Synthesis and characterization of graft polymethacrylates containing conducting diphenyldithiophene for organic thin-film transistors

Article abstract of DOI:10.1002/pola.24957

A class of [5,5′]-diphenyl-[5,5′]-dithiophene (PTTP)-modified methacrylates has been synthesized and free radically polymerized to form graft polymethacrylates with the conducting PTTP segments as pendant side chains. Both the terminal alkyl side chain an

Full text of DOI:10.1002/pola.24957

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Synthesis and Characterization of Graft Polymethacrylates Containing
Conducting Diphenyldithiophene for Organic Thin-Film Transistors
Zhaokang Hu,1 Boyi Fu,1 Avishek Aiyar,1 Elsa Reichmanis1,2,3
1School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Dr. NW, Atlanta, Georgia 30332
2School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
3School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Correspondence to: E. Reichmanis (E-mail: ereichmanis@chbe.gatech.edu)
Received 5 July 2011; accepted 12 August 2011; published online 9 September 2011
DOI: 10.1002/pola.24957
ABSTRACT: A class of [5,50]-diphenyl-[5,50]-dithiophene (PTTP)-
modified methacrylates has been synthesized and free radically
polymerized to form graft polymethacrylates with the conduct-
ing PTTP segments as pendant side chains. Both the terminal
alkyl side chain and spacer between the PTTP segments and
polymer backbone could be varied to study fundamental struc-
ture–property relationships for this class of materials. Specifi-
cally, a group of three different PTTP graft polymethacrylates
has been successfully synthesized with the alkyl side chain
varying from hexyl to dodecyl. For the dodecyl-terminated
poly(4-(50-(4-dodecylphenyl)-[2,20-bithiophen]-5-yl)phenethyl
methacrylate), p(DPTTPEM), a counterpart, poly(4-(50-(4-dode-
group, was synthesized. The experimental results indicated
that both the alkyl side chain and spacer significantly affected
the reactivity of the PTTP-modified methacrylates during free
radical polymerization as well as the physical properties of the
resultant graft polymers including solubility, morphology, and
electrochemical and electrical properties. Typical field-effect
mobilities on the order of 10ꢀ5 cm2 Vꢀ1 sꢀ1 were observed for
all the PTTP monomers in air, which was attributed to their
crystalline phase as revealed by differential scanning calorime-
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V
try and X-ray diffraction studies. 2011 Wiley Periodicals, Inc.
J Polym Sci Part A: Polym Chem 50: 199–206, 2012
cylphenyl)-[2,20-bithiophen]-5-yl)phenbutyl
methacrylate),
KEYWORDS: charge transport; free radical polymerization; semi-
p(DPTTPBM), where the ethyl spacer was replaced by a butyl
conducting graft polymer
INTRODUCTION Organic semiconductors used as the active
layer in electronic devices have attracted considerable atten-
tion for a wide range of applications including organic light-
emitting diodes, organic field-effect transistors, and photovol-
taic devices.1–5 Realization of desirable molecular architec-
tures for organic semiconductors with well-defined dimen-
sions and functions is one of the current challenges for
research in the field of organic semiconducting materials.6 It is
critically important to precisely build molecular architectures
to afford necessary performance attributes such as good solu-
bility, long-term thermal and oxidative stability, and enhanced
p-stacking for superior electrical conductivity.7,8 Through the
techniques of advanced organic synthesis which uses
approaches such as the Suzuki reaction,9 the Stille coupling,10
the Wittig reaction,11 and others,12 a variety of complex new
building blocks including fluorene,13–15 carbazole,16,17 isothia-
naphthene,18,19 cyclopentadithiophene,20–22 benzothiadiazole,23
along with many others24–29 have been successfully developed
and introduced into polymer backbones.
combination of mobilities and Ion/Ioff ratios owing both to its
molecular geometry and energy levels.30–32 For example, both
p-channel and n-channel mobilities higher than 0.1 cm2 Vꢀ1
sꢀ1 have been recently reported for disubstituted PTTPs.33,34
However, all of these studies have exclusively focused on
PTTP small molecules. Good reproducibility of device per-
formance remains a challenge for these solution-processed
materials.32 Nonuniform films were common when organic
solvents such as benzene, toluene, chloroform, and 1,2,4-tri-
chlorobenzne (TCB) were used for semiconductor processing.
