Low-bandgap benzodifuranone-based polymers

Article abstract of DOI:10.1021/ma2008628

π-Conjugated monomers and polymers containing benzodifuranone units in the main chain were prepared. Polymers were obtained using palladium-catalyzed Suzuki and Stille polycondensation reactions of dibromo-substituted benzodifuranone derivatives with either dihexylfluorene diboronester or bis(trimethylstannyl)thiophene. The optical and electronic properties were investigated using UV/vis absorption spectroscopy and cyclic voltammetry. It was found that the polymers show very broad absorption bands with a high extinction coefficient up to 32-500 L mol^-1 cm^-1. The polymers also show reversible redox behavior, giving small HOMO-LUMO gaps up to 1.30 eV with strong donor-acceptor character.

Full text of DOI:10.1021/ma2008628

COMMUNICATION TO THE EDITOR
pubs.acs.org/Macromolecules
Low-Bandgap Benzodifuranone-Based Polymers
Kai Zhang and Bernd Tieke*
Department of Chemistry, University of Cologne, Luxemburger Strasse 116, D-50939 Cologne, Germany
S Supporting Information
b
ABSTRACT: π-Conjugated monomers and polymers contain-
ing benzodifuranone units in the main chain were prepared.
Polymers were obtained using palladium-catalyzed Suzuki and
Stille polycondensation reactions of dibromo-substituted ben-
zodifuranone derivatives with either dihexylfluorene diboron-
ester or bis(trimethylstannyl)thiophene. The optical and elec-
tronic properties were investigated using UV/vis absorption
spectroscopy and cyclic voltammetry. It was found that the
polymers show very broad absorption bands with a high
extinction coefficient up to 32 500 L molꢀ1 cmꢀ1. The poly-
mers also show reversible redox behavior, giving small HOMOꢀLUMO gaps up to 1.30 eV with strong donorꢀacceptor character.
’ INTRODUCTION
benzodifuranone monomers (Scheme 1). New π-conjugated
polymers containing bifunctionalized benzodifuranone units in
the main chain were synthesized upon palladium-catalyzed
polycondensation reactions, such as Suzuki and Stille cross-
coupling (Scheme 2). The key compounds were investigated
using spectroscopic methods (NMR, UV/vis) and cyclic
voltammetry.
For the design of new polymers for electronic applications, the
incorporation of chromophores highly absorbing and emitting in
the visible and near-infrared region in π-conjugated polymers
chains has been widely used. Potentially useful chromophores
can be found among the various organic colorants, especially in
the field of so-called “high-performance pigments” developed
in the last three decades.1 Among these pigments are 2,5-
diketopyrrolo[3,4-c]pyrrole (DPP) derivatives, which were com-
mercialized in the 1980s.2,3 In recent years, a growing number of
polymer chemists and physicists became interested in DPPs
since it was shown that DPP-containing polymers exhibit light-
emitting2 and photovoltaic properties.3 Very recently, Janssen
et al. demonstrated the utility of thiophene-2-yl-DPP-containing
conjugated polymers for application in photovoltaic devices
exhibiting a power conversion efficiency up to 4.0%.4 Another
interesting chromophore with a related structure, benzodifura-
none (Scheme 1) has attracted our interest very recently. Benzo-
difuranones have been developed as a class of relatively new dyes
and chromophores in the last 30 years.5 Because of their deep
color, good brightness of shape and light fashion, they were
commercialized as disperse dyes for textiles, especially for
polyesters.6 Depending on the substitution pattern, benzodifur-
anones exhibit red to blue colors,7 and are strongly solvatochromic
in organic solvents.8 Benzodifuranones are prepared upon con-
densation of hydroquinone or 1,2-dihydroxybenzene with deriva-
tives of mandelic acid.9 The final deep color originates from the
quinoid structure of the central core unit, which is obtained upon
oxidation of the benzene unit with chloranil. The broad absorption
in the visible combined with the high color depth render benzo-
difuranones interesting as building blocks for novel π-conjugated
polymers with potential electronic applications.
’ RESULTS AND DISCUSSION
Synthesis. The synthetic route to new benzodifuranone-
based monomers is described in Scheme 1. The starting
compounds 1aꢀ1d were prepared according to literature
procedures.10 The key compounds 3aꢀ3e were synthesized
as shown in Scheme 1. In general, the synthesis of the
symmetrical monomers 3aꢀ3d required the condensation of
0.5 eq. hydroquinone and 1 eq. of a mandelic acid derivative
(1aꢀ1d), respectively, leading to a double cyclization of
benzodihydrofuranone 2aꢀ2d, followed by oxidation to the
conjugated benzodifuranones 3aꢀ3d in a yield of 65ꢀ75%. A
DeanꢀStark apparatus was used for separation of water as
byproduct favoring the double cyclization of dihydrofuranone
in 2aꢀ2d. Monomer 3a is a soluble yellow solid. In compar-
ison, the bifunctionalized monomers 3bꢀ3d were less soluble
in common solvents, and were not used for further palladium-
catalyzed polymerization reactions. Only UV/vis absorption
spectra were recorded.
