitrogen gas, and then
baked at 110 °C to remove residual water. The spacing (channel
length) between the contacts ranged from 10 to 50 μm, and the
channel width was 1000 μm. The polymers were dissolved in 1,2,4-
trichlorobenzene to form solutions at a concentration of 5 mg mL−1.
The polymer solutions were heated on a hot plate at 80 °C for
complete dissolution for more than 24 h and then spin-coated at
different spin coating rates of 1000, 1500, and 2000 rpm for 30 s (in
the case of 1000 rpm) and 60 s to form films with different
thicknesses. All four solutions were filtered before film deposition and
annealed at different temperatures ranging from 100 to 300 °C for 30
min to remove residual solvent and promote molecular packing.
PMMA (6.2 nF cm−2) was used as the gate dielectric. It was spin-
coated using the 80 mg mL−1 butyl acetate solution and spin-coated at
2000 rpm for 60 s to form a 500 nm thick dielectric layer and annealed
at 80 °C for 2 h. Al gate electrodes of 50 nm thickness were deposited
by thermal evaporation through shadow masks to complete the
devices. All of the above-mentioned fabrication procedures were
carried out under a nitrogen atmosphere.
Subsequently, it was allowed to cool to room temperature. To this
mixture, FeCl3·6H2O (7.75 g, 28.56 mmol) in 2 M aqueous HCl (15
mL) was added and stirred at 70 °C for 1 h. Then the mixture was
extracted with dichloromethane several times and dried over MgSO4.
The solvent was removed and purified by silica gel column
chromatography using dichloromethane:hexane (2:1 v/v) to obtain
the desired compound as a yellow solid. Yield: 74% (1.28 g). 1H NMR
(300 MHz, CDCl3, ppm): δ 7.39 (s, 2H), 7.35 (d, J = 5.1 Hz, 2H),
7.22 (d, J = 5.1 Hz, 2H). 13C NMR (75 MHz, CDCl3, ppm): δ 150.0,
129.3, 126.5, 122.3, 114.4, 110.0. Elem Anal. Calcd for C12H6N2S2: C,
59.5; H, 2.5; N,11.5; S, 26.5. Found: C, 59.2; H, 2.4; N,11.5; S, 26.5.
HRMS (ESI) m/z: 243.00 [M + H]+, calcd 243.00.
Synthesis of (E)-1,2-Bis(5-(trimethylstannyl)-3-cyanothiophene-
2-yl)ethene (3). A solution of compound 2 (0.24 g, 1 mmol) in
anhydrous THF (30 mL) was added dropwise at −78 °C to 2.0 M
lithium diisopropylamine (LDA, 1.05 mL, 2.1 mmol). After stirring for
1 h at −78 °C, trimethyltin chloride (3 mL, 3 mmol, 1 M in hexanes)
was added dropwise to the reaction mixture. Subsequently, the
reaction flask was warmed to room temperature and stirred overnight.
The reaction mixture was quenched with distilled water and extracted
with ethyl acetate. The organic layer was washed twice with brine and
dried over MgSO4. After drying the extracts, the obtained residue was
purified by recrystallization from ethanol to give a yellow product.
Electrical properties of the OFETs were characterized by using a
Keithley 4200-SCS parameter analyzer in a nitrogen-filled glovebox at
room temperature. The device parameters such as the mobility and
threshold voltage values were extracted in the saturation region by the
1
Yield: 74% (0.42 g). H NMR (300 MHz, CDCl3, ppm): δ 7.37 (s,
classic method using the formula Id,sat
=
W Ciμ(V − V )2, where Id is
2H), 7.21 (s, 2H), 0.37 (s, 18H). 13C NMR (75 MHz, CDCl3, ppm): δ
155.5, 141.5, 136.5, 121.8, 115.0, 110.9, − 8.1. Elem Anal. Calcd for
C18H22N2S2: C, 38.0; H, 3.9; N, 4.9; S, 11.3. Found: C, 36.8; H, 4.0; N,
4.7; S, 11.4. MS (MALDI-TOF) m/z: 569.12 [M+], calcd 569.93.
