General synthesis of extended fused oligothiophenes consisting of an even number of thiophene rings

Article abstract of DOI:10.1002/chem.200601064

The intramolecular double cyclization of bis(3-bromo-2-thienyl)acetylenes though a lithium-halogen exchange reaction with tBuLi followed by treatment with elemental sulfur produces two thiophene-fused thieno[3,2-c](1,2-dithiin)s. The subsequent dechalcogenation from the 1,2-dithiins with copper nanopowder affords tetrathienoacenes. On the basis of this two-step procedure, a series of trialkylsilylterminated, fused oligothiophenes, including hexathienoacene (a six thiophene-fused system) and octathienoacene (an eight thiophene-fused system), were synthesized. In the UV/ Vis absorption and fluorescence spectra of the fused oligothiophenes, the absorption and emission maxima shift to longer wavelengths, as the π-conjugation length increases. Their maximum wavenumbers have linear relationships with the reciprocal number of thiophene rings consisting of the π-conjugated frameworks. In the cyclic voltammograms, all the compounds show reversible oxidation waves, the first ox idation potential of which shifts to less positive as the conjugation length increases. Among them, the octathienoacene also shows a reversible second oxidation process. Indeed, its chemical oxidation with an excess amount of NO⁺SbF ₆^- produces the dication as a golden crystal. The crystal structures of the neutral octathienoacene and its dication were determined by X-ray crystallography. While in the neutral state, the octathienoacene has a benzenoid structure with a large bond alternation of about 0.04 A, its dication has a quinoid structure in which two cationic charges are mainly localized on the terminal rings.

Full text of DOI:10.1002/chem.200601064

DOI: 10.1002/chem.200601064
General Synthesis of Extended Fused Oligothiophenes Consisting of an Even
Number of Thiophene Rings
Toshihiro Okamoto, Kenichi Kudoh, Atsushi Wakamiya, and Shigehiro Yamaguchi*[a]
Abstract: The intramolecular double
cyclization of bis(3-bromo-2-thienyl)-
Vis absorption and fluorescence spec-
tra of the fused oligothiophenes, the
absorption and emission maxima shift
to longer wavelengths, as the p-conju-
gation length increases. Their maxi-
mum wavenumbers have linear rela-
tionships with the reciprocal number of
thiophene rings consisting of the p-con-
jugated frameworks. In the cyclic vol-
tammograms, all the compounds show
reversible oxidation waves, the first ox-
idation potential of which shifts to less
positive as the conjugation length in-
creases. Among them, the octathienoa-
cene also shows a reversible second ox-
idation process. Indeed, its chemical
oxidation with an excess amount of
A
exchange reaction with tBuLi followed
by treatment with elemental sulfur pro-
duces two thiophene-fused thieno[3,2-
A
À
c](1,2-dithiin)s. The subsequent dechal-
cogenation from the 1,2-dithiins with
copper nanopowder affords tetrathi-
NO+SbF6 produces the dication as a
golden crystal. The crystal structures of
the neutral octathienoacene and its di-
cation were determined by X-ray crys-
tallography. While in the neutral state,
the octathienoacene has a benzenoid
structure with a large bond alternation
of about 0.04 Š, its dication has a qui-
noid structure in which two cationic
charges are mainly localized on the ter-
minal rings.
A
step procedure, a series of trialkylsilyl-
terminated, fused oligothiophenes, in-
cluding hexathienoacene (a six thio-
phene-fused system) and octathienoa-
Keywords: alkynes · cyclization ·
fused-ring systems
·
oligothio-
cene
(an
eight
thiophene-fused
phenes · organic transistors
system), were synthesized. In the UV/
Introduction
two characteristic features concerning electronic and solid-
state structures. First, while the acenes have a large quinoid
character and thus narrow HOMO–LUMO gaps, the fused
oligothiophenes have wide HOMO–LUMO gaps, since each
thiophene ring retains the aromatic character. Second, the
fused oligothiophenes favors a face-to-face p-stacked struc-
ture in the crystal packing, in sharp contrast to the herring-
bone structures generally observed for unsubstituted
acenes.[11] While in the crystal structures of the acenes, the
intermolecular edge-to-face interaction between the periph-
Linearly fused p-conjugated systems are an important class
of materials in the area of organic electronics.[1–4] Not only
are their rigid and flat p-conjugated frameworks without
conformational disorder, but also their unique electronic
structures make them intriguing candidates for a broad spec-
trum of electronic and optoelectronic applications, including
organic field-effect transistors (OFETs),[5–7] light-emitting
diodes,[8] and solid-state lasers.[9,10] Fused oligothiophenes
are a representative example, which can be recognized as
the thiophene analogues of acenes. When compared with
acenes such as pentacene, the fused oligothiophenes have
À
eral C H bonds of one molecule with the p planes of neigh-
boring molecules may be dominant, in the fused oligothio-
phenes, the nonbonding sulfur–sulfur interaction may play
an important role. These features make them intriguing as
environmentally stable and transparent carrier transporting
materials for OFETs.[12]
[a] Dr. T. Okamoto, K. Kudoh, Dr. A. Wakamiya,
Prof. Dr. S. Yamaguchi
From a synthetic viewpoint, however, only limited atten-
tion has been paid to the chemistry of the fused oligothio-
phenes to date.[11–20] Schroth and co-workers synthesized di-
benzo-fused dithieno[3,2-b:2’,3’-d]thiophene 1 by dechalco-
