Pentacene oligomers and polymers: Functionalization of pentacene to afford mono-, di-, tri-, and polymeric materials

Article abstract of DOI:10.1021/ol702094d

(Chemical Equation Presented) The synthesis and characterization of defined-length di- and trimeric pentacenes and the corresponding polymers are described. The synthesis is divergent from two common pentacene building blocks, 1 and 2, allowing for structural diversity. The resulting materials are air stable and exhibit good solubility in common organic solvents.

Full text of DOI:10.1021/ol702094d

ORGANIC
LETTERS
2007
Vol. 9, No. 22
4583-4586
Pentacene Oligomers and Polymers:
Functionalization of Pentacene to Afford
Mono-, Di-, Tri-, and Polymeric Materials
Dan Lehnherr and Rik R. Tykwinski*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta T6G 2G2, Canada
rik.tykwinski@ualberta.ca
Received August 24, 2007
ABSTRACT
The synthesis and characterization of defined-length di- and trimeric pentacenes and the corresponding polymers are described. The synthesis
is divergent from two common pentacene building blocks, 1 and 2, allowing for structural diversity. The resulting materials are air stable and
exhibit good solubility in common organic solvents.
It was not until a decade ago when organic materials began
to have a significant practical impact in optoelectronic
applications. Today, the field of organic semiconducting
materials is very much active and has the potential to
revolutionize applications in photovoltaic cells, chemical
sensors, and thin film transistors (TFTs).1,2 The use of
polycyclic aromatic hydrocarbons, such as pentacene, has
dominated most studies, but despite the substantial research
on pentacene-based devices, relatively few derivatives of
pentacene are known.1-3 To date, much of the work to
enhance the performance of pentacene-based materials has
been through device fabrication techniques rather than
synthetic modification of the pentacene skeleton. Only
recently has functionalization been used in an attempt to
increase the semiconductive properties (i.e., charge carrier
mobilities) and processability of pentacene-based materials.3
A natural progression in the development of organic ad-
vanced materials is the incorporation of functional chro-
mophores into a polymer framework with the goal of
increasing the solubility and facile film formation of the
material. While oligoanthracenes have been reported,4 defined-
length pentacene-based oligomers remain, to our knowledge,
unknown.5 We report herein the synthesis of oligomers and
polymers that contain pentacene in the repeat unit.
It has been shown that substitution of pentacene in the
6,13-positions with ethynylsilanes improves stability versus
(3) For recent examples, see: (a) Anthony, J. E.; Brooks, J. S.; Eaton,
D. L.; Parkin, S. R. J. Am. Chem. Soc. 2001, 123, 9482-9483. (b) Anthony,
J. E.; Eaton, D. L.; Parkin, S. R. Org. Lett. 2002, 4, 15-18. (c) Payne, M.
M.; Delcamp, J. H.; Parkin, S. R.; Anthony, J. E. Org. Lett. 2004, 6, 1609-
1612. (d) Payne, M. M.; Odom, S. A.; Parkin, S. R.; Anthony, J. E. Org.
Lett. 2004, 6, 3325-3328. (e) Swartz, C. R.; Parkin, S. R.; Bullock, J. E.;
Anthony, J. E.; Mayer, A. C.; Malliaras, G. G. Org. Lett. 2005, 7, 3163-
3166. (f) Susumu, K.; Duncan, T. V.; Therien, M. J. J. Am. Chem. Soc.
2005, 127, 5186-5195. (g) Vets, N.; Smet, M.; Dehaen, W. Synlett 2005,
217-222. (h) Chan, S. H.; Lee, H. K.; Wang, Y. M.; Fu, N. Y.; Chen, X.
M.; Cai, Z. W.; Wong, H. N. C. Chem. Commun. 2005, 66-68. (i) Taka-
hashi, T.; Li, S.; Huang, W.; Kong, F.; Nakajima, K.; Shen, B.; Ohe, T.;
Kanno, K.-i. J. Org. Chem. 2006, 71, 7967-7977. (j) Briseno, A. L.; Miao,
Q.; Ling, M.-M.; Reese, C.; Meng, H.; Bao, Z.; Wudl, F. J. Am. Chem. Soc.
2006, 128, 15576-15577. (k) Miao, Q.; Chi, X.; Xiao, S.; Zeis, R.; Lefen-
feld, M.; Siegrist, T.; Steigerwald, M. L.; Nuckolls, C. J. Am. Chem. Soc.
2006, 128, 1340-1345. (l) Palayangoda, S. S.; Mondal, R.; Shah, B. K.;
Neckers, D. C. J. Org. Chem. 2007, 72, 6584-6587. (m) For supramolecular
assemblies, see: Desvergne, J.-P.; Del Guerzo, A.; Bouas-Laurent, H.; Belin,
C.; Reichwagen, J.; Hopf, H. Pure Appl. Chem. 2006, 78, 707-719.
(4) Ito, K.; Suzuki, T.; Sakamoto, Y.; Kubota, D.; Inoue, Y.; Sato, F.;
Tokito, S. Angew. Chem., Int. Ed. 2003, 42, 1159-1162.
(1) (a) Murphy, A. R.; Fre´chet, J. M. J. Chem. ReV. 2007, 107, 1066-
1096. (b) Bendikov, M.; Wudl, F. Chem. ReV. 2004, 104, 4891-4945.
(2) Anthony, J. E. Chem. ReV. 2006, 106, 5028-5048.
10.1021/ol702094d CCC: $37.00
© 2007 American Chemical Society
Published on Web 10/06/2007

