Synthesis and Characterization of Poly (2-alkylbenzimidazole-alt-9, 9

May 1, 2014 - We present the synthesis and characterization of poly(2-alkyl-benzimidazole-alt-9,9-dihexylfluorene). An improved microwave-assisted ...
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Synthesis and Characterization of Poly(2-alkylbenzimidazole-alt-9,9dihexylfluorene)s: A Dually Dopable Polymer System Jared D. Harris, Charlotte Mallet, Cathrin Mueller, Carmen Fischer, and Kenneth R. Carter* Department of Polymer Science and Engineering, Conte Center for Polymer Research, University of MassachusettsAmherst, 120 Governors Drive, Amherst, Massachusetts 01003, United States S Supporting Information *

ABSTRACT: We present the synthesis and characterization of poly(2-alkyl-benzimidazole-alt-9,9-dihexylfluorene). An improved microwave-assisted synthesis of the monomers 4,7dibromo-2-heptyl-1H-benzo[d]imidazole and 4,7-dibromo-2(heptan-3-yl)-1H-benzo[d]imidazole was developed. The monomers were polymerized with 9,9-dihexylfluorene-2,7diboronic acid bis(1,3-propanediol) ester using Suzuki crosscoupling to yield the corresponding poly(2-alkyl-benzimidazole-alt-9,9-dihexylfluorene)s. The polymers were characterized by both traditional optical and thermal properties in conjunction with experiments yielding strong evidence to suggest controllable chemical doping of benzimidazole moieties. Based on ultraviolet−visible spectroscopy (UV−vis) and cyclic voltammetry (CV) data, the band structure of these benzimidazole-containing polymers may be manipulated through treatment with acids or bases to achieve polycationic or polyanionic products. Protonic doping is shown to significantly impact the electronic properties of the parent polymer; the optical band gaps were shown to shift +0.07 and −0.33 eV for the acid- and base-doped polymers, respectively. Herein, we quantify the changes in band structure between the neutral and charged polymers.

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INTRODUCTION There are many opportunities for conjugated polymers, including electrochromic devices, field effect transistors, lightemitting diodes, and photovoltaics, among others. These macromolecules find utility in such applications due to their semiconducting properties. Conjugated polymers are often either p- or n-channel semiconductors depending on the relative location of their frontier molecular orbitals. The pchannel semiconductors often have lower ionization potentials (IPs) and tend to have relatively electron-rich backbones. This is commonly achieved by incorporating electron donating groups on the periphery of the conjugated core to encourage hole diffusion. The n-channel semiconductors tend to have higher electron affinities (EA) with somewhat electron-deficient conjugated backbones. Synthetically, this is readily achieved through the use of peripheral electron-withdrawing groups to further enable electron movement.1 Although many ambipolar materials have been developed,2 it has been difficult to achieve a singular parent polymer that can be controllably and reversibly doped into a predominantly p- or n-channel species. The ability to provide a single polymer system that can be systematically doped into an electron or hole transporting material will lead to great advances in the semiconducting polymer field. We propose a derivative of poly(benzimidazole) (PBI) as a candidate for this type of dually dopable polymer system. The fully protonated benzimidazole (BI) monomer has accessible pKa’s of ∼6.40 and 12.00; thus, the cationic and © 2014 American Chemical Society

anionic forms may be achieved through acidic or basic treatment, respectively.3 By incorporating BI into a conjugated polymer, it is relatively easy to achieve poly(ionomer)s from simple acid/base chemistry (Figure 1). Although BI monomers

Figure 1. Generalized doping scheme for dually dopable conjugated PBIs.

have been incorporated into macromolecular structures for mechanical and high temperature applications as well as proton exchange membranes, relatively few reports have been published regarding their polymerization from the 4,7 positions to yield electronically interesting macromolecules.4 Yamamoto and co-workers were the early developers of some of these conjugated systems, exploring poly(benzimidazole-4,7-diyl)s with various solubilizing side groups5−8 and later venturing into copolymers incorporating aryleneethynylene and thiophene units.6,9−11 Some of these early publications hint at the Received: January 30, 2014 Revised: April 11, 2014 Published: May 1, 2014 2915

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Scheme 1. Synthesis of BI1 and BI2 Monomers and the Corresponding Polymers PBI1F and PBI2F

