Synthesis of Ferrocene-Grafted Poly (p-phenylene-ethynylenes) and

Synthesis and photoluminescence properties of novel polyarylates bearing pendent naphthylamine chromophores. Guey-Sheng Liou , Shih-Ming Lin , Hung-Ju...
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Synthesis of Ferrocene-Grafted Poly(p-phenylene-ethynylenes) and Control of Electrochemical Behaviors of Their Thin Films Cuihua Xue,† Zhen Chen,† Ya Wen,† Fen-Tair Luo,‡ Jian Chen,§ and Haiying Liu*,† Department of Chemistry, Michigan Technological University, Houghton, Michigan 49931, Institute of Chemistry, Academia Sinica, Taipei, Taiwan, 115 Republic of China, and Zyvex Corporation, 1321 North Plano Road, Richardson, Texas 75081 Received March 12, 2005. In Final Form: May 31, 2005 New ferrocene-coated poly(p-phenylene-ethynylenes) (PPEs) with end capping groups of protected thiol were prepared by a palladium-catalyzed Sonogashira coupling reaction. Ferrocene groups were covalently attached to polymers A and B through ethylene oxide tethers and to polymer C through methylene tethers. Polymers A and B are soluble in common solvents such as tetrahydrofuran (THF), chloroform, methylene chloride, acetone, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), and polymer C is soluble in toluene, THF, chloroform, and methylene chloride. Polymers A-C display low quantum yield, caused by electron-transfer quenching of ferrocene groups as electron donors. The polymer thin films were prepared through incubation of gold electrodes in THF solutions containing the polymers for 2 days. Ferrocene in thin films of polymers A and B display significantly faster electron-transfer rate than that of polymer C. Hydrophilic ethylene oxide side chains of polymers A and B decrease formal potential of tethered ferrocene groups because of electron-donating effect from ethylene oxide side chains, which stabilizes the ferrocenium ion and leads to a cathodic shift of the redox wave.

Introduction Conjugated polymers as sensing materials have been the focus of considerable recent interest due to the high sensitivity of their optical and conducting properties to analytes.1-4 In particular, conjugated polymers have been used to facilitate electron transfer between redox centers of enzymes and electrodes.1,5 Electrochemical behavior of electron-transfer mediators such as ferrocene incorporated into the conjugated polymers plays a very important role in the sensitivity and response of amperometric enzymebased biosensors.1a,5 Fast electron transfer of polymer thin * To whom correspondence should be addressed. E-mail: hyliu@ mtu.edu. † Michigan Technological University. ‡ Academia Sinica. § Zyvex Corp. (1) (a) McQuade, D. T.; Pullen, A. E.; Swager, T. M. Chem. Rev. 2000, 100, 2537-2574. (b) Swager, T. M. Acc. Chem. Res. 1998, 31, 201-207. (c) Bunz, U. H. F. Chem. Rev. 2000, 100, 1605-1644. (d) Pinto, M. R.; Schanze, K. S. Synthesis 2002, 9, 1293-1309. (2) (a) Yang, J. S.; Swager, T. M. J. Am. Chem. Soc. 1998, 120, 1186411873. (b) Gaylord, B. S.; Heeger, A. J.; Bazan, G. C. J. Am. Chem. Soc. 2003, 125, 896-900. (c) Jones, R. M.; Bergstedt, T. S.; McBranch, D. W.; Whitten, D. G. J. Am. Chem. Soc. 2001, 123, 6726-6727. (d) Wilson, J. N.; Wang, Y. Q.; Lavigne, J. J.; Bunz, U. H. F. Chem. Commun. 2003, 1626-1627. (3) (a) Dwight, S. J.; Gaylord, B. S.; Hong, J. W.; Bazan, G. C. J. Am. Chem. Soc. 2004, 126, 16850-16859. (b) Chen, Z.; Xue, C.; Shi, W.; Luo, F.-T.; Green, S.; Chen, J.; Liu, H. Anal. Chem. 2004, 76, 6513-6518. (c) Pinto, M. R.; Schanze, K. S. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 7505-7510. (d) Disney, M. D.; Zheng, J.; Swager, T. M.; Seeberger, P. H. J. Am. Chem. Soc. 2004, 126, 13343-13346. (4) (a) Haskins-Glusac, K.; Pinto, M. R.; Tan, C.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 14964-14971. (b) Ramey, M. B.; Hiller, J.-A.; Rubner, M. F.; Tan, C.; Schanze, K. S.; Reynolds, J. R.; Macromolecules 2005, 38, 234-243. (c) Tan, C.; Atas, E.; Muller, J. G.; Pinto, M. R.; Kleiman, V. D.; Schanze, K. S. J. Am. Chem. Soc. 2004, 126, 13685-13694. (d) Wosnick, J. H.; Mello, C. M.; Swager, T. M. J. Am. Chem. Soc. 2005, 127, 3400-3405. (e) Kim, I.-B.; Wilson, J. N.; Bunz, U. H. F. Chem. Commun. 2005, 1273-1275. (5) (a) Heller, A. J. Phys. Chem. 1992, 96, 3579-3587. (b) Xue, C.; Chen, Z.; Luo, F.-T.; Palaniappan, K.; Chesney, D. J.; Liu, J.; Chen, J.; Liu, H. Biomacromolecules 2005, 6, 1810-1815. (c) Foulds, N. C.; Lowe, C. R. Anal. Chem. 1988, 60, 2473-2478.

