Synthesis and structure of alkyl-substituted fused thiophenes containing up to seven rings

Article abstract of DOI:10.1021/jo061853y

We have established a series of synthetic methods to synthesize alkyl-substituted fused thiophenes with degrees of fusion from two to seven rings. These fused thiophene ring compounds have very good solubility in common organic solvents, making possible the solution processing of these compounds for electronic applications. The UV absorption of these fused thiophenes is blue-shifted when compared with their hydrocarbon counterparts. The larger band gaps result in much better stability. Single-crystal X-ray results for 3,6-didecanyldithieno[2,3-d:2′,3′-d′]thieno[3,2-b:4, 5-b″]dithiophene (FT5) and 3,7-didecanylthieno-[3,2-b]thieno[2′, 3′:4,5]thieno[2,3-d]thiophene (FT4) demonstrate that both compounds form π-stacking structures instead of a herringbone-type of packing motif. This more favorable π-stacked structure may lead to better material electronic properties such as mobility in devices fabricated with these compounds.

Full text of DOI:10.1021/jo061853y

Synthesis and Structure of Alkyl-Substituted Fused Thiophenes
Containing up to Seven Rings
Mingqian He* and Feixia Zhang
Corning Incorporated, SP-FR-6, Corning, New York 14830
hem@corning.com
ReceiVed September 7, 2006
We have established a series of synthetic methods to synthesize alkyl-substituted fused thiophenes with
degrees of fusion from two to seven rings. These fused thiophene ring compounds have very good solubility
in common organic solvents, making possible the solution processing of these compounds for electronic
applications. The UV absorption of these fused thiophenes is blue-shifted when compared with their
hydrocarbon counterparts. The larger band gaps result in much better stability. Single-crystal X-ray results
for 3,6-didecanyldithieno[2,3-d:2′,3′-d′]thieno[3,2-b:4,5-b′′]dithiophene (FT5) and 3,7-didecanylthieno-
[3,2-b]thieno[2′,3′:4,5]thieno[2,3-d]thiophene (FT4) demonstrate that both compounds form π-stacking
structures instead of a herringbone-type of packing motif. This more favorable π-stacked structure may
lead to better material electronic properties such as mobility in devices fabricated with these compounds.
Introduction
Conjugated organic molecules such as pentacene have been
widely used as organic field-effect transistor (FET) materials.
2
1,2
Mobilities exceeding 1 cm /V‚s have been achieved. It has
been shown that good organic FET materials need to possess
good π-stacking structures in the solid state. One previous
3
report indicated that pentacene (I) films exhibited essentially
single-crystal-like macroscopic morphology.3 Although the
charge carrier mobility mechanism for organic FET materials
is still not fully understood, a fully extended π system is
essentially needed to achieve higher mobility in an all organic
system. Extension of the π system in a single molecule should
lead to increased π stacking, which could result in increased
charge carrier mobility. However, in most cases, extension of
conjugation also causes other problems. First, increased con-
jugation tends to lead to a decreased band gap, which makes
the molecule degrade easier in oxygen environments. Second,
extensively conjugated organic molecules usually have very
limited solubility, thus limiting their processibility. A recent
FIGURE 1. Pentacene, tetrathienoacene, and pentathienoacene.
report from Anthony et al.4 on the synthesis of acene-
dithiophenes with up to seven linearly fused ring units tends to
support this trend. Acenedithiophenes have absorption maxima
near 760 nm and can only be stored in oxygen-free toluene.
Pentacene itself suffers the same instability and insolubility
5
problems. In contrast, fused thiophene compounds have
exhibited very good environmental stability due to their larger
band gaps, as stated in several recent publications.6 Pen-
tathienoacene (III) (Figure 1), for example, has a maximum
-8
(
4) Payne, M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org. Lett.
(
1) Katz, H. E. Chem. Mater. 2004, 16, 4748-4756.
2004, 6, 3325-3328.
(2) Hwang, D. K.; Lee, K.; Kim, J. H.; Im, S.; Kim, C. S.; Baik, H. K.;
(5) Yamada, M.; Ikemoto, I.; Kuroda, H. Bull. Chem. Soc. Jpn. 1988,
61, 1057-1062.
Park, J. H.; Kim, E. Appl. Phys. Lett. 2006, 24, 243513/1-243513/3.
3) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur, A.
Chem. Mater. 1996, 8, 2542-2544.
