Synthesis and characterization of soluble low-bandgap oligothiophene-[all]- S,S-dioxides-based conjugated oligomers and polymers

Article abstract of DOI:10.1002/pola.24641

The synthesis and characterization of a new family of soluble oligothiophene-S,S-dioxides and their use as building blocks to form polythiophene-S,S-dioxides via microwave-assisted Stille coupling polymerization are described. Incorporation of the sulfone group into the polythiophene backbone leads to narrowing of the polymer bandgap, and while the energies of both Frontier orbitals in polythiophene-S,S-dioxide are lower with respect to polythiophenes, this tendency is considerably stronger for the lowest unoccupied molecular orbital than for the highest occupied molecular orbital, resulting in greater electron-accepting ability.

Full text of DOI:10.1002/pola.24641

Synthesis and Characterization of Soluble Low-Bandgap Oligothiophene-
[all]-S,S-dioxides-Based Conjugated Oligomers and Polymers
ELIZABETH AMIR,1,2 KULANDAIVELU SIVANANDAN,1,2 JUSTIN E. COCHRAN,3 JOHN J. COWART,2,3
SUNG-YU KU,1,2 JUNG HWA SEO,4 MICHAEL L. CHABINYC,1,2,4 CRAIG J. HAWKER1,2,3,4
1Materials Research Laboratory, University of California, Santa Barbara, California 93106-9510
2Mitsubishi Chemical Center for Advanced Materials, University of California, Santa Barbara, California 93106-9510
3Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106-9510
4Materials Department, University of California, Santa Barbara, California 93106-9510
Received 31 January 2011; accepted 22 February 2011
DOI: 10.1002/pola.24641
Published online 15 March 2011 in Wiley Online Library (wileyonlinelibrary.com).
ABSTRACT: The synthesis and characterization of a new family
of soluble oligothiophene-S,S-dioxides and their use as build-
ing blocks to form polythiophene-S,S-dioxides via microwave-
assisted Stille coupling polymerization are described. Incorpo-
ration of the sulfone group into the polythiophene backbone
leads to narrowing of the polymer bandgap, and while the
energies of both Frontier orbitals in polythiophene-S,S-diox-
ide are lower with respect to polythiophenes, this tendency is
considerably stronger for the lowest unoccupied molecular
orbital than for the highest occupied molecular orbital, result-
C
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ing in greater electron-accepting ability.
2011 Wiley Periodi-
cals, Inc. J Polym Sci Part A: Polym Chem 49: 1933–1941,
2011
KEYWORDS: conjugated polymers; crystal structures; oligomers;
synthesis; UV–vis spectroscopy
INTRODUCTION The development of solution-processable
small molecules and polymeric materials with good electron-
transporting properties is highly desired for various applica-
tions in organic-based electronics. The vast majority of mate-
rials known today exhibit p-channel (hole-transporting)
behavior.1 In direct contrast, air-stable n-channel (electron-
transporting) semiconductors and polymeric materials that
show solid-state transport of both hole and electron carriers
erties of oligo and polythiophenes to be tuned from p-type
to n-type. These findings triggered investigations into the
synthesis of various chemical architectures containing par-
tially oxygenated oligothiophene-S,S-dioxides, their character-
ization, and applications in electronic devices,10 with one no-
ticeable example being the synthesis of oligothiophene-[all]-
S,S-dioxides.11 In comparison with their partially oxygenated
counterparts, complete dearomatization of the oligothio-
phenes resulted in a significant redshift of the absorption
maxima, indicating a considerable narrowing of the energy
gap. Theoretical calculations in combination with electro-
chemical measurements indicated that the energies of the
Frontier orbitals of oligothiophene-[all]-S,S-dioxides were
reduced, with the values of the LUMO dropping almost twice
as much as the highest occupied molecular orbital (HOMO).
Despite their promising characteristics, the use of oligothio-
phene-[all]-S,S-dioxides in applications was limited due to
their poor solubility in common organic solvents.
are significantly less developed. Consequently,
a major
research challenge in this field is to synthesize electron-poor
building blocks having the energy of the lowest unoccupied
molecular orbital (LUMO) decreased to such an extent that
the compound is easily reduced and capable of acting as an
n-channel device.2 It was shown that incorporation of elec-
tron-withdrawing groups such as fluorine3 and/or car-
bonyl4,5 into oligo and polythiophene skeletons can increase
the n-type character of the corresponding device. Building
on this strategy, thiophene-S,S-dioxides6,7 have been exam-
ined as electron-poor units8 with Barbarella and coworkers
reporting the preparation of partially oxygenated oligothio-
phenes where transformation of one or several aromatic thi-
ophene rings into the corresponding S,S-dioxides inserts a
strong electron-withdrawing group into the p-conjugated
main chain.9 This leads to an increase in electron-affinity,
In this study, we describe the synthesis of bi-, ter-, and
quater-thiophenes incorporating solubilizing alkyl groups in
the oligomer skeleton and the subsequent use of these sys-
tems as modular building blocks for the preparation of con-
jugated systems based on thiophene-S,S-dioxides. Oxygen-
ation of these oligothiophenes using the acetonitrile complex
of hypofluorous acid, HOFꢀCH3CN, resulted in the formation
of a new family of oligothiophene-[all]-S,S-dioxides that are
electron delocalization, and
a reduction in the optical
bandgap of the oligothiophene, allowing the transport prop-
Correspondence to: C. J. Hawker (E-mail: hawker@mrl.ucsb.edu)
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Journal of Polymer Science Part A: Polymer Chemistry, Vol. 49, 1933–1941 (2011) 2011 Wiley Periodicals, Inc.