The occasional smooth films obtained from casting with these
solvents were not easily reproducible.32 Given the advantages
of polymers versus small molecules, particularly in terms of
uniform film formation, mechanical flexibility, and excellent
rheology properties,35,36 it is of practical significance to incor-
porate PTTP conducting blocks into polymeric systems to
facilitate creation of uniform thin films with variations in de-
vice performance. To the best of our knowledge, PTTP-based
semiconducting polymers for TFT applications are yet to be
reported. It is anticipated that such materials would exhibit
improved film formation upon solution processing in compari-
son to their small molecule PTTP counterparts.
Among these new structures, the [5,50]-diphenyl-[2,20]-bithio-
phene (PTTP) co-oligomer family has emerged as an attractive
candidate for thin-film transistors (TFTs) affording a good
Additional Supporting Information may be found in the online version of this article.
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SCHEME 1 Synthetic route of PTTP graft polymers.
Recently, we have successfully developed a strategy to syn-
thesize electroactive graft polymers via incorporation of con-
jugated oligothiophenes onto conventional polystyrene back-
bones as pendant side chains.37 In contrast to main-chain
conjugated polymers, graft polymers are regarded as attrac-
tive because of the ease in tuning mechanical and rheological
properties, by varying the polymer main-chain structures
while retaining electrical properties that are inherited pri-
marily from pendant semiconductor cores. Previously, graft
poly(methyl methacrylate)s with alkyl sexithiophene as
pendant side chains have been synthesized via anodic poly-
merization by Shirota and coworkers.38,39 However, no FET
performance has been reported in those studies. Inspired by
the concept of graft polymer-based materials, a class of poly-
methacrylate derivatives containing conjugated PTTP cores
as pendant side chains has been synthesized via direct free
radical polymerization of PTTP-modified methacrylic mono-
mers. The effect of fundamental changes in chemical struc-
ture of the PTTP graft polymers (e.g., alkyl side chain and al-
iphatic spacer) on the physical properties of the resultant
polymers has been systematically studied by synthesizing
the PTTP graft polymers having the alkyl chain length vary-
ing from hexyl to dodecyl (C6, C8, and C12). Pertinent proper-
ties that were evaluated included morphology, solubility and
electrical characteristics. Particularly for the dodecyl-termi-
nated PTTP graft polymer systems, the effect of the spacer
on the polymer’s flexibility and solubility has also been stud-
ied by substitution of the ethyl group between the methacry-
late backbone and the PTTP segments with a butyl unit.
TFTs were fabricated from both the PTTP monomers and
graft polymers by spin-coating solutions of each material in
chloroform onto the device substrate to characterize their
field-effect performance.
RESULTS AND DISCUSSION
Synthesis of PTTP Monomers and Graft Polymers
Unlike the symmetric disubstituted PTTP and derivatives
previously synthesized and tested for their field-effect tran-
sistor performance, it is more challenging to synthesize
asymmetric small molecule analogs via multiple step reac-
tions. As outlined in Scheme 1, a long alkyl chain was
attached to one end of the PTTP segment to enhance overall
solubility of the desired product, using a 4-bromo-alkylben-
zene as the starting material. The final PTTP methacrylic
monomers were synthesized via the Stille coupling reaction
between bromo-alkylphenyl bithiophene (3a, 3b, or 3c) and
4-tributyltin-phenalkyl methacrylate (5a or 5b). By varying
the sizes of the alkyl side chain and spacer, a total of four
different PTTP monomers were successfully synthesized
including
4-(50-(4-hexylphenyl)-[2,20-bithiophen]-5-yl)phe-
nethyl methacrylate (HPTTPEM), 4-(50-(4-octylphenyl)-[2,20-
bithiophen]-5-yl)phenethyl methacrylate (OPTTPEM), 4-(50-
(4-dodecylphenyl)-[2,20-bithiophen]-5-yl)phenethyl methacry-
late (DPTTPEM), and 4-(50-(4-dodecylphenyl)-[2,20-bithio-
phen]-5-yl)phenbutyl methacrylate (DPTTPBM), as shown in
Scheme 1. These reactive monomers were further free
radically polymerized to form four different PTTP graft
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FIGURE 1 1H NMR spectra of (a) DPTTPBM and (b) p(DPTTPBM).
polymethacrylates accordingly. The 1H spectra of DPTTPBM
and poly(4-(50-(4-dodecylphenyl)-[2,20-bithiophen]-5-yl)phen-
butyl methacrylate), p(DPTTPBM), shown in Figure 1, are
representative of the series of materials prepared here.