genation of the corresponding 1,2-dithiin precursor.[13] Ko-
bayashi and co-workers prepared pentathienoacene 2 (five
thiophene-fused system)[14] and investigated its electronic
Department of Chemistry, Graduate School of Science
Nagoya University, and SORST, Japan Science and Technology
Agency
Chikusa, Nagoya 464–8602 (Japan)
Fax : (+81)52-789-5947
E-mail: yamaguchi@chem.nagoya-u.ac.jp
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
548
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Chem. Eur. J. 2007, 13, 548 – 556

FULL PAPER
tion of bis(3-halothienyl)acetylenes 7 to two thiophene-
fused thieno[3,2-c](1,2-dithiin)s 8. The second step of the re-
G
actions is the dechalcogenation reaction from the 1,2-dithiins
to the target fused oligothiophenes 6. The latter transforma-
tion was originally reported by Schroth and co-workers.[13]
A typical procedure for the preparation of the tetrathi-
A
ogen exchange reaction of the dithienylacetylene 7a with
structure.[15] Oyaizu and Tsuchida reported the synthesis of
fused polythiophenes 3 based on the polymer-to-polymer
transformation by means of an electrophilic annulation reac-
tion.[16] Matzger and co-workers also reported the synthesis
of heptathienoacenes 4 (seven thiophene-fused system)
through the coupling of two dithieno[3,2-b:2’3’-d]thiophene
moieties with a sulfur atom, followed by oxidative annula-
tion,[11] and recently investigated its properties based on the
Raman and UV-Vis-NIR studies.[17] We have also presented
a synthetic route to the fused heteroacenes 5 consisting of
thiophene or selenophene rings on the basis of the intramo-
lecular triple cyclization from bis(o-haloaryl)diacetylenes
(vide infra).[18,19] Whereas these several routes to the fused
oligothiophenes and related compounds have already been
developed, the development of more general and facile syn-
thetic methods not only for longer oligomers, but also for a
wide range of derivatives would be required for further
progress in this chemistry. In this article, we disclose a gen-
eral synthetic route to a series of fused oligothiophenes 6
that consist of an even number of thiophene rings. The fun-
damental photophysical and electrochemical properties of
the produced fused oligothiophenes as well as the chemical
oxidation are also described.
Scheme 2. Synthesis of tetrathienoacene 6a and the plausible intermedi-
ates. Reagents and conditions: i) 1) tBuLi, THF; 2) S8 (3.0 mol amt.);
3) K3[Fe(CN)6], 1n aq. NaOH. ii) Cu nanopowder, 3008C.
tBuLi produced a dithiolate intermediate 9, which under-
went the 5-endo-dig mode cyclization to give a thieno[3,2-
G
b]thienylthiophenedithiolate (10).[21] The subsequent oxida-
tive workup with K3[Fe(CN)6] in a NaOH aqueous solution
yielded the thieno[3,2-c](1,2-dithiin) 8a in 65% yield. The
U
dechalcogenation of 8a with copper nanopowder (ca.
100 nm mesh) was accomplished by heating at 3008C for
20 min without solvent by using a sand bath. The extraction
with dichloromethane gave the tetrathienoacene 6a in 56%
yield.
On the basis of this procedure, hexathienoacene 6b and
octathienoacene 6c were also successfully synthesized, as
shown in Scheme 3. Thus, starting from the bis(thienothien-
Results and Discussion
C
General synthesis of fused oligothiophenes: The outline of
our synthesis is shown in Scheme 1, which includes two-step
reactions. The first step is the intramolecular double cycliza-
Scheme 1. Two-step synthesis of fused oligothiophenes from bis(3-halo-
thienyl)acetylenes.
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549

S. Yamaguchi et al.
AHCTREUNG
3) The silyl groups at the terminal positions of the starting
materials play important roles not only to retain the sol-
ubility of the products in common solvents, but also to
let the double cyclization cleanly proceed. Without the
silyl groups, the cyclization only resulted in a complex
mixture of products, probably due to the high reactivity
at the 5-position of the thiophene ring.
The produced fused oligothiophenes have a high thermal
stability. According to the thermogravimetric analysis, for
instance, the decomposition temperature for a 5% weight
loss (Td5) of the octathienoacene 6c is 3908C. While the
silyl-substituted fused oligothiophenes have a good solubility
up to the hexathienoacene 6b, the octathineoacene 6c has a
rather poor solubility (0.09 mgmLÀ1 in THF).
Photophysical properties of fused oligothiophenes: The UV/
Vis absorption and fluorescence spectra of the series of
fused oligothiophenes are shown in Figure 1 and their data
are summarized in Table 1.
Scheme 3. Synthesis of a) hexathienoacene 6b and b) octathienoacene
6c. Reagents and conditions: i) 1) tBuLi, THF; 2) S8 (3.0 mol amt.);
K3[Fe(CN)6], 1n aq. NaOH. ii) Cu nanopowder, 300–3408C.
A
There are several important points about the present pro-
cedure.
1) The intramolecular double cyclization of the bis(o-hal-
oaryl)acetylenes is a monoacetylenic version of the pre-
viously reported triple cyclization,[18a] in which the 1,4-
bis(o-haloaryl)diacetylenes can be converted into the
corresponding two aryl-ring-fused dithieno[3,2-c:2’,3’-
e](1,2-dithiin) derivatives through the similar reaction
pathway to the present one. One advantage of using the
acetylenic starting materials instead of the previous di-
Figure 1. Photophysical properties of the fused oligothiophenes: a) UV/
Vis absorption and b) fluorescence spectra in o-dichlorobenzene: 6a,
dotted line; 6b, dashed line; 6c, solid line.
Table 1. Photophysical data for fused oligothiophenes.
