Reduced band gap dithieno[3,2-b:2',3',-d]pyrroles: New n-type organic materials via unexpected reactivity

Article abstract of DOI:10.1021/ol8002018

Direct addition of tetracyanoethylene to W-(p-hexylphenyl)dithieno[3,2-b: 2,3-d]pyrrole yields not only the aromatic mono- and bis-tricyanovinyl-substituted products but also a quinoidal product with dicyanomethylene groups. The analogous reaction with dithieno[3,2-b:2, 3-d]thiophene yields exclusively the aromatic mono-tricyanovinyl product. The aromatic and quinoidal products possess red-shifted absorptions, increased electron affinities, and favorable π stacking motifs in comparison to the unsubstituted oligomers.

Full text of DOI:10.1021/ol8002018

ORGANIC
LETTERS
2008
Vol. 10, No. 8
1553-1556
Reduced Band Gap
Dithieno[3,2-b:2 ,3 -d]pyrroles: New
′
′
n-Type Organic Materials via
Unexpected Reactivity
Ted M. Pappenfus,*,† Bethany J. Hermanson,† Tyler J. Helland,†
Garett G. W. Lee,† Steven M. Drew,# Kent R. Mann,§ Kari A. McGee,§ and
Seth C. Rasmussen*,‡
DiVision of Science and Mathematics, UniVersity of Minnesota,
Morris, Minnesota 56267, Department of Chemistry, UniVersity of Minnesota,
Minneapolis, Minnesota 55455, Department of Chemistry and Molecular Biology,
North Dakota State UniVersity, Fargo, North Dakota 58105
pappe001@morris.umn.edu; seth.rasmussen@ndsu.edu
Received January 28, 2008
ABSTRACT
Direct addition of tetracyanoethylene to N-(p-hexylphenyl)dithieno[3,2-b:2
substituted products but also a quinoidal product with dicyanomethylene groups. The analogous reaction with dithieno[3,2-b:2
yields exclusively the aromatic mono-tricyanovinyl product. The aromatic and quinoidal products possess red-shifted absorptions, increased
electron affinities, and favorable -stacking motifs in comparison to the unsubstituted oligomers.
′
,3′
-d]pyrrole yields not only the aromatic mono- and bis-tricyanovinyl-
′
,3′-d]thiophene
π
Dithieno[3,2-b:2′,3′-d]thiophene (DTT, Figure 1) is an
important building block for a wide variety of functional
organic materials. In particular, DTT has been employed in
materials acting as the active layer in organic thin-film
transistors (OTFTs).1 The fused structure of DTT can
promote π-stacking1d,2 which is predicted to be a favorable
motif for high charge transport in devices.3 Far less inves-
tigated are the related dithieno[3,2-b:2′,3′-d]pyrroles (DTPs,
Figure 1)4 which are emerging as a useful structure for both
molecular4 and polymeric5 materials. Functionalization of
DTPs with electron-withdrawing groups is intriguing with
Figure 1. Structures of dithieno[3,2-b:2′,3′-d]thiophene (left) and
dithieno[3,2-b:2′,3′-d]pyrroles (right).
regards to the development of reduced band gap materials
given the fact that they possess lower oxidation potentials
than DTTs.4a This approach can help create molecular
materials with low-energy electronic transitions based on
donor-acceptor interactions.6
† University of Minnesota, Morris
‡ North Dakota State University
§ University of Minnesota
# Department of Chemistry, Carleton College, Northfield, MN 55057.
10.1021/ol8002018 CCC: $40.75
© 2008 American Chemical Society
Published on Web 03/19/2008