SYNTHESIS AND CHARACTERIZATION OF OLIGOTHIOPHENE-S,S-DIOXIDES, AMIR ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
thiophene derivatives 5a and 5b, which could be fully oxi-
dized to form terthiophene-[all]-S,S-dioxides 6a and 6b in 65
and 60% yield, respectively (Scheme 2). It should be noted
that HOFꢀCH3CN, while an extremely strong oxidizing rea-
gent, is selective and allows oligomers 3a, 3b, 6a, and 6b to
be prepared with terminal CABr groups. A consequence of
this is that these systems are now available for subsequent
transition metal-catalyzed coupling reactions and can be
used as building blocks, leading to more complex conjugated
systems.
SCHEME 1 Synthetic route to soluble bithiophene-[all]-S,S-
dioxides (3a and 3b).
To construct a derivative with four thienyl rings, commer-
cially available 3,30-dihexylquaterthiophene (7) was alkylated
with 1-iodohexane to form the tetra-substituted aromatic
oligomer (8). Significantly, oxygenation with HOFꢀCH3CN
could be carried out at room temperature using a twofold
excess of the reagent, yielding compound 9 after introduc-
tion of eight oxygen atoms and dearomatization of all four
thiophene rings in 40% (Scheme 3). Because of the presence
of the long alkyl side chains, all oligothiophene-S,S-dioxides
mentioned above were found to be completely soluble in
common organic solvents such as dichloromethane, chloro-
form, and chlorobenzene. This is in contrast to previous
study,11 where oxygenated oligothiophenes, bearing bro-
mines, methyl, or t-butyl groups only at the terminal carbons
adjacent to the sulfur atoms, were only partially or slightly
soluble in chlorinated solvents.
completely soluble in a variety of organic solvents and can
be easily processed into thin films. Oxygenated oligothio-
phenes were characterized utilizing UV–vis spectroscopy,
thermal gravimetric analysis (TGA), and X-ray diffraction. In
addition, bi- and ter-thiophene-[all]-S,S-dioxides were used
as building blocks to prepare soluble conjugated polythio-
phenes with narrow optical bandgaps.
RESULTS AND DISCUSSION
2-Bromo-3-hexylthiophene (1a) and 2-bromo-3-dodecylthio-
phene (1b) were obtained in near quantitative yield from
reaction of the corresponding 3-alkylthiophenes with N-bro-
mosuccinimide (NBS). Palladium-catalyzed CAH homocou-
pling of 1a and 1b afforded the aromatic bithiophenes 2a
and 2b, bearing alkyl groups in the 4 and 40 positions.12 Oxi-
dation of 2a and 2b was then carried out by adding freshly
prepared HOFꢀCH3CN solution, prepared by bubbling com-
mercial 15% F2 in nitrogen into a mixture of water and ace-
tonitrile at ꢁ15 ꢂC, to the dichloromethane solution of the
aromatic bithiophene at room temperature.13 The reaction
reached completion after 15 min, affording previously
unknown bithiophene-[all]-S,S-dioxides (3a, 3b) in high yield
(Scheme 1).
UV–vis measurements were performed on all the oxygenated
oligothiophenes and their aromatic precursors. We report
here the results for the hexyl-substituted derivatives 2a, 3a,
5a, 6a, 8, and 9. On comparison with the starting aromatic
oligothiophenes that show a broad absorption band, the cor-
responding oligothiophene-[all]-S,S-dioxides exhibit well-
defined vibronic fine structures typical of rigid conjugated
systems.15 Furthermore, the absorption maximum for the
oxygenated bithiophene-S,S-dioxide (3a) was redshifted by
ꢃ63 nm with respect to the aromatic bithiophene (2a). This
trend is maintained for all oligomers with even larger shifts
of 103 and 106 nm for the terthiophene-[all]-S,S-dioxide
(6a) and quaterthiophene-[all]-S,S-dioxide (9), respectively
(Fig. 1).