Chromium(III) acetylacetonate was used as an additive to enhance the nonprotonated carbon signals from BI1. GPC was performed at 40 °C and 1.0 mL/min using an Agilent 1260 series system equipped with a refractive index detector, PL Gel 5 μm guard column, two 5 μm analytical Mixed-C columns, and a 5 μm analytical Mixed-D column (Agilent) with tetrahydrofuran (THF) as the eluent. Glass transition temperatures were determined using differential scanning calorimetry (DSC) on a Thermal Analysis Q-2000 DSC in aluminum T-zero pans using a heat−cool−heat cycle at 10 °C/min. UV−vis absorption in dimethylacetamide (DMAc), THF, and solid state were done on a Cary 50 UV−vis absorption spectrometer with 1 cm path length quartz cuvettes or quartz plates. Photoluminescence from solutions in DMAc and THF was measured with a Cary Eclipse. Photoluminescence from thin films were measured with a PerkinElmer LS-50B. Cyclic voltammetry was carried out with a Bioanalytical Systems Inc. EC epsilon potentiostat in an electrolyte solution of 0.1 M TBAPF6 in dry acetonitrile using platinum wires for working and counter electrodes (Bioanalytical Systems Inc.) and a Ag/Ag+ reference electrode (Ag in 0.1 M AgCl solution, Bioanalytical Systems Inc.). Synthesis. 4,7-Dibromo-2-heptyl-1H-benzo[d]imidazole (BI1) was prepared via a modified route from Nurioglu et al.13 4,7-Dibromo2,1,3-benzothiadiazole (1)18 and 1,2-diamine-3,6-dibromobenzene (2) were synthesized in accordance with established literature in high yield.19 Ring closure with an aldehyde was achieved by combining 1.5 g of 2 (5.6 mmol) with 18 mL of acetonitrile and 0.9 mL of octanal (5.7 mmol) in a 50 mL pear-shaped flask. Once the solid dissolved, cerium(IV) ammonium nitrate (0.3 g, 0.55 mmol) was added along with 2.4 mL of 30% H2O2 (30 mmol). The reaction was heated at reflux in a CEM Discover microwave reactor for 8 h at 85 °C and 75 W. Following the reaction, solvent was removed under reduced pressure and the resulting solid dried. The solid was then redissolved in CHCl3 and purified on a SiO2 column from 5:1 CH2Cl2:hexanes. The solid was recrystallized in ethyl acetate to yield white crystals (65%). 1H NMR (300 MHz, DMSO-d6, δ) 12.83 (s, 1H), 7.28 (s, 2H), 2.82 (t, 2H), 1.76 (p, 2H), 1.16−1.42 (m, 8H), 0.85 (t, 3H). 13C NMR (100 MHz, CDCl3, δ) 156.9, 141.9, 134.3, 125.7, 110.7, 102.6, 31.5, 29.9, 29.6, 29.1, 29.0, 22.5, 14.1. 4,7-Dibromo-2-(heptan-3-yl)-1H-benzo[d]imidazole (BI2) was prepared in a similar fashion as BI1 with the exception of using 2ethylhexanal in place of octanal. Additionally, microwave parameters were slightly different, running the reaction at 88 °C and 77 W for 6 h (71%). 1H NMR (400 MHz, CDCl3, δ) 8.95 (s, 1H), 7.31 (d, 2H), 7.21 (d, 2H), 2.98 (p, 1H), 1.82 (m, 4H), 1.32 (m, 2H), 1.20 (m, 2H), 0.90 (t, 3H), 0.86 (t, 3H). 13C NMR (100 MHz, CDCl3, δ) 159.4, 142.3, 133.8, 126.1, 125.7, 111.9, 102.0, 42.6, 34.2, 29.8, 27.8, 22.8, 14.0, 12.2. Poly(2-heptylbenzimidazole-alt-9,9-dihexylf luorene) (PBI1F). BI1 (0.25 g, 0.67 mmol) and 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester (0.34 g, 0.67 mmol) were dissolved in ∼25 mL of toluene with 1−2 drops of Aliquat 336 and 4 mL of 2 M Na2CO3(aq) (8 mmol) in a 100 mL three-neck flask fitted with a

