Poly(styrene sulfonate) - American Chemical Society

Herein we report the oxidative chemical polymerization of thieno[3,4-b]thiophene (T34bT) using ... have long been known to exhibit very low band-gaps ...
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Langmuir 2005, 21, 10797-10802

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Poly(thieno[3,4-b]thiophene)-Poly(styrene sulfonate): A Low Band Gap, Water Dispersible Conjugated Polymer Byoungchul Lee, Venkataramanan Seshadri, and Gregory A. Sotzing* Department of Chemistry and the Polymer Program, Institute of Materials Science, University of Connecticut, 97 N. Eagleville Road, Storrs, Connecticut 06269 Received April 25, 2005. In Final Form: August 20, 2005 Herein we report the oxidative chemical polymerization of thieno[3,4-b]thiophene (T34bT) using several different oxidants including ferric sulfate, ammonium persulfate, and hydrogen peroxide in the presence of poly(styrenesulfonic acid) in water and properties of the resulting poly(thieno[3,4-b]thiophene)-poly(styrenesulfonic acid) (PT34bT-PSS) dispersion. The PT34bT-PSS is rendered a colloidal dispersion in water with a particle size diameter ranging between 180 and 220 nm depending on the oxidant used for polymerization. PT34bT-PSS films have band gaps of ca. 1 eV (1260 nm) as determined by the onset of the π to π* transition from the vis-NIR spectrum with absorption maxima ranging from 1.4 eV (912 nm) to 1.7 eV (724 nm). The neutral and oxidized forms of PT34bT-PSS prepared from ferric sulfate dispersed in water were blue and lime green, respectively, whereas the neutral and oxidized forms of PT34bT-PSS prepared from ammonium persulfate and hydrogen peroxide were blue and blue-green, respectively. Spectral properties of the PT34bT-PSS dispersion can be tuned by the combination of oxidants. PT34bT-PSS films showed ca. 100% cation dominant ion transport behavior as determined by electrochemical gravimetry with each charge-discharge cycle and the doping level of the polymer was calculated to be 26%. Electrical conductivities for these polymers were found to be dependent on chemical oxidants used and varied from 10-2 to 10-4 S/cm.

Introduction Potential applications in optoelectronic and electronic devices1,2 have given the impetus for research on optically transparent intrinsically conducting polymers. Several strategies to tune the band-gap of conjugated polymers reported in the literature have been reviewed elsewhere.3 Conjugated polymers based on fused heterocyclic rings have long been known to exhibit very low band-gaps owing to the stabilization effect the fused ring has on the quinoidal form of the main chain polymer. Poly(isothianaphtene)4 was the first to be shown to exhibit a low bandgap (1.0-1.2 eV) owing to the stabilization factor aforementioned, which has been further substantiated through the theoretical works by Bredas.5 Commercialization of most intrinsically conducting polymers has not been successful due to their intractability and lack of environmental stability. Several techniques have been reported for the processing of intractable ICPs such as appending flexible side groups onto the backbone,6 ionic side chains covalently linked to the backbone resulting in water solubility/dispersability,7 soluble poly* To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Argun, A. A.; Cirpan, A.; Reynolds, J. R. Adv. Mater. 2003, 15, 1338-1341. (b) Sariciftci, N. S.; Smilowitz, L.; Heeger, A. J.; Wudl, F. Science 1992, 258, 1474. (c) Ding, L.; Jonforsen, M.; Roman, L. S.; Andersson, M. R.; Inganas, O. Synth. Met. 2000, 110, 133. (d) Novak, P.; Muller, K.; Santhanam, K. S. V.; Haas, O. Chem. Rev. 1997, 97, 207. (2) (a) Zhang, F.; Johansson, M.; Andersson, M. R.; Hummelen, J. C.; Ingana¨s O. Adv. Mater. 2002, 14, 662-665. (b) Winder, C.; Sariciftci, N. S. J. Mater. Chem., 2004, 14, 1077. (c) Inganas, O.; Svensson, M.; Zhang, F.; Gadis1a, A.; Persson, N. K.; Wang, X.; Andersson, M. R. Appl. Phys. A 2004, 79, 31. (3) (a) Roncali, J. Chem. Rev. 1997, 97, 173. (b) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988, 88, 183. (c) Yamamoto, T.; Hayashida, N. React. Func. Polym. 1998, 37, 1. (d) Scherf, U.; Mullen, K. Synthesis 1992, 23. (e) Ajayaghosh, A. Chem. Soc. Rev. 2003, 32, 181. (4) Wudl, F.; Kobayashi, M.; Heeger, A. J. J. Org. Chem. 1984, 49, 3382. (5) Bredas, J. L.; Heeger, A. J.; Wudl, F. J. Chem. Phys. 1986, 85, 4673.

