Sulfur containing stable unsubstituted heptacene analogs

Article abstract of DOI:10.1021/ol2028724

Heptacene and higher polyacenes have been sought for their high degree of charge carrier mobility in thin film transistors. Such acenes are, however, extremely unstable in ambient conditions. Approaches to obtain stable heptacenes include introduction of suitable substituents, of heteroatoms, or both. Methods to synthesize stable linear and a nonlinear unsubstituted heptacene analogs are discussed. No changes in the UV-visible absorption spectra in solution over time for these materials indicate higher oxidative stability compared to analogous polyacenes.

Full text of DOI:10.1021/ol2028724

ORGANIC
LETTERS
2012
Vol. 14, No. 1
78–81
Sulfur Containing Stable Unsubstituted
Heptacene Analogs
Puran K. De and Douglas C. Neckers*
Center for Photochemical Sciences, Bowling Green State University, Bowling Green,
Ohio 43403, United States
neckers@photo.bgsu.edu
Received October 24, 2011
ABSTRACT
Heptacene and higher polyacenes have been sought for their high degree of charge carrier mobility in thin film transistors. Such acenes are,
however, extremely unstable in ambient conditions. Approaches to obtain stable heptacenes include introduction of suitable substituents, of
heteroatoms, or both. Methods to synthesize stable linear and a nonlinear unsubstituted heptacene analogs are discussed. No changes in the
UVÀvisible absorption spectra in solution over time for these materials indicate higher oxidative stability compared to analogous polyacenes.
Acenes with five or more fused aromatic rings have been
long known as promising materials for applications in high
performance and low-cost organic thin film transistors.1À8
However, due to their high HOMO energy, such systems
are prone to rapid oxidative degradation under ambient
conditions.9À15 Therefore, the search for ideal candidates
that will be suitable for solution processing as well as stable
under ambient conditions and possess high-charge mobil-
ity has been ongoing.1,3,9,12,16,17
Pentacene is the largest unsubstituted acene to have been
isolated, purified, and characterized. Its thin film transis-
tor mobility is similar to that of amorphous silicon.18À21
Neckers and co-workers reported successful photogenera-
tion of unsubstituted hexacene and heptacene and regis-
tered the photophysical properties of these elusive com-
pounds.22 Bettinger and co-workers recently reported the
photogeneration of unsubstituted octacene and nonacene.23
However, because of their lack of stability under ambi-
ent conditions, further attempts to introduce suitable sub-
stitutions to obtain oxidatively stable hexacene, heptacene,
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Figure 1. Molecular structures of compounds 1 and 2.
r
10.1021/ol2028724
Published on Web 11/30/2011
2011 American Chemical Society

