Effects of pyridyl group orientations on the optoelectronic properties of regio-isomeric diketopyrrolopyrrole based π-conjugated polymers

Article abstract of DOI:10.1039/c6tc00051g

Two novel regio-isomeric π-conjugated polymers consisting of the pyridyl unit flanked by diketopyrrolopyrrole as the electron-accepting unit and 2,5-bis(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene as the electron-donating unit were designed and synthesized. The comparison of the optical and electrochemical properties indicated that the copolymer based on the nitrogen atom proximal to the central diketopyrrolopyrrole unit (p-PDBPy) exhibited bathochromic shifted absorption spectra and narrower bandgaps than the counterpart copolymer (d-PDBPy) with the distally oriented nitrogen atom, which can be correlated with the stronger intermolecular aggregation of the former as a result of the different intrinsic molecular geometry of the polymer backbone. Of particular interest is that the copolymer p-PDBPy exhibited a moderate hole mobility of 0.35 cm² V^-1 s^-1, which is about three orders of magnitude higher than the hole mobility of 3.2 × 10^-4 cm² V^-1 s^-1 obtained based on the counterpart copolymer d-PDBPy, as measured using organic field-effect transistors. These results demonstrated that the delicate control of the pyridyl orientations along the polymer backbone is of vital importance for the molecular design of π-conjugated polymers for high-performance organic electronic devices.

Full text of DOI:10.1039/c6tc00051g

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Page 1 of 32
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DOI: 10.1039/C6TC00051G
Effects of pyridyl group orientations on the optoelectronic
properties of regio-isomeric diketopyrrolopyrrole based π-
conjugated polymers
Jing Yue, Sheng Sun, Junfei Liang, Wenkai Zhong, Linfeng Lan, Lei Ying*, Fei Huang*,
Wei Yang, Yong Cao
State Key Laboratory of Luminescent Materials and Devices, and Institute of Polymer
Optoelectronic Materials and Devices, South China University of Technology,
Guangzhou 510640, China
Corresponding authors
Prof. Lei Ying (L. Ying)
Email: msleiying@scut.edu.cn
Tel : +86-20-87114346-17
Fax: +86-20-87110606
Prof. Fei Huang (F. Huang)
Email: msfhuang@scut.edu.cn
Tel : +86-20-87114346-17
Fax: +86-20-87110606
1

Journal of Materials Chemistry C
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DOI: 10.1039/C6TC00051G
Abstract
Two novel regio-isomeric π-conjugated polymers consisting of the pyridyl unit flanked
diketopyrrolopyrrole as the electron-accepting unit and 2,5-bis(3-hexylthiophen-2-
yl)thieno[3,2-b]thiophene as the electron-donating unit were designed and synthesized.
The comparison of the optical and electrochemical properties indicated that the
copolymer based on the nitrogen atom proximal to the central diketopyrrolopyrrole unit
(p-PDBPy) exhibited bathochromic shifted absorption spectra and narrower bandgaps
than the counterpart copolymer (d-PDBPy) with the distal oriented nitrogen atom,
which can be correlated to the stronger intermolecular aggregations of the former as a
result of the different intrinsic molecular geometry of the polymer backbone. Of
particular interests is that the copolymer p-PDBPy exhibited a moderate hole mobility
of 0.35 cm2 V-1 s-1, which is about three orders of magnitude higher than the hole
mobility of 3.2×10-4 cm2 V-1 s-1 obtained based on the counterpart copolymer d-PDBPy,
as measured by organic field-effect transistors. These results demonstrated that the
delicate control of the pyridyl orientations along the polymer backbone is of vital
importance for the molecular design of π-conjugated polymers for high-performance
organic electronic devices.
Keywords: Donor-acceptor conjugated polymers, regioisomer, diketopyrrolopyrrole,
organic field effect transistors
2

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DOI: 10.1039/C6TC00051G
Table of contents
The orientation of N-atom in the pyridyl groups shows pronounced
effects on the molecular geometry and charge transport property.
N
N
Ar
Ar
n
n
N
N
d-PDBPy
μhole ~ 3×10-4 cm2 V-1 s-1
p-PDBPy
μhole ~ 0.3 cm2 V-1 s-1
= -30 V
10-5
10-6
10-7
10-8
10-9
10-10
10-11
10-8
0.0025
0.0001
8 10-5
6 10-5
4 10-5
2 10-5
0
V
= -30 V
V
D
D
0.002
0.0015
0.001
0.0005
0
10-9
10-10
10-11
10-12
10
5
0
-5 -10 -15 -20 -25 -30
VG(V)
10
5
0
-5 -10 -15 -20 -25 -30
VG(V)
3

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Introduction
Solution processable π-conjugated polymers have gained much attention due to their
specific advantages of light-weight, low-cost processing techniques and great potential
for the mass-production of large-area flexible organic electronic products.1-3 Over the
past decades, significant progress has been achieved in developing high-performance
π-conjugated polymers through rational molecular design along with device
engineering.4-9 In particular, π-conjugated polymers based on the alternating connected
electron-rich (donor, D) unit and electron-deficient (acceptor, A) unit, along the
backbone have been extensively investigated.10 - 14 The exceptional merit of this
molecular framework is that it allows for the delicate modification of both D and A
units, which can lead to well-controlled frontier molecular orbital levels, finely tailored
supramolecular stacking, associated with appropriate optoelectronic properties.15,16
However, it is worth concerning the subject of stereo-regularity when the involved unit
of either D or A unit is asymmetric, since the regioregularity of π-conjugated polymers
can significantly affect the optoelectronic properties.17,18
As a well-studied species, diketopyrrolopyrrole (DPP) moiety has been
extensively used for the construction of high-performance organic semicoductors for
organic photovoltaics and organic field-effect transistors.15, 19 - 21 Regarding to the
synthetic procedures, the DPP derivatives typically comprising of two flanked aromatic
groups, such as the thienyl15, selenophenyl22, furyl23, thiazolyl24, and phenyl25 units,
etc. In conjugated polymers, these units may form non-covalent interactions with the
central DPP unit or the adjacent species, which can potentially influence the backbone
4

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co-planarity, energy levels, crystallinity, and π–π stacking in thin films. Of pDaOrtI:i1c0u.1l0a3r9/C6TC00051G
interests is the recently emerged Py-DPP unit that consists of two flanked 2-pyridyl
units. The strong electronegativity of the amine group (C=N) in pyridyl can effectively
withdraw electrons and lead to simultaneously decreased highest occupied molecular
orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels, thus
imparting the resultant polymers n-type properties with outstanding electron
mobilities.26-29 Additionally, since the nitrogen-atom is proximal to the central DPP
unit, the lack of hydrogen in the nitrogen atom adjacent to the DPP unit can significantly
reduce the repulsive interaction between the central DPP unit and the flanked 2-pyridyl
unit, and thus lead to nearly co-planar architecture. However, it is worth noting that
the hydrogen atoms in the 3,5-position of the pyridyl unit may also give rise to the steric
interactions with adjacent species.