mers at the ends of conjugated polymers,8 solid-state polymerization onto surfaces,9 solid-state oxidative crosslinking of precursor polymers containing pendant heterocycles,10 etc. If the ionic side chains have strong protonic acid moiety such as sulfonic acid, the polymer shows selfdoping behavior with auto-doping characteristics.11 A more pragmatic approach has been to oxidatively polymerize the monomer in the presence of a water soluble polyelectrolyte such as poly(styrenesulfonic acid) (PSSA). In this case, PSS- acts as the charge compensating dopant for the oxidized conjugated polymer and also renders the resulting complex a colloidal dispersion as these polymers are themselves water-soluble. Aniline and 3,4-ethylenedioxythiophene have been polymerized using this templated approach to yield a processible polymer.12,13 (6) (a) Elsenbaumer, R. L.; Jen, K. Y.; Obood, R. Synth. Met. 1986, 15, 169. (b) Miller, G. G.; Elsenbaumer, R. L J. Chem. Soc. Chem. Commun. 1986, 1346. (c) McCullough, R. D.; Lowe, R. D. J. Chem. Soc., Chem. Commun. 1992, 70. (d) McCullough, R. D.; Lowe, R. D.; Jayaraman, M.; Anderson, D. L. J. Org. Chem. 1993, 58, 904. (7) (a) Yue, J.; Wang, Z. H.; Cromack, K. R.; Epstein, A. J.; MacDiarmid, A. G. J. Am. Chem. Soc. 1991, 113, 2665. (b) Wei, X. L.; Wang, Y. Z.; Long, S. M.; Bobeczko, C.; Epstein, A. J. J. Am. Chem. Soc. 1996, 113, 2545. (8) (a) Liu, J.; Sheina, E.; Kowalewski, T.; McCullough, R. D. Angew. Chem. Int. Ed. 2002, 41, 329. (b) Leclere, P.; Calderone, A.; Marsitzky, D.; Francke, V.; Mullen, K.; Bredas, J. L.; Lazzaroni, R. Synth. Met. 2001, 121, 1295. (c) Leclere, P.; Calderone, A.; Mullen, K.; Bredas, J. L.; Lazzaroni, R. Mater. Sci. Technol. 2002, 18, 749. (9) (a) Meng, H.; Perepichka, D. F.; Wudl, F. Angew. Chem., Int. Ed. 2003, 42, 658. (b) Meng, H.; Perepichka, D. F.; Bendikov, M.; Wudl, F.; Pan, G. Z.; Yu, W.; Dong, W.; Brown, S. J. Am. Chem. Soc. 2003, 125, 15151. (10) (a) Jang, S.-Y.; Sotzing, G. A.; Marquez, M. Macromolecules 2002, 35, 7293. (b) Jang, S.-Y.; Marquez, M.; Sotzing, G. A. J. Am. Chem. Soc. 2004, 126, 9426. (11) (a) Patil, A. O.; Ikenoue, Y.; Wudl, F.; Heeger, A. J. J. Am. Chem. Soc. 1987, 109, 1858. (b) Patil, A. O.; Ikenoue, Y.; Basescu, N.; Colaneri, N.; Chen, J.; Wudl, F.; Heeger, A. J. Synth. Met. 1987, 20, 151. (12) (a) Samuelson, L. A.; Anagnostopoulos, A.; Alva, K. S.; Kumar, J.; Tripathy, S. K. Macromolecules 1998, 31, 4376. (b) Liu, W.; Kumar, J.; Tripathy, S.; Senecal, K. J.; Samuelson, L. J. Am. Chem. Soc. 1999, 121, 71.