(
(6) Zhang, X.; Cote, A. P.; Matzger, A. J. Am. Chem. Soc. 2005, 127,
10502-10503.
10.1021/jo061853y CCC: $37.00 © 2007 American Chemical Society
4
42
J. Org. Chem. 2007, 72, 442-451
Published on Web 12/21/2006

Alkyl-Substituted Fused Thiophenes
SCHEME 1. Monosubstituted Thienothiophene
SCHEME 2 â-Disubstituted Alkylthienothiophenes
absorption at 357 nm,6,9 and the newly synthesized hep-
tathienoacene has an absorption maximum at 396 nm. Com-
of 10 . This is the first time pentathienoacene was successfully
3
6
tested as an organic semiconductor in a FET device.
pared with pentacene’s 575 nm and acenedithiophene’s 760 nm
absorption maxima,10 the fused thiophene compounds absorb
at a much higher energy level. Although, previously, pen-
tathienoacene was reported to gradually decompose to give a
The thiophene moiety, as in 3-alkylthiophene, has been
successfully used in organic-electronic and optical applications
due to its unique electronic and optical properties.1
2-16
Oligo-
meric thiophenes have also been successfully employed as the
9
17-19
greenish solution in chloroform due to possible oxidation, a
active components in organic FET and light-emitting devices.
2
0
device fabricated in a recent report indicated that pen-
Recently, McCulloch et. al. reported a new polymer system
which contains a two-ring fused thiophene moiety. A mobility
of 0.6 cm /V‚s was reported. However, the majority of the
7
tathienoacene is quite stable. Direct current (dc), electronic
2
resistivity measurements of crystalline tetrathienoacene (II) at
room temperature in dry air demonstrated a resistivity of F )
1
2
1
.7 × 10 Ω‚cm that is in the same range as that of the nine
(12) Huynh, W. U.; Dittmer, J. J.; Alivastatos, A. P. Science 2002, 295,
12 11
benzene ring compound, violanthrene (F ) 3.7 × 10 Ω‚cm).
2425-2427.
These data indicate that tetrathienoacene has the potential to
be a very good organic semiconductor. A recent report from
(13) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359-
3
69.
(
14) Tour, J. M. Acc. Chem. Res. 2001, 34, 791-804.
7
Zhu has confirmed that pentathienoacene is a semiconductor
with a measured mobility of 0.045 cm /V‚s and an on/off ratio
(
15) Tourillon, G. Handbook of Conducting Polymers; Dekker: New
2
York, 1986; p 293.
16) Mullen, K.; Wegner, G. Electronic Materials: The Oligomer
(
Approach; John Wiley: New York, 1998.
(
7) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang, F.; Gao, J.; Qiu, W.;
(17) Dimitrakopoulos, C. D.; Malenfant. P. R. L. AdV. Mater. 2002, 14,
Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.; Hu, W.; Zhu, D. J.
Am. Chem. Soc. 2005, 127, 13281-13286.
99-117.
(18) Dodabalapur, A.; Torsi, L.; Katz, H. E. Science 1995, 268, 270-
271.
(8) Osuna, R. M.; Zhang, X.; Matzger, A. J.; Hernandez, V.; Navarrete,
J. T. L. J. Phys. Chem. A 2006, 110, 5058-5065.
(19) Uchiyama, K.; Akimichi, H.; Hotta, S.; Noge, H.; Sakaki, H. Synth.
Met. 1994, 63, 57-59.
(20) Mcculloch, I.; Heeney, M.; Bailey, C.; Genevicius, K.; Macdonald,
I.; Shkunov, M.; Sparrowe, D.; Tierney, S.; Wagner, R.; Zhang, W.;
Chabinyc, M. L.; Kline, R. J.; Mcgehee, M. D.; Toney, M. F. Nat. Mater.
2006, 5, 328-333.
(
9) Mazaki, Y.; Kobayashi, K. Tetrahedron Lett. 1989, 25, 3315-3318.
(10) CRC Handbook of Chemistry and Physics, 52nd ed.; Weast, R. C.
Ph., Ed.; Chemical Rubber: Cleveland, OH, 1971-1972; p C406.
11) Mazaki, Y.; Kobayashi, K. J. Chem. Soc., Perkin Trans. 2 1992,
61-764.
