3,6-Di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione and bithiophene copolymer with rather disordered chain orientation showing high mobility in organic thin film transistors

Article abstract of DOI:10.1039/c1jm11290b

Pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione or diketopyrrolopyrrole (DPP) is a useful electron-withdrawing fused aromatic moiety for the preparation of donor-acceptor polymers as active semiconductors for organic electronics. This study uses a DPP-furan-containing building block, 3,6-di(furan-2-yl)pyrrolo[3,4- c]pyrrole-1,4(2H,5H)-dione (DBF), to couple with a 2,2′-bithiophene unit, forming a new donor-acceptor copolymer, PDBFBT. Compared to its structural analogue, 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT), DBF is found to cause blue shifts of the absorption spectra both in solution and in thin films and a slight reduction of the highest occupied molecular orbital (HOMO) energy level of the resulting PDBFBT. Despite the fact that its thin films are less crystalline and have a rather disordered chain orientation in the crystalline domains, PDBFBT shows very high hole mobility up to 1.54 cm ² V^-1 s^-1 in bottom-gate, top-contact organic thin film transistors.

Full text of DOI:10.1039/c1jm11290b

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links <
C
Journal of
Materials Chemistry
Cite this: J. Mater. Chem., 2011, 21, 10829
www.rsc.org/materials
PAPER
3,6-Di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione and bithiophene
copolymer with rather disordered chain orientation showing high mobility in
organic thin film transistors†
ab
Prashant Sonar,b Samarendra P. Singh,b Wenjin Zengb and Mui Siang Sohb
*
Yuning Li,
Received 25th March 2011, Accepted 5th May 2011
DOI: 10.1039/c1jm11290b
Pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione or diketopyrrolopyrrole (DPP) is a useful electron-
withdrawing fused aromatic moiety for the preparation of donor–acceptor polymers as active
semiconductors for organic electronics. This study uses a DPP-furan-containing building block, 3,6-di-
(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBF), to couple with a 2,20-bithiophene unit,
forming a new donor–acceptor copolymer, PDBFBT. Compared to its structural analogue, 3,6-di-
(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT), DBF is found to cause blue shifts of the
absorption spectra both in solution and in thin films and a slight reduction of the highest occupied
molecular orbital (HOMO) energy level of the resulting PDBFBT. Despite the fact that its thin films are
less crystalline and have a rather disordered chain orientation in the crystalline domains, PDBFBT
shows very high hole mobility up to 1.54 cm2 Vꢀ1 sꢀ1 in bottom-gate, top-contact organic thin film
transistors.
The two thiophene units adjacent to DPP can alleviate their steric
Introduction
repulsions with DPP to maintain high coplanarity of the polymer
backbone, which is essential for achieving efficient charge
transport properties and low band gaps.
Pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione or diketopyrrolopyrrole
(DPP) has recently become a popular conjugated moiety incor-
porated in polymer semiconductors1 for organic light-emitting
diodes (OLEDs),2,3 organic thin film transistors (OTFTs),4–9 and
organic photovoltaics (OPVs).10–15 Due to its strong electron
withdrawing effect, DPP could reduce the lowest unoccupied
molecular orbital (LUMO) energy level of the resulting poly-
mers. Donor–acceptor polymers of DPP coupled with other
proper moieties exhibit interesting ambipolar transport proper-
ties, i.e., conducting both electrons and holes, making them
useful as channel semiconductors in ambipolar OTFTs for
fabricating CMOS-like circuits.4,6,9,14,15 They also show excellent
performance in OPV devices owing to their high hole mobility for
efficient charge transport as well as their optimum band gaps for
light harvesting.10–15 The DPP-containing building block, 3,6-di-
(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT,
Fig. 1), can be readily synthesized and is particularly useful for
constructing conjugated polymers for OTFTs and OPVs.4–9,14,15
Recently, we designed several DBT-based donor–acceptor
polymers, e.g., PDBT-co-TT,7 PDQT,8 and PDPP-TBT9 (Fig. 1),
which showed high hole mobility up to ꢁ1 cm2 Vꢀ1 sꢀ1 in p-
channel OTFT devices7,8 and balanced high electron (ꢁ0.40 cm2
Vꢀ1 sꢀ1) and hole (ꢁ0.35 cm2 Vꢀ1 sꢀ1) mobilities in ambipolar
OTFT devices.9 Until recently, furan has been paid much less
attention compared to thiophene and very few furan-containing
oligomers and polymers have been reported for OTFT
aDepartment of Chemical Engineering and Waterloo Institute for
Nanotechnology (WIN), University of Waterloo, 200 University Ave
West, ON, Canada N2L 3G1. E-mail: yuning.li@uwaterloo.ca; Fax: +1
15198884347; Tel: +1 1-519-888-4567 ext. 31105
bInstitute of Materials Research and Engineering (IMRE), Agency for
Science, Technology, and Research (A*STAR),
Singapore, 117602
3 Research Link,
Fig.