Given the asymmetric molecular structures of the 2-pyridyl unit, in this manuscript,
we introduced a novel Py-DPP derivative based on the isomer of 3-pyridyl unit, which
has the nitrogen atom oriented distal to the central DPP unit. Despite the existence of
hydrogen atoms adjacent to the central DPP is unfavorable for the effective conjugation,
the lack of hydrogen atom in 3-position of the nitrogen atom is beneficial for the
formation of co-planar structure. The 2,5-bis(3-hexylthiophen-2-yl)thieno[3,2-
b]thiophene was used as donors, since it can promote intermolecular π-π stacking that
can facilitate charge transportation. 30 - 35 The constructed regio-isomeric Py-DPP
conjugated polymers provide an interesting platform to study the effects of the
orientation of nitrogen unit on the molecular structures and optoelectronic properties.
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Results and discussion
Synthesis and characterizations
Scheme 1 shows the synthesis of monomers and polymers. The compound 2,5-bis(3-
hexylthiophen-2-yl)thieno[3,2-b]thiophene (1) was synthesized via a palladium
catalyzed Stille coupling reaction of 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene
and
2-bromo-3-hexylthiophene.
The
compound
5-bis(3-hexyl-5-
(trimethylstannyl)thiophen-2-yl)thieno[3,2-b]thiophene (2, TT2T-Sn2) was prepared
o
by treating (1) with n-butyllithium at -78 C followed by treating with trimethyltin
chloride in a relatively high yield of 80%. The alkylation 3,6-bis(5-bromopyridin-2-
yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (3) by using 1-bromo-2-octyldodecyl gives
the
dibromo-monomer
3,6-bis(5-bromopyridin-2-yl)-2,5-bis(2-
octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (4) in a yield of 41%. It is worth
noting that the nitrogen-atom in (4) is proximal to the central DPP unit.
To construct the regioisomeric copolymers of compound (4) that comprising the
nitrogen-atom distal to the central DPP core unit, our initial efforts were carried out
based on the condensation reaction of the nitrile with succinic acid ester by using 6-
bromonicotinonitrile and diisopropyl succinate as the starting materials. However, this
reaction gave complex products while failed to give the target regioisomeric
intermediate corresponding to (3). Thus, we switch the synthetic strategy by
incorporating an alkylthiophene unit to the nicotinonitrile moiety. The synthesis of the
intermediate compound 6-(4-hexylthiophen-2-yl)nicotinonitrile (5) is performed
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through a Stille coupling reaction of tributyl(4-hexylthiophen-2-yl)stannane and 6-
bromonicotinonitrile, which can be successfully reacted with disopropyl succinate
through condensation reaction in 2-methylbutan-2-ol solution to give the target
intermediate compound 3,6-bis(6-(4-hexylthiophen-2-yl)pyridin-3-yl)pyrrolo[3,4-
c]pyrrole-1,4(2H,5H)-dione (6) in a moderate yield of 50%. Compound (6) was directly
used for the alkylation with 1-bromo-2-octyldodecyl without purification, giving the
intermediate compound 3,6-bis(6-(4-hexylthiophen-2-yl)pyridin-3-yl)-2,5-bis(2-
octyldodecyl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (7) in a moderate yield of 39%.
The target dibromo-monomer 3,6-bis(6-(5-bromo-4-hexylthiophen-2-yl)pyridin-3-yl)-
2,5-bis(2-octyldodecyl)pyrrol o[3,4-c]pyrrole-1,4(2H,5H)-dione (8) can be afforded by
bromination of intermediate compound (7) with N-bromosuccinimide (NBS) in
chloroform solution. The molecular structures of all intermediates, monomers and
polymers are confirmed by nuclear magnetic resonance (NMR) spectroscopy (see the
Supporting Information, SI).
