Synthesis and properties of indigo based donor-acceptor conjugated polymers

Article abstract of DOI:10.1039/c3tc32276a

Indigo is for the first time used as a building block to construct polymer semiconductors for organic thin film transistors (OTFTs). Two donor-acceptor polymers using indigo as the acceptor and bithiophene as the donor are synthesized via Stille coupling polymerization. Two types of acyl groups, 2-hexldecanoyl (for polymer P1) and 2-octyldodecanoyl (for polymer P2), are utilized as solubilizing side chains. These polymers possess very deep highest-occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, due to the strong electron accepting capability of the indigo moiety. In OTFT devices, characteristic n-type semiconductor performance with electron mobility of up to ～10^-3 cm² V^-1 s^-1 is observed.

Full text of DOI:10.1039/c3tc32276a

Journal of
Materials Chemistry C
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PAPER
Synthesis and properties of indigo based donor–
acceptor conjugated polymers†
Cite this: J. Mater. Chem. C, 2014, 2,
289
4
Chang Guo, Bin Sun, Jesse Quinn, Zhuangqing Yan and Yuning Li*
Indigo is for the first time used as a building block to construct polymer semiconductors for organic thin film
transistors (OTFTs). Two donor–acceptor polymers using indigo as the acceptor and bithiophene as the
donor are synthesized via Stille coupling polymerization. Two types of acyl groups, 2-hexldecanoyl (for
polymer P1) and 2-octyldodecanoyl (for polymer P2), are utilized as solubilizing side chains. These
polymers possess very deep highest-occupied molecular orbital (HOMO) and lowest unoccupied
molecular orbital (LUMO) energy levels, due to the strong electron accepting capability of the indigo
moiety. In OTFT devices, characteristic n-type semiconductor performance with electron mobility of up
Received 18th November 2013
Accepted 23rd February 2014
DOI: 10.1039/c3tc32276a
www.rsc.org/MaterialsC
to ꢀ10ꢁ cm V
3
2
ꢁ1 ꢁ1
s
is observed.
substituted indigo can form intramolecular hydrogen bonding
between the oxygen and the hydrogen atoms of the two 1H-
Introduction
Conjugated polymers comprising alternating electron donor (D) indol-3-one units, resulting in a highly coplanar geometry of the
3
1–34
and acceptor (A) moieties in the backbone have been receiving indigo molecule.
Indigo has a much longer wavelength of
35,36
much attention recently as active semiconductors for organic absorption maximum (lmax ¼ 600–610 nm)
than that of
in solutions, indicating the
1,2
37–40
thin lm transistors (OTFTs)
and organic photovoltaics isoindigo (lmax ¼ 520–540 nm)
3–12
(OPVs).
Intramolecular charge transfer between the donor more effective conjugation of the former. Recently, indigo and
0
and acceptor moieties allows these polymers to absorb visible its derivative, tyrian purple (6,6 -dibromoindigo), have been
28,41–43
and even near infrared portions of the sun light, making many used as semiconducting channel materials in OTFTs.
The
On devices based on these compounds showed ambipolar charge2
the other hand, it has been found that the donor and acceptor transport characteristics with a hole mobility of up to 0.40 cm
13–15
D–A polymers good candidate donor materials for OPVs.
ꢁ1
ꢁ1
44
units on the adjacent polymer chains in the solid state could
effectively strengthen the intermolecular interactions, making
the polymer chains tightly packed along the p–p stacking
direction in comparison to the polymers with only donor
V
s . According to a recent report, the hole and electron
16–18
units.
The shortened p–p stacking distance could greatly
facilitate charge carrier hopping between polymer chains,
2
ꢁ1 ꢁ1
resulting in very high carrier mobility of over 1 cm V
s
for
1,2
some D–A polymers. Although the electron donor units play
an important role for achieving such high mobility values, the
electron acceptor building blocks are considered to be the
determining factor. The majority of high mobility polymers are
based on a few types of electron acceptors such as diketo-
17,19–23
24–27
pyrrolopyrrole (DPP)
and isoindigo (IID).