(
7
J. Org. Chem, Vol. 72, No. 2, 2007 443

He and Zhang
SCHEME 3. Synthesis of Three-Membered, Alkyl-Substituted Fused Thienothiophenes
SCHEME 4. Synthesis of Alkyl-Substituted, Four-Membered Fused Thienothiophenes
thiophene materials reported in organic-electronic device
applications are still R-oligothiophenes. There are only a few
devices, we should be able to obtain more fundamental
knowledge of structure-property relationships. We also envision
that, by engineering the length of the side chains, we can achieve
good solid-state molecular packing along with high solubility
and processibility. Ultimately, these materials can be utilized
in solution-processing methods, such as ink jet printing, to
achieve high quality organic semiconductor materials.
examples using fused thiophene systems for semiconductor
applications.7
,21,22
This is due to the following reasons: (1) fused
thiophenes, such as pentathienoacene, have poor solubility and
2) synthetic methodologies, especially for the formation of
fused thienothiophenes with more than four member rings, are
(
6
,7,9,23
limited.
More recently, Yamaguchi reported an efficient
synthetic route by using a triple cyclization of bis(o-haloaryl)-
Results and Discussion
diacetylenes to synthesize fused rings,24 but this method is
Synthesis of â-Alkyl-Substituted Thienothiophene Two
Fused Ring Systems. An initial attempt to synthesize the
alkyl-substituted fused thiophenes from 3,6-dibromothieno-
limited to odd-number fused ring synthesis.
During the past several years,25 we have developed a series
of synthetic methods to prepare several alkyl-substituted fused
thiophenes with up to seven fused rings. The alkyl groups can
be distributed in either R, â, or both the R and â positions of
the fused ring. We believe that our synthetic methods supply a
systematic tool for synthesis of different size fused thiophenes.
By varying the size of fused thiophene compounds in FET
2
6
[
3,2-b]thiophene was made using a coupling reaction of
the appropriate Grignard reagent catalyzed with 1,3-bis-
diphenylphosphino)propanedichloronickel(II), which was used
successfully for the synthesis of 3-alkylthiophene and 3,4-
(
2
7,28
dialkylthiophenes.
It was found that this coupling reaction
was not suitable for fused thiophene compounds. We are not
sure of the reasons at this moment, but the rigidity change and
electron distribution change may all have contributed to this
result. Thus, an alternative approach where the alkyl groups
(
21) Li, W.-C.; Sirringhaus, H.; Garnier, F.; Holmes, A. B.; Moratti, S.
C.; Feeder, N.; Clegg, W.; Teat, S. J.; Friend, R. H. J. Am. Chem. Soc.
998, 120, 2206-2207.
22) Morrison, J. J.; Murray, M. M.; Li, X.-C.; Holmes, A. B.; Morratti,
S. C.; Friend, R. H.; Sirringhaus, H. Synth. Met. 1999, 102, 987-988.
23) Sato, N.; Mazaki, Y.; Kobayashi, K.; Kobayashi, T. J. Chem. Soc.,
Perkin Trans. 2 1992, 765-770.
24) Okamoto, T.; Kudoh, K.; Wakamiya, A.; Yamaguchi, S. Org. Lett.
005, 7, 5301-5304.
25) He, M. Preparation of Fused Thiophenes. PCT Int. Appl. WO
006031893, 2006.
1
(
(
(26) Fuller, L. S.; Iddon, B.; Smith, K. A. J. Chem. Soc., Perkin Trans.
1 1997, 1, 3465-3470.
(
(27) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L.
J. Org. Chem 1993, 58, 904-912.
2
(
(28) He, M.; Leslie, T. M.; Sinicropi, J. A. Chem. Mater. 2002, 14, 4662-
4668.
2
444 J. Org. Chem., Vol. 72, No. 2, 2007

Alkyl-Substituted Fused Thiophenes
SCHEME 5. Hellberg’s Improved Method
Crafts reaction of 8 at 0 °C gave a mixture of 9 and 10 in an
approximate 1:3 ratio based on GC/MS analysis. The separation
of these isomers was not attempted. The mixture was used
directly for the next step since the ring closure reaction can
only take place on compound 10, leaving compound 9 unreacted.
After hydrolysis of the mixture, 12 could easily be separated
from 9. Decarboxylation of 12 yielded compound 13 as a waxy
white solid.
SCHEME 6. Synthesis of Five-Membered, â-Disubstituted
Fused Thienothiophenes
Synthesis of Three- and Four-Membered â-Alkyl-Substi-
tuted Fused Thienothiophenes. Although dimethyl and diphe-
nyl dithienothiophenes (three fused thiophene rings) have been
30-32
previously synthesized,
there are no effective methodologies
to make the long alkyl-chain derivatives in a convenient way.