films bearing the mediators is desirable to enhance biosensor sensitivity.5,6 Ferrocene-terminated self-assembled monolayers (SAMs) on gold electrodes have been prepared by using small molecules such as ferrocene thiol or disulfide derivatives.7 However, compared with ferrocenegrafted polymers, they often fail to mediate electron transfer between redox centers of enzymes and electrodes because they lack flexibility to contact the enzyme redox centers.7 Thin films of conjugated polymers are expected to combine both properties of the polymers and SAMs of the small molecules, while they overcome instability of polymer films attached to electrodes by physical adsorption. Rigid rod conjugated poly(p-phenylene-ethynylenes) display unique properties, which were used for noncovalent engineering of single-walled carbon nanotube surfaces while keeping the nanotube intrinsic properties such as conductivity and mechanical strength.8 In this paper, we have covalently attached ferrocene to conjugated poly(p-phenylene-ethynylenes) (polymers A-C in Chart 1) via hydrophobic or hydrophilic connection tethers. We have demonstrated the feasibility to control the electrontransfer rate of polymer thin films on gold surfaces by controlling ferrocene tethers and side-chain compositions. Experimental Section Instrumentation. 1H NMR and 13C NMR spectra were recorded on 400 MHz Varian Unity Inova spectrometer in the indicated solvents at the indicated fields. Chemical shifts are expressed in parts per million (δ) using residual solvent peaks as internal standards. Chloroform (δ 7.24 ppm for 1H and 77.00 ppm for 13C) was employed as an internal standard for chloroform(6) (a) Mao, F.; Mano, N.; Heller, A. J. Am. Chem. Soc. 2003, 125, 4951-4957. (b) Calvo, E. J.; Etchenique, R.; Danilowicz, C.; Diaz, L. Anal. Chem. 1996, 68, 4186-4193. (7) Napper, A. M.; Liu, H.; Waldeck, D. H. J. Phys. Chem. B 2001, 105, 7699-7707. (8) (a) Chen, J.; Liu, H.; Weimer, W. A.; Halls, M. D.; Waldeck, D. H.; Walker, G. C. J. Am. Chem. Soc. 2002, 124, 9034-9035. (b) Ramasubramaniam, R.; Chen, J.; Liu, H. Appl. Phys. Lett. 2003, 83, 2928-2930.

10.1021/la050674t CCC: $30.25 © 2005 American Chemical Society Published on Web 07/22/2005

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Chart 1. Chemical Structures of Poly(p-phenylene-ethynylenes)