1
Structures of 3,6-di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
(DBT), 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4-
(2H,5H)-dione
† Electronic supplementary information (ESI) available: NMR,
MODI-TOF MS, and DSC data of compounds 1–3 and PDBFBT. See
DOI: 10.1039/c1jm11290b
(2H,5H)-dione (DBF) building blocks, and representative DBT-con-
taining polymers, PDBT-co-TT,7 PDQT,8 and PDPP-TBT,9 for OTFTs.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10829–10835 | 10829

View Article Online
applications.16–21 In this study, we use the furan-containing 3,6-
di-(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBF,
Fig. 1), a structural analogue of DBT, to synthesize a donor–
acceptor alternating copolymer, PDBFBT (Scheme 1). A few
DBF-containing polymers were reported very recently, which
showed good photovoltaic performance in OPVs.20,21 Our results
in this work demonstrate that the DBF building block could also
be potentially very useful for high performance polymer semi-
conductors in OTFTs.
Results and discussion
The DBF-bithiophene alternating copolymer, PDBFBT, was
synthesized according to Scheme 1. The DBF core, 3,6-di(furan-
2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (1), was prepared in
61% yield by reacting 2-furylnitrile with diisopropyl succinate in
the presence of sodium 2-methyl-2-butanoate in accordance to
a similar procedure for the preparation of 3,6-di(thiophen-2-yl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (DBT).5,7 Substitution
at nitrogen atoms with an alkyl side chain, 2-octyldodecyl, using
2-octyldodecyl bromide in the presence of potassium carbonate
and the subsequent bromination at the a-position of furan units
using bromine (Br2) afforded 2,5-bis(2-octyldodecyl)-3,6-di-
(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2) (48%
yield) and 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-octyldodecyl)-
Fig. 2 UV-vis-NIR absorption spectra of PDBFBT in chloroform and
in thin film.
PDQT, the structural analogue of PDBFBT, showed a longer
lmax (777 nm in chloroform; 790 nm in thin films) and a smaller
band gap (1.24 eV).8 Experimental and theoretical data have
shown that polyfuran has a wider band gap (ꢁ2.35 eV) than that
(ꢁ2.00 eV) of polythiophene.22 Our results are in good agreement
with this trend, showing that the introduction of furan units
indeed has broadened the optical band gap of PDBFBT (Fig. 2).
Electrochemical measurements of PDBFBT thin films with
cyclic voltammetry (CV) show a reversible oxidative process
(Fig. 3). The highest occupied molecular orbital (HOMO) energy
level was determined from the oxidation onset potential (ꢁ0.92
V) to be 5.32 eV, which is lower than that (5.20 eV) of PDQT.
The reductive process of PDBFBT is also reversible, but the
currents are significantly lower compared to the oxidative
process. The LUMO level of PDBFBT was estimated to be 3.91
eV from its optical band gap and HOMO level.
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione
(3)
(58%
yield),
respectively. Stille coupling polymerization of 3 with 5,50-bis-
(trimethylstannyl)-2,20-bithiophene formed the target polymer
PDBFBT, which was purified using Soxhlet extraction succes-
sively with acetone and hexane, yielding a dark solid (ꢁ99%).
PDBFBT showed excellent solubility in several common solvents
including chloroform, toluene, and tetrahydrofuran (THF) at
room temperature. The number average molecular weight (Mn)
and polydispersity index (PDI, Mw/Mn) of PDBFBT were
determined to be 130 500 and 2.27 by using gel permeation
ꢂ
chromatography (GPC) at a column temperature of 40 C with
Thermal properties of PDBFBT were characterized using
differential scanning calorimetry (DSC). PDBFBT shows an
endothermic peak at 286 ꢂC during the first heating scan and an
exothermic peak at 253 ꢂC during the subsequent cooling, which
THF as an eluent and polystyrene as standards.
PDBFBT in chloroform exhibits the maximum absorption
(lmax) at 770 nm along with one shoulder at 700 nm (Fig. 2). In
the solid thin film, PDBFBT shows only a slight red-shift of 6 nm
in lmax (776 nm), but the absorption profile is broadened
compared to its solution absorption spectrum. The optical band
gap of the polymer thin film is calculated from the absorption
cut-off (ꢁ880 nm) to be 1.41 eV. It was observed previously that
Fig. 3 Cyclic voltammograms of a PDBFBT thin film (red lines) and
a PDQT8 thin film (blue lines) showing the first oxidative and reductive
cycles at a scan rate of 100 mV sꢀ1. The electrolyte was 0.1 M
tetrabutylammonium hexafluorophosphate in anhydrous acetonitrile.
Scheme 1 Synthesis of PDBFBT: (i) t-C5H11OK/2-methyl-2-butanol/
120 ꢂC; (ii) K2CO3/DMF/130 ꢂC; (iii) chloroform/Br2/rt; (iv) Pd
(PPh3)2Cl2/toluene/90 ꢂC.
