Synthesis of solution-soluble pentacene-containing conjugated copolymers

Article abstract of DOI:10.1021/ja0725403

We have synthesized new-type of pentacene copolymers containing linear- and branched-dialkoxylphenyldiethynylene units via a modified Sonogashira coupling reaction at room temperature to prepare low-band gap polymers (<1.7 eV). The structures of polymers

Full text of DOI:10.1021/ja0725403

Published on Web 08/08/2007
Synthesis of Solution-Soluble Pentacene-Containing Conjugated Copolymers
Toshihiro Okamoto and Zhenan Bao*
Department of Chemical Engineering, Stanford UniVersity, 381 North-South Mall, Stanford, California 94305
Received April 12, 2007; E-mail: zbao@stanford.edu
Scheme 1. Synthetic Scheme of Pentacene Polymers 1 and 2
Conjugated polymers have great potential as organic field effect
transistors (OFETs),1 photovoltaic cells (OPVCs),2 and light emit-
ting devices (OLEDs)3. Thiophene-based conjugated polymers have
been extensively studied for the above applications.4 In addition,
acene- and heteroacene-based organic semiconductors, such as
pentacene and fused oligothiophenes, have been intensely inves-
tigated for OFET applications.5 Among them, pentacene and its
derivatives have the highest reported thin-film transistor mobility
at above 5 cm2 V-1 s-1.6 To date, there are few reported conjugated
acene-containing polymers, most of which feature anthracenes.7
Pentacene-containing conjugated polymers will be of great interest
as solution-soluble polymers with high charge carrier mobility and
low band gap. In addition, extending the conjugation of pentacene
reduces the electron density on the reactive 6 and 13 positions and
therefore improves the stability of pentacene, as seen later in this
work from the increased oxidation potential of the polymer.
So far, there has been only one report on a pentacene-fluorene
random copolymer with the pentacene core orthogonal to the
conjugated polymer chain through the 6 and 13 positions for OLED
devices.7a The main difficulty in synthesizing pentacene-containing
conjugated polymers is due to the poor solubility of pentacene in
common organic solvents as well as its rapid degradation when
exposed to light and oxygen.8
DMF, 60 °C, 2 days; (c) 1) (2-triisopropylsilyl)ethynylmagnesium
chloride; (2) SnCl2‚2H2O, 10% HCl(aq); (d) 1,4-diethynyl-2,5-
dialkoxybenzene (R ) dodecyl or 2-ethylhexyl), PdCl2(PhCN)2,
PtBu3, iPr2NH, CuI, toluene, room temperature for 3 days.
TIPSEBr2P did not show sign of degradation in 1H NMR spectra,
indicating that it should be stable enough under basic conditions
even at an elevated temperature.12 However, we were concerned
that the elevated temperature and long reaction times commonly
used in Sonogashira polymerization reactions might result in partial
decomposition of the pentacene monomer and polymer. It is well-
known that trace amounts of impurity may have a significant impact
on device performance. Therefore, we adapted a room-temperature
Sonogashira coupling reaction condition for aryl bromides reported
recently by Fu and Buchwald using PdCl2(PhCN)2/PtBu3 as the
catalyst.11
Recently, Anthony’s and Neckers’ groups reported a class of
soluble triisopropylsilylethynyl (TIPSE) substituted pentacene
derivatives.9 Compared to pentacene, the bulky TIPSE substituents
make the resulting pentacene derivatives very soluble in common
organic solvents and afford improved stability under ambient
conditions both in the solid state and in solution.
