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A thiophene-fused PAH molecule synthesized from sulfur.
Credit: Image courtesy of Institute of Transformative Bio-Molecules
(WPI-ITbM), Nagoya University
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Dr. Lingkui Meng, Dr. Yasutomo Segawa,
Professor Kenichiro Itami of the JST-ERATO Itami Molecular Nanocarbon
Project, Institute of Transformative Bio-Molecules (ITbM) of Nagoya
University and Integrated Research Consortium on Chemical Sciences, and
their colleagues have reported in the Journal of the American Chemical
Society, on the development of a simple and effective method for the
synthesis of thiophene-fused PAHs.
Thiophene-fused PAHs are organic molecules composed of multiple
aromatic rings including thiophene. Thiophene is a five-membered
aromatic ring containing four carbon atoms and a sulfur atom.
Thiophene-fused PAHs are known to be one of the most common organic
semiconductors and are used in various electronic materials, such as in
transistors, organic thin-film solar cells, organic electro-luminescent
diodes and electronic devices. More recently, they have found use in
wearable devices due to their lightweight and flexibility.
Thienannulation (thiophene-annulation) reactions, a transformation
that makes new thiophene rings via cyclization, leads to various
thiophene-fused PAHs. Most conventional thienannulation methods require
the introduction of two functional groups adjacent to each other to form
two reactive sites on PAHs before the cyclization can take place. Thus,
multiple steps are required for the preparation of the substrates. As a
consequence, a more simple method to access thiophene-fused PAHs is
desirable.
A team led by Yasutomo Segawa, a group leader of the JST-ERATO
project, and Kenichiro Itami, the director of the JST-ERATO project and
the center director of ITbM, has succeeded in developing a simple and
effective method for the formation of various thiophene-fused PAHs. They
have managed to start from PAHs that have only one functional group,
which saves the effort of installing another functional group, and have
performed the thienannulation reactions using elemental sulfur, a
readily available low cost reagent. The reactions can be carried out on a
multigram scale and can be conducted in a one-pot two-step reaction
sequence starting from an unfunctionalized PAH. This new approach can
also generate multiple thiophene moieties in a single reaction. Hence,
this method has the advantage of offering a significant reduction in the
number of required steps and in the reagent costs for thiophene-fused
PAH synthesis compared to conventional methods.
The researchers have shown that upon heating and stirring the
dimethylformamide solution of arylethynyl group-substituted PAHs and
elemental sulfur in air, they were able to obtain the corresponding
thiophene-fused PAHs. The arylethynyl group consists of an alkyne (a
moiety with a carbon-carbon triple bond) bonded to an aromatic ring. The
reaction proceeds via a carbon-hydrogen (C-H) bond cleavage at the
position next to the arylethynyl group (called the ortho-position) on
PAHs, in the presence of sulfur. As the ortho-C-H bond on the PAH can be
cleaved under the reaction conditions, prior functionalization
(installation of a functional group) becomes unnecessary.
Arylethynyl-substituted PAHs are readily accessible by the
Sonogashira coupling, which is a cross-coupling reaction to form
carbon-carbon bonds between an alkyne and a halogen-substituted aromatic
compound. The synthesis of thiophene-fused PAHs can also be carried out
in one-pot, in which PAHs are subjected to a Sonogashira coupling to
form arylethynyl-substituted PAHs, followed by direct treatment of the
alkyne with elemental sulfur to induce thienannulation.
"Actually, we coincidentally discovered this reaction when we were
testing different chemical reactions to synthesize a new molecule for
the Itami ERATO project," says Yasutomo Segawa, one of the leaders of
this study. "At first, most members including myself felt that the
reaction may have already been reported because it is indeed a very
simple reaction. Therefore, the most difficult part of this research was
to clarify the novelty of this reaction. We put in a significant amount
of effort to investigate previous reports, including textbooks from
more than 50 years ago as well as various Internet sources, to make sure
that our reaction conditions had not been disclosed before," he
continues.
The team succeeded in synthesizing more than 20 thiophene-fused PAHs.
They also revealed that multiple formations of thiophene rings of PAHs
substituted with multiple arylethynyl groups could be carried out all at
once. Multiple thiophene-fused PAHs were generated from three-fold and
five-fold thienannulations, which generated triple thia[5]helicene
(containing three thiophenes) and pentathienocorannulene (containing
five thiophenes), respectively. The pentathienocorannulene was an
unprecedented molecule that was synthesized for the first time by the
group's new method.
"I was extremely happy when I was able to obtain the propeller-shaped
triple thia[5]helicene and hat-shaped pentathienocorannulene, because I
have always been aiming to synthesize exciting new molecules since I
joined Professor Itami's group," says Lingkui Meng, a postdoctoral
researcher who mainly conducted the experiments. "We had some problems
in purifying the compounds but we were delighted when we obtained the
crystal structures of the thiophene compounds, which proved that the
desired reactions had taken place."
"The best part of this research for me is to discover that our C-H
functionalization strategy on PAHs could be applied to synthesize
structurally beautiful molecules with high functionalities," says
Segawa. "The successful synthesis of a known high-performance organic
semiconductive molecule, (2,6-bis(4-n-octylphenyl)-
dithieno[3,2-b:2′,3′-d]thiophene, from a relatively cheap substrate
opens doors to access useful thiophene compounds in a rapid and
cost-effective manner."
"We hope that ongoing advances in our method may lead to the
development of new organic electronic devices, including semiconductor
and luminescent materials," say Segawa and Itami. "We are considering
the possibilities to make this reaction applicable for making useful
thiophene-fused PAHs, which would lead to the rapid discovery and
optimization of key molecules that would advance the field of materials
science."
Story Source: sciencedaily.com
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