potential of doping but do not probe the fundamental transitions that occur upon treatment of poly(benzimidazole4,7-diyl)s with strong acids or bases. More recently, Toppare and co-workers have investigated H-bonding interactions between BI and ethylenedioxythiophene (EDOT).12−14 Herein we explore possible BI coupling partners for the production of dually dopable polymers. As mentioned above, these systems are fundamentally unique from most previously developed semiconducting polymers owing to their ability to be acid or base-doped materials through postpolymerization chemical doping treatments. For our initial system, we chose to employ 9,9-dihexylfluorene due to its distinctive photoluminescent properties. We note that a paper by Yang et al.15 on theoretical modeling of similar systems has appeared, although the PBIs in our work were not the focus of their publication. Early efforts of Leclerc and co-workers on strictly base-dopable poly(fluorene)s describe some similar phenomena as the systems we desire to explore;16,17 however, our system represents a deviation from the Leclerc work owing to our manipulation of the BI moiety while the fluorene remains charge-neutral. Herein we describe the synthesis and characterization of poly(2-alkylbenzimidazole-alt-9,9-dihexylfluorene). We report an improved synthesis of 2-alkylbenzimidazoles and employ both n-heptyl (PBI1F) and 1-ethylpentyl (PBI2F) side chains on the BI monomer. The polymers are prepared from a palladium catalyzed Suzuki cross-coupling reaction to yield high molecular weight products. The acid- and basedoped polyionic forms of the products are achieved through acid/base chemistry and probed with ultraviolet−visible spectroscopy (UV−vis), photoluminescence (PL), and cyclic voltammetry (CV).

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EXPERIMENTAL SECTION

Materials and Methods. All chemicals, solvents, and reagents were used as received without further purification. The following were purchased from Sigma-Aldrich Co.: 2,1,3-benzothiadiazole, bromobenzene, 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester, and ferrocene. Alfa Aesar supplied bromine, 48% w/w aqueous hydrobromic acid solution, octanal, 2-ethylhexanal, cerium(IV) ammonium nitrate, and trifluoroacetic acid. Pd(PPh3)4 was obtained from Strem Chemicals. Anhydrous N,N-dimethylacetamide and 21 wt % sodium ethoxide were bought from Acros Organics. Remaining solvents, hydrogen peroxide, sodium hydroxide, and silica was purchased from Fischer Scientific. Reactions were carried out in conventional glassware except where noted. Instrumentation. NMR spectra were recorded on a Bruker DPX300 (300 MHz) or a Bruker Avance 400 (400 MHz). Chemical shifts were determined relative to residual peaks in the deuterated solvent. 2916

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Table 1. Basic Characterization Data for PBI1F and PBI2Fa

condenser. This mixture was deoxygenated by sparging with N2 for 90 min. After 90 min, the remaining transformations were performed under N2 and Pd(PPh3)4 was added (0.04 g, 0.035 mmol). The mixture was heated at reflux for 48 h before adding a small portion of bromobenzene as a polymer end-capping agent. Bromobenzene was permitted to react for 1.5 h before adding phenylboronic acid as a second end-capping agent. After 1.5 h, the reaction mixture was precipitated into ∼400 mL of ice cold methanol and filtered. Solids were transferred to a Soxhlet thimble. The product was then extracted under sequential reflux with methanol, hexanes, and CHCl3. 1H NMR (300 MHz, CDCl3, δ) 9.15 (s, 1H), 8.30 (m, 1H), 8.10−7.90 (m, 3H), 7.68 (m, 3H), 7.51 (m, 1H), 3.03 (m, 2H), 2.13 (bm, 4H), 1.96 (t, 3H), 1.55- 1.05 (m, 26H), 0.92 (t, 3H), 0.80 (t, 6H). Poly(2-ethylpentylbenzimidazole-alt-9,9-dihexylf luorene) (PBI2F). PBI2F was synthesized in the same manner as PBI1F by substituting BI1 with BI2. 1H NMR (300 MHz, CDCl3, δ) 9.05 (s, 1H), 8.21 (d, 2H), 7.93 (d, 2H), 7.67 (m, 2H), 7.49 (d, 2H), 2.98 (m, 1H), 2.11 (bm, 4H), 1.86 (m, 4H), 1.46−1.06 (m, 20H), 1.02 (t, 3H), 0.93 (t, 3H), 0.80 (t, 6H).

