Polymers containing rigid benzodithiophene repeating unit with extended electron derealization

Article abstract of DOI:10.1021/ol901786z

Polymers containing a fused benzodithlophene core with phenylethynyl substituents were prepared. The parent poly[4,8-bis(4-pentylphenylethynyl) benzo[1,2-b:4,5-b']dithiophene] was prepared by a Stille coupling. Copolymers with the new core were also obtai

Full text of DOI:10.1021/ol901786z

ORGANIC
LETTERS
2009
Vol. 11, No. 19
4422-4425
Polymers Containing Rigid
Benzodithiophene Repeating Unit with
Extended Electron Delocalization
Nadia Hundt, Kumaranand Palaniappan, John Servello, Daniel K. Dei,
Mihaela C. Stefan,* and Michael C. Biewer*
Department of Chemistry, UniVersity of Texas at Dallas, MS BE26, Richardson, Texas 75083
biewerm@utdallas.edu; mci071000@utdallas.edu
Received August 3, 2009
ABSTRACT
Polymers containing a fused benzodithiophene core with phenylethynyl substituents were prepared. The parent poly[4,8-bis(4-pentylphenyl-
ethynyl)benzo[1,2-b:4,5-b′]dithiophene] was prepared by a Stille coupling. Copolymers with the new core were also obtained by Stille coupling
with dibrominated fluorene and carbazole monomers. The obtained polymers had a lower band gap as compared to the related benzodithiophene
cores due to the extended electron conjugation. The polymers were also highly fluorescent as observed by high quantum yields.
Conjugated 3-alkyl-substituted polythiophenes have been
extensively studied as a processable polymer with good
electronic and photonic properties.1 The regioregular poly(3-
hexylthiophene) (P3HT) polymer, for example, has good
solubility due to the hexyl substituents and also displays good
conductivities due to the strong π overlap both in intra- and
interchain delocalization as a result of the ordered lamellar
packing that results from the minimization of steric interactions
with the regioregular placement of the alkyl substituents.2 These
polymers have been studied as active components for sensors,3
TFTs,4 OLEDs,5 and solar cells.6 One limitation for these
polymers, however, is the inability to control the molecular
orbital energy levels for P3HT. With only one remaining
aromatic substitution point available with P3HT, any substitution
at this position will cause steric interactions with the adjacent
alkyl substituent, thus limiting all of the benefits of the
regioregular alkyl substituents for extended conjugation and
interdigitation of the hexyl substituents.
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J. W.; Khanam, N.; Horvath, J.; Alshareef, H.; Quevedo-Lopez, M.; Biewer,
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10.1021/ol901786z CCC: $40.75
Published on Web 08/28/2009
 2009 American Chemical Society

In order to maintain the electronic and photonic benefits
of the regioregular P3HT, but still allow control of the
molecular orbital levels for the polymer, the fused ben-
zodithiophene monomer core has been studied.7 The fused
central ring allows the incorporation of substituents on the
central benzene core, while also maintaining planarity of the
two thiophene units. In addition, the symmetric nature of
the benzodithiophene core eliminates the need to control the
regioregularity during the polymerization process. Previously,
groups have attached electron-donating alkyl and alkoxy
substituents on the central benzene ring.7 In order to create
polymers with a better molecular orbital overlap for molec-
ular electronic devices, however, we desired to create
polymers with greater electron delocalization and thus a
smaller band gap. The substitution of two phenylethynyl
groups on the central fused benzene ring will allow extended
electron delocalization.
gated cores through a Stille coupling with the corresponding
dibromo monomers. Fluorene-based polymers have been
shown to emit blue light with high quantum yields.9 In
addition, similar results have been obtained with poly(2,7-
carbazole).10 Due to the interest in fluorene and carbazole
cores, the 2,7-dibromo-9,9-dioctylfluorene and the 2,7-
dibromo-N-octylcarbazole monomers were polymerized with
4 to create polymers P2 and P3, respectively (Scheme 2).
Scheme 2. Synthesis of Benzodithiophene-Containing Polymers
Scheme 1. Synthesis of Benzodithiophene Cores
The number average molecular weight for both P2 (26300
g/mol) and P3 (40200 g/mol) were appreciably larger than
the parent P1, presumably due to the higher solubilities
during polymerization.
The UV-vis spectra for the monomer 2 and polymers P1,
P2, and P3 are shown in Figure 1. The phenylethynyl
As shown in Scheme 1, the previously prepared diketone
18 was reacted with 4-pentylphenylacetylide, followed by
aromatization with tin chloride to create compound 2.
Electropolymerization was attempted directly with compound
2, but no polymer was obtained. The dibromo-substituted
compound 3 was obtained by forming the dianion of 2 with
2 equiv of n-butyllithium, followed by reaction with carbon
tetrabromide. Grignard metathesis (GRIM)1 polymerization
of compound 3 was attempted but gave low yields. The
distannylated derivative of compound 2 was obtained by
forming the dianion with 2 equiv of n-butyllithium, followed
by reaction with trimethyltin chloride to obtain compound
4. Stille coupling of compounds 3 and 4 proved successful
as poly[4,8-bis(4-pentylphenylethynyl)benzo[1,2-b:4,5-b′]di-
thiophene] (P1) with an average number molecular weight
of 3300 g/mol was obtained. Attempts are underway to attach
longer alkyl groups than the pentyl group, which would
presumably make the polymer more soluble during polym-
erization and thus allow the synthesis of high molecular
weight polymer.
With the distannylated benzodithiophene monomer (4) in
hand, copolymers could also be prepared with other conju-
Figure 1. UV-vis absorbance of compounds in chloroform
solution.
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substituted monomer 2 displays a λmax of 398 nm which is
red-shifted by 62 nm compared to the previously reported
benzo[1,2-b:4,5-b′]dithiophene monomer.11
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Org. Lett., Vol. 11, No. 19, 2009
4423

