Synthesis of homopolymer containing diphenyl end-capped oligothiophene co-oligomer unit in the side chain

Article abstract of DOI:10.1021/ol060162l

A new polymer with the diphenyl end-capped oligothiophene co-oligomer unit in the side chain was obtained by the ROMP method. The polymer showed good photophysical characteristics, thermal stability, and film-forming properties. A photovoltaic cell fabric

Full text of DOI:10.1021/ol060162l

ORGANIC
LETTERS
2006
Vol. 8, No. 8
1585-1588
Synthesis of Homopolymer Containing
Diphenyl End-Capped Oligothiophene
Co-oligomer Unit in the Side Chain
Chunchang Zhao,† Yong Zhang,‡ Chengwei Wang,† Lewis Rothberg,†,‡ and
Man-Kit Ng*,†
Department of Chemistry and Department of Chemical Engineering, UniVersity of
Rochester, Rochester, New York 14627
ng@chem.rochester.edu
Received January 19, 2006
ABSTRACT
A new polymer with the diphenyl end-capped oligothiophene co-oligomer unit in the side chain was obtained by the ROMP method. The
polymer showed good photophysical characteristics, thermal stability, and film-forming properties. A photovoltaic cell fabricated from this
polymer showed relatively large open-circuit voltage (VOC
stability under ambient conditions.
) 0.7 V), moderate short-circuit current (ISC ) 0.7 µ
A/cm2), and excellent device
Thiophene-based π-conjugated oligomers and polymers1 have
been well investigated in the past decades due to their
chemical and environmental stability and their potential
applications in many fields such as field-effect transistors,1c,h,l
photoswitches,1e photovoltaic cells,1f or light modulators.1g
Extensive studies have been carried out on polymers obtained
by oxidative or electropolymerization of thiophenes,2
bithiophenes,3 and terthiophenes.4 However, this research on
conjugated polythiophenes was plagued by difficulties in
processing the intractable materials and imperfections in the
conjugated polymer microstructures. Conjugated oligoth-
iophene derivatives that have well-defined structures and
interesting physical, electronic, and optical properties have
emerged as model compounds to rationalize and predict the
properties of the corresponding polymeric analogues.5
The incorporation of conjugated oligomers into polymer
structures combines the properties of the specific oligomer
with the desirable properties of polymer such as mechanical
strength and film-forming properties. A few reports2,6 showed
that oligothiophenes can be attached to nonpolar vinyl-type
monomers as well as methacrylate polymers as side chains.
These polymers showed an electrical conductivity in the
† Department of Chemistry.
‡ Department of Chemical Engineering.
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Fung, A. W. P.; Katz, H. E. Science 1996, 272, 1462-1464. (d) Garnier,
F.; Yassar, A.; Hajlaoui, R.; Horowitz, G.; Deloffre, F.; Servet, B.; Ries,
S.; Alnot, P. J. Am. Chem. Soc. 1993, 115, 8716-8721. (e) Tsivgoulis, G.
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10.1021/ol060162l CCC: $33.50
© 2006 American Chemical Society
Published on Web 03/25/2006

