β-alkyl substituted dithieno[2,3-d;2′,3′-d′]benzo[1, 2-b;4,5-b′]dithiophene semiconducting materials and their application to solution-processed organic transistors

Article abstract of DOI:10.1021/cm301816t

A novel highly π-extended heteroacene with four symmetrically fused thiophene-ring units and solubilizing substituents at the terminal β-positions on the central ring, dithieno[2,3-d;2′,3′-d′] benzo[1,2-b;4,5-b′]dithiophene (DTBDT) was synthesized via int

Full text of DOI:10.1021/cm301816t

Article
pubs.acs.org/cm
β‑Alkyl substituted Dithieno[2,3‑d;2′,3′-
d′]benzo[1,2‑b;4,5‑b′]dithiophene Semiconducting Materials and
Their Application to Solution-Processed Organic Transistors
Jonggi Kim,†,∥ A-Reum Han,‡,∥ Jung Hwa Seo,§ Joon Hak Oh,*,‡ and Changduk Yang*,†
‡
†Interdisciplinary School of Green Energy and School of Nano-Bioscience and Chemical Engineering, KIER-UNIST Advanced
Center for Energy (K−U ACE), Low Dimensional Carbon Materials Center, Ulsan National Institute of Science & Technology
(UNIST), Ulsan 689-798, South Korea
§Department of Materials Physics, Dong-A University, Busan 604-714, South Korea
S
* Supporting Information
ABSTRACT: A novel highly π-extended heteroacene with four sym-
metrically fused thiophene-ring units and solubilizing substituents at the
terminal β-positions on the central ring, dithieno[2,3-d;2′,3′-d′]benzo[1,2-
b;4,5-b′]dithiophene (DTBDT) was synthesized via intramolecular
electrophilic coupling reaction. The α-positions availability in the
DTBDT motif enables the preparation of solution-processable DTBDT-
based polymers such as PDTBDT, PDTBDT-BT, PDTBDT-DTBT, and
PDTBDT-DTDPP. Even with its highly extended acene-like π-framework,
all polymers show fairly good environmental stability of their highest
occupied molecular orbitals (HOMOs) from −5.21 to −5.59 eV. In the
course of our study to assess a profile of semiconductor properties, field-
effect transistor performance of the four DTBDT-containing copolymers via solution-process is characterized, and PDTBDT-
DTDPP exhibits the best electrical performance with a hole mobility of 1.70 × 10−2 cm2 V−1 s−1. PDTBDT-DTDPP has a
relatively smaller charge injection barrier for a hole from the gold electrodes and maintains good coplanarity of the polymer
backbone, indicating the enhanced π−π stacking characteristic and charge carrier transport. The experimental results
demonstrate that our molecular design strategy for air-stable, high-performance organic semiconductors is highly promising.
KEYWORDS: dithieno[2,3-d, 2′,3′-d′]benzo[1,2-b, 4,5-b′]dithiophene, heteroacenes, thiophene-benzene annulated acenes,
organic field-effect transistors (OFETs), polymeric semiconductors
thieno[n]acenes29−32 or thiophene-benzene annulated
INTRODUCTION
■
acenes32,33 for the development of molecular organic semi-
Organic field-effect transistors (OFETs) based on π-conjugated
conductors (see Figure 1). Moreover, in S-containing fused
organic semiconductors have been intensively investigated as
heteroacenes, the nonbonding sulfur−sulfur interaction of
key elements for realizing low-cost, flexible, large-area
electronic displays.1−13 Owing to the excellent solution
neighboring molecules can play an important role in facilitating
charge transport in OFETs.30−34 Together with consideration
processability, film uniformity, and thermal stability, π-
of the ability to ink formulations with tuned rheological
conjugated polymeric materials are a promising choice to
commercialize OFETs.14−23 Among a number of molecular
classes reported so far, linearly fused higher oligoacenes,
properties, thereafter, these units as π-cores have also been
integrated into conjugated polymers for good OFET character-
istics with environmental stability (Figure 1).29 Nonetheless,
exemplified by pentacene consisting of five fused benzenes,
exhibit mobility of up to 3.0 cm2 V−1 s−1 because of their strong
none of the chemistries are readily applicable to the synthesis of
intermolecular π−π interaction in the crystalline state.24−26
Spurred by these promising results, Bao et al. reported solution-
soluble pentacene-containing conjugated polymers with slightly
improved air-stability compared to pentacene itself.27 However,
those polymers exhibited very low mobilities of ∼8 × 10−7 cm2
V−1 s−1 in a nitrogen environment and no field-effect behavior
in ambient conditions because of their intrinsically insufficient
air-stability (the highest occupied molecular orbitals (HOMO)
= −5.08 eV).28 As an alternative to the benzene-based acenes,
structurally related π-extended cores based on thiophene ring
with much better stability have been focused either on
large heteroacene molecules. Recently, Mullen et al. have
̈
successfully developed dithieno[2,3-d;2′,3′-d′]benzo[1,2-b;4,5-
b′]dithiophene (DTBDT) derivatives as a five-ring-fused
pentacene analogue by using a simple intramolecular electro-
philic coupling reaction (see Figure 1).35−37 The desirable
chemical, electronic, and morphological structures of DTBDT
yielded a high hole mobility of up to 1.7 cm2 V−1 s−1 in
Received: June 12, 2012
Revised: August 15, 2012
Published: August 16, 2012
© 2012 American Chemical Society
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Figure 1. Chemical structures of linear acenes, thieno[n]acenes, thiophene-benzene acenes, and their semiconducting polymers.