10830 | J. Mater. Chem., 2011, 21, 10829–10835
This journal is ª The Royal Society of Chemistry 2011

View Article Online
incident X-ray parallel and perpendicular to the polymer thin
films, respectively (Fig. 4). The diffraction patterns obtained with
the parallel and perpendicular X-ray irradiation modes are
similar (Fig. 4b and c), unlike PDQT which showed dramatically
different diffraction patterns with different X-ray irradiation
modes.8 The broad featureless diffractions from 2q ¼ 17 to 32ꢂ
are probably contributed from both the short range order of the
amorphous regions and the p–p stacking distance of the crys-
talline domains in the polymer thin films. The intensity ratio
(ꢁ8.9) of the primary peak to the broad peak at 2q ¼ ꢁ23ꢂ
obtained with the parallel X-ray mode (Fig. 4b) is slightly higher
than that (ꢁ5.9) obtained with a perpendicular X-ray mode
(Fig. 4c). The limited enhancement in the diffraction at 2q
¼ ꢁ23ꢂ observed with the X-ray perpendicular to the thin film
(Fig. 4c) might imply that the polymer chains in the thin films
have a slight preference in the edge-on orientation, but the face-
on and other tilted chain orientations likely co-exist (Fig. 4d).
The morphologies of PDBFBT thin films spin-coated on OTS-
modified Si/SiO2 substrates were examined with atomic force
microscopy (AFM). The AFM height and phase images are
shown in Fig. 5 and 6, respectively. The as-spun thin film without
thermal annealing shows a uniform surface with very fine
domains. As the annealing temperature increased to 100 ꢂC, large
domains appear in the AFM height image, but the phase image
remains uniform. When the annealing temperature increased to
Fig. 4 XRD diffraction patterns of PDBFBT thin films annealed at
different temperatures on Si/SiO2 substrates (a) and a stack of PDBFBT
thin films (annealed at 200 ꢂC under nitrogen for 15 min) with the inci-
dent X-ray parallel (b) and perpendicular (c) to the thin films. Insets in (b)
and (c) are 2-D XRD images obtained with the incident X-ray parallel
and perpendicular to the film stacks. (d) Illustrates the possible lamellar
crystalline domains with edge-on, face-on, and tilted chain orientations in
the polymer thin films.
ꢂ
150 C, worm-like fibrils start to appear in both the height and
phase images. The polymer thin film annealed at a further
elevated temperature of 200 ꢂC is composed of nanometre-sized
grains (ꢁ10’s nm in diameter), which are presumably due to the
evolution of crystallization. The thin film morphology (phase
images in particular) dependence on the annealing temperature
agrees well with the changes in crystallinity of the thin films upon
thermal annealing observed with XRD (Fig. 4a).
correspond to the melting and crystallization temperatures of
this polymer, respectively (see ESI†).
Molecular organization of PDBFBT was studied using X-ray
diffractometry (XRD) measurements on polymer thin films. A
heavily n-doped silicon wafer with a ꢁ200 nm thermally grown
SiO2 layer on top was used as the substrate. Prior to use, the
surface of the SiO2 layer was treated with octyltrichlorosilane
(OTS) to form a monolayer of octyl groups on the surface, which
could improve the interfacial properties of the substrate with the
subsequently coated polymer semiconductor layer and might
facilitate the establishment of an edge-on molecular packing
motif of the polymer chains.23 As shown in Fig. 4a, the as-spun
thin film exhibits a broad diffraction peak around 2q ¼ ꢁ20ꢂ (d
ꢀ
¼ ꢁ4.4 A), indicating the amorphous nature of this sample.
ꢂ
Upon annealing at 100 C, a diffraction peak at 4.56ꢂ appears.
As the annealing temperature increased further to 150 and 200
ꢂC, this primary diffraction peak becomes intensified, but the
broad peak around 2q ¼ ꢁ20ꢂ remains. Based on the previous
studies on PDQT8 and many other crystalline conjugated poly-
mers,24–28 the polymer chains in the annealed crystalline
PDBFBT thin films likely adopt a layer-by-layer lamellar
packing manner. The peak at 2q ¼ 4.56ꢂ corresponds to
ꢀ
a d-spacing of 19.4 A of the crystalline lamellae. These thin film
XRD results indicate that PDBFBT in the as-spun and annealed
thin films is less crystalline compared to its DBT-containing
polymer analogue, PDQT.8 To further elucidate the molecular
organization, two-dimensional (2-D) transmission XRD
measurements were carried out on a stack of polymer thin films
(annealed at 200 ꢂC for 30 min prior to measurements), with the
Fig. 5 AFM height images of a PDBFBT thin film (ꢁ30 nm) without
annealing (a) and annealed at 100 (b), 150 (c), and 200 ꢂC (d) for 15 min
under nitrogen.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10829–10835 | 10831

View Article Online
Fig. 6 AFM phase images of a PDBFBT thin film (ꢁ30 nm) without
annealing (a) and annealed at 100 (b), 150 (c), and 200 ꢂC (d) for 15 min
under nitrogen.
Field effect transistor performance of PDBFBT as a channel
semiconductor was evaluated in OTFT devices with a bottom-
gate, top-contact OTFT configuration. A polymer thin film (ꢁ30
nm) was deposited on top of the OTS-modified SiO2/Si substrate
by spin coating a polymer solution in chloroform (6 mg mLꢀ1).
The polymer thin films were optionally annealed on a hotplate at
100, 150, and 200 ꢂC, respectively, in a glove box filled with
nitrogen. Gold thin films were then thermally evaporated on top
of the polymer thin film to form source/drain electrode features
using a shadow mask. Current–voltage (I–V) characteristics of
OTFTs were measured under nitrogen.