To bring the
charge transport performance beyond the current level, new
acceptor building blocks need to be explored and developed.
A structural isomer of isoindigo, the indigo (ID) blue dye has
28–30
been widely used since as early as 1600 BC (Fig. 1).
The non-
Department of Chemical Engineering and Waterloo Institute for Nanotechnology
WIN), University of Waterloo, 200 University Ave West ON, Waterloo, Canada, N2L
G1. E-mail: yuning.li@uwaterloo.ca; Fax: +1-519-888-4347; Tel: +1-519-888-4567
ext. 31105
Electronic supplementary information (ESI) available: Additional computer
(
3
†
simulation data, NMR, DSC, and TGA. See DOI: 10.1039/c3tc32276a
Fig. 1 Structures of isoindigo (IID) and indigo (ID) and their polymers.
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1
mobilities of indigo and tyrian purple calculated based on the
H NMR spectra of 2a and 2b indeed indicated the presence of
45
Marcus's method are very similar to those of isoindigo. The ꢀ85% of the trans-isomer and ꢀ15% of the cis-isomer when the
high mobility values both experimentally achieved and theo- solutions were prepared under ambient light. Two PIDBT
retically predicted for indigo and its derivatives suggest that polymers, P1 and P2, were synthesized via Stille coupling poly-
0
indigo might be another promising electron acceptor building merizations of 2a and 2b, respectively, with 2,5 -bis(trimethyl)
2 3 3
block for D–A polymers for organic electronics. In this study, we stannylbithiophene using Pd (dba) /P(o-toyl) as a catalyst
ꢂ
report the synthesis and properties of two polymers comprising system in chlorobenzene at 90 C. Polymers were puried with a
indigo (acceptor) and bithiophene (donor), which are, to the Sohxlet extraction apparatus using sequentially acetone, hexane
best of our knowledge, the rst D–A polymers based on indigo. and chloroform, and then the remaining solid was heated in
ꢂ
1
,1,2,2-tetrachloroethane (TCE) at 130 C in a ask. Acetone and
hexane were used to remove the catalyst residues and oligo-
mers, respectively, chloroform and TCE were used to dissolve
the higher molecular weight polymer fraction. P1 showed poor
Results and discussion
0
D–A polymers based on 6,6 -isoindigo and bithiophene, PIIDBT solubility; it is essentially insoluble in chloroform and only 15%
(
Ar ¼ bithiophene in Fig. 1), were reported to show very high of the polymer was dissolved by hot TCE. The very poor solu-
charge transport performance in OTFTs with a hole mobility of bility of P1 indicates the very strong intermolecular D–A inter-
up to 3.62 cm V
2
ꢁ1 ꢁ1 27
s . The closest structural analogue of their actions of this polymer. P2 with longer 2-octyldodecanoyl side
indigo based polymers would be PIDBT (Ar ¼ bithiophene) as chains showed improved solubility with 13% dissolved in
shown in Fig. 1, where the indigo unit is connected at the 6 and chloroform and 59% dissolved in TCE, but ꢀ28% of the poly-
0
6
positions with the neighbouring thiophene units. In this mer still remained insoluble. The molecular weights of these
0
study, we chose 6,6 -dibromoindigo or tyrian purple (1), as the two polymers were determined by using gel permeation chro-
starting material to construct PIDBT polymers. Compound 1 matography (GPC) with chlorobenzene as the eluent and poly-
ꢂ
was readily prepared with ꢀ70% yield in one step by using a styrene as the standards at a column temperature of 40 C. The
46
literature method (Scheme 1). Substitution at the nitrogen number average molecular weight (M ) of P1 dissolved in TCE is
n
atoms with suitable side chains such as alkyl groups can 13.5 kDa with a polydispersity index (PDI) of 3.50. The fraction
improve the solubility of the resulting substituted indigo of P2 extracted with chloroform has a slightly lower M of 12.4
n
molecules and the nal polymers. However, our attempts to kDa. However, the weight average molecular weight (M ) of P2 is
w
substitute compound 1 using an alkyl bromide such as 2-octa- 131.4 kDa, which is much higher than that of P1 (47.3 kDa),
dodecyl bromide in the presence of a base (e.g., K
2 3
CO and NaH) resulting in a very broad PDI of 10.6. The molecular weight of P2
only led to mono-alkylated products. Similar observations were dissolved in TCE could not be measured because of its very poor
47
also reported for the substitution of indigo. Finally we found solubility in chlorobenzene used for the GPC measurements at
ꢂ
that the nitrogen atoms of 1 could be readily substituted with 40 C. However, we can reasonably assume that the molecular
48
acyl groups by using the chemistry we used for DPP. Because weight of P2 in the TCE fraction should be higher than that of
most D–A polymers have very strong intermolecular interac- P1. The results used for the following discussions were obtained
tions, they generally have poor solubility in organic solvents. using the fractions of P1 and P2 dissolved in TCE.