Using a similar methodology, the soluble terthiophene-based
oligomers 4a and 4b were obtained in good yield from 2,5-
dibromothiophene and Grignard reagents derived from 2-
bromo-3-hexylthiophene 1a and 2-bromo-3-dodecylthio-
phene 1b, using Kumada coupling conditions.14 Bromination
of 4a and 4b with NBS gave the corresponding dibromoter-
The bathochromic shift is fully consistent with the transfor-
mation of the starting oligothiophenes into the
SCHEME 2 Synthesis of terthio-
phene-[all]-S,S-dioxides (6a and
6b).
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SCHEME 3 Synthesis of quater-
thiophene-[all]-S,S-dioxide (9).
corresponding oligothiophene-[all]-S,S-dioxides, leading to
narrowing of the optical bandgap. Optical bandgaps were
estimated by the onset point of the absorption band and are
summarized in Table 1. For all studied oligomers, on dearo-
matization of all thiophenes rings, a reduction of more than
0.5 eV in the energy gap was observed.
The limited structural data on oligothiophene-[all]-S,S-di-
oxides with an exception of terthiophene,11 prompted a
thorough evaluation of molecular structures and cell packing
motifs for the oxygenated oligomers 3a and 9. Bithiophene-
S,S-dioxide (3a) crystallizes in a triclinic system with a P-1
space group. Thiophene rings exhibit an anti, nearly planar
conformation with the torsion angle between the mean
planes of the rings of 2ꢂ [Fig. 3(A)]. This torsion angle is
smaller than the values reported for the fully oxygenated
oligomer consisted of three thiophene rings.11 Furthermore,
cell packing reveals that oligomer 3a adopts a p–p stacking
arrangement of the molecular chains with an angle of 0ꢂ
between the main planes of the adjacent molecules
[Fig. 3(B)]. The sulfone groups of adjacent oligomers are in
syn conformation relative to each other, and to avoid unfav-
orable electronic interactions, the molecules adopt an align-
ment where a slight displacement along the long molecular
axis takes place. In this packing motif, the shortest intermo-
lecular distance measured between the oxygen atom of one
molecule and the thiophene carbon atom of another mole-
cule is 3.3 Å.
Thermal properties of the oxygenated oligomers were ana-
lyzed by TGA, revealing a two-step degradation process in all
cases. The initial weight loss of ꢃ30% can be attributed to
the elimination of SO2 from all sulfone groups along the oli-
gothiophene backbone. This assumption is supported by a
previous report showing that in sulfone-containing polymers,
due to the relatively weak CAS bonds, thermally induced
chemical disintegration leads to initial loss of SO2 groups.8b
Furthermore, on increasing the length of the oligomer, ther-
mally induced degradation was observedꢂat lower tempera-
tures, with an onset temperature of 164 C for bithiophene-
[all]-S,S-dioxide 3a, 158 ꢂC for terthiophene-[all]-S,S-dioxide
6a, and 144 ꢂC for quaterthiophene 9 (Fig. 2). A rationale
for this trend is the increasing number of the electron-with-
drawing sulfone groups, weakening the neighboring CAS
bonds, resulting in lower decomposition temperatures with
increasing oligomer length.
In comparison, the quaterthiophene-S,S-dioxide (9) crystalli-
zes in a monoclinic system with a C2/c space group. The
conformation of the thiophene-S,S-dioxide rings is again
almost planar with torsion angles of 2.7 and 5.2ꢂ [Fig. 4(A)].
Analysis of the unit cell configuration reveals an interesting
alignment, which is different from the one observed for the
oxygenated bithiophene 3a. Molecular chains are oriented in
an alternating motif with each molecule being parallel to
another molecule and perpendicular to a third one [Fig.
4(B)]. In addition, the parallel chains undergo long-axis
TABLE 1 Absorption Maxima and Optical Bandgaps for the
Starting Oligothiophenes (2a, 5a, and 8) and the
Corresponding Oligothiophene-[all]-S,S-dioxides (3a, 6a, 9)
Compound
kmax (nm)
Eg (eV)
DEg (eV)
2a
3a
5a
6a
8
332
395
348
451
388
494
3.31
2.76
2.95
2.36
2.70
2.14
0.55
0.59
0.56
FIGURE 1 UV–vis spectra, measured in chloroform, for the
bithiophene (2a), bithiophene-[all]-S,S-dioxide (3a), terthio-
phene (5a), terthiophene-[all]-S,S-dioxide (6a), quaterthiophene
(8), and quaterthiophene-[all]-S,S-dioxide (9).
9
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
FIGURE 2 TGA analysis for the oligothiophene-[all]-S,S-di-
oxides, 3a, 6a, and 9.