d. Splitting patterns are expressed as s (singlet), d (doublet), t (triplet), q (quartet), bs (broad singlet), or m (multiplet). UV spectra were taken on a Hewlett-Packard 8452A diode array spectrophotometer. Fluorescence spectra were obtained on a 1681 steady-state Spex Fluorolog fluorometer. Fluorescence quantum yields of the novel polymers were measured in dilute chloroform solution and calculated by using quinine sulfate in 0.1 N sulfuric acid as the reference absolute quantum efficiency (φn ) 55%).9 Molecular weight of the polymers was determined by gel permeation chromatography (GPC) by using a Waters Associates Model 6000A liquid chromatograph. The mobile phase was HPLC grade tetrahydrofuran (THF), which was filtered and degassed by vacuum filtration through a 0.5 µm Fluoropore filter prior to use. The polymers were detected by a Waters Model 440 ultraviolet absorbance detector at a wavelength of 254 nm and a Waters Model 2410 refractive index detector. The polymer solutions were prepared at about 1 mg/mL concentration. Molecular weight was measured relative to polystyrene standards. Scanning tunneling microscopy (STM) images were obtained with a PicoScan STM system (Molecular Imaging). The electrochemical experiments were conducted by using a CH instruments Model 660 electrochemical workstation and a conventional three-electrode (Pt auxiliary, Ag/AgCl reference) cell with a modified gold electrode as a working electrode for electrochemical measurement of polymer thin films. For cyclic voltammetry of polymers in organic solvent, a glassy carbon electrode was used as a working electrode and the supporting electrolyte solution was 0.1 M tetrabutylammonium hexafluorophosphate (Bu4NPF6) in anhydrous CH2Cl2. All experimental solutions were thoroughly deoxygenated by bubbling argon through the solution for at least 10 min. Materials. All reagents and solvents were obtained from Aldrich, Acros, Sigma, Lancaster or Fluka. All solvents were dried by standard procedures. All other chemicals are analytically pure and used without further purification. All reactions and manipulations were conducted under argon atmosphere using Schlenk techniques or in an argon-atmosphere glovebox. 2,5Diiodo-1,4-hydroquinone (1), 2,5-diiodo-1,4-bis[2-(2-hydroxyethoxy)ethoxy]benzene (2a), 2,5-diiodo-1,4-{2-[2-(2-hydroxyethoxy)ethoxy]ethoxy}benzene (2b), 1,4-bis((triethylene glycol monomethyl ether)oxy)-2,5-diethynylbenzene (4), thioacetic acid S-(4-iodophenyl)ester (5), and 2,5-diiodo-1,4-bis(11-hydroxyundecyloxy)benzene (6) were prepared according to reported procedures.3b,10 Compound 3a. To a dried round-bottom flask were added ferrocenecarboxylic acid (2.00 g, 8.69 mmol), CH2Cl2 (20 mL), and oxalyl chloride (3 mL, 34.9 mmol), and then a catalytic amount of dimethylformamide (DMF). The mixture was stirred at room (9) Demasa, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991-1024. (10) (a) Kim, J.; Swagger, T. M. Nature 2001, 411, 1030-1034. (b) Zhou, Q.; Swager, T. M. J. Am. Chem. Soc. 1995, 117, 12593-12602. (c) Gryko, D. T.; Clausen, C.; Roth, K. M.; Dontha, N.; Bocian, D. F.; Kuhr, W. G.; Lindsey, J. S. J. Org. Chem. 2000, 65, 7345-7355. (d) Breen, C. A.; Deng, T.; Breiner, T.; Thomas, E. L.; Swager, T. M. J. Am. Chem. Soc. 2003, 125, 9942-9943.