Therefore, we used long branched side chains, 2-hexldecanoyl
The thermal properties of polymers were characterized by
and 2-octyldodecanoyl groups, to substitute the nitrogen atoms using thermogravimetric analysis (TGA) and differential scan-
of compound 1, resulting in 2a and 2b with 34% and 39% yields, ning colorimetry (DSC). P1 and P2 had 5% weight losses at 160
ꢂ
ꢂ
respectively (Scheme 1). It has been reported that acyl
C and 220 C, respectively, indicating that these polymers are
substituted indigo compounds undergo photoisomerization quite thermally labile. DSC diagrams clearly showed an
49,50
ꢂ
from the stable trans-isomers to the less stable cis-isomers.
exothermic peak at 258 C in the rst heating scan for both
polymers. These results are reminiscent of the behaviour of a
DPP based polymer that also has acyl (2-octadodecanoyl)
substituents at the nitrogen atoms, where the 2-octadodecanoyl
substituents were found to be thermally unstable and the
ꢂ
48
polymer started to loose side chains at ꢀ180 C. Therefore the
thermal instability observed for P1 and P2 is most likely caused
by the thermally labile acyl groups.
Unlike the substituted isoindigo, where the substituents at
the nitrogen atoms are distant from their neighbouring indol-2-
one rings, substitution of indigo at the nitrogen atoms might
cause twisting of the indigo moiety because the acyl side chains
are very close to the C]O groups of the neighbouring indol-3-
one rings. An acetyl-substituted dimer compound, IDCOMe-BT-
IDCOMe-BT, was simulated by performing density functional
Scheme 1 Synthetic route for indigo monomers 2a and 2b and
polymers P1 and P2: (i) NaOH/acetone/room temp.; (ii) (a) NaH/NMP/
51,52
theory (DFT) calculations using Gaussian 09W
to determine
room temp., (b) RCOCl/room temp.; (iii) Pd
2 3 3
(dba) /P(o-tolyl) /chlo-
ꢂ
robenzene/90 C.
the inuence of acyl substitution on the coplanarity of the
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indigo moiety. As shown in Fig. 2, the dihedral angle between
the two indol-3-one moieties in the acetyl-substituted indigo
ꢂ
unit (4 ) is ꢀ23 . On the other hand, the dihedral angle between
1
the two indol-2-one moieties in the methyl-substituted IID unit
(4
1
) of an analogous dimer, IIDMe-BT-IIDMe-BT, is only ꢀ11ꢂ
(
similar results were reported using the same simulation
53,54
protocols).
We also simulated the methyl-substituted indigo
dimer, IDMe-BT-IDMe-BT, the dihedral angle 4
further to 29 (see ESI†). Additionally, the dihedral angles
1
increased
ꢂ
ꢂ
between the indigo and thiophene (4 ¼ 24 ) and between two
2
ꢂ
thiophene units (4 ¼ 15 ) are also greater than the respective
3
ꢂ
ꢂ
dihedral angles of its isoindigo dimer (4 ¼ 20 ; 4 ¼ 5 ). These
2
3
results indicate that the acyl-substituted PIDBT (P1 or P2) has a
more twisted backbone than that of the alkyl-substituted
PIIDBT.
