Diketopyrrolopyrrole-based semiconducting polymer bearing thermocleavable side chains

Article abstract of DOI:10.1039/c2jm33818a

The synthesis of a diketopyrrolopyrrole (DPP)-based semiconducting polymer bearing thermocleavable side chains is described. This polymer can be converted into a native semiconductive polymer by thermal annealing at 200 °C. The resulting side chain-free polymer showed a good p-type semiconductor performance in organic thin film transistors. The Royal Society of Chemistry.

Full text of DOI:10.1039/c2jm33818a

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PAPER
Diketopyrrolopyrrole-based semiconducting polymer bearing
thermocleavable side chains†
ab
Bin Sun,ab Wei Hong,ab Hany Azizbc and Yuning Li
*
Received 14th June 2012, Accepted 12th July 2012
DOI: 10.1039/c2jm33818a
The synthesis of a diketopyrrolopyrrole (DPP)-based semiconducting polymer bearing
thermocleavable side chains is described. This polymer can be converted into a native semiconductive
polymer by thermal annealing at 200 ꢀC. The resulting side chain-free polymer showed a good p-type
semiconductor performance in organic thin film transistors.
reported a polythiophene having tertiary ester side chains, which
Introduction
could result in a polythiophene with the carboxylic acid side
chains upon heating at 210 ꢀC.5a To form a completely side
chain-free (native) polythiophene, a higher temperature of
310 ꢀC was required to remove the carboxylic acid groups.5c,d Bao
et al. recently found that the secondary or tertiary alkyl groups
on the nitrogen atoms of quaterrylene diimides could be removed
at elevated temperatures (ꢁ350 to 400 ꢀC) with the chromophore
intact.4e After thermally removing the side chains, the resulting
quaterrylene diimide showed a significant increase in mobility in
OTFTs. Holdcroft et al. reported polythiophenes having side
chains comprising acid-sensitive tetrahydropyran (THP)
groups.3 In the presence of a trace amount of acid and at an
elevated temperature, the THP groups were readily detached to
form a closer polymer chain packing due to the shortened side
chains.3aꢀ The THP group was also found thermocleavable
at ꢁ300 C.5a,e
p-Conjugated polymer semiconductors have attracted much
attention in recent years as active materials in organic thin film
transistors (OTFTs),1 organic photovoltaics (OPVs),2 etc. Their
excellent solution processability and mechanical robustness
allow for fabrication of various low-cost, large-area, and flexible
organic electronic products such as displays, radio-frequency
identification (RFID) tags, memory devices, and solar cells. To
be solution-processable, polymer semiconductors require the
incorporation of a large portion of solubilizing side chains to
oppose the strong aggregation tendency of polymer backbones
from solution. For instance, the hexyl side chains in poly-
(3-hexylthiophiophene) (P3HT) constitute of up to ꢁ51% of the
polymer mass. The non-conjugated side chains would separate
the semiconductive aromatic polymer backbones and intrinsi-
cally reduce the overall charge carrier transport performance.
Labile side chain groups, which provide the needed solubility and
can be chemically3 or thermally4,5 removed under mild condi-
tions, have been used to substitute both small molecular and
polymeric semiconductors to achieve a higher density of the
aromatic components via a post-deposition treatment. For
example, cyclohexadiene4a and N-sulfinylamide4b were used to
form soluble pentacene precursors, which can undergo thermally
activated retro Diels–Alder reactions to recover the native sem-
Here we report a novel polymer semiconductor, PDQT-tc,
which has 2-octyldodecanoyl side chains that can be completely
ꢀ
thermally removed at a low temperature of ꢁ200 C to form a
side chain-free conjugated polymer PDQT-n. Our results show
that this native conjugated polymer is a promising semi-
conductor for OTFTs and other organic electronics.
ꢀ
iconductive pentacene after solution deposition. Frechet et al.
