Donor-acceptor conjugated polymers based on p - And o -benzodifuranone and thiophene derivatives: Electrochemical preparation and optical and electronic properties

Article abstract of DOI:10.1021/ma202387t

A series of π-conjugated polymers consisting of bis-EDOT or bis-EDTT units and p- or o-diphenylbenzodifuranones have been prepared electrochemically. The monomers and polymers were investigated using UV/vis absorption spectroscopy and cyclic voltammetry. The monomers were synthesized from 3,4-ethylenedioxythien-2-yl or 3,4-ethylenedithiathien-2-yl trimethylstannane and 3,7-bis(4-bromophenyl)benzo[1,2-b:4,5-b′]difuran-2,6-dione or 3,6-bis(4-bromophenyl)benzo[1,2-b:6,5-b′]difuran-2,7-dione using Stille coupling. The polymers exhibit broad absorption bands, and strong donor-acceptor characteristics with very small band gaps (in a range from 0.40 to 1.20 eV). Electrochemically grown polymer thin films exhibit reversible behavior under oxidative and reductive conditions. Under reduction, the polymer films show color changes from dark to almost transparent.

Full text of DOI:10.1021/ma202387t

Article
pubs.acs.org/Macromolecules
Donor−Acceptor Conjugated Polymers Based on p- and
o-Benzodifuranone and Thiophene Derivatives: Electrochemical
Preparation and Optical and Electronic Properties
Kai Zhang,† Bernd Tieke,*,† John C. Forgie,‡ Filipe Vilela,‡ and Peter J. Skabara‡
†Department of Chemistry, University of Cologne, Luxemburger Str. 116, D-50939 Cologne, Germany
‡WestCHEM, Department of Pure and Applied Chemistry, University of Strathclyde, Glasgow G1 1XL, United Kingdom
S
* Supporting Information
ABSTRACT: A series of π-conjugated polymers consisting of
bis-EDOT or bis-EDTT units and p- or o-diphenylbenzodifur-
anones have been prepared electrochemically. The monomers
and polymers were investigated using UV/vis absorption spectros-
copy and cyclic voltammetry. The monomers were synthesized
from 3,4-ethylenedioxythien-2-yl or 3,4-ethylenedithiathien-2-yl tri-
methylstannane and 3,7-bis(4-bromophenyl)benzo[1,2-b:4,5-b′]-
difuran-2,6-dione or 3,6-bis(4-bromophenyl)benzo[1,2-b:6,5-b′]-
difuran-2,7-dione using Stille coupling. The polymers exhibit
broad absorption bands, and strong donor−acceptor character-
istics with very small band gaps (in a range from 0.40 to 1.20 eV).
Electrochemically grown polymer thin films exhibit reversible
behavior under oxidative and reductive conditions. Under reduction, the polymer films show color changes from dark to almost
transparent.
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
region of the spectrum, combined with a high color depth,
renders benzodifuranones interesting as building blocks for
novel π-conjugated polymers suitable for electronic applica-
tions. In a first report it has been shown that low-band-gap
polymers can be prepared from dibromobenzodifuranone
derivatives via Stille and Suzuki coupling reactions.10 In another
study, benzodipyrrolidone-based conjugated polymers follow-
ing a similar synthetic route have been reported very recently.11
As an alternative way to synthesize such polymers we have
chosen the electrochemical polymerization of benzodifuranones
functionalized by EDOT or EDTT units at both ends of the
molecules because their low oxidation potential enables
electrochemical coupling12 and because thiophene-based
compounds are known for their role in organic semiconductor
materials.13 In the present work, we describe the synthesis of π-
conjugated polymers with benzodifuranone chromophores in
the main chain via electrochemical coupling. Synthesis, optical,
electrochemical, and electronic properties of the polymers will
be described. It is demonstrated that the polymers exhibit
interesting redox behavior and can be cathodically reduced with
high reversibility.
INTRODUCTION
■
The incorporation of chromophores in π-conjugated polymer
chains, which are highly absorbing and emitting in the visible
and near-infrared region, has been widely used for the design of
new polymers for electronic applications. Potentially useful
chromophores can be found among the various organic colorants,
especially in the field of so-called “high-performance pigments”
developed in the past three decades.1 Among these pigments
are 2,5-diketopyrrolo[3,4-c]pyrrole (DPP) derivatives, which
were commercialized in the 1980s.2 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-emitting3 and photovoltaic properties.4 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 5.5%.5
Another interesting chromophore with a related structure,
benzodifuranone (Scheme 1), has attracted our interest very
recently.