Compound 1 had a lmax at 586 nm in TCE (Fig. 3a), which
Fig. 3 UV-vis absorption spectra of (a) 1, 2a and 2b in TCE solutions;
55
agrees with the reported value of 585 nm. The acyl-substituted (b) P1 and P2 in TCE solutions and in thin films; (c) P1 and (d) P2 thin
a and 2b had a lmax at 576 nm in TCE solutions. The blue shi films on glass substrates annealed at different temperatures.
for 2a and 2b with respect to compound 1 is
2
of ꢀ10 nm l
max
likely to be the result of the twisted indigo moiety caused by the
acyl groups. Polymers P1 and P2 had lmax values at 527 nm and than that of PIIDBT-20. As noted earlier, small molecular indi-
5
72 nm, respectively, in TCE solutions, and 607 nm and 620 nm, goids have longer absorption wavelengths and are structurally
respectively, in the solid state (as-spun lms) as can be seen in more conjugated than their isoindigo counterparts. The
Fig. 3b. The longer absorption wavelengths of P2 than those of observed reduction in conjugation for P2 compared with
P1 are considered to be due to the higher molecular weight and PIIDBT-20 might be accounted for by the substitution of indigo
thus the longer effective conjugation length of P2. By comparing at the nitrogen atoms with the acyl groups, leading to a more
P2 with its isoindigo analogue, PIIDBT-20 (Ar ¼ bithiophene twisted polymer backbone and reduced main chain conjuga-
and R ¼ 2-octyldodecyl in Fig. 1), which had a l
value at 706 tion. Another possible reason might be due to the presence of
max
27
nm in solution and 701 nm in lm, the lmax values of P2 both some cis-indigo units on the polymer backbone, which might
in solution and in the solid state blue shied notably. This also cause serious twisting and less ordered packing of the
suggests that P2 has a shorter main chain conjugation length polymer chains in the solid state. It was reported that the lmax of
Fig. 2 The chemical structure, geometry, LUMO orbital, and HOMO orbital of two model dimers IDCOMe-BT-IDCOMe-BT and IIDMe-BT-
27
IIDMe-BT, which correspond to acyl-substituted PIDBT (P1 or P2) and alkyl-substituted PIIDBT, respectively, obtained by performing DFT
calculations with the B3LYP 6-31G* basis set.
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ꢂ
cis-IDCOMe is 438 nm (in benzene), which is signicantly blue was increased to 150 C, the primary peak shied slightly to 2q
4
9
ꢂ
shied compared with that of trans-IDCOMe (562 nm).
¼ 4.81 (d-spacing ¼ 1.84 nm). Once the annealing temperature
ꢂ
When the thin lms were thermally annealed, both polymers was raised to 200 C, the primary peak disappeared. This is due
showed apparent red shits in their UV-Vis spectra (Fig. 3c and to the loss of side chains at such a high temperature as
ꢂ
ꢂ
d). The lmax of the 100 C-annealed P1 thin lm red shied to corroborated by the thermal analysis data. The 100 C-annealed
ꢂ
6
26 nm. Further increasing the annealing temperature to 150 P2 lm had a very weak primary peak at 2q ¼ 4.30 , corre-
ꢂ
and 200 C did not change the lmax of P1 thin lms. For the P2 sponding to a d-spacing distance of 2.05 nm. Increasing the
ꢂ
thin lms, the lmax red shied progressively to 639 and 643 nm annealing temperature to 150 C improved the crystallinity of
as the annealing temperature increased to 100 and 150 C, the P2 lm signicantly, manifested by the much intensied
ꢂ
ꢂ
respectively. Thermal annealing would increase the chain peak at 2q ¼ 4.38 (d-spacing ¼ 2.02 nm). Further increasing the
ꢂ
ordering as manifested by the following XRD results and might annealing temperature to 200 C, the intensity of the primary
56
also transform the cis-indigo to the trans-indigo, leading to peak decreased and the d-spacing further decreased to 1.92 nm
ꢂ
more extended p-conjugation of the polymer backbone. The 200 (2q ¼ 4.58 ), again due to the loss of the side chains. Since the
ꢂ
C-annealed P2 sample had a dramatically different absorption XRD diagrams of the crystalline polymer thin lms only showed
prole and a further red-shied lmax at 651 nm. This is most the primary peaks, the polymer chains of P1 and P2 most likely
58
likely due to the removal of acyl side chains at such a high adopted a layer-by-layer lamellar packing motif, a commonly
annealing temperature, revealed by the TGA data. The indigo observed crystal structure for most p-conjugated polymers in
1
,2,59
units without substitution at the nitrogen atoms are almost thin lms.