Results and discussion
used thermally decomposable secondary ester groups as terminal
substituents for oligothiophenes.4c,d After thermal annealing, the
resulting oligothiophenes possessed shorter terminal groups and
showed good performance in OTFTs. The same research group
The key building block for PDQT-tc is 2,5-dihydro-pyrrolo[3,4-
c]pyrrole-1,4-dione or diketopyrrolopyrrole (DPP), which has
been an increasingly popular electron acceptor moiety for con-
structing high mobility donor–acceptor polymers for OTFTs.6–9
The tert-butoxycarbonyl (t-BOC) substituted DPP molecule was
found to be thermally decomposable at ꢁ180 ꢀC.10 Very recently
a DPP-based copolymer incorporating a t-BOC-substituted DPP
unit was reported.11 The t-BOC groups in the polymer could be
thermally removed at ꢁ200 ꢀC and the resulting polymer showed
enhanced electron transport performance. Previously we have
developed a high mobility polymer PDQT,6c which has solubil-
ising 2-octyldodecyl (OD) side chains on the DPP units. In this
aDepartment of Chemical Engineering, University of Waterloo, 200
University Ave West, Waterloo, ON, Canada, N2L 3G1. E-mail: yuning.
li@uwaterloo.ca
bWaterloo Institute for Nanotechnology (WIN), University of Waterloo,
200 University Ave West, Waterloo, ON, Canada, N2L 3G1
cDepartment of Electrical and Computer Engineering, University of
Waterloo, 200 University Ave West, Waterloo, Ontario, Canada N2L 3G1
† Electronic supplementary information (ESI) available: NMR, GPC,
FT-IR, and AFM data. See DOI: 10.1039/c2jm33818a
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Fig. 1 TGA curves of PDQTꢂ-t1c, PDQT, compound 2 and compound 3
at a heating rate of 10 ꢀC min under nitrogen.
PDQT,6c which has 2-octyldodecyl side chains, showed a much
higher onset thermal decomposition temperature at ꢁ400 ꢀC. A
weight loss of ꢁ46% for PDQT-tc levelled off at ꢁ350 ꢀC, which
is ꢁ10% less than the calculated mass fraction (ꢁ56%) of the 2-
octyldodecanoyl side chains in PDQT-tc. This discrepancy is
probably due to trapping of some decomposed side chain frag-
ments in the polymer sample, whereas for the small molecule 3
the decomposed side chains could evaporate readily and its
weight loss at ꢁ350 ꢀC is close to the calculated value (ꢁ56%).
The chemical structural changes of PDQT-tc upon heating
were studied by FT-IR spectroscopy. The thin film samples
(ꢁ63 nm) for FT-IR measurements were prepared by spin
coating a PDQT-tc solution in chloroform on boron-doped
silicon wafer substrates with a (111) orientation and a resistivity
of ꢁ10 U cm followed by thermal annealing in a glove box for
20 min at various temperatures. It can be clearly seen that the
peaks (2922 and 2851 cmꢂ1) representing the side chain C–H
groups decreased upon heating at ꢁ200 ꢀC and completely
disappeared after heating at 250 ꢀC or higher (Fig. 2: top). It
was also observed that the peak at 1598 cmꢂ1, which originated
from the N–H vibration, appeared for a film annealed at
200 ꢀC. Extending the annealing time to 3 h, the side chains
could be completely removed at 200 ꢀC (Fig. 2: bottom). The
thickness of the polymer films with side chains completely
removed is ꢁ26 nm, which is ꢁ41% of the thickness (ꢁ63 nm)
of the films before the thermal treatment (ESI†), in a good
match with the calculated value (44%), assuming that the
density of the polymer before and after thermal annealing
remains similar. As opposed to the trapping of some decom-
posed side chains observed in the TGA results, the complete
elimination of side chains of PDQT-tc confirmed by FT-IR is
likely due to the easier evaporation of the decomposed side
chain fragments in the thin films used for the FT-IR measure-
ments than in the bulk samples used for the TGA. The liberated
N–H groups in the resulting PDQT-n without side chains are
expected to strongly interact with the C]O groups on the DPP
units of the neighbouring polymer chains to form intermolec-
ular hydrogen bonding (NH/O),12 which would help the
establishment of tightly held three-dimensional networks of
native semiconducting polymer backbones (Scheme 1).