The benzodifuranone chromophore belongs to a class of
currently used dyes and chromophores, which have been deve-
loped in the past 30 years.6 Because of their deep color and
high brightness of shape, they were commercialized as disperse
dyes for textiles, especially for polyesters. Depending on the
substitution pattern, benzodifuranones exhibit red to blue colors7
and exhibit strong solvatochromism in organic solvents.8
Benzodifuranones are prepared upon condensation of 1,4-
or 1,2-dihydroxybenzene with derivatives of mandelic acid.9
Received: October 26, 2011
Revised: December 16, 2011
Published: December 23, 2011
© 2011 American Chemical Society
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a
Scheme 1. Synthetic Route to Monomers A1, A2, B1, and B2
a
Reagents and conditions: (i)1,2,4-trichlorobenzene, 200 °C, 12 h; (ii) nitrobenzene, 200 °C, 2 h; (iii) tetrakis(triphenylphosphine)palladium(0),
DMF, microwave, 1 h.
was stirred at 200 °C for another hour. After cooling to room tem-
EXPERIMENTAL SECTION
■
perature, 50 mL of methanol was added. A precipitate formed, which
Materials. 3,4-Ethylenedioxythien-2-yl trimethylstannane and 3,
was filtered off, washed with methanol, and dried in air. Compound 3
4-ethylenedithiathien-2-yl trimethylstannane were synthesized
was obtained as a dark green solid (1.75 g, 65%), which was only
according to literature procedures14 (see also Supporting
sparingly soluble in common organic solvents at room temperature.
Information). All other chemicals and solvents were purchased
However, the product showed high solubility at temperatures above
from commercial sources and used without further purification.
Air- and/or water-sensitive reactions were conducted under nitrogen
using dry solvents. Microwave-assisted syntheses were carried out using
a Biotage Initiator Sixty EXP microwave system.
100 °C (mp above 300 °C). Anal. Calculated for C22H10Br2O4: C, 53.05%;
H, 2.02%. Found: C, 52.80%; H, 2.08%. UV/vis (dichloromethane):
λmax at 483 nm.
3,7-Bis(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)benzo-
Physical Measurements. 1H NMR spectra were recorded on a
[1,2-b:4,5-b′]difuran-2,6-dione (A1). In a vial, 200 mg (0.40 mmol)
Bruker DPX instrument at 400 and 100 MHz; chemical shifts are given
in parts per million. Elemental analyses were obtained on a Perkin-
Elmer 2400 elemental analyzer. Absorption spectra were measured on
a Unicam UV 300 spectrophotometer.
of 3,7-bis(4-bromophenyl)benzo[1,2-b:4,5-b′]difuran-2,6-dione (3),
305 mg (1.00 mmol) of 3,4-ethylenedioxythien-2-yl trimethylstannane,
and 14 mg (0.012 mmol) of tetrakis(triphenylphosphine)palladium(0)
