Stable, crystalline acenedithiophenes with up to seven linearly fused rings

Article abstract of DOI:10.1021/ol048686d

(Chemical Equation Presented) We report the synthesis of a series of crystalline acenedithiophenes with up to seven linearly fused rings and silylethynyl substituents. These functional groups are designed to both improve solubility and enhance cofacial interactions in the solid. We discuss the crystal packing of these materials, as well as their physical properties such as oxidation potential, UV-vis absorption, fluorescence emission, and decomposition pathways.

Full text of DOI:10.1021/ol048686d

ORGANIC
LETTERS
2004
Vol. 6, No. 19
3325-3328
Stable, Crystalline Acenedithiophenes
with up to Seven Linearly Fused Rings
Marcia M. Payne, Susan A. Odom, Sean R. Parkin, and John E. Anthony*
Department of Chemistry, UniVersity of Kentucky, Lexington, Kentucky 40506-0055
anthony@uky.edu
Received July 8, 2004
ABSTRACT
We report the synthesis of a series of crystalline acenedithiophenes with up to seven linearly fused rings and silylethynyl substituents. These
functional groups are designed to both improve solubility and enhance cofacial interactions in the solid. We discuss the crystal packing of
these materials, as well as their physical properties such as oxidation potential, UV−vis absorption, fluorescence emission, and decomposition
pathways.
Polycyclic aromatic hydrocarbons represent a vast library
of molecular geometries with properties exploitable for ma-
terials chemistry.1 The linearly fused acenes in particular have
applications in field-effect transistors (FETs)2 and organic
light-emitting diodes (OLEDs).3 Our research has emphasized
the functionalization of acenes to improve solid-state ordering
and increase solubility and stability. For example, 6,13-bis-
(triisopropylsilylethynyl) pentacene (TIPS pentacene) packs
in a 2-dimensional π-stacked array in the solid state, yielding
improved conductivity, reduced band gap,4 and FET devices
with a hole mobility of 0.4 cm2/Vs.5 Spurred by these
promising results, we wished to apply our functionalization
approach to other fused systems, to further our structure-
property relationship studies of organic semiconductors.
One of the most promising systems for such study involves
a fusion of acenes and oligothiophenes:6 alkylated anthra-
dithiophene systems were recently reported by Katz and co-
workers as part of their search for new materials for use in
organic FETs. Their materials,7 synthesized as an inseparable
mixture of syn- and anti-isomers, exhibited hole mobilities
as high as 0.15 cm2/Vs with good device stability. A recent
theoretical treatment of these materials showed them to have
great promise if the proper molecular packing could be
attained, particularly orderings that enhanced π-face interac-
tions.8 To us, substitution of thiophene for the “terminal”
benzene rings of an acene is attractive because it should not
significantly perturb the basic molecular geometry compared
to the acene hydrocarbons. Increased stability in these
pentacene-like derivatives also indicates that this might be
(1) Clar, E. Polycyclic Hydrocarbons; Academic Press: New, York,
1964; Vols. I and II.
(2) (a) Butko, V. Y.; Chi, X.; Lang, D. V.; Ramirez, A. P. Appl. Phys.
Lett. 2003, 83, 4773. (b) Zhang, Y.; Petta, J. R.; Ambily, S.; Shen, Y.;
Ralph, D. C.; Malliaras, G. C. AdV. Mater. 2003, 15, 1632. (c) Dimitra-
kopoulos, C. D.; Kymissis, I.; Purushothaman, S.; Neumayer, D. A.;
Duncombe, P. R.; Laibowitz, R. B. AdV. Mater. 1999, 11, 1372. (d) Lin,
Y. Y.; Gundlach, D. J.; Nelson, S.; Jackson, T. N. IEEE Trans. Electron
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(3) (a) Wolak, M. A.; Jang, B.-B.; Palilis, L. C.; Kafafi, Z. H. J. Phys.
Chem. B 2004, 108, 5492. (b) Odom, S. A.; Parkin, S. R.; Anthony, J. E.
Org. Lett. 2003, 5, 4245.
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(b) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem.
Soc. 2001, 123, 9482.
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1998, 120, 664.
(8) Kwon, O.; Coropceanu, V.; Gruhn, N. E.; Durivage, J. C.; Laquin-
danum, J. G.; Katz, H. E.; Cornil, J.; Bre´das, J. L. J. Chem. Phys. 2004,
120, 8186.
10.1021/ol048686d CCC: $27.50 © 2004 American Chemical Society
Published on Web 08/24/2004