temperature for 6 h. The solvent and excess oxalyl chloride were removed to give ferrocenecarbonyl chloride as a brown solid. Without further purification, ferrocenecarbonyl chloride was dissolved in the dried THF (20 mL) and added dropwise to a dried THF solution (20 mL) containing 2a (2.20 g, 4.09 mmol) and pyridine (1.0 mL, 12.4 mmol). The reaction mixture was stirred at room temperature for 12 h. When the solvent was removed, the residue was diluted with ethyl acetate (50 mL), washed with 10% NaHCO3 (30 mL × 3) and brine (30 mL × 3), and dried over anhydrous MgSO4. The solvent was evaporated, and the crude compound was purified by column chromatography on silica gel with CH2Cl2/EtOAc (10:1, v/v) to give the target compound 3a (3.41 g, 86.5% yield) as a yellow solid. 1H NMR (400 MHz, CDCl3): δ 7.21 (s, 2H), 4.80 (m, 4H), 4.36 (m, 8H), 4.18 (m, 6H), 4.10 (m, 4H), 3.90 (m, 12H). 13C NMR (400 MHz, CDCl3): δ 63.60, 69.74, 70.09, 70.18, 70.50, 70.58, 71.17, 71.61, 86.71, 123.69, 153.33, 171.88. EIHRMS (m/z): M+ calcd for C36H36Fe2I2O8, 961.9199; found, 961.9199. Compound 3b. To a dried round-bottom flask were added ferrocenecarboxylic acid (2.00 g, 8.69 mmol), CH2Cl2 (20 mL), and oxalyl chloride (3 mL, 34.9 mmol) and then a catalytic amount of DMF. The mixture was stirred at room temperature for 6 h. The solvent and excess oxalyl chloride were removed to give the ferrocenecarbonyl chloride as a brown solid. Without further purification, ferrocenecarbonyl chloride was dissolved in the dried THF (20 mL) and added dropwise to the solution of dried THF (20 mL) containing 2b (2.60 g, 4.15 mmol) and pyridine (1.0 mL, 12.4 mmol). The reaction mixture was stirred at room temperature for 12 h. After the solvent was removed, the residue was diluted with ethyl acetate (50 mL), washed with 10% NaHCO3 (30 mL × 3) and brine (30 mL × 3), and dried over anhydrous MgSO4. The solvent was evaporated, and the crude compound was purified by column chromatography on silica gel with CH2Cl2/ EtOAc (10:1, v/v) to give the target compound 3b (3.91 g, 89.5% yield) as a yellow solid. 1H NMR (400 MHz, CDCl3): δ 7.19 (s, 2H), 4.80 (m, 4H), 4.37 (m, 8H), 4.18 (m, 10H), 4.06 (t, J ) 4.0 Hz, 4H), 3.86 (m, 4H), 3.79 (m, 8H), 3.73 (m, 4H). 13C NMR (400 MHz, CDCl3): δ 62.93, 69.17, 69.29, 69.47, 69.85, 69.91, 70.40, 70.58, 70.82, 70.97, 86.05, 123.04, 152.70, 171.26. EIHRMS (m/ z): M+ calcd for C40H44Fe2I2O10, 1049.9723; found, 1049.9721. Compound 7. To a dried round-bottom flask were added ferrocenecarboxylic acid (1.50 g, 6.52 mmol), CH2Cl2 (20 mL), and oxalyl chloride (3 mL, 34.9 mmol) and then a catalytic amount of DMF. The mixture was stirred at room temperature for 6 h. The solvent and excess oxalyl chloride were removed to give the ferrocenecarbonyl chloride as a brown solid. Without further purification, ferrocenecarbonyl chloride was dissolved in the dried THF (20 mL) and added dropwise to a dried THF solution (20 mL) containing 6 (2.30 g, 3.27 mmol) and pyridine (1.0 mL, 12.4 mmol). The reaction mixture was stirred at room temperature for 12 h. After the solvent was removed, the residue was diluted with ethyl acetate (50 mL), washed with 10% NaHCO3 (30 mL × 3) and brine (30 mL × 3), and dried over anhydrous MgSO4. The solvent was evaporated, and the crude compound was purified by column chromatography on silica gel with hexane/EtOAc (10:

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Scheme 1. Synthesis of Monomers and PPEs