range of semiconductors and have potential applications in
polynorbornene system with π-conjugated oligomers fused
electrochromic devices.Polymers containing π-conjugated
to the backbone as shown in Scheme 2. The polynorbornene
Scheme 1. Synthesis of Oligothiophene-Functionalized
Norbornene Monomer 8
Scheme 2. Ring-Opening Metathesis Polymerization of 8
was prepared by a ring-opening metathesis polymerization
(ROMP) of laterally attached rodlike norbornene monomer,
using the first-generation Grubbs’ catalyst as the initiator.
The reactions used for the preparation of monomer 8
and polymer 9 are outlined in Scheme 1 and Scheme 2.
The compound 4′-(2,5-dimethoxyphenyl)-5,5′′′-diphenyl-
2,2′;5′,2′′;5′′,2′′′-quaterthiophene 5 was prepared in five steps
commencing from 2-iodo-1,4-dimethoxybenzene by known
chemistries including a palladium-catalyzed Stille cross-
coupling8 with 3-(tributylstannyl)thiophene and regioselective
iodination on one of the R-positions of the resulting thiophene
unit.9
It is worth mentioning that in the course of the purification
of 3-(2,5-dimethoxyphenyl)thiophene 1, we observed that this
intermediate was somewhat difficult to purify because of its
similar polarity to 1,4-dimethoxybenzene, a contaminant
resulting from the selective monoiodination of 1,4-dimethox-
ybenzene. Gratifyingly, the presence of this side product has
no detrimental effect on the two following steps, and it can
be completely removed from the less polar dithienyl-
substituted dimethoxybenzene 3. A regioselective diiodina-
tion of 3 gave diiodo bithiophene 4 in 99% yield. This
intermediate was subject to another Stille coupling with
5-phenyl-2-(tributylstannyl)thiophene to afford hexamer 5 in
75% yield. This compound can be purified by column
chromatography on silica gel or by recrystallization and is
partially soluble in CH2Cl2 and CHCl3. Conversion of the
dimethoxybenzene moiety in 5 to the corresponding quinone
6 was carried out following a standard deprotection-
oxidation protocol involving removal of the dimethoxy
protecting groups (BBr3, CH2Cl2) followed by oxidation of
the intermediate hydroquinone (Ag2O, CH2Cl2).10 A Diels-
Alder reaction of quinone 6 with excess cyclopentadiene
provided a norbornene derivative 7 in nearly quantitative
oligomers in the side chain are of particular interest because
of the variety of possible oligomers, processability, supramo-
lecular self-assembly properties attainable by the appropriate
choice of the side group, and potential applications in
electronic devices.2 On the basis of these grounds, we became
interested in attaching π-conjugated oligothiophene-phe-
nylene co-oligomer in the side chain because of the rigid
rodlike structures and useful optoelectronic properties of
these oligomers.7 For this purpose, we report here a new
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J. Heterocycl. Chem. 2000, 37, 281-286. (f) Lee, S. A.; Hotta, S.;
Nakanishi, F. J. Phys. Chem. A 2000, 104, 1827-1833.
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(b) Facchetti, A.; Deng, Y.; Wang, A. C.; Koide, Y.; Sirringhaus, H.; Marks,
T. J.; Friend, R. H. Angew. Chem., Int. Ed. 2000, 39, 4547-4551.
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28, 723-729 (b) Imae, I.; Nawa, K.; Ohsedo, Y.; Noma, N.; Shirota, Y.
Macromolecules 1997, 30, 380-386. (c) Melucci, M.; Barbarella, G.;
Zambianchi, M.; Benzi, M.; Biscarini, F.; Cavallini, M.; Bongini, A.;
Fabbroni, S.; Mazzeo, M.; Anni, M.; Gigli, G. Macromolecules 2004, 37,
5692-5702. (d) Kagan, J.; Liu, H. Synth. Met. 1996, 82, 75-81. (e) Meeker,
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Macromolecules 1998, 31, 2943-2946.
1586
Org. Lett., Vol. 8, No. 8, 2006

