Very large silacylic substituent effects on response in silole-based polymer transistors

Article abstract of DOI:10.1021/cm200009k

Understanding the interrelationships between molecular structure and organic thin film transistor performance is key to the realization of novel organic semiconductors achieving superior device characteristics. Herein we report the synthesis, characterization, and charge-transporting properties in organic field-effect transistors (OFETs) of dithieno silole-based oligomers and copolymers having silacycloalkyl substituents. Silacyclization of the alkyl substituents on the silole silicon atom reduces steric encumbrance, contracts solid state intermolecular π-π contacts, and enhances the charge-transport capacity of the oligomers. Oligomer 3,3′-dihexylsilylene-2,2′:5, 2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′-sexithiophene (SM5) with two Si-n-hexyl substituents is not FET-active, while the mobilities of 3,3′-cyclopentanylsilylene-2,2′: 5,2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′′-sexithiophene (SM4) and 3,3′-cyclobutysilylene-2, 2′:5,2′′:5′,2′′′:5′′, 2′′′′:5′′′,2′′′ ′′-sexithiophene (SM3) FETs are 2.6 × 10^-4 and 3.4 × 10^-4 cm²/(V s), respectively. Single crystal structural data and melting point derived intermolecular packing trends parallel these FET results. Copolymers P1-P4 based on the same dithienosilole cycloalkyl cores exhibit optimized hole mobilities of 2 × 10^-5, 6 × 10^-4, 3 × 10^-4, and 2 × 10^-3 cm²/V?s, respectively, lower than that of analogous silole-containing polymers with long Si-alkyl substituents, implying that the solubilizing and self-assembly functions of Si-alkyl substituents are important for optimizing the mobility. Interestingly, copolymer [poly{[N,N′-bis(2- octyl-dodecyl)-1,4,5,8-naphthalenedicarboximide-2,6-diyl]-alt-5,5′-(3, 3′-cyclopentanylsilylene-2,2′-bithiophene (P5) films are the most ordered and exhibit a good electron mobility of 4 × 10^-3 cm²/V?s after thermal annealing. All of these OFETs exhibit good ambient-stability, which is attributed to their low-lying HOMOs (>0.2 eV lower than that of P3HT), a consequence of introducing silole cores into polythiophene backbones.

Full text of DOI:10.1021/cm200009k

ARTICLE
pubs.acs.org/cm
Very Large Silacylic Substituent Effects on Response in Silole-Based
Polymer Transistors
†
†
†,‡
§
,†,§
,†
Hui Huang, Jangdae Youn, Rocio Ponce Ortiz, Yan Zheng, Antonio Facchetti,* and Tobin Marks*
†
Department of Chemistry and the Materials Research Center, Northwestern University, 2145 Sheridan Road, Evanston, Illinois, 60208.
‡
Department of Physical Chemistry, University of M ꢀa laga, Malaga, 29071, Spain.
§
Polyera Corporation, 8045 Lamon Avenue, Skokie, Illinois, 60077, USA.
ABSTRACT: Understanding the interrelationships between molecular struc-
ture and organic thin film transistor performance is key to the realization of
novel organic semiconductors achieving superior device characteristics. Herein
we report the synthesis, characterization, and charge-transporting properties in
organic field-effect transistors (OFETs) of dithieno silole-based oligomers and
copolymers having silacycloalkyl substituents. Silacyclization of the alkyl
substituents on the silole silicon atom reduces steric encumbrance, contracts
solid state intermolecular πꢀπ contacts, and enhances the charge-transport
0
0
00
0
000
capacity of the oligomers. Oligomer 3,3 -dihexylsilylene-2,2 :5,2 :5 ,2 :
0
0
0000 000 00000
5
,2 :5 ,2 -sexithiophene (SM5) with two Si-n-hexyl substituents is not
0
0
00
0
000
FET-active, while the mobilities of 3,3 -cyclopentanylsilylene-2,2 :5,2 :5 ,2 :
0
0
0
0000 000 000000 0 0 00
5
5
1
,2 :5 ,2 -sexithiophene (SM4) and 3,3 -cyclobutysilylene-2,2 :5,2 :
000 00 0000 000 00000
ꢀ4
,2 :5 ,2 :5 ,2 -sexithiophene (SM3) FETs are 2.6 ꢁ 10 and 3.4 ꢁ
ꢀ
4
2
0
cm /(V s), respectively. Single crystal structural data and melting point
derived intermolecular packing trends parallel these FET results. Copolymers P1-P4 based on the same dithienosilole cycloalkyl
ꢀ
5
ꢀ4
ꢀ4
ꢀ3
2
cores exhibit optimized hole mobilities of 2 ꢁ 10 , 6 ꢁ 10 , 3 ꢁ 10 , and 2 ꢁ 10 cm /V s, respectively, lower than that of
3
analogous silole-containing polymers with long Si-alkyl substituents, implying that the solubilizing and self-assembly functions of Si-
alkyl substituents are important for optimizing the mobility. Interestingly, copolymer [poly{[N,N -bis(2-octyl-dodecyl)-1,4,5,8-
0
0
0
0
naphthalenedicarboximide-2,6-diyl]-alt-5,5 -(3,3 -cyclopentanylsilylene-2,2 -bithiophene (P5) films are the most ordered and
ꢀ
3
2
exhibit a good electron mobility of 4 ꢁ 10 cm /V s after thermal annealing. All of these OFETs exhibit good ambient-stability,
3
which is attributed to their low-lying HOMOs (>0.2 eV lower than that of P3HT), a consequence of introducing silole cores into
polythiophene backbones.
KEYWORDS: transistors, conjugated polymers, spiro siloles, sexithiophenes
’
INTRODUCTION
oligothiophenes typically exhibit low mobilities when the semicon-
ductor films are deposited from solution, presumably due to reduced
long-range structural order within the films. In principle, however,
Over the past few decades, several classes of π-conjugated
organic molecular and polymeric materials have been developed
21
thiophene-based polymers are excellent candidate materials for
as semiconductors for low-cost organic electronics applications,
22
23
1
ꢀ8
solution processing methodologies such as spin-coating, stamping,
such as organic thin-film transistors (OTFTs),
light-emitting
OTFTs are
24
9,10
11ꢀ14
and inkjet printing because of their great structural versatility and
synthetic accessibility. Among polythiophenes, regioregular polythio-
phenes such as poly(3-hexylthiophene) (P3HT) and variants thereof
diodes (OLEDs), and photovoltaics (OPVs).
essential elements of printed integrated circuits, operating RF-ID
tags, and sensors, as well as pixel drivers for active matrix displays.
Among the organic semiconductors used for OTFTs, (oligo, poly)-
thiophenes have been extensively investigated due to their synthetic
accessibility and acceptable charge carrier mobilities (μ). For
example, the oligothiophene R-sexithiophene (R-6T) was discov-
25ꢀ27
are the most commonly used in OTFTs.
Conjugated silole-containing materials have attracted great
attention because of their unique electronic structures, character-
ized by a low-lying lowest unoccupied molecular orbital (LUMO)
and a relatively small band gap resulting from the interaction between
15
ered as a hole transporting material in 1988, and one year
28ꢀ33
the silicon σ* orbital and the π* orbital of the butadiene moiety.
later thermally evaporated R-6T thin films were used to fabricate
Furthermore, the silicon atom stabilizes the highest occupied mole-
ꢀ
2
p-type OTFTs exhibiting a hole mobility of ∼1 ꢁ 10 to
34
cular orbital (HOMO) compared to the carbon counterparts,
ꢀ
3
2
16,17
1
ꢁ 10 cm /(V s).
When electron-withdrawing groups such as
which should enhance the ambient stability of silole-containing
perfluoroalkyl chains or perfluoroaromatic rings are introduced into
the semiconductor skeleton, high-performance electron-transporting
oligothiophenes were realized, which can be used to fabricate organic
Received: January 3, 2011
Revised:
March 1, 2011
18ꢀ20
complementary circuits.
Unfortunately, OTFTs based on simple
Published: March 23, 2011
r 2011 American Chemical Society
2185
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|Chem. Mater. 2011, 23, 2185–2200

Chemistry of Materials
ARTICLE
Figure 1. Structures of silole-thiophene polymers.
charge transport capacity (Figures 3 and 4). Thus, silole polymers
having different-sized cycloalkyl substituents are investigated here
and the solution and solid state properties of these materials
compared/contrasted with those of analogous linear alkyl chain-
substituted silole-thiophene derivatives. It will be seen that silicon
substituent annellation results in a new series of semiconducting
oligomers with significantly enhanced FET mobilities, however, the
effect on the corresponding copolymers charge-transport character-
istics vary considerably depending on the comonomer nature.
Figure 2. Strategy of replacing linear alkyl substituents with more
compact spiro-cycloalkyl substituents.
’
RESULTS AND DISCUSSION
polymer with respect to O oxidation. Inspired by these properties,
In the following sections we first discuss the synthesis of the
2
several groups have developed silole-containing small molecules,
small molecules and polymers, followed by thermal properties,
optical properties, and electrochemical characterization of the
semiconductors prepared in this study. This presentation is
followed by analysis of the thin film microstructure and mor-
phology in order to understand OTFT performance. Finally, the
fabrication and characterization of these devices is presented and
trends analyzed.
31,35ꢀ37
oligomers, and polymers as active materials in OLEDs,
38ꢀ42
43ꢀ47
OPVs,
and OTFTs.
To enable solution-processable polymeric semiconductors,
4
8,49
50ꢀ52
long linear
or branched alkyl
chains have been produc-
tively appended to the conjugated backbones. Furthermore, the
intermolecular self-assembly of the alkyl side chains can assist in
the formation of lamellar-ordered solid-state structures, strongly
Materials Synthesis. The new cycloalkyl silole-containing
building blocks 3, 4, and 5 were prepared according to Scheme 1.
53,54
enhancing intermolecular charge transporting properties.
0
0
For some oligothiophenes, it is known that R,ω-core functiona-
lization with alkyl chains also enhances carrier mobility because
of improved molecular packing caused by lipophilic interactions
Bithiophene was first tetrabrominated to give 3,5,3 ,5 -tetrabro-
mobithiophene, followed by selective debromination with Zn to
0
46
afford 3,3 -dibromobithiophene. Next, double lithiation of
5
5,56
0
among the alkyl chains.
On the other hand, alkyl chains
3,3 -dibromobithiophene with n-BuLi, followed by subsequent
3
connected to sp -hybridized bridgehead carbon or silicon atoms,
such as in fluorene and silole-containing polymers, extend perp-
endicular to the planar polymer backbone and hinder close solid
state πꢀπ stacking, hence compromise charge-transporting
quenching with the corresponding dichlorocycloalkylsilane,
yields monomers 3ꢀ5 as light-yellow oils in 46ꢀ60% yield.