1, v/v) to give the target compound 7 (3.31 g, 89.4% yield) as a brown oil. 1H NMR (400 MHz, CDCl3): δ 7.15 (s, 2H), 4.84 (m, 4H), 4.41 (m, 4H), 4.23 (m, 10H), 4.17 (t, J ) 4.0 Hz, 4H), 3.90 (t, J ) 4.0 Hz, 4H), 1.77 (m, 4H), 1.69 (m, 4H), 1.32 ∼ 1.48 (m, 28H). 13C NMR (400 MHz, CDCl3): δ 26.24, 26.29, 29.15, 29.35, 29.47, 29.52, 29.72, 29.78, 64.48, 69.94, 70.33, 70.52, 71.41, 71.76, 86.58, 122.96, 153.06, 171.88. EIHRMS (m/z): M+ calcd for C50H64Fe2I2O6, 1126.1491; found, 1126.1494. 1,4-Diethynyl-2,5-didecyloxybenzene (8). Compound 8 was prepared according to a reported procedure.3b 1H NMR (300 MHz, CDCl3): δ 6.96 (s, 2H), 3.98 (t, J ) 6.6 Hz, 4H), 3.34 (s, 2H), 1.85-1.76 (m, 4H), 1.49 (m, 4H), 1.32 (br, 22H) and 0.89 (t, J ) 6.73 Hz, 6H). 13C NMR (300 MHz, CDCl3): δ 153.96, 117.71, 113.23, 82.37, 79.77, 69.64, 31.90, 29.54, 29.31, 29.11, 25.88, 22.67 and 14.10. EIHRMS (m/z): M+ calcd for C30H46O2, 438.3498; found, 438.3497. Polymer A. To a round-bottom flask were added monomer 3a (0.30 g, 0.31 mmol), 1,4-bis((triethylene glycol monomethyl ether)oxy)-2,5-diethynylbenzene (4) (0.15 g, 0.33 mmol), Pd(PPh3)2Cl2 (0.02 g, 0.03 mmol), CuI (0.01 g, 0.05 mmol), and PPh3 (0.02 g, 0.08 mmol). The flask was evacuated and backfilled with nitrogen, which was repeated three times. THF (20 mL) and diisopropylamine (10 mL) were added to the flask. After the mixture was stirred at 60 °C for 24 h, thioacetic acid S-(4-iodophenyl)ester (5) (0.01 g, 0.03 mmol) was added, and the mixture was stirred for another 3 h. The solvent was removed, and the residual was dissolved in CH2Cl2, washed with saturated NH4Cl solution and NaCl solution, and dried over anhydrous MgSO4. The solvent was evaporated, and polymer A was obtained by recrystallization from CH2Cl2/hexane and dried under vacuum at room temperature. 1H NMR (400 MHz, DMSO-d6): δ 7.62 (s, end group), 7.43 (s, end group), 7.15 (s, 4H), 4.67 (s, 4H), 4.54 (m, 4H), 4.39 (s, 4H), 4.16-4.27 (m, 18H), 3.78-3.84 (m, 8H), 3.63 (s, 4H), 3.30-3.50 (m, 16H), 3.18 (s, 6H), 2.41 (s, end group). It displays maximum absorption and emission peaks at 420 and 463 nm, respectively. It is readily soluble in common solvents such as THF, acetone, chloroform, methylene chloride, DMF, and dimethyl sulfoxide (DMSO). Polymer B. Polymer B was prepared in a procedure similar to that of polymer A by polymerizing monomer 3b, 4, and endcapping agent 5 in the presence of CuI, Pd(PPh3)2Cl2, and PPh3. 1H NMR (400 MHz, DMSO-d ): δ 7.61 (s, end group), 7.42 (s, end 6 group), 7.13 (s, 4H), 4.70 (s, 4H), 4.52-4.55 (m, 4H), 4.41 (s, 4H), 4.18-4.21 (m, 18H), 3.78 (s, 4H), 3.60-3.66 (m, 8H), 3.30-3.50 (m, 24H), 3.18 (s, 6H), 2.41 (s, end group). It displays maximum absorption and emission peaks at 422 and 460 nm, respectively. It displays similar solubility to polymer A. Polymer C. To a round-bottom flask were added monomer 7 (0.50 g, 0.44 mmol, 1.0 equiv), monomer 8 (0.20 g, 0.45 mmol, 1.1 equiv), Pd(PPh3)2Cl2 (0.02 g, 0.03 mmol), CuI (0.01 g, 0.05 mmol), and PPh3 (0.02 g, 0.08 mmol). The flask was evacuated and backfilled with nitrogen. THF (20 mL) and diisopropylamine (10 mL) were added to the flask. After the mixture was stirred at

60 °C for 24 h, 5 (0.01 g, 0.05 mmol) was added, and the mixture was stirred for another 3 h. The mixture was poured into a large amount of ethanol (500 mL) to precipitate polymer C. Polymer C was obtained by filtering, washing with ethanol and acetone, and drying under vacuum at room temperature. 1H NMR (400 MHz, CDCl3): δ 7.50 (s, end group), 7.35 (s, end group), 6.99 (s, 4H), 4.77 (s, 4H), 4.34 (s, 4H), 4.16 (s, 10H), 4.01 (br,12H), 2,43 (s, end group), 1.83 (br, 8H), 1.68 (br, 4H), 1.53 (br, 24H), 1.231.35 (m, 40H), 0.85 (t, 6H). It displays maximum absorption and emission peaks at 438 and 472 nm, respectively. It is readily soluble in common solvents such as toluene, THF, chloroform, and methylene chloride. Preparation of Polymer Thin Films. A gold wire (99.99%, 0.5 mm diameter) was cleaned in concentrated nitric acid under reflux for 12 h. Subsequently it was thoroughly washed with ultrapure water (18 MΩ). The tip of the wire was melted using a H2 gas flame to make a smooth, approximately spherical surface on the electrode, and then quenched with ultrapure water. The wire was sealed into a glass capillary with the flame, and the melted tip was annealed again and cooled under argon flow. A gold ball electrode (ca. 1.5 mm diameter) modified with polymer was used as a working electrode for electrochemistry of polymer thin films. The polymer thin films were prepared by incubating gold electrodes in a THF solution containing 1.0 mM polymer for 24 h, sonicating them in THF for 3 min, and washing the electrodes with pure THF solution to remove the polymer through physical adsorption. Preparation of Samples for STM Images. The Au(111) single-crystal disk (1 cm in diameter and 2 mm thick from Monocrystals Co., Ohio) was cleaned by incubation in hot piranha solution (1:3 H2O2 and H2SO4 for 1 h) and incubation in hot HNO3 for 30 min. (Caution! Since the piranha solution is a very strong oxidizing reagent and extremely dangerous to handle, gloves and goggles should be used to handle piranha at all times.) After each step the sample was rinsed by ultrasonication in ultrapure water. The crystal was hydrogen flame annealed and allowed to cool to room temperature in air. The crystal was incubated in THF solution containing 1.0 mM polymer for 24 h, sonicated in THF solution for 3 min, and washed with THF.