Scheme 1 Synthesis of PDQT-tc and PDQT-n: (a) (i) NaH, NMP, (ii)
2-octyldodecanoyl chloride; (b) NBS, CHCl3; (c) Pd(PPh3)2Cl2, toluene,
90 ꢀC; (d) heating at T ꢀC; 200 ꢀC;C under nitrogen.
study, we synthesized a PDQT derivative, PDQT-tc, which has
2-octyldodecanoyl groups on the nitrogen atoms. The synthesis
of PDQT-tc is outlined in Scheme 1. 2,5-Dihydro-3,6-di-2-
thienyl-pyrrolo[3,4-c]pyrrole-1,4-dione (1) was first substituted
with 2-octyldodecanoyl groups at two nitrogen atoms by reacting
with NaH in N-methylpyrrolidone (NMP) followed by addition
of 2-octyldodecanoyl chloride, yielding 2,5-bis(2-octyldodeca-
noyl)-2,5-dihydro-3,6-di-2-thiophen-pyrrolo[3,4-c]pyrrole-1,4-
dione (2) in 10% yield. Bromination of 2 with 2 equivalents of
N-bromosuccinimide (NBS) gave 3,6-bis(5-bromothiophen-2-yl)-
2,5-bis(2-octyldodecanoyl)-2,5-dihydro-pyrrolo[3,4-c]pyrrole-1,4-
dione (3) in 65% yield. Stille coupling polymerization of 3 with
5,50-bis(trimethylstannyl)-bithiophene was conducted in toluene
at 90 ꢀC for 24 h in the presence of a catalytic amount of
bis(triphenylphosphine)palladium(II) dichloride. The resulting
crude polymer was purified by Soxhlet extraction sequentially
with acetone and hexane to remove oligomers and other impu-
rities. Extraction with chloroform dissolved the remaining
polymer, affording blue solid films after removal of the solvent in
a high yield of 92%. The number average molecular weight (Mn)
and polydispersity index (PDI) of PDQT-tc were determined by
gel-permeation chromatography (GPC) to be 24 334 and 3.90,
respectively, using chlorobenzene as an eluent and polystyrene as
ꢀ
standards at a column temperature of 40 C.
The thermal decomposition characteristics of PDQT-tc were
investigated by using thermogravimetric analysis (TGA). The
thermal liability of the –N–C(]O)– linkage was demonstrated
by heating compounds 2 and 3. Both compounds started to
decompose at ꢁ180 ꢀC, although compound 3 was found to
decompose at a slightly faster rate (Fig. 1). Polymer PDQT-tc
ꢀ
started to lose weight at ꢁ200 C, while its structural analogue
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resulting from the improved molecular ordering by the thermal
annealing. After annealing at 200 ꢀC for 3 h, this diffraction peak
completely disappeared. This is due to the elimination of the side
chains, which results in the shortening of the interlayer distance. It
is also noted that the diffraction intensity in the broad range
from ꢁ15 to 30ꢀ increased, which is possibly the result of
formation of the amorphous phase. The atomic force microscopic
(AFM) images in Fig. 4 show that the PDQT-tc film annealed at
150 ꢀC (without removal of side chains) is quite smooth (with an
RSM roughness of 0.89 nm), which is comprised of small nano-
grains (ꢁ20 to 30 nm). After removal of side chains by annealing
at 200 ꢀC for 3 h, the nanograins in the resulting PDQT-n film are
connected to form nanofibers. The film becomes rougher (with
an RSM roughness of 1.42 nm) and has large gaps (ꢁ10’s to
100’s nm) between domains.