were dissolved in 5 mL of dry DMF and stirred for 5 min. The mixture
was degassed and heated in the microwave synthesizer at 160 °C for
1 h. After cooling, the mixture was diluted with 50 mL of DCM and
washed with 50 mL of brine and 50 mL of water. The organic layer
was separated, dried over magnesium sulfate, and evaporated. The dark
product was recrystallized from DCM/methanol. Yield: 196 mg
(79%). 1H NMR (400 MHz, CDCl3): δ = 7.87 (d, aromatic, 8H), 6.96
(s, EDOT aromatic H, 2H), 6.41 (s, aromatic, 1H), 4.33 (m, EDOT-
CH2, 8H). UV/vis (dichloromethane): 298, 391, 590 nm. ε(590) = 63
580 L mol−1 cm−1.
Electrochemistry. Dichloromethane (DCM) (HPLC grade, Acros),
acetonitrile (HPLC grade, Aldrich) and tetra-n-butylammonium hexa-
fluorophosphate (TBAPF6, electrochemical grade, Fluka) were used as
received. Electrochemical measurements were performed on a CH
Instruments 660A electrochemical workstation using anhydrous dich-
loromethane or acetonitrile as the solvent, silver wire as the pseudo
reference electrode, and platinum wire and glassy carbon as the counter
and working electrodes, respectively. Electrochemical data were
referenced to the ferrocene/ferrocenium redox couple using metal-
locene as an internal standard. All solutions were degassed (Ar) and
contained monomer substrates in concentration of ca. 1 × 10−3 M,
together with TBAPF6 (0.1 M) as the supporting electrolyte. Spec-
troelectrochemical experiments were conducted on ITO glass. The gel
electrolyte was prepared according to a literature procedure.15 The
electrolyte contains 70 wt % of acetonitrile, 20 wt % of propylene
carbonate, 7 wt % of PMMA (Mw: 50 000 Da), and 3 wt % of tetra-
butylammonium hexafluorophosphate. HOMO and LUMO levels
were calculated according to the literature.16
3,7-Bis(4-(2,3-dihydrothieno[3,4-b][1,4]dithien-5-yl)phenyl)-
benzo[1,2-b:4,5-b′]difuran-2,6-dione (A2). The same procedure as
described for compound A1 was used except that 3,4-ethylenediox-
ythien-2-yl trimethylstannane was replaced by 3,4-ethylenedithiathien-
1
2-yl trimethylstannane. A dark solid (200 mg, 89%) was obtained. H
NMR (400 MHz, CDCl3): δ = 7.85 (d, aromatic, 8H), 6.69 (s, EDTT
aromatic H, 2H), 7.09 (s, aromatic, 1H), 3.24 (m, EDTT-CH2, 8H).
UV/vis (dichloromethane): 306, 502 nm. ε(502) = 79 220 L mol−1 cm−1.
3,6-Bis(4-bromophenyl)benzo[1,2-b:6,5-b′]difuran-2,7-dione
(4). Using a Dean−Stark apparatus, 1,2-dihydroxybenzene (1.20 g,
10.82 mmol) and 4-bromomandelic acid (5.0 g 21.64 mmol) were
dissolved in 1,2,4-trichlorobenzene (30 mL) and stirred at 200 °C for
5 h. After cooling to 60 °C, nitrobenzene (5.0 mL) was added, and the
mixture was stirred for another hour at 200 °C. After cooling to room
Synthesis. 3,7-Bis(4-bromophenyl)benzo[1,2-b:4,5-b′]difuran-2,6-
dione (3). Using a Dean−Stark apparatus, hydroquinone (0.60 g, 5.41
mmol) and 4-bromomandelic acid (2.5 g 10.82 mmol) were dissolved
in 1,2,4-trichlorobenzene (25 mL) and stirred at 200 °C for 12 h. After
cooling to 60 °C, nitrobenzene (2.2 mL) was added, and the mixture
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Figure 1. UV/vis absorption spectra of monomers A1, A2, B1, and B2 in dichloromethane solution.
a
Table 1. Optical and Electrochemical Data for Monomers
HOMO−LUMO gap/opt
onset of oxidation
(V)
HOMO
(eV)
onset of reduction
(V)
LUMO
(eV)
HOMO−LUMO gap
UV (nm)
(eV)
(eV)
A1
A2
B1
B2
298, 391, 590
306, 502
1.81
1.88
1.80
1.86
+0.83
+0.82
+0.45
+0.85
−5.63
−5.62
−5.25
−5.65
−0.50
−0.47
−0.80
−0.41
−4.30
−4.33
−4.00
−4.39
1.33
1.29
1.25
1.26
302, 588
307, 407, 558
a
Absorption spectra were taken in dichloromethane solutions. All redox potentials are referenced to the ferrocene/ferrocenium redox couple.
