Thieno[3,2-b]thiophene oligomers and their applications as p-type organic semiconductors

Article abstract of DOI:10.1039/b900979e

This study describes the synthesis, characterization and electronic properties of a novel series of soluble thieno[3,2-b]thiophene oligomers (1a and b and 2a and b) for thin film transistor (TFT) applications. All the compounds were synthesized in high yi

Full text of DOI:10.1039/b900979e

View Article Online / Journal Homepage / Table of Contents for this issue
PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Thieno[3,2-b]thiophene oligomers and their applications as p-type organic
semiconductors†
Moawia O. Ahmed,a Chunmei Wang,a Peisi Keg,a Wojciech Pisula,b Yeng-Meng Lam,a Beng S. Ong,c
d
Siu-Choon Ng, Zhi-Kuan Chen and Subodh G. Mhaisalkar
c
a
*
*
Received 16th January 2009, Accepted 26th February 2009
First published as an Advance Article on the web 3rd April 2009
DOI: 10.1039/b900979e
This study describes the synthesis, characterization and electronic properties of a novel series of soluble
thieno[3,2-b]thiophene oligomers (1a and b and 2a and b) for thin film transistor (TFT) applications.
All the compounds were synthesized in high yield using Pd-catalyzed Stille or Suzuki coupling reactions
and were substituted by two dodecyl groups at the 3- or 4-position of the thiophene unit to ensure the
solubility for facile device fabrication. Aryl units such as phenyl and naphthyl were used for ‘end-
capping’ to provide stability against oxidation. The design of these materials has focused on their self-
assembly and solution processability. All the compounds have been characterized by 1H, 13C NMR, and
elemental analysis. Their electronic and optical properties were investigated using UV-Vis and
photoluminescence spectroscopy, cyclic voltammetry, thermal gravimetric analysis (TGA), and
differential scanning calorimetry (DSC). High-resolution STM images of 1a and 2a adsorbed on
HOPG revealed highly ordered self-organized domains. Two-dimensional wide-angle X-ray scattering
(2D-WAXS) was used to study the solid state packing of 1a and 2a. Top-contact OTFT devices from 1a
were prepared by spin coating and showed promising behaviour with mobilities up to 3.11 ꢀ 10ꢁ2 cm2
Vꢁ1 sꢁ1 and on/off ratios up to 104.
makes it very challenging for device fabrication through a solu-
Introduction
tion process.13 Their oligomeric counterparts are potentially
advantageous over the polymer analogs because of their good
solution processability, facile synthesis and purification, and
well-defined structure.14 In addition, these low molecular weight
semiconductors can be deposited by both vacuum sublimation
and solution process techniques. One of the main issues with
most oligomeric semiconductors containing thiophene or thie-
nothiophene building blocks is the rapid degradation of their
TFT devices due to the relatively low ionisation potentials.3
Thus, it is an aim of the present work to provide new organic
materials for use as semiconductors, which will allow straight-
forward up-scaling due to a well-designed synthetic route, with
good air and thermal stability. The work reported here also
focuses on materials that are easily processable and can be
readily deposited by a solution process method. For this purpose,
four oligomeric compounds (1a and b and 2a and b) using
thieno[3,2-b]thiophene as the building blocks (see Scheme 1 and
2) have been designed and synthesized. Two alkyl chains were
introduced at the 3 or 4-position of the thiophene ring to ensure
a better solubility for easier device fabrication. Aryl units such as
phenyl and naphthyl groups were used as ‘end-cap’ substituents
to enhance material/device stability against oxidation. The
devices derived from oligothiophenes, end-capped with an aryl
Organic thin film transistors (OTFTs) have recently received
considerable attention due to their ease of fabrication by solution
techniques which offer the potential for roll-to-roll printing and
thus for low cost high volume manufacturing processes.1–4 In the
past two decades, a great variety of organic semiconductors have
been developed and explored. Among them, thiophene or fused
thiophene, in particular thieno[3,2-b]thiophene based oligomeric
and polymeric materials, are the widely investigated materials for
OTFTs due to their high mobility.5–11 McCulloch et al. have
demonstrated high mobility poly[2,5-bis(3-alkylthiophen-2-yl)-
thieno(3,2-b)thiophene] semiconductors, with mobilities of up to
0.6 cm2 Vꢁ1 sꢁ1 12
.
While Ong and coworkers reported a poly[2,5-
that
bis(2-thienyl)-3,6-dipentadecylthieno(3,2-b)thiophene]
showed a high mobility of 0.25 cm2 Vꢁ1 sꢁ1 under ambient con-
ditions.1c Recently, Tokiyoshi and Shizuo have reported highly
crystalline thin films of liquid-crystalline poly[2,5-bis(3-tetrade-
cylthiophene-2-yl)-thieno(3,2-b)thiophene] with mobilities of up
to 0.44 cm2 Vꢁ1 sꢁ1 8
. However, thieno[3,2-b]thiophene based
polymers show insufficient solubility at room temperature, which
aSchool of Materials Science and Engineering, Nanyang Technological
University, 50 Nanyang Avenue, 639798 Singapore. E-mail: subodh@ntu.
edu.sg
bMax Planck Institute for Polymer Research, Ackermannweg 10, 55128
Mainz, Germany
cInstitute of Materials Research and Engineering (IMRE), 3 Research
Link, 117602 Singapore. E-mail: zk-chen@imre.a-star.edu.sg
dDivision of Chemical and Biomolecular Engineering, Nanyang
Technological University, 637459 Singapore
unit, not only show very high mobilities (up to 0.4 cm2 Vꢁ1 sꢁ1
and high on/off current ratios (Ion off
up to 105) but are also
found to be stable in air and under ambient illumination.14a
)
/
Herein we report the synthesis and characterization of a series
of solution processable small compounds (1a and b and 2a
and b) based on thieno[3,2-b]thiophene, for TFT applications.
To investigate their solid state packing, we performed
† Electronic supplementary information (ESI) available: MALDI-TOF
and DSC spectra. See DOI: 10.1039/b900979e
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 3449–3456 | 3449

View Article Online
The optical and electrochemical properties of 1a and b and 2a
and b were examined by UV-Vis absorption spectroscopy, fluo-
rimetry and cyclic voltammetry. The results are summarized in
Table 1. The absorption and emission spectra of 1a and b and 2a
and b in dilute THF are depicted in Fig. 1. All the compounds
show strong absorption and emission peaks in the 350–420 nm
and 470–480 nm ranges respectively. As expected, the absorption
and emission maxima of 1b and 2b with naphthalene units are
red-shifted compared to that of 1a and 2a with phenyl units. The
HOMO–LUMO gaps estimated from the absorption edges are
2.71 eV for 1a, 2.66 eV for 1b, 2.73 eV for 2a and 2.67 eV for 2b.