a reasonable method for attaining higher acene analogues.
We report here the synthesis and characterization of a series
of functionalized acenedithiophenes, from anthradithiophene
to pentadithiophene.
Silylethynylated acenes are easily prepared from the
corresponding acenequinones. Quinone 2 was prepared from
dicarboxaldehyde 19 as described previously. Quinone 5 was
made from dicarboxaldehyde 4,10 synthesized by a four-
carbon homologation of 1 (Scheme 1). Addition of trialkyl-
tion on silica gel with minimal decomposition and were
further purified by recrystallization. Most importantly, the
fact that these materials exist as inseparable mixtures of syn-
and anti-isomers did not significantly interfere with crystal-
lization or X-ray diffraction studies.
Single-crystal X-ray diffraction studies of 3 and 8 indicated
that both compounds were planar and formed crystals of
P21/c and P21 space group symmetries, respectively (Figure
1). The structures determined for these compounds exhibited
Scheme 1a
Figure 1. Thermal ellipsoid plots of the 3 and 8 (as a mixture of
isomers) and dimerized 6a.
disorder of the thiophene rings expected as a result of the
isomeric mixture formed in their synthesis (as shown in the
Figure). Both 3 and 8 were modeled as the anti-isomer for
structure solution purposes only. These materials are stable
in the solid state or in (deoxygenated) solution for weeks
and can be evaporated or solution-cast to form stable,
uniform thin films.
The seven-ringed compound formed irregular green needles
from toluene that were poorly suited for X-ray analysis.
Although structure solution was straightforward, refinement
was hindered by disorder of both the thiophene rings and
isopropyl groups. However, the atom connectivity of the
compound could easily be determined, showing the material
to be a dimeric species (9), formed by a cyclization reaction
between the diene within the third ring of one acene and the
ethynyl group of a second acene. Further experimentation
revealed that the dimerization of 6a proceeds to completion
a Reagents and conditions: (a) 1,4-cyclohexanedione, ethanol,
1 mL 15% aq NaOH. (b) R3SiCCLi, hexanes, 65 °C, 12 h, then
SnCl2/10% HCl, 2 h. (c) 2,5-dimethoxytetrahydrofuran, AcOH
110 °C.
silylethynyllithium followed by deoxygenation with stannous
chloride yielded the desired five- and seven-ring acene-
dithiophenes. To prepare the six-ringed tetradithiophene 8,
condensation of a 1:2 mixture of dicarboxaldehydes 1 and 4
with 1,4-cyclohexanedione followed by deoxygenative ethy-
nylation yielded an easily separated mixture of 3, 6a, and 8.
The acenedithiophenes underwent chromatographic purifica-
(9) De la Cruz, P.; Martin, N.; Miguel, F.; Seoane, C.; Albert, A.; Cano,
H.; Gonzalez, A.; Pingarron, J. M. J. Org. Chem. 1992, 57, 6192.
(10) El-Borai, M.; Abdel-Megeed, M. F.; Hassien, M.; Fahmy, M. Sulfur
Lett. 1987, 6, 99.
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Org. Lett., Vol. 6, No. 19, 2004