pristine pentacene,3b and this structural motif became the
basis of the targeted oligo- and polymers. The Si atoms
pendent to the pentacene core also provide an attachment
point for two isopropyl groups to enhance solubility and
stability,3b and a “functional arm” for polymerization (1 and
2, Figure 1). For the “functional arm”, primary alcohol(s)
Scheme 2. Functionalization of Pentacene
Figure 1. Pentacene building blocks 1 and 2.
were chosen to allow for condensation reactions with bis-
acid chlorides to form polyesters.
The synthetic scheme begins with 3,6 which is available
in two steps from 1,4-dibromobenzene.7 In a one-pot
sequence, lithiated 3 (Scheme 1) was added to a solution of
(Scheme 2).8 The best results from this reaction were
obtained when an excess of the Li-acetylide derived from 5
was used (2.9 equiv per ketone moiety, 5.9 equiv total). The
excess 5 could be recovered almost quantitatively during
purification.7
The tin(II)-mediated reduction of purified trans-diol 6 in
THF provided protected monomer 7 in 83% yield. The
success of this step was solvent dependent, both in terms of
reaction rate and yield/purity. For example, reactions in
acetonitrile or acetone were complete in less than 30 min,
but gave numerous byproducts and low yields. Removal of
the two TBS groups to unveil the diol functionality of
monomer 1 was accomplished by using dilute HCl in THF
in 98% yield. If the reaction time was limited to 45 min,
monodeprotected 2 was isolated in 49% yield, along with
36% of unreacted 7 and 15% of monomer 1; all could be
easily separated by column chromatography.
Scheme 1. Synthesis of Functionalized Silylacetylene 5
Pentacene di- and trimers were synthesized as model
compounds to bridge the gap between monomer 1 and
polymers 8 and 9 (Scheme 5). Because of the simplicity of
this reaction, the synthesis of the dimers with 1-8 methylene
groups was done to probe systematically the effects of
tether length on solubility, aggregation, and thin-film forma-
tion. Slow addition of a solution of the requisite bis-acid
chloride to a solution of 2 in the presence of DMAP provided
pentacene dimers 10-17 in excellent yield (79-97%,
Scheme 3). All of the dimers were highly soluble in
CH2Cl2, CHCl3, and THF. The thermal stability of the dimers
was assessed by TGA analysis and showed no significant
weight loss (<5%) below 350 °C in all cases.
i-Pr2SiCl2 in THF at -78 °C, followed by the addition of
LiCtCSiMe3 to afford a ∼2:1 mixture of 4 and desilylated
5. This mixture was subjected to mild desilylation conditions,
completing the conversion of 4 to 5 and providing 58% yield
(based on 3). This method is scaleable to tens-of-grams, and
it has the added benefit that column chromatography is not
required over the four steps from 1,4-dibromobenzene.
Compound 5 was then deprotonated at low temperature
with BuLi to give the Li-acetylide, which was added to
6,13-pentacenequinone. This provided a mixture of trans-
and cis-diol 6 (∼95:5 ratio) from which the trans-diol
could be isolated in 88% yield by column chromatography
Dimers with shorter linker chains (10, 14, and especially
11) had a tendency to aggregate in concentrated solution
(∼0.06 M) as observed by H NMR spectroscopy. In these
(5) Two reports of pentacene-based polymers have appeared: (a) Tokito,
S.; Weinfurtner, K.-H.; Fujikawa, H.; Tsutsui, T.; Taga, Y. Proc. SPIE -
Int. Soc. Opt. Eng. 2001, 4105, 69-74. (b) Okamoto, T.; Bao, Z. J. Am.
Chem. Soc. 2007, 129, 1308-1309.
1
(8) This assignment of the stereochemistry of the addition is based
tentatively on the polarity of the two products on silica gel TLC plates.
The major isomer is the less polar of the two and thus was assigned as the
centrosymmetric trans-isomer.
(6) Sessler, J. L.; Wang, B.; Harriman, A. J. Am. Chem. Soc. 1995, 117,
704-714.
(7) See Supporting Information for details.
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Org. Lett., Vol. 9, No. 22, 2007