The unsymmetrical benzodifuranone monomers 3e and 3f,
however, showed a very high solubility in common solvents such
as chloroform, dichloromethane, DMF, THF and toluene, for
Received: April 15, 2011
In this Communication, we report the synthesis and char-
acterization of a series of symmetrical and unsymmetrical
Revised:
Published: June 07, 2011
May 27, 2011
r
2011 American Chemical Society
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COMMUNICATION TO THE EDITOR
example. This could be caused by the high polarity of the
molecules, which can resonance easily into a polarized form
and interact with the adjacent solvent molecules (see a sug-
gested mechanism in Supporting Information). The 1H NMR
spectra of 3e and 3f displayed all the expected resonances with
no discernible peaks corresponding to impurities (see Support-
ing Information). Significantly, the two adjacent singulet signals
with a chemical shift at about 4.00 ppm are typical for the CH3
groups of the benzene ring on one side of the benzodifuranone
core. The other signals are similar to those of the unsubstituted
monomer 3a.
3f was polymerized with comonomers 511 and 612 using
palladium-catalyzed cross-coupling methods. The yields were
70% for P1, and 76% for P2, respectively. The synthetic route is
described in Scheme 2. P1 and P2 are dark solids and soluble in
common organic solvents. The 1H NMR spectra (see Supporting
Information) show broad signals for aromatic protons from 6 to
8 ppm. Typical peaks at about 4 ppm originate from the CH3
protons of the methoxy groups. For P1, the NMR spectrum
exhibits typical proton signals of the alkyl groups attached to the
fluorene units at 0.7ꢀ1.6 ppm.
Scheme 1. Synthesis of Monomers 3aꢀ3f
UV/Vis Spectroscopy. The monomers 3aꢀ3f show a strong
absorption and weak emission in the visible with large Stokes
shifts in a range from 80 to 140 nm. The optical data are listed in
Table 1 and the UV/vis absorption and emission spectra are
displayed in the Supporting Information. The nonsubstituted
benzodifuranone derivative 3a shows an absorption maximum
at 469 nm, and the brominated monomer 3b shows a bath-
ochromically shifted absorption maximum at 483 nm, which
can be caused by the inductive effect of the two bromine atoms
of the phenyl rings attached to the benzodifuranone core. In
comparison, the absorption of monomer 3c with 3-Br-5-meth-
oxyphenyl substituents is divided into two bands with maxima
at 477 and 391 nm. The maximum at 477 nm can be ascribed to
the main absorption of the benzodifuranone core, and the
maximum at 391 nm should belong to the two phenyl
rings attached to the benzodifuranone core. In this case, an
inductive effect caused by bromine and a mesomeric effect
by the methoxy group should be considered. The same effects
are even stronger for the monomer 3d with 3-Br-4,5-dimethox-
yphenyl substituents, where the absorption maxima of the
benzodifuranone core and the two phenyl rings are more
Table 1. Optical Data of Monomers 3aꢀ3fa
3a
3b
3c
3d
3e
3f
absorption/nm 469 483 391, 477 382, 521 525 441, 490
emission/nm 600 586 610 606 635 645
a Absorption spectra were taken in dichloromethane solutions.
Scheme 2. Synthesis of P1 and P2
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COMMUNICATION TO THE EDITOR
largely divided and placed at 521 and 382 nm. For the
unsymmetrical monomer 3e, only the mesomeric effect is
shown. It shows an absorption maximum at 525 nm. In com-
parison, the brominated unsymmetrical monomer 3f shows an
absorption maximum at 441 nm with a broad shoulder at
500 nm, where an additional inductive effect caused by bromine
occurs.
(Figure 2). The measurement conditions are described in the
Supporting Information. The electrochemical data of the
polymers P1 and P2 are listed in Table 2. Both polymers
show reversible oxidative and reductive waves. For P1, the
oxidative cycle exhibits a reversible peak at þ0.90 V, which is
reverted at þ0.71 V, and a quasi-reversible peak at þ0.57 V.
The reductive cycle shows two reversible waves at ꢀ0.80/
ꢀ0.99 and ꢀ1.23/ꢀ1.52 V. P2 shows a quasi-reversible peak
at þ0.89 V and a reversible peak at þ0.50 V. Significantly, the
reductive cycle shows two reversible cathodic waves at ꢀ0.78/
ꢀ0.92 and ꢀ1.19/ꢀ1.33 V. For both polymers, reversible
reductive waves were observed indicating a high stability of the
anion radical. This could possibly be explained with a stabiliz-
ing negative charge of the oxygen atoms in the carbonyl
groups of both lacton groups in the benzodifuranone units.