Detailed NMR information for all other PTTP compounds can
be found in Supporting Information.
p(DPTTPEM) shared the same degree of polymerization (n,
23). Alternatively, introduction of the butyl spacer was seen
to decrease the reactivity of DPTTPBM, whose degree of po-
lymerization was found to be only 12. Compared to
DPTTPEM, the analog that possessed the same dodecyl side
chain, the butyl spacer in DPTTPBM was believed to increase
steric hindrance which in turn lowered the reactivity of the
monomer. The significantly lower molecular weight of
p(DPTTPBM) could also explain its much improved solubility
compared to the other PTTP graft polymers examined here.
All the aforementioned PTTP monomers had good solubility
in common organic solvents such as dichloromethane, chlo-
roform, and tetrahydrofuran. After free radical polymeriza-
tion, the solubility of the resultant PTTP graft polymers was
significantly affected by the length of both the alkyl side
chain and spacer. Specifically, the yellow p(HPTTPEM) pow-
der displayed very limited solubility in many common or-
ganic solvents at room temperature. However, poly(4-(50-(4-
All PTTP graft polymers demonstrated good thermal stability
with decomposition temperatures (5 wt % loss) higher than
350 ꢁC, as determined by thermogravimetric analysis (Fig.
1S in Supporting Information). Crystallization and melting
features of the PTTP monomers and graft polymers were
elucidated through differential scanning calorimetry (DSC). A
typical DSC curve of a monomer displayed two distinct tran-
sitions in either the heating or cooling process (DPTTPEM
was an exception, where the endothermic transition at low
temperature during cooling was undetectable). As shown in
Figure 2(a), transitions for HPTTPEM were clearly observed
at 120, 225 ꢁC during heating and 62, 220 ꢁC during the
cooling cycle. The DSC curves of these PTTP small molecules
appeared to be very similar to that of a dihexyl PTTP semi-
conductor previously reported by Marks and coworkers,32
dodecylphenyl)-[2,20-bithiophen]-5-yl)phenethyl
methacry-
late), p(DPTTPEM), could dissolve in hot TCB and chloroben-
zene and was partially soluble in chloroform. The solubility
was further enhanced for p(DPTTPBM) where the ethyl
spacer was replaced with a butyl unit. As deduced from GPC
evaluation as seen Table 1, the length of the alkyl side chain
had no significant effect on the reactivity of the PTTP metha-
crylic monomers. Both poly(4-(50-(4-octylphenyl)-[2,20-bithio-
phen]-5-yl) phenethyl methacrylate), p(OPTTPEM), and
TABLE 1 Molecular Weight, Polydispersity Index (PDI), Degree
of Polymerization (DP), and Thermal Stability of PTTP Graft
Polymers
ꢁ
where the exothermic transition at 120 C was attributed to
an SA/nematic liquid crystalline phase transition. In this
study, the liquid crystalline phase was not visually observed
by temperature-dependent polarized optical microscopy
(POM). Rather, crystallites with a domain size in the micro-
meter range were consistently observed for a sublimed
HPTTPEM film by both POM and atomic force microscopy
(Fig. 2S in Supporting Information), indicative of the strong
dependence of the formation of a liquid crystalline phase on
parameters such as chemical structure, temperature, and
Mn
(kg molꢀ1
Mw
(kg molꢀ1
Td
(ꢁC)
)
)
PDI
DP
p(HPTTPEM)
p(OPTTPEM)
p(DPTTPEM)
p(DPTTPBM)
–
–
–
–
359
355
378
405
12,500
13,500
7,600
36,200
34,500
26,500
2.9
2.5
3.4
23
23
12
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FIGURE 2 DSC curves of (a) PTTP monomers and (b) graft polymers. The figure displays the results for the first heating cycle and
the y-axis was shifted accordingly.
substrate surface. The second thermal peak observed at ꢂ225
ꢁC during heating was related to the crystallization–melting of
HPTTPEM. In the cooling cycle for this compound, a shoulder
on the high temperature side could be discerned, suggesting
that melting and crystallization occurred in two steps, possibly
via an intermediate-layered mesophase of smectic symme-
try.40 As the alkyl chain length increased, both thermal transi-
tions gradually moved to lower temperatures.ꢁThe transitions
for DPTTPEM ꢁwere observed at 114 and 214 C, compared to
120 and 225 C for HPTTPEM upon heating. Meanwhile, the
spacer was also found to have impact on the thermal transi-
tion temperatures. Replacing the ethyl linker in DPTTPEM
with a butyl unit, as in DPTTBM, afforded a decrease in the
thermal transition temperatures by ꢂ20 ꢁC.