The side chain-free PDQT-n films are insoluble in any solvents
for NMR analysis. To indirectly elucidate the chemical structure
ꢀ
of PDQT-n, compound 2 was thermally decomposed at 250 C
for 20 min in a sealed vial on a hotplate. The mixture after
heating was analyzed using 1H NMR, which unambiguously
showed that 2 was completely decomposed and the new peaks
that appeared in the aromatic region are identical to the peaks of
the known compound 1 with free N–H groups (ESI†). The
thermal decomposition of compound 2 to form compound 1 was
also investigated using FT-IR, which confirmed that the removal
of the side chains and the appearance of the N–H groups started
at ꢁ200 ꢀC (ESI†). Therefore, it is reasonable to assume that
PDQT-tc was converted into the side chain-free native conju-
gated polymer PDQT-n as proposed in Scheme 1.
Fig. 2 Top: FT-IR spectra of PDQT-tc films on Si wafer annealed at
different temperatures for 20 min in nitrogen. Bottom: FT-IR spectra of
PDQT-tc films on Si wafer at 200 ꢀC for 20 min and 3 h, respectively.
The UV–Vis absorption spectra of PDQT-tc in chloroform
solution and of thin films are shown in Fig. 5. In solution,
PDQT-tc exhibited an absorption maximum (lmax) at 762 nm.
The as-spun thin film showed a lmax at a longer wavelength of
800 nm, suggesting an improved coplanarity of the polymer
backbone and chain packing order. Thermal annealing at 150 ꢀC
led to a further red-shift in the lmax to 820 nm. As the annealing
temperature was increased to 200 ꢀC, the absorption peak of the
resulting PDQT-n film significantly blue-shifted to 772 nm. This
dramatic change suggests the shortening of the effective p-
conjugation length of the polymer chains, which is probably the
direct result of the formation of a less ordered, amorphous phase
where the polymer backbones are rather twisted. The strong
interchain hydrogen bonding between the C]O and the N–H
The X-ray diffractometry (XRD) measurements of the as-spun
film and the films annealed at 150 and 200 ꢀC are shown in Fig. 3.
The as-spun film showed a diffraction peak at 2q ¼ ꢁ4.40ꢀ, which
ꢁ
corresponds to a d-spacing of 20.1 A. This peak is considered to
reflect the inter-lamellar distance as observed for PDQT.6c The
much weaker diffraction intensity for PDQT-tc is probably due to
the presence of the C]O groups on the 2-octyldodecanoyl side
chains, which interfere with the molecular ordering. The primary
peak of the thin film annealed at 150 ꢀC became intensified,
Fig. 3 XRD diagrams obtained from PDQT-tc thin films on DTS-
treated SiO2/Si substrates annealed at 150 ꢀC for 20 min and 200 ꢀC for
3 h in nitrogen along with one of the as-spun thin film (r. t.).
Fig. 4 AFM images (2 mm ꢃ 2 mm) of PDQT-tc thin films annealed at 150
ꢀC for 20 min (left) and 200 ꢀC for 3 h (right) in nitrogen. The root mean
square (RMS) roughness of the films is 0.89 nm (left) and 1.42 nm (right).
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Fig. 5 UV–Vis spectra of the PDQT-tc chloroform solution and thin
films on glass substrates without annealing (as-spun), annealed at 150 ꢀC
for 20 min, and annealed at 200 ꢀC for 3 h in nitrogen.
groups of the native conjugated backbones in the PDQT-n film
might have segregated the p–p stacks during the elimination of
the side chains. The resulting backbone-only polymer PDQT-n
would have very strong intermolecular interactions, which
inhibit any chain motion to reorganize the polymer chains into
ordered crystalline structures.
Fig. 6 Output and transfer curves of an OTFT device with a PDQT-tc
thin film annealed at 150 ꢀC for 20 min (a and b) and an OTFT device
with PDQT-n obtained by heating a PDQT-tc thin film at 200 ꢀC for 3 h
in nitrogen (c and d). Device dimensions: channel width (W) ¼ 4 mm;
channel length (L) ¼ 100 mm.