We and others have found that the tricyanovinyl (TCV)
functionality is a good acceptor group for creating reduced
band gap materials of oligothiophenes.7 The resulting materi-
als display intriguing nonlinear optical properties, as well
as favorable electron transport in OTFTs.6,8 One approach
to introduce TCV functionality in conjugated oligomers
involves the formation of organolithium species followed by
quenching with tetracyanoethylene (TCNE).7,9 For activated
aromatic rings, a more convenient method is to add TCNE
directly to the molecule in a polar solvent.10 This method
has been applied to a variety of systems ranging from
bithiophenes11 to ferrocene.12 Our present study was influ-
enced by the investigation of Ogura et al. which reports the
direct addition of TCNE to 1-aryl-2,5-di(2-thienyl)pyrroles
(Scheme 1).13 The reactivity toward bis-TCV products in this
conditions in an effort to further tailor their electronic and
redox properties.
In order to best match the pyrrole substitution in the work
of Ogura and co-workers,13 a known aryl-functionalized DTP,
N-(p-hexylphenyl)dithieno[3,2-b:2′,3′-d]pyrrole4a (DTP, 1)
was chosen for this study. The reactivity of DTP with TCNE,
however, is unique in comparison to previous studies. As
shown in Scheme 2, addition of TCNE to DTP resulted in
the formation of three products. All products were separable
Scheme 2. Addition of TCNE to DTP
Scheme 1. Addition of TCNE to Dithienylpyrroles
by column chromatography, and the first two products were
quickly identified as the mono- and bis-TCV-substituted
products, TCV-DTP (2) and BTCV-DTP (3), with 2 being
the major product. The low production of the bis-TCV
product 3 can be rationalized by deactivation of the remaining
R-carbon upon TCV substitution due to the more strongly
coupled π-system of the fused ring structure.
The identity of the last product DTP-Q (4) was established
using NMR, combustion analysis, and X-ray crystallography
(vide infra). This product was unexpected since dicyano-
methylene-capped quinoidal species are typically prepared
from bis-halo precursors using either Pd-catalyzed methods
or TCNE oxide.14 To our knowledge, preparation of quinoidal
systems by direct TCNE addition to unsubstituted oligomers
is unprecedented.
Although no precedent could be found for this reactivity
in the literature, further investigation of its potential scope
was of interest. As a result, DTT (5)15 was treated with TCNE
under the same conditions as DTP (Scheme 3). The reaction,
however, yielded only one product, which was confirmed
to be the mono-TCV product 6. Prolonged reaction times
yielded identical results, and no additional products were
observed.
case is in contrast to other reports where mono-TCV products
prevail. The bis-TCV products are reported as high-melting-
point materials with metallic appearance. In this communica-
tion, we report the reactivity of DTPs under similar reaction
(1) (a) Zhan, X.; Tan, Z.; Domercq, B.; An, Z.; Zhang, X.; Barlow, S.;
Li, Y.; Zhu, D.; Kippelen, B.; Marder, S. R. J. Am. Chem. Soc. 2007, 129,
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J.; Joo, J.-s.; Choi, D. H. Chem. Mater. 2007, 19, 4925. (c) Sun, Y.; Ma,
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Pappenfus, T. M.; Bader, M. M.; Bredas, J.-L.; Mann, K. R.; Frisbie, C. D.
J. Phys. Chem. B 2006, 110, 14590.
To further investigate this unique reactivity and confirm
the lack of multiple product formation in the previous work
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Y.; Yamashita, K.; Kunai, A. Org. Lett. 2002, 4, 1891. (b) Raposo, M. M.
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Yamaguchi, K. Tetrahedron Lett. 2003, 44, 3595.
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(15) DTT was prepared through a modification of the synthesis of 2,6-
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1554
Org. Lett., Vol. 10, No. 8, 2008