Likewise, alkyl-substituted, four fused thiophene ring com-
pounds have not been reported in the literature so far. The
previous three-ring syntheses included two, two-step reactions.
First, butyllithium was used to generate an anion from 3-bromo-
4-alkylthiophene, followed by the addition of SCl2 or (PhSO3)2S
as a coupling reagent. Second, the connected bisthienylsulfide
was treated again with butyllithium, followed by addition of
CuCl2 to oxidize the dianion for ring closure. Since the dianions
are generated by butyllithium without any selectivity, this
reaction results in only moderate yields. Alternatively, Holmes
SCHEME 7. Synthesis of
3
-Bromo-6-alkylthieno[3,2-b]thiophene
3
3
et al. reported using 2,5-diformyl-3,4-dibromothiophene to
synthesize the unsubstituted fused three-ring compound. Fol-
lowing our strategy for alkyl-substituted thienothiophenes, we
used diketones for ring closure. An attempt to run a Friedel-
Crafts diacylation reaction on 3,4-dibromothiophene produced
only a monoketone on the thiophene ring. Apparently, the first
aliphatic carbonyl deactivates the ring toward further acylation.
Since cation chemistry proved unfruitful, anion chemistry was
employed. Starting with 2,3,4,5-tetrabromothiophene (14) and
butyllithium, the 2,5-dilithio-3,4-dibromothiophene dianion was
generated at -78 °C (Scheme 3). The dianion was quenched
with the appropriate aldehyde to yield the desired dialcohols.
The dialcohol was easily oxidized to the corresponding diketone
using Jones’ reagent. The â-dialkyl-substituted fused thienothio-
phenes were obtained in reasonably good yields following the
ring closure procedure used for the two-ring system.
SCHEME 8. Unsuccessful Synthesis of Seven-Membered,
â-Disubstituted Fused Thienothiophenes
Fused thiophenes comprised of four fused rings were also
synthesized using the same method, as illustrated in Scheme 4.
The desired starting compound, 2,3,5,6-tetrabromothieno[3,2-
b]thiophene, 20, was obtained according to Fuller and Smith’s
26
report. This compound was mixed with dry THF before being
reacted with butyllithium at -78 °C. The resulting dianion was
then quenched with the aldehyde. Although the starting material
was not totally dissolved in the solvent, the dialcohol was
obtained in a good yield.
are added during the ring closure step was taken. The mono-
substituted, two fused thiophene ring compound was synthesized
from 3-bromothiophene using the methodology reported by
Synthesis of Five- and Seven-Membered Alkyl-Substituted
Fused Thienothiophenes. The design of a synthetic route to
prepare alkyl-substituted, five-membered fused thiophene ma-
2
9
Matzger et al. (Scheme 1).
Although the di-â-substituted, two fused thiophene ring
system was also reported by Matzger,29 we decided to develop
an alternative synthetic route by using a ketone-based ring
closure procedure for the synthesis of di-â-substituted alkylth-
ienothiophenes (Scheme 2). Although this route is less efficient,
these reaction steps can be finished very quickly with no
separations required until the desired acid compound 12 is
formed. Compound 7, resulting from the bromination of 6, was
partially debrominated by butyllithium to give 8. A Friedel-
34
terials was more challenging. Hellberg’s improvements to
35
Jong’s and Janssen’s previous synthesis of dithienothiophene
(
30) Barbarella, G.; Favaretto, L.; Sotgiu, G.; Antolini, L.; Gigli, G.;
Cingolani, R.; Bongini, A. Chem. Mater. 2001, 13, 4112-4122.
31) Sotgiu, G.; Favaretto, L.; Barbarella, G.; Antolini, L.; Gigli, G.;
Mazzeo, M.; Bongini, A. Tetrahedron 2003, 59, 5083-5090.
32) Sotgiu, G.; Zambianchi, M.; Barbarella, G.; Aruffo, F.; Cipriani,
(
(
F.; Ventola, A. J. Org. Chem. 2003, 68, 1512-1520.
(33) Frey, J.; Bond, A. D.; Holmes, A. B. Chem. Commun. 2002, 2424-
425.
2
(
34) Allared, F.; Hellberg, J.; Remonen, T. Tetrahedron Lett. 2002, 43,
(
29) Zhang, X.; Kohler, M.; Matzger, A. J. Macromolecules 2004, 37,
1553-1554.
