Electron acceptors based on an all-carbon donor-acceptor copolymer

Article abstract of DOI:10.1002/anie.201206145

The Sonogashira cross-coupling polymerization of a dibrominated cyclopenta[hi]aceanthrylene and a diethynylfluorene derivative produced a donor-acceptor copolymer composed solely of cyclopenta-fused polycyclic aromatic hydrocarbons. The resulting polymer

Full text of DOI:10.1002/anie.201206145

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
Homepage: http://www.kyleplunkett.com
[**] 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
under http://dx.doi.org/10.1002/anie.201206145.
Angew. Chem. Int. Ed. 2012, 51, 12321 –12324
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
12321

.
Angewandte
Communications
delocalized p-conjugated polymer backbone. The spectrum
demonstrates the classical dual-band absorption found in
many other donor–acceptor copolymers.[17] The simultaneous
absorption of red (l < 500 nm) and blue (l > 600 nm) light
leads to the saturated green color analogous to other green
polymeric electrochromic materials.[18a,b] The optical band gap
(Egap,opt) is estimated to be 1.49 eV. When a film was created,
the absorption was further red-shifted (Egap,opt = 1.42 eV),
which suggests even further planarization and aggregation in
the solid state. Similar to small-molecule analogues of CP-
PAHs,[7,15b,19] 9 demonstrated no fluorescence in solution. We
were therefore unable to perform solvatochromic fluores-
cence studies to confirm the presence of charge-separated
states.[20a,b] Furthermore, absorption spectra of 9 in various
solvents and concentrations were all identical. Cyclic voltam-
metry (Figure 2) was utilized to determine the HOMO
Scheme 1. Synthetic pathway to polymers 9 and 10. Conditions:
a) trimethylsilyl(TMS)-acetylene, [Pd(PPh3)2Cl2], PPh3, NEt3, benzene,
pressure tube at 1108C, 16 h (62%); b) NBS, THF, 08C, 3 h (66%);
c) 9,9-didodecyl-2,7-diethynylfluorene, [Pd(PhCN)2Cl2], PtBu3, iPrNH,
CuI, toluene, RT, 1–6 h; d) 1,4-diethynyl-2,5-bis((2-octyldodecyl)oxy)-
benzene, [Pd(PhCN)2Cl2], PtBu3, iPrNH, CuI, toluene, RT, 1–6 h.
at room temperature with a catalyst system consisting of
[Pd(PhCN)2Cl2], PtBu3, and CuI in a toluene/diisopropyl-
amine solvent mixture.[16] Reaction times as low as 1 h
provided polymers with reasonable molecular weights (Mn =
19750, PDI = 2.0) and good thermal stability (Td = 4218C)
after purification by repeated precipitation. Longer reaction
times led to modest increases in the molecular weight, larger
polydispersities (6 h, Mn = 20980, PDI = 2.75), and less solu-
ble materials. The resulting polymers are dark green and
soluble in THF as well as chlorinated solvents to create
emerald green solutions (see the Supporting Information).
In the UV/Vis spectrum of the resulting polymer 9
(Figure 1) the low-energy absorption is dramatically red-
shifted and much greater in intensity than the analogous
absorption of 7. Such an absorption shift suggests a highly
Figure 2. Cyclic voltammograms of 7 (top), 9 (middle), and 10
(bottom) in THF with 0.1m tetrabutylammonium hexafluorophosphate.
Values listed are the onset values of oxidation (left) or reduction
(right). Scan rate=50 mVsÀ1. Ferrocene added as internal standard
and referenced to 0 V.
(À5.27 eV) and LUMO (À3.69 eV) energy levels in THF
(see the Supporting Information). The characteristic two-
electron reduction (Scheme 2) of the cyclopenta[hi]acean-
Scheme 2. Benzocyclopentadienyl anion stabilization.
thrylene, which is assigned to the formation of two stabilized,
(4n + 2)electron benzocyclopentadienyl anions, was present
in both monomer 7 and polymer 9, albeit shifted to lower
potentials for the polymer.
The copolymer 9 possesses an impressively small band gap
for an all-carbon system, although it is similar in energy to the
that of the all-carbon poly(indenofluorenes) homopolymers
Figure 1. Absorbance of 7 in THF (gray), 9 in THF (middle) and in film
(dashed line), and 10 in THF (top) and in film (dashed line).
12322 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12321 –12324