The palladium-catalyzed Stille polymerization of the bis-stannylled monomer (2)
with dibromo-monomer (4) gives the copolymer p-PDBPy, where the “p” represents
that the N-atom in the pyridyl unit is proximal to the central DPP unit. Based on similar
procedure, the palladium-catalyzed Stille polymerization of the 2,5-
bis(trimethylstannyl)thieno[3,2-b]thiophene and dibromo-monomer (8) gives the
copolymer d-PDBPy, where the “d” denotes that the N-atom in the pyridyl unit is distal
to the central DPP unit. After polymerization, the solids were precipitated in methanol
and collected by filtration. The purification of these copolymers was carried out by
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Soxhlet extraction using methanol, acetone and hexane successively to remove the
small-molecule and catalyst residues. Then the copolymers were collected by using
hexane for d-PDBPy and dichloromethane for p-PDBPy followed by re-precipitation
and filtration in methanol. Both of the two resultant copolymers can be readily
dissolved in common organic solvents, such as chloroform (CF), tetrahydrofuran (THF),
chlorobenzene (CB) owing to the tethered bulky 2-octyldodecyl side groups on the DPP
unit and hexyl side groups on the thiophene units. The number average molecular
weight (Mn) and polydispersity indices (PDIs) of these polymers were evaluated by gel
permeation chromatography (GPC) using THF as the eluent and polystyrene as the
standard. The estimated Mn are 37.7 and 21.4 kDa with PDI of 2.5 and 3.1 for d-PDBPy
and p-PDBPy, respectively.
Scheme 1 Synthetic route of pyridine-bridged DPP polymers.
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Thermal properties
Thermal properties of two copolymers were investigated by thermogravimetric analysis
(TGA) at a heating rate of 20 oC min-1 and differential scanning calorimetry (DSC) at a
scanning speed of 10 °C min-1 under a nitrogen atmosphere. As shown in Fig. S1a (see
the Supporting Information, SI), both copolymers exhibited good thermal stability with
5% mass loss temperature higher than 400 °C. Copolymer p-PDBPy exhibited an
o
obvious glass transition temperature (Tg) of about 120 C, indicating good thermal
property of the copolymer. In contrast, no discernable glass transition temperature can
be realized for copolymer d-PDBPy up to 250 oC (Fig. S1b in the SI). The different Tg
of these copolymers might be correlated to the different intermolecular
interactions.29, 36 , 37 Detailed molecular weight and thermal analysis data are
summarized in Table S1 (see the SI).
Optical properties
The UV-Vis absorption spectra of the copolymers d-PDBPy and p-PDBPy in dilution
chlorobenzene (CB) solution with concentration of 1×10-5 g mL-1 and as thin films are
shown in Fig. 1. Copolymer p-PDBPy based on 2-pyridyl as the flanked unit exhibited
much broader and bathochromic-shifted absorption spectra both in CB solution and as
thin film than that of d-PDBPy based on 3-pyridyl moiety as the flanked unit. In
addition, both in solution and as thin films, these copolymers exhibit dual absorption
bands, which are characteristic absorption profiles for the donor-acceptor type of
conjugated polymers. The relatively weak absorption band located at 350-450 nm can
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be ascribed to the delocalized π-π* transition, and the intense absorption bands at 500-
750 nm can be correlated to the intramolecular charge transfer (ICT) effects between
electron deficient DPP-based flanked pyridyl units and electron donating TT2T unit in
the polymer backbones. The optical band gaps (Egopt) are 1.84 and 1.62 eV for d-PDBPy
and p-PDBPy, respectively, as evaluated from the onset of absorption in thin film. The
maximum absorption wavelength (λmax) and the optical band gap (Egopt) are summarized
in Table 1.
The absorption profiles of d-PDBPy in CB solution and as thin film are quite
analogous to a maximal absorption (λmax) of about 570 nm, which are distinctly
hypsochromic shift with respect to those for p-PDBPy. One notes that there is a sharp
absorption signal emerged at 693 nm, which can be correlated to the formation of strong
intermolecular aggregation of the p-PDBPy in thin film. It is also worth noting that
there is a relatively weak shoulder peak for the absorption profile of p-PDBPy in CB
solution, which is supposed to be attributed to the existence of aggregation. This is
understandable since for copolymer d-PDBPy, one notes that there are the two
hydrogen atoms in the pyridal unit that are adjacent to the central DPP unit, which
would lead to steric hindrance with less favorable aggregation tendency. In contrast,
since there is only one hydrogen atom in the pyridal unit that is adjacent to the central
DPP unit, thus segments comprising of Py-DPP-Py may have strong tendency to
aggregate. This assumption can be confirmed by the UV-vis absorption spectra of
copolymers at raised temperatures (Fig. 1c and d). As can be seen in Fig. 1c, the
absorption spectra of d-PDBPy showed trivial variations upon raised temperature from
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28 to 130 oC, which is understandable since the lack of strong aggregation in the CB
solution. In contrast, for the UV-vis spectra of copolymer p-PDBPy, it was realized that
the shoulder peak located at about 690 nm gradually disappeared, while the λmax
progressively hyposchromic shift from 626 nm to about 590 nm, which can be ascribed
to the breakup of aggregations. These observations clearly indicated that the strong
aggregation can only be formed in solution of p-PDBPy, implying that the 2-pyridyl
unit can facilitate the formation of co-planar geometry that is favorable for the overlap
of π-orbitals along the polymeric chain.
Fig. 1 UV-vis absorption spectra of the copolymers in CB solution with concentration
of 1 × 10-5 g mL-1 (a) and as thin films (b); temperature-dependent UV-vis absorption
spectra of copolymers in chlorobenzene (CB) solution with concentration of 1 × 10-5 g
mL-1 for d-PDBPy (c) and p-PDBPy (d).
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Table 1 Optical and electrochemical properties of copolymers
cv d
b
c
opt
Eg
polymer
λmax (nm)
EHOMO ELUMO Eg
λonset
(eV)
(eV)
(eV)
(nm) (eV)
in solutiona as films b
d-PDBPy
p-PDBPy
576
626
570
672
765
1.84
1.62
–5.44
–5.32
–3.01
–3.09
2.43
2.23
635, 693
a. Measured in chlorobenzene solution;
b. Measured as thin film spin-coated from chlorobenzene solution;
c. Calculated from the UV-vis absorption band edge of the copolymers film by the
equation, Egopt = 1240/λonset
;
d. Estimated by cyclic voltammetry.