6
306-6315.
(35) Jong, F. D.; Janssen, M. J. J. Org. Chem. 1971, 36, 1645-1648.
J. Org. Chem, Vol. 72, No. 2, 2007 445

He and Zhang
SCHEME 9. Synthesis of Tetra-Substituted Fused Thiophene
was considered. In the original work, 3-thienyllithium was
generated from 3-bromothiophene at -78 °C, followed by
reaction with bis(phenylsulfonyl)sulfide to form 3,3′-dithienyl
sulfide. This sulfide was reacted again with butyllithium at low
temperature (-78 °C) and oxidized by CuCl2 to produce the
desired three-membered fused thiophene. Usually, good yields
are obtained for the first step due to the high reactivity of
promising result, we designed a method to synthesize five-
membered fused thiophenes, as shown in Scheme 6.
The key intermediate for this synthesis is compound 26. The
corresponding synthetic scheme for synthesis of 26 is very
straightforward and is shown in Scheme 7. The 3,4-dibro-
mothiophene, 32, was treated under Friedel-Crafts acylation
conditions and only gave a single acylated product, 33. This
ketone was reacted with ethyl thioglycolate to close the ring.
Finally, compound 26 was obtained by decarboxylation of acid
35 in quinoline with a copper catalyst. However, when we tried
the same set of reactions to form a seven-membered fused
thiophene ring (Scheme 8), the ring closure reaction did not
proceed well. Since compound 30 is a highly crystalline solid,
it can only dissolve into hot toluene and other aromatic solvents.
It is possible to dissolve compound 30 in hot THF, but the
3
-thienyllithium with bis(phenylsulfonyl)sulfide. However, the
second step usually gives poor yields due to low selectivity
during lithiation of the 3,3′-dithienyl sulfide. In order to improve
the yield, Hellberg et al.34 used 2,3-dibromothiophene as the
starting material. In this case, both the oxidation reaction (CuCl2)
and the ring closure reaction using bis(phenylsulfonyl)sulfide
gave good yields due to better selectivity in generating the
appropriate anions during the lithiation (Scheme 5).
Hellberg’s approach still has some limitations. First, the R,â-
dibromothiophene is very difficult to obtain. Second, all of the
reactions were run at -78 °C, which limits the synthesis of
some larger fused thiophene ring systems due to solubility
issues. To circumvent these issues, we considered the work of
Fuller et al. again.26 In their report, when lithium diisopropy-
lamide (LDA) reacts with 3-bromothiophene at 0 °C, the
deprotonation occurs almost exclusively in the R position.
Although we did not find any reports using LDA/CuCl2 for
oxidative coupling reactions, we tested the feasibility using
3
4
Hellberg reaction is usually conducted at -78 °C, and at this
low temperature, all of the material precipitated out from the
solvent and no reaction was observed. Almost all of the starting
material could be recovered after workup without loss of
bromine after treating 30 with butyllithium at -78 °C. The
reaction was also attempted at 0 °C in hexane, and similar results
were obtained.
In order to synthesize seven-membered fused thiophene rings
with appropriate solubility, two more alkyl substituents were
added to the ring. This synthetic route is illustrated in Scheme
9. Two versions of compound 26 were prepared, 26 with R )
n-C10H21 and 26a with R ) CH3. Treatment of 26a with LDA
followed by 1-formylpiperidine at 0 °C resulted in incorporation
3
0
-bromothiophene as the starting material to react with LDA at
°C. The coupled compound could be successfully obtained
by reaction of the anion with CuCl2. On the basis of this very
446 J. Org. Chem., Vol. 72, No. 2, 2007

Alkyl-Substituted Fused Thiophenes
thiophene at -78 °C, followed by quenching with water.
Although we made several attempts to repeat this reaction,
almost all gave a mixture of 60-70% of 43 and 40-30% of
1
4
4 based on GC/MS and H NMR. Finally, we found that, when
the reaction was conducted at 0 °C in the presence of N,N,N′,N′-
tetramethylethylenediamine (TMEDA) to stabilize the anion,
4
3 could be converted to 44. Although the yield is moderate,
separation was much easier due to a minimized amount of the
6
mixture of 43 and 44. We then used Matzger’s coupling
reaction procedure, followed by the ring closure procedure, to
produce compound 46a. We also prepared a tetradecanyl seven
ring fused thiophene compound 46 when compound 26 (R )
C10H21) was used as the starting material.