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Ferraris14 and Pomerantz15 reported the polymerization of thieno[3,4-b]thiophene with the 2-position derivatized with phenyl and decyl/dodecyl side groups to yield a 4-6 coupled polymer. These polymers were found to exhibit very low band-gaps of 0.85 and 0.92 eV, respectively. In our previous report, unsubstituted T34bT was electrochemically polymerized to PT34bT and was found to exhibit a low band-gap of 0.85 eV as measured by the onset of the π to π* transition of the neutral polymer.16 Furthermore, films of PT34bT (thickness ∼800 nm) were found to be sky blue in the neutral form and highly transparent in the oxidized form. Electrochemically prepared PT34bT was found to be capable of being both p- and n-doped at low positive and negative potentials, respectively, with very good stability toward p-doping and moderate stability to n-doping. Recently, we reported the sulfonation of intractable poly(thieno[3,4-b]thiophene) to produce the sulfonated water processable low band gap version that can be assembled into layer-by-layer structures.17 In this approach, the electronic structure of the poly(thieno[3,4-b]thiophene) is changed by the incorporation of sulfonic acid groups directly onto the backbone, which affects the spectral properties of polymers. Polymerization of thieno[3,4-b]thiophene in the presence of PSSA would render the resulting polymer dispersible in water with optoelectronic properties close to that of the electrochemically prepared polymer. We believe that water processable PT34bT could be useful in electrochromic devices,18 hole injection layers/buffer layers in LEDs,19 photon harvesting materials in photovoltaics,20 and transparent charge dissipation coatings.21 Herein we report the synthesis and the properties of processable colloidal dispersions of poly(thieno[3,4-b]thiophene) in water, which were prepared from several different oxidants systems. Experimental Section Chemicals. Thieno[3,4-b]thiophene was synthesized in accordance to a previously reported and modified procedure by our group from 3,4-dibromothiophene.16,22 Ferric sulfate hydrate (iron content 21.7 wt %), ammonium persulfate, and sodium dodecylbenzene sulfonate were purchased from ACROS and were used as received. Lithium trifluoromethanesulfonate (LITRIF) and hydrogen peroxide (35 wt % solution in water) were purchased from Aldrich and used as received. Poly(styrenesulfonic acid) (30 wt % in water, Mw ) 70 000) was purchased from Polysciences Inc. and used without further purification. Acetonitrile (ACN) was purchased from Fisher scientific and was distilled over calcium hydride prior to use. Millipore water (18 MΩ) was used for the polymerizations. (13) (a) Hwang, J. H.; Yang, S. C. Synth. Met. 1989, 29, E271. (b) Jonas, F.; Krafft, W.; Muys, B. Macromol. Symp. 1995, 100, 169. (14) Neef, C. J.; Brotherston, I. D.; Ferraris, J. P. Chem. Mater. 1999, 11, 1957. (15) (a) Pomerantz, M.; Gu, X. Synth. Met. 1997, 84, 243. (b) Pomerantz, M.; Gu, X.; Zhang, S. X. Macromolecules 2001, 34, 1817. (16) (a) Lee, K.; Sotzing, G. A. Macromolecules 2001, 34, 5746. (b) Sotzing, G. A.; Lee, K. Macromolecules 2002, 35, 7281. (17) Lee, B.; Seshadri, V.; Palko, H.; Sotzing, G. A. Adv. Mater. 2005, 17, 1792-1795. (18) (a) Krishnamoorthy, K.; Ambade, Ashootosh V.; Kanungo, Mandakini; Contractor, A. Q.; Kumar, A. J. Mater. Chem. 2001, 29092912. (b) Schwendeman, I.; Hwang, J.; Welsh, D. M.; Tanner, D. B.; Reynolds, J. R. Adv. Mater. 2001, 13, 634-637. (c) Schottland, P.; Zong, K.; Gaupp, C. L.; Thompson, B. C.; Thomas, C. A.; Giurgiu, I.; Hickman, R.; Abboud, K. A.; Reynolds, J. R. Macromolecules 2000, 33, 70517061. (19) Yu, G.; Heeger, A. J. Synth. Met. 1997, 85, 1183-1186. (20) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 10771086. (21) Jonas, F.; Heywang, G., Electrochim. Acta 1994, 39, 1345-1347. (22) (a) Brandsma, L.; Verkruijsse, H. D. Synth. Commun. 1990, 20, 2275. (b) Wynberg, H.; Zwanenburg, D. J. Tetrahedron Lett. 1967, 9, 761.