Electrochemical properties
Cyclic voltammetry (CV) measurements were performed to evaluate the highest
occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO)
energy levels of these copolymers. The characteristics of CV curves were recorded
under an inert atmosphere at a scan rate of 100 mV s-1, with relevant curves shown in
Fig. 2 and relevant data summarized in Table 1. Here the ferrocene/ferrocenium (Fc/Fc+)
redox couple was used as the standard for the measurement, which was assumed to have
an absolute energy level of –4.8 eV of the redox potential relative to vacuum.38 Under
exactly the same conditions for the measurements of samples, the potential of Fc/Fc+
was measured as 0.39 V relative to the saturated calomel electrode. Thus, the HOMO
energy levels and the LUMO energy levels of the copolymers can be calculated
according to the equation of HOMO = –e(Eox + 4.41) (eV) and LUMO = –e(Ered + 4.41)
(eV), where the Eox and Ered represent the onset of oxidation and reduction, respectively.
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The calculated HOMO/LUMO energy levels are –5.44/–3.01 eV and –5.32/–3.09 eV
for d-PDBPy and p-PDBPy, respectively. Relative to the copolymer d-PDBPy, the
HOMO energy level of p-PDBPy slightly increased while the LUMO energy level
slightly decreased. The combination of these factors leads to reduced electrochemical
cv
band gaps (Eg ) of 2.23 eV for p-PDBPy relative to that of 2.43 eV for d-PDBPy.
Fig. 2 Cyclic voltammograms (a) and HOMO/LUMO energy levels of copolymers (b).
Theoretical calculation
To get insight into the effects of the position of N atom in pyridine unit on the molecular
geometry and the electronic structures, density functional theory (DFT) theoretical
calculations were performed at the B3LYP/6-31G (d) basis on the Gaussian 09
package.39 Here the alkyl side chains on the modeling dimer compounds were replaced
by methyl groups to simplify the calculation. Fig. 3 illustrated the resulted frontier
molecular orbitals and the dihedral angles between the flanked 3-pyridyl or 2-pyridyl
unit with the adjacent thiophene unit (φ2) and the DPP moiety (φ1). For both dimers
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[d-DBPy]2 and [p-DBPy]2, HOMO orbitals are delocalized over the coDnOjuI: g10a.1t0e3d9/C6TC00051G
backbone, while the LUMO orbitals more preferentially resided in the DPP unit.
The comparison of these dimers indicated that the dihedral angle of φ1
significantly reduced from 21.4o for [d-DBPy]2 to 1.2o for [p-DBPy]2, which can be
attributed to the reduced steric interaction since the lack of hydrogen in the 2-position
of the 2-pyridyl moiety. In contrast, due to the potential noncovalent interaction of N…S
in [d-DBPy]2 can promote the formation of coplanar architecture between the adjacent
thienyl and pyridyl moieties,40 the nearly planar conformation was realized with the
dihedral angle of φ2 is 0.2o for [d-DBPy]2, which significantly increased to 18.2o due to
the repulsion of the adjacent hydrogen atoms for [p-DBPy]2.
Fig. 3 The frontier molecular orbitals of (d-DBPy)2 and (p-DBPy)2 obtained from DFT
calculations with a chain length n = 2 at the B3LYP / 6-31G (d) level.
Charge carrier mobility
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To investigate the effects of the orientation of pyridyl groups on the charge transport
properties of DPP-based copolymers, bottom-gate and top-contact (BGTC) organic
field effect transistors (OFET) with architecture of Al:Nd/AlOx:Nd/ODPA
/polymer/Au were fabricated. Here the Al:Nd was used as the gate instead of pure Al,
since it can effectively suppress the formation of hillock and can reduce the surface
roughness. The Al:Nd film was anodized to produce a layer of AlOx:Nd on the surface
of the Al:Nd film as the gate insulator, which was then modified with
octadecylphosphonic acid (ODPA, molecular structure shown in Fig. 4a). The DPP-
based polymers were spin-coated from chloroform (CF) or chlorobenzene (CB)
solution onto ODPA modified AlOx:Nd film. The films were thermally annealed at
100°C in nitrogen atmosphere for 10 min to facilitate the formation of ordered micro-
structures. Then Au was thermally evaporated onto the polymer film through a shadow
mask. The recorded output and transfer characteristics are shown in Fig. 4 (and Fig. S2
in the SI), with relevant parameters summarized in Table 2.