Characterization. The UV-vis absorption spectra obtained
for the alkyl-substituted fused thiophenes in chloroform are
shown in Figure 2, and the absorption maxima are summarized
in Table 1. The spectra are shown in two plots for simplicity.
Although the optical properties (UV-vis) of fused thiophenes
have been reported before, almost all of the reported spectral
9,24
data are for unsubstituted fused ring systems. It has not been
possible to systematically study the effect of alkyl substitution
on the absorption of fused thiophenes due to the unavailability
of materials. Even though flexible alkyl chains were placed on
the fused ring systems, the rigid π systems still demonstrated
the same absorption patterns (in solution) as those of the
unsubstituted versions. The vibronic fine structure of the
absorption is evident for the compounds containing two to seven
fused rings. Although the fused thiophene compounds demon-
strate the expected red shifts as the number of fused rings is
increased, when compared to their benzene analogues such as
pentacene and the recently synthesized acenedithiophene,4 the
π-π* transitions are still significantly blue-shifted. FT5 has
its longest absorption at 362 nm, while pentacene has its at 575
,10
10
nm. Likewise, FT7 has its longest absorption at 409 nm, while
4
acenedithiophene has its at 762 nm. Why do the fully
conjugated fused thienothiophenes consistently display a sig-
nificantly higher energy band when compared to that of the all
carbon analogues? Matzger et al. recently stated that planariza-
tion of R-linked thiophenes acts to narrow the HOMO-LUMO
gap, while adding sulfur atoms acts to widen it. This widening
is thought to be due to the LUMO of R-linked thiophenes having
the appropriate symmetry for interacting with the sulfur atoms
introduced into fused thiophene structures, resulting in LUMO
FIGURE 2. (a and b) UV-vis spectra of fused thienothiophenes.
(a) FT2, FT3, and FT4 absorption spectra. (b) FT5 and two FT7
absorption spectra.
of the aldehyde function adjacent to the bromine only (36). After
ring closure, the reactions were conducted as in the previous
procedures. Compound 39 was dissolved into DMF and reacted
with NBS in the dark to give 40. Compound 40 easily reacted
with 1-decyne under Sonogashira36 reaction conditions to give
good yields of the acetylene coupling product. However, when
the alkyne group is linked in the R position, the reduction
reaction becomes more difficult than that with the â-linked
alkyne-fused thiophene compound.29 Is the R-linked alkyne
compound more effectively conjugated than the â-linked alkyne
compound, which resulted in much harsher reaction conditions?
More detailed study is needed before a conclusion can be
reached. Reduction of the alkyne 41 by a Pd/C-catalyzed
hydrogenation took 7 days, yielding the desired alkane. The
one-side dialkyl-substituted fused thiophene compounds were
reacted with NBS again to yield 43. Halogen migration reactions
have been previously reported6 to be successful in moving
the R-bromine to the â position in both thiophene and fused
3
8
destabilization.
8
Osuna et al. have recently reported the HOMO-LUMO
energy band gaps of unsubstituted FT5 and FT7. When our
results are compared with their report, it appears that alkyl
substitution shifts the absorption to slightly lower energies. The
unsubstituted FT5 and FT7 have their longest absorptions at
355 and 396 nm, respectively, whereas the substituted versions
absorb at 362 and 409 nm, respectively. The additional 7 nm
red shift in the substituted FT5 should be due to the extra
electron donation from the â-alkyl groups, while the 13 nm red
shift in the substituted FT7 could be a combination of both R-
and â-alkyl group effects.
Stability and Solubility. The thermal behavior of the alkyl-
substituted fused thiophene materials was investigated by
thermal gravimetric analysis (TGA) and differential scanning
calorimetry (DSC). These results are shown in Figures 3 and
,37
(36) Sonogashira, K. In Handbook of Organopalladium Chemistry for
4
. These data demonstrate that the substituted FT4, FT5, and
Organic Synthesis; Negishi, E., de Meijere, A., Eds.; Wiley-Interscience:
New York, 2002; Vol. 1, pp 493-529.
FT7 (note, DMDC10FT7 is compound 46a) have exceptional
(38) Zhang, X.; Matzger, A. J. J. Org. Chem. 2003, 68, 9813-9815.
J. Org. Chem, Vol. 72, No. 2, 2007 447
(
37) Kano, S.; Yuasa, Y.; Yokomatsu, T.; Shibuya, S. Heterocycles 1983,
1
0, 2035-2037.