The polymerization was carried out between TIPSEBr2P and
1,4-diethynyl-2,5-dialkoxybenzene (R ) dodecyl or 2-ethylhexyl)13
at room temperature to obtain the desired pentacene-containing
copolymers 1 and 2 in 47% and 62% yields, respectively, after
washing with appropriate solvents. We limited the reaction time to
6 h so that a large fraction of the resulting polymer was soluble in
common organic solvents. When a longer reaction time was used
(12 h), less than 30 wt % of the polymer was soluble. The molecular
weights of the polymers were determined by gel permeation
chromatography (GPC) using tetrahydrofuran (THF) as the eluent
and polystyrene standards. Polymer 1 was found to have a number
averaged molecular weight (Mn) of 1.13 × 104 g mol-1 with a
polydispersity (PDI) of 2.21, while polymer 2 had Mn ) 2.39 ×
104 g mol-1 and PDI ) 2.42. To our knowledge, these are the first
examples of pentacene-containing phenylethynylene polymers as
well as the first application of room-temperature Sonogashira
coupling reaction using an aryl bromide as a starting monomer to
the preparation of polyarylethynylenes (PAEs) polymers.
Polymer 2 showed good solubility (>5.0 mg/mL) in most
chlorinated organic solvents as well as in THF and toluene, while
polymer 1 had only limited solubility (∼0.3 mg/mL) in halogenated
benzene solvents. This difference is attributed to the difference in
bulkiness of the alkoxy group substituted on the main polymer
backbone.
In this Communication, we report the synthesis of pentacene-
containing conjugated polymers with low-bandgaps of less than
1.7 eV, using alternating TIPSE-substituted pentacene with linear
or branched alkoxyl-substituted diethynylphenyl moieties10 using
Sonogashira coupling reaction conditions at room temperature.11
Our pentacene copolymers exhibited much better air stability than
TIPSE-substituted pentacene (TIPSEP) in the solution state.
The polymerization scheme and conditions are summarized in
Scheme 1. The TIPSE-substituted dibromopentacene (TIPSEBr2P)
was prepared from dibromopentacene quinone. First, 4-bromo-o-
xylene was converted to dibromopentacene quinone in two steps,
yielding a mixture of 2,9- and 2,10-dibromopentacene quinone.
Because of similar solubility and polarity, we were not able to
separate these isomers. They were subsequently converted to
TIPSEBr2P using conditions reported in literature.9a From the 1H
NMR of TIPSEBr2P in deuterated chloroform (CDCl3), we
estimated that the ratio of 2,9- to 2,10-isomer was around five to
four.12 Before polymerization, we checked the stability of TIPSEBr2P
under the commonly used Sonogashira polymerization conditions
without addition of the comonomer in a degassed mixture of
toluene-iPr2NH at 70-80 °C.
The reactions were run using the following reagents and
conditions: (a) NBS, BPO, CCl4, reflux; (b) 1,4-benzoquinone, NaI,
9
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10.1021/ja0725403 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Summary of Electrochemical and Optical Properties of
Pentacene Copolymers and TIPSEPa
exhibited better stability (about three times the half-life) than
TIPSEP under our measurement conditions (see Supporting
Information).
Ered
/
Eox
/onset
optical
onset
compound
(V)
(V)
λmax
/
λonset
energy gap
In conclusion, we have synthesized a new type of conjugated
polymer containing pentacene using a room-temperature Sono-
gashira coupling reaction. Polymer 2 exhibited good solubility in
common organic solvents (>5.0 mg/mL) which should make it a
good candidate for solution-processable device applications. The
optical gap of these polymers was lowered with respect to TIPSEP
and was as small as 1.68 eV for polymer 1. At the same time, the
oxidative stability of these polymers in solution was better than
TIPSEP both from CV measurements and monitoring UV-vis
absorption. Determination of the charge transport properties of these
new polymers is underway.
polymer 1
-1.80
-1.37
-1.82
-1.42
-1.54
+0.32
671/693 (o-DCB)
672/736 (film)
674/695 (o-DCB)
674/706 (film)
1.68
1.76
1.86
polymer 2
+0.28
+0.24
TIPSEP
649/667 (o-DCB)
a Ered/onset is the onset reduction potential; Eox/onset is the onset oxidation
potential; λonset is the absorption edge.