Angewandte
Chemie
previously described.[9a] To demonstrate the benefit of the
donor–acceptor copolymer system, we employed a stronger
donor unit to modulate the energy gap. Although it is not
entirely composed of carbon, we chose the well-known
bis(alkoxy)benzene donor due to its synthetic accessibility
and because it is known to be a stronger donor than fluorene.
Utilizing similar polymerization conditions as described
above, 8 was copolymerized with 1,4-diethynyl-2,5-bis((2-
octyldodecyl)oxy)benzene to create the donor–acceptor co-
polymer 10 (Scheme 1). The molecular weight and polydis-
persity increased, similar to 9, when the reaction time was
extended from 1 h (Mn = 25260, PDI = 2.47) to 6 h (Mn =
25057, PDI = 3.26). The stability (Td = 3658C) and solubility
of 10 is poorer than 9, and are a result of the more
symmetrical alkoxy monomer. As expected for a copolymer
with a stronger donor unit, a bathochromic shift of the
absorption in solution (Egap,opt = 1.38 eV) and in the film
(Egap,opt = 1.27 eV) was observed (Figure 1). Furthermore, the
intensity of the low-energy absorption increased compared to
9, which has been found in other donor–acceptor copolymer
systems when stronger donor units are utilized. Overall, the
copolymer absorbs intensely over the entire UV and visible
region of the spectrum. In the cyclic voltammogram of 10,
reduction onset is similar to that of 9 (À1.13 V vs. À1.11 V,
respectively) while the oxidation signal of 10 is considerably
lowered compared to that of 5 (+ 0.33 V vs. + 0.47 V)
(Figure 2). The electrochemistry clearly demonstrates the
selective destabilization of the HOMO energy level with the
increased donor strength of the bis(alkoxy)benzene unit.
Finally, to demonstrate the electron-accepting behavior of
9, we performed solution-phase fluorescence quenching of the
demonstrates the efficient electron-accepting behavior of the
described CP-PAH polymers.
In conclusion, we have demonstrated the utility of an
externally fused CP-PAH as a unique electron-accepting unit
in donor–acceptor copolymers. The electron-accepting behav-
ior is directly attributed to the stabilization of the reduced
(4n + 2) aromatic benzocyclopentadienyl anions rather than
traditional stabilization modes such as inductively withdraw-
ing groups or p-accepting heteroatoms. The resulting poly-
mers display low band gaps (< 1.5 eV) and are efficient
electron acceptors that may be of utility as n-type materials in
OPVs and organic field-effect transistors.
Received: July 31, 2012
Published online: October 29, 2012
Keywords: copolymerization · donor–acceptor systems ·
.
polycyclic aromatic hydrocarbons
[1] E. E. Havinga, W. ten Hoeve, H. Wynberg, Synth. Met. 1993, 55,
299 – 306.
[2] a) B. C. Thompson, J. M. J. Frꢁchet, Angew. Chem. 2008, 120,
62 – 82; Angew. Chem. Int. Ed. 2008, 47, 58 – 77; b) M. Zhang,
H. N. Tsao, W. Pisula, C. Yang, A. K. Mishra, K. Mꢂllen, J. Am.
Chem. Soc. 2007, 129, 3472 – 3473; c) C. Li, M. Liu, N. G.
Pschirer, M. Baumgarten, K. Mꢂllen, Chem. Rev. 2010, 110,
6817 – 6855; d) A. Facchetti, Chem. Mater. 2011, 23, 733 – 758.
[3] a) C. J. Brabec, C. Winder, N. S. Sariciftci, J. C. Hummelen, A.
Dhanabalan, P. A. van Hal, R. A. J. Janssen, Adv. Funct. Mater.
2002, 12, 709 – 712; b) J. Peet, J. Y. Kim, N. E. Coates, W. L. Ma,
D. Moses, A. J. Heeger, G. C. Bazan, Nat. Mater. 2007, 6, 497 –
500.
prototypical
donor
poly(3-hexylthiophene)
(P3HT,
[4] a) A. Facchetti, M. Mushrush, M.-H. Yoon, G. R. Hutchison,
M. A. Ratner, T. J. Marks, J. Am. Chem. Soc. 2004, 126, 13859 –
13874; b) Y. Wang, M. D. Watson, Macromolecules 2008, 41,
8643 – 8647.
[5] a) X. Guo, F. S. Kim, S. A. Jenekhe, M. D. Watson, J. Am. Chem.
Soc. 2009, 131, 7206 – 7207; b) F. S. Kim, X. Guo, M. D. Watson,
S. A. Jenekhe, Adv. Mater. 2010, 22, 478 – 482; c) H. Yan, Z.
Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dçtz, M. Kastler, A.
Facchetti, Nature 2009, 457, 679 – 686.
10 mgmLÀ1) with varying concentrations of 9 (1–10 mgmLÀ1,
Figure 3). The LUMO energy level of 9 (À3.69 eV) is
comparable in energy to those of PCBM-C61 (À3.7 eV) and
perylene bisimide (À3.83 eV), which are well-known electron
acceptors. A dramatic decrease in the fluorescence of P3HT
with increasing concentrations of 9 can be observed. Stern–
Volmer analysis of the quenching provided a KSV value of
134 mLmgÀ1 (KSV = 1.1 ꢀ 105 mÀ1 based on repeat unit), which
[6] L. Gao, W. Senevirathna, G. Sauvꢁ, Org. Lett. 2011, 13, 5354 –
5357.
[7] J. D. Wood, J. L. Jellison, A. D. Finke, L. Wang, K. N. Plunkett, J.
Am. Chem. Soc. 2012, 134, 15783 – 15789.
[8] a) B. D. Steinberg, E. A. Jackson, A. S. Filatov, A. Wakamiya,
M. A. Petrukhina, L. T. Scott, J. Am. Chem. Soc. 2009, 131,
10537 – 10545; b) A. Ayalon, A. Sygula, P.-C. Cheng, M.
Rabinovitz, P. W. Rabideau, L. T. Scott, Science 1994, 265,
1065 – 1067.
[9] a) H. Reisch, U. Wiesler, U. Scherf, N. Tuytuylkov, Macro-
molecules 1996, 29, 8204 – 8210; b) E. J. Meijer, D. M. De Leeuw,
S. Setayesh, E. V. Veenendaal, B.-H. Huisman, P. W. M. Blom,
J. C. Hummelen, U. Scherf, T. M. Klapwijk, Nat. Mater. 2003, 2,
678 – 682.
[10] a) A. R. Mohebbi, J. Yuen, J. Fan, C. Munoz, M. F. Wang, R. S.
Shirazi, J. Seifter, F. Wudl, Adv. Mater. 2011, 23, 4644 – 4648;
b) A. R. Mohebbi, F. Wudl, Chem. Eur. J. 2011, 17, 2642 – 2646.
[11] L. T. Scott, M. S. Bratcher, S. Hagen, J. Am. Chem. Soc. 1996,
118, 8743 – 8744.
[12] H. A. Wegner, L. T. Scott, A. de Meijere, J. Org. Chem. 2003, 68,
883 – 887.
[13] a) D. T. Chase, B. D. Rose, S. P. McClintock, L. N. Zakharov,
M. M. Haley, Angew. Chem. 2011, 123, 1159 – 1162; Angew.
Figure 3. Emission spectrum of P3HT (10 mgmLÀ1) in THF with
varying concentrations of the electron acceptor 9. Concentrations of 9
(from top to bottom): a) 0, b) 1.0 mgmLÀ1, c) 2.5 mgmLÀ1
,
d) 5.0 mgmLÀ1, e) 7.5 mgmLÀ1, f) 10.0 mgmLÀ1. Corrected for inner
filter effect.
Angew. Chem. Int. Ed. 2012, 51, 12321 –12324
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 12323

