Synthesis, properties, and structures of benzo[1,2-b:4,5-b′]bis[b] benzothiophene and benzo[1,2-b:4,5-b′]bis[b]benzoselenophene

Article abstract of DOI:10.1021/ol701815j

(Chemical Equation Presented) Employing two consecutive cyclization reactions, benzo[1,2-b:4,5-b′]bis[b]benzochalcogenophenes, which are π-extended heteroarenes, were efficiently synthesized. Their electronic and crystal structures were elucidated on the

Full text of DOI:10.1021/ol701815j

ORGANIC
LETTERS
2
007
Vol. 9, No. 22
499-4502
Synthesis, Properties, and Structures of
4
Benzo[1,2-b:4,5-b
and
′
]bis[b]benzothiophene
Benzo[1,2-b:4,5-b
′
]bis[b]benzoselenophene
†
†
†
,†,‡
Hideaki Ebata, Eigo Miyazaki, Tatsuya Yamamoto, and Kazuo Takimiya*
Department of Applied Chemistry, Graduate School of Engineering, Hiroshima
UniVersity, Higashi-Hiroshima, 739-8527, Japan, and Institute for AdVanced Materials
Research, Hiroshima UniVersity, Higashi-Hiroshima, 739-8530, Japan
ktakimi@hiroshima-u.ac.jp
Received August 7, 2007
ABSTRACT
Employing two consecutive cyclization reactions, benzo[1,2-b:4,5-b′]bis[b]benzochalcogenophenes, which are π-extended heteroarenes, were
efficiently synthesized. Their electronic and crystal structures were elucidated on the basis of UV
−vis spectra, electrochemical measurements,
and X-ray structural analyses.
5
The synthesis and characterization of π-extended hetero-
arenes containing chalcogenophenes (i.e., thiophene and/or
selenophene) in fused aromatic ring systems are of current
interest owing to their potential applications as organic
semiconductors for various optoelectronic devices, such as
thiophene-containing peri-fused aromatics, have been de-
veloped and tested as active semiconducting channels in
OFET devices.
From the viewpoint of synthetic chemistry, however,
efficient synthetic strategies for highly extended analogues
possessing more than four aromatic rings are limited. In
6
organic field-effect transistors (OFETs), light emitting diodes
1
(
LEDs), and photovoltaic cells. In particular, their use in
this context, the Bergman cycloaromatization, which involves
OFETs has been actively investigated owing to their
structural resemblance to oligoacenes, such as pentacene, one
of the best organic semiconductors for OFET devices that
(
3) (a) Li, X.-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. (b) Xiao, K.; Liu, Y.; Qi, T.; Zhang, W.; Wang,
F.; Gao, J.; Qiu, W.; Ma, Y.; Cui, G.; Chen, S.; Zhan, X.; Yu, G.; Qin, J.;
1
2
-1 -1 2
show very high-field-effect mobility (∼3.0 cm V s ).
Hu, W.; Zhu, D. J. Am. Chem. Soc. 2005, 127, 13281-13286.
3
New π-extended heteroarenes, e.g., thieno[n]acenes, thio-
(4) (a) Laquindanum, J. G.; Katz, H. E.; Lovinger, A. J.; Dodabalapur,
1
a,4
phene-benzene annulated acene-like compounds,
and
A. AdV. Mater. 1997, 8, 36-39. (b) Sirringhaus, H.; Friend, R. H.; Wang,
C.; Leuninger, J.; Mullen, K. J. Mater. Chem. 1999, 9, 2095-2101. (c)
Takimiya, K.; Kunugi, Y.; Konda, Y.; Niihara, N.; Otsubo, T. J. Am. Chem.
Soc. 2004, 126, 5084-5085. (d) Payne, M. M.; Parkin, S. R.; Anthony, J.
E.; Kuo, C.-C.; Jackson, T. N. J. Am. Chem. Soc. 2005, 127, 4986-4987.
(e) Takimiya, K.; Ebata, H.; Sakamoto, K.; Izawa, T.; Otsubo, T.; Kunugi,
Y. J. Am. Chem. Soc. 2006, 128, 12604-12605. (f) Takimiya, K.; Kunugi,
Y.; Konda, Y.; Ebata, H.; Toyoshima, Y.; Otsubo, T. J. Am. Chem. Soc.
2006, 128, 3044-3050. (g) Yamamoto, T.; Takimiya, K. J. Am. Chem.
Soc. 2007, 129, 2224-2226.
†
Department of Applied Chemistry, Graduate School of Engineering,
Hiroshima University.
‡
Institute for Advanced Materials Research, Hiroshima University.
(
1) (a) Anthony, J. E. Chem. ReV. 2006, 106, 5028-5048. (b) See also
a special issue on organic electronics: Chem. Mater. 2004, 16, 4381-4846.
2) (a) Lin, Y. Y.; Gundlach, D. J.; Nelson, S. F.; Jackson, T. N. IEEE
(
Electron DeVice Lett. 1997, 18, 606-608. (b) Kelly, T. W.; Boardman, L.
D.; Dunbar, T. D.; Muyres, D. V.; Pellerite, M. J.; Smith, T. P. J. Phys.
Chem. B 2003, 107, 5877-5881.
(5) Takimiya, K.; Kunugi, Y.; Toyoshima, Y.; Otsubo, T. J. Am. Chem.
Soc. 2005, 127, 3605-3612.
1
0.1021/ol701815j CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/03/2007