The oxidation potentials measured by cyclic voltammetry (CV)
(Fig. 2) of 1a and b and 2a and b are 0.88, 0.80, 0.84 and 0.81 V
(reference to SCE) respectively, from which the HOMO levels
were calculated as shown in Table 1. The HOMO levels of all
compounds range from ꢁ5.20 eV to ꢁ5.28 eV, which is signifi-
cantly lower than most of the thiophene based oligomers and
Scheme 1 Synthesis of oligomers 1a and b.
Table 1 Photophysical properties of 1a and b and 2a and b
HOMO–
LUMO
(band gap)/
UV-Vis
PL
Compound Tm/ꢂCa Td/ꢂCb lmax/nmc lmax/nmc eVd
1a
1b
2a
2b
124
114
117
118
360
395
397
400
389
409
401
407
474
480
465
478
ꢁ5.25/ꢁ2.54
(2.71)
ꢁ5.20/ꢁ2.61
(2.66)
ꢁ5.28/ꢁ2.55
(2.73)
ꢁ5.21/ꢁ2.54
(2.67)
Scheme 2 Synthesis of oligomers 2a and b.
a
b
Obtained from DSC measurement.
Obtained from TGA
d
c
measurement. Measured as a THF solution. Calculated from CV
and UV-Vis absorption spectra band edges.
two-dimensional wide-angle X-ray scattering (2D-WAXS)
studies on two compounds (1a and 2a).
Results and discussion
Synthesis
The syntheses of compounds 1a and b and 2a and b were
conveniently achieved as outlined in Scheme
1 and 2.
Compounds 1a and b were synthesized in good yields by utilizing
a one-pot Stille coupling15 of 2,5-dibromothieno[3,2-b]thiophene
4 and the corresponding freshly prepared tri-n-butylstannyl
derivatives of 3a [tributyl(4-dodecyl-5-phenyl-thiophen-2-yl)-
stannane] for 1a and 3b [tributyl(4-dodecyl-5-naphth-2-yl-thio-
phen-2-yl)stannane] for 1b, in the presence of PdCl2(PPh3)2,
under reflux in THF (Scheme 1). Compounds 2a–b were
synthesized by the Suzuki coupling of 2,5-bis(5-bromo-3-dode-
cylthiophen-2-yl)thieno[3,2-b]thiophene 6 and the appropriate
aryl boronic acid 7 [phenyl boronic acid (2a) or 2-naphthyl
boronic acid (2b)] using Pd(PPh3)4 under reflux in THF in good
yields (Scheme 2). All the compounds are soluble in organic
solvents such as CHCl3, toluene, THF, and can be easily purified
by column chromatography and recrystallization. The structures
of 1a and b and 2a–b were characterized by 1H, 13C NMR,
elemental analysis and MALDI-TOF mass spectrometry. The
results were consistent with the predicted chemical structures.
Fig. 1 UV-Vis and PL spectra of (a) 1a and b, (b) 2a and b.
This journal is ª The Royal Society of Chemistry 2009
3450 | J. Mater. Chem., 2009, 19, 3449–3456

View Article Online
adsorption due to their atomically flat surface and chemical
inertness. Adsorption studies of conjugated polythiophenes,
oligothiophenes and anthracene molecules on HOPG substrates
have been found to correlate successfully with their three-
dimensional crystal packings, X-ray diffraction studies and
device performances.19 It has also been found that molecules
typically possess the same molecular order of the two-dimen-
sional adsorbed monolayer on the HOPG surface as one layer of
its three-dimensional crystal. In addition to that, previous studies
using STM and AFM have also shown the close relationship
between the organization of polyquaterthiophenes on HOPG
surfaces and SiO2 surfaces.19a
On deposition, both compounds appear to self-organize into
highly ordered domains extending over 500 nm with a long
range order of lamellar rows (Fig. 4). Due to the electron rich
system concentrated at the main chain backbone of the
thienothiophene molecule, the backbones tend to appear as
brighter areas from the higher tunnelling currents. Alkyl chains
thus appear as spots, where only half the number of –CH2
groups from the alkyl chains are visible, which is typically the
case for alkyl chain adsorption onto the HOPG surfaces,
arising from the commensurability between the alkyl chains
and graphite sheet.20
Fig. 2 Cyclic voltammograms of 1a and b and 2a and b.
polymers, indicating higher stabilities of 1a and b and 2a and
b.3,4,16 In addition, the HOMO levels of all the compounds match
well with the workfunction of metallic gold (ꢁ5.1 eV) and
efficient hole charge injection between the electrode and the
semiconductor is anticipated.17
Alkylated oligothiophenes have been generally observed to
adsorb flatly on HOPG surfaces, with the alkyl chains spread out
and extended, parallel to the substrate surface.21 These alkyl
chains were also observed to interdigitate due to van der Waals
interactions, hence forming neat rows of organized lamellae.
Visible alkyl chain interdigitation from adjacent rows of
compounds 1a and 2a were observed in both images. Despite
different substituents sites, both compounds appear to possess
a similar packing order with slight differences in the unit cell
dimensions. The 2D unit cell parameters measured by STM for
1a are a ¼ 1.4 ꢃ 0.1 nm; b ¼ 2.5 ꢃ 0.1 nm; a ¼ 79 ꢃ 2ꢂ and for 2a
are a ¼ 1.4 ꢃ 0.1 nm; b ¼ 2.9 ꢃ 0.1 nm; a ¼ 81 ꢃ 2ꢂ. It can be
inferred that the compounds lie seemingly flat and planar on the
surfaces upon adsorption.
The thermal behaviour of compounds 1a and b and 2a and
b was determined by thermal gravimetric analysis (TGA) and
differential scanning calorimetry (DSC) (results are summarized
in Table 1). All the compounds demonstrated excellent thermal
ꢂ
stabilities, with decomposition temperatures above 360 C and
melting points higher than 110 ꢂC. Fig. 3 presents the DSC traces
for 1a and 2a respectively. It is interesting to note that at the same
cooling rate of 10 C minꢁ1, only 1a undergoes crystallization,
ꢂ
while 2a remains amorphous. Therefore, during the second
heating cycle 2a reveals two cold-crystallization peaks. The
variation of the thermal properties might indicate a different
influence of the side chains on the packing.