Lee et al. Instrumentation. 13C CP-MAS solid-state NMR spectra were recorded on a Varian 75 MHz FT-NMR spectrometer. The data were acquired at a magic angle spinning rate of 7 kHz. The chemical shift data for each signal are given in units of δ (ppm) and referenced to the carbon chemical shift of adamantane. Fourier transform infrared spectroscopy (FT-IR) was performed using a MAGNA-IR560. Optical properties of the polymer solution were measured by a Perkin-Elmer Lambda 900 UV-vis-NIR spectrometer. Polymer film thickness was measured using Zygo New View 3-D surface profilers. Elemental analysis was performed at Galbraith Laboratories Inc. Dynamic light scattering measurements were made on dilute dispersions at 25 °C with a Brookhaven BI9000-AT auto-correlator. The light source was a Coherent Innova 70-3 Ar+ laser operating at 514.5 nm. The PT34bT-PSS particles were imaged using JEOL 2010 Fas and Philips EM420 transmission electron microscopy. Conductivities were measured using a four-line collinear array utilizing a Keithley Instruments 224 constant current source and a 2700 Multimeter. The polymer was coated on the glass substrate having four gold coated leads on the surface across the entire width of the polymer and 0.25 cm apart from each other. The current is applied across the outer leads and voltage is measured across the inner leads. Films were prepared by drop casting PT34bT-PSS onto glass slides and allowing for slow evaporation over a period of 24 h to yield 5 µm thick films. For the comparison of conductivity, PEDOT-PSS (BAYTRON-P, V4071) films with same thickness were prepared, and their conductivity was measured at the same condition. The humidity is kept constant at 65% within our laboratory, and the temperature is approximately 25 ( 2 °C. Electrochemistry. All electrochemical experiments were performed using a three electrode cell configuration in 0.1 M LITRIF/CH3CN/H2O (10 wt % H2O) with a CHI 400 potentiostat. The reference electrode was a nonaqueous Ag/Ag+ electrode consisting of a silver wire immersed in a glass capillary body fitted with a Vycor tip and filled with a 10 mM silver nitrate, 0.1 M LITRIF/ACN solution. The Ag/Ag+ reference electrode was calibrated to be 0.443 V vs the normal hydrogen electrode (NHE) using 10 mM ferrocene/CH3CN/H2O (10 wt %). Electrochemical quartz crystal microbalance (EQCM) studies were carried out using a CHI 400 Potentiostat equipped with an oscillator circuit. Polished quartz crystals coated with 0.201′′ diameter gold key electrodes on both sides and operating at a resonance frequency of 7.995 MHz were purchased from International Crystal manufacturing. The key electrode comprised of a 1000 Å thick gold coating with a 100 Å chromium underlay was soldered to electrical contact leads that were sealed away from the solution. A 1 cm2 platinum flag was used as the counter electrode, and the Ag/Ag+ (0.443 V vs NHE) was used as the reference electrode. All of the EQCM studies were carried out using 0.1 M sodium dodecylbenzenesulfonate/ACN/H2O (20 wt % H2O). Preparation of Poly(thieno[3,4-b]thiophene), PT34bT. T34bT, 50.0 mg (0.36 mmol), and 418.0 mg of 30 wt % PSSA aqueous solution were added to a 25 mL one neck flask equipped with a magnetic stir-bar. To this suspension were added the oxidant and deionized water (18 MΩ), and the mixture was vigorously stirred for 24 h at room temperature. The total mass of all of the reactants was adjusted to be 10 g using the appropriate amount of deionized water. The oxidant used for the polymerization was (a) 159.0 mg (0.31 mmol) of Fe2(SO4)3; (b) 113.0 mg (0.49 mmol) of (NH4)2S2O8; or (c) 67.3 mg (0.69 mmol) of 35 wt % hydrogen peroxide. After polymerization, the resulting aqueous dispersions were purified by passing them through cation and anion exchange columns (Lewatit K-2629 H+ form and Lewatit K-7333 OH- form). These dispersions were undoped by adding three drops of hydrazine hydrate and purified by a dialysis membrane having a molecular weight cutoff of 3500 Da. During dialysis, the surrounding water (1 L) was exchanged with fresh deionized water every 5 h over 3 days, and the water from the dialysis tube was then evaporated to yield 157.2, 147.3, and 137.5 mg of solids from the dispersions prepared using ferric sulfate, ammonium persulfate, and hydrogen peroxide, respectively. The calculated yields based on the initial amounts of T34bT and PSSA for the dispersions prepared from ferric sulfate, ammonium persulfate, and hydrogen peroxide were 89.6%, 84.0%, and 78.4%, respectively.