The morphology of the PTTP compounds was further studied
by micro-X-ray diffraction. All monomers showed strong
reflection signals up to multiple orders, consistent with the
crystalline nature of these compounds revealed by both DSC
and POM (Fig. 2S in Supporting Information). As shown in
Table 2, the first order d-spacing varied from about 28 Å in
HPTTPEM to about 35 Å as the alkyl side chain length
increased. These values are consistent with those previously
reported for other, analogous PTTP derivatives.32,34 Upon
replacing the ethyl spacer with a butyl unit, the d-spacing
decreased slightly from about 35 Å in DPTTPEM to about 31
Å in DPTTPBM. As revealed by DSC, it is speculated that the
enhanced flexibility of DPTTPBM induced by the longer butyl
spacer allowed for folding of the alkyl moieties as well as
possible interdigitated alkyl chain packing, which eventually
resulted in the decreased interlayer d-spacing. It is notewor-
thy that the alkyl side chain also played a significant role in
the formation of the crystalline phase. As shown in Figure 3,
besides the first-order reflection, two sets of the high-order
reflection peaks were observed specifically for DPTTEM, sug-
gesting the formation of a mixture of at least two different
crystalline phases; the NMR spectra demonstrated the high
As shown in Figure 2(b), the thermal behavior of the asso-
ciated PTTP graft polymers was also investigated by DSC.
Similar to those of the monomers, the lengths of both the
alkyl side chain and spacer clearly influenced the melting
temperatures of the polymers. Polymerization of the PTTP
monomers was thought to immobilize both the aliphatic
moieties and aromatic PTTP cores by reducing the degree
of segment relaxation, which was supported by the increase
of about 20 ꢁC in the melting temperature for each PTTP
graft polymer compared to its allied monomer. Crystalline
phase transitions at low temperature, similar to those
found for the monomers, were not found for the PTTP graft
polymers; a result which is consistent with a previous
report and thought to result from the lack of ordered align-
ment of the PTTP segments along a given direction.41 Addi-
tionally, with the enhanced flexibility of the PTTP pendant
side chains attributed to the longer aliphatic segments, a
glass transition temperature became detectable for
TABLE 2 X-Ray Diffraction h–2h Data for Sublimed PTTP Films
Sublimed Film at 25 ꢁC
Other Observable
˚
Compound
d (A)
Reflections (Order)
HPTTPEM
OPTTPEM
DPTTPEM
DPTTPBM
27.8
31.8
35.0
31.3
3, 4
3, 4
3, 4, 5, 6 (30, 40, 50, 60)
ꢁ
ꢁ
p(DPTTPEM) at 181 C and further decreased to 169 C for
p(DPTTPBM).
3, 4
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Interchain interactions of polymer chains were further stud-
ied using UV–vis spectroscopy. As shown in Table 3, the
changes in chemical structure, that is, alkyl side chain and
spacer, had little effect on the absorbance spectra of the
monomers, which repeatedly showed an absorbance maxi-
mum at 380 nm. This result is not unexpected because the
alkyl side chains have only limited contribution to the PTTP-
conjugated system and therefore are not expected to affect
the energy levels of the units in solution. A blue shift was
previously observed for an oligofluorene graft polystyrene
compared to its monomer due to the twisted confirmation of
polymer backbone upon polymerization.44 Twisting of poly-
mer backbone was thought to be negligible for these PTTP
graft polymer systems because no significant blue shift was
observed between each monomer/polymer pair in solution,
with both maxima at ꢂ380 nm, whereas a significant blue
shift was observed upon comparison of the absorbance spec-
tra of either monomer or polymer in solution to those in the
solid state. As seen in Figure 4, the absorbance maximum
shifted from 385 nm in solution to 350 nm in thin-film state
for p(DPTTPBM), a result which is significantly different
from the normally observed red shift for most conjugated
polymer solid films.45 A similar 50-nm blue shift was
observed for DPTTPBM. According to Okamoto and co-
workers, two distinct modes of aggregation of molecules
could occur: J- and H-aggregation.43 These different forms
result in distinct photoelectronic properties. Specifically, J-
aggregation, which occurs by an end-to-end arrangement of
p-molecules, displays red-shifted J-bands, whereas a face-to-
face assembly affords blue-shifted H-bands. The blue shift
observed for the PTTP materials is thought to be related
with H-aggregation, that is, the formation of face-to-face
stacking between the pendant PTTP segments.34,46 This ex-
planation is realistic, specifically for the graft polymers
under investigation, because the end-to-end aggregation of
the PTTP pendant side chains is expected to be severely
impaired by the shielding of the alkyl terminal groups
attached to the PTTP cores, whereas face-to-face aggregation
between neighboring PTTP segments is highly possible.37
FIGURE 3 Micro-X-ray diffraction y–2y scans of sublimed
DPTTPEM film and p(DPTTPEM) powder at room temperature.