HOMO−LUMO gap according to the equation16 −ELUMO = Eonset(red) + 4.8 eV and −EHOMO = Eonset(ox) + 4.8 eV, where Eonset(ox) and Eonset(red) are
the onset potentials for the oxidation and reduction processes vs ferrocene.
temperature, 50 mL of methanol was added. The solid formed was
condensation of 1 equiv of p-bromomandelic acid with 0.5 equiv
of hydroquinone or 1,2-dihydroxybenzene, respectively, leading
to a double cyclization to benzodihydrofuranones 1 and 2. A
Dean−Stark apparatus was used for the removal of water as a
byproduct, favoring the double cyclization into 1 and 2. 1 and 2
were not isolated but directly oxidized with nitrobenzene. This
resulted in the conjugated system of benzodifuranone in 65%
yield for para- (3) and 61% for the ortho-isomer (4). The low
solubility of benzodifuranone 3 in common solvents at room
temperature could be overcome upon heating to temperatures
above 100 °C. This allowed us to use the monomer for further
synthetic procedures.
filtered off, washed with methanol, and dried in air. Compound 4 was
1
obtained as a dark red solid (3.28 g, 61%). H NMR (400 MHz,
CDCl3): δ = 7.85 (d, aromatic, 4H), 7.47 (d, aromatic, 4H), 7.16 (d,
aromatic, 2H). (mp over 300 °C). Anal. Calculated for C22H10Br2O4:
C, 53.05%; H, 2.02%. Found: C, 52.10%; H, 2.09%. UV/vis
(dichloromethane): λmax at 493 nm.
3,6-Bis(4-(2,3-dihydrothieno[3,4-b][1,4]dioxin-5-yl)phenyl)benzo-
[1,2-b:6,5-b′]difuran-2,7-dione (B1). The same procedure as de-
scribed for compound A1 was used. A dark solid (93 mg, 82%) was
1
obtained. H NMR (400 MHz, CDCl3): δ = 7.88 (d, aromatic, 8H),
6.98 (s, EDOT aromatic H, 2H), 7.29 (s, aromatic, 1H), 4.34 (m,
EDOT-CH2, 8H). UV/vis (dichloromethane): 302, 588 nm. ε(588) =
23 080 L mol−1 cm−1.
3,6-Bis(4-(2,3-dihydrothieno[3,4-b][1,4]dithien-5-yl)phenyl)-
benzo[1,2-b:6,5-b′]difuran-2,7-dione (B2). The same procedure as
described for compound A1 was followed except that compound 3 was
The 3,4-ethylenedioxythiophene (EDOT) and 3,4-ethyl-
enedithiathiophene (EDTT) substituted monomers A1, A2,
B1, and B2 were synthesized upon a microwave-assisted Stille
coupling of 3 and 4 with 3,4-ethylenedioxythien-2-yl trimethyl-
stannane (5) and 3,4-ethylenedithiathien-2-yl trimethylstan-
nane (6) using Pd(PPh3)4 as the catalyst and DMF as the solvent.
The coupling gave high yields of 79, 89, 82, and 86% for A1,
A2, B1, and B2, respectively. The four monomers are dark
bluish solids being slightly up to moderately soluble in common
solvents such as chloroform, dichloromethane, DMF, THF, and
toluene, for example.
1
replaced by 4. A dark solid (120 mg, 86%) was obtained. H NMR
(400 MHz, CDCl3): δ = 7.84 (d, aromatic, 8H), 7.44 (s, EDTT
aromatic H, 2H), 7.09 (s, aromatic, 1H), 3.27 (m, EDTT-CH2, 8H).
UV/vis (dichloromethane): 307, 407, 558 nm. ε(558) = 115 560 L
mol−1 cm−1.
RESULTS AND DISCUSSION
■
Synthesis. The starting compounds 1−4 and key compounds
A1, A2, B1, and B2 containing para- and ortho-benzodifuranone
units were synthesized as shown in Scheme 1. In general, the
synthesis of the starting compounds 3 and 4 required the
The 1H NMR spectra of A1, A2, B1, and B2 displayed all the
expected resonances with no discernible peaks corresponding
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Figure 2. A suggested reduction mechanism for p-benzodifuranone monomers.
Figure 3. Growth of PA1 (a), PA2 (b), PB1 (c), and PB2 (d) by cyclic voltammetry in dichloromethane using a carbon working electrode, Ag wire
pseudoreference electrode. Supporting electrolyte: 0.1 M TBAPF6. Scan rate: 100 mV s−1; T = 20 °C.
to impurities (see Supporting Information). Significantly, the
singlet signal at about 7.0 ppm can be ascribed to the thiophene
unit in the EDOT and EDTT substituents. The two multiplet
signals with a chemical shift of about 4.34 ppm are typical for
the ethylene bridge of the EDOT unit. The two multiplet
signals at about 3.37 ppm are typical for the ethylene bridge of
the EDTT unit and are similar to those reported in the
literature.14b Unfortunately, the solubility in solvents such as
chloroform-d or dimethyl-d sulfoxide was not high enough to
measure 13C NMR spectra for further structural character-
ization.