As shown in Fig. 7a and b, an OTFT device with a non-
annealed polymer thin film displays typical p-type semiconductor
characteristics with hole mobility of 0.13 cm2 Vꢀ1 sꢀ1 in the
saturation regime and a current on-to-off ratio (Ion/Ioff) of 1.8
Fig. 7 Output (VGS ¼ 0 V to ꢀ60 V) and transfer characteristics of
OTFTs with a PDBFBT thin film without annealing (a and b), annealed
at 100 ꢂC (c and d), 150 ꢂC (e and f), and 200 ꢂC (g and h) for 15 min
under nitrogen (L ¼ 125 mm; W ¼ 4 mm).
ꢃ 106. Annealing the polymer thin film at 100, 150, and 200 C
ꢂ
increases the mobility progressively to 0.51, 0.65, and 1.54 cm2
Vꢀ1 sꢀ1, respectively, with high current on-to-off ratios main-
tained at ꢁ106 (Fig. 7c–h). The threshold voltages for all devices
are in the range of ꢀ3.42 to ꢀ5.79 V. Interestingly, the slope of
attributed to the contact resistance between the semiconductor
and the source/drain electrodes.32 The gate bias dependent
mobility observed for PDBFBT is likely related to a non-linear
increase of the charge carrier concentration against the
increasing gate voltage, which might be originated from (i) the
relatively low HOMO level (5.32 eV) of this polymer that poorly
matches the work function (ꢁ4.5 ꢄ 0.1 eV)35 of the gold contacts
and/or (ii) the poor interfacial properties of the polymer and the
dielectric layers. The polymer thin films annealed at 200 ꢂC
showed mobility of 1.02–1.54 cm2 Vꢀ1 sꢀ1 in the saturation regime
among 8 OTFT devices fabricated in two batches. It has been
conventionally considered that the establishment of an edge-on
chain orientation is favoured for efficient charge transport of
conjugated polymers such as P3HT24 in an OTFT where the
charge transport occurs parallel to the dielectric layer. It was only
very recently reported that a few polymers with a face-on chain
the (IDS)
1/2 vs. VGS curves (and thus the calculated mobility) for
all devices decreases as the gate voltage increases. For instance,
a mobility of 1.54 cm2 Vꢀ1 sꢀ1 for the OTFT device with a poly-
mer thin film annealed at 200 ꢂC is calculated from the linear
1/2
fitting of (IDS
)
with VGS in the range from ꢀ8 to ꢀ17 V
(Fig. 7g). As the gate voltage increases beyond ꢀ18 V, the slope
1/2
of the (IDS
)
vs. VGS curve starts to drop considerably. The
mobility calculated in the VGS range from ꢀ19 to ꢀ39 V is 0.17
cm2 Vꢀ1 sꢀ1. This phenomenon is rather unusual, as for most
organic semiconductors that show gate-dependent charge
transport behaviour, their mobility increases with increasing gate
voltage due to the presence of charge traps and structural
defects.29–31 Decreases in mobility as the gate voltage increases
were only observed for a few organic semiconductors, e.g.,
sexithiophene,32 rubrene,33 and pentacene,34 which were
10832 | J. Mater. Chem., 2011, 21, 10829–10835
This journal is ª The Royal Society of Chemistry 2011

View Article Online
orientation could also be highly efficient charge transport
materials in OTFTs.36,37 The high charge carrier transport
performance observed for PDBFBT having appreciable amor-
phous regions and without a preferential edge-on chain orien-
tation is rather unexpected. We think the high performance is
partially contributed from (i) the strong intermolecular interac-
tions of PDBFBT arisen from the fused DPP rings and the donor
(furan and thiophene units)–acceptor (DPP units) interactions
and (ii) the well-interconnected thin film morphology.7,8 The
enhanced intermolecular interactions would bring the polymer
chains into a close proximity, while the well-interconnected
morphology facilitates charge hopping between crystalline
domains.
thin film, because this polymer is relatively polar due to the
nature of donor–acceptor moieties in the conjugated backbone.
Heating the air-exposed devices would remove the moisture
absorbed on the polymer thin films and restore the device
performance.
Conclusions
We have demonstrated that 3,6-di(furan-2-yl)pyrrolo[3,4-c]-
pyrrole-1,4(2H,5H)-dione (DBF) is a promising building block
for preparing high performance conjugated polymers for
OTFTs. The copolymer of DBF with bithiophene, PDBFBT,
shows high hole mobility of up to 1.54 cm2 Vꢀ1 sꢀ1 even though
the polymer thin films are less crystalline and rather disordered in
the polymer chain orientation. The high field effect transistor
performance is considered to be partially originated from its
strong intermolecular interactions and the well-interconnected
thin film morphology.
OTFT devices were also characterized in ambient conditions
to investigate the air stability of this polymer. It was found that
the mobility decreased when the devices were measured in air.