.
Angewandte
Communications
Chem. Int. Ed. 2011, 50, 1127 – 1130; b) D. T. Chase, A. G. Fix,
B. D. Rose, C. D. Weber, S. Nobusue, C. E. Stockwell, L. N.
Zakharov, M. C. Lonergan, M. M. Haley, Angew. Chem. 2011,
123, 11299 – 11302; Angew. Chem. Int. Ed. 2011, 50, 11103 –
11106; c) F. G. Brunetti, A. Varotto, N. A. Batara, F. Wudl,
Chem. Eur. J. 2011, 17, 8604 – 8608.
d) H. Xia, D. Liu, X. Xu, Q. Miao, Chem. Commun. 2012, DOI:
10.1039/C2CC34992B.
[16] T. Okamoto, Z. Bao, J. Am. Chem. Soc. 2007, 129, 10308 – 10309.
[17] P. M. Beaujuge, C. M. Amb, J. R. Reynolds, Acc. Chem. Res.
2010, 43, 1396 – 1407.
[18] a) G. Sonmez, C. K. F. Shen, Y. Rubin, F. Wudl, Angew. Chem.
2004, 116, 1524 – 1528; Angew. Chem. Int. Ed. 2004, 43, 1498 –
1502; b) G. Sonmez, H. B. Sonmez, C. K. F. Shen, R. W. Jost, Y.
Rubin, F. Wudl, Macromolecules 2005, 38, 669 – 675.
[19] B. F. Plummer, M. J. Hopkinson, J. H. Zoeller, J. Am. Chem. Soc.
1979, 101, 6779 – 6781.
[20] a) S. A. Jenekhe, L. Lu, M. M. Alam, Macromolecules 2001, 34,
7315 – 7324; b) G. L. Gibson, T. M. McCormick, D. S. Seferos, J.
Am. Chem. Soc. 2012, 134, 539 – 547.
[14] a) T. Kawase, A. Konishi, Y. Hirao, K. Matsumoto, H. Kurata, T.
Kubo, Chem. Eur. J. 2009, 15, 2653 – 2661; b) M. Saito, M.
Nakamura, T. Tajima, Chem. Eur. J. 2008, 14, 6062 – 6068.
[15] a) H. Dang, M. A. Garcia-Garibay, J. Am. Chem. Soc. 2001, 123,
355 – 356; b) H. Dang, M. Levitus, M. A. Garcia-Garibay, J. Am.
Chem. Soc. 2002, 124, 136 – 143; c) C. L. Eversloh, Y. Avlase-
vich, C. Li, K. Mꢂllen, Chem. Eur. J. 2011, 17, 12756 – 12762;
12324 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 12321 –12324