coplanar (see ESI†).
The atomic force microscopic (AFM) images of
ꢂ
ꢂ
the 100 C- and 150 C-annealed P1 thin lms contain large
The energy levels of the two polymers were determined by grains (Fig. 6). The 200 C-annealed P1 thin lm became more
cyclic voltammetry (CV) in the solid thin lm state using uniform, probably due to the removal of the side chains. On the
ferrocene as the standard that has a HOMO level of ꢁ4.8 eV. P2 other hand, all of the P2 thin lms are very smooth and the
showed reversible oxidative and reductive cycles (Fig. 4). The surface morphology was not much inuenced by thermal
HOMO/LUMO levels of P2 were calculated by using the onset annealing.
ꢂ
57
oxidative/reductive potentials to be ꢁ5.78 eV and ꢁ4.02 eV,
P1 and P2 were tested as channel semiconductors in top-
respectively. The band gap of 1.68 eV determined by CV is close gate, bottom-contact OTFTs. Heavily n-doped Si/SiO2 wafer
to the optical band gap of 1.74 eV calculated from the onset patterned with gold source and drain electrode pairs (with a
absorption wavelength of the as-spun lm. The HOMO/LUMO channel length of 30 mm and a channel width of 1 mm) was used
levels of P2 obtained by CV are slightly lower than those of as the substrate. A polymer solution in TCE was spin coated on
PIIDBT-20 (EHOMO ¼ ꢁ5.70 eV and ELUMO ¼ ꢁ3.70 eV relative to the substrate to form a polymer thin lm (ꢀ30–50 nm), which
27
ꢂ
the ferrocene standard). The HOMO and LUMO levels of the was annealed at 100, 150, or 200 C on a hot plate, followed by
as-spun P1 lm were determined to be ꢁ5.69 eV and ꢁ4.00 eV, spin coating a CYTOP layer (ꢀ500 nm) as the gate dielectric in a
respectively.
i
glove box under nitrogen. The capacitance per unit area (C ) of
ꢁ
2
The crystallinity of polymers was studied by using X-ray the dielectric layer (3.2 nF cm ) was determined from a metal–
diffractometry (XRD) of polymer thin lms spin coated on SiO / insulator–metal (MIM) structure. The devices were character-
2
ꢂ
Si substrates. The P1 lm annealed at 100 C exhibited a weak ized in ambient conditions in the absence of light. Devices with
ꢂ
ꢂ
primary peak at 2q ¼ 4.86 , which corresponds to a d-spacing P1 or P2 thin lms annealed at 100 C showed no eld effect
ꢂ
distance of 1.82 nm (Fig. 5). When the annealing temperature performance. Devices based on P1 thin lms annealed at 150 C
showed characteristic electron transport behaviour with a
ꢁ
4
2
ꢁ1 ꢁ1
mobility as high as 6.6 ꢃ 10 cm V
s
(current on-to-off
4
ratios of ꢀ10 ) (Fig. 7a and b). Devices based on P2 thin lms
ꢂ
annealed at 150 C also exhibited electron transport perfor-
ꢁ
3
2
ꢁ1 ꢁ1
mance with improved mobility of up to 1.1 ꢃ 10 cm V
s
4
(
on-to-off ratios of ꢀ10 ) (Fig. 7c and d). No hole transport was
observed for both polymers, which is presumably due to the
Fig. 4 Cyclic voltammograms of as-spun P1 and P2 thin films
measured in anhydrous CH
electrolyte.