within a few hours of isolation of this compound, even in
relatively dilute solution. Further, formation of the adduct
was not reversible at temperatures of up to 180 °C. These
results are especially surprising because the triisopropylsilyl
group has been used to shield ethynyl substituents from
dienes in similar Diels-Alder reactions,11 which emphasizes
the reactive nature of the higher acenes. Because compound
9 still contains an intact seven-ringed chromophore, we
postulated that a pentadithiophene could be isolated as a
stable material if this intermolecular cyclization were pre-
vented by the use of a more sterically demanding alkyne
substituent.
The bulky tri(tert-butyl)silyl group has been employed to
generate low-coordinate metal complexes and previously
unrealizable alkynyl-substituted zirconium and hafnium
complexes and therefore seemed a viable alternative.12 Thus,
tri(tert-butyl)silylethynyl derivative 6b was prepared as
outlined in Scheme 1. Although this material is significantly
less soluble than the corresponding TIPS derivative 6a,
recrystallization from CS2 yielded dark green needles suitable
for single-crystal X-ray diffraction, and we were pleased to
discover that this compound did not undergo dimerization.
This derivative crystallizes in the P1h space group and is a
remarkable and rare example of a stable, crystalline, linearly
fused seven-ringed compound (Figure 2). Acene 6b is stable
0.14 Å.13 The only significant deformation of the acene
involves a slight pyramidalization of the aromatic carbon
attached to the alkyne, and the degree of pyramidalization
increases with increasing acene length. An examination of
the crystal packing for these three derivatives reveals that
the molecules in the crystal assemble into 1-dimensional
π-stacked arrays, as predicted by our model for the packing
of ethynylated pentacenes.4a This solid-state arrangement not
only minimizes the distortion of the aromatic core in the
crystal but also leads to close π-face contacts critical for
device performance. The average interplanar spacing for
compounds 3, 8, and 6b are 3.46, 3.46, and 3.42 Å,
respectively (Figure 3). The degree of π-overlap varies with
Figure 3. Solid-state ordering of 3, 8, and 6b. View along the
acene short axis, with frontmost silylethynyl groups omitted for
clarity.
the size of the acene, with the seven-ringed 6b exhibiting
the greatest number of overlapping rings.
UV-visible absorption spectra for thienyl acenes 3, 8, and
6b were recorded in dichloromethane solution (Figure 4).
As expected, the extinction coefficient for the longest-
wavelength absorption decreases significantly as the length
of the acene increases. The ∼100 nm red shift upon addition
of a single aromatic ring in the oligomer series implies that
longer oligo(acenedithiophene)s will have impressively small
optical gaps. Indeed, a simple plot of long-wavelength cutoff
vs 1/(oligomer length) reveals an anticipated HOMO-
LUMO gap of 0.9 eV for the infinite-length polymer.14 This
number compares well with energies predicted, both at
various levels of theory and by extrapolation of UV-vis
Figure 2. Thermal ellipsoid plot of 6b (mixture of isomers).
both in the solid state and in deoxygenated toluene solution
for a period of several weeks.
The molecular structures for 3, 8, and 6b all show that
the acene backbones of these molecules are planar to within
(11) Morgenroth, F.; Berresheim, A. J.; Wagner, M.; Mu¨llen, K. Chem.
Commun. 1998, 1138.
(12) (a) Vaid, T. P.; Veige, A. S.; Lobvoysky, E. B.; Glassey, W. V.;
Wolczanski, P. T.; Liable-Sands, L. M.; Rheingold, A. L.; Cundari, T. R.
J. Am. Chem. Soc. 1998, 120, 10067. (b) Wiberg, N.; Amelunxen, K.;
Lerner, H.-W.; Schuster, H.; Noth, H.; Krossing, I.; Schmidt-Amelunxen,
M.; Seifert, T. J. Organomet. Chem. 1997, 542, 1
(13) The planarity of large polycyclic aromatic systems is easily disrupted
by crystal packing effects: Holmes, D.; Kumaraswamy, S.; Matzger, A. J.;
Vollhardt, K. P. C. Chem. Eur. J. 1999, 5, 3399.
Org. Lett., Vol. 6, No. 19, 2004
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energy levels from approximately 5.2 to 4.9 eV for the series.
Access to such a wide range of potentials over a structurally
homologous series of compounds may prove to be useful in
device applications because it allows for a flexible approach
to matching the proper material with a particular application
and desired electrode metal. These alkyne-substituted acene-
Table 1. Selected Spectral Data for 3, 8, and 6b
λmax abs
λmax ema
Eox (mV)b
compound
(nm)
(nm)
vs SCE
3
8
6b
555
653
762
561
665
802
900
725
613
a In dichloromethane. b In 0.1 M solution of Bu4NPF6 in dichloromethane,
Pt electrode, scan rate of 150 mV/s, ferrocene as internal standard.
Figure 4. Absorption spectra of the acenedithiophenes (top),
compared to the related parent hydrocarbons (bottom).
dithiophenes are soluble and exhibit strong π-stacking
interactions in the solid state. By careful selection of the
substituent on the alkyne, even the system with a backbone
1.7 nm long consisting of seven linearly fused aromatic rings
can be stabilized and solubilized. Studies of the transport
properties, including charge carrier mobility and conductivity
band gap for each of these materials, are currently underway
and will be the subject of a future report.
spectral data, for the parent polyacenes.15 This correlation
is unsurprising, because the absorption spectra of the
acenedithiophenes are remarkably similar to those of the
related acenes (see the absorption spectra for bis(triisopro-
pylsilylethynyl)tetracene and TIPS pentacene, Figure 4). The
acenedithiophenes have absorption behavior that corresponds
to the absorptions of acenes that contain one less aromatic
ring, likely due to the unique nature of the heteroaromatic
thiophene rings. Whereas five-ringed compound 3 exhibits
bright pink/orange fluorescence, the longer acenedithiophenes
(8 and 6b) were only weakly fluorescent in solution.
The oxidation potentials for these compounds span a range
of values from ∼600 to 900 mV, corresponding to HOMO
Acknowledgment. The authors are grateful to the Office
of Naval Research and the Defense Advanced Research
Projects Agency for providing funding for this research.
Supporting Information Available: Full experimental
details, molecule characterization, and crystallographic in-
formation in CIF format. This material is available free of
charge via the Internet at http://pubs.acs.org.
(14) Schreiber, M.; Anthony, J.; Diederich, F.; Spahr, M. E.; Nesper,
R.; Hubrich, M.; Bommeli, F.; Degiorgi, L.; Wachter, P.; Kaatz, P.;
Bosshard, C.; Gu¨nter, P.; Colussi, M.; Suter, U. W.; Boudon, C.;
Gisselbrecht, J.-P.; Gross, M. AdV. Mater. 1994, 6, 786.
(15) Houk, K. N.; Lee, P. S.; Nendel, M. J. Org. Chem. 2001, 66, 5517
and references therein.
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