PDQT-tc was tested in bottom-gate, top-contact OTFT
devices. A heavily p-doped silicon wafer with a ꢁ200 nm
thermally grown SiO2 layer was used as the substrate, where
the conductive silicon functions as the gate electrode and the
SiO2 layer as the dielectric. The SiO2 layer was modified with
under a high pressure4e will also be used to improve the quality and
the crystallinity of the final polymer thin films.
dodecyltrichlorosilane (DTS) prior to use.
A PDQT-tc
solution in chloroform was spin coated on the substrate at 1500
rpm for 60 s to form a ꢁ60 to 70 nm polymer thin film, which
ꢀ
Experimental
was subjected to thermal annealing at 200 C for 3 h in a glove
box to form a ꢁ30 nm PDQT-n thin film. Then the source-
drain electrode pairs were deposited on top of the polymer thin
film by thermal evaporation of silver through a shadow mask
to form OTFT devices with a channel width (W) of 4 mm and a
channel length (L) of 100 mm. For comparison, OTFT
devices using PDQT-tc annealed at 150 ꢀC for 20 min
(without eliminating the side chains) were fabricated. All
devices were encapsulated with a TiOx top layer,9 which was
deposited by spin coating a precursor solution and then
Instrumentation and materials
All materials were purchased from Sigma-Aldrich and used
without further purification. 3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]
pyrrole-1,4(2H,5H)-dione (1)13 and 5,50-bis(trimethylstannyl)
bithiophene14 were synthesized according to the literature. NMR
data were collected on a Bruker DPX 300 MHz spectrometer
with chemical shifts relative to tetramethylsilane (TMS, 0 ppm).
FT-IR spectra were performed on an FTIR-8400S (Shimadzu)
Fourier Transform Infrared Spectrophotometer. UV–Vis spectra
were recorded on a Thermo Scientific GENESYSꢀ 20 Spectro-
photometer. XRD diagrams of polymer thin films (ꢁ100 nm) on
the DTS-modified Si/SiO2 substrate deposited by spin-coating
polymer solutions in chloroform were obtained with a Bruker D8
Advance powder diffractometer using standard Bragg-Brentano
ꢀ
annealing at 80 C for 20 min. The devices were characterized
in air in the dark.
As shown in Fig. 6, the device with PDQT-tc annealed at 150 ꢀC
showed typical hole transport characteristics under the hole
accumulation mode. The hole mobility in the saturation regime is
0.096 cm2 Vꢂ1 sꢂ1 with a current on-to-off ratio of 8.6 ꢃ 105. The
mobility value is about one order of magnitude lower than that of
PDQT, probably due to the poor crystallinity of the PDQT-tc thin
film. The device having the PDQT-n film obtained by annealing a
PDQT-tc film at 200 ꢀC for 3 h also showed a typical hole trans-
port behaviour. The hole mobility of this device is 0.078 cm2 Vꢂ1
ꢁ
geometry with Cu Ka radiation (l ¼ 1.5406 A). Gel-permeation
chromatography (GPC) measurements were performed on a
Waters Breeze HPLC system using chlorobenzene as an eluent
with polystyrene as standards at a column temperature of 40 ꢀC.
Thermogravimetry analysis (TGA) was conducted onꢂa1 TGA
ꢂ1, slightly lower than that of the device annealed at 150 ꢀC.
Q500 (TA Instruments) at a heating rate of 10 C min under
ꢀ
s
However, if the amorphous nature of the PDQT-n film is taken
into account, the charge transport of thispolymer isquite efficient.
It will be our next subject of study to maintain the crystallinity and
the p–p stacking during the thermal elimination of the side chains
by fine-tuning the heating profile. Thermal removal of side chains
nitrogen. AFM images were performed on polymer thin films on
a DTS-modified SiO2/Si substrate using a Dimension 3100
Scanning Probe Microscope. The polymer thin films (ꢁ30 to
40 nm) were deposited by spin-coating polymer solutions in
chloroform and optionally annealed at different temperatures.
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Synthesis of 2-octyldodecanoic acid
3.88–3.76 (2H, m), 1.90–1.70 (4H, m), 1.70–1.54 (4H, m), 1.45–
1.10 (56H, m), 0.95–0.80 (12H, t, J ¼ 6.36 Hz).