Figure 2. β-alkyl substituted DTBDT and DTBDT-containing polymers.
solution-processed OFETs.37 Besides, the comparison of the
relatively low HOMO levels and larger band gaps of DTBDT
derivatives to other fused heteroacenes suggested DTBDT
motif as one of the most stable oligoacene semiconductors.
Although further development of the materials based on
DTBDTs would be expected to afford superior printable
organic semiconductors, their related materials formed by
introducing DTBDT moieties into molecules have not been
reported so far. This is a result of incorporating the solubilizing
substituents at the terminal α-positions of the DTBDT unit,
which makes further chemical modification no longer possible.
Replacement of the α-substituents in DTBDT with β-
solubilizing groups is a promising strategy to increase the
diversity of the modification of the molecular and electronic
structures, which can lead to the development of superior
organic semiconductors. In this work, we design and effectively
new polymer semiconductor based on DTBDT but also several
commonly used conjugated building blocks such as 2,1,3-
benzothiadiazole (BT), 4,7-dithienyl-2,1,3-benzothiadiazole
(DTBT), and 3,6-dithiophen-2-yl-pyrrolo[3,4-c]pyrrole-1,4-
dione (DTDPP) copolymerized with DTBDT units to
investigate the relationships between their structures and
properties. A series of conjugated polymers, that is, PDTBDT,
PDTBDT-BT, PDTBDT-DTBT, and PDTBDT-DTDPP are
shown in Figure 2. It should be pointed out that, to the best of
our knowledge, β-substituted DTBDT as a building block and
its derivatives are so far the first examples synthesized and used
for the fabrication of OFET devices.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The synthetic routes to
the β-alkyl substituted DTBDT monomer and DTBDT-based
polymers (PDTBDT, PDTBDT-BT, PDTBDT-DTBT, and
PDTBDT-DTDPP) are depicted in Scheme 1. Compounds 1,
2, and 3 were easily prepared in moderate to good isolated
yields according to the established methods.38 The key
synthesize a β-alkyl substituted DTBDT by employing Mullen’s
̈
previously explained synthetic methodology. The α-positions
availability of the resulting DTBDT allows us to prepare various
DTBDT-containing polymers. Here, we not only report on a
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Scheme 1. Synthetic Routes of β-Dodecyl Substituted DTBDT and DTBDT-Containing Polymers (PDTBDT, PDTBDT-BT,
PDTBDT-DTBT, and PDTBDT-DTDPP)
1
Figure 3. H NMR spectrum of β-dodecyl substituted DTBDT (CDCl3, 600 MHz, 300 K).
intermediate 1,4-dibromo-2,5-bis(methylsulfinyl)benzene 3
effectively cross-coupled with 5-tributylstannyl-3-dodecylthio-
phene to afford the precursor 4 in a yield of 70%. Dodecyl was
chosen as the long side chain of the precursor to ensure good
solubility of the target semiconducting molecules in organic
solvents and solution processability. In our initial synthetic
attempt, the trifluoromethanesulfonic acid induced intra-
molecular ring-closing reaction of the dimethylsulfinyl benzene
with the adjacent thiophene rings was attempted and followed
by a reflux in pyridine, but no desired product was observed. A
likely rationale for this result is the occurrence of the
intermolecular cross-linking reaction. Thus, the intramolecular
ring closure was performed with Eaton’s reagent (7.7 wt %
P2O5 in CH3SO3H) as a relatively mild Lewis acid at a low
concentration, successfully resulting in the regioisomer-free β-
dodecyl-substituted DTBDT (5) as an off-white solid in 40%
1
yield. As shown in Figure 3, the H NMR spectrum provides
unambiguous evidence for no isomers since all the typical
singlets can be assigned to the corresponding aromatic protons
in the molecule (see the 13C NMR in the Supporting
Information for further information). Finally, the organotin
DTBDT monomer 6 was synthesized by deprotonation with n-
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Figure 4. UV−vis absorption spectra of DTBDT-based polymers in o-DCB solution (a) and as thin solid films (b). Cyclic voltammograms of
DTBDT-based polymer thin films (c). Energy level diagrams for DTBDT-based polymers (d).
Table 1. Optical and Electrochemical Properties of DTBDT-Based Polymers
a
b
c
d
abs
abs
polymer
PDTBDT
λmax (o-DCB, nm)
λonset (film, nm)
Egopt (film, eV)
HOMO (eV)
LUMO (eV)
Egelec (eV)
e
396
411
2.63
2.28
1.91
1.51
−5.59
−5.55
−5.29
−2.96
PDTBDT-BT
393
412
−3.34
−3.38
−3.57
2.21
1.90
PDTBDT-DTBT
PDTBDT-DTDPP
394, 532
334, 636
431, 572
335, 636
b
−5.21
1.64
ferrocene
a
c
ox
ferrocene
Calculated from onset wavelength in absorption spectra. HOMO = −(Eonset − E1/2
+ 4.8) eV. LUMO = −(Eonsetred − E1/2
+ 4.8)
d
e
eV. Egelec (eV) = Eonset − Eonsetred. LUMO = Egopt + HOMO.