3
CN solution using Bu
4
NPF
6
as the Fig. 5 XRD diagrams obtained from spin-coated P1 and P2 thin film
ꢂ
on silicon substrates annealed at 100, 150 and 200 C.
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the side chains. Substitution of the indigo units with more
desirable side chains to minimize the steric effect and eliminate
the cis-isomers is expected to improve the backbone coplanary
and molecular organization for more efficient charge transport
of the indigo based polymers.
Experimental
Materials and characterization
All chemicals were purchased from Sigma-Aldrich and were
used without further purication. CYTOP (a peruoronated
amorphous resin solution) was purchased from AGC Chem-
0
0
0
icals. 6,6 -Dibromo-[2,2 -biindolinylidene]-3,3 -dione (1) was
46
synthesized according to the literature. NMR spectra were
Fig. 6 AFM images (2 mm ꢃ 2 mm) of P1 and P2 thin films (ꢀ50–60 nm) obtained on a Bruker DPX 300 MHz spectrometer with chemical
ꢂ
spin coated on Si/SiO
under nitrogen.
2
substrate annealed at 100, 150 and 200
C
shis relative to tetramethylsilane (TMS, 0 ppm). UV-Vis spectra
were collected on a Thermo Scientic GENESYS 20 Spectro-
photometer. Cyclic voltammetry (CV) data were obtained with a
BAS-CV-50W potentiostat/galvanostat using an Ag/AgCl refer-
ence electrode, a Pt wire counter electrode, and a Pt foil working
electrode in 0.1 M tetrabutylammonium hexauorophosphate
ꢁ1
in dry acetonitrile at a scan rate of 50 mV s . Ferrocene was
used as a reference, which has a highest occupied molecular
57
orbital (HOMO) of ꢁ4.8 eV. A Bruker D8 Advance powder
diffractometer with standard Bragg–Bretano geometry using Cu
˚
Ka radiation (l ¼ 1.5418 A) was used to collect the XRD patterns
of polymer thin lms (ꢀ100 nm) spin-coated on the dodecyl-
2
trichlorosilane (DTS)-modied SiO /Si substrates using polymer
solutions in 1,1,2,2-tetrachloroethane (TCE). GPC measure-
ments were performed on a Waters SEC system in chloroben-
ꢂ
zene at 40 C. A TGA Q500 instrument (TA Instruments) was
used to conduct the thermogravimetry analysis (TGA) at a
ꢂ
ꢁ1
heating rate of 10 C min under nitrogen. AFM images were
recorded on polymer thin lms on DTS-modied SiO /Si
2
substrates using a Dimension 3100 scanning probe microscope.
OTFT fabrication and characterization
A top-gate, bottom-contact OTFT structure was adopted for
2
evaluating P1 and P2. A heavily n-doped Si/SiO wafer was used
Fig. 7 Transfer and output curves of OTFT devices with P1 (a and b)
and P2 (c and d) thin films annealed at 150 C for 20 min. Device
dimensions: channel width (W) ¼ 1 mm; channel length (L) ¼ 30 mm.
ꢂ
as the substrate. The source/drain electrode pairs were depos-
ited using conventional photolithography to obtain the dened
device dimensions with a channel length (L) of 30 mm and a
channel width (W) of 1 mm. The substrate was cleaned using an
ultrasonic bath with de-ionized (DI) water, and rinsed with
acetone and isopropanol. Polymer lms with a thickness of
large hole injection barrier between their low-lying HOMO
6
0,61
levels and the work function of gold (ꢀ4.7–5.1 eV).