Scheme 3. Synthesis of a Homologous Series of Pentacene
Scheme 5. Synthesis of Pentacene Polymers
Dimers
soluble materials showed thermal stability analogous to that
of both monomer 1 and the analogous dimers 11 and 12.
Preliminary efforts toward the formation of polymers were
then conducted via the reaction of monomer 1 with bis-acid
chlorides in the presence of DMAP (Scheme 5). Following
aqueous workup, purification of the products was ac-
complished by recrystallization. MALDI MS analysis showed
macromolecules of over m/z 17 000 were achieved (e.g., m
) 20 for 8).9 DSC analysis showed no glass transition
temperature for these polymers, and TGA demonstrated the
polymers were as stable as the oligomers, with no significant
weight loss (<5%) before 350 °C.
cases, multiple signals were observed where a single reson-
ance was expected. Upon dilution of the NMR sample, these
resonances coalesced into a single signal. Furthermore, in
more dilute solutions, such as concentration ranges used for
UV-vis spectroscopy (∼10-5 to 10-6 M), Beer’s Law was
maintained, confirming that aggregation was not occurring.
Pentacene trimers were synthesized (Scheme 4). Desym-
metrized pentacene 2 was reacted with succinic or glutaric
anhydride to afford pentacene 18 and 19 with pendent
carboxylic acid groups in 98% and 99% yields, respectively.
A subsequent DCC-mediated coupling of 18 or 19 with
monomer 1 afforded trimers 20 and 21, respectively. These
Scheme 4. Synthesis of Pentacene Trimers
Figure 2. Absorption spectrum of monomer 1 in CH2Cl2. (Inset)
Absorption (left trace) and emission (right trace) spectra of
monomer 1 in CH2Cl2, illustrating the small Stokes shift.
Figure 2 shows the absorption and emission spectra of
monomer 1, which has a strong absorption at λmax ) 309
nm and a series of low-energy absorptions with the lowest
at 645 nm. The emission is broad and featureless with the
λem,max ) 652 nm which represents a small Stokes shift of
only 7 nm and suggests minimal molecular rearrangement
occurs upon photoexcitation of the molecule.
(9) MALDI MS analyses showed that, in addition to the generalized
polymer structure shown in Scheme 5 for 8 and 9 (terminated with an acid
and an alcohol), the product mixtures also consisted of structures terminated
with two acids, two alcohols, and macrocyclic structures resulting from
intramolecular ester formation. See Supporting Information for MS spectra
and a more detailed discussion.
Org. Lett., Vol. 9, No. 22, 2007
4585