ox
BuLi followed by quenching with trimethyltin chloride, which is
capable of functionalization either asymmetrically or symmetri-
cally with various π-cores at both terminal positions of DTBDT
as well as incorporation into versatile oligomers and polymers
through metal-mediated cross-coupling reactions. For solution-
processable polymeric semiconductors, the DTBDT-based
polymer PDTBDT was first synthesized by oxidative coupling
of DTBDT using FeCl3 in anhydrous chloroform. A
combination of an electron-rich and an electron-deficient unit
in a copolymer main chain is a well-known synthetic strategy to
yield low band gap with relatively high charge carrier mobilities
due to the intramolecular charge transfer (ICT). Thus, such
desirable properties afford the necessary motivation for further
work toward the preparation of novel donor−acceptor
copolymers based on DTBDT. Therefore, a series of donor−
acceptor polymers, PDTBDT-BT, PDTBDT-DTBT, and
PDTBDT-DTDPP were prepared by copolymerizing a newly
conceived soluble DTBDT donor moiety with various acceptor
moieties, namely, 2,1,3-benzothiadiazole (BT), 4,7-dithienyl-
2,1,3-benzothiadiazole (DTBT), and 3,6-dithiophen-2-yl-
pyrrolo[3,4-c]pyrrole-1,4-dione (DTDPP), respectively. All
the polymers were purified by sequential Soxhlet extraction
processes with methanol, acetone, and hexane to remove the
fractions of small molecules. After purification, the molecular
weights of the resulting solids from either chloroform or
chlorobenzene fraction were investigated by gel permeation
chromatography (GPC) against PS standard (for details, see
Experimental Section). The difference of the molecular weights
may result from either the reactivity of the acceptor segments
or the different solubility of the resulting polymers. Attempts
are underway to attach longer branched alkyl groups to β-
positions of DTBDT than dodecyl groups, which would
presumably make the polymers more soluble during polymer-
ization and thus allow the synthesis of high molecular weight
polymers. Also, the DTBDT-based polymers have good
thermal stability which was examined by thermogravimetric
analysis (TGA) techniques, and these polymers showed a
decomposition temperature of about 400 °C under nitrogen
atmosphere (Supporting Information, Figure S4a).
Optical and Electrochemical Properties. The UV−vis
spectra for four polymers PDTBDT, PDTBDT-BT,
PDTBDT-DTBT, and PDTBDT-DTDPP in o-dichloroben-
zene (o-DCB) solution and as spin-coated films on glass are
shown in Figure 4a−b and the absorption maxima (λmax) are
summarized in Table 1. Compared with DTBDT as a small
molecule, all polymer solutions show large red-shifts which
demonstrates that all the polymers have extended their
conjugation through polymer chain formation. In solution,
the parent polymer PDTBDT shows a strong absorption π−π*
transition band at λmax = 396 nm, which is mostly identical to
that of the as-spun thin film. This suggests that PDTBDT has a
highly ordered backbone structure and strong π−π interaction
in the solution because of the flat skeleton.39 On the other
hand, the absorption spectra of the DTBDT-cored donor−
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acceptor copolymers (PDTBDT-DTBT and PDTBDT-
DTDPP) in the solutions are characterized with two bands,
one near 334−394 nm and the other centered at 532−636 nm.
The former bands can be assigned to π−π* transitions whereas
the lowest energy bands are due to ICT between the donor and
the acceptor moieties. Relative to the solution absorption, the
absorption spectrum of PDTBDT-DTBT film is slightly
broadened and exhibits a red shift (∼ 40 nm), indicating the
presence of stronger interactions between molecules (packing
effect) in the solid state. However, the absorption of PDTBDT-
DTDPP remains almost unchanged from the solution to the
film as the similar feature observed from PDTBDT. This
implies that there are aggregations of the polymer chains
formed in solution that can be accounted for by π−π stacking
of fused ring moieties (DTBDT and DPP). Most surprising is
that the optical features of PDTBDT-BT in both solution and
films reveal remarkable similarity to those of PDTBDT even
though PDTBDT-BT has an alternating donor−acceptor
bichromophore system. This means that DTBDT is a weak
donor unit like other fused aromatic compounds such as
naphtho[2,1-b:3,4-b′]dithiophene (NDT), dithieno[3,2-f:2′,3′-
h]quinoxaline (QDT), and benzo[1,2-b:4,5-b′]dithiophene
(BnDT).
the molecular backbone of DTBDT and two 2-ethylhexyl
substituents on the DPP are replaced by hydrogen and methyl
groups, respectively. As expected from the fused-acene
structure of the DTBDT unit, its geometry is entirely planar.
In addition, good coplanarity also exists in the DTBDT-
containing polymers (PDTBDT, PDTBDT-DTBT, and
PDTBDT-DTDPP) (Figure 5). Thus, we propose that they
The electronic states and ionization potential/electron
affinity (HOMO/LUMO levels) of the DTBDT-based polymer
films on a Pt electrode were investigated by cyclic voltammetry
(CV) (Figure 4c) and the electrochemical data are also listed in
Table 1. All polymers exhibit quasi-reversible or reversible p-
doping/dedoping (oxidation/rereduction) processes in the
positive potential range, while irreversible n-doping/dedoping
(reduction/reoxidation) behaviors in the negative potential
range are observed. Note that like the other p-type fused
heteroacene frameworks,33 as the reduction potentials of
PDTBDT are out of our scan range, the lowest unoccupied
molecular orbital (LUMO) energy level is calculated from the
HOMO energy level and the optical band gap. The reversibility
of CV in the oxidation processes can be interpreted as a good
sign of the stability of the radical cation of DTBDT.
Furthermore, in terms of electrochemical properties, the
HOMO level of PDTBDT is empirically estimated to be
−5.59 eV, which is lower than those of pentacene and most
oligothiophenes. The low-lying HOMO and relatively large
band gap afford environmental stability.
Compared with PDTBDT, the HOMO level of PDTBDT-
BT (−5.55 eV) is almost unchanged but its LUMO level
(−3.34 eV) shows a significant decrease of 0.38 eV, most likely
being rationalized by considering the electron-deficient BT
species in the polymers. In both cases of PDTBDT-DTBT and
PDTBDT-DTDPP, the noticeably higher HOMO levels are
ascribed to the two unsubstituted thiophene units in the π-
system because these thiophene rings render the resulting
conjugated polymer backbone more electron-rich. From these
results, we conclude that the insertion of unsubstituted
thiophene segments allows a moderate modulation of the
band gap and energy level of the resulting polymers. This
finding will assist the future design of semiconductive polymers
with tunable electronic properties.