purity of this compound (>99%) after column chromatogra-
phy. No second set of crystalline structures was observed for the
other PTTP compounds explored (Fig. 3S in Supporting Informa-
tion). The three major reflection peaks at ꢂ19ꢁ, 22ꢁ, and 27ꢁ,
observed for all four PTTP monomers, were very similar to those
previously observed for an oligothiophene, indicative of the pres-
ence of a layered morphology.42 Pertaining to the related graft
polymers, major reflection features diminished in the small-angle
regions, indicating the formation of amorphous state for these
materials. These outcomes were well correlated with the DSC
results, in which the crystalline phase transition observed at
lower temperatures for relevant monomers disappeared for the
related polymers. However, the three major peaks in the wide-
angle regions (19ꢁ–27ꢁ), as shown in Figure 3, remained essen-
tially unchanged. According to the folded chain structure for a
PTTP block copolymer proposed by Okamoto and coworkers,43
this result may be indicative of a face-to-face alignment of the
PTTP pendant side chains along the polymer backbone.
TABLE 3 Electrochemical and Electrical Properties of PTTP Monomers and Graft Polymers
UV Absorption
CV
FET Device
Solution
(nm)
Film
(nm)a
Band Gap
(eV)b
HOMO
(eV)
LUMO
(eV)
Hole Mobility
Threshold
(cm2 Vꢀ1 sꢀ1
)
I
on/Ioff
Voltage (VT, V)
HPTTPEM
OPTTPEM
DPTTPEM
DPTTPBM
p(HPTTPEM)
p(OPTTPEM)
p(DPTTPEM)
p(DPTTPBM)
a
379
379
380
380
384
384
384
380
330*, 359
331*, 359
330*, 355
340*, 358, 429
–
2.82
2.80
2.81
2.80
2.77
2.76
2.75
2.74
5.38
5.36
5.38
5.34
5.42
5.47
5.48
5.47
b
2.56
2.56
2.57
2.54
2.65
2.71
2.73
2.73
5.4 ꢃ 10ꢀ5
2.2 ꢃ 10ꢀ5
1.3 ꢃ 10ꢀ5
9.2 ꢃ 10ꢀ6
–
8.2 ꢃ 102
ꢀ12.8
0.7
8.4 ꢃ 102
1.2 ꢃ 103
ꢀ14.9
ꢀ10.9
–
3.2 ꢃ 102
–
364*, 431
349*, 393, 423
350*, 394, 425
–
–
–
5.6 ꢃ 10ꢀ7
13
5
ꢀ1.52
ꢀ32.8
1.7 ꢃ 10ꢀ7
Absolute maximum indicated by asterisk. Uniform and reproducible
films could not obtain for p(HPTTPEM) due to its poor solubility in
p-xylene.
Optical band gaps of p(OPTTPEM), p(DPTTPEM), and p(DPTTPBM)
were identified from the film absorption, and they were identified from
the solution absorbance spectra for all the other listed materials.
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lay in the narrow range of 5.34–5.38 eV as shown in Figure 5.
The relatively deeper HOMO levels of these compounds com-
pared to oligothiophenes, normally in 4.8–5.1 eV, demonstrated
improved oxidative stability. It was thought to be a result of the
delocalization of electron density on the conjugated PTTP sys-
tem through the incorporation of phenylene units.30,32 All the
PTTP compounds studied here (monomers and polymers) could
not be reduced using the experimental conditions described;
thus, the LUMO energy levels were extracted from the difference
between the HOMO level and the optical band gap determined
from the onset of the UV–vis absorbance spectrum. As summar-
ized in Table 3, a negligible difference in the HOMO levels was
observed for all the PTTP monomers and graft polymers.