and higher acenes are also ongoing.16,17,23,24 As an alter-
native, higher thienoacenes containing five to seven fused
systems have been prepared.9,25À29 It should be clear
however that such thioenoacenes exist in more than one
isomer.19 Recently, thienothiophenes have attracted great
attention due to high hole mobility and stability in the
air.30,31 Compared to the oligothiophenes, the limited num-
ber of sulfur atoms in such compounds results in smaller
reorganizational energy and hence higher mobilities.32
This has inspired much previous work33,34 substituting
different molecular fragments, such as thiophene and
acene, in order to develop organic semiconductors with
high mobility and stability. Bao, Aspuru-Guzik, and co-
workers35 have shown in their recent work, using compu-
tational discovery and experimental characterization, that
suitable substitution of thiophene and acenes can lead to
new high-performance organic semiconductors for various
electronic applications. The compounds discussed in this
report35 have thiophene rings at the center of the acene
fragments. Itwill, therefore, be importanttoinvestigatethe
hole mobility and air stability of thienoacenes with the
thiophene ring at the center of the acene fragments. Taken
together, the present work is motivated by the fact that
while there are several reports on thienoacenes,25À29 few
attempts have been made to synthesize unsubstituted
thienoacenes with the thiophene ring at the center of the
acene fragments due to the challenges encountered in their
synthesis and their existence in several isomeric forms.
We have now prepared two stable unsubstituted hepta-
cene analogs with the thiophene ring at the center of the
acene fragments; thienoacenes, one containing seven lin-
early fused aromatic rings (DAT-2,3; 1), and another with
seven nonlinearly fused aromatic rings (DAT-1,2; 2)
(Figure 1). We discuss the synthetic strategy, spectral
1
characterization using mass and H NMR spectra, and
absorption spectra. Monitoring the absorption spectra in the
UVÀvis region over time in solution (o-dichlorobenzene)
indicates higher stability for compounds 1 and 2 compared
to analogous polyacenes. Due to a heptacene-like struc-
ture, each might have favorable two-dimensional interac-
tions in the thin films in the solid state and is thus expected
to have a high degree of charge carrier mobility when
assembled in organic transistors.
Toavoidsolubility issues, weintroduced suitableremov-
able substitution (TMS-acetylene in this case) and utilized
a zirconia mediated coupling method36 for the synthesis of
1, Scheme 1. Commercially available o-xylenedibromide
was silylated using trimethyl silylacetylene to obtain bis-
silylated diyne (3). Diyne 3 was cyclized through zirco-
nium-mediated cyclization and coupling reactions devel-
oped by Takahashi et al.37 The four silyl groups were
then effectively removed by treatment with triflic acid38
(CF3COOH) to obtain a tetrahydro heptacene analog (5).
Subsequent aromatization of 5 afforded the target com-
pound, DAT-2,3 (1). Further purification through sub-
limation produced yellowish orange 1. DAT-2,3 (1) was
characterized by mass and 1H NMR spectra (Supporting
Information, SI). However, due to its low solubility in
common NMR solvents, the 13C NMR spectra could not
be recorded.
(16) Kaur, I.; Stein, N. N.; Kopreski, R. P.; Miller, G. P. J. Am.
Chem. Soc. 2010, 132, 1261.
(17) Kaur, I.; Stein, N. N.; Kopreski, R. P.; Miller, G. P. J. Am.
Chem. Soc. 2009, 131, 3424.
The synthesis of 2, a nonlinear isomer of 1, is outlined in
Scheme 2. Commercially available bromomethyl naphtha-
lene was converted to phosphonium ylide 6 using known
literature methods via treatment with triphenyl phosphine.38
Ylide 6 was then treated with 2,5-thiophene dicarboxalde-
hyde, synthesized following previously reported proce-
dures,39 in a Wittig type reaction to obtain 7. Photocycliza-
tionof7 inthepresenceof iodine inethylene oxideafforded
2 as the major product. The conversion of 7 to 2 involves a
trans to cis conversion of 7, followed by photocyclization
under light (λ ≈ 300 nm). Compound 7 undergoes photo-
isomerization to cis-conformations around each of the
carbonÀcarbon double bonds to adopt an all-cis confor-
mation (cis-7). Cis-7 undergoes facile photoinduced elec-
trocyclization to form the tetrahydro heptacene analog 8
which aromatizes under oxidative conditions (I2). A mer-
cury arc lamp (λ ≈ 300 nm; filter with 325 nm cutoff,
intensity ∼1.03 mW C mÀ2) was used as the light source,
and compound 7 was irradiated overnight to achieve
(18) Tang, M. L.; Mannsfeld, S. C. B.; Sun, Y.-S.; Becerril, H. A.;
Bao, Z. J. Am. Chem. Soc. 2009, 131, 882.
(19) Tang, M. L.; Okamoto, T.; Bao, Z. J. Am. Chem. Soc. 2006, 128,
16002.
(20) Nelson, S. F.; Lin, Y. Y.; Gundlach, D. J.; Jackson, T. N. Appl.
Phys. Lett. 1998, 72, 1854.
(21) Klauk, H.; Halik, M.; Zschieschang, U.; Schmid, G.; Radlik,
W.; Weber, W. J. Appl. Phys. 2002, 92, 5259.
(22) Mondal, R.; Shah, B. K.; Neckers, D. C. J. Am. Chem. Soc. 2006,
128, 9612.
(23) Tonshoff, C.; Bettinger, H. F. Angew. Chem., Int. Ed. 2010, 49,
412.
(24) Purushothaman, B.; Bruzek, M.; Parkin, S. R.; Miller, A.-F.;
Anthony, J. E. Angew. Chem., Int. Ed. 2011, 50, 7013.
(25) Payne, M. M.; Parkin, S. R.; Anthony, J. E.; Kuo, C. C.;
Jackson, T. M. J. Am. Chem. Soc. 2005, 127, 4986.
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2005, 127, 8028.
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2004, 6, 3325.
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J.; Bredas, J. L. J. Chem. Phys. 2003, 118, 3764.
(30) Yamamoto, T.; Takimiya, K. J. Photopolym. Sci. Technol. 2007,
20, 57.
(31) Yamamoto, T.; Takimiya, K. J. Am. Chem. Soc. 2007, 129, 2224.
(32) Takimiya, K.; Yamamato, T.; Ebata, H.; Izawa, T. Sci. Technol.
Adv. Mater. 2007, 8, 273.
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Phys. Lett. 2009, 95, 022111.
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2009, 11, 3702.
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B.; Ohe, T.; Kanno, K.-i. J. Org. Chem. 2006, 71, 7967.
(35) Sokolov, A. N.; Atahan-Evrenk, S.; Mondal, R.; Akkerman,
H. B.; Sanchez-Carrera, R. S.; Granados-Focil, S.; Schier, J.; Mannsfeld,
S. C. B.; Zoombelt, A. P.; Bao, Z.; Aspuru-Guzik, A. Nat. Commun.
2011, 2, 437.
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Org. Lett., Vol. 14, No. 1, 2012
79