He and Zhang
TABLE 1. Maximum UV-Vis Absorption of Fused Thienothiophene in CHCl3 Solution
FIGURE 3. TGA results of FT4, FT5, and FT7 in air.
thermal stability in air, even up to 340 °C. In the TGA traces,
we noticed that there are two decompositions in the curves. We
put all of the three compounds into a sublimation apparatus
under nitrogen for tests. After heating them up to their
decomposition temperature, all three compounds became dark,
and no evidence of sublimation was observed.
phases. The higher melting point of FT4, when compared to
that with FT5, is most likely due to a different molecular
symmetry.
All of the fused thiophene compounds exhibited some
solubility in organic solvents. Due to their smaller ring size,
FT2 and FT3 are the most soluble compounds in the series.
Both are soluble in hexane, ethyl acetate, toluene, chlorobenzene,
methylene chloride, and many other organic solvents. FT4 and
FT5 are less soluble than FT2 and FT3, but they still dissolve
in hot hexane, methylene chloride, toluene, benzene, chloroben-
zene, and other aromatic solvents. FT7 has the poorest solubility
Based on DSC analysis, FT4 has a melting point at 112 °C,
while FT5 has a melting point at 109 °C, and FT7 does not
show an obvious melting point even when heated on a hot stage
and observed using polarized light microscopy. None of these
compounds have demonstrated any liquid-crystalline transition
448 J. Org. Chem., Vol. 72, No. 2, 2007

Alkyl-Substituted Fused Thiophenes
FIGURE 4. DSC results of FT4, FT5, and FT7.
FIGURE 5. (a) Two layers of FT4. (b) Packing structure of FT4 viewed through the tilted BC plane.
in the series. It is soluble in warm toluene, xylene, and
chlorobenzene. All of these compounds can be processed into
thin films through different solution methods.
Single-Crystal X-Ray. It was possible to obtain single
crystals of FT4 and FT5 suitable for X-ray analysis by slow
evaporation from a toluene solution. The detailed crystal-
J. Org. Chem, Vol. 72, No. 2, 2007 449

He and Zhang
FIGURE 6. (a) Two layers of FT5. (b) Packing structure of FT5 viewed through the tilted AB plane.
lographic data can be found in the Supporting Information. FT4
and FT5 showed different symmetry and packing motifs. FT4
crystallizes in the P1 space group (triclinic crystal system), while
FT5 crystallizes in the C2/c space group (monoclinic crystal
system). The crystal packings are illustrated in Figures 5 and
â-substituted fused thiophenes do not show either the â-type
6
,11
or the herringbone-type of packing behavior. Both FT4 and
FT5 are very planar molecules that form layered structures. In
the FT4 crystal structure, the interlayer distance is found to be
only 3.46(5) Å, and the closest distance between a sulfur atom
in one molecule and a sulfur atom in an adjacent molecule in
the same layer is 3.58(3) Å (Figure 5). This intralayer S-S
distance is almost the same as that reported by Mazaki for the
6
. Unlike in the extensively studied polyaromatic hydrocarbon
39-41
compounds
and the in previously reported packing patterns
of unsubstituted FT4 and R-substituted fused thiophenes, the
1
1
unsubstituted tetrathienoacene (3.482 Å). Because we are
interested in carrier mobility for the fused thiophene materials,
we thought it would be relevant to consider the interlayer overlap
between π systems. We used the shortest interlayer distance
between S atoms on the fused core as a metric. In FT4, this
(
39) Gavezzotti, A.; Desiraju, G. R. Acta. Crystallogr., Sect. B 1988,
4, 427-434.
40) Desiraju, G. R.; Gavezzotti, A. Acta. Crystallallogr., Sect. B 1989,
5, 473-482.
41) Desiraju, G. R.; Gavezzotti, A. J. Chem. Soc., Chem. Commun. 1989,
21-623.