Results and Discussion Synthesis of Ferrocene-Grafted Poly(p-phenyleneethynylenes). Poly(p-phenylene-ethynylenes) (PPEs) bearing ferrocene groups were synthesized by a Sonogashira coupling polymerization of a diiodobenzene (3a, 3b, or 7) with a diethynyl benzene derivative (4 or 8) according to Scheme 1. To prepare thin films of PPEs at gold electrodes, 5 was added to ensure that the polymers have well-defined, protected thiol polymer backbone end groups. To allow for covalent attachment of ferrocene to the PPE backbone via a methylene or ethylene oxide

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Figure 2. Cyclic voltammogram of polymer C (1.0 mM) in methylene chloride containing 0.1 M Bu4NPF6 at scan rate of 100 mVs and room temperature. Figure 1. UV-visible absorption and fluorescence spectra of polymers in CHCl3 solution.

tether, we used a synthesis strategy based on the use of intermediates (2a, 2b, and 6), which were prepared by the alkylation of 2,5-diiodohydroquinone (1) with 2-(2chloroethoxy)ethanol, 2-[2-(2-chloroethoxy)ethoxy]ethanol, and 11-bromo-1-undecanol in K2CO3/DMF solution, respectively (Scheme 1). Monomers 3a, 3b, and 7 were obtained by coupling the intermediates 2a, 2b, and 6 with ferrocenecarbonyl chloride in THF solution in the presence of pyridine, respectively (Scheme 1). Polymer C was prepared by polymerizing monomer 7 (1.0 equiv), monomer 8 (1.1 equiv), and compound 5 (0.1 equiv) via Sonogashira reaction in the presence of 5% Pd(PPh3)2Cl2, 10% PPh3 and 5% CuI in THF and diisopropylamine at 60 °C for 2 days (Scheme 1). Compound 5 (0.3 equiv) was added to the mixture, which reacted for another 3 h. It was precipitated in a large amount of ethanol, filtered, and dried under vacuum at room temperature. Gel permeation chromatography analysis (mobile phase: THF, polystyrene standards) indicates that Mn of polymers C is 17 300 and its polydispersity is 1.66. It is readily soluble in common solvents such as THF, chloroform, and methylene chloride. Polymers A and B were prepared in a way similar to polymer C. Polymers A and B are soluble in common solvents such as THF, chloroform, methylene chloride, acetone, DMF, and DMSO. Gel permeation chromatography analysis (mobile phase: THF, polystyrene standards) indicates that Mn of polymers A, and B are 34 020 and 27 000 and their polydispersities are 2.29 and 1.93, respectively. UV-Visible and Fluorescence and Electrochemistry of Polymer Solutions. Polymer C displays maximum UV-visible absorption at 438 nm and maximum emission peaks at 472 nm. (Figure 1), which were ascribed to the π-π* transition of the conjugated polymer backbone. It also shows a shoulder peak at 510 nm, which is due to excimer formation.11 Ferrocene in chloroform displays no fluorescence but shows a maximum UV-visible absorption peak at 450 nm, which overlaps with the conjugated polymer absorption. However, fluorescence quantum yield of the polymer C is 10%, which is caused by electrontransfer quenching with tethered ferrocene groups as electron donors. We reported that PPE-bearing bromoalkyl groups shows high fluorescence quantum yield of 62%.5b PPE-bearing ferrocene groups obtained by a quaterniza(11) Li, H.; Powell, D. R.; Hayashi, R. K.; West, R. Macromolecules 1998, 31, 52-58.