Mn/Mw (kg/mol)

xn/xw

Đ

hexanes chloroform

28.8/49.4 30.5/64.7

52/90 56/118

hexanes chloroform

10.3/16.2 17.0/23.0

18/29 30/41

Tg (°C)

yield (%)

1.7 2.1

100

74 25

1.6 1.4

95

69 3

PBI1F

PBI2F

a

Molar masses and degrees of polymerization fractions separated in hexanes and chloroform for PBI1F and PBI2F as determined by GPC against PS standards. Glass transitions of hexanes fractions were determined via DSC (H/C/H cycle at 10 °C/min in between 30 and 200 °C).

molecular weights in PBI2F due to a higher concentration of chain ends increasing free volume. UV−vis/Photoluminescence. The basic optical properties of PBI1F and PBI2F were characterized through UV−vis and PL experiments. The polymers absorb in the near-UV with maximum absorbance wavelength, λmax, of 385 nm for PBI1F and PBI2F in THF (Figure 2a). Both compounds fluoresce blue in THF solution with maximum emission bands centered at 422 nm (Figure 2b). The fluorescence profile of both compounds greatly resembles the well-structured emission of poly(fluorene) (PF) homopolymers. There are distinctive peaks at 422, 446, and 484 nm, presumably from the 0−0, 0−1, and 0−2 intrachain singlet transitions, respectively.20 Based on these data, it can be inferred that the chromophore of the conjugated polymer backbone is unaffected by the differing alkyl side chains in solution. In the solid state, there is a minimal difference between the absorbance and subsequently the optical band gaps of PBI1F and PBI2F (2.88 and 2.85 eV, respectively) which may be attributed to their difference in molecular weight. Solution Doping. Dilute solutions of PBI1F and PBI2F were prepared in anhydrous DMAc. Solvent selection was crucial for the success of this experiment; the neutral polymers were insoluble in solvents more polar than DMAc (i.e., dimethyl sulfoxide) while the doped polymers required polar solvents to avoid precipitation. Solubility upon doping became a problem when attempting to dope PBI1F solutions in chlorobenzene, tetrahydrofuran, chloroform, and dichloromethane. Base doping was achieved through the addition of 0.13 M NaOEt (in ethanol) in 10 μL increments (not all increments shown) to a septum-capped cuvette containing PBI1F solution. All experiments were performed under N2. The samples were given 5 min for acid/base equilibration following each aliquot of dopant prior to measuring absorbance and photoluminescence (Figure 3a). The absorption spectra show an initial λmax of 394 nm, which shifts 51 nm bathochromically to 445 nm upon completion of the experiment. We posit that the origin of this shift is due to deprotonation of BI units to BI(−). This shift complements observations by Ranger et al.16 in basedoped PFs and Nurulla in base-doped poly(benzimidazole-altthiophene)s.11 Nurulla et al. attribute their shifts to strain relaxation leading to more planar backbones, which may play a role in the phenomena observed here (Figure 4a). When considering Ranger et al.’s work, where torsional backbone strain is not alleviated through doping, there is likely an electronic contribution to the shift. In terms of band energy, deprotonation of PBI1F results in a reduction in the optical