Absorption[a]
Fluorescence[a]
Stokes shift
[c]
lmax [nm][b]
loge
lmax [nm][b]
FF
Dlmax [cmÀ1
]
6a
6b
6c
352
398
431
4.52
4.68
4.76
368 (sh)[d]
0.08
0.32
0.39
1230
970
830
A
414[d]
tended starting materials like 7c, which can be easily
synthesized by the iterative Sonogashira reaction/alkaline
desilylation procedure starting from appropriate arylha-
lides with trimethylsilylacetylene.
447
[a] In o-dichlorobenzene. [b] Only the longest absorption and shortest
emission maximum wavelengths are given. [c] Determined using quinine
sulfate in 0.5m H2SO4 aqueous solution as a standard. [d] The highest
emission bands are located at 382 nm for 6a and 437 nm for 6b.
2) It is also worth noting that the present procedure is com-
plementary to the previous one in terms of the number
of rings found in the produced fused oligothiophenes.
Thus, while the previous method converts the 3-halo-
thienyl-substituted symmetrical diacetylenes into the
fused oligothiophenes with an odd number of thiophene
rings, such as the pentathienoacenes, in the present pro-
cedure the 3-halothienyl-substituted symmetrical mono-
In the absorption spectra, all the fused oligothiophenes
show vibronically structured absorption bands. Their longest
absorption maximum wavelength red-shifts by about 80 nm
from 352 nm for 6a to 431 nm for 6c, associated with a
slight increase in the molar absorption coefficient. In the
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Fused Oligothiophenes
FULL PAPER
A
emission maximum shifts to longer wavelength from 368 nm
for 6a to 447 nm for 6c and the fluorescence quantum yield
also increases from 0.08 for 6a to 0.39 for 6c. Notably, both
of the absorption and emission maximum wavenumbers
show good linear relationships with the reciprocal number
(1/n) of thiophene rings, as shown in Figure 2. Extrapolation
Figure 3. Cyclic voltammograms of fused oligothiophenes of 6a (top), 6b
(middle), and 6c (bottom). Measurement conditions: nBu4NPF6 (0.1m) in
o-dichlorobenzene; scan rate 50 mVsÀ1
.
Figure 2. Plots of absorption and fluorescence maxima of fused oligothio-
phenes 6 as a function of the reciprocal number of thiophene rings.
of the linear plots to 1/n=0 estimates the absorption and
emission maxima for the polythienoacenes to be 552 and
566 nm, respectively, which are consistent with the values
obtained from the odd-numbered thiophene-ring-containing
fused oligothiophenes, as reported by Matzger and co-work-
ers.[11] The other notable fact is that the Stokes shift decreas-
es from 1230 cmÀ1 for 6a to 830 cmÀ1 for 6c, indicative of a
smaller structural difference between the ground state and
the excited state, as the p conjugation is extended.
Figure 4. Plot of the first oxidation potential of fused oligothiophenes 6
as a function of the reciprocal number of thiophene rings.
Electrochemical properties of fused oligothiophenes: To
obtain a deeper insight into the behavior of the fused oligo-
thiophenes against oxidation, their electrochemical proper-
ties were studied by cyclic voltammetry. All the fused oligo-
thiophenes 6, which have silyl groups at the terminal posi-
tions, show reversible oxidation waves, as shown in Figure 3,
indicative of the significant stability of the produced cationic
species in these rigid ladder skeletons. Notably, their first
oxidation potentials also show a linear dependence on the
reciprocal number of thiophene rings, as shown in Figure 4.
The extrapolation of the linear plot indicates that the oxida-
tion potential will decrease up to 0.14 V (vs. ferrocene/ferro-
cenium) upon infinitely extending the p conjugation. While
the tetrathienoacene 6a and hexathienoacene 6b only show
the first oxidation process, the octathienoacene 6c exhibits a
second oxidation wave, demonstrating that the extension of
the p conjugation with more than eight fused thiophene
rings allows the formation of the dication. In fact, upon
chemical oxidation by treating 6c with an excess amount of
dicationic complex 6c[SbF6]2 as air-sensitive golden crystals,
the structure of which was determined by the X-ray crystal-
lography (vide infra).
G
Crystal structures of octathienoacence and its dication:
Crystal structures of the octathienoacene 6c and its dication
6c[SbF6]2 were determined by the X-ray crystallography.
G
Their structural data are summarized in Table 2, and their
ORTEP drawings are shown in Figure 5. Both compounds
have nearly flat p-conjugated frameworks with the length of
approximately 1.8 nm. For example, the dihedral angle be-
tween the two outermost thiophene rings is only 6.98 for 6c
and 08 for 6c[SbF6]2.
R
The degree of bond alternation in the p-conjugated
framework is important for discussing the nature of the p
system. Figure 6 shows the plots of the bond length versus
the bond number in the octathienoacene framework. In the
neutral 6c, there is substantial bond alternation. The odd-
numbered bonds are shorter than the even-numbered bonds
and their averaged difference is about 0.04 Š, which is com-
À
NO+SbF6 as an oxidant, we have succeeded in obtaining its
Chem. Eur. J. 2007, 13, 548 – 556
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551

S. Yamaguchi et al.
Table 2. Structural data for 6c and 6c[SbF6]2.