The reaction of compound 3 with NBS afforded an unidenti-
fied mixture, possibly because the strained four-membered ring is
unstable in the presence of the NBS. Several other attempts were
made to synthesize the dibromo four-membered ring silole
compounds (Scheme 2). Compound 3 was treated with n-BuLi
5
7,58
capacity.
The carrier mobility in organic field-effect transis-
tors (OFETs) is generally found to increase with decreasing
interplanar distances in semiconductor crystals because of great-
er πꢀπ orbital overlap between neighboring molecules, thus
in THF and quenched with Me SnCl to afford an unidentified
3
59
facilitating charge transfer. For these reasons, replacing the bulky
phenyl substituents on the dithienosilole Si atom with less bulky
alkyl substituents reduces the intermolecular distances in the solid
mixture of products. Tetrabromobithiophene was next lithiated
6
2
with 2 equiv. of n-BuLi and quenched with Bu SnCl to give 6.
3
Treating 6 with n-BuLi, followed by quenching with the appro-
priate cycloalkyl dichlorosilane afforded a complex mixture of
products (Scheme 2). However, dibromo-functionalized mono-
mers 7 and 8 could be prepared in 92ꢀ94% yield by bromination
of 4 and 5, respectively, using NBS in DMF (Scheme 3).
With the new dibromosilole building blocks in hand, annel-
lated silole-based thiophene oligomers were synthesized via Stille
coupling with the various trimethylstannyl thiophene reagents as
shown Scheme 3. Compound SM1, obtained by reacting 8 with
5-hexyl-2-trimethylstannyl-thiophene, was isolated as a dark red
60,61
state and increases the charge transporting capacity.
Re-
cently, we reported dithienosilole and dibenzosilole-based poly-
mers functionalized with linear alkyl chains as the active layers to
ꢀ
3
fabricate OTFTs exhibiting hole mobilities ∼1 ꢁ 10 ꢀ 0.08
3
2
ꢀ1 ꢀ1
46
cm V
s
(Figure 1). The linear alkyl chains bonded to the
sp silicon center are oriented perpendicular to the silole planes,
conferring solubility to the silole-containing polymers. Never-
theless, the long alkyl chains appear to hinder close inter-
molecular πꢀπ stacking of the polymer backbones, and thus
only moderate p-type mobility is achieved.
0
oil in 80% yield. Similarly, reacting 8 with 5-trimethylstannyl-5 -
0
In this contribution, we report the synthesis and characteriza-
tion of new molecular and polymeric semiconductors based on
dithienosilole cycloalkyl cores (Figure 2). Silacyclization of
the alkyl substituents on the silicon atom should reduce steric
demands, contract solid state intermolecular contacts, and facilitate
hexyl-2, 2 -bithiophene affords SM2 as a dark-red solid in 79%
yield. The R,ω-unsubstituted oligomers SM3 and SM4 were also
obtained by reaction of 7 and 8 with 2-trimethylstannyl bithio-
phene in 55ꢀ69% yield. For comparative purposes, dialkyl silole
compound SM5 was synthesized via reaction of known silole
2
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Chemistry of Materials
ARTICLE
Figure 3. Structures of spiro-silole-based molecules SM1-SM5.
1
13
H and C NMR spectroscopy (broad features are observed
as expected), optical spectroscopy, GPC, TGA, DSC, and
elemental analysis. Details are given in the Experimental
Section.
Silole Thermal Properties. The thermal properties of oligo-
mers SM2ꢀSM5 and polymers P1ꢀP5 were investigated by
thermogravimetric analysis (TGA), differential scanning calo-
rimetry (DSC), and conventional melting point determinations.
Figures 5 and 6 show representative TGA and DSC plots, and all
data are collected in Table 1. Thermogravimetirc analysis (TGA,
heating ramp rate = 10 °C/min under N ) was used to investigate
2
the thermal stability of the thiophene oligomers and polymers
(
Figure 5). In the thermolysis data, a 5% mass loss is defined as
the thermolysis threshold. The thermolysis onset temperature
for polymer P1 is ∼250 °C, while oligomers SM2ꢀSM5 and all
other polymers have onset temperatures above 300 °C, indicative
of excellent thermal stability. This characteristic allows thin film
annealing and microstructure analysis to be carried out over a
broad range of temperatures, 50ꢀ300 °C. The observed melting
points of compounds SM3 and SM4 are within 0ꢀ20 °C of the
values observed by DSC (Table 1). We find that silicon
substituent annellation has a substantial influence on oligomer
thermal properties, doubtless reflecting an interplay of intermo-
lecular πꢀπ stacking interactions and alkyl chain interdigitation.
The melting point of the dialkyl compound SM5 is
Figure 4. Structures of spiro-silole-based copolymers P1-P5.
Scheme 1. Synthesis of Spiro-dithienosiloles with Cycloalkyl
Substituents
1
∼
34ꢀ136 °C. Interestingly, this parameter increases to
240 °C for the annellated compounds SM3 and SM4,
indicating substantial differences in solid-state cohesive en-
ergies. Since the dipole moments of SM3-SM5 are similar
(vide infra), the significant increases in melting temperatures
are likely the result of the less sterically demanding cyclic
moieties of SM3 and SM4, which thereby enhances inter-
molecular πꢀπ stacking. Interestingly, the melting points of
SM3 and SM4 are independent of cycloalkyl ring size, which
implies that similar intermolecular πꢀπ stacking energetics
dominate. Note that the melting temperature of R,ω-di-n-
hexyl substituted compound SM2 is 144ꢀ146 °C. This
indicates a significant reduction in the intermolecular cohe-
sive forces, a phenomenon also observed in other conjugated
4
6
building block 9 with 2-trimethylstannyl bithiophene, and was
obtained as a dark-red solid in 85% yield.
Dithienosilole-based polymers P1ꢀP5 were synthesized via
Stille polycondensation of 8 with 2,5-bis(trimethylstannyl)thiophene,
0
0
0
0
5
,5 -bis(trimethylstannyl)-2,2 -bithiophene, 3,3 -dodecoxy-5,5 -
bis(trimethylstannane)- 2,2 -bithiophene, 2,6-bis(trimethyltin)-4,8-
dioctylbenzo[1,2-b:4,5-b ]dithiophene, and N,N -bis(2-octyl-
dodecyl)- 1,4,5,8-naphthalenedicarboximide in 52ꢀ87% yields as
shown in Scheme 4. The physicochemical properties of these
polymers are summarized in Table 1. Polymers P1, P3, P5 are
very soluble in common nonprotic organic solvents such as THF,
toluene, xylenes, chloroform, chlorobenzene, and o-dichloroben-
zene (DCB). The molecular weights (M ) of polymers P1, P3,
P5 by GPC are 4.9 KD (PDI = 1.13), 32 KD (PDI = 6.0), 18 KD
0
0
51
0
52
4
9,63
organic semiconductors.
The thermal properties of the present oligomers and polymers
were also examined by differential scanning calorimetry (DSC)
at a scanning speed of 10 °C/min (Figure 6). Oligomers SM2
and SM5 evidence no endotherms/exotherms in the heating/
cooling cycles. Oligomer SM3 exhibits a single endothermic peak
at 238 °C, which is tentatively attributed to backbone melting,
w
(
PDI = 2.26), respectively. Polymers P2 and P4 are only soluble
in hot chlorinated aromatic solvents such as chlorobenzene
and DCB. Each of the new polymers was fully characterized by
2
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Chemistry of Materials
ARTICLE
Scheme 2. Synthetic Strategies to Prepare Spiro-cycloalkyl Silole Semiconductors Based on Compound 3
Scheme 3. Synthesis of Spiro-dithienosilole Monomers
and an exothermic recrystallization peak upon cooling at 158 °C.
Oligomer SM4 exhibits a single endotherm at 246 °C, with a
broad exotherm at 124 °C. Importantly, all four polymers have
no endotherms/exotherms in the respective heating/cooling
cycles, which implies that annealing of the semiconductors
during the device fabrication will have little effect on OFET
performance.
minimize the influence of molecular aggregation and differences
in molecular dipole moments (solvents with large dielectric cons-
tants can preferentially stabilize polar molecular conformations),
and thereby revealing the “intrinsic” molecular structural features
such as planarity, optical HOMOꢀLUMO gap, and extent of
65
π-conjugation.
Compounds SM3 and SM4 have similar optical absorption
spectra and exhibit essentially the same optical absorption maxima
at 473 nm. The corresponding spin-coated thin films exhibit abso-
rption maxima at 487 nm (with a strong shoulder at 524 nm) and
484 nm (with a strong shoulder at 522 nm), respectively. Com-
pound SM2 has an optical absorption maximum at 482 nm in DCB
solution and at 491 nm for thin films. Both of the SM2 maxima are
atslightly larger energies than those of SM4, reflecting the electron-
donating properties of the terminal n-hexyl chains. All of the
absorption maxima for oligomers SM2-SM4 exhibit small red-shifts
Silole Optical Properties. Solution and thin-film UVꢀvis
absorption spectra of the present silole oligomers and polymers
are shown in Figure 7, and data are collected in Table 1. Because
of the presence of multiple absorption transitions and large
Stokes shifts, solution and solid-state optical band gaps (Eg)
were estimated from the low-energy band edges in the optical
6
4
spectra. The solution-based absorption and emission measure-
ꢀ
5
ments were carried out at very low concentrations (<10 M) in
a solvent of intermediate dielectric constant (DCB, ε = 9.8) to
2
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Chemistry of Materials
ARTICLE
Scheme 4. Synthesis of Annellated Spiro-dithienosilole-based Copolymers
Table 1. Summary of Thermal, Optical Absorption/Emission, and Electrochemical Properties of Silole Compounds SM2-SM5,
P1ꢀP5, and the Corresponding Estimated Frontier Molecular Orbital Energies
a
red-1/2
ox-1
CV
solution
thin-film
abs
sol
λ
em
T
DSC (°C)
T
TGA
b
E
E
E
g
E
LUMO
d
E
HOMO
e
λ
abs
λ
c
c
c
f
g
h
compd mp (°C)
heating (cooling)
(°C)
(V)
(V)
(eV)
(eV)
(eV)
(nm)
g
(nm) (E (eV))
(nm)
SM2
SM3
SM4
SM5
P1
144ꢀ146
236ꢀ238
244ꢀ246
134ꢀ136
NA
413
350
303
320
250
320
320
415
412
ꢀ1.4
ꢀ1.2
ꢀ1.3
ꢀ1.2
ꢀ1.0
ꢀ0.9
ꢀ1.2
ꢀ1.1
ꢀ0.4
1.0
1.1
1.1
1.2
1.0
1.1
0.7
0.8
1.2
2.4
2.3
2.4
2.4
2.0
2.0
1.9
1.9
1.6
ꢀ3.4
ꢀ3.4
ꢀ3.4
ꢀ3.5
ꢀ3.5
ꢀ3.6
ꢀ3.4
ꢀ3.3
ꢀ4.0
ꢀ5.4
ꢀ5.5
ꢀ5.5
ꢀ5.6
ꢀ5.4
ꢀ5.5
ꢀ5.1
ꢀ5.2
ꢀ5.6
482
491 (2.0)
521
520
537
603
612
623
688
657
258 (158)
246, 124
NA
473
487,524 (2.1)
473
484, 522 (2.1)
447 (2.1)
466
NA
508
532 (1.9)
P2
NA
542
560 (1.9)
P3
NA
586
608 (1.7)
P4
NA
548
542 (1.9)
i
i
P5
NA
392, 681
400, 755 (1.4)
439, 539 (392)
93, 913 (681)
7
a
b
From DSC scans under nitrogen at a scan rate of 10 °C/min. Onset decomposition temperature measured by thermogravimetric analysis under
nitrogen. For oligomers, 0.1 M Bu N PF in THF (vs SCE) at a scan rate of 100 mV/s. For polymers, as thin films with 0.1 M Bu N PF in
acetonitrile (vs SCE). ELUMO is calculated from: E
absorption in DCB. From optical absorption as spin-coated thin-film on glass, optical band gap is estimated from the low energy band edge of the
c
þ
ꢀ
þ
ꢀ
6
4
6
4
d
e
red-1/2 f
g
= ELUMO ꢀ EHOMO. Estimated from the equation: EHUMO = ꢀ4.4 eV ꢀ Esol.