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RESULTS AND DISCUSSION Synthesis. BI monomer synthesis involved a three-step process (Scheme 1) beginning with bromination of commercially available 2,1,3-benzothiadiazole to achieve 4,7-dibromo2,1,3-benzothiodiazole (1) and subsequent reduction to 1,2diamine-3,6-dibromobenzene (2) by reported methods.18,19 Attempts to condense 2 with either octanal or 2-ethylhexanal by reported procedures were unsuccessful.6−11 Using those methods, we were unable to get any appreciable yields and only obtained unclosed imidic acid side products after repeated attempts. We pursued alternative methods to synthesize the monomers BI1 and BI2 using a procedure modified from Nurioglu et al.13 We reacted 2 with aldehyde in the presence of cerium ammonium nitrate and hydrogen peroxide in a microwave reactor and achieved modest yields (65 and 71% for BI1 and BI2, respectively). Previous studies have reported similar condensations using conventional heating, and we achieved similar yields in 1/3 of the reaction time using microwave heating. Straight and branched C7H15 chains were used to modify solubility with the expectation that a branched chain would lead to greater solubility. The monomer structures were confirmed by 1H NMR and 13C NMR (Figures S1−S4). Polymerization of the BI monomers with 9,9-dihexylfluorene-2,7-diboronic acid bis(1,3-propanediol) ester was easily achieved via Suzuki cross-coupling (Scheme 1). The product was end-capped with bromobenzene and phenylboronic acid to yield phenyl end groups. Crude polymer was purified by sequential Soxhlet extraction with methanol, hexanes, and chloroform. In both cases, insoluble solid material remained in the Soxhlet thimbles, implying insoluble higher molecular weight products. Hexanes and chloroform fractions were isolated and the molecular weight was determined by GPC (Table 1; THF eluent, PS standards). Polymer structures were validated with 1H NMR (Figures S5 and S6). The hexanes fraction contained a majority of the polymer yield, and it was this fraction that was used for subsequent measurements. Within the limit of 1 mg/mL, the solubilities of PBI1F and PBI2F are comparable in many common organic solvents (Table S1). Glass transition temperatures followed the expected trend, with PBI2F’s branched pendant chains lowering the Tg by 5 °C relative to the straight-chained PBI1F derivative (Table 1, Figures S7 and S8). The branched chains act as an internal plasticizer by increasing free volume, resulting in the depressed Tg. A portion of the Tg disparity may also be attributed to lower 2917

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Figure 2. (a) UV−vis spectra of PBI1F and PBI2F in THF (dashed) and thin films (solid) spun from CHCl3 at 3000 rpm. (b) PL spectra of the same samples excited at λmax.

Figure 3. (a) UV−vis and PL of base-doped PBI1F in DMAc. (b) UV−vis and PL of acid-doped PBI1F in DMAc. All PL spectra were collected with λex = λabs,max.

anionic BI has an IP comparable to thiophene, a wellestablished donating moiety.22 As discussed earlier, the neutral PL spectrum profile is consistent with previously reported PF derivatives. Upon base doping, the key 0−0 emission mode at 430 nm rapidly decreases in intensity with respect to dopant concentration until 40 μL of dopant has been added (2.1 mM NaOEt) where the emission profile shifts entirely and a new 0−0 mode appears at 484 nm (Figure 3a). As more dopant is added, this and the other modes (0−1 and 0−2) continue to develop at 508 and 551 nm, respectively. The final emission profile is very similar to that of PTF, which has reported maxima in chloroform of 469 nm (0−0) and 498 nm (0−1).21 The ∼10 nm discrepancy between the observed shifts may be attributed to solvatochromism, considering the spectra here were taken in a relatively polar solvent in comparison to Davis et al.21 Acid-doping experiments using 1 M trifluoroacetic acid (TFA) in anhydrous DMAc as the acidic dopant were carried out in a similar fashion to those described above for base doping. The results of this experiment are somewhat less dramatic than what was observed for the polyanionic system. Considering the evolution of positive charge within the conjugated chain, we expected BI to transition into a stronger acceptor as it becomes doped to BI(+). This should result in a lowering of the LUMO while maintaining a relatively constant HOMO. However, the optical band gap widens from 2.91 to 2.98 eV (Figure 3b). We suggest that this shift is caused by chain twisting due to benzimidazole N−H and fluorene C−H

Figure 4. Trimers indicating potential torsional backbone strain in PBIFs. (a) Strain is reduced in the BI(−) form compared to BI (b) and BI(+) (c) where strain is exacerbated by the additional proton.

band gap of 0.33 eV from neutral (2.91 eV) to fully doped (2.58 eV). Any decrease in Eg may be attributed to an increase in EA and/or decrease in IP. In this case, we suggest that BI(−) has a lower IP (shallower HOMO) than BI, possibly resulting in a charge transfer, donor−acceptor type of interaction with fluorene. Davis et al.21 have reported optical band gap of poly(fluorene-alt-thiophene) (PFT) as 2.58 eV; this, in conjunction with CV data presented below, suggests that the 2918

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Figure 5. (a) UV−vis of individual solutions of PBI1F(−) and PBI1F(+) (red and blue, respectively) and then following combination with equal volumes (PBI1F(+/−), black). (b) PL of from (a) where λex = λmax.