G
6c
6c[SbF6]2
A
bond distances [Š]
À
C1 C2
À
C2 C3
1.372(5)
1.412(5)
1.379(5)
1.427(5)
1.379(5)
1.419(5)
1.385(5)
1.420(5)
1.382(5)
1.750(4)
1.713(4)
1.748(4)
1.732(4)
1.731(4)
1.737(4)
1.732(4)
1.737(4)
1.373(11)
1.398(10)
1.397(10)
1.375(10)
1.414(10)
1.386(10)
1.437(10)
1.364(10)
1.452(14)
1.752(7)
1.743(8)
1.718(8)
1.753(7)
1.734(7)
1.717(8)
1.723(8)
1.748(7)
À
C3 C4
À
C4 C5
À
C5 C6
À
C6 C7
À
C7 C8
À
C8 C9 (C9*)
À
C9 C10 (C9*)
À
C1 S1
À
C4 S1
À
C3 S2
À
C6 S2
À
C5 S3
À
C8 S3
À
C7 S4
À
C10 (C9) S4
bond angles [8]
C1-C2-C3
C2-C3-C4
C3-C4-C5
C4-C5-C6
C5-C6-C7
C6-C7-C8
C7-C8-C9 (C9*)
C8-C9 (C9*)-C10 (C9)
C1-S1-C4
112.8(3)
114.0(3)
112.2(3)
112.1(3)
111.7(3)
112.4(3)
112.0(3)
112.2(3)
92.58(17)
90.33(17)
90.16(17)
90.35(17)
115.3(6)
111.9(7)
113.3(7)
111.6(6)
112.4(6)
109.4(7)
110.9(7)
112.9(8)
91.7(4)
Figure 6. Comparison of bond alternation in the crystal structures of neu-
tral 6c (left) and dicationic complex 6c[SbF6]2 (right).
A
parable to those in the non-fused oligothiophenes.[22,23] This
result demonstrates that, unlike the acenes, the fused oligo-
thiophenes have little contribution of the quinoid structure
in the neutral state, and each thiophene ring retains a sub-
C3-S2-C6
C5-S3-C8
C7-S4-C10 (C9)
89.7(3)
88.2(4)
89.4(3)
stantial aromaticity. In contrast, the dication 6c[SbF6]2 has a
G
large quinoid structure. Thus, the even-numbered bonds are
much shorter than the odd-numbered bonds and there are
marked bond alternations of 0.02–0.09 Š, except for the
bonds in the terminal thiophene rings. In the terminal thio-
phene rings, while the bond length of bond 1 is in the range
of an ordinal C=C double bond, those of the bonds 2 and 3
are comparable to each other and slightly shorter than those
of the other odd-numbered bonds. These facts suggest that
the two cationic charges seem to mostly localize at the ter-
minal thiophene rings, particularly at the C3 and C3* posi-
tions, as schematically shown in Figure 5. This structure is
consistent with that expected by the theoretical calcula-
tions[16] and might be important as an experimental descrip-
tion of the bipolaron in the fused oligothiophene structure.
While in the crystal packing, the dication 6c[SbF6]2 does
N
not have any special intermolecular interaction, it is interest-
ing to note that the neutral 6c shows a distinct intermolecu-
lar S···S nonbonding interaction. Thus, as shown in Figure
7a, the 6c molecule aligns in parallel and slightly overlaps
with the neighboring molecules with the closest nonbonding
S···S distance of 3.4 Š, which is shorter than the sum of the
van der Waals radii of two sulfur atoms (3.6 Š). Conse-
quently, 6c forms a stairlike p sheet, as shown in Figure 7b,
two of which pair up with an interplane distance of 7.0 Š.
The space between the two sheets is filled with o-dichloro-
benzene solvent molecules. Finally, the pairs of two p sheets
are arranged in a herringbone-like packing structure, as
shown in Figure 7c. It may be rationalized that the bulky
triisopropylsilyl groups prevent the face-to-face p stacking,
Figure 5. ORTEP drawing of a) octathienoacene 6c and b) its dicationic
complex 6c[SbF6]2 (50% probability for thermal ellipsoids).
A
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Chem. Eur. J. 2007, 13, 548 – 556

Fused Oligothiophenes
FULL PAPER
tural elucidation of the neutral and dicationic octathienoa-
cenes, which provides us with important information about
the electronic structures of the fused oligothiophenes. Fur-
ther studies on the synthesis of a series of derivatives as well
as their application in organic field-effect transistors are
now in progress in our laboratory.
Experimental Section
General methods: Melting point (m.p.) determination was performed
using a Yanaco MP-S3 instrument. 1H and 13C NMR spectra were mea-
sured with a JEOL EX-270 spectrometer, a JEOL A-400 spectrometer,
or a VARIAN INOVA-500 spectrometer. UV/Vis absorption spectra and
fluorescence spectra were recorded with a Shimadzu UV-3150 spectrome-
ter and a F-4500 Hitachi spectrometer, respectively, in degassed spectral
grade THF. Cyclic voltammetry were recorded on a CHI600 A instru-
ment. Thin-layer chromatography (TLC) was performed on plates coated
with 0.25 mm thick silica gel 60F-254 (Merck). Column chromatography
was performed using a Fuji Silysia silica gel PSQ60B (60 mm). Recycling
preparative gel permeation chromatography (GPC) was performed by
using polystyrene gel columns (JAIGEL 1H and 2H, Japan Analytical In-
dustry) with toluene or chloroform as an eluent. Copper nanopowder
(100 nm) was purchased from Aldrich. Compound 7b was obtained ac-
cording to Scheme 4. 3-Bromo-2-(trimethylsilylethynyl)thieno
phene (11) was prepared by Sonogashira coupling of 3-bromo-2-
iodothieno[3,2-b]thiophene with trimethylsilylacetylene in the presence
of PdCl2(PPh)3/CuI catalyst system in triethylamine. All reactions were
A
AHCTREUNG
AHCTREUNG
carried out under argon atmosphere except where noted.
Figure 7. Crystal packing of octathienoacene 6c: arrangements of a) mol-
ecules and b) p sheets, and c) a view of the packing structure along the a
axis.
Scheme 4. Synthesis of compound 7b. Reagents and conditions: i) LDA,
(iPr)3SiOTf, THF. ii) 1) K2CO3, THF/MeOH; 2) 2-bromothieno
ophene, [PdCl2(PPh3)2], CuI, Et3N.