Acknowledgment. The authors thank Dr. Nobuyuki Miyaki for
helpful discussions. Z.B. acknowledges financial support from the
Air Force Office of Scientific Research Grant FA 9550-06-1-0126,
the Global Climate and Environmental Program, and NSF Center
for Polymer and Macromolecular Assemblies.
Note Added after ASAP Publication. An error was detected
in paragraph 6 in the version published August 8, 2007; CuI was
added as a reagent in the version published August 14, 2007.
Figure 1. (a) Cyclic voltammograms of polymer 2 (top) and TIPSEP
(bottom) in 0.05 M tetra-n-butylammonium hexafluorophosphate (TBAPF6)
of o-dichlorobenzene (o-DCB); (b) UV-vis spectra of polymer 1 (solid
line) and polymer 2 (dashed line) films and in o-DCB (inset) at room
temperature.
Supporting Information Available: Details of experimental
procedures, synthesis of all compounds, UV-vis spectra, FL spectra,
cyclic voltammogram, TGA and DSC of polymer 1 and 2. This material
is available free of charge via the Internet at http://pubs.acs.org.
To estimate the highest occupied molecular orbital (HOMO)
levels, lowest unoccupied molecular orbital (LUMO) levels, and
optical energy gaps for polymers 1 and 2, we examined their
electrochemical and optical properties by cyclic voltammetry (CV)
and UV-vis spectrometry in solution and in thin film. We
compared this data to that of TIPSEP. Table 1 summarizes the
CV and UV-vis results and the optical energy gaps. In terms of
electrochemical properties, polymer 2 exhibited one reversible
oxidative step and two reversible reductive steps, as shown in Figure
1, which indicates high chemical stability in the solvated oxidized
and reduced states. On the other hand, polymer 1 had one reversible
oxidation step and two pseudoreversible reduction steps. The onset
of oxidation for polymer 2 is 0.04 eV higher than that of TIPSEP
while the onset of reduction for 2 was 0.12 eV lower. On the basis
of the onset values, the HOMOs and LUMOs were calculated to
be 5.28 and 3.57 eV for polymer 1 and 5.24 and 3.54 eV for
polymer 2, respectively. As a comparison, the HOMO level of
TIPSEP was 5.20 eV versus vacuum, indicating that it is more
easily oxidized than the polymers.14 The slightly increased oxidation
potentials of the polymers is due to the incorporation of pentacene
into the conjugated polymer leading to delocalization of π-electrons
on pentacene ring and making it less electron rich. The band gaps
determined from CV were 1.69 and 1.70 eV for polymer 1 and 2,
respectively. The UV-vis spectra in solution for polymer 1 and 2
were very similar with their λmax and λonset slightly red-shifted by
about 22-28 nm compared to that of TIPSEP. With respect to the
properties of polymer films deposited by spin-coating or dip-coating,
the absorption of polymer 1 has an onset at approximately 736 nm,
and the spectrum displays a broad tail in the range of 500-800
nm. On the other hand, the absorption onset of polymer 2 is about
30 nm blue-shifted compared to polymer 1 indicating that polymer
1 has a slightly stronger interchain interaction.
References
(1) (a) Babel, A.; Jenekhe, S. A. J. Am. Chem. Soc. 2003, 125, 13656. (b)
Chua, L.-L.; Zaumseil, J.; Chang, J.-F.; Ou, E. C.-W.; Ho, P. K.-H.;
Sirringhaus, H.; Friend, R. H. Nature 2005, 434, 194. (c) Li, Y.; Wu, Y.;
Liu, P.; Birau, M.; Pan, H.; Ong, B. S. AdV. Mater. 2006, 18, 3029.
(2) (a) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.;
Yang, Y. Nat. Mater. 2005, 4, 864. (b) Winder, C.; Sariciftci, N. S. J.
Mater. Chem. 2004, 14, 1077.