Figure 6. (a) UV−vis and PL of PBI1F(−) in DMAc over the course of 1 week. (b) UV−vis and PL of PBI1F(+) in DMAc over the course of 1 week.

photoluminescence of the acid-doped polycationic species are largely unaffected by aging (Figure 6b). However, a marked decrease in photoluminescence was observed. We believe that this observation stems from less than ideal solvation. Electrochemistry. Cyclic voltammetry (CV) was utilized to probe the electrochemical transitions of both products (PBI1F and PBI2F). Films were drop-casted from dichloromethane onto the glassy carbon work electrode, and the HOMO energy level, EHOMO, was estimated23 through external calibration with the ferrocene/ferrocenium redox couple (assumed to be at −4.8 eV relative to vacuum) using the following equation: EHOMO = [(Eox − E1/2fc) + 4.8](−q) (eV/J), where Eox is the analyte oxidation onset, E1/2fc is the measured average potential of the anodic and cathodic peaks for the ferrocene/ferrocenium redox couple, q is the elementary charge (1.6022 × 10−19 C), and eV/J represents the conversion constant (6.2415 × 1018 eV/J) from joules to electronvolts. EHOMO was determined to be −5.54 and −5.48 eV for PBI1F and PBI2F, respectively (Figure S10). EHOMO was also determined for both PBI1F(−) and PBI2F(+) by drop-casting films of the doped polymer onto the work electrode (Figure 7). As predicted above, the HOMO of PBI1F(−) is raised by 0.25 eV to −5.29 eV while the HOMO of PBI1F(+) largely remained the same at −5.61 eV. Again, the values for PBI1F(−) agree strongly with Davis et al.’s reported HOMO/LUMO for PTF (−5.24 eV/−2.66 eV), furthering the comparison between BI(−) and thiophene in this system.21 These changes are consistent with the changes in optical band gap (Table 2) and confirm the effective doping of the polymers.

steric repulsion (Figure 4c). This interaction would be less strongly felt in the neutral form for two reasons: (1) there is only one steric interaction per BI as opposed to two in the protonated state, and (2) the N−H bonds are likely longer in the protonated form, increasing the N−H)(H−CF overlap (Figure 4b,c). The nature of this steric interaction will be studied in greater detail in future work using molecular modeling and small molecule model compounds. The disruption in effective conjugation results in a loss of PL intensity in the blue region at 430 nm. There is a slight rise in green emission possibly due to localized D−A interactions between fluorene and BI(+) moieties. Analogous acid- and base-doping results for PBI2F were observed and are shown in Figure S9. The doping process was shown to be reversible by the combination of equal molar volumes of PBI1F(−) and PBI1F(+) in DMAc (Figure 5). In this scenario, “dedoping” may occur through excess small molecule acid (TFA) and base (−OEt) and/or through macromolecular polyacids (PBI1F(+)) protonating the analogous polybase (PBI1F(−)). Stability of the doped species was examined through an aging process where doped polymer solutions were stored passively under N2. As seen in Figure 6, the optical properties are largely unaffected by time, implying chemical stability of the poly(ionomer) species. The absorbance and photoluminescence of the base-doped species are remarkably stable within the experimentation window (Figure 6a). This is somewhat surprising considering the general instability associated with anionic organic species. The fundamental absorbance and 2919

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Science Foundation (CHE1213072) for support. We also thank Prof. Kevin Kittilstved for use of his UV−vis and photoluminescence spectrometers.