To further understand the two-dimensional self-organization
of the compounds, the adsorption of 1a and 2a on highly ordered
pyrolytic graphite (HOPG) was investigated using scanning
tunnelling microscopy (STM) in the solid–liquid interface.18 Due
to the rough surface conditions of SiO2 substrate surfaces, it was
not possible to conduct STM investigations on those surfaces;
hence an alternative substrate was used. HOPG substrates have
been widely used as surfaces for the study of molecular
Since supramolecular order is one key parameter for the
performance of organic semiconductors in devices, the organi-
zation of 1a and 2a was investigated by using two-dimensional
wide-angle X-ray scattering. The samples were prepared by
filament extrusion22 below their melting points at a temperature
of 100 ꢂC. Fig. 5a shows a characteristic pattern for 2a recorded
Fig. 4 Typical constant current STM images of self-organized mono-
layers of (a) 1a adsorbed at the n-tetradecane–HOPG interface (12 nm ꢀ
12 nm; It ¼ 75 pA; Vt ¼ 78 mV) and (b) 2a adsorbed at the n-tetradecane–
HOPG interface (12 nm ꢀ 12 nm; It ¼ 80 pA; Vt ¼ ꢁ200 mV).
Fig. 3 DSC scans for 1a and 2a at a rate of 10 ꢂC minꢁ1. The heating–
cooling directions are indicated by the arrows.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 3449–3456 | 3451

View Article Online
Fig. 5 (a) 2D-WAXS pattern of 2a at 30 ꢂC (arrow indicates the extruded direction of the sample towards the detector, the white dashed circle marks
reflections which are related to the p-stacking and the red circle corresponds to the intra-lamella spacing), (b) 1a at 30 ꢂC, and (c) schematic illustration
of the supramolecular organization of 2a. In the side view drawing, the red line corresponds the intra-lamella spacing of 0.48 nm oriented along the
alignment direction, whereas the blue line represents the p-stacking distance of 45ꢂ tilted oligomers. The parameters a and b are randomly distributed
around c which coincides with the fiber axis.
at 30 ꢂC, whereby no change of the solid state superstructure was
observed with the variation of temperature. In both cases, the 2D
patterns indicate highly ordered structures, whereby the distri-
bution of the scattering intensities implies a perpendicular
orientation of the oligomers with their molecular planes to the
fiber axis. The supramolecular organization in the extruded
filament of 1a is schematically illustrated in Fig. 5b. The first
wide-angle meridional reflections related to a spacing of 0.48 nm
are attributed to perpendicularly aligned oligomers which are
packed on top of each other due to p-stacking interactions
forming a lamella superstructure. The p-stacking distance of 0.34
nm is represented by additional off-meridional reflections which
appear at an angle of 45ꢂ towards the meridional pattern plane.
This angle is in agreement with the molecular tilting of 2a
towards the lamella axis (c axis) and to the herringbone single
crystal structure of thieno[3,2-b]thiophene based molecules
reported previously.3 An identical intra-lamella organization has
been determined for 1a. The two other axes of the unit cell, a and
b, are randomly oriented around the fiber direction leading to
reflections in the equatorial plane of the 2D pattern. For both
cases, an orthorhombic unit cell was derived from the positions
of the reflections with lattice parameters of a ¼ 2.12 nm and b ¼
0.85 nm for 1a and a ¼ 2.23 nm and b ¼ 0.92 nm for 2a. These
packing parameters cannot be compared directly to the organi-
zation of thienothiophene based polymers possessing the same
dodecyl alkyl substituents.23 In our case, the a parameter is in
agreement with the molecular length of the oligomers, while
b corresponds to the distance between lamellae.
higher the steric hindrance of the alkyl substituents, the more
pronounced the phase separation and the more improved the
macroscopic alignment in the extruded filament. Apparently, in
the case of 2a, the side chains possess slightly larger steric
hindrance due to their substitution position, softening the
material and resulting in better alignment during mechanical
shearing in comparison to 1a. The different roles of the side
chains can be clearly observed in the thermal behaviour and
crystallization behaviour as discussed above (Fig. 3). DSC
revealed that during cooling 1a crystallized at a temperature of
75 ꢂC, while the crystallization of 2a was suppressed at the same
cooling rate. This verifies the slightly larger steric influence of the
side chains for 2a. During the second heating two exothermic
peaks appeared which were assigned to a cold-crystallization of
2a. Furthermore, the melting temperature of 1a was higher (124
ꢂC) in comparison to 2a (117 ꢂC) also indicating the stronger
interactions of the 1a molecules.
To obtain further information about the supramolecular
organization of the compounds, the phases were investigated
using polarized optical microscopy (POM). Surprisingly, in both
cases well-ordered spherulites nucleated randomly over the
whole sample during the cooling of the materials from the
isotropic state (Fig. 6). The spherulites revealed high anisotropy
and therefore coherent long range order. This was expressed by
the Maltese cross, where the isogyres followed the extinction of
the analyzer/polarizer direction, indicating a radial alignment
of the assemblies from the center. Using an a-plate and
analyzing the red–blue distributions in the optical spherulite
image, it was possible to determine that the spherulites are
optically negative (Fig. 6b). The refractive index parallel to the
radial direction is smaller than that perpendicular to it. An
These lamella structures are formed due to p-stacking inter-
actions of the aromatic rods and the local phase separation
between the flexible alkyl side chains and the rigid rods. The
Fig. 6 (a) POM images of 2a between cross-polarizers at 25 ꢂC crystallized at a cooling rate of 25 ꢂC minꢁ1, (b) with an a-plate and (c) at 90 ꢂC.
3452 | J. Mater. Chem., 2009, 19, 3449–3456 This journal is ª The Royal Society of Chemistry 2009

View Article Online
identical optical behaviour of such optically negative spherulitic
domains was also observed for small molecules organized in
columnar systems.24 The analysis of these optical textures of 1a
and 2a suggests that the oligomers assemble in lamella super-
structures which in turn might be oriented macroscopically in
the spherulite growth direction with an edge-on arrangement of
the molecules. The observed domains of both oligomers excee-
ded several hundreds of micrometres, whereby the spherulite
size and morphology was dependent on the cooling rate. Upon
closer inspection of the spherulites of 2a, a macroscopic peri-
odicity along the radius was discovered (Fig. 6). The periodic
contrast, due to birefringent effects, appears on single fibrous
structures, suggesting a helical twist of crystallites within the
fibers. It is known that extinction occured in such banded
spherulites when the optical axis of crystallites coincided with
the polarization plane.25 In the case of 2a, the macroscopic
helical pitch was ca. 25 mm. However, after heating the film of
2a over the second phase transition observed in the DSC scan,
the morphology changed to a more crystalline texture and the
helical features disappeared (Fig. 6c).