A
AHCTREUNG
but the attractive nonbonding S···S interaction still exists, re-
sulting in the formation of this packing structure.
Bis(3-bromo-5-trimethylsilyl-2-thienyl)acetylene (7a):
a
solution of
nBuLi in hexane (1.60m, 21.8 mL, 34.8 mmol) was added dropwise to a
solution of diisopropylamine (5.43 mL, 38.3 mmol) in THF (38 mL) at
À788C. After stirring for 15 min, the solution was allowed to warm to
08C over 20 min. The produced solution of lithium diisopropylamide was
Conclusion
added dropwise to
a solution of 1,2-bis(3-bromo-2-thienyl)acetylene
We have described a general and versatile synthetic route to
the extended fused oligothiophenes on the basis of the intra-
molecular double cyclization. This methodology enabled us
to synthesize a series of derivatives consisting of an even
number of thiophene rings, up to eight-ring fused octathi-
(5.50 g, 15.8 mmol) and trimethylsilyl chloride (4.12 g, 37.9 mmol) in
THF (80 mL) at À788C. After stirring for 1 h, the mixture was gradually
warmed to room temperature over 5 h. The mixture was quenched with a
1m NH4Cl aqueous solution (50 mL), and extracted with diethyl ether.
The organic layer was washed with water and brine, dried over MgSO4,
filtered, and concentrated under reduced pressure. The resulting mixture
was subjected to a silica gel column chromatography (hexane, Rf =0.49)
to give 7.59 g (15.4 mmol) of 7a in 98% yield as pale yellow solid. M.p.
86–878C; 1H NMR (270 MHz, CDCl3): d=7.08 (s, 2H), 0.32 ppm (s,
18H); 13C NMR (100 MHz, CDCl3): d=144.60, 136.21, 124.73, 117.50,
89.23, À0.47 ppm; MS (EI): m/z: 492 [M]+; elemental analysis calcd (%)
for C16H20Br2S2Si2: C 39.02, H 4.09; found: C 39.18, H 4.15.
A
but also the oxidation potentials show linear relationships
with the reciprocal number of thiophene rings, indicative of
the perfect extension of the p conjugation in this ladder
framework. Reversible redox behavior is another notable
feature for the present fused oligothiohenes that promise
the potential use as a carrier transporting material in organ-
ic electronics. In addition, we have succeeded in the struc-
3-Bromo-5-triisopropylsilyl-2-(trimethylsilylethynyl)thieno[3,2-b]thio-
phene (12): A solution of lithium diisopropylamide in THF, freshly pre-
pared from diisopropylamine (0.95 mL, 6.69 mmol) and nBuLi (1.58m
A
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553

S. Yamaguchi et al.
hexane solution, 4.0 mL, 6.37 mmol) in THF (6.7 mL), was added drop-
wise to a solution of 3-bromo-2-(trimethylsilylethynyl)thieno[3,2-b]thio-
(51 mg, 0.268 mmol) in
a
5:2 triethylamine/toluene mixed solvent
A
(70 mL) at room temperature. The mixture was stirred at 708C for 18 h.
The mixture was quenched with a 1n HCl aqueous solution, and extract-
ed with toluene. The organic layer was washed with water and brine,
dried over MgSO4, filtered, and concentrated under reduced pressure.
Recrystallization from a 1:3 toluene/ethyl acetate mixed solvent gave
1.42 g (1.45 mmol) of 7c as yellow crystals. The residue was further puri-
fied by a silica gel column chromatography (hexane, Rf =0.55) to give
1.61 g (1.64 mmol) of 7c. In total, 3.03 g (3.09 mmol) of 7c was obtained
in 46% yield. M.p. 179–1808C; 1H NMR (270 MHz, CDCl3): d=7.13 (s,
2H), 1.26–1.40 (m, 6H), 1.11 ppm (d, J=7.3 Hz, 36H); 13C NMR
(68 MHz, C6D6): d=140.39, 139.26, 138.11, 125.16, 123.91, 118.99, 109.92,
92.43, 90.38, 18.70, 11.98 ppm; HRMS (FAB): m/z calcd for
C36H44Br4S4Si2: 975.8598; found:975.8609.
phene (1.83 g, 5.79 mmol) and triisopropylsilyl trifluoromethanesulfonate
(8.87 g, 29.0 mmol) in THF (29 mL) at À788C. After stirring for 80 min,
the mixture was gradually warmed to room temperature with stirring
over 2 h. The mixture was quenched with a 1m NH4Cl aqueous solution,
and extracted with diethyl ether. The organic layer was washed with
water and brine, dried over MgSO4, filtered, and concentrated under re-
duced pressure. The resulting mixture was subjected to a silica gel
column chromatography (hexane, Rf =0.59) to give 2.21 g (4.69 mmol) of
12 in 81% yield as pale yellow solid. M.p. 101–1038C; 1H NMR
(400 MHz, CDCl3): d=7.34 (s, 1H), 1.29–1.42 (m, 3H), 1.14 (d, J=
7.2 Hz, 18H), 0.31 ppm (s, 9H); 13C NMR (100 MHz, CDCl3): d=144.66,
141.53, 138.70, 127.48, 121.94, 108.77, 104.65, 96.54, 18.52, 11.76,
À0.17 ppm; HRMS (EI): m/z calcd for C20H31BrS2Si2: 470.0589; found:
470.0588.