Poly(thieno[3,4-b]thiophene)-Poly(styrene sulfonate)

Langmuir, Vol. 21, No. 23, 2005 10799 Table 1. Elemental Analysis of PT34bT-PSS Samples Prepared from Different Oxidants Systems calculateda oxidant

Figure 1. Oxidative chemical polymerization of thieno[3,4b]thiophene (T34bT) in water in the presence of poly(styrenesulfonic acid) to produce a colloidal dispersion of PT34bTPSS. Preparation of Samples for 13C CP-MAS Solid-State NMR. To prepare solid-state NMR samples, we attempted to remove poly(styrenesulfonic acid) used as a template in order to simplify the spectrum. First, the as-prepared PT34bT-PSS dispersions were purified via ion exchange column and dialysis as mentioned above and undoped using hydrazine hydrate to remove strong ionic interaction between oxidized PT34bT and PSS. This was confirmed by vis-NIR spectrum which showed complete neutralization. The undoped dispersions were then added to 60 mL of tetrahydrofuran, and a blue-black solid was precipitated. The filtered precipitates were then washed with copious amounts of tetrahydrofuran followed by ethanol. Finally, the filtrate was washed with deionized water on the funnel and dried in the vacuum oven at 60 °C for overnight. After drying, 156.0 mg, 124.1 mg, and 129.7 mg of solids were collected from 20 g of dispersions prepared from the reaction of 100 mg of T34bT and 836.0 mg of 30 wt % PSSA aqueous solution using ferric sulfate, ammonium persulfate, and hydrogen peroxide, respectively.

Results and Discussion Preparation of PT34bT-PSS Dispersions in Water. Generally, the use of an organic solvent is inevitable for the polymerization of monomers in the laboratory or industrial plant except polymerization techniques such as melt polymerization. Such organic solvents are, however, toxic and tend to cause environmental contamination. Use of solvents such as supercritical fluids and water are not toxic and hence are more feasible to address industrial standards. Water is a great solvent for polymerization reactions since it is abundant and does not involve the use of pressure. In practice, EDOT (3,4-ethylenedioxythiophene) is polymerized in the presence of PSSA in the aqueous phase and sold as water dispersion (BAYTRON-P). Here, T34bT was polymerized in water using three different chemical oxidant systems such as ferric sulfate hydrate, ammonium persulfate, and hydrogen peroxide in the presence of PSSA as shown in Figure 1. The reaction mixture forms a white suspension with vigorous stirring at room temperature, and the color of the mixture changes from milky white to black within 10 min. After completion of polymerization for 24 h, the polymer dispersion was purified via dialysis and ion exchange columns of strong acid and base types. There was no apparent sedimentation of polymer from water having utilized ammonium persulfate and hydrogen peroxide as the oxidant and these dispersions appeared to be stable in this regard after having been stored for 12 months under normal laboratory conditions. However, PT34bT-PSS dispersions prepared from ferric sulfate hydrate show partial precipitation after 24 h during normal laboratory storage. Elemental analysis results of PT34bT-PSS samples prepared from each oxidant are shown in Table 1. From the elemental analysis, we found that the resulting PT34bT-PSS are all different in their composition. This difference in composition could be explained by the different ratio of PT34bT to PSS in the final dispersion. The three oxidants which were used in this experiment

found

C, %

H, %

S, %

C, %

H, %

S, %

S/C ratio

Fe2(SO4)3 55.22 (NH4)2S2O8 55.22 H2O2 55.22

4.21 4.21 4.21

27.07 46.54 27.07 47.31 27.07 44.24

3.65 3.10 4.82

29.99 38.24 24.12

0.24 0.30 0.20

a These values were calculated based on the feed amount of T34bT and PSSA.