Synthesis of 2,5-Bis(7-dodecylhenicosyl)-3,6-di(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (4). A mixture of 3,6-di-
(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (3.00 g, 0.1
mol) and potassium carbonate (8.25 g, 0.6 mol) was dissolved in 100
mL of anhydrous N,N-dimethylformamide (DMF) and heated to 120
°C under nitrogen for 1 h. Subsequently, 13-(6-bromohexyl)-
heptacosane (15.54 g, 0.3 mol) was added dropwise, and the mixture
was stirred overnight at 120 °C. The mixture was allowed to cool
down to room temperature and quenched with distilled water. The
organic layer was then extracted with chloroform and dried over
MgSO4. The crude product was purified by silica column
chromatography with dichloromethane/hexane (1:1) to obtain a
purple compound 2-a. Yield: 26% (3.2 g). 1H NMR (300 MHz,
CDCl3, ppm): δ 8.95 (d, 2H), 7.65 (d, 2H), 7.30 (t, 2H), 4.07 (t, 4H),
1.75 (m, 4H), 1.25 (m, 114H), 0.88 (t, 12H). 13C NMR (75 MHz,
CDCl3, ppm): δ 161.34, 140.06, 135.25, 130.59, 129.81, 128.58,
107.71, 42.31, 37.35, 33.31, 31.93, 30.17, 29.72, 29.69, 29.68, 29.42,
29.38,26.45, 24.08, 22.71, 14.15. MS (MALDI-TOF) m/z: 1225.06
[M+], calcd 1225.04.
g
th
2L
drain-to-source current, W and L are the channel width and length,
respectively, Ci is the gate dielectric capacitance per unit area, μ is the
field effect mobility in the saturation regime, and Vg and Vth are gate-
to-source and threshold voltages, respectively.
Monomer and Copolymer Syntheses. Synthesis of (E)-1,2-
Bis(3-bromothiophen-2-yl)ethene (1). TiCl4 (12.07 mL, 109.8 mmol)
was added dropwise to a slurry of 3-bromothiophene-2-carbaldehyde
(14 g, 73.2 mmol) in THF (200 mL) with stirring over a period of 30
min at −18 °C. After stirring at this temperature for 30 min, Zn
powder (15 g, 219.6 mmol) was added in small portions over a period
of 30 min. The mixture was stirred at −18 °C for another 30 min,
warmed to room temperature, and then refluxed for 4 h. The reaction
was quenched by adding ice-cold H2O (100 mL). The mixture was
extracted with dichloromethane several times and dried over MgSO4.
The solvent was removed and purified by silica gel column
chromatography using hexane as the eluent to obtain the desired
product as a yellow solid. Yield: 39% (10 g) of the pure (E) isomer. 1H
NMR (300 MHz, CDCl3, ppm): δ 7.20 (d, J = 5.4 Hz, 2H), 7.12 (s,
2H), 6.98 (d, J = 5.4 Hz, 2H). 13C NMR (75 MHz, CDCl3, ppm): δ
136.5, 130.9, 124.4, 121.4, 111.5. HRMS (ESI) m/z: 348.86 [M + H]+,
calcd 348.83.
Synthesis of (E)-1,2-Bis(3-cyanothiophene-2-yl)ethene (2). The
mixture of compound 1 (2.5 g, 7.14 mmol) and CuCN (3.19 g, 35.7
mmol) in anhydrous DMF (100 mL) was stirred for 12 h at 150 °C.