Because of the shorter band gap, the PTTP graft polymers pos-
sessed slightly higher LUMO levels than those of the monomers.
A bottom-gate, bottom-contact TFT device configuration was
utilized in the characterization of FET properties of PTTP
monomers and graft polymers. The thermal silicon oxide
layer (SiO2, ꢂ300 nm in thickness) on the silicon wafer sur-
face functioned as the gate dielectric. The p-channel semi-
conductors were deposited by spin coating from chloroform
solution. TFTs were characterized in air without taking
FIGURE 4 UV–vis absorption spectra of DPTTPBM and
p(DPTTPBM).
Consistent with the observation in the UV–vis spectra, a struc-
tural change in the alkyl side chain or spacer did not signifi-
cantly affect the HOMO energy levels of the monomers, which
FIGURE 5 Cyclic voltammograms of (a) PTTP monomers and (b) graft polymers.
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FIGURE 6 I–V characteristics of a representative TFT device fabricated from HPTTPEM in chloroform solution (3 mg mLꢀ1): (a) Out-
put curves at different gate voltages and (b) transfer curve in the saturated regime at a constant source—drain voltage of ꢀ50 V
and square root of the absolute value of the drain current as a function of gate voltage.
precautionary measures to isolate the devices from exposure
to atmospheric oxygen, moisture, or ambient light. Except for
the two graft polymers, p(HPTTPEM) and p(OPTTPEM), hav-
ing shorter alkyl side chains, which could not be solution
processed due to their relatively poor solubility in chloro-
form, all other materials exhibited characteristic p-channel
FET behavior. As the example shown in Figure 6, the output
curves of HPTTPEM displayed good saturation behavior with
no obvious contact resistance. The extracted mobilities in the
solvents among all the PTTP graft polymers examined. A
crystalline nature was observed for all the PTTP monomers
with the transition temperatures being significantly affected
by both the alkyl side chains and spacers. Longer aliphatic
chains enhanced the overall flexibility of the PTTP molecules
and therefore lowered their crystallization–melting tempera-
tures. The face-to-face alignment of the pendant PTTP side
chains along the polymer chains resulted in a distinct blue
shift in their UV–vis spectra. Although the aliphatic moieties
negligibly affected the HOMO/LUMO energy levels, longer
side chain did result in decreased p-channel mobilities for
TFT devices prepared from the materials. Compared to alter-
native well-developed organic semiconductor materials, there
is still substantial space to improve on the FET performance
characteristics through optimization of the structure of the
PTTP materials investigated here. The introduction of the
concept of a graft polymer approach could also contribute to
the development of new chemical architectures and/or build-
ing blocks for applications related to organic electronics.
saturated regime were on the order of 10ꢀ5 cm2 Vꢀ1 sꢀ1
,
which were similar to values previously reported for PTTP
derivatives without device optimization.47 It should be noted
here that the chemical structure of the PTTP small molecules
did affect their FET performance. The mobility gradually
decreased as the size of the PTTP molecule increased. This
result is consistent with previous results showing that rather
than aligning in a geometry perpendicular to the substrate,
PTTP cores having aliphatic appendages that occupy a large
volume tend to tilt toward the substrate.47 Similar to a pre-
vious report related to a dimethacrylate-functionalized PTTP
monomer,48 polymerization enhanced the thermal and oxida-
tive stability of the resultant polymers, while accordingly, the
rigid polymer backbone distorted the lamellar structures
observed in the monomer state. Also, the increase in grain
boundaries associated with polymer domains could be a
source of charge trapping. Both facts were thought to delete-
riously affect the mobility of the resultant polymers.
This research was supported by the American Chemical Society
Petroleum Research Fund (PRF 49158ND7). Parts of the work
were carried out in the X-ray Characterization Facility, Univer-
sity of Minnesota, which receives partial support from NSF
through the MRSEC program.
REFERENCES AND NOTES
1 Burroughes, J. H.; Bradley, D. D. C.; Brown, A. R.; Marks, R.
N.; Mackay, K.; Friend, R. H.; Burns, P. L.; Holmes, A. B. Nature
1990, 347, 539–541.