(3) (a) Zhang, X.; Jenekhe, S. A. Macromolecules 2000, 33, 2069. (b) Ego,
C.; Marsitzky, D.; Becker, S.; Zhang, J.; Grimsdale, A. C.; Mullen, K.;
MacKenzie, J. D.; Silva, C.; Friend, R. H. J. Am. Chem. Soc. 2003, 125,
437. (c) Kulkarni, A. P.; Zhu, Y.; Jenekhe, S. A. Macromolecules 2005,
38, 1553.
(4) Fichou, D.; Ziegler, C. In Handbook of Oligothiophenes and Poly-
thiophenes, 1st ed.; Fichou, D., Ed.; Wiley-VCH: Weinheim, Germany,
1999; Chapter 4, p 183.
(5) (a) Mallik, A. B.; Locklin, J. J.; Mannsfeld, S.; Reese, C.; Roberts, M.;
Senatore, M.; Zi, H.; Bao, Z. Design, Synthesis, and Transistor Perfor-
mance of Organic Semiconductors. In Organic Thin Film Transistors,
Bao, Z., Locklin, J. J., Eds.; CRC Press: Boca Raton, FL, 2007. (b)
Dimitrakopoulos, C. D.; Malenfant, P. R. L. AdV. Mater. 2002, 14, 99.
(c) Katz, H. E.; Bao, Z.; Gilat, S. L. Acc. Chem. Res. 2001, 34, 359. (d)
Anthony, J. E. Chem. ReV. 2006, 106, 5028.
(6) (a) Yen-Yi, L.; Gundlach, D. I.; Nelson, S. F.; Jackson, T. N. IEEE Trans.
Electron DeVices 1997, 44, 1325. (b) Kelley, T. W.; Muyres, D. V.; Baude,
P. F.; Smith, T. P.; Jones, T. D. Mater. Res. Soc. Symp. Proc. 2003, 771,
L6.5.1.
(7) (a) Tokito, S.; Weinfurtner, K.-H.; Fujikawa, H.; Tsutsui, T.; Taga, Y.
Proc. SPIE-Int. Opt. Soc. Eng. 2001, 4105, 69. (b) Hodge, P.; Power,
G. A.; Rabjohns, M. A. Chem. Commun. 1997, 73. (c) Klarner, G.; Davey,
M. H.; Chen, W.-D.; Scott, J. C.; Miller, R. D. AdV. Mater. 1998, 10,
993. (d) Ishow, E.; Bouffard, J.; Kim, Y.; Swager, T. M. Macromolecules
2006, 39, 7854.
(8) Maliakal, A.; Raghavachari, K.; Katz, H.; Chandross, E.; Siegrist, T. Chem.
Mater. 2004, 16, 4980.
(9) (a) Anthony, J. E.; Brooks, J. S.; Eaton, D. L.; Parkin, S. R. J. Am. Chem.
Soc. 2001, 123, 9482. (b) Jiang, J.; Kaafarani, B. R.; Neckers, D. C. J.
Org. Chem. 2006, 71, 2155.
(10) (a) Giesa, R. J. Macromol. Sci. ReV. Macromol. Chem. Phys. 1996, C36,
631. (b) Bunz, U. H. F. Chem. ReV. 2000, 100, 1605. (c) Bunz, U. H. F.
AdV. Polym. Sci. 2005, 177, 1.
(11) Hundertmark, T.; Littke, A. F.; Buchwald, S. L.; Fu, G. C. Org. Lett.
2000, 2, 1729.
We have also investigated the stability of these pentacene
polymers. We monitored UV-vis absorption over time for the
polymer solution in o-dichlorobenzene as compared to TIPSEP,
which was reported to be approximately 50 times more stable than
pentacene.8 Consistent with the CV results, our pentacene polymers
(12) See Supporting Information.
(13) Weder, C.; Wrighton, M. S. Macromolecules 1996, 29, 5157.
(14) Payne, M. M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E. Org. Lett.
2004, 6, 1609.
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