UV/vis Absorption of Monomers. In the UV/vis
absorption spectra of A1, A2, B1, and B2, a large bathochromic
shift is introduced upon the addition of EDOT and EDTT to
the conjugated system. Compared with their starting
compounds 3 and 4, the shifts are 154 nm for A1, 64 nm for
A2, 95 nm for B1, and 65 nm for B2. The spectra are displayed
in Figure 1. In dichloromethane A1 exhibits an absorption
maximum at 590 nm, A2 at 502 nm, B1 at 588 nm, and B2 at
558 nm (Table 1). The bathochromic shifts result from a com-
bination of two phenomena: (i) an extension in conjugation
through the thiophene units and (ii) a push−pull donor−acceptor
interaction between the electron-rich thiophenes and the
carbonyls of the furanone units. Interestingly, the EDTT-sub-
stituted monomers A2 and B2 are not shifted bathochromically
as largely as the EDOT-substituted monomers A1 and B1. The
reason could be a larger electron-withdrawing effect caused by
sulfur in the EDTT unit compared to oxygen in the EDOT
units. Alternatively, it is well-known that conjugated compounds
incorporating EDOT and EDTT units provide planar and twisted
geometries, respectively. Therefore, it can be assumed that there is
a lower degree of conjugation in the EDTT-containing com-
pounds. From the absorption edge of the monomers (about
690 nm) similar optical HOMO−LUMO gaps between 1.80
and 1.90 eV (Table 1) can be determined.
Electrochemistry. Cyclic Voltammograms of Mono-
mers. The electrochemistry of compounds A1, A2, B1, and B2
was studied using cyclic voltammetry in dichloromethane solution.
The electrochemical details are given in the Experimental
Section, the cyclic voltammograms of the monomers are shown
in the Supporting Information, and the electrochemical data are
compiled in Table 1. For the para-substituted compound A1,
the oxidative cycle exhibits an irreversible peak at +0.91 V. The
reductive cycle shows two reversible waves at −0.61/−0.55
and −0.94/−0.88 V. A2 shows an irreversible oxidation peak
at +0.84 V and two reversible cationic waves at −0.61/−0.55
and −0.93/−0.87 V. The ortho-substituted analogue B1 shows
an irreversible oxidation peak at +0.65 V and two reversible
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Figure 4. Cyclic voltammograms of PA1 (a), PA2 (b), PB1 (c), and PB2 (d) as thin films deposited on a glassy carbon electrode. Solvent: 0.1 M
TBAPF6/acetonitrile. Potential calculated versus ferrocene. Scan rate: 100 mV s−1; T = 20 °C.
reductive waves at −0.93/−0.86 and −1.22/−1.18 V. B2 un-
dergoes an irreversible oxidation at +1.00 V, and the reduction
shows two reversible waves at −0.55/−0.49 and −0.87/−0.82 V.
Within this monomer series, a high stability of the anion
radical and dianion showing reversible reductive waves was
observed. The driving force for facile and stable reduction is
the aromatization of the core benzene ring in the four
benzodifuranone monomers. This represents a strong acceptor
character of the benzodifuranone unit. A suggested reduction
mechanism for the p-benzodifuranone is shown in Figure 2. It
also should be noted that the four monomers show narrow
band gaps in the range from 1.25 to 1.33 eV.
confirming the stability of adhered films and that charge transport
through the film is not diffusion limited.
Cyclic Voltammograms of Polymers. The cyclovoltammet-
ric response of the four polymers was studied using films of the
polymers on a glassy carbon working electrode in monomer-
free acetonitrile vs Ag/AgCl. The CV diagrams of PA1 (a),
PA2 (b), PB1 (c), and PB2 (d) are shown in Figure 4, and the
electrochemical data are listed in Table 2. The four polymers
a
Table 2. Optical and Electrochemical Data for Polymers
onset of
onset of
HOMO−
LUMO
gap (eV)
UV
(nm)
oxidation HOMO reduction LUMO
(V)
(eV)
(V)
(eV)
Electropolymerization. The four monomers were electro-
polymerized at the anode using the same cell as for the cyclic
voltammetry experiments. Repetitive cycling over the oxidation
range of the materials produced the desired polymers, which
were deposited as thin films on the working electrode. The
polymer growth plots for the series are displayed in Figure 3.
