pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrochemical Tuning of the Optoelectronic Properties of a Fluorene-Based Conjugated Polymer Shinsuke Inagi,* Kazuya Koseki, Shotaro Hayashi, and Toshio Fuchigami* Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan Received October 12, 2010. Revised Manuscript Received November 16, 2010 The electrochemical reduction of a fluorene-based conjugated polymer, poly(9-fluorenone-alt-9,9-dioctylfluorene), was investigated for the first time. The carbonyl group in the fluorenone unit was selectively and quantitatively converted to the methylene group, as determined by 1H NMR, IR, and energy-dispersive X-ray (EDX) analysis. The optical and electrochemical properties of the polymers were studied by UV-vis, photoluminescence (PL), and cyclic voltammetry (CV) measurements and were found to be tunable by varying the reduction level.
Introduction Postfunctionalization of conjugated polymers has drawn much attention because of its potential advantages for creating versatile functional materials stemming from a single parent polymer.1 In general, such postfunctionalization methods aim for complete reactions that generate a fully converted form of the precursor polymer. However, the precursor polymers themselves are often useful materials. Thus, partially converted polymers possessing properties intermediate between those of the precursor and the final form may offer important advantages if the conversion can be tightly controlled. One successful example of tuning thermal and optoelectronic properties via a click-type reaction has recently been reported.2 Such reactions make the tuning of the percent conversion and polymer properties possible by using the proper amount of the reactant. However, there are limitations to the polymers and reactions that may be used in this method. Electroorganic synthesis, which uses electrochemically generated reactive species for organic reactions, is an advanced synthesis method.3 The ability to choose a potential or current to effect oxidation or reduction is what makes the methodology attractive. Furthermore, the supply of charge is readily controllable by simple on-off switching. With these advantages in mind, we recently reported a novel polymer reaction method based on the concept of electroorganic synthesis that involved the electrochemical doping and subsequent reaction of a film.4 Selective *Corresponding authors. Tel: þ81-45-924-5427. Fax: þ81-45-924-5427. E-mail:
[email protected] (S.I.),
[email protected] (T.F.). (1) (a) Tolosa, J.; Kub, C.; Bunz, U. H. F. Angew. Chem., Int. Ed. 2009, 48, 4610–4612. (b) Li, H.; J€akle, F. Angew. Chem., Int. Ed. 2009, 48, 2313–2316. (c) Li, H.; Sundararaman, A.; Venkatasubbaiah, K.; J€akle, F. J. Am. Chem. Soc. 2007, 129, 5792–5793. (d) Taylor, M. S.; Swager, T. M. Angew. Chem., Int. Ed. 2007, 46, 8480– 8483. (e) Li, Y.; Vamvounis, G.; Holdcroft, S. Macromolecules 2002, 35, 6900–6906. (2) Michinobu, T. J. Am. Chem. Soc. 2008, 130, 14074–14075. (3) For books and recent reviews in this area, see the following: (a) Organic Electrochemistry; Sch€afer, H. J., Ed.; Encyclopedia of Electrochemistry; Wiley-VCH: Weinheim, Germany, 2003; Vol. 8. (b) Organic Electrochemistry, 4th ed.; Lund, H., Hammerich, O., Eds.; Marcel Dekker: New York, 2001. (c) Sperry, J. B.; Wright, D. J. Chem. Soc. Rev. 2006, 35, 605–621. (d) Yoshida, J.; Kataoka, K.; Horcajada, R.; Nagaki, A. Chem. Rev. 2008, 108, 2265–2299. (4) (a) Inagi, S.; Hayashi, S.; Fuchigami, T. Chem. Commun. 2009, 1718–1720. (b) Hayashi, S.; Inagi, S.; Fuchigami, T. Macromolecules 2009, 42, 3755–3760. (c) Inagi, S.; Hayashi, S.; Hosaka, K.; Fuchigami, T. Macromolecules 2009, 42, 3881– 3883. (d) Hayashi, S.; Inagi, S.; Hosaka, K.; Fuchigami, T. Synth. Met. 2009, 159, 1792–1795. (e) Inagi, S.; Hosaka, K.; Hayashi, S.; Fuchigami, T. J. Electrochem. Soc. 2010, 157, E88–E91. (f) Hayashi, S.; Inagi, S.; Fuchigami, T. Polym. J. 2010, 42, 772– 775.