Conclusion
In summary, we have successfully synthesized a series of new
organic semiconductors. The HOMO energy levels of all the
materials are in the range of ꢁ5.20 to ꢁ5.28 eV, which match well
with the work function of the gold electrodes, favoring the charge
injection of holes. This study indicates that combining
thieno[3,2-b]thiophene, thiophene and phenyl units in the
conjugated compound is a promising approach for the creation
of new stable semiconducting materials with good device
performance. The presence of long alkyl chains in such
compounds improves their solubilities as well as their molecular
ordering, thus positively influencing their semiconducting prop-
erties. The thermal analyses as well as the electrochemical
measurement data indicated better thermal and oxidation
stability of the designed materials. Furthermore, we found that
the OTFT devices based on compound 1a have a high stability in
air and exhibit excellent field-effect performances, with
a mobility as high as 3.1 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1 for a top-contact
OTFT made by spin coating under normal air and 1.4 ꢀ 10ꢁ4 cm2
Vꢁ1 sꢁ1, for a bottom-contact OTFT deposited by thermal
evaporation. Further studies of materials synthesis, device opti-
mization and performance improvement are under way.
OTFT characteristics
Experimental section
Fig. 7 shows the typical FET electrical characteristics of
a bottom-contact TFT (Fig. 7a) and a top-contact TFT (Fig. 7b)
made from 1a spin coated from toluene under air. ꢂAnnealing
took place in a vacuum oven at a temperature of 70 C for 1 h,
followed by 100 ꢂC for 20 min. It was then cooled under vacuum
overnight to ensure molecular ordering. The bottom-contact
device has a saturation charge carrier mobility of 5.0 ꢀ 10ꢁ4 cm2
Vꢁ1 sꢁ1, a subthreshold slope of 0.98 V decadeꢁ1, and an on/off
ratio of 2 ꢀ 103. Comparatively, the top-contact device outper-
forms the bottom-contact TFTs, with a higher saturation
mobility of 3.1 ꢀ 10ꢁ2 cm2 Vꢁ1 sꢁ1, a lower subthreshold slope of
0.4 V decadeꢁ1, and a higher on/off ratio of 4.5 ꢀ 104. The better
performance of top-contact organic TFT devices is typically
attributed to small charge injection areas and poor ordering of
the organic semiconductor on the electrodes in the bottom-
contact geometry.26 In comparison to the solution processed
devices, bottom-contact OTFT made from 1a deposited by
thermal evaporation shows a mobility of 1.4 ꢀ 10ꢁ4 cm2 Vꢁ1 sꢁ1
with a subthreshold slope of 1.1 V decadeꢁ1 and an on/off ratio of
2.5 ꢀ 103 (Fig. S7, ESI†).
General
Reactions and manipulations were performed using standard
Schlenk techniques under an atmosphere of nitrogen or argon.
Solvents were purified, dried, and distilled under an argon or
nitrogen atmosphere prior to use. All new compounds were
1
characterized by H and 13C NMR, MALDI-TOF mass spec-
trometry and elemental analysis.
Instruments
1H and 13C NMR spectra were collected on a Bruker DPX 400
MHz spectrometer with chemical shifts referenced to CDCl3.
Photoluminescence (PL) spectra were measured on a Perkin-
Elmer (LS50B) fluorescence spectrometer. Thermal gravimetric
analysis (TGA) was carried out using a TGA Q500 instrument
(heating rate of 10 C minꢁ1). Differential scanning calorimetry
ꢂ
(DSC) was carried out under nitrogen on a DSC Q100 instru-
ment (scanning rate of 10 ꢂC minꢁ1). Cyclic voltammetry
experiments were performed using an Autolab potentiostat
(model PGSTAT30) by Echochimie. All CV measurements were
recorded in dichloromethane with 0.1 M tetrabutylammonium
hexafluorophosphate as the supporting electrolyte (scan rate of
100 mV sꢁ1). The experiments were performed at room temper-
ature with a conventional three electrode configuration consist-
ing of a platinum wire working electrode, a gold counter
electrode, and a Ag–AgCl in 3 M KCl reference electrode. Matrix
assisted laser desorption/ionisation time-of-flight (MALDI-
TOF) mass spectra were obtained on a Bruker Autoflex TOF/
TOF instrument using dithranol as a matrix and silver tri-
fluoroacetate as an ionizing salt when necessary.
The optical textures of the compound were investigated using
a Zeiss microscope with polarizing filters equipped with a Hitachi
KP-D50 Colour digital CCD camera. The samples were
Fig. 7 Transfer characteristics of 1a for (a) a bottom-contact device and
(b) a top-contact device.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 3449–3456 | 3453

View Article Online
sandwiched between two glass slides and then thermally treated
on a Linkam hotstage regulated with a Linkam TMS 91
temperature controller.
119.90, 32.46, 31.48, 31.16, 30.69, 30.21, 30.02, 29.88, 29.14,
23.21, 14.59. GC/MS m/z: 328.10 (M); calcd for C22H32S ¼ 328
3-Dodecyl-2-(naphth-2-yl)thiophene (3b) (85% yield): 1H NMR
(CDCl3, ppm) d 8.01 (s, 1H), 7.99–7.73 (m, 4H), 7.62–7.60 (m,
2H), 7.38 (d, J ¼ 4.8 Hz, 1H), 7.16 (d, J ¼ 4.8 Hz, 1H), 2.89 (t, J
¼ 8 Hz, 2H), 1.82–1.88 (quintet, 2H), 1.44–1.39 (m, 18 H), 1.07 (t,
J ¼ 6.8 Hz, 3H). MALDI-TOF-MS (dithranol) m/z: 378.29 (M);
calcd for C26H34S ¼ 378.60
The 2D-WAXS experiments were performed by means of
a rotating anode (Rigaku 18 kW) X-ray beam with a pinhole
collimation and a 2D Siemens detector. A double graphite
monochromator for the Cu Ka radiation (l ¼ 0.154 nm) was
used.
The samples were prepared by filament extrusion using
a home-built mini-extruder. Therein, if necessary, the material is
heated up to a phase at which it becomes plastically deformable
and is extruded as 0.7 mm thin fiber by a constant-rate motion of
the piston along the cylinder.
General method for 1a and 1b
3-Dodecyl-2-phenylthiophene (3a) or 3-dodecyl-2-(naphth-2-yl)-
thiophene (3b) (10.67 mmol) was dissolved in dry THF (50 mL)
and cooled in an acetone–dry ice bath for 20 min. n-Butyllithium,
1.6 M solution in hexanes, (6.68 mL, 10.67 mmol) was then
added dropwise and the mixture was allowed to stir for 1 h. At
this point the mixture was removed from the ice bath, allowed to
warm to room temperature, and then returned to the ice bath.
