Angewandte
Chemie
DOI: 10.1002/anie.201206145
Polycyclic Aromatic Hydrocarbons
Electron Acceptors Based on an All-Carbon Donor–Acceptor
Copolymer**
Jessica L. Jellison, Che-Hsiung Lee, Xinju Zhu, Jordan D. Wood, and Kyle N. Plunkett*
Donor–acceptor-conjugated polymers[1] have been success-
fully employed to improve organic field effect transistors
(OFETs) and organic photovoltaic devices (OPVs).[2a–d]
These polymers provide access to low-band-gap materials
by increasing the energy level of the highest occupied
molecular orbital (HOMO) through electron-donor units
while decreasing the energy level of the lowest unoccupied
molecular orbital (LUMO) with electron-acceptor units. The
reduced band gap leads to beneficial properties such as longer
wavelength absorption to match the solar spectrum as well as
the opportunity to create n-type or ambipolar semiconductor
materials. Over the past decade, several impressive acceptor
comonomers, such as benzothiadiazole,[3a,b] fluorinated aro-
matics,[4a,b] imide and bisimide aromatics,[5a–c] as well as novel
ring systems[6] have been copolymerized into conjugated
polymers. In general, these electron-accepting monomers
require the utility of heteroatoms that are incorporated in
pendant p-acceptor functionalities (cyano, imide, etc.) or
inductively withdrawing substituents (fluorine) to stabilize
the LUMO. We have recently become interested in creating
new materials that utilize alternative modes of LUMO
stabilization including stabilization through aromatic cyclo-
pentadienyl anions.[7]
to new scalable and functionalizable small-molecule CP-PAH
systems including indenofluorenes 3 (TIPS = triisopropyl-
silyl),[13a-c] dibenzopentalenes 4,[14a,b] and cyclopenta[hi]ace-
anthrylene units 5[15a–d] that show promising electronic and
photophysical properties.
Nonalternant cyclopenta-fused polycyclic aromatic
hydrocarbons (CP-PAHs) have been extensively studied in
the past several decades owing to their structural similarities
to fullerenes, interesting optical properties, and remarkable
ability to accept electrons.[8a,b] Although these properties
would lend themselves favorably to conducting polymers,
they have rarely been utilized. Notable exceptions include the
low-band-gap poly(indenofluorenes)[9a,b] 1 and newer poly-
mers based on emeraldicene[10a,b] (2). Two major drawbacks
can be cited for the overall absence of other CP-PAH systems.
First, these materials were typically difficult to prepare in
large quantities owing to their inefficient (flash-vacuum-
pyrolysis methodology)[11] or sometimes time-consuming
(Suzuki–Heck methodology)[12] syntheses. Second, they were
often difficult to selectively functionalize after formation of
the five-membered rings. Recent synthetic advances have led
In an effort to create new functionalizable CP-PAHs, our
group has recently extended the cyclopentenelation method-
ology of Garcia-Garibay[15a,b] to create gram quantities of 2,7-
dibromocyclopenta[hi]aceanthrylene (8).[7] The synthesis can
be accomplished in two steps from the commercially available
9,10-dibromoanthracene (6) (Scheme 1). The methodology
takes advantage of a unique ring-closure mechanism that is
accessible by eliminating CuI from a typical Sonogashira-like
cross-coupling with trimethylsilylacetylene. The resulting 2,7-
bis(trimethylsilyl)cyclopenta[hi]aceanthrylene (7) can then be
subjected to bromination under mild conditions with N-
bromosuccinimide (NBS) in THF to give the dibrominated
monomer 8 as a green/black solid.
We have found that 8 can be subjected to Sonogashira
cross-couplings to create discrete small molecules with
relatively small band gaps.[7] With these promising results,
we were intrigued by the possibility of utilizing the cyclo-
penta[hi]aceanthrylene unit as an acceptor in donor–acceptor
copolymers. We purposely chose a fluorene donor unit that is
also composed solely of carbon, and ironically, is also a CP-
PAH. However, the five-membered ring in this monomer
serves only as a scaffold to help planarize the two benzene
rings and does not lend electron-accepting behavior to the
polymer. We carried out the Sonogashira cross-coupling
polymerization of 9,9-didodecyl-2,7-diethynylfluorene with 8
[*] J. L. Jellison, C.-H. Lee, X. Zhu, J. D. Wood, Prof. K. N. Plunkett
Department of Chemistry and Biochemistry
Southern Illinois University
Carbondale, IL (USA)
E-mail: kplunkett@chem.siu.edu
[**] This work was supported by startup funds and a seed grant provided
by Southern Illinois University.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 12321 –12324
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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