However, once the devices were heated and then characterized
again in nitrogen, high mobility could be recovered. As shown in
Fig. 8, when measured in air, the device (with the PDBFBT thin
film pre-annealed at 100 ꢂC in nitrogen) showed a mobility of
0.17 cm2 Vꢀ1 sꢀ1 in the saturation regime, with a current on-to-off
ratio of 3.2 ꢃ 106. After the same device was heated at 100 ꢂC for
30 min and then measured again under nitrogen, a high mobility
of 0.74 cm2 Vꢀ1 sꢀ1 and a current on-to-off ratio of 2.0 ꢃ 107 were
obtained. These results suggest that the drop in mobility in air is
not caused by catastrophic photooxidative degradation of the
furan units.38 The observed low mobility of the OTFTs when
measured in air is also unlikely due to a reversible oxygen doping
mechanism,39 since PDBFBT possesses an even lower HOMO
level (5.32 eV) than that (5.1–5.25 eV) of several polymers25–28,40
which are stable against oxygen doping. The air sensitivity
observed for the PDBFBT OTFT devices is thought to be caused
by the physical absorption of water molecules on the polymer
Experimental
Instrumentation and materials
NMR data were collected on a Bruker DPX 300 MHz or 400
MHz NMR spectrometer with chemical shifts referenced to
tetramethylsilane (TMS). Matrix assisted laser desorption/
ionization time-of-flight (MALDI-TOF) mass spectra were
obtained on a Bruker Autoflex TOF/TOF. UV-vis-NIR spectra
were recorded on a Shimadzu model 2501-PC. Cyclic voltam-
metry (CV) experiments were performed using an Ecochimie
PGSTAT30 Autolab potentiostat in 0.1 M tetrabutylammonium
hexafluorophosphate in dry acetonitrile at a scanning rate of 100
mV sꢀ1. An Ag/AgCl in 3 M KCl electrode, a platinum wire, and
a platinum foil were used as the reference electrode, counter
electrode, and working electrode, respectively. The working
electrode was coated with a polymer film using a polymer solu-
tion in chloroform. The HOMO energy level wa0s calculated
0
using the equation EHOMO ¼ Ep + 4.4 eV, where Ep is the onset
potential for oxidation relative to the Ag/AgCl reference elec-
trode.41 Differential scanning calorimetry (DSC) was carried out
on a TA Instrument DSC Q100 under nitrogen. X-Ray diffrac-
tion patterns of thin films deposited on the octyltrichlorosilane
(OTS)-modified Si/SiO2 substrate was obtained with a PAN-
ꢀ
alytical X’PERT PRO system using Cu Ka source (l ¼ 1.5418 A).
Two-dimensional (2-D) transmission XRD measurements were
carried out similarly as described in ref. 28. The samples for 2-D
XRD measurements were prepared by evaporating the solvent
from a dilute polymer solution in chloroform to form a thin layer
of polymer thin film on the flask, followed by carefully rinsing off
the thin film with methanol. The polymer thin film (ꢁ100 nm
thick) was then cut into small pieces and stacked (ꢁ100 mm thick)
for 2-D XRD measurements. Gel permeation chromatography
(GPC) measurements were performed on a Waters 2690 System
at a column temperature of 40 ꢂC using tetrahydrofuran as eluent
and polystyrene as standards. Fourier-transform infrared (FT-
IR) analysis was carried out using a Bio-Rad FTS 3000MX
spectrometer. All chemicals were purchased from Sigma-Aldrich
and Strem as received. 5,50-Bis(trimethylstannyl)-2,20-bithio-
phene was synthesized according to the literature method.42
Fig. 8 Transfer (VDS ¼ ꢀ60 V) characteristics of an OTFT device (L ¼
75 mm; W ¼ 4 mm) with a PDBFBT thin film annealed at 100 ꢂC
measured in air (dotted lines) and under nitrogen after being re-heated at
100 ꢂC for 30 min (solid lines). The calculated mobility in the saturation
regime is 0.17 cm2 Vꢀ1 sꢀ1 measured in air and 0.74 cm2 Vꢀ1 sꢀ1 measured
under nitrogen after being re-heated.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10829–10835 | 10833

View Article Online
Synthesis of 3,6-di(furan-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (1)
Removal of the solvent afforded the crude product which was
further purified using column chromatography on silica gel (a
mixture of hexane and chloroform as eluent) to give the product
as a dark red solid (3.08 g, 58%). Mp (DSC): 105.0 ꢂC. 1H NMR
(400 MHz, CDCl3): d 0.85 (t, J ¼ 6.2 Hz, 12H), 1.10–1.40 (m,
64H), 1.76 (s, 2H), 3.96 (d, J ¼ 6.7 Hz, 4H), 6.59 (d, J ¼ 3.4 Hz,
2H), 8.27 (d, J ¼ 3.4 Hz, 2H). 13C NMR (100 MHz, CDCl3):
d 160.87, 146.23, 132.78, 126.13, 122.14, 115.45, 106.34, 46.63,
38.78, 31.87, 31.84, 31.47, 30.07, 29.59, 29.52, 29.46, 29.29, 29.24,
26.46, 22.62, 14.02. MS (MALDI-TOF, m/z): calcd. for
C54H86Br2N2O4, 984.50; found, 986.38 (M + 2).