Figure 5. DFT-optimized geometries and charge-density isosurfaces
for the HOMO and LUMO levels and the side views of (a) PDTBDT,
(b) PDTBDT-BT, (c) PDTBDT-DTBT, and (d) PDTBDT-
DTDPP.
will allow efficient π−π stacking between facing backbones and
enhance the charge carrier transport in the π−π direction.
However, in the case of PDTBDT-BT, steric interactions
between directly neighboring BT and DTBDT moieties induce
a 21° twist in the ground state of the isolated molecule (Figure
5b), supporting our aforementioned interpretation about
absorption features reflecting the absence of ICT in
PDTBDT-BT. PDTBDT shows a similar density of states
distribution for both the HOMO and LUMO wave functions,
implying the weak donor nature of DTBDT. On the other
hand, the HOMOs of the donor−acceptor copolymers
(PDTBDT-BT, PDTBDT-DTBT, PDTBDT-DTDPP) are
well-distributed along the conjugated chains; their LUMOs
are centralized on each acceptor unit such as BT, DTBT, and
DTDPP, being proposed effective p-type organic semi-
conductors. Theoretical calculations also show that the
HOMO−LUMO gaps decrease with the electron-withdrawing
abilities of the acceptors groups on the molecules, supporting
the notion that DPP possesses enhanced electron-accepting
ability relative to BT. Those speculations are in agreement with
the optical and electrochemical results above (Table1).
Considering the packing motif and the enhanced ICT effect,
we predicted that PDTBDT-DTDPP would give the best
performance in OFETs (vide infra).
Morphology and X-ray Diffraction (XRD) Analyses of
Thin Films. The morphologies of the DTBDT-based polymer
thin films were investigated with atomic force microscopy
(AFM). The granular domain sizes in the as-cast polymer thin
films became larger upon thermal treatment at 150 °C (Figure
6 and Supporting Information, Figure S5). For instance, the as-
cast PDTBDT-DTDPP thin film showed randomly entangled
and nanofibrillar clusters, whereas the annealed thin film
exhibited significantly larger grains, implying enhanced
DFT Frontier Molecular Energy Calculation. To provide
an insight into the molecular architecture of the polymers, a
molecular simulation was carried out for the DTBDT-based
polymers with a chain length of n = 1 using density functional
theory (DFT) at the B3LYP/6-31G* with the Gaussian 03
package. To simplify the calculations, the β-dodecyl chains in
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spacing of polymer thin film was decreased (by up to ∼7 Å)
after thermal treatment, indicating the formation of a denser
molecular packing (Supporting Information, Table S1). The
annealed PDTBDT thin film exhibited the first, second, and
third order diffraction peaks at 2θ = 4.38°, 8.72°, 12.92°,
respectively, revealing that the film has a relatively long-range
order (Supporting Information, Figure S6). PDTBDT-BT and
PDTBDT-DTBT thin films showed the primary diffraction
peak at 2θ = 4.44° and 4.32° after annealing, corresponding to
d(001)-spacing values of 19.89 Å and 20.43 Å, respectively.
(Figure 7a and Supporting Information, Table S1). Despite the
relatively large dihedral angle (21°) between BT and DTBDT
moieties predicted by our DFT calculations, the crystalline
ordering was observed in the thin film. On the other hand, the
annealed PDTBDT-DTDPP thin films displayed two primary
(001) diffraction peaks at 2θ = 3.24° and 3.94°, corresponding
to d(001)-spacing values of 27.24 Å and 22.40 Å, respectively.
This reflects the existence of crystalline polymorphs in the
polymer thin film. Additional higher order diffraction peaks
were observed at larger 2θ, indicating that PDTBDT-DTDPP
thin film has the largest long-range order among the DTBDT-
based polymer thin films. The polymorphism of PDTBDT-
DTDPP was also investigated using differential scanning
calorimetry (DSC) analysis. In the DSC thermogram, two
exothermic crystallization peaks were observed, indicating the
existence of two polymorphs (See Supporting Information,
Figure S4b).
OFET Characterizations. To investigate how the acceptor
units copolymerized with DTBDT affect charge transport, we
fabricated bottom-gate/top-contact OFETs. The polymer thin
films were drop-cast from a chloroform solution onto n-
octadecyltrimethoxysilane (OTS)-treated SiO2/Si substrates.
The films were annealed on a hot plate at 150 °C for 30 min in
N2 atmosphere. Further details on the surface treatment and
OFET fabrication are included in the Experimental Section.
The OFET performance of the annealed thin films of DTBDT-
based polymers is listed in Table 2. All the DTBDT-based
polymers exhibited hole-transporting (p-channel) character-
istics. The typical transfer characteristics are illustrated in
Figure 8 (The typical output characteristics are illustrated in
Supporting Information, Figure S8). The annealed thin films of
DTBDT-based polymers showed significantly enhanced mobi-
lities, compared with the as-cast polymer thin films (See
Supporting Information, Table S2: typically 10−3−10−8 cm2 V−1
s−1 for the as-cast polymer thin films). The average hole
mobility of the annealed polymer thin films was found to be in
the order of 10−2−10−3 cm2 V−1 s−1 except for PDTBDT-BT
(∼10−4 cm2 V−1 s−1). This is presumably due to the influence
of the torsional hindrance by the BT unit in PDTBDT-BT,
which is in line with the results of the DFT calculation.