Scheme 3. Addition of TCNE to DTT
Table 1. Spectroscopic and Electrochemical Data of the DTP
and DTT Molecules
λmax band
λmax, nm (film) gap
E°ox, Vd
E°red, Vd
molecule
DTP (1)
(log ꢀ)a
nmb (eV)c (∆E, mV) (∆E, mV)
g
300 (4.4)
262 (4.0)
318 3.56 1.07f
of Ogura,13 1-(p-hexylphenyl)-2,5-di(2-thienyl)pyrrole16 was
prepared and treated with TCNE. In agreement with Ogura,
the reaction yielded a single product which was confirmed
by 1H NMR, HRMS, and combustion analysis to be the bis-
TCV product.
TCV-DTP (2) 538 (4.7)
285 (4.2)
440 1.88 1.51f
-0.54 (87)
-1.20 (118)
BTCV-DTP (3) 577 (5.0)
606 1.72 1.88 (78) -0.12 (80)
-0.41 (82)
540e (4.9)
380 (4.1)
363e (4.0)
-1.46 (150)
From the studies above, it is clear that the reactivity of
DTP with TCNE is unique. When scaling up the reaction, it
has been confirmed that this process is reproducible in
affording products 2, 3, and 4. In attempts to improve the
reaction yields, the treatment of dilithiated DTPs with TCNE
was investigated. Contrary to expectations, however, this
gave poor results, giving even lower yields and no production
of 4.
269 (4.3)
DTP-Q (4)
545 (5.0)
510 (4.9)
477e (4.6)
312 (3.9)
270 (4.0)
291 (4.5)
499 (4.6)
275 (4.1)
465 1.74 1.76 (101) -0.23 (90)
-0.53 (93)
h
h
h
h
g
DTT (5)
TCV-DTT (6)
1.44f
1.78f
-0.47 (94)
-1.13 (108)
Typical reactivity of π systems with TCNE to produce
TCV products is thought to occur via an electron-transfer
mechanism,12 which would account for the formation of 2,
3, and 6. However, while much less common, cycloaddition
processes have also been reported,17 and it is proposed that
such processes could be responsible for the formation of the
quinodal product 4 (see Supporting Information for proposed
mechanism). This would be consistent with both the in-
creased reactivity of DTPs4,5 and the lack of product 4 from
the dilithiated DTP.
UV-vis absorption and cyclic voltammetry (CV) data of
the various DTP and DTT species are summarized in Table
1. Consistent with previous TCV oligomers, red-shifts in the
electronic spectra are observed with increasing TCV sub-
stitution.6,7 For example, the lowest energy λmax values
increase across the DTP/2/3 series: 300 f 538 f 577 nm.
Compounds 4 (λmax ) 545 nm) and 6 (λmax ) 499 nm) also
display red-shifts in comparison to DTP and DTT, respec-
tively. This general trend has been previously attributed to
the large stabilization of the LUMO by the electron-
withdrawing groups in both aromatic18 and quinoidal19
systems. Density functional theory (DFT) calculations at the
B3LYP/6-31G(d, p) level were performed for the DTP series
to illustrate this point (Figure 2). Although the HOMO levels
are also stabilized in these molecules, the magnitude of the
LUMO stabilization is much greater, resulting in a net
reduction in HOMO-LUMO energy upon TCV substitution.
The redox properties of the DTP and DTT series were
investigated by CV (Table 1). Both DTT and DTP display
a Measured in dry CH2Cl2. b Solvent cast film. c Optical. d Potentials vs
Ag/AgCl in 0.1 M TBAPF6/CH2Cl2 solution. e Shoulder. f Irreversible
process; Epa value provided. g No observable reduction processes. h Not
measured due to poor film formation.
irreversible oxidations with no observable reductions under
the experimental conditions. Upon TCV substitution, how-
ever, reductions are observed at decreasing potentials with
increased TCV substitution. To a lessor degree, increased
TCV substitution results in the oxidations shifting to higher
potentials. The DFT calculations accurately reflect both redox
trends for the DTP series as predicted by Koopmans’
theorem, which suggests that the HOMO/LUMO energies
relate, respectively, to the first oxidation/redution potentials.
For example, here the DFT calculations accurately predict
that 3 should be the easiest to reduce (i.e., lowest energy
LUMO) and DTP should be the easiest to oxidize (i.e.,
highest energy HOMO).
The solid-state structures of the DTP series were examined
via X-ray crystallography, and selected bond lengths are
given in Table 2. The crystal structure of DTP has been
(16) Ogura, K.; Zhao, R.; Yanai, H.; Maeda, K.; Tozawa, R.; Matsumoto,
S.; Akazome, M. Bull. Chem. Soc. Jpn. 2002, 75, 2359.
(17) Nishida, S.; Asanuma, N.; Murakami, M.; Tsuji, T.; Imai, T. J. Org.
Chem. 1992, 57, 4658.
(18) Casado, J.; Pappenfus, T. M.; Miller, L. L.; Mann, K. R.; Orti, E.;
Viruela, P. M.; Pou-Amerigo, R.; Hernandez, V.; Lopez Navarrete, J. T. J.
Am. Chem. Soc. 2003, 125, 2524.
(19) Casado, J.; Zgierski, M. Z.; Ewbank, P. C.; Burand, M. W.; Janzen,
D. E.; Mann, K. R.; Pappenfus, T. M.; Berlin, A.; Perez-Inestrosa, E.; Ortiz,
R. P.; Lopez Navarrete, J. T. J. Am. Chem. Soc. 2006, 128, 10134.
Figure 2. Calculated HOMO and LUMO levels for DTP, TCV-
DTP (2), BTCV-DTP (3), and DTP-Q (4).
Org. Lett., Vol. 10, No. 8, 2008
1555