strong electronic coupling mediated by strong intermolecular
interactions between adjacent molecules.2 The solid-state
structure of the common building block of the oligomers, 1,
was determined by X-ray analysis of crystals grown from
THF/MeOH.10 Monomer 1 cocrystallized with two molecules
of MeOH, and crystallographic analysis of these crystals
revealed that 1 arranged in one-dimensional slipped stacks11
with an intermolecular distance between the acenes of ca.
3.4 Å (Figure 3). Thus, in terms of π-overlap and orientation
in the solid state, compound 1 is well suited for intermo-
lecular communication.
In conclusion, we provide the first reported synthesis of
defined-length pentacene oligomers and representative poly-
meric derivatives, as well as a description of their physical
properties. All molecules exhibit good solubility and stability.
The synthetic scheme that has been developed is also
amenable to inclusion of other terminal groups on the
monomer unit toward forming, for example, polyamides. A
full account of these efforts will be reported in due course.
Acknowledgment. This work has been generously sup-
ported by the University of Alberta and the Natural Sciences
and Engineering Research Council of Canada (NSERC)
through the Discovery Grant program. D.L. thanks NSERC
(CGS-M), the Alberta Ingenuity Fund, the University of
Alberta, and the Killam Trust for scholarship support. We
thank Dr. Robert McDonald (University of Alberta) for
determining the X-ray structure of 1 and Prof. John E.
Anthony (University of Kentucky) for insightful and helpful
discussions.
Figure 3. (Above) X-ray structure of 1. (Below) Solid-state
packing of 1 as viewed along the b-axis.
The electronic structure of the basic pentacene unit as
found in 1 is essentially unaffected by oligomer or polymer
formation. Comparing the UV-vis spectra of the homolo-
gous series made up of monomer 1, dimer 12, trimer 21,
and polymer 8 reveals that all molecules share nearly
identical absorption maxima ((1 nm).7
Thin films of the polymers 8 and 9 could be cast from a
number of solvents, including CH2Cl2, CHCl3, and THF.
While a complete description of this analysis is beyond the
scope of the current discussion, it is worth pointing out that
UV-vis characteristics of these films were very similar to
those of the monomer and were red-shifted by <10 nm
compared to that of the solution-state spectra.7
Supporting Information Available: Experimental pro-
cedures, spectroscopic data for new compounds, MS spectra
for polymers, and UV-vis spectra of thin films of the
polymers. This material is available free of charge via the
Internet at http://pubs.acs.org.
OL702094D
(10) X-ray crystallographic data for 1: C52H54O2Si2‚2CH3OH, M )
831.22; monoclinic space group P21/c (No. 14); Fc ) 1.156 g cm-3; a )
8.8223(6) Å, b ) 19.0409(13) Å, c ) 14.6224(10) Å; â ) 103.5400(16)°;
V ) 2388.1(3) Å3; Z ) 2; µ ) 0.118 mm-1. Final R(F) ) 0.0433 (6400
2
observations [Fo2 g 2σ(Fo )]); wR2 ) 0.0985 for 275 variables and 10024
2
data with [Fo2 g -3σ(Fo )]; CCDC 654502. X-ray data have been deposited
Solid-state packing plays an important role for materials
used in molecular electronics because good electronic
performance is often observed for compounds that have
at the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge
CB21EZ, UK; fax: (+44)1223-336-033.
(11) See ref 2 for solid-state packing terminology of acenes.
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Org. Lett., Vol. 9, No. 22, 2007