yield. This intermediate was then deprotonated to give bis-
enolates followed by methylation with dimethyl sulfate in a
one-pot procedure to furnish monomer 8.11
for optical absorption of the polymer (422 nm) in CHCl3
was slightly blue-shifted when compared to that of the
monomer (430 nm). On the other hand, the λmax for the
emission of the polymer (551 nm) was red-shifted and the
band was essentially structureless when compared to that of
the monomer (510, 538 nm) (Supporting Information S2).
The UV-vis absorption and emission spectra of polymer 9
in CHCl3 solution and as thin film are depicted in Figure 1.
The monomer was polymerized by using the ROMP
method in accordance with Scheme 2. The polymerization
was performed with commercially available bis(tricyclo-
hexylphosphine)benzylidine ruthenium(IV) dichloride (first-
generation Grubbs’ catalyst)12 as initiator in dry CH2Cl2 under
an inert atmosphere. The ratio of monomer to initiator was
close to 100:1. The polymerization was allowed to run at
25 °C for 22 h and then terminated by quenching the reaction
mixture with excess ethyl vinyl ether. The homopolymer
were conveniently obtained in pure form by filtration of the
precipitate formed from the reaction mixture followed by
washing with ether for the removal of catalyst and unreacted
monomer. Our attempts to directly polymerize monomer 7
have been unsuccessful so far, which was in agreement with
other endo-monomers of norbornene derivatives.12a,13 On the
contrary, the polymerization of monomer 8 went smoothly
under the same conditions within 22 h, providing oligoth-
iophene-phenylene co-oligomer functionalized polynor-
bornene in 83% yield.
1
The H NMR spectrum of the polymer shows broadened
peaks compared to those of the monomer and the signal of
vinylic protons appeared at 5.59 ppm, which is shifted
upfield. The molecular weight of the polymer was determined
by gel permeation chromatography (GPC), using THF as the
eluent and polystyrene as the standard. The number average
molecular weight (Mn) was determined to be 65 217, and
the polydispersity was narrow (PDI ) 1.08). The thermal
properties were evaluated by means of thermogravimetric
analysis (TGA) under a nitrogen atmosphere. The TGA plot
(Supporting Information S1) of the polymer measured at a
heating rate of 10 °C min-1 revealed that this polymer
exhibited good thermal stability up to 411 °C.
The photophysical properties of both the monomer and
polymer were investigated in solutions and also as a solid
thin film for the polymer, prepared by spin-coating from a
CHCl3 solution. Owing to the comparatively large size and
higher rigidity of the laterally attached co-oligomer relative
to the more fexible polymeric backbone, the thiophene-
phenylene co-oligomer moieties on the side chains are
expected to have slightly weaker interactions with one
another when compared to the individual monomers. This
is supported by the observation that the λmax value observed
Figure 1. (a) UV-vis absorption spectra of polymer 9 in CHCl3
solution and as a thin film. (b) Normalized photoluminescence
spectra of polymer 9 in CHCl3 solution and as a thin film.
The λmax of the polymer in solution was around 422 nm,
while the thin film was bathochromically shifted to 435 nm,
and the λmax for the emission of polymer thin film (558 nm)
also undergoes a red-shifting in comparison with that in
solution (551 nm). The red-shifting effect was presumably
due to increased π-π stacking in the solid state. Accordingly,
the optical band gap values of the polymer in solution and
as a film were found to be 2.48 and 2.10 eV. Cyclic
voltammetric measurement of polymer 9 (Supporting Infor-
mation S4) showed two reversible one-electron oxidations
at +0.96 and +1.13 V (versus SCE).
This polymer has been incorporated into a simple organic
photovoltaic cell by spin-coating a CHCl3 solution of the
polymer 9 on an indium-tin oxide (ITO) covered glass
substrate and depositing an aluminum film (50 nm) on top
of the polymer materials. Figure 2 shows the current-voltage
characteristics of the Al/polymer/ITO device in the dark and
under illumination at an intensity of 50 mW/cm2. The device
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Am. Chem. Soc. 2000, 122, 8717-8727. (b) Capitosti, G. J.; Guerrero, C.
D.; Binkley, D. E.; Rajesh, C. S., Jr.; Modarelli, D. A. J. Org. Chem. 2003,
68, 247-261. (c) Chan, C.; Tse, A. K. S.; Chan, K. J. Org. Chem. 1994,
59, 6084-6089. (d) Vickery, E. H.; Pahler, L. F.; Eisenbraun, E. J. J. Org.
Chem. 1979, 44, 4444-4446. (e) McIntosh, A. R.; Siemiarczuk, A.; Bolton,
J. R.; Stillman, M. J.; Ho, T. F.; Weedon, A. C. J. Am. Chem. Soc. 1983,
105, 7215-7223. (f) Schmidt, J. A.; Siemiarczuk, A.; Weedon, A. C.;
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14119.
(12) (a) Kanaoka, S.; Grubbs, R. H. Macromolecules 1995, 28, 4707-
4713. (b) Schwab, P.; Grubbs, R. H.; Ziller, J. W. J. Am. Chem. Soc. 1996,
118, 100-110. (c) Weck, M.; Schwab, P.; Grubbs, R. H. Macromolecules
1996, 29, 1789-1793.
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1746.
Org. Lett., Vol. 8, No. 8, 2006
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Figure 2. Current-voltage (I-V) characteristics of a Al/polymer/
ITO device test structure when measured in the dark and under
white light illumination (50 mW/cm2).
Figure 3. Current-voltage (I-V) curves of a Al/polymer/ITO
structure under illumination with white light at an intensity of 50
mW/cm2 obtained by repeated measurements at 5-min intervals.
clearly shows a rectifying behavior in the dark and exhibits
significant photovoltaic effects under illumination. These
preliminary results showed that our photovoltaic cell has a
large open-circuit voltage (VOC ) 0.7 V) and moderate short-
circuit current (ISC ) 0.7 µA/cm2). Interestingly, we observed
that this photovoltaic cell showed excellent device stability
under ambient conditions as depicted in Figure 3. By
repeatedly measuring the device performance at 5-min
intervals, one finds that their I-V characteristics remain
essentially unchanged and this sort of device stability is in
sharp contrast to other solar cells fabricated from poly-
thiophenes where their device performance showed a much
faster decay upon repeated measurements.
taic cell fabricated from this polymer showed relatively large
open-circuit voltage, moderate short-circuit current, and
excellent device stability under ambient conditions. In light
of the good film-forming and promising photovoltaic proper-
ties of this polymer system, further study toward the
optimization of polymer structure by changing the spacers
between the polymer backbone and co-oligomer unit and
construction of corresponding solar cell devices is currently
underway.
Acknowledgment. We wish to thank the University of
Rochester and ACS (PRF Type G) for financial support.
In summary, a novel polynorbornene functionalized with
thiophene-phenylene co-oligomer in the side chain was
synthesized by the ROMP method. Overall, the polymer
showed good photophysical properties for the side-chain
appended co-oligomer, as well as excellent thermal stability
and film-forming properties due to the polynorbornene
backbone. More importantly, it is observed that a photovol-
Supporting Information Available: Experimental pro-
cedures, absorption and photoluminescence spectra of the
monomer 8, and cyclic voltammogram of polymer 9. This
material is available free of charge via the Internet at
http://pubs.acs.org.
OL060162L
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Org. Lett., Vol. 8, No. 8, 2006