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(1) Guo, X.; Baumgarten, M.; Müllen, K. Prog. Polym. Sci. 2013, 38, 1832−1908. (2) Zhao, Y.; Guo, Y.; Liu, Y. Adv. Mater. 2013, 25, 5372−5391. (3) Sari, H.; Covington, A. K. J. Chem. Eng. Data 2005, 50, 1425− 1429. (4) Li, Q.; Jensen, J. O.; Savinell, R. F.; Bjerrum, N. J. Prog. Polym. Sci. 2009, 34, 449−477. (5) Kanbara, T. Y.; Takakazu. Chem. Lett. 1993, 419−422. (6) Hayashi, H.; Yamamoto, T. Macromolecules 1998, 31, 6063− 6070. (7) Yamamoto, T.; Sugiyama, K.; Kanbara, T.; Hayashi, H.; Etori, H. Macromol. Chem. Phys. 1998, 199, 1807−1813. (8) Tanimoto, A. S.; Kouichi; Yamamoto, T. Bull. Chem. Soc. Jpn. 2004, 77, 597−598. (9) Morikita, T.; Hayashi, H.; Yamamoto, T. Inorg. Chim. Acta 1999, 296, 254−260. (10) Nurulla, I.; Morikita, T.; Fukumoto, H.; Yamamoto, T. Macromol. Chem. Phys. 2001, 202, 2335−2340. (11) Nurulla, I.; Tanimoto, A.; Shiraishi, K.; Sasaki, S.; Yamamoto, T. Polymer 2002, 43, 1287−1293. (12) Akpınar, H.; Balan, A.; Baran, D.; Ü nver, E. K.; Toppare, L. Polymer 2010, 51, 6123−6131. (13) Nurioglu, A. G.; Akpinar, H.; Sendur, M.; Toppare, L. J. Polym. Sci., Part A: Polym. Chem. 2012, 50, 3499−3506. (14) Akpinar, H.; Nurioglu, A. G.; Toppare, L. J. Electroanal. Chem. 2012, 683, 62−69. (15) Yang, L.; Feng, J.-K.; Ren, A.-M. J. Mol. Struct.: THEOCHEM 2007, 816, 161−170. (16) Ranger, M.; Rondeau, D.; Leclerc, M. Macromolecules 1997, 30, 7686−7691. (17) Ranger, M.; Leclerc, M. Macromolecules 1999, 32, 3306−3313. (18) Liu, B.; Bazan, G. C. Nat. Protoc. 2006, 1, 1698−1702. (19) Tsubata, Y.; Suzuki, T.; Miyashi, T.; Yamashita, Y. J. Org. Chem. 1992, 57, 6749−6755. (20) Neher, D. Macromol. Rapid Commun. 2001, 22, 1365−1385. (21) Davis, A. R.; Peterson, J. J.; Carter, K. R. ACS Macro Lett. 2012, 1, 469−472. (22) Zhang, Z. G.; Wang, J. J. Mater. Chem. 2012, 22, 4178−4187. (23) Cardona, C. M.; Li, W.; Kaifer, A. E.; Stockdale, D.; Bazan, G. C. Adv. Mater. 2011, 23, 2367−2371.

Figure 7. Cyclic voltammograms of PBI1F, PBI1F(−), and PBI1F(+). Scans were measured on films drop-casted from CH2Cl2, basic DMAc, and TFA, respectively, at 200 mV/s.

Table 2. Summary of Optoelectronic and Electrochemical Properties of PBI1F and PBI2F

PBI1F PBI1F(−) PBI1F(+) PBI2F PBI2F(−) PBI2F(+)

λmaxa (nm)

λonseta (nm)

optical Ega (eV)

Eonsetb (V)

HOMO/LUMO (−eV)c

393 445 378 393 460 380

426 480 417 425 487 420

2.91 2.58 2.98 2.92 2.55 2.95

1.18 0.93 1.25 1.12

5.54/2.63 5.29/2.71 5.61/2.63 5.48/2.56

a

Optical data were extracted from solutions in DMAc. bEonset was determined from CV of thin films. cHOMO levels were determined from Eonset while LUMOs were estimated from the optical Eg.

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CONCLUSION Through the incorporation of 4,7-benzimidazole moieties in poly(fluorene) semiconducting polymer systems, we demonstrated the feasibility of a dually dopable polymeric system. By the addition of simple acidic or basic reagents, the neutral polymer was effectively doped in solution, yielding stable doped species. The neutral polymers, as well as both the acid- and base-doped derivatives, were soluble in suitable solvents and could be processed into thin films. Although the systems presented here were not optimized, they showed the potential for reversibly and systematically manipulating band gaps of a single polymeric species through postpolymerization chemical methods. This ability to control the electronic band structure is difficult to achieve by current synthetic techniques which often require synthesis of distinct macromolecules to achieve alternate band structures. With the demonstration of their dopability, we believe that these semiconducting 4,7-benzimidazole-based materials represent an important evolution in the progress of conjugated polymers and will offer the exciting possibility of tunable control over hole and electron transporting behavior of these materials in device applications.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

Figures S1−S10 and Table S1. 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] (K.R.C.). 2920

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