Tributyltin chloride (2.89 mL, 10.67 mmol) was then added
dropwise and the mixture was allowed to warm to room
temperature and stir for 1 h. The resulting tributyl(4-dodecyl-5-
phenyl-thiophen-2-yl)stannane (3a) or tributyl(4-dodecyl-5-
naphth-2-yl-thiophen-2-yl)stannane (3b) solution was added by
cannula under nitrogen to a solution of PdCl2(PPh3)2 (0.19 g,
0.267 mmol) and 2,5-dibromothieno[3,2-b]thiophene (4) (1.59 g,
5.34 mmol) in THF (20 mL). The mixture was heated to reflux
under argon for 20 h. The solution was then allowed to cool,
quenched with water, and extracted with dichloromethane. The
organic layer was separated and dried over MgSO4. The solvent
was removed via rotary evaporation and the crude product was
purified by column chromatography over silica gel and recrys-
tallized from dichloromethane–ethanol.
Materials
NiCl2(dppp), 1-bromododecane, bromobenzene, tri-n-butyltin
chloride, 3-bromothiophene, tert-butyllithium, lithium diiso-
propylamide (LDA), tetrakis(triphenylphosphine)palladium,
PdCl2(PPh3)2, quinoline, 2-sulfonyl acetate, N-formylpiperidine,
copper powder and N-bromosuccinimide (NBS), phenyl boronic
acid and 2-naphthyl boronic acid, were all purchased from
Aldrich. All reagents purchased commercially were used without
further purification except for tetrahydrofuran (THF), which
was dried over sodium–benzophenone.
Synthesis
3-dodecylthiophene (1), tributyl(3-dodecylthiophen-2-yl)stan-
nane (2), and 2,5-dibromothieno[3,2-b]thiophene (4) were
prepared according to the procedures reported in the
literature.27,28
2,5-Bis(4-dodecyl-5-phenylthiophen-2-yl)thieno[3,2-b]thiophene
1
(1a) (3 g, 71% yield): H NMR (CDCl3, ppm) d 7.61 (d, J ¼ 7.6
General method for the synthesis of 3
Hz, 4H), 7.39 (t, J ¼ 7.6 Hz, 4H), 7.28(t, J ¼ 7.2 Hz, 2H), 7.19 (s,
2H), 6.98 (s, 2H), 2.82 (t, J ¼ 7.6 Hz, 4H), 1.73–1.67 (m, 4H),
1.42–1.26 (overlapped peaks 36H), 0.87 (t, J ¼ 7.2 Hz, 6H). 13C
NMR (CDCl3, ppm): d 142.86, 141.37, 139.43, 137.97, 134.26,
130.56, 129.08, 127.79, 126.32, 125.83, 118.00, 32.10, 30.86,
29.84, 29.75, 29.66, 29.52, 22.84, 14.22. Anal. calcd for C50H64S4:
C, 75.70; H, 8.13; S, 16.17. Found: C, 75.82; H, 8.03; S, 15.94.
MALDI-TOF-MS (dithranol) m/z: 792.30 (M); calcd for
C50H64S4 ¼ 792.40
3-Dodecylthiophene (5 g, 19.92 mmol) was dissolved in dry THF
(40 mL) and cooled in an acetone–dry ice bath for 20 min. tert-
Butyllithium (1.6 M) (12.5 mL, 19.92 mmol) was then added
dropwise and the mixture was allowed to stir for 1 h. Tributyltin
chloride (5.43 mL, 19.92 mmol) was then added dropwise and the
mixture was allowed to warm to room temperature and stir for
1 h. Meanwhile, in a separate flask, PdCl2(PPh3)2 (0.8 g, 0.56
mmol) was dissolved in THF (20 mL) and the appropriate bro-
moaryl (1-bromobenzene or 2-bromonaphthalene) (19.92 mmol)
was added. The tributyl(3-dodecylthiophen-2-yl)stannane solu-
tion was transferred into the catalyst–bromoaryl solution and
refluxed for 12 h. After the reaction was cooled to room
temperature, water (250 mL) and hexane (100 mL) were added.
The organic layer was separated, and the aqueous layer was
extracted with dichloromethane. The organic layers were
combined and dried over MgSO4, filtered and concentrated
in vacuo. The crude product was purified by column chromato-
graphy on silica gel.
2,5-Bis(4-dodecyl-5-(naphth-2-yl)thiophen-2-yl)thieno[3,2-b]thio-
phene (1b) (3.2 g, 67% yield): 1H NMR (CDCl3, ppm) d 7.88–7.86
(m, 8H), 7.61–7.58 (m, 2H), 7.54–7.50 (m, 4H), 7.32 (s, 2H), 7.14
(s, 2H), 2.72 (t, J ¼ 7.6 Hz, 4H), 1.72–1.64 (m, 4H), 1.34–1.23
(overlapped peaks 36H), 0.88 (t, J ¼ 7.2 Hz, 6H). 13C NMR
(CDCl3, ppm): d 140.51, 139.45, 138.78, 137.72, 136.26, 133.86,
132.96, 132.18, 128.54, 128.44, 128.12, 127.66, 126.85, 126.61,
115.84, 32.31, 31.27, 30.01, 29.87, 29.74, 29.29, 23.05, 14.43.
Anal. calcd for C58H68S4: C, 77.97; H, 7.67; S, 14.36. Found: C,
77.81; H, 7.83; S, 14.31. MALDI-TOF-MS (dithranol) m/z:
892.31 (M); calcd for C58H68S4 ¼ 892.40
3-Dodecyl-2-phenylthiophene (3a) (90% yield): 1H NMR
(CDCl3, ppm) d 7.63 (d, J ¼ 7.6 Hz, 2H), 7.39 (t, J ¼ 7.6 Hz, 2H),
7.29 (t, J ¼ 7.6 Hz, 1H), 7.19 (d, J ¼ 4 Hz, 1H), 6.89 (d, J ¼ 4 Hz,
1H), 2.65 (t, J ¼ 7.6 Hz, 2H), 1.71–1.65 (quintet, 2H), 1.38–1.32
(m, 18H), 0.94 (t, J ¼ 7.2 Hz, 3H); 13C NMR (100 MHz, CDCl3,
ppm): d 144.65, 135.23, 129.94, 129.20, 128.80, 126.42, 125.20,
2,5-Bis(3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene 5
3-Dodecylthiophene (3 g, 11.95 mmol) was dissolved in dry
THF (40 mL) and cooled in an acetone–dry ice bath for 20 min.