Table 2. λmax of the Oxidized and Neutral Polymer Films on ITO λmax (nm) oxidant

oxidized state

neutral state

Fe2(SO4)3 (NH4)2S2O8 H2O2

1315 1078 965

912 812 724

have different oxidizing power and thereby the composition of the resulting polymer could be changed by the oxidants used for polymerization. Especially, unreacted monomer and oligomers whose molecular weights are less than 3500 Da are removed using the dialysis membrane during the purification process. From the FT-IR analysis, we did not observe any noticeable changes among the three different dispersions. All three dispersions show the strong absorption bands located at ca. 1033 and ca. 1163 cm-1 which are assigned to SdO symmetric and asymmetric stretching from the polystyrene sulfonate.23 Weak characteristic absorption peaks corresponding to aliphatic C-H stretching from PSSA and aromatic C-H stretching from both PT34bT and PSSA were observed at 2850∼2950 and 3050∼3100 cm-1, respectively. Spectral Properties of PT34bT. Figure 2 shows visNIR spectra of PT34bT-PSS films on glass substrates in the oxidized and neutral forms. Polymerizations from ferric sulfate hydrate show a significant change in optical properties, as evidenced by 100-188 and 237-350 nm bathochromic shifts of the λmax of the neutral and oxidized polymer, respectively, in comparison to polymerizations carried out using the other two oxidants. The neutral and oxidized forms of PT34bTPSS prepared from ferric sulfate dispersed in water were blue and pale green, respectively, whereas the neutral and oxidized forms of PT34bT-PSS prepared from ammonium persulfate and hydrogen peroxide were blue and blue-green, respectively. It is clear from these data that different oxidants produce optically different polymers. The shifts observed in the vis-NIR spectra upon changing chemical oxidants can be attributed to the different oxidizing power of each oxidant used for polymerization. Taken into consideration that different oxidants give optically different polymers, generally, PT34bT-PSS has a band edge gap of ca. 0.94 eV (1320 nm) to 0.98 eV (1260 nm) as determined by the onset of the π to π* from the vis-NIR spectrum with a peak at an energy from 1.36 eV (912 nm) to 1.71 eV (724 nm). Tuning of Spectral Properties. Ammonium persulfate and hydrogen peroxide cause blue shifts of λmax in vis-NIR spectra of the resulting dispersion compared to ferric sulfate. Thus, ferric sulfate could be used for polymerization and hydrogen peroxide or ammonium persulfate could then be added at different time increments to control the spectral properties of the resulting polymer. (23) (a) Melinda, B. G.; Reynolds, J. R. Macromolecules 1992, 25, 4832. (b) Melinda, B. G.; Reynolds, J. R. Macromolecules 1993, 26, 5633.

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Figure 3. Changes of λmax of polymer dispersions obtained from the polymerization of T34bT using ferric sulfate hydrate as the oxidant with the addition of hydrogen peroxide (A) and ammonium persulfate (B) after fixed periods of time (9, oxidized state; b, neutral state).

Figure 2. vis-NIR spectra of as prepared and undoped PT34bT-PSS films on glass substrate from (A) Fe2(SO4) (thickness 260 nm); (B) (NH4)2S2O8 (thickness 202 nm); (C) H2O2 (thickness 376 nm).

Figure 3 shows the λmax of the conjugated polymer as a result of adding hydrogen peroxide and ammonium persulfate at different times during the ferric sulfate oxidative polymerization of thieno[3,4-b]thiophene. For this experiment, 141 mg of Fe2(SO4)3 was added to 10 g of aqueous suspension containing 50 mg of T34bT and 418 mg of PSSA at 0 h and 52.0 mg of 35 wt % hydrogen peroxide (0.54 mmol) or 124.5 mg (0.54 mmol) of ammonium persulfate were added after 0.5, 1, 2, and 3 h to different polymerizations. A bathochromic shift in λmax of both the oxidized and neutral polymer was observed as a result of the delayed addition of hydrogen peroxide or ammonium persulfate. It is clear from these data that spectral properties of the polymer can be adjusted by the combination of two oxidants which are different in their oxidizing power. Solid State NMR Study of PT34bT. It was found that, even after removing the ionic interaction between doped PT34bT and PSSA by treatment with hydrazine and precipitation followed by washing of precipitated PT34bT, PSSA is not completely removed. From the X-ray photoelectron spectroscopy with these samples, we found that the carbon-to-sulfur ratios are 4.26 from ferric sulfate, 4.62 from ammonium persulfate, and 5.71 from hydrogen