To a three-necked flask containing 2-methyl-1-butanol (60 mL)
was added sodium (3.45 g, 0.15 mol) under argon. The mixture
ꢂ
was heated to 90 C and iron(III) chloride (FeCl3) (50 mg) was
added. After sodium disappeared, the solution was cooled to
85 ꢂC. 2-Furonitrile (9.31 g, 0.10 mol) was added to the reaction
mixture, followed by drop-wise addition of diisopropyl succinate
(8.1 g, 0.04 mol) in 2-methyl-1-butanol (5 mL) over 1 h at 85 ꢂC.
When the addition was completed, the mixture was stirred for
additional 2 h at this temperature. The reaction mixture was then
cooled to 50 ꢂC, diluted with methanol (50 mL), and then slowly
neutralized with glacial acetic acid (ꢁ15 mL) and refluxed for 15
min. The reaction mixture was cooled down to room temperature
and filtered. The solid was washed respectively with hot meth-
anol and de-ionized (DI) water several times, and dried in vacuo
Synthesis of PDBFBT
To a 50 mL flask equipped with a condenser were charged 3
(0.2961 g, 0.3 mmol), 5,50-bis(trimethylstannyl)-2,20-bithiophene
(0.1476 g, 0.3 mmol), and bis(triphenylphosphine)palladium(II)
dichloride (7 mg, 0.01 mmol). After degassing and refilling argon
for 3 times, toluene (20 mL) was added and the reaction mixture
was raised to 90 ꢂC and stirred for 48 h. To the reaction mixture
was then added bromobenzene (0.5 mL) to react with the residual
trimethylstannyl end group. The mixture was further stirred at
90 ꢂC for 8 h before cooling down to room temperature. The
mixture was then poured into stirring methanol (200 mL), filtered
off, washed with methanol, and dried. The solid was then further
purified by Soxhlet extraction using acetone (24 h), hexane (24 h),
and then dissolved with chloroform. A blue solid was obtained.
Yield: 0.294 g (98.7%). 1H NMR (400 MHz, CDCl3): d 8.95 (br),
6.85 (br), 4.01 (br), 3.05 (br), 1.22 (br), 0.85 (br). GPC (at 40 ꢂC;
THF as eluent; polystyrene as standards): Mn ¼ 130 500; Mw/Mn
¼ 2.27. UV-vis-NIR: lmax ¼ 770 nm (chloroform); 776 nm (thin
film). Anal. Calcd (%) for C62H90N2O4S2: C 75.10, H 9.15, N
2.83, S 6.47; found: C 75.46, H 9.14, N 3.31, S 6.16. FT-IR: 3068,
2954, 2925, 2853, 1585, 1666, 1558, 1480, 1397, 1350, 1103, 1034,
ꢂ
at 50 C for 15 h. A dark red solid was obtained. Yield: 6.54 g
(61.0%). 1H NMR (300 MHz, DMSO-d6): d 11.17 (s, 2H), 8.04 (d,
J ¼ 1.3 Hz, 2H), 7.65 (d, J ¼ 3.4 Hz, 2H), 6.83 (dd, J ¼ 1.3, 3.4
Hz, 2H). 13C NMR (75 MHz, DMSO-d6): d 107.57, 113.71,
116.79, 131.27, 143.75, 146.91, 161.71.
Synthesis of 2,5-bis(2-octyldodecyl)-3,6-di(furan-2-yl)pyrrolo-
[3,4-c]pyrrole-1,4(2H,5H)-dione (2)
To a dry 250 mL three-neck round bottom flask were added 1
(3.0 g. 11.18 mmol), anhydrous K2CO3 (4.63 g, 33.54 mmol), and
anhydrous N,N-dimethylfoꢂrmamide (DMF) (250 mL). The
mixture was heated to 120 C under argon for 1 h. 2-Octyldo-
decylbromide (12.12 g, 33.54 mmol) was then added drop-wise,
and the reaction mixture was further stirred overnight at 130 ꢂC.
The reaction mixture was allowed to cool down to room
temperature and poured into DI water (500 mL) and stirred for
30 min. The product was extracted with chloroform, washed with
DI water, and dried over anhydrous MgSO4. Removal of the
solvent afforded the crude product which was further purified
using column chromatography on silica gel (a mixture of hexane
and chloroform as eluent) to give the product as a red solid (4.5 g,
48%). Mp (DSC): 110.5 ꢂC. 1H NMR (400 MHz, CDCl3): d 0.86
(t, J ¼ 6.4 Hz, 12H), 1.15–1.40 (m, 64H), 1.79 (s, 2H), 4.01 (m,
4H), 6.68 (dd, J ¼ 1.4, 1.6 Hz, 2H), 7.59 (s, 2H), 8.32(d, J ¼ 3.6
Hz, 2H). 13C NMR (100 MHz, CDCl3): d 160.89, 146.25, 132.82,
126.16, 122.17, 115.48, 106.36, 46.65, 38.80, 31.90, 31.87, 31.48,
30.09, 29.62, 29.61, 29.54, 29.49, 29.32, 29.27, 26.48, 22.65, 22.63,
14.05. MS (MALDI-TOF, m/z): calcd for C54H88N2O4, 828.67;
found, 828.53 (M).