3454 | J. Mater. Chem., 2009, 19, 3449–3456
This journal is ª The Royal Society of Chemistry 2009

View Article Online
tert-Butyllithium (1.6 M) (7.47 mL, 11.95 mmol) was then added
dropwise and the mixture was allowed to stir for 1 h. Tributyltin
chloride (3.24 mL, 11.95 mmol) was then added dropwise and the
mixture was allowed to warm to room temperature and stir for 1
h. Meanwhile, in a separate flask, PdCl2(PPh3)2 (0.175 g, 0.25
mmol) was dissolved in THF (20 mL) and 2,5-dibromo-
thieno[3,2-b]thiophene (4) (1.50 g, 5 mmol) was added. The
tributyl(3-dodecylthiophen-2-yl)stannane (2) was transferred
into the catalyst–2,5-dibromothieno[3,2-b]thiophene solution
and refluxed for 20 h. The solution was then allowed to cool,
quenched with water, and extracted with dichloromethane. The
organic layer was separated and dried over MgSO4. After solvent
removal the crude product was purified by column chromatog-
raphy over silica gel and recrystallized from dichloromethane–
ethanol (2.6 g, 81% yield). 1H NMR (CDCl3, ppm) d 7.20 (s, 2H),
7.04 (d, J ¼ 7.6 Hz, 2H), 6.83 (d, J ¼ 7.6 Hz, 2H), 2.59 (t, J ¼ 7.6
Hz, 2H), 1.65–1.60 (m, 2H), 1.42–1.26 (overlapped peaks 18H),
0.88 (t, J ¼ 7.2 Hz, 3H). Calcd for C38H56S4: C, 71.19; H, 8.80; S,
20.01. Anal. found: C, 70.91; H, 8.66; S, 19.80. MALDI-TOF-
MS (dithranol) m/z: 640.19 (M); calcd for C38H56S4 ¼ 640.30.
2b: 1H NMR (CDCl3, ppm) d 7.92–7.86 (m, 8H), 7.60–7.58 (d,
J ¼ 8.4 Hz, 2H), d.7.52–7.48 (m, 4H), 7.33 (s, 2H), 7.14 (s, 2H),
2.72 (t, J ¼ 7.6 Hz, 4H), 1.67–1.60 (m, 4H), 1.32–1.24 (over-
lapped peaks 36H), 0.88 (t, J ¼ 7.2 Hz, 6H). 13C NMR (CDCl3,
ppm): d 140.44, 139.41, 138.84, 137.74, 136.23, 133.78, 132.97,
132.14, 128.56, 128.48, 128.39, 128.10, 127.67, 126.87, 126.63,
115.84, 32.32, 31.28, 30.04, 29.87, 29.75, 29.31, 23.01, 14.50.
Calcd for C58H68S4: C, 77.97; H, 7.67; S, 14.36. Found: C, 78.01;
H, 7.54; S, 14.43. MALDI-TOF-MS (dithranol) m/z: 892.37 (M);
calcd for C58H68S4 ¼ 892.40
Scanning tunneling microscopy (STM)
In situ STM measurements were performed at the n-tetradecane–
substrate (liquid–solid) interface at room temperature in
constant current mode using a Pico LE SPM system (Molecular
Imaging). STM tips were prepared from a Pt–Ir (80 : 20) wire by
mechanical cutting. Compounds 1a and 2a were dissolved in n-
tetradecane (n-C14H30, Aldrich, 99.99%) prior to deposition. A
droplet of approximately 5 mL of a 0.1 mg mLꢁ1 solution of the
compound was then dropped onto a freshly cleaved HOPG
substrate (Goodfellow).
2,5-Bis(5-bromo-3-dodecylthiophen-2-yl)thieno[3,2-b]thiophene 6
OFET device performance
N-Bromosuccinimide (0.28 g, 1.56 mmol) was added in portions
over 20 min to a solution of 5 (0.5 g, 0.78 mmol) in DMF (20 mL)
at 0 ꢂC. After stirring the solution for 3 h at ambient temperature,
water (50 mL) was added, and the aqueous layer was extracted
with ethyl acetate (50 mL). The organic layer was dried (anhy-
drous Na2SO4) and the solvent was removed in vacuo. The
residue was purified by chromatography on silica gel eluting with
Field-effect transistors were made both in bottom-contact
(semiconductor deposited above the drain and source electrodes)
and top-contact (drain and source deposited above the semi-
conductor) device geometries by solution deposition. A heavily
doped Si wafer was used as a substrate and gate electrode with
100 nm thermally grown SiO2 serving as a gate dielectric. Before
thin film deposition, the Si wafer was cleaned by a piranha
solution followed by SC1. For the bottom-contact structure, the
gold layer (source and drain) with a thickness of 100 nm was
sputter-deposited and patterned by photolithography and lift-off
to define the source and drain electrodes. For the top-contact
structure, the gold electrodes (source and drain) were thermally
evaporated and defined by using a shadow mask with a film
thickness of 40 nm.
1
hexanes to give 6 (0.56 g, 90% yield). H NMR (CDCl3, ppm)
d 7.20 (s, 2H), 6.89 (s, 2H), 2.54 (t, J ¼ 7.6 Hz, 4H), 1.62–1.55 (m,
4H), 1.33–1.26 (overlapped peaks 36H), 0.88 (t, J ¼ 7.2 Hz, 6H).
MALDI-TOF-MS (dithranol) m/z: 798.29 (M); calcd for
C38H54S4Br2 ¼ 798.90
General method for 2a and 2b
To a deoxygenated solution of aryl boronic acid 7 [phenyl
boronic acid (0.19 g, 1.57 mmol) (7a) or 2-naphthyl boronic acid
(0.27 g, 1.57 mmol (7b)] and Na2CO3 (20 mL, 2 M), a deoxy-
genated solution 6 (0.5 g, 0.63 mmol) in toluene (60 mL) and
tetrakis(triphenylphosphine palladium(0)) (0.036 g, 0.031 mmol)
was added. The reaction mixture was refluxed for 24 h under an
argon atmosphere. The cooled solution was poured into water,
extracted with dichloromethane and dried over magnesium
sulfate. The final solution was purified by column chromatog-
raphy (silica gel, hexane) and recrystallized from dichloro-
methane–ethanol to yield 2a (0.38 g, 77%) as yellow crystals and
2b (0.41, 73%) as orange crystals.
All transistors were characterized under a N2 environment.