The much
ꢀ
30–50 nm were deposited on the substrate by spin-coating a
lower mobility values observed for P1 and P2 in comparison to
their isoindigo counterpart, PIIDBT-20, and small molecular
indigoids is considered to originate from its poor main chain
conjugation which is caused by the backbone twisting as dis-
cussed previously. The signicantly poorer crystallinity
ꢁ1
polymer solution in TCE (10 mg mL ) at 1500 rpm for 50 s and
ꢂ
subsequently annealed at 100, 150 or 200 C for 10 min. A
CYTOP layer (ꢀ500 nm) was then spin-coated as the gate
ꢂ
dielectric, and then dried at 100 C on a hotplate for 30 min
before depositing the Al gate electrode (ꢀ70 nm). All the devices
(
molecular ordering) of P1 and P2, which originates from the
were characterized in air in the absence of light using an Agilent
main chain twisting and the possible existence of cis-isomers,
4
155C Semiconductor Analyzer. Carrier mobility was calculated
also accounts for their low mobility values. Further increasing
the annealing temperature to 200 C resulted in the absence of
ꢂ
in the saturation regime according to the following equation:
ꢀ
ꢁ

eld effect performance for both polymers, due to the deterio-
WC
i
2
I
D
¼
mðV
G
ꢁ V Þ
T
rated molecular ordering caused by thermal decomposition of
2L
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ꢂ
heated in TCE at 130 C and ltered aer cooling. Very little
where I
D
is the drain current, m is the charge carrier mobility, C
i
is the capacitance per unit area of the insulator determined polymer was dissolved in chloroform. The yield of the polymer
from a metal-insulator-metal structure (CYTOP, 500 nm, C ¼ dissolved in TCE was 52.4 mg (15.0%). The remaining polymer
i
ꢁ
2
3
.2 nF cm ), V is the gate voltage, respectively.
is insoluble.
G
Synthesis
Synthesis of P2
0
0
0
46
0
6
,6 -Dibromo-[2,2 -biindolinylidene]-3,3 -dione
(1).
4- P2 was synthesized using 2b (0.352 g, 0.349 mmol) and 5,5 -
Bromo-2-nitrobenzaldehyde (2 g, 8.7 mmol) was dissolved in bis(trimethylstannyl)bithiophene (0.172 g, 0.349 mmol),
acetone (90 mL) and water (100 mL) was added slowly. A 2 N following a similar procedure as that for the synthesis of P1.
aqueous NaOH solution was added dropwise to adjust the pH to Yield: 49.1 mg (12.6%) from the chloroform extraction, 228.2
10. The suspension was then stirred overnight. Filtration gave a mg (58.7%) from the TCE extraction. The remaining polymer is
dark purple solid, which was washed with excess acetone and insoluble.
water. The resulting solid was dried in vacuo to give the product.
Yield: 1.32 g (72.1%).
Conclusions
0 0 0
,6 -Dibromo-1,1 -bis(2-hexyldecanoyl)-[2,2 -biindolinylidene]-
,3 -dione (2a). Compound 1 (0.672 g, 1.6 mmol) was dissolved
6
0
3
We reported two new donor–acceptor polymers using indigo as
the electron acceptor and bithiophene as the electron donor.