Sulphuric acid (10 g, 98%) was diluted with water (75 mL) and
added to a solution of 2-octyl-1-dodecanol (30 g) in dichloro-
methane (60 mL). To this mixture was added drop-wise 1 M
potassium permanganate solution (15 mL) over 1 h and then this
mixture was stirred for an additional 3 h. The mixture was
filtered to remove precipitated manganese dioxide, washed with
water, and treated with 2 M NaOH (400 mL). The solution was
washed with hexane, neutralized with 2 M HCl, and extracted
with ethyl acetate three times. The combined organic phase was
dried over anhydrous Na2SO4. Distillation under a reduced
pressure gave a colourless liquid (23 g, 74%). 1H NMR (CDCl3)
2.38–2.35, 1.59–1.54, 1.47–1.43, 1.26, 0.90–0.85.
Synthesis of poly(2,5-bis(2-octyldodecanoyl)-3,6-di(thiophen-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione-alt-2,20-bithiophene)
(PDQT-tc)
3,6-Bis(5-bromothiophen-2-yl)-2,5-bis(2 octyldodecanoyl)pyr-
rolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3) (0.2470 g, 0.236 mmol)
and 5,50-bis(trimethylstannyl)-bithiophene (0.1160 g, 0.236
mmol) were charged in a 50 mL flask. After degassing and
refilling argon for
3 times, toluene (12 mL) and bis-
(triphenylphosphine)palladium(II) dichloride (4.8 mg) were
added and the reaction mixture was stirred at 80 ꢀC for 1 h, and
ꢀ
90 C for 23 h. Then 0.5 mL of bromobenzene was added and
stirred at 90 ꢀC for an additional 3 h. The mixture was then
poured into stirring methanol (200 mL). The solid was filtered off
and further purified by Soxhlet extraction using acetone and
hexane, and then dissolved with chloroform. A dark blue solid
was obtained (227 mg, 91.5%). GPC: Mn ¼ 24 334; PDI ¼ 3.90.
Synthesis of 2-octyldodecanoyl chloride
2-Hexyldecanoic acid (82 mmol, 21 g) was added drop-wise into
thionyl chloride (80 mL). The reaction mixture was refluxed for
3 h and the excess thionyl chloride was removed at 70 ꢀC in vacuo
to leave a brown liquid (22.3 g, ꢁ100%), which was used in the
1
next step without further purification. H NMR (CDCl3): 2.76–
Fabrication and characterization of OTFT devices
2.71, 1.70, 1.53, 1.24, 0.88–0.84.
A top contact, bottom-gate OTFT configuration was used to
evaluate the polymer semiconductors. A heavily p-doped Si
wafer with a thermally grown SiO2 layer (ꢁ200 nm) having a
capacitance of ꢁ17 nF cmꢂ2 was used as a substrate, where the
conductive Si layer functions as the gate electrode and the SiO2
layer as the dielectric. After cleaning sequentially with DI water,
acetone, and isopropanol in an ultrasonic bath, the substrate was
cleaned by O2 plasma. Subsequently, the substrate was merged in
Synthesis of 2,5-bis(2-octyldodecanoyl)-3,6-di(thiophen-2-yl)
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (2)
3,6-Di(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (1)
(1.2 g, 4 mmol) was dissolved in anhydrous N-methyl-2-pyrro-
ꢀ
lidone (NMP) (25 mL) at 60 C. Then the solution was cooled
down to room temperature, and sodium hydride (0.36 g, 9 mmol)
ꢀ
a dodecyltrichlorosilane (DTS) solution in toluene (10 mg mLꢂ1
)
was added. The reaction mixture was stirred for 0.5 h at 60 C
at 70 ꢀC for 20 min. The substrate was then washed with toluene
and dried under a nitrogen flow. A PDQT-tc solution in chlo-
roform (5 mg mLꢂ1) was spin coated on the substrate at 1500
rpm for 60 s, resulting in a ꢁ30 to 40 nm thick PDQT-tc thin film.