FIGURE 3 (A) Molecular structure and (B) cell packing diagram
for the 5,50-dibromo-4,40-dihexyl-2,20-bithiophene-[all]-S,S-di-
oxide (3a).
sliding most likely to afford better cofacial stacking and to
minimize steric hindrance from the alkyl chains, allowing
greater interdigitation. A similar perpendicular alignment
trend was previously reported when electron-withdrawing
The availability of well-defined, strongly electron-deficient
oligomers now permits their use as building blocks for
the construction of conjugated polymeric materials to be
examined. Bromine atoms at the terminal carbons of the
fluorine atoms were incorporated into
skeleton.16
a
pentacene
FIGURE 4 (A) Molecular structure
and (B) cell packing diagram for the
quaterthiophene-[all]-S,S-dioxide (9).
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SCHEME 5 Chemical structures for the polymers, P2 and P4.
between the polymers P1 and P2, we focused on the dodecyl
derivative P2 in this work. For direct comparison, polymer,
P4, with all aromatic thiophene rings was synthesized using
similar polymerization conditions. P4 was obtained with
comparable molecular weight (Mn ¼ 11.0 kDa, PDI ¼ 1.4) in
SCHEME 4 Synthetic route to polymers P1–P3.
oligomers 3a, 3b, 6a, and 6b make them suitable candidates
for various transition metal-catalyzed coupling reactions. Ini-
tial attempts to polymerize 5,50-dibromo-4,40-dihexyl-2,20-
bithiophene-[all]-S,S-dioxide (3a) using Pd(PPh3)4 with 2,5-
bistrimethylstannylthiophene (10),17 resulted in low-molecu-
lar weight material (Mn ¼ 3.1 K) that was isolated in 30%
yield after 4 days of reflux in N,N-dimethylformamide. In
contrast to the reactions carried out under conventional
heating, microwave irradiation allows reaction times to be
reduced significantly, yielding products in high yield with
less side products.18 Consequently, by using microwave-
assisted Stille coupling conditions using Pd2(dba)3/P(o-tol)3
as a catalyst system and chlorobenzene as a solvent, we
were successful in increasing molecular weight and product
yield considerably. Polymerization of 3a and 10 now gave
polythiophene-S,S-dioxide, P1, after only 20 min in 92%
yield with Mn ¼ 8.5 K, PDI ¼ 2.7 (Scheme 4). Following the
same reaction conditions, polymer P2 with 4,40-didodecyl-
2,20-bithiophene-[all]-S,S-dioxide incorporated in the back-
bone was formed in 90% yield with higher molecular weight
and lower polydispersity index (Mn ¼ 10.8 K, PDI ¼ 1.9). In
the case of polymerization of terthiophene-S,S-dioxide (6b),
the reaction time was extended to 45 min with the product
polymer, P3, having a relatively high polydispersity index
(PDI ¼ 4.1) and moderate molecular weight Mn ¼ 5.7 kDa.
This is most likely due to the fact that in the precursor
monomer, 6b, carbon atoms that undergo the coupling reac-
tion are more deactivated due to the presence of an addi-
tional electron withdrawing sulfone unit in comparison with
bithiophene-S,S-dioxides. We should emphasize that all the
sulfone-containing polymers are soluble in chloroform or
chlorobenzene, air-stable, and can be stored at room temper-
ature without degradation.
Finally, we examined the influence of incorporation of elec-
tron-withdrawing bithiophene-S,S-dioxide unit in polythio-
phene on the optical bandgap and energy levels of the Fron-
tier molecular orbitals. Because the structural similarity
FIGURE 5 (a) UV–vis spectra, measured in chloroform, for
bithiophene, 2b (green), bithiophene-[all]-S,S-dioxide, 3b
(blue), P4 (orange), P2 (red). (b) HOMO and LUMO energy lev-
els for the as cast P2 and P4.
SYNTHESIS AND CHARACTERIZATION OF OLIGOTHIOPHENE-S,S-DIOXIDES, AMIR ET AL.
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JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
high yield (see Experimental Section). Chemical structures of
the P2 and P4, as well as the precursor monomers are
shown in Scheme 5.
and 5,50-dibromo-4,40-didodecyl-2,20-bithiophene (2b) were
made according to the coupling procedure,12 with reaction
times being extended to 12 h. Preparation of 3,300-dihexyl-
2,20:50,200-terthiophene (4a)23 and 3,300-didodecyl-2,20:50,200-
terthiophene (4b)24 was carried out following Kumada cou-
pling conditions.14 Crystal structures for the compounds 3a
and 9 were deposited at the Cambridge Crystallographic
Data Center and allocated the deposition numbers CCDC
810108 and CCDC 810107, respectively. Flash chromatogra-
phy was performed with EM science 37–75 lm silica gel. An-
alytical thin layer chromatography was performed on EM sci-
ence silica plates with F-254 indicator, and the visualization
was accomplished by UV lamp or using the molybdic acid
stain mixture.