i
. From optical
g
h
UVꢀvis spectrum. From optical emission in DCB. Broad, weak emission peak.
maximum at 447 nm in thin films on glass. This hypsochromatic
shift of the dominant absorption band suggests that H-aggregate
66
formation may occur in the solid state, in which the molecules
are cofacially aligned.
To better understand the electronic spectra of the present
silole molecules, time-dependent density functional theory (DFT)
calculations (TD-DFT) were performed, and the 12 frontier MO
energies estimated. Selected data are shown in Table 2. These
calculations predict that the electronic transitions having the high-
est oscillator strengths are located at 535, 533, and 532 nm for SM3,
SM4, and SM5 respectively, and correspond to monoelectronic
excitation from the highest occupied molecular orbital (HOMO)
to the lowest unoccupied molecular orbital (LUMO). The differ-
ences found between theoretical and experimental wavelengths
can be ascribed both to the fact that the calculations are carried out
considering isolated molecules in vacuo vs the solution phase or
solid-state in which experimental data are acquired, and to the
Figure 5. Thermogravimetric analysis (TGA) of oligomers and poly-
mers at a temperature ramp of 10 °C/min under N2.
upon going from the solution to thin film phase, indicating on
average a somewhat greater degree of organization in the thin
film phase. Note, however, that the optical absorption spectrum
of SM5 has a maximum at 466 nm in DCB solution, but a
67
tendency of DFT to overestimate π-conjugation. Figure 8 shows
the topologies of the frontier molecular orbitals involved in the
2
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Chemistry of Materials
ARTICLE
2
Figure 6. Differential scanning calorimetry (DSC) of the spiro-silole oligomers (A) SM3 and (B) SM4 at a temperature ramp of 10 °C/min under N .
Figure 7. Optical absorption spectra of annellated silole oligomers and polymers (A) in DCB solution and (B) as thin films on glass substrates, and (C)
photoluminescence spectra in DCB solution.
4
6
highest oscillator strength electronic transition for molecules SM3,
SM4, and SM5. As also found in oligothiophenes, the HOMOs are
aromatic in nature, with no significant contributions of the S or Si
orbitals, and are basically linear combinations of the fused core
HOMOs with the thiophene HOMOs. This result suggests that Si
inclusion in the oligothiophene backbone should not significantly
affect oxidation potentials, in agreement with the present (see
below) and previous results on related systems. Nevertheless, it is
interesting to note that the greatest contribution to the HOMO
comes from the central fused silole moiety and the nearby
thiophene rings, with lesser contributions from the external
thiophene rings. In the case of the LUMOs, the topologies for
the three compounds have a distinctive quinoidal character,
with a significant contribution from the S atoms, while the
contribution of the Si atoms remains negligible. As expected,
the substitution of linear or branched alkyl chains by cycloalkyl
ones does not induce any significant changes in either the
energies or molecular topologies of the frontier orbitals, making
it clear that any variations in TFT electrical performance should
be solely ascribed to changes in crystal packing and/or micro-
structure effects rather than to differences in molecular electro-
nic structure.
46
As noted above and summarized in Table 1, the optical
absorption spectra of the annellated silole polymers exhibit
absorption maxima that are red-shifted versus those of the
corresponding oligomers, suggesting extended π-conjugation.
The absorption maxima of P1, P2, P3 and P4 in solution are
located at 508 nm, 542 nm, 586 nm, and 548 nm, respectively,
2
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Chemistry of Materials
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Table 2. Summary of Computed (DFT//B3LYP/6-31G**) Frontier Molecular Orbital Energies, HOMO-LUMO Gaps, Highest
Oscillator Strength Transition Energies, Oscillator Strengths, and Excited-State Configurations as Determined by TDDFT
Calculations. Theoretical (DFT//B3LYP/6-31G**) Reorganization Energies for Hole Injection for Molecules SM3, SM4, and SM5
compd
HOMO (eV)
LUMO (eV)
Eg,theor.(eV)
λ (nm)
osc. str.
configuration
λ (eV)
SM3
ꢀ4.71
ꢀ2.14
2.57
535
1.70
0.33
HOMOfLUMO
H-2fL
0.308
324
H-1fLþ1
HfLþ4
SM4
SM5
ꢀ4.70
ꢀ4.68
ꢀ2.13
ꢀ2.11
2.57
2.57
533
23
1.74
0.31
HOMOfLUMO
H-2fL
0.318
0.321
3
H-1fLþ1
HfLþ4
HOMOfLUMO
H-2fL
532
22
1.74
0.29
3
H-1fLþ1
HfLþ2
HfLþ4
smallest band gap of P5 (1.4 eV) is comparable to those reported
52,70
for NDI-based copolymers.
The silole solution PL emission spectra are shown in Figure 7C
and data are collected in Table 1. Because 0ꢀ0 transitions are rarely
observed in room temperature solution spectra, it is an accepted
procedure to use Δ = λ ꢀ λ as an estimation of the magnitude
em
abs
71
of the Stokes shift. The silole-based oligomers SM2, SM3, and
SM4 emit green-yellow light with maxima at 521, 520, and 537 nm,
respectively, while SM5 emits orange-red light with maximum
at 603 nm. These transitions are assigned to the 0ꢀ1 transition
in dilute solutions. The silole-based oligomer SM5 exhibits
much larger Stokes’ shifts (137 nm) than SM4 (64 nm),
suggesting that the long alkyl chains on the tetrahedral Si
provide greater degrees of molecular freedom than do the
constrained cycloalkyl substituents (more rotational/vibra-
tional states from which to dissipate energy). Such large Stokes’
shifts are also observed in fluorene systems having long alkyl
Figure 8. B3LYP/6-31G** electronic density contours for the frontier
molecular orbitals of: (a) SM3, (b) SM4, and (c) SM5.
7
2
chains at the bridgehead carbon atom of the backbone.
Molecule SM2 with pedant alkyl chains in the backbone exhibits
a smaller Stokes’ shift (39 nm) than does SM4 (64 nm). Similar
trends are also observed for the polythiophenes functionalized
with pedant alkyl chains, where longer chains afford smaller
while those of the corresponding thin films are located at 532 nm
for P1, 560 nm for P2, 608 nm for P3, and 542 nm for P4. The
absorption maxima of P5 occur at 680 and 392 nm in DCB solution
and at 755 and 400 nm for the thin film, corresponding to the
smallest πꢀπ* transition energy in the silole-based polymer series
and indicating that the strongest electron-accepting NDI building
block affords the lowest LUMO energy and leads to the smallest
HOMOꢀLUMO gap.
73
Stokes’ shifts, suggesting that the SM2 backbone is more
74,75
rigid.
Similarly, the Stokes’ shift of SM3 (47 nm) is slightly
smaller than that of SM4 (64 nm), indicative of the greater SM3
backbone rigidity.
Silole-based polymers P1 and P2 emit orange light with
emission maxima located at 612 and 623 nm, while P3 and P4
emit red light with emission maxima located at 688 and 657 nm.
The optical band gaps for the oligomers and polymers were
estimated from the low-energy band edges of the thin-film optical
spectra, taking 10% of the maximum as the band edge. The
optical band gaps of oligomers SM2, SM3, SM4, and SM5 are
All of these polymers exhibit large Stokes’ shifts of ∼80ꢀ110 nm,
46,76
which are not unusual for silicon-containing polymers,
may possibly be due to eximer emission.
and
77,78
2
.08, 2.08, 2.05, and 2.07 eV, respectively, significantly smaller
However, when
68
than that of R-6T (2.30 eV). However, the optical band gaps of
polymers P1, P2, P4 are 1.89, 1.90, and 1.87 eV, respectively,
similar to that of P3HT (1.9 eV), and to those of TS6T1 and
TS6T2 (1.8ꢀ1.9 eV; Figure 1).
electron-deficient polymer P5 is excited at the short-wavelength
(
(
392 nm) or at the long-wavelength absorption maximum
681 nm), only very weak, broad emissions having shapes similar
69
44,46
These small band gaps likely
to the absorption spectra are observed with maxima at 439,
539 nm or 793, 913 nm, respectively. This is consistent with the
general observation that increased Stokes shift amplitudes gen-
erally correlate with longer excited-state lifetimes, resulting in an
increase in the relative probability of nonradiative decay and
reflect the appreciable planarity of these macromolecules (vide
infra) and a backbone similarity to that of polythiophenes. Note
that polymer P3 has a particularly small optical band gap of 1.72
eV, indicating that the oxyalkyl subsituent is a strongly electron-
donating group, resulting in a higher-lying HOMO. Also, the
79
lower PL quantum efficiencies. This suggests for the present
2
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þ
6ꢀ
Figure 9. Cyclic voltammograms of molecules SM2-SM5 as solution in THF with 0.1 M Bu
4
N PF suppoting electrolyte and polymers P1ꢀP5 as
þ
ꢀ
6
thin films in acetonitrile with 0.1 M Bu N PF supporting electrolyte at scan rates of 100 mV/s.
4
present oligomers and polymers can be estimated by taking the
SCE energy level to be ꢀ4.4 eV below the vacuum level and using
81
the standard relationship of eqs 1 and 2.
