Synthesis and properties of symmetric and unsymmetric dibenzothienopyrroles

Article abstract of DOI:10.1021/ol901133m

Symmetrical and unsymmetrical heteroacenes containing thiophene and pyrrole rings were synthesized. The unsymmetrical heteroacene was synthesized in two steps involving an unexpected palladium catalyzed amination of alkyl or aryl amines with benzo[b]thiop

Full text of DOI:10.1021/ol901133m

ORGANIC
LETTERS
2009
Vol. 11, No. 15
3358-3361
Synthesis and Properties of Symmetric
and Unsymmetric Dibenzothienopyrroles
Ganapathy Balaji and Suresh Valiyaveettil*
Department of Chemistry, National UniVersity of Singapore,
3 Science DriVe 3, Singapore 117 546
chmsV@nus.edu.sg
Received May 28, 2009
ABSTRACT
Symmetrical and unsymmetrical heteroacenes containing thiophene and pyrrole rings were synthesized. The unsymmetrical heteroacene was
synthesized in two steps involving an unexpected palladium catalyzed amination of alkyl or aryl amines with benzo[b]thiophene followed by
a copper catalyzed coupling. The symmetrical heteroacene was obtained by a palladium catalyzed amination reaction and also by a copper
catalyzed amidation reaction. The crystal structure, photophysical and electrochemical properties of symmetrical and unsymmetrical heteroacenes
are described.
Key advantages of using organic materials for electronic
applications are their easy accessibility in pure form,
tunability of properties and easy fabrication.1 Acenes such
as pentacene2 are common benchmarks in the field of organic
electronics with a high mobility of ∼3 cm2 V-1 s-1. But
pentacene and other higher acenes suffer from low environ-
mental stability and poor solubility in common organic
solvents which limits their practical applications.3 Het-
eroacenes are π-conjugated ladder molecules, with heteroa-
toms (e.g., N, S, O, etc.) incorporated into the structures.4
Some heteroacenes such as thieno[n]acenes,5 pyrrole based
indolocarbazole6 and thiophene-benzene annulated acene7
have been successfully tested for application as organic field
effect transistors (OFET). Most of the heteroacenes reported
in the literature are thiophene based symmetrical systems.8
Unsymmetrical heteroacenes provide an inherent advantage
of having a high dipole moment which is expected to provide
dense packing and higher transport properties.9 Synthetic
strategies for a highly extended π system possessing more
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T. J. Am. Chem. Soc. 2006, 128, 3044. (h) Yamamoto, T.; Takimiya, K.
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10.1021/ol901133m CCC: $40.75
Published on Web 07/02/2009
 2009 American Chemical Society