UV–vis absorption spectra in chloroform of the polymers P2
and P4, and their respective monomers, 3b and 2b are
shown in Figure 5(A). In analogy with the prior oligomer
studies, the results show a strong bathochromic shift of
ꢃ60 nm for the bithiophene-[all]-S,S-dioxide building block,
3b, relatively to its aromatic counterpart, 2b. The extended
conjugation length of the polymers leads to a further redshift
as reflected by the absorption maximum at 631 nm for poly-
thiophene-S,S-dioxide P2 compared with 496 nm for aro-
matic polythiophene P4 (Dkmax ¼ 135 nm). The optical
bandgap was estimated by the onset point of the absorption
band and corresponded to 1.55 eV for P2. This value is
lower than the optical bandgap of the aromatic polymer P4
(1.85 eV), owing to the incorporation of the bithiophene-
[all]-S,S-dioxide as an electron-withdrawing subunit along
the polymer backbone.
Instrumentation
All materials were characterized by nuclear magnetic reso-
nance (NMR) spectroscopy using Varian 200-, Varian 400-,
and Varian 500-MHz spectrometers as indicated. Chemical
shifts are reported in ppm and referenced to the solvent
(proton and carbon). Microwave-assisted reactions were con-
ducted on a Biotage Microwave reactor at a frequency of 2.5
GHz. VG70 Magnetic Sector and Waters GCT Premier TOF
instruments were used for low and high resolution mass
analysis by electron ionization (EI). Micromass QTOF2 Quad-
rupole/Time-of-Flight Tandem mass spectrometer was used
for high-resolution mass analysis using electrospray ioniza-
tion (ESI). Gel permeation chromatography (GPC) was per-
formed in THF on a Waters system (Millford, MA, USA)
equipped with four 5-mm Waters columns (300 ꢄ 7.7 mm2)
connected in series with increasing pore size (102, 103, 104,
and 106 Å). Waters 410 differential refractometer index and
Waters 996 photodiode array detectors were used. The mo-
lecular weights of the polymers were calculated relative to
linear PS standards. UV–vis spectra were recorded on Agilent
8453 spectrophotometer using chloroform solutions.
Ultraviolet photoelectron spectroscopy (UPS) was then used
to obtain the energy of the HOMO of the polymers.19,20 UPS
measurements revealed that the HOMO energy levels for the
as cast polymers P2 and P4 are 5.58 and 4.90 eV, respec-
ꢂ
tively, [Fig. 5(B)]. In addition, annealing at 110 C for 5 min
led to a change in the HOMO energies in both polymers to a
value of 5.77 eV (P2) and 4.97 eV (P4). This result is
expected, because on annealing, conducting polymers typi-
cally show slight changes to the HOMO, associated with an
increase in crystallinity.20 LUMO values were estimated by
using the optical gap and HOMO energies (Formula 1).19
ELUMO ¼ Eg ꢁ EHOMO
(1)
The LUMO energy of the aromatic polythiophene P4 is 3.05
eV, while the LUMO of the polythiophene-S,S-dioxide deriva-
tive P2 is lower by about 1 eV (4.03 eV).
General Procedure for Producing HOFꢀCH3CN Complex
Mixture of 20% F2 with nitrogen was used in this work. It
was passed at a rate of about 400 mL per minute through a
cold (ꢁ15 ꢂC) mixture of CH3CN (60 mL) and H2O (6 mL).
The development of the oxidizing power was monitored by
reacting aliquots with an acidic aqueous solution of KI. The
liberated iodine was then titrated with thiosulfate. Typical
concentrations of the oxidizing reagent were around 0.4–
0.6 M.
Significantly, these results indicate that the energies of the
Frontier orbitals in polythiophenes can be fine-tuned by the
incorporation of sulfones in the repeating unit. Furthermore,
the presence of the electron-withdrawing S,S-dioxide group
has a stronger effect on the lowering of the LUMO than the
HOMO level, leading to potentially better electron-accepting
properties for the thiophene-S,S-dioxide containing polymers
with respect to the aromatic polythiophenes. These materials
show promise when compared with traditional fullerene
structures21 in terms of modularity and tuning of electronic
and physical properties.
General Procedure for Oxidations
The oligothiophene (usually 2–4 mmol) was dissolved in
CH2Cl2 at room temperature. The HOFꢀCH3CN solution was
slowly added to the reaction mixture, and the reaction was
stopped after 15–30 min by adding a saturated solution of
NaHCO3. The organic material was extracted with CH2Cl2,
washed with water, and dried over MgSO4. The crude prod-
uct was usually purified either by vacuum flash chromatog-
raphy, using silica-gel 60-H (Merck), or recrystallization.
EXPERIMENTAL
Materials
Chemicals were purchased from Sigma-Aldrich (St. Louis,
MO, USA) and used without further purification. Deuterated
solvents were obtained from Cambridge Isotope Laboratorie-
s,and 2-bromo-3-hexylthiophene (1a) and 2-bromo-3-dode-
cylthiophene (1b) were synthesized following the literature
procedure.22 5,50-Dibromo-4,40-dihexyl-2,20-bithiophene (2a)
General Procedure for Polymerizations
A 5-mL glass vial was charged with a stirrer bar, 5,50-
dibromo-oligothiophene (usually 0.2–0.4 mmol), equimolar
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amount of 2,5-bis(trimethylstannyl)thiophene,17 Pd2(dba)3 (4
mol % Pd), P(o-tol)3 (8 mol %), and chlorobenzene (2 mL).