Synthesis of 3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(7-dodecyl-
henicosyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (5). Compound 3
G
DOI: 10.1021/acs.macromol.7b01524
Macromolecules XXXX, XXX, XXX−XXX

Macromolecules
Article
(3.2 g, 2.6 mmol) was dissolved in 50 mL of CHCl3, and the reaction
flask was covered with aluminum foil. N-Bromosuccinimide (1.12 g,
5.98 mmol) was added to the flask at 0 °C, and the mixture was stirred
at room temperature overnight. Subsequently, the reaction was
quenched with distilled water. The organic layer was extracted with
chloroform and dried over MgSO4. The crude product obtained after
evaporating the solvent was recrystallized with CHCl3/MeOH to
pyrrole-1,4-(2H,5H) -dione (300 mg, 1.0 equiv), tris(dibenzylidene-
acetone)dipalladium(0) (3.97 mg, 4.33 μmol), tri(o-tolyl)phosphine
(5.28 mg, 17.3 μmol), and CB (4.5 mL). The polymer yield was 90%.
Elem Anal. Calcd for C92H142N4O2S4: C, 75.5; H, 9.8; N,3.8; S, 8.7.
Found: C, 75.3; H, 10.1; N, 3.4; S, 8.2. Mn = 53 kg mol−1, PDI = 1.3,
and Td = 405 °C.
1
obtain a deep purple solid. Yield: 87% (3.2 g). H NMR (300 MHz,
ASSOCIATED CONTENT
* Supporting Information
■
CDCl3, ppm): δ 8.67 (d, 2H), 7.25 (d, 2H), 3.96 (t, 4H), 1.70 (m,
2H), 1.25 (m, 114H), 0.87 (t, 12H). 13C NMR (75 MHz, CDCl3,
ppm): 161.75, 140.44, 135.62, 130.91, 130.20, 128.99., 108.18, 42.67,
37.85, 34.55, 32.23, 30.51, 30.35, 30.12, 29.73, 29.67, 27.30, 27,31,
27.05, 23.60, 14.51. MS (MALDI-TOF) m/z: 1381.02 [M+], calcd
1380.86.
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.macro-
mol.7b01524.
1H and 13C NMR spectra, TGA and CV results, DFT
calculations, OFET data, 2D-GIXD and AFM data
(PDF)
General Polymerization Procedure. The four copolymers were
synthesized via Stille coupling polymerization, as shown in Scheme 2.
(E)-1,2-Bis(5-(trimethylstannyl)thiophen-2-yl)ethene (TVT-Tin),
(E)-1,2-bis(5-(trimethylstannyl)-3-cyanothiophene-2-yl)ethene
(2CNTVT-Tin), 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-dodecyl-
hexadecyl)pyrrolo[3,4-c]pyrrole-1,4-(2H,5H)-dione (2DPP), and 3,6-
bis(5-bromothiophen-2-yl)-2,5-bis(7-dodecylhenicosyl)pyrrolo[3,4-c]-
pyrrole-1,4-(2H,5H)-dione (7DPP) were synthesized using previously
reported procedures. Tris(dibenzylideneacetone)dipalladium(0) and
tri(o-tolyl)phosphine were dissolved in anhydrous chlorobenzene
(CB) and stirred at 120 °C for 2 days. Subsequently, a solution of
excess 2-bromothiophene and tributyl(thiophen-2-yl)stannane (the
end-capper) in 1 mL of anhydrous toluene was added to the reaction
mixture and after 12 h of stirring, it was cooled to ∼50 °C and 200 mL
of methanol was added slowly with vigorous stirring. The crude
polymer was collected by filtration and purified by Soxhlet extraction
using methanol, acetone, hexane, and chloroform, successively. The
polymers were obtained by reprecipitation of the chloroform solution
into methanol.