CONCLUSIONS
Here, we have synthesized a class of PTTP-modified metha-
crylic monomers with variable alkyl side chains and spacer
units. The monomers were further free radically polymerized
to form PTTP graft polymers. A relatively low-molecular-
weight polymer, p(DPTTPBM), was obtained from the
dodecyl-PTTP butyl methacrylate due to increased steric
hindrance induced by the bulky spacer even though this
polymer possessed the best solubility in common organic
2 Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C. W.; Ho, P. K.
H.; Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194–199.
3 Ling, M. M.; Bao, Z. Chem Mater 2004, 16, 4824–4840.
4 Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Sci-
ence 1995, 270, 1789–1791.
5 Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery,
K.; Yang, Y. Nat Mater 2005, 4, 864–868.
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6 Lehn, J.-M. Science 2002, 295, 2400–2403.
28 Ramos, A. M.; Rispens, M. T.; van Duren, J. K. J.; Humme-
len, J. C.; Janssen, R. A. J. J Am Chem Soc 2001, 123,
6714–6715.
7 Electronic Materials: The Oligomer Approach; Mullen, K.;
Wegner, G., Eds.; Wiley-VCH: New York, 1998.
¨
29 Zhan, X.; Tan, Z. a.; Domercq, B.; An, Z.; Zhang, X.; Barlow,
S.; Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J Am Chem Soc
2007, 129, 7246–7247.
8 Summary of the Fundamental Research Needs in Organic
Electronic Materials; Vardeny, Z. V.; Heeger, A. J.; Dodabala-
pur, A., Eds.; Elsevier: Amsterdam, 2005.
30 Hong, X. M.; Katz, H. E.; Lovinger, A. J.; Wang, B.-C.; Ragha-
9 Zheng, Q.; Jung, B. J.; Sun, J.; Katz, H. E. J Am Chem Soc
vachari, K. Chem Mater 2001, 13, 4686–4691.
2010, 132, 5394–5404.
31 Facchetti, A.; Letizia, J.; Yoon, M.-H.; Mushrush, M.; Katz, H.
10 Hou, J.; Chen, T. L.; Zhang, S.; Huo, L.; Sista, S.; Yang, Y.
E.; Marks, T. J. Chem Mater 2004, 16, 4715–4727.
Macromolecules 2009, 42, 9217–9219.
32 Mushrush, M.; Facchetti, A.; Lefenfeld, M.; Katz, H. E.;
11 Shahid, M.; Ashraf, R. S.; Klemm, E.; Sensfuss, S. Macromo-
Marks, T. J. J Am Chem Soc 2003, 125, 9414–9423.
lecules 2006, 39, 7844–7853.
33 Katz, H. E. In Plastic Electronics Conference, Frankfurt, Ger-
12 Mikroyannidis, J. A.; Stylianakis, M. M.; Sharma, G. D.; Bal-
many, 2006.
raju, P.; Roy, M. S. J Phys Chem C 2009, 113, 7904–7912.
34 Facchetti, A.; Letizia, J.; Yoon, M.-H.; Mushrush, M.; Katz, H.
13 Tang, W.; Ke, L.; Tan, L.; Lin, T.; Kietzke, T.; Chen, Z.-K. Mac-
E.; Marks, T. J. Chem Mater 2004, 16, 4715–4727.
romolecules 2007, 40, 6164–6171.
35 Hamilton, R.; Smith, J.; Ogier, S.; Heeney, M.; Anthony, J.
E.; McCulloch, I.; Veres, J.; Bradley, D. D. C.; Anthopoulos, T.
D. Adv Mater 2009, 21, 1166–1171.
14 Campbell, A. J.; Bradley, D. D. C.; Antoniadis, H. Appl Phys
Lett 2001, 79, 2133–2135.
15 Hou, Q.; Zhou, Q.; Zhang, Y.; Yang, W.; Yang, R.; Cao, Y.
36 Zhang, L.; Di, C.-a.; Yu, G.; Liu, Y. J Mater Chem 2010, 20,
Macromolecules 2004, 37, 6299–6305.
7059–7073.
16 Blouin, N.; Michaud, A.; Gendron, D.; Wakim, S.; Blair, E.;
Neagu-Plesu, R.; Belletete, M.; Durocher, G.; Tao, Y.; Leclerc,
M. J Am Chem Soc 2007, 130, 732–742.
37 Hu, Z.; Reichmanis, E. J Polym Sci Part A: Polym Chem
2011, 49, 1155–1162.