Based on the film spin-casted from CB solution, the copolymer p-PDBPy showed
a moderate hole mobility (μhole) of 0.35 cm2·V-1·s-1 with a high on/off ratio (Ion/off) of
104~105. In contrast, copolymer d-PDBPy that is processed under exactly the same
conditions exhibited about three orders of magnitude lower hole mobility of about 3.2
× 10-4 cm2 V-1 s-1 with a Ion/off of 102~103. Similar trend was also realized for the films
processed from chloroform (CF) solution, with μhole of was 0.23 cm2·V-1·s-1 for p-
PDBPy that is much higher that of 3.7 × 10-4 cm2 V-1 s-1 for d-PDBPy. Given that the
devices are fabricated under exactly the same conditions, and the molecular weight of
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the two copolymers are quite comparable, thus the difference in charge carrier DmOoI: b10i.l1i0t3y9/C6TC00051G
must be correlated to the orientation of the asymmetric pyridyl unit along with the
backbone of the regio-isomeric polymer structures. Since the nitrogen atom in pyridyl
unit proximal the adjacent DPP core unit in copolymer p-PDBPy, the 2-pyridyl
substituents can effectively reduce the steric interaction with the DPP core unit that
resulted in a highly coplanar structure, which may potentially promote close packing of
the DPP unit in solid state with obviously increased intermolecular interaction that can
offer effective pathways for charge transport. In contrast, despite the nitrogen unit that
in 3-pyridyl unit is distal to the central DPP unit can facilitate the formation of co-planar
structure with the adjacent thiophene unit, the repulsive interaction of the hydrogen
atoms adjacent to the DPP core unit can resulted in increased dihedral angle and thus a
larger interchain π–π stacking distance, which would impede both intra- and
intermolecular charge transport events. It is also worth mentioning that despite the in
hole-mobility can be significantly affected by the pyridyl orientations, one may
anticipate such effect of the isomeric structures on the electron-mobility. However,
since the resultant polymers in this work do not exhibited apparent n-type properties, at
the current stage we do not have good explanations on this issue. Nonetheless, these
observations clearly indicated that the flanked 2-pyridyl unit that can lead to smaller
dihedral angle with the central DPP unit is more promising than 3-pyridyl unit for the
attainment of higher hole mobility.
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Table 2. Summary of the device properties of the DPPT-TT OFETs with different
processing conditions.
Polymer
CB
μ(cm2·V-1·s-1)
CF
μ(cm2·V-1·s-1)
Ion/Ioff
102~103
104~105
Ion/Ioff
102~103
104~105
d-PDBPy
p-PDBPy
3.2×10-4
0.35
3.7×10-4
0.23
Fig. 4 (a) Devices structure of the copolymer OFETs. Output and transfer
characteristics of (b and c) d-PDBPy and (d and e) p-PDBPy devices (spin-coated from
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o
CB solutions) at VSD=-30 V (L=70 μm, W=500 μm) after thermal annealing at 100 C
for 10 min.
Morphology
Tapping mode atomic force microscopy (AFM) was used to investigate the film
morphology of the isomeric conjugated polymers. The films were processed from the
chlorobenzene or chloroform solution on the top of the glass/Al:Nd/AlOx:Nd/ODPA
substrate according to the same procedure as that of the OFET device. For copolymer
d-PDBPy with the nitrogen atom distal to the central DPP unit, one can observe very
smooth surface with a root mean square (RMS) values of 0.3 and 0.6 nm for the film
spin-casted from CB (Fig. 5a) and CF (Fig. 5c) solution, respectively. In contrast, for
copolymer p-PDBPy that with the nitrogen atom proximal to the central DPP unit, both
films spin-casted from CB (Fig. 5b) and CF (Fig. 5d) showed apparent granular features
across the entire film associated with much rougher film morphology with RMS value
of 2.0 and 4.6 nm, respectively, indicating the formation of much stronger
intermolecular aggregations that is favorable for charge transportation. Given that the
only difference in the two films is the intrinsic orientation of pyridyl unit, thus it is
rational to surmise that the obviously different film morphology of the regio-isomeric
copolymers can be attributed to the discrepancy of the intrinsic orientation of pyridyl
unit along polymer backbone. We note that additional film morphology studies may
facilitate further understanding of the effects of the orientation of the pyridyl on the
microstructures and charge transportations, nonetheless, the current observations
clearly indicated that the importance of attaining high co-planarity between the adjacent
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flanked unit with the central DPP unit for the achievement of high mobility.
Fig. 5 AFM topography images (2×2 μm2) of (a, c) d-PDBPy and (b, d) p-PDBPy
copolymer thin film. The films were spin-coated from (a, b) CB and (c, d) CF.
Conclusion
In summary, two regio-isomeric π-conjugated polymer consisting of pyridyl
flanked diketopyrrolo[3,4-c]pyrrole unit have been designed and synthesized, where
the nitrogen atom in the pyridyl unit was proximal or distal to the central
diketopyrrolopyrrole unit. It was noted that the molecular geometry, absorption spectra,
optical band-gaps, frontier molecular orbitals as well as the film morphology of the
resulted copolymers can be significantly affected by the orientation of nitrogen atom in
the pyridyl unit. Of particular interests is that the copolymer based on proximal oriented
nitrogen atom exhibited a moderate field-effect transistor hole mobility for the films
processed from both chloroform and chlorobenzene solution, which is about three
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orders of magnitude higher than those obtained based on the counterpart copolymer
with distal oriented nitrogen atom. These observations illustrated that the charge
transport properties can be significantly affected by the orientation of pyridyl unit along
the polymer backbone, which highlighted the importance of the delicate control of
molecular structures for the high-performance organic semiconductors.
Experimental section
Materials
All the solvents were purchased from Guangzhou Chemical Reagent Cooperation.
Anhydrous THF and xylene were distilled from sodium and benzophenone was used as
the indicator. All reagents were purchased from Acros, Strem Chemicals, and were used
without further purification. Compound 3,6-bis(5-bromopyridin-2-yl)pyrrolo[3,4-
c]pyrrole-1,4(2H,5H)-dione (3) was synthesized according to the reported procedures.