the conversion of o-bis(ethynyl)benzene into naphthalene,
is one of the potential reactions for the synthesis of
π-extended aromatic compounds. Although to our best
Scheme 2. Syntheses of 2a and 2b
7
knowledge only a few examples of the Bergman cyclo-
8
aromatization on chalcogenophenes have been reported, we
thought that the reaction was worth considering in the
synthesis of highly extended heteroarenes and planned to
examine the Bergman double cyclization on benzo[1,2-b:
4
,5-b′]dichalcogenophenes (1, Scheme 1),9 which gives
Scheme 1. Consecutive Double Cyclization Approach to 2
in the Larock protocol to form benzo[1,2-b:4,5-b′]dichal-
1
2
cogenophene derivatives (5). The trimethylsilyl groups on
were substituted with iodine to give tetraiodo compounds
6), and these were readily converted into tetrakis(trimeth-
5
(
ylsilylethynyl) derivatives (7) by the Sonogashira coupling
reaction. Formation of the outermost benzene rings of 2 was
achieved in reasonable yields with the Bergman cycloaro-
matization, using in situ generated sodium telluride as
1
3
reagent. Under the reaction conditions of the final step,
desilylation was concomitantly induced to give 2 directly.
Both sulfur (2a) and selenium (2b) analogues were stable
pale yellow solids that were well-characterized by spectro-
scopic and combustion elemental analyses. Single-crystal
X-ray analyses enabled unquestionable characterization of
these highly extended heterocycles (vide infra).
benzo[1,2-b:4,5-b′]bis[b]benzochalcogenophenes (2)10 pos-
sessing five aromatic rings fused in a linear fashion. In this
approach, the tetraethynyl derivative of 1 is a rational
precursor. Since various derivatives of 1 are readily synthe-
sized by double electrophilic heterocyclization of the central
9
Cyclic voltammetry of 2 showed irreversible oxidation
waves (for charts, see Figure S1 in the Supporting Informa-
tion). The oxidation peaks of 2a and 2b were found at +1.15
benzene core, the combination of these two cyclization
protocols can provide a unique and efficient synthetic
approach to 2 (five aromatic rings) from a benzene derivative
+
and +0.89 V (vs Fc/Fc ). The potential of 2a was much
(one aromatic ring) via 1 (three aromatic rings). We here
higher than that of 1a (+0.95 V), whereas the potential of
report the efficient synthesis of 2 by this approach, their
properties and crystal structures, and the preliminary results
of FET characteristics of their evaporated thin films.
Scheme 2 shows the synthetic route to 2. Readily available
2
b was comparable to that of 1b (+0.89 V). On the premise
+
that the Fc/Fc energy level is 4.8 eV below the vacuum
1
4
level, HOMO levels of 2a and 2b were estimated by using
oxidation onsets (+0.96 V for 2a and +0.76 V for 2b) 5.8
and 5.6 eV below the vacuum level, respectively. These
values are consistent with the HOMO levels that were
calculated by using DFT methods (5.57 eV for 2a and 5.41
eV for 2b). It should be noted that the HOMO levels of 2
were similar to or slightly lower than those of 1, even though
the former have a more extended π-system than the latter,
indicating that the electronic structures of 2 are different from
those of 1.
11
1
,4-dibromo-2,5-bis(trimethylsilylethynyl)benzene (3) was
converted into 1,4-bis(methylthio)- or 1,4-bis(methylseleno)-
,5-bis(trimethylsilylethynyl)benzene (4), which was utilized
2
(6) (a) Okamoto, T.; Kudoh, K.; Wakamiya, A.; Yamaguchi, S. Org.
Lett. 2005, 7, 5301-5304. (b) Okamoto, T.; Kudoh, K.; Wakamiya, A.;
Yamaguchi, S. Chem.-Eur. J. 2007, 13, 548-556.
(
7) (a) Bergman, R. G. Acc. Chem. Res. 1973, 6, 25-53. (b) For an
excellent review, see: Grissom, J. W.; Gunawardena, G. U.; Klingberg,
D.; Huang, D. Tetrahedron 1996, 52, 6453-53. (c) Bowles, D. M.; Anthony,
J. E. Org. Lett. 2000, 2, 85-87.
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Org. Chem. 2005, 70, 10482-10487.
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Chem. 2005, 70, 10569-10571.
10) The synthesis of 2a with conventional synthetic strategies has been
(12) (a) Yue, D.; Larock, R. C. J. Org. Chem. 2002, 67, 1905-1909.
(b) Kesharwani, T.; Worlikar, S. A.; Larock, R. C. J. Org. Chem. 2006,
71, 2307-2312.
(13) (a) Tsuchiya, T.; Sashida, H.; Kurahashi, H. J. Chem. Soc., Chem.
Commun. 1991, 802. (b) Landis, C. A.; Payne, M. M.; Eaton, D. L.;
Anthony, J. E. J. Am. Chem. Soc. 2004, 126, 1338-1339.
(14) (a) Br e´ das, J.-L.; Silbey, R.; Boudreaux, D. S.; Chance, R. R. J.
Am. Chem. Soc. 1983, 105, 6555-6559. (b) Pommerehne, J.; Vestweber,
H.; Guss, W.; Mark, R. F.; B a¨ ssler, H.; Porsch, M.; Daub, J. AdV. Mater.
1995, 7, 551-554.
(
(
reported: (a) Ahmed, M.; Ashby, J.; Ayad, M.; Meth-Cohn, O. J. Chem.
Soc., Perkin Trans. 1 1973, 1099-1102. (b) Rao, D. S.; Tilak, B. D. J. Sci.
Ind. Res. 1958, 17B, 260-266. (c) Pandya, L. J.; Pillai, G. N.; Tilak, B. D.
J. Sci. Ind. Res. 1959, 18B, 198-202.
(11) Goldfinger, M. B.; Crawford, K. B.; Swager, T. M. J. Am. Chem.
Soc. 1997, 119, 4578-4593.
4500
Org. Lett., Vol. 9, No. 22, 2007