A typical procedure for the intramolecular double cyclization—com-
pound 8a: A solution of tBuLi in pentane (1.53m, 26.5 mL, 40.6 mmol)
was added dropwise to a solution of compound 7a (5.0 g, 10.2 mmol) in
THF (100 mL) at À788C. After being stirred for 1 h, sulfur (976 mg,
30.5 mmol) was added as a crystal under a stream of argon. The mixture
was kept at the same temperature for 5 min and then allowed to warm to
room temperature with stirring over 2 h. The mixture was quenched with
[3-Bromo-5-(triisopropylsilyl)-2-thieno
A
ACHTREUNG
was added to a solution of 12 (1.28 g, 2.71 mmol) in a 1:1 THF/methanol
mixed solvent (16 mL) at room temperature. The mixture was stirred for
2 h, and then concentrated under reduced pressure. After addition of a
1m NH4Cl aqueous solution, the mixture was extracted with diethyl
ether. The organic layer was washed with water and brine, dried over
MgSO4, filtered, and concentrated to give 1.08 g (2.70 mmol) of 3-bromo-
a
1m sodium hydroxide aqueous solution, followed by addition of
K3[Fe(CN)6] (13.4 g, 40.6 mmol). The resulting mixture was extracted
with diethyl ether for three times. The combined organic layer was
washed with water and brine, dried over MgSO4, filtered, and concentrat-
ed under reduced pressure. The resulting mixture was subjected to a
silica gel column chromatography (hexane, Rf =0.54) to give 2.83 g
(6.61 mmol) of 8a in 65% yield as red oil. 1H NMR (270 MHz, CDCl3):
d=7.33 (s, 1H), 7.08 (s, 1H), 0.37 (s, 9H), 0.34 ppm (s, 9H); 13C NMR
2-ethynyl-5-(triisopropylsilyl)thieno[3,2-b]thiophene in 99% yield as pale
U
orange solid. This compound was used for the next reaction without fur-
ther purification: 1H NMR (270 MHz, CDCl3): d=7.33 (s, 1H), 3.69 (s,
1H), 1.31–1.42 (m, 3H), 1.12 ppm (d, J=7.3 Hz, 18H). A solution of the
produced 3-bromo-2-ethynyl-5-(triisopropylsilyl)thieno
(1.08 g, 2.71 mmol) in toluene (6.3 mL) was added dropwise to a solution
of 3-bromo-2-iodothieno[3,2-b]thiophene (849 mg, 2.46 mmol), [PdCl2-
(PPh3)2] (17.3 mg, 0.0246 mmol) and CuI (9.4 mg, 0.0492 mmol) in diiso-
A
(100 MHz, CDCl3):
d 144.94, 143.02, 139.67, 139.53, 138.24, 136.10,
132.72, 125.53, 125.33, 116.10, À0.27, À0.33 ppm; HRMS(EI): m/z calcd
AHCTREUNG
for C16H20S5Si2: 427.9707; found: 427.9708.
ACHTREUNG
propylamine (12.4 mL) at room temperature. The mixture was stirred for
12 h at the same temperature. Insoluble salts were removed by filtration
with a Celite, and washed with benzene. The filtrate was concentrated
under reduced pressure. The mixture was passed through a silica gel
column (dichloromethane) and further purified by recycling preparative
gel permeation chromatography (toluene as an eluent) to give 1.28 g
(2.07 mmol) of 13 in 84% yield as yellow solid. M.p. 136–1388C;
1H NMR (400 MHz, CDCl3): d=7.56 (d, J=5.4 Hz, 1H), 7.37 (s, 1H),
7.28 (d, J=5.4 Hz, 1H), 1.35–1.42 (m, 3H), 1.14 ppm (d, J=7.3 Hz,
18H); 13C NMR (100 MHz, CDCl3): d=145.07, 142.12, 139.63, 138.18,
130.09, 127.54, 121.06, 120.86, 120.37, 90,46, 90.22, 18.54, 11.77 ppm; MS
(EI): m/z: 616 [M]+; elemental analysis calcd (%) for C23H24Br2S4Si: C
44.80, H 3.92; found: C 45.06, H 3.92.
Compound 8b: This compound was obtained essentially in the same
manner as described for 8a starting from 7b in 42% yield as a red
powder. M.p. 142–1448C; 1H NMR (400 MHz, CDCl3): d=7.41 (s, 1H),
7.36 (s, 1H), 1.45–1.34 (m, 6H), 1.16–1.14 ppm (m, 18H); 13C NMR
(100 MHz, CDCl3): d=143.48, 143.37, 141.03, 139.90, 138.59, 138.08,
136.58, 135.79, 134.70, 134.43, 128.25, 127.06, 126.58, 117.29, 115.87, 18.56,
18.54, 11.81, 11.78 ppm; HRMS(EI): m/z calcd for C32H44Si2S7: 708.1026;
found: 708.1017.
Compound 8c (as a mixture of three regioisomers): A solution of tBuLi
in pentane (1.43m, 5.9 mL, 8.42 mmol) was added dropwise to a solution
of compound 7c (1.03 g, 1.05 mmol) in THF (40 mL) at À788C. After
stirring for 1.5 h, sulfur (810 mg, 25.3 mmol) was added as a crystal under
a stream of argon. The mixture was kept at the same temperature for
5 min and then allowed to warm to room temperature with stirring over
2.5 h. The mixture was quenched with K3[Fe(CN)6] in a 1m sodium hy-
droxide aqueous solution. The mixture was extracted with diethyl ether.
The organic layer was washed with water and brine, dried over MgSO4,
filtered, and concentrated under reduced pressure. The resulting mixture
was subjected to a silica gel column chromatography (hexane, Rf =0.36).