PDTBDT as a homopolymer exhibits a high threshold voltage
(VT) of −55.8 V, probably because of the larger charge
injection barrier given by the low-lying HOMO (−5.59 eV).40
PDTBDT-DTBT showed a relatively higher mobility than
PDTBDT-BT. This can be attributed to the fact that two
unsubstituted thiophene units adjacent to the BT unit truly
reduce the influence of torsional hindrance and enhance
coplanarity of the polymer backbone. Among the DTBDT-
based polymers, PDTBDT-DTDPP exhibited the best OFET
performance with a hole mobility as high as 1.70 × 10−2 cm2
V−1 s−1 and a reasonably small VT of −7.1 V. PDTBDT-
DTDPP has a relatively smaller charge injection barrier for hole
from the gold electrodes because of the well-matching HOMO
Figure 6. AFM height (left) and phase (right) images of the
PDTBDT-DTDPP thin films (a) as-cast, (b) annealed at 150 °C on
OTS-treated SiO2/Si substrates.
intermolecular interactions and potentially facilitated charge
transport (Figure 6).
To further elucidate the crystalline nature and molecular
orientation in the polymer thin films, out-of-plane X-ray
diffraction (XRD) analysis was carried out on the DTBDT-
based polymer thin films. DTBDT-based polymer thin films
display relatively well-defined XRD peaks (Figure 7 and
Supporting Information, Figure S6). The out-of-plane d(001)-
Figure 7. XRD patterns obtained from (a) PDTBDT-BT, (b)
PDTBDT-DTDPP thin films as-cast (black) and annealed at 150 °C
(red). Symbol * for PDTBDT-DTDPP thin film denotes the
secondary phase (P2) of crystalline polymorphs in the polymer thin
film.
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Table 2. OFET Performance of DTBDT-Based Polymer Thin Films Annealed at 150 °C
polymer
PDTBDT
μmax [cm2 V−1 s−1
]
μavg [cm2 V−1 s−1
]
Ion/Ioff
VT [V]
3.40 × 10−3
4.53 × 10−4
1.13 × 10−2
1.70 × 10−2
1.94 × 10−3
3.18 × 10−4
9.24 × 10−3
1.54 × 10−2
6.40 × 104
4.27 × 105
1.42 × 104
4.98 × 104
−55.8
−24.7
−22.5
−7.1
PDTBDT-BT
PDTBDT-DTBT
PDTBDT-DTDPP
the DTBDT an excellent candidate as a new class of p-type
semiconductors. Further studies to optimize DTBDT-based
devices are under way.
EXPERIMENTAL SECTION
■
Materials and Instruments for Characterizations. All reagents
and chemicals were purchased from Sigma-Aldrich chemical company
and used without further purification. THF was freshly distilled over
sodium and benzophenone, prior to use. 1H and 13C NMR were
recorded on Varian VNRS 600 MHz spectrophotometer using the
deuterated chloroform (CDCl3) with TMS as an internal standard.
Elemental analyses of carbon, hydrogen and sulfur were carried out
with Flesh 2000 elemental analyzer, and MALDI-TOF spectra were
measured with a Bruker Ultraflex III. To investigate molecular weights
and polydispersity indices (PDI) of new polymers, gel permeation
chromatography (GPC) was carried out with Agilent 1200 series and
miniDAWN TREOS using tetrahydrofuran (THF) as solvent against
PS as a standard. UV−vis absorption spectra were recorded on a
Varian Carry 5000 spectrophotometer at room temperature. Cyclic
voltammetry (CV) measurement was performed on AMETEK
VersaSTAT 3 with a three-electrode cell in a nitrogen bubbled 0.1
M tetra-n-butylammonium hexafluorophosphate (n-Bu4NPF6) solu-
tion in acetonitrile at a scan rate of 50 mV/s at room temperature. As
electrodes, the Ag/Ag+ (0.1 M of AgNO3 in acetonitrile) reference
electrode, a platinum counter electrode, and polymers coated platinum
working electrodes were used respectively. The Ag/Ag+ reference
electrode was calibrated using a ferrocene/ferrocenium redox couple
as an internal standard, whose oxidation potential is set at −4.8 eV
with respect to zero vacuum level. The HOMO energy levels were
obtained from the equation HOMO = −(Eoxonset − E(ferrocene)onset + 4.8)
eV. The LUMO levels of polymers were obtained from the equation
Figure 8. Transfer characteristics of OFET devices of DTBDT-based
polymers annealed at 150 °C (VDS = −100 V); (a) PDTBDT, (b)
PDTBDT-BT, (c) PDTBDT-DTBT, and (d) PDTBDT-DTDPP.
level. In addition, introduction of a DPP segment to the
copolymer backbone allows a long-range molecular ordering in
the polymer thin films, as shown by X-ray diffraction (XRD)
analysis. These electrochemical and molecular packing
characteristics of PDTBDT-DTDPP thin films are found to
facilitate charge transport.
onset
onset
LUMO = −(Ered
− E(ferrocene)
+ 4.8) eV. The surface
topographic images on the films were acquired in the tapping mode
using a Veeco Multimode V AFM. All measurements were made at
room temperature in ambient conditions. XRD analysis for all of the
films was performed with a Bruker D8 Advance high resolution X-ray
diffractometer.