The glass vial was purged with nitrogen and securely sealed.
The glass vial was placed into a microwave reactor and
heated at 200 ꢂC for 20–45 min with stirring. After cooling
to room temperature, the reaction mixture was precipitated
into a mixture of methanol (50 mL) and 37% HCl (5 mL)
and stirred for 2 h. The polymer was filtered off and
washed with methanol, following by further purification by
sequential Soxhlet extraction with methanol, hexanes, and
dichloromethane.
1H NMR (400 MHz, CDCl3, ppm): d 7.43 (1H, s), 6.96 (1H, s),
2.84 (2H, t, J ¼ 8 Hz), 1.66–1.58 (2H, m), 1.43–1.38 (2H, m),
1.32–1.26 (16H, m), 0.88 (3H, t, J ¼ 7 Hz). 13C NMR
(100 MHz, CDCl3, ppm): d 144.3, 132.4, 131.3, 127.2, 125.1,
123.3, 32.09, 32.07, 29.81, 29.79, 29.75, 29.6, 29.5, 29.4,
28.5, 22.8, 14.3. HRMS (ESI) (m/z): (MþNa)þ calcd for
C36H54Br2NaO6S3 859.1347; found, 859.1334.
3,3000,5,5000-Tetrahexyl-2,20:50,200:500,2000-quaterthiophene (8) was
obtained from quaterthiophene 7 in 75% yield. A solution of
quaterthiophene 7 (3 g, 6 mmol) in 10 mL of dry THF was
ꢂ
cooled to ꢁ78 C, and a solution of n-BuLi (1.6 M in hexanes,
5,50-Dibromo-4,40-dihexyl-2,20-bithiophene-[all]-S,S-dioxide (3a)
was prepared from 2a as described above in 78% yield. 1H
NMR (200 MHz, CDCl3, ppm): d 7.12 (1H, s), 2.46 (2 H, t, J ¼
8 Hz), 1.62–1.55 (2H, m), 1.40–1.33 (6H, m), 0.90 (3H, t, J ¼ 7
Hz). 13C NMR (50 MHz, CDCl3, ppm): d 142.9, 131.0, 128.4,
118.3, 32.0, 30.7, 29.5, 27.0, 23.1, 14.7. HRMS (EI) (m/z): (M)þ
calcd for C20H28Br2O4S2 553.9796; found, 553.9803.
20 mmol) was added dropwise. The reaction mixture was
stirred for 10 min at ꢁ78 ꢂC, following by the addition of t-
BuOK (24 mmol) in THF. The stirring was continued for 15
min at ꢁ78 ꢂC, and then 1-iodohexane (13.2 mmol) was
added. The reaction mixture was allowed to reach room tem-
perature and stirred overnight, quenched by slow addition of
water, and extracted with hexanes. The organic phase was
dried with MgSO4, following by solvent removal under
reduced pressure. The crude solid was purified by column
chromatography.
5,50-Dibromo-4,40-didodecyl-2,20-bithiophene-[all]-S,S-dioxide
(3b) was prepared from 2b as described above in 84%
yield. 1H NMR (200 MHz, CDCl3, ppm): d 7.12 (1H, s), 2.46
(2H, t, J ¼ 8 Hz), 1.62–1.57 (2H, m), 1.31–1.26 (18H, m),
0.88 (3H, t, J ¼ 7 Hz). 13C NMR (50 MHz, CDCl3, ppm): d
142.9, 131.0, 128.4, 118.2, 32.6, 30.7, 30.3, 30.2, 30.1, 30.0,
29.9, 27.1, 23.4, 14.8. HRMS (ESI) (m/z): (MþNa)þ calcd for
1H NMR (500 MHz, CDCl3, ppm): d 7.08 (1H, d, J ¼ 4 Hz),
6.96 (1H, d, J ¼ 4 Hz), 6.62 (1H, s), 2.77–2.69 (4H, m,),
1.69–1.60 (4H, m), 1.39–1.30 (12H, m), 0.91–0.86 (6H, m).
13C NMR (125 MHz, CDCl3, ppm): d 144.6, 139.7, 136.5,
135.9, 127.8, 127.5, 126.0, 123.8, 34.9, 31.9, 31.8, 31.7, 30.8,
30.3, 29.5, 29.1, 25.5, 22.8, 14.32, 14.31. HRMS (EI) (m/z):
(M)þ calcd for C40H58S4 666.3421; found, 666.3419.
C
32H52Br2NaO4S2 745.1571; found, 745.1552.