HOMOox ¼ ꢀ Eox-onset ꢀ 4:4
ð1Þ
ð2Þ
Eg ¼ ELUMO ꢀ EHOMO
Using relationships (1) and (2), the HOMO/LUMO energies are
estimated as ꢀ5.4/-3.4 eV for SM2, ꢀ5.5/-3.4 eV for SM3, ꢀ5.5/-
3
.4 eV for SM4, ꢀ5.6/ꢀ3.5 for SM5. Similarly, polymers P1, P2, P3,
P4, and P5 have HOMO/LUMO energies at ꢀ5.4/-3.5 eV,
ꢀ
5.5/ꢀ3.6 eV, ꢀ5.1/ꢀ3.4 eV, ꢀ5.2/ꢀ3.3 eV, ꢀ5.6/ꢀ4.0 eV. respec-
tively, indicative of significantly greater ionization energies than P3HT
82
(
∼4.9 eV). Clearly, introduction of the silole core into the
thiophene backbone stabilizes the system with respect to ioniza-
tion without sacrificing π-conjugation, as argued by the similar
optical band gaps (vide supra). The HOMO/LUMO energies of
the present polymers with cycloalkyl substituents are comparable
to those of the silole-based polymers having long alkyl chain
Figure 10. Diffraction-derived single-crystal structures of (A) SM3, (B)
SM4, and (C) SM5. Hydrogen atoms are omitted for clarity. The
distance indicated by d is the shortest interplanar πꢀπ contact.
44,46
substituents.
siloles that the core electron-deficiency facilitates intramolecular
As shown in Table 2, DFT calculations estimate the HOMO
charge-transfers which result in large Stokes shifts and eventual
and LUMO energies to be ꢀ4.71/ꢀ2.14 eV, ꢀ4.70/ꢀ2.13, and
8
0
quenching.
ꢀ
4.68/ꢀ2.11 eV for compounds SM3, SM4, and SM5, respec-
Silole Electrochemical Properties. Oxidation and reduction
potentials for the present oligomers and polymers were mea-
sured by cyclic voltammetry (CV) experiments. Cyclic voltam-
mograms of the oligomers in THF and the polymers as thin films
are shown in Figure 9, and electrochemical data are summarized
in Table 1. The ferrocene/ferrocenium couple was used as the
internal standard. The silole-based oligomers SM2, SM3, SM4,
and SM5 exhibit reduction/oxidation potential onsets at ꢀ1.4/
tively, leading to a uniform HOMOꢀLUMO energy gap of
2
.57 eV. This value is in good agreement with the experimental
band gaps obtained from the electrochemical results (Table 1),
which range from 2.3 to 2.4 eV.
Single-Crystal Molecular Structures. Single crystals of the
new silole compounds SM3, SM4, and SM5 were grown by slow
diffusion of hexane or methanol into DCB solutions, and the
crystal structures are shown in Figure 10. The introduction of the
silole functional group into the sexithiophene backbone causes
twisting of the SM3, SM4, and SM5 backbones from planarity,
and the change in Si substituents from linear long alkyl chains to
cycloalkyls results in major changes in the solid state packing. For
molecule SM3, the twist angles between rings 2 and 3, and
between rings 4 and 5 are 20.17 and 11.80°, respectively, which is
1
.0, ꢀ1.2/1.1, ꢀ1.3/1.1, and ꢀ1.2/1.2 V, respectively (vs SCE).
The onset potentials for the polymers are at ꢀ1.0/1.0, ꢀ0.9/1.1,
ꢀ
1.2/0.7, ꢀ1.1/0.8, and ꢀ0.4/1.2 V (vs SCE) for P1, P2, P3,
P4, and P5, respectively. The smallest onset oxidative potentials
of P3 suggest a lower stability with respect to oxidative doping.
ox
From the oxidation potentials, the ionization potentials (IP ≈
ox
83
ꢀ
HOMO , assuming that Koopmans’ theorem holds) for the
much larger than those observed in sexithiophene (∼3ꢀ4°)
2
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Chemistry of Materials
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Figure 11. Tapping-mode AFM images of SM3 thin films grown
at the indicated temperatures and on various substrates (scan areas:
Figure 13. (A) θꢀ2θ X-ray diffraction (XRD) scans of spin-coated P5
thin films after annealing at the indicated temperatures. Note that peak
intensities are not normalized). (B) Tapping mode AFM images of P5
thin films after annealing at the indicated temperatures on different
substrates (scan areas: 4.5 μm ꢁ 4.5 μm).
9
μm ꢁ 9 μm).
be seen in Figure 10, the backbones of successive SM4 molecules
assemble in a slipped herringbone conformation along the a axis
with a minimum cofacial distance between the closest thiophene
rings of 5.24 Å, which is significantly larger than that in SM3. For
compound SM5 having two Si-n-hexyl chains, the backbone is
significantly twisted with thiophene-thiophene twist angles
of 31.7, 25.3, 2.8, 21.8, and 27.3° on going from ring 1 to 6,
respectively, indicating weak intramolecular π conjugation.
Futhermore, the molecular backbones stack along the a axis in
a slipped herringbone motif having a large minimum distance
between the neighboring backbones of 12.72 Å, indicating
negligible πꢀπ stacking interactions. The long n-hexyl chains
occupy/create the large free volume between adjacent molecules.
Thus, through cyclization of the silicon alkyl substitents, it can be
seen that closer distances between the neighboring π backbones
are achieved (SM3 < SM4 < SM5), representing a heretofore
unexplored strategy for enhancing πꢀπ stacking. This is con-
sistent with the change of the calculated density according to the
Figure 12. Tapping-mode AFM images of SM4 thin films grown
at the indicated temperatures and on various substrates (scan areas:
9
μm ꢁ 9 μm).
2
0,84
and in alkyl-subsituted oligothiophenes (∼5ꢀ6°).
The SM3
3
3
twist angle values are midway between a planar configuration and
30°, the latter being the maximum value beyond which there is
insufficient intramolecular π-overlap to achieve a significantly
crystal data, SM3 (1.526 g/mm ) > SM4 (1.477 g/mm ) > SM5
3
∼
(1.301 g/mm ). Furthermore, it will be seen that these crystal
structure observations are consistent with the charge transport-
ing properties of the TFT devices fabricated with these oligomers
(vide infra).
Thin-Film Microstructure and Morphology. Thin film mi-
crostructure, morphology, and molecular orientations for the
present new class of materials were studied by θꢀ2θ X-ray
diffraction (XRD) and atomic force microscopy (AFM). Mea-
surements were carried out on vacuum-deposited and spin-
85
π-conjugated structure. The twist angles between SM3 rings
1
4
and 2, 3 and 4, and 5 and 6, are much smaller, 1.53, 5.50, and
.68°, respectively. The exocyclic silacyclopentane ring is twisted
and the distance between atoms C25 and C28 is 2.774 Å, while
the theoretical distance from DFT calculations is 2.822 Å. The
twisted confomation of the silacyclopentane is similar to that of
61,86,87
other reported silacylopentanes.
In regard to packing, the
conjugated backbones of the SM3 molecules are partially πꢀπ
stacked in a slipped cofacial orientation along the b axis. The
minimum πꢀπ distance between the closest thiophene rings is
coated thin films grown on OTS-treated Si/SiO substrates.
2
The thicknesses of all films were 50ꢀ60 nm as measured by
profilometry. The XRD scans reveal that the films of all the
oligomers as well as polymers P1-P4 are essentially amorphous,
even films of the polymers annealed overnight at 250 °C. AFM
images of the vacuum-deposited films of the oligomers indicate
that the morphologies of the films depend on the substrate
surface energy and the deposition temperature as shown in
Figures 11 and 12.
∼
3.78 Å. For compound SM4, the thiophene-thiophene twist
angles are 7.2, 21.2, 4.64, 13.0, and 10.8° on proceeding from ring
1
to 6, respectively. The highly twisted structure suggests
unoptimized intramolecular conjugation, probably hindering
charge transporting efficiency. The silacyclohexane ring adopts
a chair conformation with a distance between C25 and C29 of
2
.971 Å, while the theoretical distance from the DFT calculation
is 2.993 Å. Such a chair confomation of the silacyclohexane
The morphology of the vacuum-deposited SM3 films consists
of rounded grains on bare SiO and OTS-treated substrates
2
86ꢀ88
substituent has been observed in related molecules.
As can
when grown at room temperature, however worm-like grains are
2
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Chemistry of Materials
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observed on HMDS-treated substrates. For growth at 80 °C,
significant plate-shaped grains with sizes of >1 μm are observed,
but also large grain boundaries are found on bare SiO , portend-
2
ing poor charge transport efficiency. Nevertheless, the film
morphologies having large worm-like grains on OTS-treated
substrates may be favorable for charge transport. In room
temperature depositions, the morphology of the vacuum-depos-
ited SM4 films changes from rounded grains with dimensions
<
0.1 μm on bare SiO to dimensions >0.2 μm on OTS-treated
2
substrates, while the morphology of the films has worm-like
grains on HMDS-treated substrates. When films of the same
compound are vacuum-deposited at 80 °C, the film morphology
is characterized by grains with very large intervening boundaries.
In marked contrast to the above results, XRD scans reveal that
P5 films are crystalline as shown in Figure 13. A single family of
diffraction peaks is observed without the presence of an obvious
πꢀπ stacking distance, and the d-spacing is estimated from the
(
100) reflection to be 21.6 Å (after 200 °C annealing), implicat-
ing a crystalline polymorph where the majority of the molecules
have an edge-on orientation on the substrate with the πꢀπ
stacking direction parallel to the substrate plane, thereby favoring
in-plane charge transport from source to drain. The degrees of
crystallinity of the solution-cast P5 films are dependent on the
annealing temperature. As shown in Figure 13, a possible change
in the preferential crystalline phase/orientation is observed upon
annealing, with the d-spacing decreasing from 23.6 to 21.9 Å from
90 to 150 °C, with a significant increase in the (100) reflection
intensity (13, 000 to 65, 000 counts/s), and with the appearance
of a higher order reflection (200), indicative of a higher degree of
ordering, and presumably increased alkyl-chain interdigitation. Note
however that annealing at even higher temperature (200 °C)
decreases the reflection intensity (65 000 to 36 000 counts/s) and
diminishes the higher order reflections, whereas the d-spacing
continuously decreases to 21.6 Å. AFM analysis shows the upon
annealing, the film morphology changes, but not in major ways. At
1
50 °C, the grain sizes appear to have the maximum dimensions vs
the other annealing temperatures. Note that these morphological
results are consistent with the transistor performance (vide infra).