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quantitative conversion was accomplished not only at the surface but also within the bulk of the polymer even in this solid-phase reaction. In the current work, we carried out the electrochemical reduction of poly(9-fluorenone-alt-9,9-dioctylfluorene) (P1). The fluorenone unit in conjugated polymers serves as an electron-withdrawing moiety, imparting specific properties5,6 such as intramolecular charge transfer (ICT) capability5a-c and n-type doping characteristics.6 The reduction of a ketone to an alcohol or a methylene group would cause electron injection into the main polymer chain; consequently, the properties of the polymer would vary with the reduction level. We report here the tuning of the optoelectronic properties of this conjugated polymer by our electrochemical method.
Experimental Section General Information. NMR spectra were recorded on a JEOL EX-270 spectrometer. UV-vis spectra were obtained on a Shimadzu UV-1800 spectrophotometer. Photoluminescence (PL) spectra were obtained on a Jasco FP-6500 spectrophotomerter. GPC analyses were performed on a Shimadzu Prominence GPC system (Shim-pack GPC 803C column) using chloroform as the eluent after calibration with a polystyrene standard. IR spectra were obtained on a Shimadzu FTIR-8100A. Cyclic voltammetry measurements were recorded on an ALS 600A electrochemical analyzer. Preparative electrolysis experiments were carried out with a constant current power supply (model 5944, Metronnix Corp.) by monitoring the electricity with a coulomb/amoperehour meter (model HF-201, Hokutodenko). EDX analysis was performed with a Genesis XM2 (Keyence). The film thickness was estimated with a laser focus displacement meter (model LT-8100, Keyence). The quantum yield of fluorescence of the polymers in a dilute chloroform solution (10-6 M) was determined on excitation of the polymer at its λabs max in comparison with the emission of quinine sulfate dehydrate/ 0.1 M sulfuric acid solution (Φfl = 0.55) as a standard. (5) (a) Dias, F. B.; Maiti, M.; Hintschich, S. I.; Monkman, A. P. J. Chem. Phys. 2005, 122, 054904. (b) Zojer, E.; Pogantsch, A.; Hennebicq, E.; Beljonne, D.; Bredas, J. L.; de Freitas, P. S.; Scherf, U.; List, E. J. W. J. Chem. Phys. 2002, 117, 6794. (c) Grisorio, R.; Piliego, C.; Striccoli, M.; Cosma, P.; Fini, P.; Gigli, G.; Mastrorilli, P.; Suranna, G. P.; Nobile, C. F. J. Phys. Chem. C 2008, 112, 20076–20087. (d) Kulkarni, A. P.; Kong, X.; Jenekhe, S. A. J. Phys. Chem. B 2004, 108, 8689–8701. (6) (a) Uckert, F.; Setayesh, S.; M€ullen, K. Macromolecules 1999, 32, 4519–4524. (b) Loganathan, K.; Pickup, P. G. Electrochim. Acta 2007, 52, 4685–4690. (c) Demadrille, R.; Delbosc, N.; Kervella, Y.; Firon, M.; De Bettignies, R.; Billon, M.; Rannou, P.; Pron, A. J. Mater. Chem. 2007, 17, 4661–4669.
Published on Web 11/23/2010
DOI: 10.1021/la104099q
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Letter
Inagi et al.
Materials. Dry solvents were used as received. 2,7-Dibromofluoreneone, 9,9-dioctylfluorene-2,7-diboronic acid ester, and Pd(PPh3)4 were used as received. Supporting electrolytes tetraethylammonium p-toluenesulfonate (Et4NOTs) and tetraethylammonium tetrafluoroborate (Et4NBF4) were used as received. Poly(9-fluorenone-alt-9,9-dioctylfluorene) (P1) was prepared according to the literature.4b Poly(fluorene-alt-dioctylfluorene) (P2) was independently prepared by Suzuki-Miyaura coupling polymerization according to the literature (Mw = 3800, Mn = 2900, and Mw/Mn=1.30).4f General Procedure for the Cathodic Reduction of P1 by Constant-Current Electrolysis. A chloroform solution containing 2 mg of P1 (Mw = 6900, Mn = 4000, and Mw/Mn = 1.7) was cast on a zinc plate electrode (2 cm 2 cm) and dried under reduced pressure. An electrolytic solution of 0.1 M Et4NOTs/2-propanol (10 mL) was added to an undivided cell and deaerated by bubbling argon. The polymer-coated zinc cathode was paired with a bare platinum plate electrode (2 cm 2 cm) as the anode. A constant current (10 mA/cm2) was passed at room temperature from 1 to 16 F/mol, after which the polymer film was washed with methanol and dried in vacuo. Spectroscopic data for P2 (16 F/mol) are given as follows. 1H NMR (270 MHz, CDCl3): δ 7.9-7.4 (Ar-H, br), 4.1 (Ar-CH2-Ar, br), 2.1 (CH2(CH2)6CH3, br), 1.3-0.6 (alkyl, br). IR (KBr): 2926, 2853, 1460, 1250, 845, 814, 758 cm-1. GPC data for P2: Mw = 7800, Mn = 4100, and Mw/Mn = 1.9.
