Donor–Acceptor Conjugated Polymers Based on p- and o

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-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.

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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-DPPcontaining 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 developed 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 1,4or 1,2-dihydroxybenzene with derivatives of mandelic acid.9 © 2011 American Chemical Society

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 applications. 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 following 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 πconjugated 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. Received: October 26, 2011 Revised: December 16, 2011 Published: December 23, 2011 743

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Scheme 1. Synthetic Route to Monomers A1, A2, B1, and B2a

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.

a

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was stirred at 200 °C for another hour. After cooling to room temperature, 50 mL of methanol was added. A precipitate formed, which was filtered off, washed with methanol, and dried in air. Compound 3 was obtained as a dark green solid (1.75 g, 65%), which was only sparingly soluble in common organic solvents at room temperature. However, the product showed high solubility at temperatures above 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[1,2-b:4,5-b′]difuran-2,6-dione (A1). In a vial, 200 mg (0.40 mmol) 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, EDOTCH2, 8H). UV/vis (dichloromethane): 298, 391, 590 nm. ε(590) = 63 580 L mol−1 cm−1. 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-ethylenedioxythien-2-yl trimethylstannane was replaced by 3,4-ethylenedithiathien2-yl trimethylstannane. A dark solid (200 mg, 89%) was obtained. 1H 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

EXPERIMENTAL SECTION

Materials. 3,4-Ethylenedioxythien-2-yl trimethylstannane and 3, 4-ethylenedithiathien-2-yl trimethylstannane were synthesized according to literature procedures 14 (see also Supporting Information). All other chemicals and solvents were purchased 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. Physical Measurements. 1H NMR spectra were recorded on a Bruker DPX instrument at 400 and 100 MHz; chemical shifts are given in parts per million. Elemental analyses were obtained on a PerkinElmer 2400 elemental analyzer. Absorption spectra were measured on a Unicam UV 300 spectrophotometer. Electrochemistry. Dichloromethane (DCM) (HPLC grade, Acros), acetonitrile (HPLC grade, Aldrich) and tetra-n-butylammonium hexafluorophosphate (TBAPF6, electrochemical grade, Fluka) were used as received. Electrochemical measurements were performed on a CH Instruments 660A electrochemical workstation using anhydrous dichloromethane 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 metallocene 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. Spectroelectrochemical 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 tetrabutylammonium hexafluorophosphate. HOMO and LUMO levels were calculated according to the literature.16 Synthesis. 3,7-Bis(4-bromophenyl)benzo[1,2-b:4,5-b′]difuran-2,6dione (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 744

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Figure 1. UV/vis absorption spectra of monomers A1, A2, B1, and B2 in dichloromethane solution.

Table 1. Optical and Electrochemical Data for Monomersa UV (nm) A1 A2 B1 B2

298, 306, 302, 307,

391, 590 502 588 407, 558

HOMO−LUMO gap/opt (eV)

onset of oxidation (V)

HOMO (eV)

onset of reduction (V)

LUMO (eV)

HOMO−LUMO gap (eV)

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

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 filtered off, washed with methanol, and dried in air. Compound 4 was obtained as a dark red solid (3.28 g, 61%). 1H 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 described for compound A1 was used. A dark solid (93 mg, 82%) was obtained. 1H 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 replaced by 4. A dark solid (120 mg, 86%) was obtained. 1H 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.

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. The 3,4-ethylenedioxythiophene (EDOT) and 3,4-ethylenedithiathiophene (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 trimethylstannane (5) and 3,4-ethylenedithiathien-2-yl trimethylstannane (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. The 1H NMR spectra of A1, A2, B1, and B2 displayed all the expected resonances with no discernible peaks corresponding

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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 745

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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.

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 compounds. 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 Monomers. 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

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 characterization. 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 combination 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-sub746

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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 undergoes 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. Electropolymerization. The four monomers were electropolymerized 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),

confirming the stability of adhered films and that charge transport through the film is not diffusion limited. Cyclic Voltammograms of Polymers. The cyclovoltammetric response of the four polymers was studied using films of the polymers on a glassy carbon working electrode in monomerfree 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 Table 2. Optical and Electrochemical Data for Polymersa

PA1 PA2 PB1 PB2 a

UV (nm)

onset of oxidation (V)

HOMO (eV)

onset of reduction (V)

LUMO (eV)

HOMO− LUMO gap (eV)

589 348, 417 515, 587 530

+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

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 represents 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 747

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Figure 5. UV/vis absorption spectra of monomers PA1, PA2, PB1, and PB2 as thin films.

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, particularly 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 spectroelectrochemical 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

EDOT-substituted benzodifuranone show smaller electrochemical band gaps and lower oxidation potentials than the EDTT analogues, but the reduction onsets were very similar throughout 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 significant 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 components 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 absorption 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 spectroscopy reveals the band gap as the energy corresponding to the 748

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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.

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.

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.

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ASSOCIATED CONTENT

S Supporting Information *

UV/vis absorption spectra and 1H 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.

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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ACKNOWLEDGMENTS Financial support by BASF Schweiz AG, Basle, Switzerland, is gratefully acknowledged. Drs. M. Düggeli and P. Hayoz from BASF are kindly thanked for helpful discussions.

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CONCLUSIONS In summary, we have presented the synthetic route and the key properties of a series of electropolymerizable monomers containing p- and o-diphenylbenzodifuranones. We also have synthesized 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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REFERENCES

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dx.doi.org/10.1021/ma202387t | Macromolecules 2012, 45, 743−750