5,500-Dibromo-3,300-dihexyl-2,20:50,200-terthiophene (5a)25 and
5,500-dibromo-3,300-didodecyl-2,20:50,200-terthiophene (5b) were
prepared in 92 and 98% yield, respectively. The synthesis of
5b is described below. NBS (0.29 g, 3.2 mmol) was added in
portions to a solution of the 4b (0.96 g, 1.6 mmol) in DMF (12
mL) in dark and stirred for 12 h at room temperature. After
the reaction completion, the reaction mixture was partitioned
between water and diethyl ether. The organic layer was
extracted, dried over MgSO4, and evaporated under reduced
pressure to obtain the crude product, which was purified by
silica gel column chromatography using hexanes.
3,3000,5,5000-Tetrahexyl-2,20:50,200:500,2000-quaterthiophene-[all]-S,S-
dioxide (9) was made by the oxidation of 5b using a freshly
prepared solution of HOFꢀCH3CN complex in 40% yield.
1H NMR (200 MHz, CDCl3, ppm): d 7.43 (1H, d, J ¼ 4 Hz),
7.30 (1H, d, J ¼ 4 Hz), 6.47 (1H, s), 2.84 (2H, t, J ¼ 8 Hz),
2.59 (2H, t, J ¼ 7 Hz), 1.72–1.25 (16H, m), 0.90 (6H, t, J ¼ 6
Hz). 13C NMR (100 MHz, CDCl3, ppm): d 148.0, 144.2, 133.5,
129.3, 127.1, 126.0, 125.1, 124.2, 32.0, 31.6, 31.5, 29.3, 28.9,
28.5, 26.7, 24.7, 22.6, 22.0, 14.2, 14.1. HRMS (ESI) (m/z):
(MþNa)þ calcd for C40H58Br2NaO8S4 817.2912; found,
817.2885.
1H NMR (500 MHz, CDCl3, ppm): d 6.98 (2H, s), 6.90 (2H, s),
2.70 (4H, t, J ¼ 8 Hz), 1.60 (4H, q, J ¼ 8 Hz), 1.40–1.20 (36H,
m), 0.88 (6H, t, J ¼ 7 Hz). 13C NMR (125 MHz, CDCl3, ppm): d
140.4, 135.1, 132.6, 131.6, 126.3, 110.6, 31.9, 30.5, 29.7, 29.6,
29.5, 29.4, 29.4, 29.3, 29.2, 22.7, 14.1. HRMS (EI) (m/z): (M)þ
calcd for C36H54Br2S3 740.1754; found, 740.1751.
P1 was synthesized from 5,50-dibromo-4,40-dihexyl-2,20-
bithiophene-[all]-S,S-dioxide (3a) and 2,5-bistrimethylstan-
nylthiophene (10) in 92% yield.
1H NMR (200 MHz, CDCl3, ppm): d 7.81–7.71 (1H, m), 7.39–
7.19 (1H, m), 2.76 (2H, broad s), 1.70–1.36 (8H, m), 0.91
(3H, broad s). GPC Mn ¼ 8.5K, Mw/Mn ¼ 2.72.
5,500-Dibromo-3,300-dihexyl-2,20:50,200-terthiophene-[all]-S,S-di-
oxide (6a) was made by oxidation of 5a using a freshly pre-
pared solution of HOFꢀCH3CN complex in 65% yield.
P2 was synthesized from 5,50-dibromo-4,40-didodecyl-2,20-
bithiophene-[all]-S,S-dioxide (3b) and 2,5-bistrimethylstan-
nylthiophene (10) in 90% yield.
1H NMR (400 MHz, CDCl3, ppm): d 7.42 (1H, s), 6.96 (1H, s),
2.84 (2H, t, J ¼ 8 Hz), 1.65–1.58 (2H, m), 1.45–1.38 (2H, m),
1.32–1.29 (4H, m), 0.90–0.87 (3H, m). 13C NMR (100 MHz,
CDCl3, ppm): d 144.4, 132.4, 131.3, 127.2, 125.1, 123.3, 32.1,
31.5, 29.2, 28.4, 22.6, 14.1. HRMS (ESI) (m/z): (MþNa)þ
calcd for C24H30Br2NaO6S3 690.9469; found, 690.9446.
1H NMR (400 MHz, CD2Cl2, ppm): d 7.79–7.65 (1H, m), 7.40–
7.30 (1H, m), 2.78 (2H, broad s), 1.73 (2H, broad s), 1.49–1.28
(18H, m), 0.88 (3H, broad s). GPC Mn ¼ 10.8K, Mw/Mn ¼ 1.96.
5,500-Dibromo-3,300-didodecyl-2,20:50,200-terthiophene-[all]-S,S-di-
oxide (6b) was made by oxidation of 5b using a freshly pre-
pared solution of HOFꢀCH3CN complex in 60% yield.