Thin-Film Transistor Characterization. Top-contact/bot-
tom-gate OTFTs devices were fabricated on bare SiO as well
2
þ
as on HMDS-, and OTS-treated SiO /p -Si substrates. All oligomer
2
ꢀ
6
films were vapor-deposited under high vacuum (1 ꢁ 10 Torr)
with the substrates maintained at temperatures of 21 or 80 °C.
Finally, Au source and drain contacts were deposited by thermal
evaporation using a shadow mask to define channel lengths (L) of
2
5ꢀ100 μm and widths (W) of 500ꢀ2000 μm. Bottom-contact/
top-gate polymer OTFTs were fabricated by spin-coating with
mg/mL polymer in DCB solutions. In this case, the spin-coating
7
processes were carried out in air on glass substrates having thermally
deposited Au source and drain electrodes with a W/Lratio of 10. The
thin polymer films were next annealed under vacuum and a PMMA
gate dielectric spin-coated on top of the semiconductor film, and the
assembly dried in a vacuum oven. Next, gold gate electrodes were
deposited by thermal evaporation. FET properties were then eval-
uated under positive or negative gate bias in ambient and vacuum to
define the majority charge carrier type, device performance, and
environmental stability. The field-effect mobility and threshold
voltages are calculated in the saturation regime. FET data are
summarized in Table 3, and representative transfer plots are shown
in Figure 14.
Devices fabricated with oligomers SM2 and SM5 are found
to be FET-inactive, whereas compounds SM3 and SM4 are
2
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Chemistry of Materials
ARTICLE
Figure 14. OTFT transfer plots of polymers (A) SM4, (B) P2, (C) P5 at VSD = ꢀ100 V.
Table 4. Summary of Crystallographic Data for SM3, SM4,
and SM5
FETs. After reducing the size of the silacycloalkyl substituents from
6- to 5-membered rings, SM3ꢀbased FETs exhibit enhanced
ꢀ
4
2
performance with hole mobilities of ∼4 ꢁ 10 cm /V s and
3
SM3
SM4
SM5
3
I /I ratios of 2 ꢁ 10 on OTS-treated substrates. It is not
on off
surprising that the FET performance of oligomers SM3 and SM4 is
below that of sexithiophene (μ = 0.02 cm /V s), not only
formula
fw
C
28
H
20
S
6
Si
C
29
H
22
S
6
Si
C
36
H
38
S
6
Si
2
89
576.89
P2(1)/n
590.92
Pna2(1)
691.11
3
because
of
the
space group
a (Å)
P1
bulky cycloalkyl chains, but also because of the twisted conforma-
tions of the SM3 and SM4 backbones. However, the mobilities of
SM3 and SM4 are superior to those reported for dithienosilole
20.858(2)
5.0475(6)
25.530(3)
90
11.1140(5)
32.1250(14)
7.4428(4)
90
13.681(3)
13.705(3)
19.382(4)
100.457(13)
98.896(13)
89.817(13)
3529.7(12)
4
b (Å)
c (Å)
ꢀ7
ꢀ5
2
oligomers sDTS-1 and DTS-3T (1 ꢁ 10 to 1 ꢁ 10 cm /
R (deg)
β (deg)
γ (deg)
43,
(
V s)) (Figure 1). All of the present devices show similar
110.948(7)
90
90
mobilities when measured in vacuum or under ambient conditions,
indicative of good environmental stability, and consistent with the
low-lying HOMOs (ꢀ5.4 to ꢀ5.6 eV) of these oligomers.
90
3
V (Å )
2510.2(5)
4
2657.4(2)
4
Z
The four polymers P1ꢀP4 are found to be p-channel semi-
conductors, while P5 is an n-channel semiconductor. Thermal
annealing is found to marginally improve the TFT response of
the devices fabricated with polymers P1ꢀP4, consistent with the
XRD results discussed above, which reveal minimal crystallinity.
Bottom-contact/top-gate TFTs fabricated with polymer P1
ꢀ3
D (calcd), g cm
1.526
1.477
1.301
ꢀ
1
μ (mm
T (K)
)
5.628
5.330
0.446
100(2)
Bruker Apex2
CCD
100(2)
Bruker Apex2
CCD
100(2)
diffractometer
Bruker Apex2
CCD
ꢀ
5
2
exhibit a hole mobility of 4 ꢁ 10 cm /(V s) and an I /I ratio
on off
radiation
CuKR
CuKR
MoKR
3
∼
1 ꢁ 10 . Devices fabricated with the more π-extended polymer
(
1.5418 Å)
(1.5418 Å)
0.47 ꢁ 0.15
ꢁ 0.05
1224
(0.71073 Å)
0.32x 0.21x 0.11
ꢀ4
2
P2show improved performance with μ ∼ 6 ꢁ 10 cm /(V s) and
3
cryst size (mm )
0.19 ꢁ 0.08
5
I /I ratio ∼1 ꢁ 10 . This trend was also observed in silole
on off
44,46
ꢁ
1192
2186
2350
0.04
polymers TS6T1 and TS6T2 (Figure 1).
However, the
F(000)
1456
solubility of polymer P2 is poor due to lack of long alkyl subs-
tituents. Thus, long alkoxyl chains were introduced in polymer
P3 to enhance the solubility. However, note that the TFT
performance of P3 is similar to that of polymer P2 with a mobility
measd reflns
10048
13029
6672
independent reflns
2855
a
R(F) (%)
0.0666
0.1091
0.0450
0.2432
ꢀ
4
2
2
b
of 3 ꢁ 10 cm /(V s) and an I /I ratio ≈ 10. This is perhaps
R (wF ) (%)
0.0459
2
0.4306
on off
a
b
2
2 2
not surprising because the P3 films are amorphous, even after high
temperature annealing. In contrast, polymer P4 containing a more
rigid building block in the backbone exhibits superior TFT perfor-
R = ∑ F
o
| ꢀ |F
c
/ ∑|F
) ]} ; ω = 1/[σ (F
o
|. R(ωF ) = {∑[ω(F
o
o
ꢀ F
c
) ]/
2
2
1/2
2
2
2
2
∑
(
[ω(F
o
) þ (aP) þ bP], P = [2F
c
þ max-
F
o
,0)]/3.
2
2
mance with a mobility of ∼0.001 cm /(V s) and I /I ≈ 1 ꢁ 10
on off
after annealing at 110 °C for 2 h, and ∼0.002 cm /V s and
2
3
2
3
p-channel semiconductors. It is not surprising that compound
SM5 is an inactive charge-transporting material since the long Si-
alkyl chains prevent close πꢀπ stacking of the cores as discussed
above. In contrast, the SM4 silacyclohexane structure allows close
πꢀπ stacking, and the charge transport is significantly enhanced
I /I ≈ 10 ꢀ10 after annealing at higher temperatures
on off
(190 °C, 2 h). Comparing the device performance of these
p-channel polymers, the general trends are that the mobility
increases as more thiophene rings are introduced into the backbone
(polymers P1 and P2) and when a more rigid building block is used
(polymers P3 and P4). Nevertheless, the amorphous morphology
of these polymer films leads to modest TFT device performance.
ꢀ
4
2
with TFTs exhibiting mobilities of ∼3 ꢁ 10 cm /(V s) and I /
on
5
I
off
ratios of ∼1 ꢁ 10 on OTS-treated substrates. Note that all
films deposited at 80 °C are FET-inactive, in agreement with the
AFM morphology results discussed above. The introduction of
R,ω-hexyl substituents in molecule SM2 is detrimental to the
charge transport capacity and negligible performance is observed in
Top contact/bottom gate TFTs fabricated with polymer P5 exhibit
2
an electron mobility of 0.0001 cm /(V s) and an I /I ratio
on off
2
∼1 ꢁ 10 after annealing at 120 °C for 2 h. The mobility increases
2
to 0.004 cm /(V s) after annealing the film at 150 °C for 2 h.
2
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Chemistry of Materials
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However, thermal annealing at higher temperature (200 °C, 2 h)
(100 mL). The mixture was then extracted with ethyl ether (100 mL ꢁ 3),
and the organic portions were combined, washed with water (100 mL ꢁ
2
decreases the mobility (0.002 cm /(V s)). Importantly, the device
performance is consistent with the microstructures and morphol-
ogies as deduced by XRD and AFM as discussed above.
4
3 times), and dried over MgSO . After filtration, the solvent was removed
by rotary evaporation, and the crude product was purified by column
chromatography (silica gel, CH Cl :hexane = 1:4) to give a light yellow
2
2
liquid. Yield: 1.41 g, 60%. Anal. Calcd for C H S Si: C, 56.36%; H, 4.30%.
11 10 2
’
CONCLUSIONS
1
Found: C, 57.17%; H, 4.54%. H NMR (CDCl
3
, 500 MHz, 21 °C): δ 7.24
A new series of silole-containing oligomers SM2ꢀSM5 and
(
4
1
d, 2H, J = 5 Hz); 7.19 (d, 2H, J = 5 Hz); 2.39 (hept, 4H, J = 9 Hz); 1.60 (t,
13
copolymers P1ꢀP5 with silacycloalkyl substituents has been
synthesized and characterized by DSC, TGA, XRD, solution/
thin film optical spectroscopy, photoluminescence, and electro-
chemical measurements. The charge-transporting properties of
the oligomers were characterized in FET devices, which reveal
that the charge transporting efficiency is significantly enhanced
by introducing cycloalkyl substituents at the oligomer silicon
atoms. Oligomer SM5 with two Si-n-hexyl substituents is not
FET-active, whereas the mobilities of SM4 and SM3 FETs are
H, J = 9 Hz). C NMR (CDCl , 125.6 MHz, 21 °C): δ 149.9; 139.4;
3
29.5; 125.7; 18.9; 15.
0
0
Synthesis of 3,3 -Cyclobutylsilylene-2,2 -bithiophene (4).
A THF (15 mL) solution of dibromo-bithiophene (1.14 g, 3.5 mmol)
was added dropwise to a ꢀ78 °C THF (50 mL) solution of n-BuLi (8.5
mmol) dropwise vigorous stirring over a period of 30 min. The mixture
was then stirred at ꢀ78 °C for 1 h to give a white suspension. Next, a THF
(20 mL) solution of dichlorocyclotetramethylsilane was added dropwise
over 5ꢀ10 min. The reaction mixture was stirred at ꢀ78 °C and gradually
warmed up to room temperature. After stirring overnight, the reaction
ꢀ4
ꢀ4
2
2
.6 ꢁ 10 and 3.4 ꢁ 10 cm /(V s), respectively. These results
are also supported by single-crystal structure determinations showing
that the intermolecular πꢀπ packing distances decrease substantially
on introducing less encumbered silacycloalkyl substituents, and
melting point studies that indicate SM4 and SM3 have more
dense intermolecular πꢀπ stacking than SM5. The charge
transporting properties of the silole-containing copolymers were
also studied in FETs. The highest measured mobility was that
4
mixtue was quenched with saturated aqueous NH Cl solution (100 mL).