The parent polymer P1 is red-shifted by 84 nm compared
to P2 or P3 (517 nm vs 433 nm). In addition, all three
polymers display vibronic structure for the absorbance bands.
Most surprising is the strong red shift for the P1 ben-
zodithiophene polymer in solution relative to regioregular
P3HT (517 nm vs 450 nm) even though P1 has a relatively
low molecular weight of 3300 g/mol. This strong red shift,
and vibronic structure in solution, indicates a long-range
ordering of the P1 polymer chains in solution that is not
present for regioregular P3HT in solution. Films prepared
by drop casting P1 from chloroform solution also demon-
strate vibronic structure with a red shift of 29 nm when the
films were annealed at 120 °C for 5 min, thus indicating
some long-range ordering for the polymer film. The annealed
P1 films have similar features to the films of regioregular
P3HT. The films prepared from P2 or P3 show no shift in
absorbance compared to solution either before or after
annealing.
Table 2. Fluorescence Data for Polymersa
Ex (nm)
Em (nm)
quantum yield (%)
P1
P2
P3
400
440
430
505
493
488
48
91
22
a Fluorescence was measured in chloroform solution.
was 21%.13 The P2 sample with the two fused thiophene rings
in a benzodithiophene structure has a greater than 4 times larger
quantum yield. The P3 copolymer containing the carbazole ring
has a lower quantum yield of 22%.
In conclusion, a benzodithiophene core with an extended
conjugated phenylethynyl appendage has been synthesized.
A Stille coupling of the respective dibromo and ditinylated
derivatives generated homopolymer. A Stille coupling was
also performed with the new core with both 2,7-dibromo-
9,9-dioctylfluorene and 2,7-dibromo-N-octylcarbazole to cre-
ate new conjugated copolymers. P1 had a red-shifted
The band gap for the polymers was determined by
electrochemical cyclic voltammetry. From the value of
oxidation potential and reduction potential of the polymers,
the HOMO and LUMO energy levels were determined as
shown in table 1.12 The band gap for P1 was determined to
Table 1. Molecular Weight, Optical, and Electonic Energy
Levels of Compounds
a
b
Mw
λmax
HOMOc LUMOd
band
gap (eV)
(g/mol) PDI (nm) Eox/V Ered/V
(eV)
(eV)
2
530 N.A. 398 1.33 –1.23
–6.04
–5.42
–5.39
–5.43
–3.48
–3.38
–3.54
–3.44
n/a
P1 3300 1.9 517 0.708 –1.33
P2 26300 1.6 433 0.675 –1.17
P3 40200 2.6 433 0.718 –1.25
2.04
1.85
2.03
a Determined by SEC (THF eluent). b Absorptions in chloroform
solution. c Estimated from peak average oxidation wave. d Estimated from
peak average reduction wave.
be 2.04 eV. This value is smaller than the previously reported
value of 2.49 eV for the alkoxy-substituted benzodithiophene
polymer.7b More importantly, P1 which has an extended
electron delocalization displays a lower LUMO energy level
than the previously reported alkoxy-substituted ben-
zodithiophene analogue (-3.38 vs -2.67 eV).7b Presumably
the band gap would lower further with a larger molecular
weight of the polymer.
The new polymers prepared in this study are fluorescent.
P1 displayed emission at 505 nm with a shoulder at 545 nm
upon excitation at 400 nm. Both P2 and P3 also displayed
a strong emission upon excitation. The quantum yields for
the polymers are shown in Table 2. The parent P1 polymer
has a quantum yield of 48%. The fluorescence quantum yield
of the copolymer P2, containing alternating fluorene and
benzodithiophene cores, was 91%. This value is larger than
those observed for polyfluorene (PFO) samples of 43%.13
In addition, a copolymer containing alternating fluorene and
two thiophene units, poly-9,9-dioctylfluorene-co-bithiophene
(F8T2), has been previously prepared.14 This polymer is
similar to P2, which contains alternating fluorene with two fused
thiophene rings. The reported quantum yield for F8T2 samples
Figure 2. UV-vis absorbance of polymer films. P1 and P1
annealed at 120 °C for 5 min.
absorption in solution (Figure 2). The electrochemical
measurements also indicate a lower band gap for the polymer
with extended conjugation. All new polymers had a strong
fluorescence with high quantum yield. The quantum yield
for the P2 with alternating benzodithiophene and dioctylfluo-
rene cores was 91%, significantly larger than observed for
either parent polymer or for the analogous F8T2 polymer
that contains dioctylfluorene and two thiophene units in
repetition. Presently the electrical mobilities of the new
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4424
Org. Lett., Vol. 11, No. 19, 2009

polymers are being tested, and homologues of P1 are being
prepared with a longer alkyl tail appendage to increase the
solubility and presumably the average molecular weight of
the formed polymer.
Supporting Information Available: Experimental pro-
1
cedures, spectroscopic data, and H and 13NMR spectra for
all new compounds. CV, fluorescence, and quantum yield
measurements for all polymers. This material is available
free of charge via the Internet at http://pubs.acs.org.
Acknowledgment. Start-up funds for M.C.S. from UTD
are appreciated. J.S. (UTD) gratefully acknowledges the UT
LSAMP program for financial support (NSF No. 0832951).
We thank Prof. Sibert (UTD) for access to CV.
OL901786Z
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