Figure 1. Degree of the conversion of ketone to methylene in P1 vs charge passed.
Results and Discussion Poly(9-fluorenone-alt-9,9-dioctylfluorene) (P1) was prepared by the Suzuki-Miyaura coupling polymerization of 2,7-dibromofluorenone with 9,9-dioctylfluorene-2,7-diboronic acid ester.4b A film of P1 (ca. 5-10 μm) on a zinc (Zn) cathode was then incorporated into an electrochemical polymer reaction system equipped with a platinum (Pt) anode in 0.1 M Et4NOTs/ 2-propanol in an undivided cell in which a constant-current cathodic reduction (10 mA/cm2) was carried out at room temperature. After the passage of a fixed quantity of charge (1, 4, 8, or 16 F/mol), the polymer film on the electrode was rinsed with methanol and dried under reduced pressure. The reaction involved electron injection from the Zn cathode into the conducting P1 film, resulting in the formation of its reduced form in the presence of an appropriate proton source.7 1H NMR analysis of the polymers dissolved in CDCl3 revealed that the electrochemical reduction of the 9-fluorenone unit exclusively produced fluorene units (Figure S1 in Supporting Information). Even in early stages of the reaction (4 F/mol), signals from the protons of the bridging methylene group (4.1 ppm) were observed but none were observed from hydroxylated bridging moieties, suggesting that the reduction proceeded directly from ketone to methylene. Energy-dispersive X-ray analysis of P2 (16 F/mol) also showed complete reduction (Figure S2). The missing oxygen signal at 0.05 keV in P2 indicated the disappearance of the carbonyl group. In the IR spectrum of P2, the carbonyl stretching band at 1720 cm-1 was nearly absent and no hydroxyl stretching band was observed (Figure S3). Although there have been several reports on the electrochemical reduction of ketone to methylene in small molecules,8,9 such a complete reaction has never before been reported in macromolecules. The degree of conversion was (7) Without the passage of charge, the reduction of P1 on the zinc plate did not occur in the reaction system. (8) (a) Islam, N.; Sopher, D. W.; Utley, J. H. P. Tetrahedron 1987, 43, 2741– 2748. (b) Comninellis, Ch.; Plattner, E. J. Appl. Electrochem. 1985, 15, 771–773. (9) The constant-current reduction of 9-fluorenone in solution using a Zn cathode under conditions similar to those in this study resulted in the recovery of the starting compound without its reduced form even after the passage of 10 F/ mol of charge.
18632 DOI: 10.1021/la104099q
Figure 2. UV-vis absorption spectra of 0.1 mg/L P1 in CHCl3, as-prepared (black curve) and after the passage of different amounts of charge (colored curves in legend).
determined by comparing the integral intensities of the methylene protons of the octyl group (at 2.0 ppm) and the protons at the 1 and 8 positions of 9-fluorenone (at 8.1 ppm).10 The plot of conversion versus charge passed indicated that the extent of reaction could be electrically controlled (Figure 1). The cathodic reduction of the fluorenone unit may compete with the reduction of the proton (hydrogen evolution), which results in a relatively low current efficiency. The GPC molecular weights of P1 (Mw = 6900, Mn = 4000, and Mw/Mn = 1.7) and P2 (Mw = 7800, Mn = 4100, and Mw/Mn = 1.9) before and after electrochemical reduction (16 F/mol), respectively, were similar, showing unimodal profiles. This result suggested that neither decomposition nor chain extension of the polymer occurred even after excess charge was passed. After each reaction level, UV-vis absorption and photoluminescence (PL) measurements were carried out on the resulting polymers in chloroform solution. In Figure 2, a broad absorption (10) The signal for methylene protons at 4.1 ppm tends to be weak, as can be seen in the spectrum of P2 independently prepared via Suzuki-Miyaura coupling polymerization (Figure S5). Therefore, it is suitable to compare the integral intensities of the methylene protons of the octyl group (at 2.0 ppm) and the protons at the 1 and 8 positions of 9-fluorenone (at 8.1 ppm).