The mixture was then extracted with ethyl ether (100 mL ꢁ 3). The organic
portions were then combined, washed with water (100 mL ꢁ 3 times), and
dried over MgSO4. After filtration, the solvent was removed by rotary
evaporation, and the crude product was purified by column chromatogra-
phy (silica gel, CH Cl :hexane =1:4) to give a light yellow liquid. Yield: 450
2
2
mg, 52%. Anal. Calcd for C H S Si: C, 58.01; H, 4.87%. Found: C, 58.70;
1
2 12 2
1
ꢀ
3
2
H, 4.95. HNMR(CDCl , 500 MHz, 21 °C): δ7.27 (d, 2H, J= 5 Hz); 7.12
d, 2H, J = 5 Hz); 1.94 (m, 4H); 1.07 (m, 4H). C NMR (CDCl ,
3
3
of copolymer P4 at 2 ꢁ 10 cm /V s, lower than that of other
3
13
(
silole-containing polymers with long Si-alkyl substituents, im-
plying that the solubility and self-assembly functions of alkyl
substituents are also important for optimizing the mobility of
polymeric semiconductors. These experimental and theoretical
results afford better fundamental understanding of the functions
of the alkyl substituents in the small molecule and polymeric
semiconductors, and provide strategic guidelines for designing
high mobility, environmentally stable organic semiconductors.
1
25.6 MHz, 21 °C): δ 149.7; 141.0; 129.4; 125.6; 27.5; 9.65. High-
resolution MS: (M þ H) 249.0232.
0
0
Synthesis of 3,3 -Cyclopentanylsilylene-2,2 -bithiophene
(5). A THF (20 mL) solution of dibromo-bithiophene (1.62 g, 5.00 mmol)
was added dropwise to a dry ꢀ78 °C THF (50 mL) solution of n-BuLi
(10 mmol) with vigorous stirring over a period of 30 min. The mixture was
then stirred at ꢀ78 °C for 1 h to give a white suspension. A THF (20 mL)
solution of dichlorocyclopentamethylsilane was next added dropwise in
5
ꢀ10 min. The reaction mixture was then stirred at ꢀ78 °C and gradually
warmed up to room temperature. After overnight, the reaction was
quenched by saturated aqueous NH Cl solution (100 mL). The mixture
’
EXPERIMENTAL SECTION
All reagents were purchased from commercial sources and used
4
was then extracted with ethyl ether (100 mL ꢁ 3). The organic portions
were combined and washed with water (100 mL ꢁ 3 times) and dried over
MgSO4. After filtration, the solvent was removed, and the crude product
was purified by column chromatography (silica gel, CH Cl :hexane =1:4)
without further purification unless otherwise noted. Anhydrous THF
was distilled from Na/benzophenone. Conventional Schlenk techniques
were used, and reactions were carried out under N2 unless otherwise
noted. Optical spectra were recorded on a Cary Model 1 UVꢀvis
spectrophotometer. Fluorescence measurements were recorded on a
Photon Technology International model QM-2 fluorimeter. NMR
2
2
to give a light yellow liquid. 600 mg, 46%. Anal. Calcd for C H S Si: C,
13 14 2
1
5
9.49; H, 5.38. Found: C, 58.80; H, 5.14. H NMR(C
6 6
D , 500 MHz,
1
2
5 °C): δ 7.24 (d, 2H, J = 5 Hz); 7.17 (d, 2H, J = 5 Hz); 2.01 (m, 4H); 1.67
spectra were recorded on a Varian Unity Plus 500 spectrometer ( H,
13
13
(m, 2H); 0.97 (t, 4H, J = 7 Hz). C NMR (C D , 125.7 MHz, 25 °C): δ
5
00 MHz; C, 125 MHz). GPC analyses of polymer samples were
6 6
149.5; 141.8; 129.7; 124.5; 29.8; 25.2; 11.0
performed on a Waters Alliance GPCV 2000 (3 columns, Waters
Styragel HT 6E, HT 4, HT 2; operation temperature, 150 °C; mobile
phase, 1,2,4-trichlorobenzene or THF at room temperature; flow rate,
0
0
0
Synthesis of 5,5 -Dibromo-3,3 -cyclobutylsilylene-2,2 -
bithiophene (7). NBS (1.35 g, 7.56 mmol) was added to a DMF
(20 mL) solution of 4 (750 mg, 3.02 mmol) in one portion. The mixture
was stirred at room temperature overnight to give a brownish-yellow
solution, which was quenched with water (50 mL). The mixture was then
extracted with ether (200 mL ꢁ 3) to give a yellow ether solution, which
1
mL/min) and are reported relative to polystyrene standards purchased
from Aldrich. Electrospray mass spectrometry was performed with a
Thermo Finnegan model LCQ Advantage mass spectrometer. Electro-
chemistry was performed on a C3 Cell Stand electrochemical station
equipped with BAS Epsilon software (Bioanalytical Systems, Inc.,
Lafayette, IN).
4
was dried over MgSO overnight. After filtration, the solvent was removed
by rotary evaporation to give a brownish-yellow solid. The crude product
was purified by column chromatography using hexane as eluent to give a
yellow solution. The solvent was removed to give a light yellow solid, which
was recrystallized from hexane to give light yellow crystals. Yield: 1.13 g,
0
0
Synthesis of 3,3 -Cyclopropylsilylene-2,2 -bithiophene
3). A THF (40 mL) solution of dibromo-bithiophene (3.24 g, 10.0 mmol)
(
was added to a dry THF (40 mL) solution of n-BuLi (9 mL, 22.5 mmol)
dropwise with vigorous stirring over a period of 30 min. The mixture was
then stirred at ꢀ78 °C for 1 h. to give a white suspension. A THF (20 mL)
solution of dichlorocyclotrimethylsilane (1.6 g, 11.3 mmol) was next added
dropwise over 5ꢀ10 min. The reaction mixture was stirred at ꢀ78 °C and
gradually warmed up to room temperature. After stirring overnight, the
92%; Anal. Calcd for C12
H
10Br
2
S
2
Si: C, 35.48; H, 2.48; S, 15.79. Found: C,
1
35.59; H, 2.54; S, 15.63. HNMR(C , 300 MHz, 25 °C): δ7.02 (2H, s);
6 6
D
13
1.85 (4H, m); 0.99 (4H, t, J =7 Hz). C NMR (C D , 125.7 MHz, 25 °C):
6 6
δ 149.3; 140.2; 131.9; 111.8; 27.3; 9.39.
0
0
Synthesis of 5,5 -Dibromo-3,3 -cyclopentanylsilylene-
0
reaction mixture was quenched with a saturated aqueous NH Cl solution
2,2 -bithiophene (8). NBS (1.07 g, 6.00 mmol) was added to a
4
2
196
dx.doi.org/10.1021/cm200009k |Chem. Mater. 2011, 23, 2185–2200

Chemistry of Materials
ARTICLE
DMF (20 mL) solution of 5 (600 mg, 2.29 mmol) in one portion. The
mixture was then stirred at room temperature overnight to give a
brownish-yellow solution, which was quenched with water (50 mL).
The mixture was then extracted with ether (200 mL ꢁ 3) to give a yellow
(165 mg, 0.51 mmol), and Pd(PPh
3
)
2
Cl
2
(5 mg) was heated at 90 °C.
After 1 h, an orange precipitate formed. After continuing overnight, a
large quantity of orange precipitate was obtained, which was collected by
filtration and washed with hexane to give an orange solid. Yield: 100 mg,
69%.; Anal. Calcd for C H S Si: C, 58.94; H, 3.75; S, 32.56. Found: C,
solution, which was dried over MgSO overnight. After filtration, the
4
29 22 6
1
solvent was removed by rotary evaporation to give a brownish-yellow
solid. The crude product was purified by column chromatography (silica
gel) using hexane as eluent to give a yellow solution. The solvent was
removed to give a light yellow solid, which was recrystallized from
hexane to give a light yellow solid. Yield: 900 mg, 94%. Anal. Calcd for
58.86; H, 3.87; S, 32.36. H NMR (CDCl , 500 MHz, 21 °C) δ 7.23
3
(2H, d, J = 5 Hz); 7.22 (2H, s); 7.19 (d, J = 4 Hz); 7.10 (2H, d, J = 4 Hz);
7.08 (2H, d, J = 4 Hz); 7.04 (2H, q, d = 4 Hz); 2.01 (4H, m, 2CH
2
); 1.67
1
3
(2H, m, CH ); 1.00 (4H, t, J = 6 Hz, 2CH ). C NMR (C D , 125.7
2
2
6 6
MHz, 25 °C) δ 147.9; 143.0; 138.1; 137.2; 136.2; 135.9; 128.0; 126.4;
124.5; 124.4; 124.0; 123.6; 29.; 25.5; 11.1. Mp: 244ꢀ246 °C.
C
13
H
12Br
2
S
2
Si: C, 37.15; H, 2.88; S, 15.26. Found: C, 37.46; H, 2.93; S,
4.90. H NMR(C , 300 MHz, 25 °C): δ 7.09 (2H, s); 1.94 (4H, m);
.62 (2H, m); 0.92 (4H, t, J = 7 Hz). C NMR (C D , 125.7 MHz,
1
0
0
00
0
000 00
1
1
2
6
D
6
Synthesis of 3,3 -Dihexylsilylene-2,2 :5,2 :5 ,2 :5 ,
0000 000 00000
13
6
6
2
:5 ,2 -sexithiophene (SM5). A DMF (2 mL) solution of
5 °C): δ 149.1; 140.8; 132.2; 111.6; 29.4; 25.4; 10.9.
Synthesis of 5 ,5 -Dihexyl-3,3 -cyclopentanylsilylene-
bistributylthiophene (200 mg, 0.200 mmol), bromobithiophene 9
(98 mg, 0.400 mmol), and Pd(PPh Cl (10 mg) was heated at
90 °C. After continuing overnight, a dark red solution was obtained.
The solvent was removed under vacuo to give a dark red oil, which was
purified by column chromatography (silica gel, CH Cl :hexane = 1:3) to
2 2
give a red solution. The solvent was removed to give dark red oil, which
was recrystallized from hexane to give a dark red solid. Yield, 100 mg, 85%.
0
0
000
0
3
)
2
2
0
00
0
000
2
,2 :5,2 :5 ,2 -tetrathiophene (SM1). A toluene (20 mL) solu-
tion of dibromothiophene 8 (264 mg, 0.63 mmol), hexyl-trimethyl-
tinthiophene (417 mg, 1.26 mmol), and Pd(PPh Cl (5 mg) was
refluxed under N overnight to give a red solution. The reaction mixture
wasquenchedwithwater(50mL),andextractedwithCH Cl (200mLꢁ 2).