Poly{2,5-bis(2-dodecylhexadecyl)-3,6-di(thiophen-2-yl)pyrrolo-
[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-bis(thiophen-2-yl)-
ethene} (2DPP-TVT). (E)-1,2-Bis(5-(trimethylstannyl)thiophen-2-yl)-
ethene (83.3 mg, 0.145 mmol) was mixed with 3,6-bis(5-bromo-
thiophen-2-yl)-2,5-bis(7-dodecylhenicosyl)pyrrolo[3,4-c]pyrrole-1,4-
(2H,5H)-dione (200 mg, 1.0 equiv), tris(dibenzylideneacetone)-
dipalladium(0) (2.94 mg, 2.91 μmol), tri(o-tolyl)phosphine (3.91
mg, 11.6 μmol), and CB (3 mL). Polymer yield = 91%. Elem Anal.
Calcd for C80H124N2O2S4: C, 75.4; H, 9.8; N, 2.2; S, 10.1. Found: C,
75.3; H, 10.0; N, 2.1; S, 10.3. Mn = 127 kg mol−1, PDI = 2.4, and Td =
398 °C.
Poly{2,5-bis(2-dodecylhexadecyl)-3,6-di(thiophen-2-yl)pyrrolo-
[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-bis(3-cyanothiophen-2-
yl)ethene} (2DPP-2CNTVT). (E)-1,2-Bis(5-(trimethylstannyl)-3-cyano-
thiophene-2-yl)ethene (137 mg, 0.241 mmol) was mixed with 3,6-
bis(5-bromothiophen-2-yl)-2,5-bis(7-dodecylhenicosyl)pyrrolo[3,4-c]-
pyrrole-1,4-(2H,5H)-dione (300 mg, 1.0 equiv), tris(dibenzylidene-
acetone)dipalladium(0) (4.42 mg, 4.82 μmol), tri(o-tolyl)phosphine
(5.87 mg, 19.3 μmol), and CB (4.5 mL). Polymer yield = 90%. Elem
Anal. Calcd for C82H122N4O2S4: C, 74.4; H, 9.3; N, 4.2; S, 9.7. Found:
C, 74.2; H, 9.4; N, 4.1; S, 9.8. Mn = 37 kg mol−1, PDI = 1.4, and Td =
401 °C.
Poly{2,5-bis(7-dodecylhenicosyl)-3,6-di(thiophen-2-yl)pyrrolo-
[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-(E)-1,2-bis(thiophen-2-yl)-
ethene} (7DPP-TVT). (E)-1,2-Bis(5-(trimethylstannyl)thiophen-2-yl)-
ethene (74.85 mg, 0.144 mmol) was mixed with 3,6-bis(5-bromo-
thiophen-2-yl)-2,5-bis(7-dodecylhenicosyl)pyrrolo[3,4-c]pyrrole-1,4-
(2H,5H)-dione (200 mg, 1.0 equiv), tris(dibenzylideneacetone)-
dipalladium(0) (2.64 mg, 2.89 μmol), tri(o-tolyl)phosphine (3.51
mg, 11.5 μmol), and CB (3 mL). Polymer yield = 93%. Elem Anal.
Calcd for C92H142N4O2S4: C, 76.4; H, 10.3; N,2.0; S, 9.1. Found: C,
76.2; H, 10.5; N, 1.9; S, 8.7. Mn = 314 kg mol−1, PDI = 2.6, and Td =
397 °C.
AUTHOR INFORMATION
Corresponding Authors
*(D.-H.H.) E-mail dohoonhwang@pusan.ac.kr.
*(Y.-Y.N.) E-mail yynoh@dongguk.edu.
■
ORCID
Yong-Young Noh: 0000-0001-7222-2401
Do-Hoon Hwang: 0000-0003-4183-0185
Author Contributions
H.S.K. and G.H. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
(NRF) grants funded by the Korea Government (NRF-
2017R1A2A2A05001345), the Korea Institute of Energy
Technology Evaluation and Planning (KETEP) and the
Ministry of Trade, Industry & Energy (MOTIE) of the
Republic of Korea (No. 20173010012960) and the Center for
Advanced Soft-Electronics (2013M3A6A5073183) funded by
the Ministry of Science and ICT of Republic of Korea.
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