2,5-Bis(3-hexylthiophen-2-yl)thieno[3,2-b]thiophene (1)
In a 100 mL three-necked flask was added compound 2-bromo-3-hexylthiophene (0.80
g, 3.22 mmol) was mixed with 2,5-bis(trimethylstannyl)thieno[3,2-b]thiophene (0.60 g,
1.29 mmol) and Pd(PPh3)4 (30 mg, 26 μmol) in 50 mL toluene, and then the solution
was heated to 110 °C for 8 h under argon protection. After cooling to room temperature,
the solvent removed by reduced pressure to give the crude product and purified by
column chromatography using petroleum ether as an eluent to obtain compound (1) as
a white solid in a yield of 66 % (0.40 g). 1H NMR (500 MHz, CDCl3): δ 7.24 (s, 2H),
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7.21 (d, J = 5.2 Hz, 2H), 6.96 (d, J = 5.2 Hz, 2H), 2.79 (t, J = 8 Hz, 4H), 1.65 (dt, J =
15.5, 7.6 Hz, 4H), 1.43 – 1.24 (m, 12H), 0.88 (t, J = 7.5 Hz, 6H). 13C NMR (126 MHz,
CDCl3): δ 140.43, 139.29, 137.76, 130.87, 130.19, 124.49, 118.20, 31.83, 30.94, 29.40,
22.78, 14.23.
2, 5-Bis(3-hexyl-5-(trimethylstannyl)thiophen-2-yl)thieno[3,2-b]thiophene (2)
In a 50 mL one-necked flask compound (1) (0.2 g, 0.42 mmol) was dissolved in 20 mL
THF under argon protection. Then the solution was cooled to -78 °C, to which 0.93 mL
n-BuLi (1.48 mmol, 1.6 M in hexanes) was added dropwise. The mixture was kept at -
78 °C for 30 minutes and then allowed to warm to 0 °C for another 30 minutes, and
then cooled to -78 °C. Then trimethyltin chloride (1.69 mmol, 1 M in THF) was added
dropwise. The mixture was stirred overnight at room temperature and poured into water,
extracted with hexane, washed with water, dried over MgSO4. The solvent removed by
reduced pressure and to give the product (2) as a brown-yellow solid in a yield of 80 %
(0.27 g). 1H NMR (500 MHz, CDCl3): δ 7.21 (s, 2H), 7.02 (s, 2H), 2.80 (t, J = 8.0 Hz,
4H), 1.71 – 1.61 (m, 4H), 1.46 – 1.24 (m, 12H), 0.93 – 0.82 (m, 6H), 0.45 (s, 18H).13C
NMR (126 MHz, CDCl3): δ 141.46, 139.28, 138.51, 137.90, 137.36, 136.66, 117.64,
31.84, 31.09, 29.56, 29.34, 22.79, -8.05.
3,6-Bis(5-bromopyridin-2-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-c]pyrrole-
1,4(2H,5H)-dione (4)
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In a 250 mL three-necked flask compound 3,6-bis(5-bromopyridin-2-yl)pyrrolo[3,4-
c]pyrrole-1,4(2H,5H)-dione (2.4 g, 5.36 mmol) was mixed with potassium carbonate
(2.22 g, 16.07 mmol) and 1-bromo-2-octyldodecyl (5.81 g, 16.07 mmol) in 100 mL
DMF under argon. The mixture was poured into 150 mL water after stirred at 130 °C
for 12 h. The organic phase was extracted with ethyl acetate, washed with brine, dried
over MgSO4. After solvent was evaporated, crude solid was purified by column
chromatography using petroleum ether and dichloromethane as an eluent to obtain the
target compound (4) as a dark red solid in a yield of 41 % (2.2 g). 1H NMR (500 MHz,
CDCl3): δ 8.93 (d, J = 8.6 Hz, 2H), 8.74 (d, J = 2.3 Hz, 2H), 8.01 (dd, J = 8.5, 2.4 Hz,
2H), 4.28 (d, J = 7.3 Hz, 4H), 1.59 (d, J = 16.6 Hz, 2H), 1.40 – 1.01 (m, 64H), 0.87(q,
13
J = 7.1 Hz, 12H). C NMR (126 MHz, CDCl3): δ 162.63, 150.25, 146.21, 145.11,
139.84, 128.61, 122.73, 111.57, 46.44, 38.36, 32.07, 32.05, 31.59, 30.16, 29.83, 29.81,
29.77, 29.71, 29.51, 29.47, 26.52, 22.84, 22.83, 14.27.
6-(4-Hexylthiophen-2-yl)nicotinonitrile (5)
In a 250 mL three-necked flask compound 6-bromonicotinonitrile (5 g, 27.32 mmol)
was mixed with tributyl(4-hexylthiophen-2-yl) stannane (18.74 g, 40.98 mmol) and
Pd(PPh3)4 (0.63 g, 0.55 mmol) in 100 mL toluene, and then the solution was heated to
110 °C for 8 h under argon protection. After the reaction mixture was cooled to room
temperature, the solvent removed by reduced pressure to give the crude product and
purified by column chromatography using petroleum ether and dichloromethane as an
eluent to give the target compound as a white solid in a yield of 93 % (6.90 g). 1H NMR
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(500 MHz, CDCl3): δ 8.77 (dd, J = 1.9, 1.0 Hz, 1H), 7.89 – 7.87 (m, 1H), 7.67 (dd, J =
8.4, 0.6 Hz, 1H), 7.54 (s, 1H), 7.13 (s, 1H), 2.66 – 2.59 (m, 2H), 1.70 – 1.57 (m, 2H),
1.42 – 1.22 (m, 6H), 0.89 (t, J = 7 Hz, 3H).13C NMR (126 MHz, CDCl3): δ 155.74,
152.61, 145.17, 142.42, 139.66, 128.71, 125.69, 118.21, 117.17, 106.96, 31.75, 30.60,
30.51, 29.02, 22.70, 14.20.