tion reaction of the bromoalkyl groups with (dimethylaminomethyl)ferrocene exhibits low fluorescence quantum yield of 11%,5b which further confirms that electrontransfer rate of ferrocene groups as electron donors is faster than excited lifetime of the polymer backbone π-π* transition. Polymer A shows maximum absorption peak at 420 nm, maximum emission peak at 460 nm, and an emission shoulder peak at 490 nm. Polymer B exhibits maximum absorption peak at 422 nm, maximum emission peak at 463 nm and an emission shoulder peak at 500 nm (Figure 1). Polymers A and B also display low fluorescence quantum yields (8.4 and 9.2%, respectively), resulting from electron-transfer quenching with tethered ferrocene groups as electron donors. The redox properties of ferrocene-grafted PPEs were characterized by cyclic voltammetry using methylene chloride solutions containing 1.0 mM polymer and 0.1 M Bu4NPF6. Figure 2 shows the cyclic voltammogram of polymer C, recorded during potential cycling between -0.1 and +1.2 V. The scan shows two pairs of redox waves: the first fully reversible wave peak at 0.55 V, corresponding to the oxidation of the ferrocene groups, and the second irreversible wave peak at 1.02 V, related to the oxidation/ reduction (p-doping/dedoping) of the polymer backbone. Polymers A and B display electrochemical behavior similar to polymer C in methylene chloride solutions with 0.1 M Bu4NPF6 as a supporting electrolyte. STM Images of Polymer Thin Films. Thin films of the polymers on a single gold crystal or electrodes were prepared by immersing in THF solutions of the polymers (ca. 5 mM) for 24 h, sonicating in THF for 3 min, and then washing with THF to remove physically adsorbed polymers before conducting electrochemical or STM experiments. It is well-known that thioacetyl-terminated oligo(phenylene-ethynylenes) form self-assembled monolayers on gold surfaces via a sulfur-gold covalent bond.12 Polymers with two thioacetyl-terminated groups are expected to lie flat on a gold surface through both sulfurs. This hypothesis is confirmed by STM images and electrochemical data. Figure 3 shows the images in the absence and presence of thin films of polymer A on a Au(111) surface. The cross-section shows single-crystal gold (111) has a very flat and smooth surface with an interface step of about 2 Å (Figure 3A), the lines indicated by arrows are the characterized reconstruction line of Au(111), and it is about 0.1-0.2 Å higher than the unreconstructed area. (12) (a) Fan, F. F.; Yang, J.; Dirk, S. M.; Price, D. W.; Kosynkin, D.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 2454-2455. (b) Cai, L.; Yao, Y.; Yang, J.; Price, D. W., Jr.; Tour, J. M. Chem. Mater. 2002, 14, 2905-2909.

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Figure 3. STM images of Au(111) without and with thin films of polymer A. (A) (300 × 300 nm2), an image of Au (111) without thin films of polymer A was obtained at a bias of 0.1 V and a set point of 0.1 nA. (B) (150 × 150 nm2), an image of Au(111) modified with thin films of polymer A was obtained at a bias of 0.2 V and a set point of 0.1 nA. (C) (330 × 330 nm2), an image of Au (111) modified with thin films of polymer A was obtained at a bias of 0.1 V and a set point of 0.1 nA.

The Au(111) with polymer A thin film (Figure 3B,C) show a rough surface, on which there are some holes with nanometer scale in diameter. This nanometer scale hole is a typical characteristic of thiol self-assembly on Au(111).13 The cross-section shows that the polymer A film is about 1.5-3.5 Å thick, which indicates that polymer A lies flat on the gold surface. Otherwise the polymer film should be much thicker if the polymer stood up through one sulfur.

Electrochemistry of the Polymer Thin Films. We did not observe any electrochemical responses of polymers when glassy carbon electrodes were used to prepare thin (13) (a) Gorman, C. B.; He, Y.; Carroll, R. L. Langmuir 2001, 17, 5324-5328. (b) He, Y.; Ye, T.; Borguet, E. J. Phys. Chem. B 2002, 106, 11264-11271. (c) Tivanski, A. V.; He, Y.; Borguet, E.; Liu, H.; Walker, G. C.; Waldeck, D. H. J. Phys. Chem. B 2005, 109, 5398-5402. (d) Wei, J.; Liu, H.; Dick, A. R.; Yamamoto, H.; He, Y.; Waldeck, D. H. J. Am. Chem. Soc. 2002, 124, 9591-9599.