than four aromatic rings are limited.10 Moreover, synthesis
of unsymmetrical heteroacenes is challenging owing to the
requirement of selective reaction conditions or a long
synthetic route. Heteroacenes containing more than two
different heteroatoms have also been reported11 though only
a few unsymmetrical heteroacenes are known.9,12
Scheme 2. Synthesis of 1a and 1b
In this regard unsymmetrical heteroacenes 1, and sym-
metrical heteroacenes 2 were synthesized (Figure 1).13 Their
3-bromo-2-nitrobenzo[b]thiophene is known for aromatic
nucleophilic substitution with amines.15 The 3,3′-diben-
zo[b]thiophene was obtained from the oxidative coupling
of 5.14 Reactions involving aryl amine resulted in higher
yields due to the absence of a competing ꢀ-elimination
pathway.14a
Figure 1. Structures of symmetric and unsymmetric heteroacenes.
properties, along with their molecular organization in the
crystal lattice are presented in this study. Palladium catalyzed
amination of 3-bromothiophene 3 with alkyl or aryl amine
resulted in N-(alkyl/aryl)-N-(3′-thienyl)-3-aminothiophene
4.14 Under identical conditions, 3-bromobenzo[b]thiophene
5 failed to give N-(alkyl/aryl)-N-(3′-benzo[b]thienyl)-3-
aminobenzo[b]thiophene 6 but yielded N-(alkyl/aryl)-N-(2′-
benzo[b]thienyl)-3-aminobenzo[b]thiophene 7 (Scheme 1).
Since the Pd(OAc)2/P(tBu)3 catalyst system was successful
in the amination of 3-bromothiophene, initially the same
catalyst was used for the amination of 3-bromobenzo[b]-
thiophene.14 Other catalysts such as Pd2(dba)3/ ( BINAP,
Pd2(dba)3/P(tBu)3, Pd(OAc)2/ ( BINAP, Pd(OAc)2/PCy3
were also employed and only product 7 was obtained, but
in lower yield. Toluene and o-xylene were employed as
solvents for the amination reaction and both solvents yielded
the same amination products without significant difference
in yield. The target compounds 1a and 1b were synthesized
via cyclization of 7, using butyl lithium and anhydrous cupric
chloride.
Scheme 1. Comparison of Reactivity of 3-Bromothiophene and
3-Bromobenzo[b]thiophene under Palladium Catalyzed
Amination Condition
As the palladium catalyzed amination reaction resulted in
rearranged product, symmetrical heteroacenes were prepared
by a different synthetic approach. Symmetrical heteroacenes
2a and 2b were synthesized from 2,3-dibromobenzo[b]-
thiophene 8. The synthetic route leading to these compounds
is represented in Scheme 3. Compound 916 was synthesized
Scheme 3. Synthesis of 2a and 2b
This unexpected reactivity formed the basis for the
synthesis of unsymmetrical heteroacenes. When 3-bro-
mobenzo[b]thiophene, was treated with half an equivalent
of alkyl or aryl amine, 7a or 7b was obtained in 40-45%
yield along with 3,3′-bibenzo[b]thiophene (20-30%) and
3-aminobenzo[b]thiophene (10-15%) (Scheme 2). A
stepwise procedure also resulted in the same rearranged
product 7, possibly due to the rearrangement of N-
functionalized 3-aminobenzo[b]thiophene under the cou-
pling conditions (in the catalytic cycle). No such reaction
has previously been reported for amination reactions in
the literature. The rearrangement of a similar system,
from 2,3-dibromobenzo[b]thiophene by treatment with BuLi
followed by addition of anhydrous CuCl2.
Org. Lett., Vol. 11, No. 15, 2009
3359