4
4
6
(
(
450 J. Org. Chem., Vol. 72, No. 2, 2007

Alkyl-Substituted Fused Thiophenes
distance was found to be 5.15(2) Å. This is relatively large
compared to the interlayer distance of 3.46(5) Å, indicating that
the interlayer overlap of the π systems is small. FT4 seems to
be packed in a ring-to-tail fashion with very little π-π orbital
overlap. This particular packing behavior may be due to the
much stronger intermolecular chain-to-chain interactions. The
strong side-chain crystallization tendency appears to have pushed
the ring to a position where the side-chain crystallization forces
and π-π stacking tendency have reached a balance. The
interlayer distance for FT5 was found to be 3.44(5) Å, which
is approximately the same as that for FT4 and is not very
different from the π-π distance of 3.52 Å reported by Matzger
for unsubstituted pentathienoacene. Within a single layer of
FT5, the shortest intermolecular S-S distance is 3.74(9) Å. This
is slightly longer than what was seen for unsubstituted and alkyl-
substituted FT4. The most significant difference that is evident
between the packing in FT4 and FT5 is the shortest S-S
interlayer distance. In contrast to FT4, this distance is much
shorter (3.90(6) Å vs 5.15(2) Å). By carefully examining the
packing motif for FT5 (Figure 6), one sees substantial π-π
overlap, which greatly contributes to the shorter S-S distance
between the molecules in different layers. In the unsubstituted
pentathienoacene, the molecules within a stack seem to line up
directly on top of each other.42 In FT5, the addition of two long
alkyl chains at the â positions of the pentathienoacence ring
leads to a similarly π-stacked structure; however, the molecules
in a stack are alternately rotated by 180° (Figure 6a). From a
structure-property point of view, FT5 has a different molecular
symmetry and net dipole direction from that of FT4. Perhaps
the orientation of the FT5 molecules alternates between layers
in order to minimize the dipolar interactions. Thus, for FT5,
the tendency of the side chains to crystallize no longer appears
to be the dominant force between molecules.
Conclusion
We have established a series of synthetic methods to obtain
alkyl-substituted thienothiophenes with up to seven fused rings.
By using these reaction methods, we have made both even and
odd â- and R,â-alkyl-substituted fused thiophene compounds.
Thermal characterization of these new fused thiophene com-
pounds has demonstrated that they are very stable. UV-vis
spectra show a large blue shift in absorption of all of the fused
thiophenes when compared to that of their hydrocarbon coun-
terparts. These larger band-gap materials are expected to have
much better photo- and electrical stability than those of the
hydrocarbon series. Since all of the compounds have long alkyl
substituents, they exhibit good solubility in common organic
solvents. These compounds may be able to be processed through
solution deposition methods such as ink-jet printing and solution
drop casting. Single-crystal X-ray structures of FT4 and FT5
demonstrated that their molecular packing behaviors are different
from previously reported data for the unsubstituted materials.
Layered molecular structures, instead of a â-type or a her-
ringbone packing motif, have been observed in both FT4 and
FT5. These facts lead us to believe that adding â-alkyl
substituents should benefit electron mobility due to a more ideal
molecular arrangement. We are actively characterizing these new
compounds and have already found some “odd-even” effects
in our FT4 and FT5 compounds. We will continue to report
our synthesis, characterization, and electrical measurements as
they become available.
42
Acknowledgment. We want to thank Professor Emil Lobk-
ovsky of Cornell University for single-crystal X-ray diffraction
studies. The authors also want to acknowledge Dr. Susan Gasper
for useful conversations and help, Mr. Michael Sorensen for
UV measurements and X-ray simulation, Ms. Liepin Huang and
Dr. Robert Burkhalter for mass spectral data, Ms. Carrie Hogue
for NMR, and Dr. Thomas Leslie and Dr. David Schissel for
useful discussions throughout all of our synthesis works.
Due to the effect of the long side chains in the â position of
these fused rings, these molecules are not packed as either a
â-type or in a herringbone fashion. Layered structures were
found for both of them. It is believed that this layered molecular
packing is a more favorable morphology for electron transport.
The short molecular layer distances indicate that all of the layers
43
are packed even tighter than in poly(3-hexylthiophene) (3.8 Å).
It should be possible to modify the chain length so as to achieve
more ring-to-ring overlap instead of ring-to-tail packing. Our
results so far indicated that layered face-to-face packing can be
achieved with the addition of â-alkyl substituents to the fused
thiophene ring.
Supporting Information Available: The detailed experimental
1
13
section, all H NMR, C NMR spectra, selected GC/MS results,
and X-ray data are available. This material is available free of charge
via the Internet at http://pubs.acs.org.
(
42) Zhang, X.; Johnson, J. P.; Kampf, J. W.; Matzger, A. J. Chem.
Mater. 2006, 18, 3486-3488.
43) Marsella, J. M.; Wang, Z.; Reid, J. R.; Yoon, K. Org. Lett. 2001,
, 885-887 (Supporting Information).
(
3
JO061853Y
J. Org. Chem, Vol. 72, No. 2, 2007 451