From the electrical transfer characteristics (Id–Vg), the parame-
ters such as carrier mobility, threshold voltage, current on/off
ratio, and subthreshold swing were extracted. The carrier
mobility was calculated from the saturation regime at a drain–
source voltage of ꢁ30 V and a gate–source voltage of ꢁ30 V. In
order to minimize the leakage current, every device was isolated
by scratching a trench around the active device area with a probe
tip to remove the organic semiconductor from the trench.26
Solution processed OFET
2a: 1H NMR (CDCl3, ppm) d 7.62 (d, J ¼ 8 Hz, 4H), 7.39 (t, J
¼ 7.6 Hz, 4H), 7.28 (t, J ¼ 7.2 Hz, 2H), 7.18 (s, 2H), 7.01 (s, 2H),
2.64 (t, J ¼ 7.6 Hz, 4H), 1.67–1.62 (m, 4H), 1.34–1.25 (over-
lapped peaks 36H), 0.88 (t, J ¼ 7.2 Hz, 6H). 13C NMR (CDCl3,
ppm): d 139.86, 139.20, 138.60, 137.58, 135.73, 134.50, 129.41,
128.73, 127.69, 126.54, 115.58, 32.09, 31.01, 30.54, 29.80, 29.72,
29.62, 28.97, 22.84, 14.23. Anal. calcd for C50H64S4: C, 75.70; H,
8.13; S, 16.17. Found: C, 75.64; H, 8.27; S, 16.06. MALDI-TOF-
MS (dithranol) m/z: 792.24 (M); calcd for C50H64S4 ¼ 792.40
All solution processed devices using bottom-contact geometry, in
general, showed lower charge carrier mobility than those using
the top-contact geometry. Other researchers also reported this
phenomenon for other semiconductive materials; this could be
attributed to the small charge injection areas and poor ordering
of the organic semiconductor on the electrodes in the bottom-
contact geometry.29
The bottom-contact devices have a channel length of 11120 mm
and a channel width of 30 mm. The smoothest and most
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 3449–3456 | 3455

View Article Online
M. L. Chabinyc, R. J. Kline, M. D. McGehee and M. F. Toney,
Nat. Mater., 2006, 5, 328.
continuous film was obtained by spin coating from toluene
solutions. Other solvents like THF, chloroform, chlorobenzene
and dichlorobenzene resulted in poor-connectivity films
observed by optical microscope. Various concentrations ranging
from 0.05 wt% to 0.5 wt% were also tested since it was reported
that generally the solubility and solution concentration is crucial
to the quality of the films.30 The uniform and good-connectivity
film was only obtained from the toluene solution with the highest
concentration of 0.5 wt%. Lower concentrations resulted in
uneven films, distinguishable by eye, or islands of crystalline
materials by optical microscope. Different annealing tempera-
tures from 100 ꢂC to 180 ꢂC, annealing times from 15 to 40 min
and annealing atmospheres, both under vacuum and N2, were
also studied. It was found that higher annealing temperatures
(Tanneal > 120 ꢂC) resulted in poor device performance and there
was no obvious enhancement in device performance through
longer annealing time. Vacuum oven annealing and cooling
down overnight resulted in a better performance, since the
cooling rate played an important role in improving the film
quality. Studies showed that slower cooling rates gave a better
molecular ordering, and hence higher device performance.31
13 (a) D. R. Rutherford, J. K. Stille, C. M. Elliott and V. R. Reichert,
Macromolecules, 1992, 25, 2294; (b) C. Taliani, R. Zamboni,
R. Danieli, P. Ostoja, W. Porzio, R. Lazzaroni and J. L. Bredas,
Phys. Scr., 1989, 40, 781; (c) R. Danieli, C. Taliani, R. Zamboni,
G. Giro, M. Biserni, M. Mastragostino and A. Testoni, Synth.
Met., 1986, 13, 325.
14 (a) H. Tian, J. Shi, D. Yan, L. Wang, Y. Geng and F. Wang, Adv.
Mater., 2006, 18, 2149; (b) S. Mohapatra, B. T. Holmes,
C. R. Newman, C. F. Prendergast, C. D. Frisbie and M. D. Ward,
Adv. Funct. Mater., 2004, 14, 605; (c) M. Mushrush, A. Facchetti,
M. Lefenfeld, H. E. Katz and T. J. Marks, J. Am. Chem. Soc.,
2003, 125, 9414.
15 K. R. Radke, K. Ogawa and S. C. Rasmussen, Org. Lett., 2005, 7,
5253.
16 Y. Wu, Y. Li, S. Gardner and B. S. Ong, J. Am. Chem. Soc., 2005, 127,
614.
17 Y. Sun, Y. Ma, Y. Liu, Y. Lin, Z. Wang, Y. Wang, C. Di, K. Xiao,
X. Chen, W. Qiu, B. Zhang, G. Yu, W. Hu and D. Zhu, Adv.
Funct. Mater., 2006, 16, 426.
18 (a) N. Katsonis, A. Marchenko and D. Fichou, J. Am. Chem. Soc.,
2003, 125, 13682; (b) L. Piot, J. Wu, A. Marchenko, K. Mullen and
D. Fichou, J. Am. Chem. Soc., 2005, 127, 16245; (c) A. Nion,
P. Jiang, A. Popoff and D. Fichou, J. Am. Chem. Soc., 2007,
129, 2450; (d) A. Popoff and D. Fichou, Colloids Surf., B, 2008,
63, 153.
19 (a) P. Keg, A. Lohani, D. Fichou, Y. M. Lam, Y. Wu, B. S. Ong and
S. G. Mhaisalkar, Macromol. Rapid Commun., 2008, 29, 1197; (b)
A. Dell’Aquila, F. Marinelli, J. Tey, P. Keg, Y. M. Lam,
O. L. Kapitanchuk, P. Mastrorilli, C. F. Nobile, A. Marchenko,
D. Fichou, S. G. Mhaisalkar, G. P. Suranna and L. Torsi, J.
Mater. Chem., 2008, 18, 786; (c) E. Mena-Osteritz, Adv. Mater.,
2002, 14, 609.
Acknowledgements
This work was supported by an A*STAR grant (Research Grant
No. 052 117 0032).
20 (a) J. P. Rabe and S. Buchholz, Science, 1991, 253, 424; (b) D. M. Cyr,
B. Venkataraman, G. W. Flynn, A. Black and G. M. Whitesides, J.
Phys. Chem., 1996, 100, 13747.
References
1 (a) K. Takimiya, H. Ebata, K. Sakamoto, T. Izawa, T. Otsubo and
Y. Kunugi, J. Am. Chem. Soc., 2006, 128, 12604; (b) G. Horowitz
and P. Delannoy, in Handbook of Oligo- and Polythiophenes, ed. D.