CONCLUSIONS
■
In summary, a soluble weak donor, β-dodecyl substituted
DTBDT, which is a highly π-extended heteroacene with four
symmetrically fused thiophene-ring units, has been synthesized
via intramolecular electrophilic coupling reaction. The α-
positions availability of the DTBDT enable us to obtain the
parent polymer as well as to create “weak donor−acceptor”
copolymers by incorporating various electron-deficient co-
monomers. Despite their highly extended aromatic features,
four DTBDT-based polymers (PDTBDT, PDTBDT-BT,
PDTBDT-DTBT, and PDTBDT-DTDPP) exhibit noticeably
low-lying HOMO energy levels (−5.21 to −5.59 eV), implying
fairly good environmental stability. The XRD patterns and
AFM images for the surface morphology show that thermally
treated DTBDT-based polymer thin films have enhanced
intermolecular interactions, thereby facilitating charge transport
in the thin-film transistor setting. The solution-processed
OFETs using DTBDT-based polymers as an active layer
exhibit p-channel characteristics, and the PDTBDT-DTDPP
exhibits the best electrical performance with a hole mobility of
1.70 × 10−2 cm2 V−1 s−1 and a small VT of −7.1 V because of
the small charge injection barrier from the electrodes and
excellent coplanarity of the polymer backbone. High hole
mobility, air stability, and ease of structural modification make
OFET Device Preparation and Measurement. Highly n-doped
(100) Si wafers (<0.004 Ω·cm) with a thermally grown SiO2 (300 nm,
Ci = 10 nF cm−2) were cleaned with piranha solution and UV-ozone
plasma. The surface of SiO2/Si wafer was modified with octadecyl-
trimethoxysilane (OTS) as previously reported.41,42 3 mM of OTS
solution, prepared by using trichloroethylene (TCE) as the solvent,
was placed to cover the entire surface of SiO2/Si substrate on a spin
coater. After waiting for at least 10 s, spin coating was performed at
3000 rpm for 30 s. The wafer was then placed into a larger desiccator
in which a vial filled with NH4OH (1−2 mL) was put together. House
vacuum was applied to the desiccator for 5 min, and the whole system
was set aside in ambient conditions for 12 h. The samples were
removed, and washed sequentially with toluene, acetone, isopropyl
alcohol, and then dried using a nitrogen gun. DTBDT-based polymers
were dissolved in chloroform (5 mgmL−1) and stirred at 50 °C for 24
h. The prepared polymer solution was drop-cast onto the hydrophobic
OTS-treated SiO2/Si, and placed in a vacuum oven at 50 °C for 12 h.
Then, the polymer film (∼ 35 nm) was annealed on a hot plate at 150
°C for 30 min under N2 atmosphere. Au (40 nm) was deposited to
form source and drain electrodes with a channel length (L) of 50 μm
and a channel width (W) of 1000 μm. The OFET characteristics were
measured in a N2-filled glovebox by using a Keithley 4200
semiconductor parametric analyzer. The field-effect mobility was
calculated in the saturation regime using the following equation:
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Chemistry of Materials
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1
2
After stirring for 24 h, brown powder was collected by filtration. The
powder was purified by Soxhlet extraction with MeOH, acetone and
hexane in sequence. Aqueous EDTA solution was treated for the
chloroform faction. The separated phase was concentrated and poured
into MeOH. Finally, the light brown polymer (130 mg, 65%) was
collected by filtration and vacuum-dried. GPC analysis: Mn = 17,000
g/mol, Mw = 52,000 g/mol, and Mw/Mn = 3.05 (for only THF-soluble
fraction, against PS standard). H NMR (CDCl3, 600 MHz): δ ppm
8.26 (br, 2H), 2.88−2.78 (br, 4H), 2.16−2.0 (br, 4H), 1.45−1.08 (br,
36H), 0.87−0.84 (br, 6H).
IDS
=
(W/L)μCi(VG − VT)2
where IDS is the drain-to-source current, μ is the mobility, and VG and
VT are the gate voltage and threshold voltage, respectively.
Synthesis of 5,5′-(2,5-Bis(methylsulfinyl)-1,4-phenylene)bis(3-do-
decylthiophene) (4). A dry 250 mL three-neck flask was flushed with
argon and charged with the compound 3 (5.0 g, 13.88 mmol) and 5-
tributylstannyl-3-dodecylthiophene (15.8 g, 29.14 mmol) in anhydrous
solvent of toluene (60 mL) and DMF (15 mL). Pd(pph3)4 (0.8 g, 0.69
mmol) was added and then the mixture was refluxed overnight.
Reaction progress was monitored by TLC. The reaction was quenched
with water, which was followed by washing with dichloromethane
several times. After drying it over MgSO4 and concentrated in vacuo,
the crude was purified by column chromatography (silca gel,
dichloromethane) to afford white solid product (6.9 g, 70%). 1H
NMR (CDCl3, 600 MHz): δ ppm 8.16 (s, 2H), 7.13 (s, 2H), 7.05 (s,
2H), 2.63 (t, J = 7.8 Hz, 4H), 2.54 (s, 6H), 1.63 (t, J = 7.2 Hz, 4H),
1.33−1.26 (m, 36H), 0.87 (t, J = 7.2 Hz, 6H). 13C NMR (CDCl3, 150
MHz): δ ppm 147.07, 144.52, 136.94, 132.54, 129.68, 125.94, 122.77,
41.68, 31.90, 30.46, 30.39, 29.65, 29.63, 29.60, 29.43, 29.33, 29.24,
22.67, 14.10. MALDI-TOF (m/z) calcd: 703.2; found: 703.23.
Elemental analysis calcd for C40H62O2S4: C, 68.32; H, 8.89; O, 4.55;
S, 18.24 Found: C, 68.02; H, 8.60; S, 17.98.