A suggested reduction mechanism is shown in the Support-
ing Information. For both polymers P1 and P2, the reversi-
bility of the oxidative and reductive cycles with very small
HOMOꢀLUMO gaps of 1.13 eV for P1 and 1.13 eV for P2
suggests a strong donorꢀacceptor character of the polymers,
respectively.
The soluble monomers 3a, 3e, and 3f show high extinction
coefficients with 35 000, 42 000, and 32 000 L molꢀ1 cmꢀ1
,
respectively. The fluorescence quantum yields of the monomers
are very low (from 0.3 to 2%). From the absorption edge of the
monomers in a range from 540 to 600 nm, optical HOMOꢀ
LUMO gaps between 2.00 and 2.20 eV can be calculated.
Compared to the corresponding monomer 3f, the absorp-
tion spectra of P1 and P2 are bathochromically shifted, giving
large Stokes shifts of 179 nm for P1, and 162 nm for P2
(Figure 1). The polymers exhibit high extinction coefficients
with 28 000 L molꢀ1 cmꢀ1 for P1, and 32 500 L molꢀ1 cmꢀ1 for
P2. From the absorption edge at about 660 nm optical HOMO
ꢀLUMO gaps of 1.70 eV for P1, and 1.90 eV for P2 (Table 2)
can be calculated.
Cyclic Voltammetry. The electrochemical response of com-
pounds 3e, P1, and P2 was studied using cyclic voltammetry
Figure 1. UV/vis absorption and fluorescence spectra of P1 and P2 in
dichloromethane solution. Photographs of polymer solutions are
also shown.
Figure 2. Cyclic voltammograms of P1 and P2 as thin films deposited
on ITO. Solvent: 0.1 M TBAPF6/acetonitrile. Potential calculated
versus ferrocene. Scan rate: 100 mV sꢀ1; T = 20 °C.
Table 2. Optical and Electrochemical Data of Polymers P1 and P2a
UV/nm
HOMOꢀLUMO gap/opt/eV
Onset of oxidation/V
HOMO/eV
Onset of reduction/V
LUMO/eV
HOMOꢀLUMO gap/eV
P1
P2
323, 532
404, 557
1.70
1.90
þ0.26
þ0.29
ꢀ5.06
ꢀ5.09
ꢀ0.87
ꢀ0.81
ꢀ3.93
ꢀ3.99
1.13
1.10
a Absorption spectra were taken in dichloromethane solutions. All redox potentials are referenced to the ferrocene/ferrocenium redox couple.
HOMOꢀLUMO gap according to the following equation: ꢀELUMO = Eonset(red) þ 4.8 eV and ꢀEHOMO = Eonset(ox) þ 4.8 eV, where Eonset(ox) and
E
onset(red) are the onset potentials for the oxidation and reduction processes vs ferrocene.
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COMMUNICATION TO THE EDITOR
’ CONCLUSIONS
(11) Pham, C. V.; Macomber, R. S.; , H. B. M., Jr; Zimmer, H. J. Org.
Chem. 1984, 49, 5250.
In summary, we have presented the synthetic route and the
key properties of a series of new conjugated monomers contain-
ing the benzodifuranone unit in the molecule. We also have
synthesized the first π-conjugated polymers using palladium-
catalyzed cross-coupling methods. The polymers exhibit a broad
absorption, low HOMO- and high LUMO-levels and small band
gaps (1.13 eV for P1 and 1.10 eV for P2). They also show
interesting electrochemical properties with reversible oxidation
and reduction behavior, which suggests a strong donorꢀacceptor
character. According to their broad absorption bands, small band
gaps and the reversibility of oxidation and reduction processes,
P1 and P2 might be useful for electronic applications. Further
studies on the conductivity of these materials are under way.
(12) Jo, J.; Chi, C.; H€oger, S.; Wegner, G.; Yoon, D., Y. Che. Eur. J.
2004, 10, 2681.
’ ASSOCIATED CONTENT
S
Supporting Information. Synthetic procedures of some
b
starting compounds, the monomers 3aꢀ3f, and polymers
P1ꢀP2, UV/vis absorption spectra of 3aꢀ3f, 1H NMR spectra
of 3a, 3e, 3f, P1, and P2, a suggested mechanism of polarization
of unsymmetrical benzodifuranone, and a suggested reduction
mechanism of benzodifuranone. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: tieke@uni-koeln.de.
’ ACKNOWLEDGMENT
Financial support by BASF Schweiz AG, Basle, Switzerland, is
gratefully acknowledged. Drs. M. D€uggeli, R. Lenz and P. Hayoz
from BASF Schweiz AG are kindly thanked for helpful
discussions.
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