Supporting Information). The molecular structures of 2a and
b are the same except for slight differences in C-S and
2
C-Se bond lengths. However, the crystal structures of these
two compounds are quite different (Figure 2). The crystal
Figure 1. UV-vis spectra of 1 and 2 in dichloromethane.
The UV-vis spectra of 2 and 1 are shown in Figure 1 for
comparison. 2a and 2b have similar absorption bands,
indicating no pronounced perturbation by the substitution of
chalcogen atoms, although the absorption bands of 2b are
slightly red-shifted. Energy gaps estimated from absorption
edges are 3.3 and 3.2 eV for 2a and 2b, respectively, which
are smaller than those for 1a (3.6 eV) and 1b (3.5 eV). The
expected LUMO levels of 2a and 2b based on oxidation
potentials and absorption edges in the UV-vis spectra are
Figure 2. Crystal structures of 2a (a) and 2b (b).
2.5 and 2.4 eV below the vacuum level, respectively, and
are lower than those of 1 (2.0 eV). MO calculations using
DFT methods reproduced these experimentally estimated
energy levels of frontier molecular orbitals (see the Sup-
porting Information). From these results, we can conclude
that the fusion of two benzene rings at the periphery of 1 is
not simply π-extension in terms of the electronic structure
of the frontier molecular orbitals: the HOMO levels of 2
are comparable to those of 1, whereas the LUMO levels of
structure of 2a is based on π-stacking along the b-axis
direction in which the plane-to-plane distance is ca. 3.52 Å.
The lack of contact between the π-stacks indicates the one-
dimensional (1D) electronic structure of the crystal. In
contrast, 2b assumes the so-called sandwiched herringbone
structure in which its dimers are packed tightly in the crystal.
The plane-to-plane distance in the dimer is ca. 3.47 Å, and
short CH-π and C-Se contacts exist in a face-to-edge
manner between the dimers, indicative of the two-dimen-
sional electronic structure of the crystal.
2
are lower (Table 1).
Preliminary studies on FET devices made of 2 were carried
out. Physical vapor deposition of 2 gave homogeneous thin
Table 1. Electrochemical and UV-Vis Data of 1 and 2
Together with Estimated HOMO and LUMO Levels
films with a metallic luster on the Si/SiO
2
substrate that was
Eox/Va
λmax /nm
b
modified with octyltrichlorosilane (OTS) or hexamethyl-
c
d
anodic/onset peak/edge HOMO /eV Eg /eV LUMO/eV
1
6
disilazane (HMDS), although no film was deposited when
the substrate temperature exceeded 40 °C. Considering the
fact that small heteroaromatic systems, such as 1 (three
aromatic rings) and [1]benzochalcogenopheno[3,2-b][1]-
benzochalcogenophene (four aromatic rings, Figure 3), did
not afford homogeneous thin films by vapor deposition,
extension of the π-conjugated system in heteroarenes is
beneficial for practical application to thin-film-based organic
devices.
2
2
1
1
a
b
+1.15/+0.96 369/380
+0.89/+0.76 374/390
+0.95/+0.80 335/344
+0.89/+0.73 350/357
-5.8
-5.6
-5.6
-5.5
3.3
3.2
3.6
3.5
-2.5
-2.4
-2.0
-2.0
e
a
b
e
a
+
b
c
Versus Fc/Fc . Absorption spectra. Estimated from the onset of the
oxidation peak. Determined from the absorption edge. e Reference 9.
d
Single-crystal X-ray analyses revealed the exact molecular
15
On top of the evaporated thin films gold source and drain
electrodes were deposited through a shadow mask to
complete the fabrication of “top-contact” devices with W/L
structures of 2a and 2b, where the π-extended frameworks
are almost planar and the maximum deviation from the mean
plane is ca. 0.03 Å for 2a and ca. 0.05 Å for 2b (Figure S4,
(15) See the Supporting Information for details.
(16) Thin film XRDs are available: see the Supporting Information.
Org. Lett., Vol. 9, No. 22, 2007
4501