Figure 4. 75 MHz solid-state 13C CP/MAS spectra of the PT34bT-PSS mixture prepared using (A) ferric sulfate, (B) ammonium persulfate, and (C) hydrogen peroxide.

peroxide, respectively, which are higher than the theoretical value of 3.0 expected for pure PT34bT with PSSA removed. We observed that PSSA in water is not precipitated from THF at the same condition. PT34bT chains could be highly entangled with the PSSA chains and hence some amount of PSSA coprecipitates from THF without complete separation.12 Using this sample, solid-state CPMAS 13C NMR spectra were obtained to investigate structure of the PT34bT-PSS. Figure 4 shows the 13C CP-MAS solid-state NMR for PT34bT from each of the three different oxidants. The resonance peaks at 47 ppm are assigned to the aliphatic carbons of PSSA, and the broad peaks ranging from 125 to 160 ppm are assigned to aromatic carbons from both PT34bT and PSSA.24 (24) (a) Adriaensens, P.; Carleer, R.; Storme, L.; Vanderzande, D.; Gelan, J. Polymer 2002, 3, 7003. (b) Smith, R. S.; McCormick, M.; Barrett, J.; Reven, L.; Spiess, H. W. Macromolecules 2004, 37, 4830.

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Figure 6. Hydrodynamic radius (Rh) of PT34bT-PSS in water from dynamic light scattering (9, ferric sulfate; 2, ammonium persulfate; b, hydrogen peroxide).

Figure 5. Chronogravimetry (A) and chronocoulometry (B) during a constant potential redox switching of PT34bT-PSS prepared fromferric sulfate in sodium dodecylbenzene sulfonate/ ACN/water (20 wt %).

Polymer Electrochemistry, Ion Transport, and Doping Level. Thin films of PT34bT-PSS were prepared on ITO/glass substrates by drop casting and drying overnight at room temperature. Cyclic voltammetry of polymeric films in 0.1 M lithium triflate/ACN-water (90: 10 w/w) was obtained at a scan rate of 50 mV/s. PT34bTPSS prepared from ferric sulfate exhibited a broad redox peak between -0.7 and +0.2 V (vs Ag/Ag+), and there was no significant loss in electroactivity upon 10 repeated scans of the polymer. The PT34bT-PSS dispersion prepared from ferric sulfate was passed through the ion exchange columns of Na+ and Cl- types and the gravimetric response of that film upon potential sweeping between -0.7 and +0.2 V in 0.1 M sodium dodelcybenzenesulfonate/acetonitrile/water (20 wt %) was obtained. The frequency of the QCM crystal increases upon potential sweep from -0.7 to +0.2 V and decreases upon reversing the potential sweep. This indicates that the PT34bT-PSS film loses mass upon oxidation due to the expulsion of sodium ions from the film and gains mass upon reduction due to the inclusion of sodium ions from the solution. Figure 5 shows one complete redox step for PT34bTPSS prepared from ferric sulfate using the electrochemical quartz crystal microbalance. The percent cations transported during the redox process can be calculated from the charge passed to oxidize or reduce the polymer and the mass change during this process using the following equation:25

∆m ) -XanMan + (1 - Xan)(Q/F)Mcat

(1)

where ∆m represents the mass change due to ion transport during oxidation or reduction. Man and Mcat are the molar mass of the anion (dodecylbenzene sulfonate) and cation (25) (a) Berlin, A.; Schiavan, G.; Zecchin, S.; Zotti, G. Synth. Met. 2001, 119, 153. (b) Seshadri, V.; Wu, L.; Sotzing, G. A. Langmuir 2003, 19, 9479.