Synthesis of 3,8-Didodecyldithieno[2,3-d;2′,3′-d′]benzo[1,2-
b;4,5-b′]dithiophene (5). The compound 4 (3.0 g, 4.26 mmol) was
stirred with Eaton’s reagent (25 mL) at room temperature under dark
for 3 days. The mixture was poured into ice−water and dark brown
solid collected by suction-filtration was dried in vacuum oven. A dry
250 mL three-neck flask was flushed with argon and charged with the
brown sulfonium salt, which was followed to be redissolved in pyridine
(100 mL) and then the mixture was refluxed overnight. When the
reaction mixture turned red, it was cooled down. After extracting with
a large volume of dichloromethane, the separated organic phase was
dried over MgSO4, and solvent was removed in vacuo. The crude was
purified by column chromatography on silica gel using dichloro-
methane as an eluent. Finally, recrystallization from hexane gave off-
1
Synthesis of Poly[3,8-didodecyldithieno[2,3-b;2′,3-b′]benzo[1,2-
b;4,5-b′]dithiophen-2,7-diyl-alt-2,1,3-benzothiadiazol-4,7-diyl]
(PDTBDT-BT). The compound 6 (200 mg, 0.20 mmol) and 4,7-
dibromo-2,1,3-benzothiadiazole (58.8 mg, 0.20 mmol) in a tube-type
Schlenk flask were dissolved with anhydrous mixture of toluene(8
mL)/DMF(2 mL). After bubbling with argon for 10 min, Pd(pph3)4
(11.55 mg, 0.01 mmol) was added and then the mixture was refluxed
for 2 days. The reaction mixture was poured into MeOH to precipitate
into red powder, which was followed by filtration. The collected
powder was purified by Soxhlet extraction with MeOH, acetone, and
hexane in sequence. The fraction from chloroform was washed with
aqueous DETA solution for 10 h. Finally, the concentrate phase was
poured into MeOH to afford a red polymer (120 mg, 80%). GPC
analysis: Mn = 16,500 g/mol, Mw = 35,500 g/mol, and Mw/Mn = 2.15
(for only THF-soluble fraction, against PS standard). 1H NMR
(CDCl3, 600 MHz): δ ppm 8.28−12 (br, 2H), 7.94−7.88 (br, 2H),
2.81 (br, 4H), 1.94−1.79 (br, 4H), 1.25 (br, 36H), 0.85 (br, 6H).
Synthesis of Poly[3,8-didodecyldithieno[2,3-b;2′,3-b′]benzo[1,2-
b;4,5-b′]dithiophen-2,7-diyl-alt-4,7-dithienyl-2,1,3-benzothiadiazol-
2′,5′-diyl] (PDTBDT-DTBT). The compound 6 (200 mg, 0.20 mmol)
and 4,7-di(2′-bromothien-5′-yl)-2,1,3-benzothiadiazole (96.2 mg, 0.21
mmol) in a tube-type Schlenk flask were dissolved with anhydrous
toluene(6 mL) and DMF(1.5 mL) under argon. After addition of
Pd(pph3)4 (12.13 mg, 0.01 mmol), the reaction mixture was refluxed
for 2 days. The mixture was poured into MeOH to precipitate dark red
powder. After filtration, the powder was washed in sequence with
MeOH for 1 day and acetone for 1 day by Soxhlet extraction. EDTA
aqueous solution was treated for chloroform and chlorobenzene
fraction. Final precipitation in MeOH gave dark red polymer (20 mg
for chloroform fraction and 80 mg for chlorobenzene fraction, 50%).
GPC analysis: Mn = 9,000 g/mol, Mw = 16,500 g/mol, and Mw/Mn =
1
white powder (1.10 g, 40%). H NMR (CDCl3, 600 MHz): δ ppm
8.26 (s, 2H), 7.11 (s, 2H), 2.76 (t, J = 7.2 Hz, 4H), 1.79 (t, J = 7.2 Hz,
4H), 1.42−1.25 (m, 36H), 0.87 (t, J = 7.2 Hz, 6H). 13C NMR (CDCl3,
150 MHz): δ ppm 139.98, 138.92, 136.10, 133.17, 130.58, 122.28,
115.39, 31.83, 29.76, 29.57, 29.54, 29.46, 29.30, 29.26, 29.24, 28.77,
22.57, 13.92. MALDI-TOF (m/z) calcd: 639.1; found: 639.23.
Elemental analysis calcd for C38H54S4: C, 71.41; H, 8.52; S, 20.07
Found: C, 71.71; H, 8.56; S, 19.87.
1
1.83 (for only THF-soluble fraction, against PS standard). H NMR
(CDCl3, 600 MHz): δ ppm 8.27−8.14 (br, 2H), 7.98−7.71 (br, 2H),
7.52−7.32 (br, 4H), 3.04−2.53 (br, 4H), 2.06−1.62 (br, 36H), 0.88−
0.83 (br, 6H).
Synthesis of 2,7-Bis(trimethylstannyl)-3,6-didodecyldithieno[2,3-
b;2′,3-b′]benzo[1,2-b;4,5-b′]dithiophene (6). The compound 5 (0.83
g, 1.29 mmol) flushed with argon in a 250 mL Schlenk flask was
dissolved with freshly distilled THF (100 mL). n-BuLi (1.6 M, 2.43
mL, 3.87 mmol) was added dropwise to the mixture at −78 °C. After
stirring for an hour, trimethyltin chloride (1.03 g, 5.16 mmol) was
added in one portion at −78 °C. The reaction mixture was allowed to
warm to room temperature and stirred overnight. The reaction was
quenched with water, and the organic phase was separated after
washing with dichloromethane several times. After drying over
MgSO4, solvent was removed in vacuo. The caramel-like phase (1.0
g, 80%) was obtained as product, and this was used for polymerization
without any further purification. 1H NMR (CDCl3, 600 MHz): δ ppm
8.21 (s, 2H), 2.76 (t, J = 7.8 Hz, 4H), 1.75 (t, J = 7.8 Hz, 4H), 1.42−
1.26 (m, 36H), 0.88 (t, J = 7.2 Hz, 6H), 0.46 (s, 18H). 13C NMR
(CDCl3, 150 MHz): δ ppm 142.64, 140.40, 140.00, 138.91, 136.30,
130.02, 115.70, 32.20, 31.93, 30.16, 29.73, 29.68, 29.65, 29.54, 29.49,
29.36, 22.69, 14.12, −7.72. MALDI-TOF (m/z) calcd: 964.7; found:
964.09. Elemental analysis calcd for C44H70S4Sn2: C, 54.78; H, 7.31; S,
13.30; Sn, 24.61. Found: C, 54.90; H, 7.29; S, 13.21.