Scheme 1. Synthetic Strategy for DAT-2,3
Scheme 2. Synthetic Strategy for DAT-1,2
purified through a silica column to obtain compound 2 as a
significantly pure yellow solid. DAT-1,2 (2) was character-
ized by mass and 1H NMR spectra (SI). Thus, a nonlinear
heptacene analog was synthesized conveniently in three
steps.
Spectra of 1 and 2 in ortho-dichlorobenzene (o-DCB)
show λmax to be in the range of ∼400À450 nm, Figure 2.
The absorption spectra, therefore, correspond with that of
anthra-[2,3-b]-thiophene19 suggesting that compounds 1
and 2 exhibit conjugation equivalently to that of anthra-
[2,3-b]-thiophene.
DATs can, therefore, be considered as two anthra-[2,3-
b]-thiophene units. However, since their structures are
analogous to that of a heptacene, they might have superior
two-dimensional interactionsinthe solid state. Hence, they
are expected to exhibit high-charge carrier mobility in thin
film transistors. The fact that each is partially soluble in
most of the common solvents makes them good candidates for
solution processing in transistor devices. Most importantly,
Figure 2. Normalized UVÀvis absorption spectra of 1 and 2 in
ortho-dichlorobenzene.
significant conversion. Long-UV light sources (λ g 350
nm) were avoided as the product 2 also absorbs in that
range. The product was isolated as a dark yellow solid and
80
Org. Lett., Vol. 14, No. 1, 2012

these unsubstituted heptacene analogs are stable under
ambient conditions. Cyclic voltametry was performed on
both to ascertain the highest occupied molecular orbital
(HOMO) energy. The HOMO energies (estimated) were
found to be 5.18 and 5.12 eV for 1 and 2 respectively, which
is comparable to pentacene. Based on the long wavelength
absorption edge in the UVÀvisible spectra in o-DCB, the
HOMOÀLUMO energy gap is found to be 2.38 and 2.26
eV for 1 and 2 respectively, the respective LUMO energies
being 2.69 and 2.78 eV.
Further studies on experimental charge carrrier mobilities in
the organic thin film transistor devices are underway.
Acknowledgment. Contribution No. 831 from the Cen-
ter for Photochemical Sciences, and the last research paper
to be published by D.C.N. (current www.sglinc.com). P.K.
D. thanks Dr. Rajib Mondal, currently at Milliken & Co.,
for his valuable suggestions and criticism of this work.
Supporting Information Available. Experimental pro-
1
cedures and spectral data for all compounds. H NMR,
MALDI-TOF spectra, and cyclic voltametry for com-
pounds 1 and 2. This materials is available free of charge
via the Internet at http://pubs.acs.org.
Therefore, while compounds 1 and 2 each resemble an-
thra-[2,3-b]-thiophene with respect to their optical band gap,
their heptacene-like structure is expected to assist in higher
degree charge carrier mobility in the thin film transistors.
Org. Lett., Vol. 14, No. 1, 2012
81