Whereas A1, A2, and B1 polymerized readily, B2 required
more cycles (120 cycles for B2, 60 cycles for A1, A2, and B1)
of continuous scanning between −0.20 and +1.10 V before a
satisfactory film was obtained. It should be noted that the
electropolymerization of thiophenes, especially for EDTTs, at
relatively low oxidation potentials can lead to oligomeric
products rather than to long chain polymers. This could be
the reason for the slow growth rate of B2. In comparison
with B2, the EDTT-substituted A2 exhibited much better
growth characteristics. In this case the angled structure of B2
could be sterically unfavorable for polymer growth in comparison
with the rodlike structure of A2. For all compounds, a plot of
scan rate vs current gives a linear fit (see Supporting Information),
PA1 589
+0.41
+0.50
−0.01
+0.87
−5.21
−5.30
−4.79
−5.67
0.03
−0.41
−0.37
−0.41
−4.77
−4.39
−4.43
−4.39
0.44
0.91
0.36
1.28
PA2 348, 417
PB1 515, 587
PB2 530
a
Absorption spectra were taken from thin films.
show a quasi-reversible oxidative and two reductive waves,
although the reduction process for PB1 was represented by a
broad single peak. The higher stability under reductive
conditions, compared with the less stable oxidation processes,
was similar to the behavior of the monomers. Good reversibility
for reduction processes was observed for polymers PA1 and
PB2. The difference between the onsets of the oxidation and
reduction potentials of conjugated polymers normally repre-
sents the electrochemical band gap of the semiconductor. From
cyclic voltammetry, small electrochemical band gaps are inferred
for all four polymers (0.44, 0.91, 0.36, and 1.28 eV for PA1,
PA2, PB1, and PB2, respectively). The polymers containing
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Figure 5. UV/vis absorption spectra of monomers PA1, PA2, PB1, and PB2 as thin films.
EDOT-substituted benzodifuranone show smaller electroche-
mical band gaps and lower oxidation potentials than the EDTT
analogues, but the reduction onsets were very similar throu-
ghout the series. This could be caused by more planar structures
of the polymer main chains due to the oxygen−sulfur interaction
between adjacent EDOT units.17 EDTT does not provide this
advantage for planarity, and in fact there is normally a signi-
ficant twist between the thiophene units.17,18 In a comparison,
the electrochemical HOMO−LUMO gap of PB2 (1.28 eV) is
similar to the value found for monomer B2 (1.26 eV). Again,
the reason could be that the polymer chain of PA2 is not fully
conjugated due to the expected twists between EDTT units in
the polymer chain. The narrow HOMO−LUMO gaps arise
from localized donor and acceptor sites within the polymer
chain through the bithiophene units and the benzodifuranones,
respectively. This is similar to our previous work, where even in
conjugated polythiophenes containing strongly redox-active com-
ponents the donor and acceptor units can exhibit independent
electroactivity.19
Spectroelectrochemistry. The absorption spectra of the
four polymers were taken from thin films on ITO-coated glass
substrates. The spectra are displayed in Figure 5, and the ab-
sorption data are listed in Table 2. The UV/vis absorption
spectra of the polymers show very broad bands with no clear
edges so that optical band gaps could not be calculated with a
satisfactory degree of confidence; the four polymers showed
very broad shoulders ranging from about 680 to 750 nm.