Herein, pyrrole fusion was achieved by employing a
recently reported amidation procedure.17 Cyclization pro-
ceeded with concomitant hydrolysis and gave 10 in 25%
yield. Heteroacene 10 was then alkylated with butyl bromide
to yield 2a. Compound 2b was synthesized by palladium
catalyzed double N-arylation of 4-hexylaniline with 3,3′-
dibromo-[2,2′]bibenzo[b]thiophene 9.18 It is notable that our
entire synthetic route leading to five ring fused systems
involves only two steps. The synthesized hetereoacenes were
characterized by NMR spectroscopy (1H and 13C), elemental
analysis and X-ray crystallography. These compounds are
soluble in common organic solvents such as hexane, chlo-
roform, toluene, ethyl acetate and methanol.
Single crystals of compounds 1a, 1b and 2b suitable for
single crystal X-ray diffraction studies were obtained by slow
evaporation from a chloroform solution. The crystal packing
of symmetric and asymmetric heteroacene are compared.
Structural analysis indicated that these heteroacenes are
planar and their molecular packing in the crystal lattice is
shown in Figure 2. The bond length of N-C(Ph) in
with respect to the acene plane. This may be due to
unsymmetrical electron cloud distribution in 1b or steric
interactions between the ortho hydrogen atoms of the
terminal benzene rings in 2b. Heteroacene 1a packs in an
edge-to-face manner, leading to a sandwiched herringbone
arrangement with a distance of 3.54 Å between sandwiched
molecules. In the case of 1b, the molecules form π-stacked
dimers which interact with the dimers of adjacent columns
by S···H (2.87 Å) interactions. The sulfur atom anti to the
pyrrole nitrogen is involved in this interaction. The molecular
packing, which is a critical parameter to consider for
applications,19 is influenced by the substituents on the
nitrogen atom of the pyrrole ring. In the case of symmetrical
molecule 2b, a molecular dimer is formed by short S···S
contacts (3.3 and 3.6 Å). Such dimers are well separated by
a pendant phenyl group and the alkyl chain attached to it. In
short, packing in the symmetrical acene (2) is mainly effected
by S···S contact whereas in the case of an unsymmetrical
acene (1a,b) packing is dominated by S···H interaction.
The UV-vis spectra of 1 and 2 in CHCl3 are shown in
Figure 3. All unsymmetrical compounds showed similar
Figure 3. UV-vis spectra of heteroacenes 1 and 2.
absorption properties with λmax at 266 and 323 nm and an
absorption onset at 343 nm. The symmetrical compounds
showed absorption peaks at 263 and 326 nm with an
absorption onset at 360 nm. The absorption maxima of
symmetrical heteroacenes are red-shifted compared to those
of unsymmetrical heteroacenes indicating a better conjugation
in 2. The bandgap estimated from absorption onset was found
to be 3.62 and 3.44 eV for 1 and 2 respectively. The
relatively lower band gap of 2 compared to 1 also suggests
a better delocalization of π electrons in 2.
Electrochemical properties of the compounds 1 and 2 were
investigated using cyclic voltammetry with Ag/AgCl as the
reference electrode. Electrochemical data and energy levels
are summarized in Table 1. All heteroacenes exhibit similar
electrochemical behaviors with reversible oxidation waves.
Compound 1a showed an oxidation peak maximum at 1.19
V with an onset potential around 0.87 V, while 1b showed
an oxidation peak at 1.17 V with an oxidation onset of 0.94
V. The oxidation peaks of 2a and 2b were found at 1.14
and 1.25 V. HOMO energy level of 1a and 1b calculated
from the oxidation onset potential using the empirical formula
Figure 2. ORTEP drawings and packing of (A) 1a, (B) 1b and (C)
2b.
unsymmetrical compound 1b is similar (1.43 Å) to sym-
metrical compound 2b (1.44 Å). The pendant phenyl group
in 1b and 2b are twisted by an angle of 46.7° and 81.8°,
3360
Org. Lett., Vol. 11, No. 15, 2009

energy level and the optical band gap. Unlike unsymmetrical
compound 1b, symmetrical compound 2b is influenced
considerably by an aryl substituent, which leads to a
noticeable difference in oxidation potential and oxidation
onset.
In summary, we report a two-step synthesis and charac-
terization of symmetrical and unsymmetrical heteroacenes
containing both pyrrole and thiophene rings. These acenes
are highly soluble and stable at ambient conditions. Further
studies on the reactivity of benzo[b]thiophene substrates
under palladium catalyzed coupling conditions and incor-
poration of these compounds in OFET application are in
progress in our labaratory.
Table 1. Photophysical and Electrochemical Data of
Compounds 1 and 2
b
onset
λabs/nm
Eox Eox
band
EHOMO ELUMO
(eV)
compound (in CHCl3) (V)
(V) gapa (eV) (eV)
1a
1b
2a
2b
266, 323
266, 323
263, 326, 346 1.14 1.01
263, 326, 346 1.25 1.13
1.19 0.87
1.17 0.94
3.62
3.62
3.44
3.44
-5.25 -1.63
-5.32 -1.7
-5.39 -1.95
-5.51 -2.07
a Calculated from onset wavelength in absorption spectra. b Derived
from bandgap and the HOMO.
EHOMO ) -(Eoxonset + 4.38) eV20 are comparable (-5.25 and
-5.32 eV). As the reduction peaks are out of our scan range,
the LUMO energy levels are calculated from the HOMO
Acknowledgment. We thank Agency for Science, Tech-
nology and Research (A*STAR), National University of
Singapore for financial support and Miss Tan Geok Kheng
of Department of Chemistry, National University of Sin-
gapore for the X-ray crystal diffraction experiment and data
refinement.
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