3
)
2
2
2
2
2
1
The organic portions were combined, washed with water (200 mL ꢁ 2),
Anal. Calcd for C36
NMR (CDCl
Hz);7.14(2H, s);7.09(2H, d,J= 4 Hz); 7.06 (2H, d, J= 4 Hz); 7.04 (2H, q,
H
38
S
6
Si: C, 62,56; H, 5.54. Found: C, 62.35; H, 5.63. H
and dried over MgSO overnight. After filtration, the solvent was
4
3
, 500 MHz, 21 °C): δ 7.24 (2H, d, J = 5 Hz); 7.19 (d, J = 4
evaporated to give a dark red oil, which was purified by column chromatog-
raphy (silica gel, CH Cl :hexane =1:10) to give a red solution. The solvent
13
d = 4 Hz); 1.21ꢀ1.41 (16H, m); 0.86ꢀ0.96 (10H, m). C NMR (C
6 6
D ,
2
2
was removed to give dark red oil. Yield: 300 mg, 80%. Anal. Calcd
125.7 MHz, 25 °C): δ 147.7; 143.2; 137.9; 137.2; 136.4; 135.7; 127.9; 126.4;
124.6; 124.4; 123.8; 123.6; 32.9; 31.4; 24.1; 22.6; 14.1; 11.8. Mp: 134ꢀ136 °C.
Synthesis of P1. A THF solution (40 mL) of dibromothiophene 8
(280 mg, 0.67 mmol), bistrimethyltinthiophene (278 mg, 0.67 mmol),
and Pd(PPh ) Cl (2 mg) was refluxed under N for 7 days to give a dark
1
C
33
H
42
S
4
Si: C, 66.61; H, 7.11. Found: C, 67.01; H, 7.20. H NMR
(
CDCl , 500 MHz, 21 °C): δ 7.14 (2H, d, J = 5 Hz); 6.97 (2H, d, J = 5
3
Hz; 6.69 (2H, s); 2.81 (4H, t, J = 7 Hz); 2.00 (4H, m); 1.70 (6H, m);
1
.41ꢀ1.28 (m, 12H); 0.99ꢀ0.92 (m, 10H). MALDI: 594.2 (100%); 595.2
3
2
2
2
(36.2%); 596.2 (21.6%).
purple solution. The reaction mixture was then cooled to room temperature
and poured into methanol (500 mL). The resulting black solid was collected
by filtration. The black solid was dissolved in THF again and precipitated with
methanol, and this procedure was repeated twice more to give a dark brown
solid. 200 mg, 87%. M , 4.9 KDa; PDI, 1.13; Anal. Calcd for C H S Si: C,
0
000 00000
0
Synthesis of 5 ,5 -Dihexyl-3,3 -cyclopentanylsilylene-
,2 :5,2 :5 ,2 :5 ,2 :5 ,2 -sexithiophene (SM2). A to-
0 00 0 000 00 0000 000 00000
2
luene solution (20 mL) of dibromothiophene 8 (420 mg, 1.00 mmol),
hexyl-trimethyltinbithiophene (826 mg, 2.00 mmol), and Pd(PPh ) Cl
3
2
2
w
18 14 3
1
(
5 mg) was refluxed under N
reaction mixture was then quenched with water (50 mL). The mixture
was extracted with CH Cl
(200 mL ꢁ 2) and the organic portions were
combined, washed with water (200 mL ꢁ 2), and dried over MgSO
2
overnight to give a red solution. The
59.60; H 4.12. Found: C, 60.31; H, 6.50. H NMR (C
25 °C): δ 7.24 (2H, brm, ArH); 7.11 (2H, brm, ArH); 2.01 (4H, brm,
2CH ); 1.61 (2H, brm, CH ); 1.01 (4H, brm, 2CH ).
Synthesis of P2. A THF solution (50 mL) of dibromothiophene 8
D Cl , 400 MHz,
2 2 4
2
2
2
2
2
4
0
0
overnight. After filtration, the solvent was evaporated to give a dark red
2
solid, which was purified by column chromatography (silica gel, CH Cl :
(420 mg, 1.00 mmol), 5,5 -bis(trimethylstannyl)-2,2 -bithiophene (492 mg,
1.00 mmol), and Pd(PPh ) (40 mg) was refluxed under N for 5 days to
2
3
4
2
hexane =1:4) to give a red solution. The solvent was removed to give a
dark red solid. Yield: 600 mg, 79%. Anal. Calcd C H S Si: C, 64.86; H,
give a purple solution. The mixture was then poured into methanol
(500 mL), and the resulting precipitate collected and subjected to Soxhlet
extraction with methanol, ethyl acetate, 1,4-dioxane, and chloroform, each for
1 day. The remaining dark red solid was dried under a vacuum. Yield: 200 mg,
59%. Anal. Calcd for C H S Si: C, 59.39; H, 3.80. Found: C, 57.10; H,
4
1 46 6
1
6
.11; S, 25.34. Found: C, 64.62; H, 6.20; S 24.80. H NMR (CDCl
MHz, 21 °C): δ 7.19 (2H, s); 7.04 (2H, d, J = 4 Hz); 7.01 (2H, d, J = 4 Hz);
.99 (2H, d, J = 4 Hz); 6.70 (2H, d, J = 4 Hz); 2.81 (4H, t, J = 8 Hz); 2.00
3
, 500
6
2
1 16 4
13
1
(
4H, m); 1.68 (6H, m); 1.41ꢀ1.32 (12H, m); 1.00ꢀ0.92 (10H, m).
C
3.63. H NMR (C D Cl , 400 MHz, 100 °C): δ 7.32 (2H, brm, ArH);
2
2
4
NMR (C , 125.7MHz, 25°C): δ146.7; 144.6; 141.8; 137.2; 135.4; 134.5;
1
6
D
6
2 2
7.28 (2H, brm, ArH); 7.16 (2H, brm, CH ); 2.07 (4H, brm,2CH ); 1.67
33.5; 135.2; 125.2; 123.8; 122.9; 122.6; 122.2. Mp: 144ꢀ146 °C.
2 2
(2H, brm, CH ); 1.08 (4H, brm, 2CH ).
0
0
00
0
000 00
Synthesis of 3,3 -Cyclobutylsilylene-2,2 :5,2 :5 ,2 :5 ,
000 000 00000
Synthesis of P3. A toluene solution (20 mL) of bithiophene 8
0
0
0
0
2
:5 ,2 -sexithiophene (SM3). A DMF (5 mL) solution of
(90 mg, 0.21 mmol), 3,3 -dodecoxy-5,5 -bis(trimethylstannane)-2,2 -
bithiophene (185 mg, 0.21 mmol), and Pd(PPh ) Cl (2 mg) was
dibromothiophene 7 (102 mg, 0.25 mmol), trimethyltin bithiophene
165 mg, 0.51 mmol), and Pd(PPh ) Cl (5 mg) was heated at 90 °C
3
2
2
(
heated at 95 °C for 2 days and then cooled to room temperature. The
mixture was next poured into a mixture of methanol (500 mL) and HCl
(5 mL, 12 M). The mixture was stirred at room temperature for 4 h and
then filtered to give a dark purple solid. The solid was then subjected to
Soxhlet extraction with methanol, acetone, ethyl acetate, each for 1 day.
3
2
2
overnight. After cooling to room temperature, a large amount of orange
precipitate was obtained, which was collected by filtration and washed
with hexane to give an orange solid. Yield: 80 mg, 55%. Anal. Calcd
1
C H S Si: C, 58.29; H, 3.49. Found: C, 58.21; H, 3.59. H NMR
2
8
20
6
(
(
4
1
1
CDCl
3
, 500 MHz, 21 °C): δ 7.23 (2H, d, J = 5 Hz); 7.18 (d, 4 Hz); 7.14
The solid was extracted with CHCl and concentrated, and then added
to methanol (500 mL) dropwise to give a dark blue precipitate, which
was collected by filtration and dried under a vacuum to give a metallic
3
2H, s);7.09 (2H, d, J = 4 Hz); 7.06 (2H, d, J = 4 Hz); 7.03 (2H, q, d =
Hz); 1.89 (4H, m, 2CH ); 1.06 (4H, m, 2CH ). C NMR (C D ,
13
2 2 6 6
25.7 MHz, 25 °C): δ 148.1; 142.3; 138.4; 137.1; 136.2; 135.8; 127.9;
26.1; 124.5; 124.4; 123.9; 123.6; 27.4; 9.59. Mp: 236ꢀ238 °C.
dark blue solid. Yield: 100 mg, 71%. M , 32 KDa; PDI, 6.0. Anal.
w
1
Calcd for C H O S Si: C, 68.13; H, 8.13. Found: C, 67.44; H, 8.01. H
4
5 66 2 4
0
0
00
0
000
Synthesis of 3,3 -Cyclopentanylsilylene-2,2 :5,2 :5 ,2 :
0000 000 00000
NMR (CHCl
3
, 500 MHz, 25 °C): δ 6.94 (2H, brm, ArH); 6.93 (2H, brm,
0
0
5
,2 :5 ,2 -sexithiophene (SM4). A DMF solution (12 mL)
ArH); 4.19 (4H, brm, CH ); 2.01 (4H, brm, CH ); 1.93 (4H, brm, CH );
1.59ꢀ1.25 (36H, brm, CH ); 0.89ꢀ0.87 (10H, brm, CH þCH ).
2
2
2
3
1
of dibromothiophene 8 (105 mg, 0.25 mmol), trimethyltin bithiophene
C
2
2
3
2
197
dx.doi.org/10.1021/cm200009k |Chem. Mater. 2011, 23, 2185–2200

Chemistry of Materials
ARTICLE
NMR (CHCl
3
, 125.7 MHz, 25 °C): δ 150.95; 146.5; 141.8; 138.1; 137.3;
electrodes (30 nm) were deposited by thermal evaporation with a W/
L ratio of 10. Semiconductors were prepared as 7 mg/mL solutions in 1,2-
dichlorobenzene and were filtered through a 0.22 μm PTFE syringe filter.
The semiconductor films were deposited by spincoating at 1500 rpm and
annealed in a vacuum oven maintained at 110 or 190 °C for 2 h. For the
1
2
31.7; 125.0; 124.2; 112.4; 111.0; 110.3;.71.1; 30.9; 30.6; 28.7; 28.5; 28.4;
8.3; 25.2; 25.2; 24.5; 21.7; 21.6; 13.1; 10.1.