P3 was synthesized from 5,500-dibromo-3,300-didodecyl-
2,20:50,200-terthiophene-[all]-S,S-dioxide (6b) and 2,5-bistrime-
thylstannylthiophene (10) in 84% yield.
SYNTHESIS AND CHARACTERIZATION OF OLIGOTHIOPHENE-S,S-DIOXIDES, AMIR ET AL.
1939

JOURNAL OF POLYMER SCIENCE PART A: POLYMER CHEMISTRY DOI 10.1002/POLA
1H NMR (400 MHz, CDCl3, ppm): d 7.71–7.64 (1H, m), 7.48–
7.42 (1H, m), 6.89–6.77 (1H, m), 2.92 (2H, broad s), 1.69–
1.25 (17H, m), 0.90 (6H, broad s). GPC Mn ¼ 5.7K, Mw/Mn ¼
4.1.
82–89; (f) Treat, N. D.; Campos, L. M.; Dimitriou, M. D.; Ma, B.;
Chabinyc, M. L.; Hawker, C. J. Adv Mater 2010, 22, 4982–4986.
2 (a) Wang, C.; Kim, F. S.; Ren, G.; Xu, Y.; Pang, Y.; Jenekhe, S.
A.; Jia, L. J. Polym Sci Part A: Polym Chem 2010, 48,
4681–4690; (b) Nedeltchev, A. K.; Han, H.; Bhowmik, P. K.
J Polym Sci Part A: Polym Chem 2010, 48, 4611–4620; (c) Rusu,
R. D.; Damaceanu, M. D.; Marin, L.; Bruma, M. J Polym Sci Part
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P4 was synthesized from 5,50-dibromo-4,40-didodecyl-2,20-
bithiophene (2b) and 2,5-bistrimethylstannylthiophene (10)
in 95% yield.
1H NMR (200 MHz, CDCl3, ppm): d 7.09 (1H, broad s), 7.02
(1H, broad s), 2.82–2.75 (2H, m), 1.73–1.66 (2H, m), 1.45–
1.27 (18H, m), 0.87 (3H, t, J ¼ 6 Hz). GPC Mn ¼ 11.0K, Mw/
Mn ¼ 1.38.
3 (a) Facchetti, A.; Yoon, M.-H.; Stern, C. L.; Hutchison, G. R.;
Ratner, M. A.; Marks, T. J.
J Am Chem Soc 2004, 126,
CONCLUSIONS
13480–13501; (b) Ie, Y.; Nitani, M.; Ishikawa, M.; Nakayama, K.;
Tada, H.; Kaneda, T.; Aso, Y. Org Lett 2007, 9, 2115–2118.
We have introduced a new synthetic route for the prepara-
tion of soluble oligothiophenes-[all]-S,S-dioxides. Incorpora-
tion of hexyl and dodecyl groups into bi-, tert-, and quater-
thiophene backbones leads to enhanced solubility, while the
4 Yoon, M.-H.; DiBenedetto, S. A.; Facchetti, A.; Marks, T. J.
J Am Chem Soc 2005, 127, 1348–1349.
5 Ie, Y.; Umemoto, Y.; Okabe, M.; Kusunoki, T.; Nakayama, K.;
oligothiophene-[all]-S,S-dioxides also display
a
reduced
Pu, Y.-J.; Kido, J.; Tada, H.; Aso, Y. Org Lett 2008, 10, 833–836.
energy gap with respect to the aromatic oligothiophene pre-
cursors. The solubility of these materials allows for their fac-
ile processability, thus, making them useful for various appli-
cations in organic-based electronic devices. The high
solubility also allows them to be used as building blocks for
the preparation of thiophene-based conjugated polymers
containing bithiophene-S,S-dioxide units in the backbone.
Incorporation of electron-withdrawing sulfones along the
polymer backbone leads to the narrowing of the energy gap
with the energies of both frontier orbitals in bithiophene-
[all]-S,S-dioxide containing polymers being lower with
respect to the aromatic polythiophene. This tendency is con-
siderably stronger for the LUMO than for the HOMO, leading
to materials with greater electron-accepting ability. The mod-
ularity of our approach can potentially allow the energy lev-
els of polythiophenes to be controlled based on the number
and position of the thiophene-S,S-dioxide units. Small mole-
cule- and polymer-based devices, such as thin film transis-
tors, based on oligothiophene-[all]-S,S-dioxides are currently
under investigation.
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Zhang, C.; Nguyen, T. H.; Sun, J.; Li, R.; Black, S.; Bonner, C. E.;
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This work was supported by the ACS PRF grant PRF-49719,
NSF-DMR award (0906224) funded under the American Recov-
ery and Reinvestment Act of 2009 and the NSF SOLAR program
(CHE-1035292). This work was partially supported by the
MRSEC Program of the National Science Foundation under
Award No. DMR05-20415.
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1941