3,6-Bis(6-(4-hexylthiophen-2-yl)pyridin-3-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-
dione (6)
In a 500 mL three-necked flask, 150 mL 2-methylbutan-2-ol was added to a mixture of
sodium (1.02 g, 44.5 mmol) and FeCl3 (80 mg, 0.49 mmol), after sodium was
completely dissolved by heating the solution at 90 °C protected by argon, cooling the
temperature to 50 °C, compound (5) (6.42 g, 23.73 mmol) was added to solution, and
then the solution was stirred for 1 h at 90 °C. Diisopropyl succinate (2 g, 9.89 mmol)
in 2-methylbutan-2-ol (10 mL) was added drop-wise over 30 min, the resulting black-
purple suspension was stirred for another 20 h at 90 °C. After cooling to 60 °C 15 mL
acetic acid was added to precipitate the product, and then the solution was refluxed for
1 h. The solution was then cooled to room temperature, and precipitates were filtrated
and washed with hot water and methanol until the filtrate remained colorless, and dried
at 60 °C in vacuum oven to obtain the target compound (6) as a dark purple solid in a
yield of 50 % (3.10 g). The product is insoluble in common NMR solvents and was
used in the next step without further purification.
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3,6-Bis(6-(4-hexylthiophen-2-yl)pyridin-3-yl)-2,5-bis(2-octyldodecyl)pyrrolo[3,4-
c]pyrrole-1,4(2H,5H)-dione (7)
In a 250 mL three-necked flask compound (6) (2.54 g, 4.08 mmol) was mixed with
potassium carbonate (1.47 g, 10.6 mmol) and 1-Bromo-2-octyldodecyl (3.98 g, 11.01
mmol) in 100 mL DMF under argon. The mixture was poured into 150 mL water after
stirred at 130 °C for 12 h. The organic phase was extracted with ethyl acetate, washed
with brine, dried over MgSO4. After solvent was evapoted, crude solid was purified by
column chromatography using petroleum ether and dichloromethane as an eluent to
obtain the target compound (7) as a bright red solid in a yield of 39 % (1.86 g). 1H NMR
(500 MHz, CDCl3): δ 8.88 (s, 2H), 8.13 (dd, J = 26.9, 8.3 Hz, 2H), 7.56 (dd, J = 41.7,
8.1 Hz, 2H), 7.37 (d, J = 35.2 Hz, 2H), 7.03 (d, J = 11.2 Hz, 2H), 3.74 (t, J = 8.4 Hz,
4H), 2.59 (dt, J = 12.9, 7.9 Hz, 4H), 1.64 (dd, J = 13.5, 6.8 Hz, 4H), 1.44 – 0.96 (m,
78H), 0.91 (t, J = 6.7 Hz, 6H), 0.85 (dt, J = 10.3, 7.0 Hz, 12H). 13C NMR (126 MHz,
CDCl3): δ 162.51, 162.47, 154.21, 154.14, 149.20, 148.99, 145.78, 144.82, 144.76,
143.49, 136.81, 136.61, 127.52, 127.45, 125.41, 124.26, 124.18, 122.44, 122.33,
121.30, 118.30, 118.19, 112.52, 110.42, 110.24, 45.51, 45.37, 37.32, 32.07, 32.02,
31.84, 31.36, 30.71, 30.51, 30.48, 30.05, 29.80, 29.78, 29.73, 29.66, 29.50, 29.44, 29.20,
29.18, 26.21, 26.20, 22.84, 22.80, 22.78, 14.27, 14.26, 14.25.
3,6-Bis(6-(5-bromo-4-hexylthiophen-2-yl)pyridin-3-yl)-2,5-bis(2-octyldodecyl)
pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione (8)
In a 250 mL one-necked flask compound (7) (1.5 g, 1.27 mmol) was dissolved in
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CHCl3/AcOH (1:1, 50 mL/50 mL) under argon protection. And then the flask was
placed in an ice bath to keep the reaction mixture at 0 °C. N-bromosuccinmide (0.52 g,
2.91 mmol) was added into the solution in small portion and the mixture was stirred for
overnight. The reaction mixture was poured into water, extracted with ethyl acetate,
washed with water, dried over MgSO4. The solvent was removed by reduced pressure
and the crude product was purified by column chromatography using dichloromethane
as an eluent to give the target compound (8) as a bright red solid in a yield of 55 %
1
(0.93 g). H NMR (500 MHz, CDCl3): δ 8.83 (d, J = 1.7 Hz, 2H), 7.99 (dd, J = 8.3,
1.6Hz, 2H), 7.39 (d, J = 8.2 Hz, 2H), 7.09 (s, 2H), 3.69 (d, J = 6.9 Hz, 4H), 2.52 – 2.48
(m, 4H), 1.57 (dd, J = 14.4, 6.9 Hz, 4H), 1.39 – 1.04 (m, 78H), 0.91 (t, J = 6.9 Hz, 6H),
0.84 (dt, J = 9.9, 7.1 Hz, 12H). 13C NMR (126 MHz, CDCl3): δ 162.34, 153.06, 149.66,
145.66, 143.88, 143.28, 136.32, 126.38, 122.26, 117.31, 114.00, 109.95, 45.06, 37.19,
32.08, 32.02, 31.78, 31.35, 30.04, 29.92, 29.82, 29.79, 29.75, 29.68, 29.52, 29.46, 29.20,
26.17, 22.85, 22.82, 22.77, 14.27, 14.26, 14.25.
Synthesis of d-PDBPy
To a microwave vial was added compound 8 (0.2 g, 0.15 mmol), 2,5-
bis(trimethylstannyl)thieno[3,2-b]thiophene (69.5 mg, 0.15 mmol). Xylenes (3 mL)
was added and the solution degassed before addition of Pd(PPh3)4 (9 mg, 0.05 mmol).