38 Imae, I.; Nawa, K.; Ohsedo, Y.; Noma, N.; Shirota, Y. Macro-
molecules 1997, 30, 380–386.
17 Barbarella, G.; Favaretto, L.; Sotgiu, G.; Zambianchi, M.;
Arbizzani, C.; Bongini, A.; Mastragostino, M. Chem Mater 1999,
11, 2533–2541.
39 Ohsedo, Y.; Imae, I.; Shirota, Y. J Polym Sci Part B: Polym
Phys 2003, 41, 2471–2484.
18 Vangeneugden, D. L.; Vanderzande, D. J. M.; Salbeck, J.;
van Hal, P. A.; Janssen, R. A. J.; Hummelen, J. C.; Brabec, C.
J.; Shaheen, S. E.; Sariciftci, N. S. J Phys Chem B 2001, 105,
11106–11113.
40 Wu, Z.; Petzold, A.; Henze, T.; Thurn-Albrecht, T.; Loh-
wasser, R. H.; Sommer, M.; Thelakkat, M. Macromolecules
2010, 43, 4646–4653.
19 Qin, Y.; Kim, J. Y.; Frisbie, C. D.; Hillmyer, M. A. Macromole-
41 Gabaston, L. I.; Foot, P. J. S.; Brown, J. W. Chem Commun
cules 2008, 41, 5563–5570.
1996, 429–430.
20 Coppo, P.; Cupertino, D. C.; Yeates, S. G.; Turner, M. L. Mac-
romolecules 2003, 36, 2705–2711.
42 Garnier, F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre,
F.; Servet, B.; Ries, S.; Alnot, P. J Am Chem Soc 1993, 115,
8716–8721.
21 Zhu, Z.; Waller, D.; Gaudiana, R.; Morana, M.; Muhlbacher,
D.; Scharber, M.; Brabec, C. Macromolecules 2007, 40,
1981–1986.
43 Watanabe, J.-i.; Hoshino, T.; Nakamura, Y.-s.; Sakai, E.;
Okamoto, S. Macromolecules 2010, 43, 6562–6569.
22 Zhang, M.; Tsao, H. N.; Pisula, W.; Yang, C.; Mishra, A. K.;
44 Sugiyama, K.; Hirao, A.; Hsu, J.-C.; Tung, Y.-C.; Chen, W.-C.
Mullen, K. J Am Chem Soc 2007, 129, 3472–3473.
Macromolecules 2009, 42, 4053–4062.
23 Hou, J.; Park, M.-H.; Zhang, S.; Yao, Y.; Chen, L.-M.; Li,
45 Hotta, S.; Rughooputh, S. D. D. V.; Heeger, A. J.; Wudl, F.
J.-H.; Yang, Y. Macromolecules 2008, 41, 6012–6018.
Macromolecules 1987, 20, 212–215.
24 Liang, Y.; Wu, Y.; Feng, D.; Tsai, S.-T.; Son, H.-J.; Li, G.; Yu,
46 Hotta, S.; Ichino, Y.; Yoshida, Y.; Yoshida, M. J Phys Chem
L. J Am Chem Soc 2008, 131, 56–57.
B 2000, 104, 10316–10320.
25 Liang, Y.; Feng, D.; Guo, J.; Szarko, J. M.; Ray, C.; Chen, L.
X.; Yu, L. Macromolecules 2009, 42, 1091–1098.
47 Vaidyanathan, S.; Dotz, F.; Katz, H. E.; Lawrentz, U.;
Granstrom, J.; Reichmanis, E. Chem Mater 2007, 19,
4676–4681.
26 Wienk, M. M.; Turbiez, M.; Gilot, J.; Janssen, R. A. J. Adv
Mater 2008, 20, 2556–2560.
48 McCulloch, I.; Coelle, M.; Genevicius, K.; Hamilton, R.;
Heckmeier, M.; Heeney, M.; Kreouzis, T.; Shkunov, M.; Zhang,
W. Jpn J Appl Phys 2008, 47, 488–491.
27 Hou, J.; Tan, Z. a.; He, Y.; Yang, C.; Li, Y. Macromolecules
2006, 39, 4657–4662.
206
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