790, 745, 733 cmꢀ1
.
Fabrication and characterization of OTFT devices
PDBFBT was tested in OTFT devices with a top-contact,
bottom-gate configuration, fabricated following
a similar
procedure as described in ref. 7. A heavily n-doped silicon wafer
with a thermally grown silicon oxide (SiO2) layer (ꢁ200 nm)
having a capacitance of 17.25 nF cmꢀ2 was used, wherein the
wafer acts as the gate electrode and the SiO2 layer acts as the gate
dielectric. A solution of PDBFBT in chloroform (6 mg mLꢀ1
)
was filtered through a 0.2 mm syringe filter, and then spin-coated
on the OTS-treated substrate at 1000 rpm for 60 s at room
temperature. The polymer thin film (ꢁ30 nm in thickness) was
Synthesis of 3,6-bis(5-bromofuran-2-yl)-2,5-bis(2-octyldodecyl)-
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3)
ꢂ
annealed at 100, 150, and 200 C, respectively, under nitrogen.
To a 100 mL three neck flask equipped with a stirring bar,
a condenser, and a dropping funnel were charged with 2 (4.5 g,
5.42 mmol) and chloroform (40 mL). Bromine (Br2) (0.55 mL,
10.84 mmol) in chloroform (20 mL) was then added to the flask
at room temperature through the dropping funnel. The mixture
was stirred at room temperature overnight, then slowly poured
into an aqueous solution of sodium thiosulfate and stirred for
additional 30 min. The product was extracted with chloroform,
then washed with DI water, and dried over anhydrous MgSO4.
Gold source/drain electrodes (ꢁ100 nm in thickness) were
deposited on top of the polymer semiconductor layer by vacuum
deposition through a shadow mask. The evaluation of transistor
performance was carried out in a glove box filled with nitrogen
using a Keithley 4200 SCS semiconductor characterization
system.
The charge carrier mobility was calculated from the data in the
saturated regime (gate voltage, VGS < source-drain voltage, VSD
according to the literature method.7
)
10834 | J. Mater. Chem., 2011, 21, 10829–10835
This journal is ª The Royal Society of Chemistry 2011

View Article Online
22 U. Salzner, J. B. Lagowski, P. G. Pickup and R. A. Poirier, Synth.
Met., 1998, 96, 177.
Acknowledgements
23 Handbook of Liquid Crystals, ed. D. Demus, J. W. Goodby, G. W.
Gray, H. W. Spiess and V. Vill, Wiley-VCH, Weinheim, Germany,
1998.
24 H. Sirringhaus, P. J. Brown, R. H. Friend, M. M. Nielen,
K. Bechgaard, B. M. W. Langeveld-Voss, A. J. H. Spiering,
R. A. J. Janssen, E. W. Meijer, P. Herwig and D. M. de Leeuw,
Nature, 1999, 401, 685.
The authors thank the IMRE for financial support, Mr. Poh
Chong Lim for assistance with 2-D XRD. SS and WZ equally
contributed to this work.
Notes and references
25 B. S. Ong, Y. Wu, Y. Li, P. Liu and H. Pan, Chem.–Eur. J., 2008, 14,
4766.
1 B. Tieke, A. R. Rabindranath, K. Zhang and Y. Zhu, Beilstein J. Org.
Chem., 2010, 6, 830.
26 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. Macdonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. M. Zhang,
M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney,
Nat. Mater., 2006, 5, 328.
€
2 T. Beyerlein, B. Tieke, S. Forero-Lenger and W. Brutting, Synth.
Met., 2002, 130, 115.
3 D. Cao, Q. Liu, W. Zeng, S. Han, J. Peng and S. Liu, Macromolecules,
2006, 39, 8347.
27 Y. Li, Y. Wu, P. Liu, M. Birau, H. Pan and B. S. Ong, Adv. Mater.,
2006, 18, 3029.
€
4 L. Burgi, M. Turbiez, R. Pfeiffer, F. Bienewald, H. Kirner and
C. Winnewisser, Adv. Mater., 2008, 20, 2217.
28 H. Pan, Y. Li, Y. Wu, P. Liu, B. S. Ong, S. Zhu and G. Xu, J. Am.
Chem. Soc., 2007, 129, 4112.
5 Y. Li, US Pat. Application 2009/65766 A1, 2009 and 2009/0065878A1,
2009.
29 A. R. Brown, C. P. Jarrett, D. M. de Leeuw and M. Matters, Synth.
Met., 1997, 88, 37.
30 C. D. Dimitrakopoulos, S. Purushothaman, J. Kymissis, A. Callegari
and J. M. Shaw, Science, 1999, 283, 822.
31 G. Horowitz, R. Hajlaoui, D. Fichou and A. ElKassmi, J. Appl.
Phys., 1999, 85, 3202.
6 J. C. Bijleveld, A. P. Zoombelt, S. G. J. Mathijssen, M. M. Wienk,
M. Turbiez, D. M. de Leeuw and R. A. J. Janssen, J. Am. Chem.
Soc., 2009, 131, 16616.