However, the optical band gap of the polymer, determined
from the onset of the longest wavelength absorption band, is
estimated to be wider than the electrochemically determined
value. The discrepancy in HOMO−LUMO gap values determined
by the two different methods, cyclic voltammetry and absorption
spectroscopy, is explained by the following. Absorption spectros-
copy reveals the band gap as the energy corresponding to the
π−π* transition, and this is normally represented by the longest
wavelength absorption band. The π−π* transition arises from
the conjugated chain of the polymer, and the length and
planarity of this chain dictate the position of this band. Cyclic
voltammetry, on the other hand, provides information on the
HOMO and LUMO energy levels. In simple structures, parti-
cularly in homopolymers, the optical band gap and the HOMO−
LUMO gap are the same, since the redox processes involve
electrons being extracted from or injected into the conjugated
polymer chain. In more complex structures, in which the redox
sites are discrete parts of the polymer, the sites of oxidation and
reduction are localized and the conjugated segment may not
extend over both these components. Therefore, the association
between the donor−acceptor sites is no longer representative of
the effective conjugation length of the polymer. In such cases,
the HOMO−LUMO and band gap values are not the same and
must be considered as separate properties of the polymer. In
our case, the bithiophene units within the polymers and the
benzodifuranone heterocycles act as localized donor and acceptor
redox entities, and this explains the differences observed in the
data relating to the “HOMO−LUMO” gap.
Because of the high stability under reduction, it was of great
interest to study the spectroelectrochemical properties for this
series of polymers. For this study, the polymer films were grown
on ITO-coated glass substrates, and the electronic absorption
spectra were taken with a gel electrolyte. 3-D absorption spec-
troelectrochemical plots for PA1, PA2, PB1, and PB2 are
shown in Figure 6.
The absorption of PA1 did not change much until a potential
of −1.0 V was reached. From −1.0 to −1.3 V a large drop in
absorbance was observed, which could point to the reduction
from mono- to dianion involving a break in the conjugated
system. Starting at −1.0 V, the film becomes more transparent.
For PA2, it can be seen that the change in the absorption
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Figure 6. 3-D absorption spectroelectrochemical plots for PA1, PA2, PB1, and PB2 as thin films on ITO in gel electrolyte. Ag wire pseudoreference
electrode. Potential calculated versus ferrocene. Scan rate: 100 mV s−1; T = 20 °C.
spectra takes place during the first 10 measurements until a
potential of −0.7 V is reached. This is just past the first
reduction peak in the CV. By taking it to −1.9 V, there is only
little more change in the absorption which corresponds to the
fact that the second reduction peak was smaller compared to
the first one. The color change was only small; the film turned
from a dark to a lighter purple/brown color. The 3D plot of
PB1 shows that there is a drop in the absorption at −0.5 V as
the polymer goes from a purple color to a more transparent
film. The spectroelectrochemistry of PB2 shows that at the first
reduction peak down to −0.8 V a decrease in intensity of the
π−π* band occurs, and a new band ranging from 770 nm into
the near-IR forms, which might originate from the formation of
a radical anion in the polymer chain. This peak diminishes at
−1.0 V, and with the formation of a dianion (second reductive
wave) there is a continuous drop in the π−π* transition toward
−1.9 V. The color of the film changed from dark purple to a
light orange, transparent color. For this series of polymers, 2-D
electronic absorption spectra are shown in the Supporting
Information.
color changes from dark to almost transparent. Because of their
reversibility in oxidation and reduction processes, PA1, PA2,
PB1, and PB2 might be useful for electronic applications.
Further studies to optimize film formation and conductivity of
these materials are underway.
ASSOCIATED CONTENT
* Supporting Information
■
S
1
UV/vis absorption spectra and H NMR spectra of some key
compounds, cyclic voltammograms of monomers A1, A2, B1,
and B2, scan rate studies of polymers PA1, PA2, PB1, and
PB2, and 2-D electronic absorption plots for polymers PA1,
PA2, PB1, and PB2. 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.
■
ACKNOWLEDGMENTS
Financial support by BASF Schweiz AG, Basle, Switzerland, is
gratefully acknowledged. Drs. M. Duggeli and P. Hayoz from
BASF are kindly thanked for helpful discussions.
■
CONCLUSIONS
̈
■
In summary, we have presented the synthetic route and the key
properties of a series of electropolymerizable monomers con-
taining p- and o-diphenylbenzodifuranones. We also have syn-
thesized the first polymers based on EDOT or EDTT and p- or
o-benzodifuranone units electrochemically. The polymers
exhibit broad absorption bands, and very small HOMO−
LUMO gaps (about 0.40 eV for PA1 and PB1). Interesting
electrochromic properties were found under oxidative and
reductive conditions. Under reduction, the polymers showed
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