(Na+), respectively. Xan is the fraction of anion transported, Q is the charge passed during oxidation or reduction, and F is Faraday’s constant. The mass of the film coated onto the QCM crystal was calculated from the change in resonant frequency of the blank and polymer-coated QCM crystal and was found to be 2.8 µg. After placement into the EQCM cell, the potential was stepped 10 times between -0.7 and +0.2 V in 10 s pulses. As shown in Figure 5A, when 0.2 V was applied, the mass of the polymer film decreased due to the expulsion of Na+ into the solution matching the cyclovoltammetric experiment. Upon switching to -0.7 V, an increase in the mass of the polymer film is observed due to uptake of Na+. From the gravimetric response of the PT34bT-PSS film, we can conclude that redox switching is dominated by cations since PSS- is bulky and highly entangled with the conjugated polymer as was reported earlier by Reynolds and co-workers for polypyrrole.26 The time taken for a 90% switch from the neutral to oxidized state calculated from the chronogravimetry was found to be ca. 3 s and the time taken for the reverse process, i.e., oxidized to neutral, was ca. 1 s. The ion transport out of the film upon oxidation and into the film on reduction was found to be ca. 100% cation dominant from the eq 1 and the doping level, calculated as the ratio of moles of charges injected per T34bT repeat unit, was found to be ca. 26%. The coulometric curves of the charging and discharging process of PT34bT-PSS film in Figure 5B were found to be nonsymmetrical, whereas the mass response attains a steady state. The charge response not attaining a steady state for PT34bT-PSS is indicative of a non-Faradaic process. The same behavior was reported for PEDOT-PSS.27 Particle Size and Stability of Dispersion. The hydrodynamic radius (Rh) of particles at 0.3 g of asprepared dispersion in 1000 mL of water was measured to range from 50 to 80 nm. The range of Rh values increases to 90-110 nm upon three month storage as shown in Figure 6. Diameters of PT34bT-PSS nanoparticles prepared from ferric sulfate were found to be larger than those from ammonium persulfate and hydrogen peroxide. All of the three dispersions were stable at this concentration and no visual evidence of precipitation was observed. It should be noted that the stability of the dispersion from ferric sulfate is reported after filtering of the initially precipitated solids. Transmission electron microscope (TEM) images of three different samples prepared from ferric sulfate, ammonium persulfate, and hydrogen peroxide are shown (26) (a) Baker, C. K.; Qiu, Y.-J.; Reynolds, J. R. J. Phys. Chem. 1991, 95, 4446. (27) Lisowska-Oleksiak, A.; Kazubowska, K.; Kupniewska, A. J. Electroanal. Chem. 2001, 501, 54.

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Figure 7. Transmission electron micrographs pf PT34bT-PSS prepared using (A) ferric sulfate, (B) ammonium persulfate, and (C) hydrogenperoxide.

in Figure 7. PT34bT-PSS prepared from ferric sulfate shows a common aggregation shape of particles with sizes ranging from ca. 50 nm to 400 nm. Particles from ammonium persulfate were spherical with diameters ranging from 60 to 300 nm. Interestingly, hydrogen peroxide polymerization resulted in wormlike particles approximately 200 nm in length and 30 nm in width. Conductivity. PT34bT-PSS films were fabricated via drop casting on normal glass substrate, and their thicknesses were measured by optical profilometry after drying at room temperature. Approximately, 5 µm thick PT34bTPSS films were prepared from PT34bT-PSS dispersions. PT34bT-PSS film prepared from ferric sulfate had conductivities averaging ca.10-2 S/cm, whereas PT34bTPSS from ammonium persulfate and hydrogen peroxide had conductivities averaging ca. 10-4 S/cm. Conductivities of PEDOT-PSS films prepared by drop casting commercially available Baytron-P were measured in our lab to be ca.0.1 S/cm.

persulfate ion or hydrogen peroxide have much better dispersibility than those prepared from ferric sulfate hydrate under normal laboratory conditions and do not precipate after 1 year of storage. PT34bT-PSS was found to be highly transmissive, green in the oxidized state and blue in the neutral state. Ion transport behavior of PT34bT-PSS was found to be ca. 100% cation dominant, and the calculated doping level was 26%. The conductivites of films prepared via drop casting on glass substrates were found to be ca. 10-2 S/cm for PT34bT-PSS prepared from ferric sulfate and ca. 10-4 S/cm for those prepared from ammonium persulfate and hydrogen peroxide. The ability to tune the optoelectronic properties of these polymers as a result of modifying the oxidant provides potential for the utilization of this system for better matching the energies of indium doped tin oxide with the light emitting polymer in light emitting diode applications to control the transport of holes.

Conclusion

Acknowledgment. We thank the University of Connecticut for helping to make this research possible. We thank Dr. Roger Ristau for TEM measurements and many valuable discussions. We thank the National Science Foundation (CHE-0349121) through the CAREER award for partial support of this work.

We have prepared low band gap (