Synthesis of Poly[3,8-didodecyldithieno[2,3-b;2′,3-b′]benzo[1,2-
b;4,5-b′]dithiophen-2,7-diyl-alt-2,5-diethylhexyl-3,6-dithiophen-2-
yl-pyrrolo[3,4-c]pyrrole-1,4-dione-5′,5″-diyl] (PDTBDT-DTDPP). The
compound 6 (200 mg, 0.20 mmol) and 3,6-bis(5-bromothiophen-2-
yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (136 mg, 0.20
mmol) were placed in a tube-type Schlenk flask connected with argon.
The mixture was dissolved with anhydrous toluene (6.5 mL) and
DMF (1.5 mL). Pd(pph3)4 (11.55 mg, 0.01 mmol) as a catalyst was
added into the mixture and then the reaction mixture was heated up
with stirring to reflux for 2 days. The mixture was poured into MeOH/
NH4OH (4:1) to precipitate green powder. Soxhlet extraction was
performed in sequence with MeOH and acetone for 1 day each to
remove small molecules and collect the chloroform fraction. From final
precipitation in MeOH gave green polymer powder (200 mg, 83%).
GPC analysis: Mn = 16,500 g/mol, Mw = 59,000 g/mol, and Mw/Mn =
1
3.57 (for only THF-soluble fraction, against PS standard). H NMR
(CDCl3, 600 MHz): δ ppm 9.09 (br, 2H), 8.30−8.10 (br, 2H), 7.14−
7.08 (br, 2H), 3.75−3.73 (br, 4H), 3.01−2.75 (br, 4H), 2.03−1.99 (br,
1H), 1.82−1.77 (br, 4H), 1.40−1.08 (br, 52H), 0.96−0.83 (br, 18H).
Synthesis of Poly[3,8-didodecyldithieno[2,3-b;2′,3-b′]benzo[1,2-
b;4,5-b′]dithiophen-2,7-diyl] (PDTBDT). FeCl3 (200 mg, 1.23 mmol)
was placed in a 100 mL Schlenk flask. The compound 5 (200 mg, 0.31
mmol) in 30 mL of anhydrous chloroform was added with stirring
under argon. The dark mixture was stirred at room temperature for 2
days. The reaction mixture was poured into MeOH/NH4OH (4:1).
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra of monomers. TGA, DSC, AFM, and
XRD data. Output characteristics of DTBDT-based polymers
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Chemistry of Materials
Article
(24) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik, W.;
Weber, W. J. Appl. Phys. 2002, 92, 5259.
(25) Kelley, T. W.; Boardman, L. D.; Dunbar, T. D.; Muyres, D. V.;
on FET. This material is available free of charge via the Internet
at http://pubs.acs.org.
Pellerite, M. J.; Smith, T. Y. P. J. Phys. Chem. B 2003, 107, 5877.
AUTHOR INFORMATION
Corresponding Author
*E-mail: yang@unist.ac.kr (C.Y.), joonhoh@unist.ac.kr
(J.H.O.).
■
(26) Herwig, P. T.; Mullen, K. Adv. Mater. 1999, 11, 480.
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L.; Tang, M. L.; Toney, M. F.; Siegrist, T.; Bao, Z. J. Mater. Chem.
2011, 21, 7078.
Author Contributions
(29) He, M. Q.; Li, J. F.; Sorensen, M. L.; Zhang, F. X.; Hancock, R.
R.; Fong, H. H.; Pozdin, V. A.; Smilgies, D. M.; Malliaras, G. G. J. Am.
Chem. Soc. 2009, 131, 11930.
(30) Li, X. C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S.
C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc.
1998, 120, 2206.
(31) Xiao, K.; Liu, Y. Q.; Qi, T.; Zhang, W.; Wang, F.; Gao, J. H.;
Qiu, W. F.; Ma, Y. Q.; Cui, G. L.; Chen, S. Y.; Zhan, X. W.; Yu, G.;
Qin, J. G.; Hu, W. P.; Zhu, D. B. J. Am. Chem. Soc. 2005, 127, 13281.
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2011, 23, 4347.
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(35) Gao, P.; Beckmann, D.; Tsao, H. N.; Feng, X. L.; Enkelmann,
∥The first two authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the National Research Foundation
of Korea (NRF) grant (Code Nos.: 2010-0002494, 2010-
0019408, 2010-0026163, 2010-0026916, 2011-0026424, 2011-
0017174) funded by the Korean Government (MEST), and by
a grant (Code No.: 2011-0031628) from the Center for
Advanced Soft Electronics under the Global Frontier Research
Program of the MEST, Korea.
V.; Pisula, W.; Mullen, K. Chem. Commun. 2008, 13, 1548.
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(36) Gao, P.; Feng, X. L.; Yang, X. Y.; Enkelmann, V.; Baumgarten,
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