A more concentrated PDQT-tc solution (10 mg mLꢂ1) was used
to deposit a ꢁ70 nm thick polymer film, which was annealed at
200 ꢀC for 3 h on a hotplate in a glove box, resulting in a PDQT-n
film with a thickness of ꢁ30 nm. Subsequently, the source/drain
electrode pairs were deposited on the polymer film by thermal
evaporation of silver through a shadow mask. Finally, the device
before 2-octyldodecanoyl chloride (3.3 g, 10 mmol) was added
into the reaction mixture. After stirring for 24 h at 60 ꢀC, the
reaction mixture was cooled down to room temperature and
poured into DI water (500 mL), and extracted with ethyl acetate
three times. The organic layer was washed with brine and DI
water to remove NMP. The combined organic layer was dried
over anhydrous MgSO4 and filtered. After evaporating the
solvent, the residue was purified by column chromatography on
silica gel with a mixture of ethyl acetate and hexane (1 : 6, v/v) as
an eluent to give the title compound as a dark purple solid. Yield:
0.35 g (ꢁ10%). 1H NMR (CDCl3): 8.28 (2H, d, J ¼ 3.0 Hz), 7.62
(2H, d, J ¼ 3.9 Hz), 7.19–7.16 (2H, t, J ¼ 4.5 Hz), 3.89–3.77
(2H, m), 1.90–1.70 (4H, m), 1.70–1.54 (4H, m), 1.50–1.10
(56H, m), 0.95–0.80 (12H, m).
ꢀ
was encapsulated by TiOX (ref. 9) and annealed at 80 C for 20
min under nitrogen. The OTFT devices have a channel length (L)
of 100 mm with a channel width (W) of 4 mm. The devices were
characterized in air using an Agilent 4155C Semiconductor
Analyzer. The field effect mobility was calculated according to
the previously employed method.6a
Synthesis of 3,6-bis(5-bromothiophen-2-yl)-2,5-bis(2-
octyldodecanoyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3)
Conclusions
2,5-Bis(2-octyldodecanoyl)-3,6-di(thiophen-2-yl)pyrrolo[3,4-c]-
pyrrole-1,4(2H,5H)-dione (2) (1.1 g, 1.24 mmol) was dissolved in
chloroform (50 mL) and NBS (0.441 g, 2.48 mmol) was added.
After stirring for 24 h at room temperature in the absence of
light, the solvent in the reaction mixture was removed and the
residue was purified with column chromatography on silica gel
using a mixture of dichloromethane and hexane (1 : 4, v/v) as
We have presented a novel polymer semiconductor wꢀith side
chains thermally cleavable at a low temperature of 200 C. The
complete cleavage and removal of the insulating 2-octyldodeca-
noyl side chains were verified with TGA, FT-IR, and NMR data.
The liberated N–H groups on the polymer backbone are expected
to form intermolecular hydrogen bonding with the C]O groups
on the neighbouring polymer chains to establish three-dimen-
sional charge transport networks. The resulting side chain-free
conjugated polymer is proven an active p-type semiconductor
1
an eluent. A dark purple solid was obtained (0.85 g, 65%). H
NMR (CDCl3): 8.17 (2H, d, J ¼ 4.2 Hz), 7.14 (2H, d, J ¼ 4.2 Hz),
18954 | J. Mater. Chem., 2012, 22, 18950–18955
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material for OTFTs, exhibiting a hole mobility of up to 0.078 cm2
Vꢂ1 ꢂ1. The resulting side chain-free polymer thin film is
ꢀ
Soc., 2002, 124, 8812; (c) A. R. Murphy, J. M. J. Frechet,
P. Chang, J. Lee and V. Subramanian, J. Am. Chem. Soc., 2004,
126, 1596; (d) P. C. Chang, J. Lee, D. Huang, V. Subramanian,
s
morphologically very stable, resistant to any solvents, and has a
favourable band gap for light harvesting, which may also be a
useful electron donor material for stable organic photovoltaics.
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