Further purification by a recycling preparative gel permeation chroma-
tography (chloroform as an eluent) gave 416 mg (0.488 mmol) of 8c as a
mixture of three regioisomers in 46% yield as purple powders. This mix-
ture was directly used for the subsequent dechalcogenation without fur-
ther separation of each isomer. M.p. 2358C (decomp); 1H NMR
(270 MHz, CDCl3): d=7.41 (s), 7.37 (s), 7.35 (s), 7.10 (s), 1.47–1.34 (m),
Bis(3-bromo-5-triisopropylsilyl-2-thieno[3,2-b]thienyl)acetylene (7b): A
A
solution of lithium diisopropylamide in THF, freshly prepared from diiso-
propylamine (0.15 mL, 1.03 mmol) and nBuLi (1.58m hexane solution,
0.62 mL, 0.98 mmol) in THF (1.0 mL), was added dropwise to a solution
of 13 (601 mg, 0.98 mmol) and triisopropylsilyl trifluoromethanesulfonate
(359 mg, 1.17 mmol) in THF (10 mL) at À788C. After stirring for 1 h, the
mixture was gradually warmed to room temperature over 3.5 h. The mix-
ture was quenched with a 1m NH4Cl aqueous solution, and extracted
with dichloromethane. The organic layer was washed with water and
brine, dried over MgSO4, filtered, and concentrated under reduced pres-
sure. The resulting mixture was passed through a silica gel column (di-
chloromethane) and further purified by recycling preparative gel permea-
tion chromatography (toluene as an eluent) to give 350 mg (0.45 mmol)
1.16–1.13 ppm (m); MS
(MALDI): m/z: 852 [M]+.
A
1
of 7b in 46% yield as yellow solid. M.p. 268–2708C; H NMR (270 MHz,
A typical procedure for dechalcogenation—compound 6a: The mixture
of 8a (300 mg, 0.70 mmol) and copper nanopowder (178 mg, 2.80 mmol)
was heated at 3008C for 20 min using a sand bath. The reaction mixture
was dissolved into CH2Cl2, and the insoluble materials were removed by
filtration with a Celite. The filtrate was concentrated under reduced pres-
sure and the resulting mixture was subjected to a silica gel column chro-
matography (hexane, Rf =0.45) to give 155 mg (0.391 mmol) of 6a in
56% yield as a colorless powder. M.p. 200–2028C; 1H NMR (270 MHz,
CDCl3): d=7.38 (s, 2H), 0.38 ppm (s, 18H); 13C NMR (100 MHz,
CDCl3): d=142.58, 142.27, 136.83, 132.90, 126.69, 100.55, À0.16 ppm; MS
CDCl3): d=7.36 (s, 2H), 1.33–1.41 (m, 6H), 1.14 ppm (d, J=7.3 Hz,
36H); 13C NMR (100 MHz, CDCl3): d=142.04, 139.59, 129.03, 127.55,
121.17, 109.07, 90.56, 18.54, 11.78 ppm; HRMS (EI): m/z calcd for
C32H44Br2S4Si2: 772.0212; found: 772.0217.
3,6-Dibromo-2,5-bis[(3-bromo-5-triisopropylsilyl-2-
thienyl)ethynyl]thieno
triisopropylsilyl-2-thienylacetylene (4.83 g, 14.1 mmol) in toluene (10 mL)
was added dropwise to a mixture of 2,3,5,6-tetrabromothieno[3,2-b]thio-
phene (3.05 g, 6.70 mmol), [PdCl2(PPh3)2] (94 mg, 0.134 mmol), and CuI
A
AHCTREUNG
AHCTREUNG
554
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ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2007, 13, 548 – 556

Fused Oligothiophenes
FULL PAPER
(EI): m/z: 396 [M]+; elemental analysis calcd (%) for C16H20S4Si2: C
48.44, H 5.08; found: C 48.50, H 5.00.
V=1534.3(10) Š3, Z=1, 1cald =1.732 gcmÀ3. The refinement converged to
R1 =0.0591, wR2 =0.1219 [I>2s(I)], GOF=1.098.
CCDC-615569 and 615570 contain the supplementary crystallographic
data for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
requeat/cif.
Compound 6b: This compound was obtained essentially in the same
manner as described for 6a starting from 8b in 60% yield as white solid.
M.p. 283–2848C; 1H NMR (270 MHz, CDCl3): d=7.41 (s, 2H), 1.46–1.35
(m, 6H), 1.17–1.14 ppm (m, 18H); 13C NMR (100 MHz, CDCl3): d=
142.11, 137.11, 136.75, 134.04, 132.31, 131.00, 128.18, 18.58, 11.81 ppm;
HRMS (EI): m/z calcd for C32H44Si2S6: 676.1306; found: 676.1317.
Compound 6c: This compound was prepared essentially in the same
manner as described for 6a starting from 8c (as a mixture of three re-
gioisomers) and obtained in 51% yield by sublimation as yellow crystals.
M.p. >3008C; 1H NMR (400 MHz, [D4]-o-dichlorobenzene): d=7.28 (s,
2H), 1.28–1.19 (m, 6H), 1.08 ppm (d, J=7.2 Hz, 36H); HRMS(EI): m/z
calcd for C36H44Si2S8: 788.0747; found:788.0750. 13C NMR spectrum was
not able to obtain due to its poor solubility.
Acknowledgements
This work was supported by from the Ministry of Education, Culture,
Sports, Science, and Technology of Japan, and SORST, Japan Science and
Technology Agency Grants-in-Aids (Nos. 15205014 and 17069011). T.O.
thanks the JSPS Research Fellowships for Young Scientists.
Chemical oxidation of octathienoacene 6c: A mixture of 6c (0.58 mg,
À
0.74 mmol) and NO+SbF6 (1 mg, 3.8 mmol) was placed in a Pyrex glass
tube. Dichloromethane (0.75 mL) was transferred into the tube. The tube
was evacuated and sealed. After sonication for 5 min, the tube was
placed in a refrigerator at À308C for 12 h. Upon standing, golden crystals
[1] J. Roncali, Chem. Rev. 1997, 97, 173.
