Du et al.
JOCArticle
However, n-type or bipolar materials8 needed for complemen-
tary circuits present a challenge for chemists. On one side, it is
difficult to find a suitable electrode for the injection of electrons
into the lowest unoccupied molecular orbital (LUMO) levels
of the n-type semiconductors because the LUMO levels
(>3.0 eV)9 of most n-type semiconductors are incompatible
with the work function of gold (4.8-5.1 eV). Low work function
electrodes such as Ca, Mg, and Al are not environmentally
stable.8a On the other side, the susceptibility of organic semi-
conductors to water and oxygen10 under ambient conditions
makes them technologically unattractive. One strategy to over-
come the problems is to increase the electron affinity of a
semiconducting material, which could not only lower the
LUMO levels but also improve its sensitivity to oxygen and
water. It has been reported that adding strong electron-with-
drawing groups such as fluorine, cyano, or diimide moieties to a
semiconducting core could lower the electron affinity.8a As we
know, acenes11 and heteroacenes12 with strong electron-with-
drawing substituents are scarcely studied.
been reported. Functionalization of tetracene with electron-
withdrawing groups is intriguing for the development of stable
n-type or bipolar semiconducting materials because these
functionalized acenes may possess lower LUMO levels than
their correlative parent acenes. This strategy can also help to
create molecular materials with low energy electronic transi-
tions based on donor-acceptor interactions.17
Dicyanomethylene- and tricyanovinyl-substituted18 oligo-
thiophenes with high electron mobilities have been reported.
However, dicyanovinyl-substituted oligomers19 usually intro-
duce bulky substituent spacers such as triarylamine, and dicya-
novinyl-substituted acenes and heteroacenes have not been
considered. Here, we selected the dicyanovinyl group as the
substituent and heterotetracene with sulfur or/and nitrogen as
core because of the following advantages: (a) The long, platelike
molecular shape of heterotetracene can retain the conjugation of
the whole π-system and thus facilitate carrier transport. (b) The
heteroatom affects not only on the electronic structures but also
on the solid-state structures led to easy control of the highest
occupied molecular orbital (HOMO)-LUMO energy gap and
an increase the air stability of the materials. (c) The cyano
groups have strong electron-accepting properties and may be
useful for lower electron affinity.
As an analogue of pentacene, tetracene13 and its heterocycle
derivatives received less attention due to their limited π systems.
For the reported heterotetracenes, Takimiya and co-workers
introduced two sulfur and selenium atoms in the backbone of
tetracene and obtained a series of derivatives of heterotetracene
and also constructed high-performance OFET devices.14 Bao’s
group3b and Tao’s group15 reported anthra[2,3-b]thiophene, an
analogy of tetracene, and its device application simultaneously.
Bunz’s group reported the synthesis, solid-state structures, and
aromaticity studies of two dialkynylated diazatetracenes and
their parent diazatetracenes.16 To the best of our knowledge,
the introduction of sulfur or/and nitrogen and also an electron-
withdrawing group together in the tetracene-like system has not
In this paper, we report the synthesis, solid-state structures,
and photophysical properties of novel indolo[3,2-b]benzo-
[b]thiophene or indolo[3,2-b]naphthalene-based heteroacenes
asymmetrically end-capped with a dicyanovinyl group, i.e.,
BTCN and NCN. To the best of our knowledge, this is the first
report of tetracene-like compounds with an electron-withdraw-
ing group. The target compounds are characterized in detail.
Their electronic properties are studied by using UV-vis/fluor-
escence spectroscopy, cyclic voltammetry, and density func-
tional theory (DFT). The solid-state structures of BTCN and
NCN have been investigated, and the solvatochromic behaviors
of the two compounds in different solvents were studied.
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Results and Discussion
The synthesis of the target compounds BTCN and NCN is
outlined in Scheme 1. Suzuki coupling reaction between
4-bromo-3-nitrobenzaldehyde and benzo[b]thiophene-2-
boronic acid or naphthalene-2-boronic acid gave compound
3a or 3b, respectively, in good yields (70-71%). Cyclization of
compound 3a or 3b with an improved Cadogan reaction5,20
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