Figure 3. Structure of [1]benzochalcogenopheno[3,2-b][1]benzo-
chalcogenophenes.
)
ca. 30. Field-effect mobilities (µFET) evaluated from the
-
3
2
-1 -1
saturation regime were of the order of 10 cm V
s
(Table 2 and Figure 4). These inferior FET characteristics
Figure 4. Transfer characteristics of 2-based FETs fabricated on
HMDS-treated Si/SiO substrate: (a) 2a and (b) 2b.
2
Table 2. FET Characteristics of Devices Fabricated on Si/SiO
Substrates with Different Surface Treatments
2
informative for the design of stable p-channel organic
semiconductors based on new heteroaromatic systems.
Preliminary results of thin-film deposition and evaluation
of FET devices of 2a and 2b indicated that these heterocycles
with five aromatic rings can form homogeneous thin films
by vapor deposition. The obtained thin films act as semi-
conducting channels, although the field-effect mobilties are
low. Further experiments to optimize device fabrication and
to elucidate the relationship between structure and properties,
particularly molecular arrangement in the thin-film state, as
well as FET characteristics, are underway.
a
µFET /
2
-1
material
surfactant
cm V s-1
Ion/Ioff
-
-
-
-
3
3
3
3
5
2
2
a
b
OTS
HMDS
OTS
2.4 × 10
2.1 × 10
3.0 × 10
3.8 × 10
10
10
6
5
7
5 × 10
HMDS
5 × 10
a
Estimated from the saturation regime.
can be related to the molecular arrangements in the solid
state: 1D π-stacking (2a) or the sandwiched herringbone
structure (2b) may not be suitable for high-performance
OFET devices.
Acknowledgment. This work was partially supported by
a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
In summary, we have established an efficient method to
synthesize highly π-extended heteroarenes, 2a and 2b, using
two cyclization protocols. Detailed studies of the physico-
chemical properties of the heterocycles revealed that the
perturbation effect of fused benzene rings on the electronic
structure is not simple: HOMO levels of 2 are almost the
same as or slightly lower than those of 1, whereas LUMO
levels are markedly low, resulting in small HOMO-LUMO
gaps. As the control of the HOMO level is regarded to be a
key factor for the stabilization of p-channel organic semi-
conductors in FET applications,17 the present results are
Supporting Information Available: Experimental details
for the synthesis and characterization of 2a and 2b, Crystal-
lographic Information Files (CIFs) for 2a and 2b, device
fabrication, DFT calculations, and FET characteristics of
devices. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL701815J
(17) Meng, H.; Bao, Z.; Lovimger, A. J.; Wang, B.-C.; Mujsce, A. M.
J. Am. Chem. Soc. 2001, 123, 9214-9215.
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Org. Lett., Vol. 9, No. 22, 2007