The reaction mixture was further degassed and subsequently sealed. The vial was
heated in a microwave reactor at 180 °C for 45 min. After cooling to room temperature,
the reaction mixture was poured into methanol. The crude product was filtered off and
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purified by subsequent Soxhlet extraction with methanol, acetone and hexane. To the
hexane fraction was added an aqueous solution of sodium diethyldithiocarbamate and
the mixture was heated to 60 °C with vigorous stirring for 8 hr. After cooling to room
temperature, the organic fraction was extracted with water. The polymer was obtained
as dark purple solid (yield 82 %) upon evaporation of the hexane extract and
1
precipitation into methanol. H NMR (500 MHz, CDCl3): δ 8.85 (br, ArH), 7.96 (br,
ArH), 7.14 (br, ArH), 3.92 (br, N-CH2), 3.45 (br, N-CH2), 2.76 (br, CH), 1.79 – 1.00
(br, CH2), 0.88-0.81 (br, CH3). GPC: Mn = 37.7 kDa, Mw = 94.1 kDa, PDI = 2.5.
Synthesis of p-PDBPy
To a microwave vial was added compound 4 (100.9 mg, 0.1 mmol), compound 2 (79.8
mg, 0.1 mmol). Xylenes (3 mL) was added and the solution degassed before addition
of Pd(PPh3)4 (9 mg, 0.05 mmol). The reaction mixture was further degassed and
subsequently sealed. The vial was heated in a microwave reactor at 180 °C for 45 min.
After cooling to room temperature, the reaction mixture was poured into methanol. The
crude product was filtered off and purified by subsequent Soxhlet extraction with
methanol, acetone, hexane and dichloromethane (CH2Cl2). To the CH2Cl2 extract was
added an aqueous solution of sodium diethyldithiocarbamate and the mixture was
heated to 60 °C with vigorous stirring for 8 hr. After cooling to room temperature, the
organic fraction was extracted with water. The polymer was obtained as dark green-
purple solid (yield 69%) upon evaporation of the hexane extract and precipitation into
methanol.1H NMR (500 MHz, CDCl3): δ 9.07 (br, ArH), 8.95 (br, ArH), 8.03 (br, ArH),
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7.61 (br, ArH), 7.53 (br, ArH), 7.43 – 7.23 (br, ArH), 4.38 (br, N-CH2), 2.86 (br, CH),
1.93 – 1.06 (br, CH2), 0.96 – 0.82 (br, CH3). GPC: Mn = 21.4 kDa, Mw = 67.5 kDa, PDI
= 3.1.
Instruments
1H NMR and 13C NMR spectra were collected on a Bruker AV-500 (500 MHz) in
deuterated chloroform with tetramethylsilane as a reference. The molecular weights of
the polymers were determined by Acquity APC using THF as eluent.
Thermogravimetric analyses (TGA) were performed on a Netzsch TG 209 at a heating
rate of 20oC min-1 under a nitrogen atmosphere. The differential scan calorimetry (DSC)
characteristics were measured on a Netzsch DSC 204 under N2 flow at a heating rate of
10 oC min-1 and cooling rate of 10 oC min-1. UV-vis absorption spectra were obtained
with a Shimadzu UV-3600 UV-vis-NIR spectrometer. Cyclic voltammetry (CV) data
were measured on a CHI630E electrochemical workstation equipped with a glassy
carbon working electrode, a saturated calomel electrode as the reference electrode, and
a platinum wire counter electrode, and carried out with anhydrous acetonitrile
tetrabutylammonium hexafluorophosphate (0.1 M) as the supporting electrolyte under
an argon atmosphere at a scan rate of 100 mV s-1. The polymers were deposited on the
electrode. Potentials were referenced to the ferrocene/ferrocenium couple by using
ferrocene as standard. The oxidation potential of ferrocene was set at −4.80 eV with
respect to zero vacuum level. The HOMO energy levels were calculated according to
the equation of EHOMO = –[4.8 + e (EOX –EFc/Fc+)] (eV).
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Fabrication and Characterization of Organic Field Effect Transistors (OFETs)
For characterization of OFETs performance, the bottom-gate and top-contact (BGTC)
device geometry was employed. Firstly, A 150-nm-thick Al:Nd (3 wt%) film was
deposited with DC sputtering and patterned with conventional photolithography
method. Afterwards, Al:Nd film was anodized for 1.5 h to produce a layer of 200-nm-
thick AlOx:Nd on the surface of the Al:Nd film as the gate insulator. After plasma
treatment, the substrate was immersed in a 2-propanol solution containing 5 mM ODPA
(shown in Fig. 4a) molecules at 60 oC for 30 min. The capacitance density (Ci) of the
ODPA-treated AlOx:Nd was 42 nF cm-2. After that, it was removed from the solution,
and rinsed thoroughly with 2-propanol, and blown dry with a stream of nitrogen. The
copolymer was dissolved in dichlorobenzene (0.5 mg/mL) and spin-coated (2000 rpm)
onto ODPA modified AlOx:Nd with a thickness of 20~30 nm. Then a 40-nm-thick Au
film was thermally evaporated onto the OSC film through a shadow mask, defining a
channel width/length (W/L) of 500/70 μm. The PTFT characterizations were measured
with a semiconductor parameter analyzer (Agilent 4155C) and a probe station at 25oC
in air.
Acknowledgments
The work was financially supported by the Natural Science Foundation of China (No.
51303056, 21520102006, 21490573 and 21125419), the Ministry of Science and
Technology (No. 2014CB643501), and Guangdong Natural Science Foundation (Grant
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No. S2012030006232 and 2015A030313229).
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