Fichou, Wiley-VCH, Weinheim, Germany, 1999; (c) Y. Li, Y. Wu,
P. Liu, M. Birau, H. Pan and B. S. Ong, Adv. Mater., 2006, 18,
3029; (d) M. Mushrush, A. Facchetti, M. Lefenfeld, H. E. Katz and
T. J. Marks, J. Am. Chem. Soc., 2003, 125, 9414.
2 (a) X. Zhang and A. J. Matzger, J. Org. Chem., 2003, 68, 9813; (b)
Z. Bao, Adv. Mater., 2000, 12, 227; (c) G. Barbarella, L. Favaretto,
G. Sotgiu, M. Zambianchi, V. Fattori, M. Cocchi, F. Cacialli,
G. Gigli and R. Cingolani, Adv. Mater., 1999, 11, 1375; (d)
21 (a) M. M. S. Abdel-Mottaleb, G. Gotz, P. Kilickiran, P. Bauerle and
E. Mena-Osteritz, Langmuir, 2006, 22, 1443; (b) R. Azumi, G. Gotz,
T. Debaerdemaeker and P. Bauerle, Chem.–Eur. J., 2000, 6, 735; (c)
R. Azumi, G. Gotz and P. Bauerle, Synth. Met., 1999, 101, 569; (d)
T. Kirschbaum, R. Azumi, E. Mena-Osteritz and P. Bauerle, New
J. Chem., 1999, 241; (e) R. Stecher, B. Gompf, J. S. R. Munter and
F. Effenberger, Adv. Mater., 1999, 11, 927; (f) A. Stabel and
J. P. Rabe, Synth. Met., 1994, 67, 47; (g) P. Bauerle, T. Fischer,
B. Bidlingmeier, A. Stabel and J. P. Rabe, Angew. Chem., Int. Ed.,
1995, 34, 303.
€
U. Mitschke and P. Bauerle, J. Chem. Soc., Perkin Trans. 1, 2001,
22 W. Pisula, Z. Tomovic, C. Simpson, M. Kastler, T. Pakula and
€
740; (e) V. Dediu, M. Murgia, F. C. Matacotta, C. Taliani and
S. Barbanera, Solid State Commun., 2002, 122, 181; (f) K. Ogawa
and S. C. Rasmussen, J. Org. Chem., 2003, 68, 2921.
3 C. Videlot-Ackermann, J. Ackermann, H. Brisset, K. Kawamura,
N. Yoshimoto, P. Raynal, A. El Kassmi and F. Fages, J. Am.
Chem. Soc., 2005, 127, 16346.
K. Mullen, Chem. Mater., 2005, 17, 4296.
23 M. L. Chabinyc, M. F. Toney, R. J. Kline, I. McCulloch and
M. Heeney, J. Am. Chem. Soc., 2007, 129, 3226.
24 (a) W. Pisula, M. Kastler, D. Wasserfallen, T. Pakula and K. Mullen,
€
J. Am. Chem. Soc., 2004, 126, 8074; (b) W. Pisula, F. Dierschke and
€
K. Mullen, J. Mater. Chem., 2006, 16, 4058; (c) W. Pisula,
4 H. Sirringhaus, N. Tessler, D. S. Thomas, P. J. Brown and
R. H. Friend, Adv. Solid State Phys., 1999, 39, 101, and in the
Merck patent, EP 1 279 689 A2, 2003.
5 J. Nakayama, H. B. Dong, K. Sawada, A. Ishii and S. Kumakura,
Tetrahedron, 1996, 52, 471.
Z. Tomovic, M. Wegner, R. Graf, M. J. Pouderoijen, E. W. Meijer
and A. P. H. J. Schenning, J Mater. Chem., 2008, 18, 2968; (d)
H. Shen, K.-U. Jeong, H. Xiong, M. J. Graham, S. Leng,
J. X. Zheng, H. Huang, M. Guo, F. W. Harris and S. Z. D. Cheng,
Soft Matter, 2006, 2, 232.
6 (a) X. Zhang, M. Kohler and A. J. Matzger, Macromolecules, 2004,
37, 6306; (b) X. Zhang, J. P. Johnson, J. W. Kampf and
A. J. Matzger, Chem. Mater., 2006, 18, 3470.
25 J. L. Hutter and J. Bechhoefer, J. Cryst. Growth., 2000, 217, 332.
26 H. Meng, J. Zheng, A. J. Lovinger, B. C. Wang, P. G. Van Patten and
Z. Bao, Chem. Mater., 2003, 15, 1778.
7 W. H. Tang, L. Ke, L. W. Tan, T. T. Lin, T. Kietzke and Z.-K. Chen,
Macromolecules, 2007, 40, 6164.
27 P. Bauerle, F. Pfau, H. Schlupp, F. Wurthner, K.-U. Gaudl,
M. B. Carot and P. Fischert, J. Chem. Soc., Perkin Trans. 2, 1993, 489.
28 L. S. Fuller, B. Iddon and K. A. Smith, J. Chem. Soc., Perkin Trans. 1,
1997, 3465–3470.
29 S. A. Ponomarenko, S. Kirchmeyer and A. Elschner, Chem. Mater.,
2006, 18, 579–586.
30 C. Reese, M. Roberts, M. Ling and Z. Bao, Mater. Today, 2004, 7,
20–27.
31 N. Zhao, G. A. Botton, S. Zhu, A. Duft, B. S. Ong, Y. Wu and P. Liu,
Macromolecules, 2004, 37, 8307.
8 U. Tokiyoshi and T. Shizuo, J. Appl. Phys., 2007, 101, 054517.
9 E. Lim, B. J. Jung and H. K. Shim, Macromolecules, 2003, 36, 4288.
10 H.-S. Kim, Y.-H. Kim, T.-H. Kim, Y.-Y. Noh, S. Pyo, M. H. Yi,
D.-Y. Kim and S.-K. Kwon, Chem. Mater., 2007, 19, 3561.
11 Y.-Y. Noh, R. Azumi, M. Goto, B.-J. Jung, E. H. Lim, H.-K. Shim,
Y. Yoshida, K. Yase and D.-Y. Kim, Chem. Mater., 2005, 17, 3861.
12 I. McCulloch, M. Heeney, C. Bailey, K. Genevicius, I. MacDonald,
M. Shkunov, D. Sparrowe, S. Tierney, R. Wagner, W. Zhang,
3456 | J. Mater. Chem., 2009, 19, 3449–3456
This journal is ª The Royal Society of Chemistry 2009