7 Y. Li, S. P. Singh and P. Sonar, Adv. Mater., 2010, 22, 4862.
8 Y. Li, P. Sonar, S. P. Singh, M. S. Soh, M. van Meurs and J. Tan, J.
Am. Chem. Soc., 2011, 133, 2198.
32 G. Horowitz, M. E. Hajlaoui and R. Hajlaoui, J. Appl. Phys., 2000,
87, 4456.
9 P. Sonar, S. P. Singh, Y. Li, M. S. Soh and A. Dodabalapur, Adv.
Mater., 2010, 22, 5409.
33 K. Kima, M. K. Kima, H. S. Kanga, M. Y. Choa, J. Joo, J. H. Kimb,
K. H. Kimb, C. S. Hongb and D. H. Choi, Synth. Met., 2007, 157,
481.
34 Y. Kimura, M. Niwano, N. Ikuma, K. Goushi and K. Itaya,
Langmuir, 2009, 25, 4861.
10 M. M. Wienk, M. Turbiez, J. Gilot and R. A. J. Janssen, Adv. Mater.,
2008, 20, 2256.
11 E. Zhou, S. Yamakawa, K. Tajima, C. Yang and K. Hashimoto,
Chem. Mater., 2009, 21, 4055.
12 Y. Zou, D. Gendron, R. Neagu-Plesu and M. Leclerc,
Macromolecules, 2009, 42, 6361.
13 L. Huo, J. Hou, H. Chen, S. Zhang, Y. Jiang, T. L. Chen and
Y. Yang, Macromolecules, 2009, 42, 6564.
14 A. P. Zoombelt, S. G. J. Mathijssen, M. G. R. Turbiez, M. M. Wienk
and R. A. J. Janssen, J. Mater. Chem., 2010, 20, 2240.
15 J. C. Bijleveld, V. S. Gevaerts, D. D. Nuzzo, M. Turbiez,
S. G. J. Mathijssen, D. M. de Leeuw, M. M. Wienk and
R. A. J. Janssen, Adv. Mater., 2010, 22, E242.
35 S. Braun, W. R. Salaneck and M. Fahlman, Adv. Mater., 2009, 21,
1450.
36 J. Rivnay, M. F. Toney, Y. Zheng, I. V. Kauvar, Z. Chen, V. Wagner,
A. Facchetti and A. Salleo, Adv. Mater., 2010, 22, 4359.
37 W. Zhang, J. Smith, S. E. Watkins, R. Gysel, M. McGehee, A. Salleo,
J. Kirkpatrick, S. Ashraf, T. Anthopoulos, M. Heeney and
I. McCulloch, J. Am. Chem. Soc., 2010, 132, 11437.
38 U. H. F. Bunz, Angew. Chem., Int. Ed., 2010, 49, 5037.
39 (a) M. S. A. Abdou, F. P. Orfino, Y. Son and S. Holdcroft, J. Am.
Chem. Soc., 1997, 119, 4518; (b) C.-K. Lu and H.-F. Meng, Phys.
Rev. B: Condens. Matter Mater. Phys., 2007, 75, 235206; (c)
H.-H. Liao, C.-M. Yang, C.-C. Liu, S.-F. Horng, H.-F. Meng and
J.-T. Shy, J. Appl. Phys., 2008, 103, 104506.
16 S. Andoa, A. Nishida, E. Fujiwara, H. Tadab, Y. Inoue, S. Tokito
and Y. Yamashita, Synth. Met., 2006, 156, 327.
17 T. Minari, Y. Miyata, M. Terayama, T. Nemoto, T. Nishinaga,
K. Komatsu and S. Isoda, Appl. Phys. Lett., 2006, 88, 083514.
18 O. Gidron, A. Dadvand, Y. Sheynin, M. Bendikov and
D. F. Perepichka, Chem. Commun., 2011, 47, 1976.
19 T.-L. Choi, K.-M. Han, J.-I. Park, D.-H. Kim, J.-M. Park and S. Lee,
Macromolecules, 2010, 43, 6045.
40 B. S. Ong, Y. Wu, P. Liu and S. Gardner, J. Am. Chem. Soc., 2004,
126, 3378.
41 (a) Y. Li, J. Ding, M. Day, Y. Tao, J. Lu and M. D’iorio, Chem.
Mater., 2004, 16, 2165; (b) D. M. Leeuw, M. M. J. Simenon,
A. R. Brown and R. E. F. Einerhand, Synth. Met., 1997, 87, 53; (c)
Y. Cui, X. Zhang and S. A. Jenekhe, Macromolecules, 1999, 32, 3824.
42 Y. Wei, Y. Yang and J.-M. Yeh, Chem. Mater., 1996, 8, 2659.
20 C. H.Woo, P. M. Beaujuge, T. W. Holcombe, O. P. Lee and
ꢁ
J. M. J. Frechet, J. Am. Chem. Soc., 2010, 132, 15547.
21 J. C. Bijleveld, B. P. Karsten, S. G. J. Mathijssen, M. M. Wienk,
D. M. de Leeuw and R. A. J. Janssen, J. Mater. Chem., 2011, 21, 1600.
This journal is ª The Royal Society of Chemistry 2011
J. Mater. Chem., 2011, 21, 10829–10835 | 10835