Synthesis of P4. A toluene solution (15 mL) of bibromobithio-
phene 8 (105 mg, 0.25 mmol), 2,6-bis(trimethyltin)-4,8-dioctylbenzo-
0
[
1,2-b:4,5-b ]dithiophene (185 mg, 0.25 mmol), and Pd(PPh
3
)
2
Cl
2
dielectric, 400ꢀ500 nm thick PMMA (Aldrich, M
w
350K) films were
(
2 mg) was heated at 110 °C under N for 2 days. The reaction mixture
spuncoat on top of the semiconductor film. The dielectric films are dried in a
110 °C vacuum oven for 2 h, and 30 nm gold films were then deposited by
thermal evaporation as the gate electrode. For P5, thin film semiconductor
layers were deposited by spin-coating of a 10 mg/mL polymer solution in
chloroform under ambient conditions onto hexamethyldisilazane (HMDS)-
treated p-doped Si (001) wafers with a 300 nm thermally grown SiO2
2
was then cooled to room temperature and poured into a mixture of
methanol (500 mL) and HCl (5 mL, 12M). This mixture was stirred for
4
h and then filtered to give a dark purple solid. Crude yield, 160 mg. The
solid was subjected to Soxhlet extraction with methanol, acetone, ethyl
acetate, each for one day, and CHCl for 3 days. The residual solid was
3
dissolved in dichlorobenzene and added to methanol (100 mL) to precipitate
it. The product was next collected by filtration. Yield: 100 mg, 52%. Anal.
dielectric layer. The capacitance of the 300 nm SiO gate insulator was
2
ꢀ
2
11 nFcm . Prior to the semiconductor deposition, the wafers were solvent
cleaned by sonicating (in two beakers, sequentially, for 30 s each) in EtOH
and were then dried with a filtered stream of nitrogen, followed by 5 min
plasma cleaning in a Harrick PDC-32G Plasma Cleaner/Sterilizer. After
semiconductor deposition, the films were annealed either at 150 °C under in
vacuum for 30 min or at 200 °C under nitrogen for 1 h. Top-contact OTFTs
1
Calcd for C H S Si: C, 69.00; H, 7.20. Found: C, 69.00; H, 7.40. H NMR
39 50 4
(
CH
C
2
D
2
Cl
4
, 400 MHz, 100 °C): δ 7.5ꢀ7.32 (4H, brm, ArH); 3.24 (4H, brm,
2
ꢀAr); 2.13ꢀ1.46 (34H, brm, 17CH
2
); 1.00 (6H, brm, 2CH
3
)
Synthesis of P5. A toluene solution (5 mL) of bithiophene 8 (140 mg,
.24 mmol), N,N -bis(2-octyldodecyl)- 1,4,5,8-naphthalenedicar-
0
0
ꢀ7
boximide (220 mg, 0.24 mmol), and Pd(PPh ) Cl (2 mg) was heated at
were fabricated by vapor deposition of gold electrodes (∼1 ꢁ 10 Torr,
3
2
2
95 °C for 2 days and then cooled to room temperature. The mixture was
0
.2 Å/s, ∼50 nm thick) onto the semiconductor thin films through a shadow
then poured into a mixture of methanol (500 mL) and HCl (5 mL, 12 M).
This mixture was stirred at room temperature for 4 h and then filtered to give
a dark purple solid. The solid was subjected to Soxhlet extraction with
methanol, acetone, ethyl acetate, each for 1 day. The solid was then extracted
mask to obtain devices with channel widths and lengths of 5000 and 100 μm,
respectively.
OTFT Characterization. IꢀV plots of device performance were
measured under vacuum, and transfer and output plots were recorded
for each device. The currentꢀvoltage (IꢀV) characteristics of the devices
were measured using a Keithley 6430 subfemtoammeter and a Keithly
2400 source meter, operated by a local Labview program and GPIB
communication. Key device parameters, such as charge carrier mobility (μ)
with CHCl
to give a dark blue precipitate, which was collected by filtration and dried
under a vacuum to give a metallic dark blue solid. Yield: 200 mg, 80%. M , 18
3
, concentrated, and then added to methanol (500 mL) dropwise
w
KDa; PDI, 2.26. Anal. fCalcd or C67
H
96
O
4
S
4
Si: C, 74.12; H, 9.09. Found: C,
, 500 M, 25 °C):8.78(2H, brm, ArH);7.44
2H, brm, AH); 4.06 (4H, brm, 2CH ꢀN); 1.99 (8H, brm, 4CH ); 1.55
1
73.77; H, 8.61. HNMR(CHCl
3
and on-to-off current ratio (I /I ), were extracted from the source-drain
on off
(
(
2
2
current (ISD) versus source-gate voltage (VSG) characteristics employing
13
4H, brm, 2CH
, 125.7 MHz, 25 °C): δ 162.2; 161.9; 151.7; 142.3; 141.7;
38.5; 135.8; 131.3; 126.6; 142.2; 120.8.
Single-Crystal Structure Determinations. Single crystals of
2 2 3
); 1.28 (84H, brm, 42CH ); 0.78 (12H, brm, 4CH ); C
standard procedures. Mobilities were obtained from the formula defined by
2
NMR (CHCl
1
3
the saturation regime in transfer plots, μ = 2ISDL/[C
i
W(VSG ꢀ V
T
) ],
where ISD is the source-drain current, VSG is source-gate voltage, and V is
T
the threshold voltage. Threshold voltage was obtained from x intercept of
1/2
SM3, SM4, and SM5 were grown by slow diffusion of hexane into
dichlorobenzene solution. X-ray single crystal diffraction measurements
were performed on a Bruker CCD area detector instrument with graphite-
monochromated MoKR (0.71073 A) radiation. The data were collected at
VSG vs ISD plots. AFM measurements were performed by using a JEOL-
Microscope (JEOL Ltd. Japan) in the tapping mode.
Electrochemistry. Cyclic voltammetry was performed in an elec-
trolyte solution of 0.1 M tetrabutylammonium hexafluorophosphate
þ ꢀ
4 6
Bu N PF ) in dry acetonitrile. Platinum wire electrodes were used as
153(2)K, and the structures were solved by direct methods and expanded
(
using Fourier techniques.
both working and counter electrodes, and Ag wire was used as the pseud-
oreference electrode. A ferrocene/ferrocenium redox couple was used as
an internal standard, and potentials obtained in reference to a silver
electrode were converted to the saturated calomel electrode (SCE)
scale. Thin films of the polymers were coated onto the Pt working
electrode by drop-casting from 0.5 to 1.0 wt % THF solutions and dried
under a vacuum at 80 °C for 2 h.
Device Fabrication and Thin-Film Characterization. Oligo-
mers. Thin film transistors were fabricated in the bottom-gate/top-contact
configuration. Highly doped p-type (100) silicon wafers (<0.004 Ω cm)
were used as gate electrodes as well as substrates, and 300 nm SiO
2
thermally
grown on Si was used as the gate insulator. The unit area capacitance is
2
1
1 nF/cm . The substrate surface was treated with octadecyltrichlorosilane-
(OTS) and hexamethyldisilazane (HMDS) purchased from Aldrich. A few
Electronic Structure Calculations. DFT calculations were car-
9
1
drops of HMDS were loaded inside a self-assembly chamber under an N2
ried out using the Gaussian 03 program. Becke’s three-parameter
blanket. The SiO /Si substrates were exposed to this atmosphere for at least
exchange functional combined with the LYP correlation functional
2
9
2
7.0 days to give a hydrophobic surface. After HMDS deposition, the
(B3LYP) was employed. We also made use of the standard 6-31G**
9
3
advancing aqueous contact angle is 95°. OTS was deposited under an N
2
basis set. Vertical electronic excitation energies were computed using
9
4ꢀ97
blanket inside a self-assembly chamber. The SiO /Si substrates were exposed
the timeꢀdependent DFT (TDDFT) approach.
The twelve low-
2
to ethanol solution for 10 h to give a hydrophobic surface. After OTS
deposition, the advancing aqueous contact angle is 105°. Semiconductor thin
2
films (50 nm) were next vapor-deposited onto the Si/SiO held at well-
est-energy electronic excited states were computed for molecules SM3,
SM4, and SM5. TDDFT calculations were carried out using the B3LYP
functional and the 6-31G** basis set on the previously optimized
molecular geometries obtained at the same level of calculation.
defined temperatures of 25 or 80 °C with a deposition rate of 0.3 Å/s at
ꢀ
6
6
ꢁ 10 Torr, employing a high-vacuum deposition chamber (Denton
Vacuum, Inc., USA). Gold source and drain electrodes (50 nm) were vapor-
’
AUTHOR INFORMATION
ꢀ
6
deposited at 2 ꢁ 10 Torr through a shadow mask in the vacuum
deposition chamber. Devices were fabricated with typical channel lengths
of 50 and100 μm, and a channel width of 2000 μm.
Corresponding Author
*To whom correspondence should be addressed. E-mail: Prof.
Antonio Facchetti (email: a-facchetti@northwestern.edu), Prof.
Tobin J. Marks (email: t-marks@northwestern.edu).
Polymers. For P1ꢀP4, OTFTs were fabricated on glass substrates
(
Precision Glass & Optics, Eagle 2000). Gold source and drain
2
198
dx.doi.org/10.1021/cm200009k |Chem. Mater. 2011, 23, 2185–2200

Chemistry of Materials
ARTICLE
’
ACKNOWLEDGMENT
(30) Zhan, X.; Risko, C.; Amy, F.; Chan, C.; Zhao, W.; Barlow, S.;
Kahn, A.; Bredas, J.-L.; Marder, S. R. J. Am. Chem. Soc. 2005, 127, 9021.
We thank Polyera Corp. and ONR AFOSR (FA9550-08-1-0331)
(
31) Tamao, K.; Uchida, M.; Izumizawa, T.; Furukawa, K.; Yamaguchi,
S. J. Am. Chem. Soc. 1996, 118, 11974.
32) Yamaguchi, S.; Goto, T.; Tamao, K. Angew. Chem., Int. Ed.
000, 39, 1695.
33) Zhan, X.; Haldi, A.; Risko, C.; Chan, C. K.; Zhao, W.;
for support of this research at Northwestern University. We thank
the NSF-MRSEC program through the Northwestern Materials
Research Center (DMR-0520513) for support of characteriza-
tion facilities. The research leading to these results has received
funding from the European Community’s Seventh Framework
Programme under Grant Agreement 234808.
(
2
(
Timofeeva, T. V.; Korlyukov, A.; Antipin, M. Y.; Montgomery, S.;
Thompson, E.; An, Z.; Domercq, B.; Barlow, S.; Kahn, A.; Kippelen,
B.; Bredas, J.-L.; Marder, S. R. J. Mater. Chem. 2008, 18, 3157.
(
(
34) Yamaguchi, S.; Tamao, K. Bull. Chem. Soc. Jpn. 1996, 69, 2327.
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