to give stacks of π-dimers with two short stacking dis-
tances: an intradimer distance of 3.423 Å and an interdimer
distance of 3.439 Å. Columns of π-stacking molecules
arrange so that hexyl chains of the p-hexylphenyl groups
aggregate together giving layers of side chains and π-stacks.
This organization appears to be driven by the tendency of
the nitriles of the dicyanomethylene substituents to interact
with atoms in adjacent molecules. Several additional short
intermolecular interactions occur (Table S4) but will not be
discussed here.
The structural data of the DTP series can shed light on
the aromatic and quinoidal character of each molecule. Bond-
by-bond data analysis shows that the C-C bond lengths
change so that the “long” and “short” bonds of DTP become
the “short” and “long” bonds of 4. Compound 3 represents
an intermediate case for all selected bonds. An alternative
analysis is to treat each C-C bond as a variable and analyze
using principal component analysis (PCA) as described
previously.20c The two scores that contain most of the
variance are plotted by projecting the data set (3 × 7 total
variables) onto the PC1-PC2 plane (Figure S30). In this
analysis, 99% of the variance lies within PC1 which means
99% of the changes in C-C bonds can be described by a
single variable relating to the quinoidal character of the DTP
core in each molecule.
Table 2. Select Bond Lengths (Å) for the DTP Series
BTCV-DTP DTP-Q
bond
DTPa
(3)
(4)
C1-C2
C2-C3
C3-C4
C4-C5
C5-C6
C6-C7
C7-C8
1.348
1.416
1.377
1.422
1.379
1.421
1.352
1.381
1.395
1.397
1.409
1.401
1.388
1.385
1.438
1.356
1.446
1.360
1.452
1.353
1.422
a Reference 4a.
reported previously,4a and exhibits no π-stacking as p-
hexylphenyl groups occupy the space between the DTP units.
The crystal packing of 3 and 4, however, each contain
π-stacking, a property typical of thiophene-based oligomers
with electron-withdrawing substituents.7,20
Molecules of 3, co-crystallized with toluene, are very flat
with a mean deviation from planarity of 0.0114 Å. Infinite
π-stacks of 3.252 Å are present where the molecules are
slipped along the long molecular axis such that little more
than the TCV groups overlap to give a stairstep-type stack
(Figure 3). The toluene molecules interact with the molecular
In summary, the addition of TCNE to DTP affords two
unique types of functional organic materials: TCV-aromatic
and dicyanomethylene-quinoidal DTPs. Both types exhibit
reduced band gaps in comparison to DTP. These properties,
in conjuction with favorable redox and solid-state properties,
make the materials attractive for electronic device applica-
tions. Further efforts will focus on more efficient syntheses
of these materials, as well as device construction and
evaluation.
Acknowledgment. T.M.P. and S.C.R. thank the Donors
of the Petroleum Research Fund, administered by the
American Chemical Society, for partial support of this
research. T.M.P. acknowledges University of Minnesota,
Morris (UMM) Faculty Research Enhancement Funds sup-
ported by the University of Minnesota Office of the Vice
President for Research, the University of Minnesota Initiative
for Renewable Energy and the Environment, Katsu Ogawa
(NDSU) for useful synthetic discussions, and Demetrio A.
da Silva Filho (Georgia Tech) for assistance with calcula-
tions. K.A.M. acknowledges the University of Minnesota
X-ray lab in the Department of Chemistry and a University
of Minnesota Doctoral Dissertation Fellowship.
Figure 3. π-Stacking of 3 (a) and 4 (b).
core of the DTP with “π-stacking” interactions less than the
van der Waals radii, resulting in a second type of π-stack
via alternating DTP and toluene molecules. The π-stacking
occurs in layers so that the hexyl substituents of the side
chains aggregate together giving alternating layers of hexyl
chains and π-stacking molecules. Several additional short
intermolecular interactions occur between adjacent molecules
(Table S3) but will not be discussed here.
Similar to 3, the core of 4 is very flat with a mean
deviation from planarity of 0.0076 Å. Molecules of 4 pack
Supporting Information Available: Experimental pro-
cedures, characterization data, crystallographic CIF files, and
plots of physical data. This material is available free of charge
via the Internet at http://pubs.acs.org.
(20) (a) Yoon, M.-H.; Facchetti, A.; Stern, C. E.; Marks, T. J. J. Am.
Chem. Soc. 2006, 128, 5792-5801. (b) Pappenfus, T. M.; Chesterfield, R.
J.; Frisbie, C. D.; Mann, K. R.; Casado, J.; Raff, J. D.; Miller, L. L. J. Am.
Chem. Soc. 2002, 124, 4184-4185. (c) Pappenfus, T. M.; Raff, J. D.;
Hukkanen, E. J.; Burney, J. R.; Casado, J.; Drew, S. M.; Miller, L. L.;
Mann, K. R. J. Org. Chem. 2002, 67, 6015-6024.
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