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Letter
Figure 4. Cyclic voltammograms of P1 before and after electrolFigure 3. PL spectra of 0.1 mg/L P1 in CHCl3, as-prepared (black curve) and after the passage of different amounts of charge (colored curves in legend). (Inset) Photograph of all solutions irradiated with UV light (365 nm).
band arising from the 9-fluorenone unit of P1 was observed at around 420-550 nm.11 As the reduction of this unit proceeded, the intensity of the broad band gradually decreased,12 accompanied by a drastic color change of the solution from orange to light yellow. The presence of an isosbestic point at 418 nm in the UV-vis spectra indicated the absence of any undesired side reactions. Interestingly, PL behavior in CHCl3 also changed from a lack of emission by P1 (Φfl < 0.001)4b to strong blue emission by P2 (Φfl = 0.38) and was dependent on the reaction level (Figure 3, see also inset picture). The foregoing results strongly suggested that the optical properties of the fluorene-based polymer were tunable by this electrochemical method. Conjugated polymers containing the electron-deficient fluorenone unit are known to show n-type doping characteristics.6 Thus, any structural change effected by electrochemical reduction would affect the reduction potential of the polymer. To survey the electrochemical properties of the polymers generated in this study, cyclic voltammetry (CV) measurements were carried out in 0.1 M Et4NBF4/acetonitrile solution before and after electrolysis (Figure 4). Polymer films after electrochemical reduction (4 and 16 F/mol) were redissolved in CHCl3, and the solution was coated (11) The origin of the low-energy band is proposed to be an intramolecular charge transfer π-π* transition (in refs 5a and 5b) or an n-π* transition (in refs 5c and 6a). (12) The remaining slight absorption at around 420-550 nm after the passage of 16 F/mol of charge indicates not an incomplete reduction in P2 but an intrinsic absorption band of the completely reduced form of P2 as evidenced by comparison with the spectrum of independently prepared P2 (Figure S4). The slight difference in absorption behavior shown in Figure S4 is due to the difference in the molecular weight and the polydispersity index.
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ysis, measured in the solid state on a glassy carbon disk working electrode (φ = 3 mm) in 0.1 M Et4NBF4/acetonitrile at a scan rate of 100 mV/s.
onto a glassy carbon working electrode (φ = 3 mm). The onset potentials for oxidation of the polymers were similar, whereas those for reduction were significantly shifted to more negative values as the reduction proceeded (P1: Ered,onset = -1.52 V vs SCE; P2: Ered,onset = -2.07 V vs SCE). These results suggested that the conversion of 9-fluoreneone units to fluorene units in the conjugated polymer significantly raised the LUMO level whereas the HOMO level remained unchanged.
Conclusions The quantitative and controlled electrochemical reduction of ketone to methylene in P1 was successfully carried out for the first time. The simple electrochemical reduction of the reducible unit in a conjugated polymer was demonstrated to be a practical new method of tuning optoelectronic properties. Detailed studies on further expanding the utility of this electrochemical reduction method for other conjugated polymer films are in progress. Acknowledgment. We thank Prof. Mahito Atobe at the Tokyo Institute of Technology for EDX measurements. This study was supported by a Grant-in-Aid for Young Scientists (start-up, no. 20850015). Supporting Information Available: 1H NMR spectra of P1 in CDCl3, as-prepared (top) and after the passage of different amounts of charge. EDX profiles of P1 and P2. IR spectra of P1 (top) and P2 (bottom). UV-vis absorption spectra of P2 (16 F/mol) and independently prepared P2 in chloroform. 1H NMR spectrum of chemically prepared P2 in chloroform. This material is available free of charge via the Internet at http://pubs.acs.org.
DOI: 10.1021/la104099q
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