Acyl groups, 2-hexldecanoyl (for polymer P1) and 2-octyldode-
canoyl (for polymer P2), were used as the side chains to render
these polymers soluble in organic solvents. The strong electron-
withdrawing capability of the indigo moiety was manifested by
the low-lying HOMO/LUMO levels, ꢁ5.69 eV/ꢁ4.00 eV for P1
and ꢁ5.78 eV/ꢁ4.02 eV for P2. Due to the strong intramolecular
D-A charge transfer, rather low band gaps (ꢀ1.7 eV) were
observed for both polymers. The acyl side chains were found to
cause serious twisting of the polymer backbone. These side
in anhydrous N-methyl-2-pyrrolidone (NMP) (20 mL) at room
temperature. Then sodium hydride (0.154 g, 6.4 mmol) was
added and the mixture was stirred for 2 h. 2-Hexyldecanoyl
chloride (1.319 g, 4.8 mmol) was added to the reaction
mixture. Aer stirring for 24 h at room temperature, the
reaction mixture was poured into de-ionized (DI) water, 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 sodium
sulfate and ltered. Aer evaporating the solvent, the residue
was puried by column chromatography on silica gel with
ꢂ
chains are thermally labile and start to decompose at 160 C for
toluene as an eluent to give the title compound as a dark
purple solid. Yield: 0.491 g (34.2%). H NMR (300 MHz, CDCl
ꢂ
P1 and 220 C for P2. Thermal annealing led to notable red
1
3
)
shis of the absorption spectra, originating from the formation
of more coplanar backbone structures. P1 and P2 showed
characteristic electron transport performance in OTFTs with
d 8.43 (s, 2H), 7.60 (d, J ¼ 8.1 Hz, 2H), 7.39 (dd, J ¼ 8.1, 1.3 Hz,
1
3
2
H), 3.10–2.97 (m, 2H), 0.84 (t, J ¼ 6.9 Hz, 12H). C NMR (75
MHz, CDCl ) d 182.64, 150.03, 132.39, 128.26, 125.17, 119.80,
4
ꢁ4
2
ꢁ1 ꢁ1
3
electron mobilities of up to 6.6 ꢃ 10 cm V
s
and 1.1 ꢃ
3.85, 31.79, 29.77, 29.41, 22.62, 14.06, 13.96.
ꢁ3
2
ꢁ1 ꢁ1
10
cm V
s , respectively. The lower than expected eld
0
0
0
6
,6 -Dibromo-1,1 -bis(2-octyldodecanoyl)-[2,2 -biindolinyli-
effect performance of these polymers in comparison to their
counterpart isoindigo polymers was considered to be due to
the backbone twisting and the presence of cis-indigo units
that are undesirable for extended delocalization of electrons.
Work on exploring other types of side chains to minimize the
backbone twisting and the existence of cis-isomers to improve
the charge transport performance of this new class of polymers
is under way.
0
dene]-3,3 -dione (2b). Compound 2b was synthesized following
a similar procedure as that for the synthesis of 2a, except for
using 2-octyldodecanoyl chloride instead of 2-hexyldecanoyl
1
chloride. Yield: 0.984 g (39.0%). H NMR (300 MHz, CDCl
3
) d
8
2
.43 (s, 2H), 7.60 (d, J ¼ 8.1 Hz, 2H), 7.38 (dd, J ¼ 8.1, 1.4 Hz,
13
H), 3.11–2.96 (m, 2H), 0.87 (t, J ¼ 7.5 Hz, 12H). C NMR (75
1
3
MHz, CDCl ) d C NMR (75 MHz, CDCl ) d 182.53, 149.93,
3
3
1
2
32.30, 128.18, 125.10, 119.70, 43.72, 31.78, 29.67, 29.47, 29.22,
2.56, 22.51, 13.98, 13.94.
Acknowledgements
Synthesis of P1
The authors thank the Natural Sciences and Engineering
0
Research Council (NSERC) of Canada for nancial support
To a 25 mL dry ask was added 2a (0.360 g, 0.401 mmol), 5,5 -
bis(trimethylstannyl)bithiophene (0.197 g, 0.401 mmol) and
tri(o-totyl)phosphine (9.8 mg, 0.0321 mmol). Aer degassing
and relling with argon 3 times, anhydrous chlorobenzene (8
mL) and tris(dibenzylideneacetone)-dipalladium (7.3 mg, 0.008
mmol) were added under an argon atmosphere. The mixture
(
Discovery Grants) of this research, and Dr Jianfu Ding at the
National Research Council Canada (NRC) for the GPC
measurements. The authors also thank Angstrom Engineering
Inc. for providing the thermal deposition system for the fabri-
cation of the OTFT devices.
ꢂ
was stirred for 60 h at 90 C. Aer being cooled to room
temperature, the reaction mixture was poured into methanol
and stirred for 0.5 h. The precipitated solid was collected by
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