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of 6c[SbF6]2 were obtained.
G
X-ray crystallographic analysis of compound 6c: Single crystals of 6c
suitable for X-ray crystal analysis were obtained by recrystallization from
a solution of 6c in o-dichlorobenzene (ODCB). Intensity data were col-
lected at 173 K on a Rigaku Single-Crystal CCD X-ray Diffractometer
(Saturn 70 with MicroMax-007) with MoKa radiation (l=0.71073 Š) and
a graphite monochromater. A total of 34327 reflections were measured
at a maximum 2q angle of 50.08, of which 8980 were independent reflec-
tions (Rint =0.0607). The structure was solved by direct methods
(SHELXS-97)[24] and refined by the full-matrix least-squares on F2
(SHELXL-97).[24] Two ODCB molecules were included, one of which
was disordered and was solved by using appropriate disordered models.
Thus, for the ODCB molecule consisting of C43–C48 and Cl3, Cl4, two
sets of chlorine atoms, that is, Cl3A, Cl4A and Cl3B, Cl4B, were placed
and their occupancies were refined to be 0.55 and 0.45, respectively. This
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Perkin Trans. 2 1992, 765.
À
disordered molecule was restrained by DFIX instruction with fixed C C
À
and C Cl bond lengths and also by using the ISOR instruction during re-
finement. All non-hydrogen atoms were refined anisotropically and all
hydrogen atoms, except for those of the disordered ODCB, were placed
by using AFIX instructions. The crystal data are as follows:
C48H52Cl4S8Si2; Mr =1083.36, crystal size 0.20”0.10”0.05 mm3, monoclin-
ic, P21/c, a=7.421(2), b=27.469(8), c=25.354(8) Š, b=96.159(4)8, V=
5138(3) Š3, Z=4, 1cald =1.400 gcmÀ3. The refinement converged to R1 =
0.0615, wR2 =0.1474 [I>2s(I)], GOF=1.084.
X-ray crystal structure analysis of compound 6c
6c[SbF6]2 suitable for X-ray crystal analysis were obtained by recrystalli-
zation from a solution of 6c[SbF6]2 in dichloromethane in a sealed
A
ACHTREUNG
AHCTREUNG
vacuum tube. Intensity data were collected at 100 K on a Rigaku Single-
Crystal CCD X-ray Diffractometer (Saturn 70 with MicroMax-007) with
MoKa radiation (l=0.71073 Š) and graphite monochromater. A total of
10408 reflections were measured at a maximum 2q angle of 50.08, of
which 5288 were independent reflections (Rint =0.0565). The structure
was solved by direct methods (SHELXS-97)[24] and refined by the full-
matrix least-squares on F2 (SHELXL-97).[24] Four solvent molecules of di-
chloromethane were included. Three isopropyl moieties of the triisopro-
pylsilyl group consisting of C10–C18 were disordered and solved using
appropriate disordered models. Thus, two sets of carbons, that is, C10A–
C18A and C10B–C18B, were placed and their occupancies were refined
to be 0.76 and 0.24, respectively. The two sets of disordered triisopropyl-
silyl groups (C10A–C18 and Si1), and (C10B–C18B and Si1) were re-
strained by SADI instruction as well as DELU instruction during refine-
ment. All non-hydrogen atoms, except for the disordered isopropyl moi-
eties of the triisopropylsilyl group consisting of C10B–C18B, were refined
anisotropically and all hydrogen atoms were placed by using AFIX in-
[16] K. Oyaizu, T. Iwasaki, Y. Tsukahara, E. Tsuchida, Macromolecules
2004, 37, 1257.
[17] R. M. Osuna, X. Zhang, A. J. Matzger, V. Hernandez, J. T. Lꢄpez
Navarrete, J. Phys. Chem. A 2006, 110, 5058.
structions. The crystal data are as follows: C40H52Cl8F12S8Sb2Si2; Mr =
3
¯
1600.58, crystal size 0.05”0.05”0.01 mm , triclinic, P1, a=9.165(4), b=
10.862(4), c=15.599(6) Š, a=83.285(11), b=89.482(12), g=84.235(11)8,
Chem. Eur. J. 2007, 13, 548 – 556
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
555

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[21] Sashida and co-workers reported the similar cyclization, which di-
rectly produce
a
thieno[3,2-b]thiophene skeleton instead of the
A
thieno[3,2-c](1,2-dithiin) skeleton: H. Sashida, S. Yasuike, J. Hetero-
AHCTREUNG
cycl. Chem. 1998, 35, 725.
[22] J. L. Brꢃdas, G. B. Street, B. Thꢃmans, J. M. Andrꢃ, J. Chem. Phys.
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1992, 259, 181.
[24] G. M. Sheldrick, SHELX-97, Program for the Refinement of Crystal
Structures, University of Gçttingen, Gçttingen (Germany), 1997.
[19] K. Kudoh, T. Okamoto, S. Yamaguchi, Organometallics 2006, 25,
2374.
[20] Other thiophene-based heteroacenes: a) X.-C. Li, H. Sirringhaus, F.
Garnier, A. B. Holmes, S. C. Moratti, N. Feeder, W. Clegg, S. J. Teat,
R. J. Friend, J. Am. Chem. Soc. 1998, 120, 2206; b) R. Rajca, H.
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Chem. 2003, 68, 9813; d) V. G. Nenajdenko, V. V. Sumerin, K. Y.
Chernichenko, E. S. Balenkova, Org. Lett. 2004, 6, 3437.
Received: July 22, 2006
Published online: September 27, 2006
556
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Chem. Eur. J. 2007, 13, 548 – 556