Ferrocene-Grafted Poly(p-phenylene-ethynylenes)

Langmuir, Vol. 21, No. 17, 2005 7865

Figure 4. Cyclic voltammograms of thin films of polymer A (A) and polymer C (B) at gold electrodes in 0.1 M HClO4 solution. Scan rates of Figure 4A from the inner to the outer are 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mV/s. Scan rates of Figure 4B from the inner to the outer are 15, 35, 55, 75, and 95 mV/s.

films of polymers instead of gold electrodes since physically adsorbed polymers on glassy carbon electrodes are easily washed away by THF, even without sonication. Figure 4 shows cyclic voltammetric responses of thin films of polymers A and C on gold electrodes, respectively. Figure 4A shows that thin films of polymer A exhibit the classical features of a kinetically fast redox couple strongly bound to an electrode surface since its current is proportional to scan rate. At a scan rate of 500 mV/s, the potential peak-to-peak separation of thin films of polymer A was less than 23 mV, demonstrating that charge transfer and counterion movement through the film, as well as charge transfer from the film to the electrode, were very fast. Thin films of polymer A at a scan rate of 1000 mV/s display the peak width at half-height of both oxidation and reduction wave (92.4 mV), which is close to the ideal value of 90.6 mV expected for Nernstian behavior of an ensemble of identical, independent sites at room temperature.14 Using Laviron’s formalism, the electron-transfer rate (Ks) of immobilized ferrocenes can be estimated from the magnitude of the peak separation and the dependence of the peak potential on the scan rate.15 Electron transfer of ferrocene from the thin film of polymer A was calculated as 31.69 ( 4.1 Hz for eight determinations when the scan rates were from 100 to 900 mV/s. Surface coverage (Γ) of redox active centers on the surface is obtained as (1.5 ( 0.15) × 10-10 mol/cm2 by integrating the charge passed on reduction of thin film of polymer A, which further confirms that polymer A lies flat on the gold surface. A thin film of polymer B displayed features similar to those of polymer A (Figure 5). Thin film of polymer B, for which the ethylene oxide side chain is longer than polymer A by one ethylene oxide, displayed features similar to those of polymer A (Figure 5). The electron-transfer rate of the thin film of polymer B was 26.64 ( 3.5 Hz for seven determinations at the same scan rates, a little slower than that of polymer A. In contrast, thin films of polymer C exhibit asymmetric surface waves and a kinetically slow redox couple strongly bound to the gold surface since its peak-to-peak potential separation was 129 mV even at a slower scan rate of 95 mV/s (Figure 3B). The electron-transfer rate of ferrocene from the thin film of polymer C was 0.37 ( 0.05 Hz for seven determinations, which is significantly smaller than those of polymers A and B. An increasing hydrophobic environment by alkyl side chains of polymer C causes poor solvation of the polymer thin films, results in irreversible electrochemical behavior of the polymer thin

Figure 5. Cyclic voltammograms of thin film of polymer B at gold electrodes. Scan rates from the inner to the outer scans are 100, 200, 300, 400, 500, 600, 700, 800, 900, and 1000 mV/s.

films, and hinders ferrocene oxidation.16 Ferrocene moieties of thin films of polymer C with hydrophobic alkyl polymer side chains display higher formal potential (Eo ) 536 mV) compared with that (Eo ) 384 mV) of polymer A with hydrophilic ethylene oxide side chains because of electron-donating effect from ethylene oxide side chains, which stabilizes the ferrocenium ion and leads to a cathodic shift of the redox wave.17 The thin films derived from polymers A-C were remarkably stable, displaying almost the same current response even after more than 100 potential scan cycles. More impressive, these thin films retained their stability over the course of a month. In summary, very stable electrochemically active thin films of poly(p-phenylene-ethynylenes) have been prepared and the electron-transfer rate of ferrocene can be greatly enhanced by covalently attaching it to a polymer backbone via an ethylene oxide tether. Acknowledgment. We acknowledge the Research Excellence Fund of Michigan Technological University, Zyvex Corp., NSF, DARPA, NASA (Grant No. NNJ04JA18C) for kind support of this work. We would like to thank reviewers for their valuable comments. LA050674T (14) Bard, A. J.; Faulkner, L. R. Electrochemical Methods; John Wiley & Sons: New York, 1980. (15) Laviron, E. J. Electroanal. Chem. 1979, 101, 19-28. (16) Rowe, G. K.; Creager, S. E. Langmuir 1991, 7, 2307-2312. (17) (a) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 55005507. (b) Beulen, M. W.; Veggel, F. C. J. M.; Reinhoudt, D. N. Chem. Commun. 1999, 503-504.