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Photochromism and Electrochemistry of a Dithienylcyclopentene Electroactive Polymer Philana Wesenhagen,† Jetsuda Areephong,† Tatiana Fernandez Landaluce,‡ Nicolas Heureux,† Nathalie Katsonis,†,| Johan Hjelm,§ Petra Rudolf,‡ Wesley R. Browne,*,† and Ben L. Feringa*,†,‡ Stratingh Institute for Chemistry, UniVersity of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands, Zernike Institute for AdVanced Materials, UniVersity of Groningen, Nijenborgh 4, NL-9747AG Groningen, The Netherlands, and Fuel Cells and Solid State Chemistry Department, Risø National Laboratory for Sustainable Energy, Technical UniVersity of Denmark, Roskilde DK-4000, Denmark ReceiVed December 21, 2007. ReVised Manuscript ReceiVed March 9, 2008 A bifunctional substituted dithienylcyclopentene photochromic switch bearing electropolymerisable methoxystyryl units, which enable immobilization of the photochromic unit on conducting substrates, is reported. The spectroscopic, electrochemical, and photochemical properties of a monomer in solution are compared with those of the polymer formed through oxidative electropolymerization. The electroactive polymer films prepared on gold, platinum, glassy carbon, and indium titanium oxide (ITO) electrodes were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The thickness of the films formed is found to be limited to several monolayer equivalents. The photochromic properties and stability of the polymer films have been investigated by UV/vis spectroscopy, electrochemistry, and XPS. Although the films are electrochemically and photochemically stable, their mechanical stability with respect to adhesion to the electrode was found to be sensitive to both the solvent and the electrode material employed, with more apolar solvents, glassy carbon, and ITO electrodes providing good adhesion of the polymer film. The polymer film is formed consistently as a thin film and can be switched both optically and electrochemically between the open and closed state of the photochromic dithienylethene moiety.
Introduction Smart surfaces—surfaces that respond to external stimuli to deliver a change in surface properties such as wettability,1 adhesion,2 roughness3 and biocompatibility4—are attracting increasing interest5 in applications as diverse as sensor technologies,6 cell culture7 and electro- and photochromic devices.8 These “smart surfaces” offer considerable potential for addressable biomaterials, biosensors, information storage devices, microfluidic systems, adhesive materials, and so forth.5 In particular, thin films based on polymers can be used to tune a wide range of surface properties through the reorganization of the internal (e.g., counterion and solvent migration into or out of the polymer layer) or external (surface) structure.9 * Corresponding author. E-mail:
[email protected] (W.R.B.);
[email protected] (B.L.F.). † Stratingh Institute for Chemistry, University of Groningen. ‡ Zernike Institute for Advanced Materials, University of Groningen. § Technical University of Denmark. | Present address: CEMES, CNRS, University of Toulouse, 29 rue J. Marvig, 31055 Toulouse Cx 4, France. (1) (a) Yuan, W.; Jiang, G.; Wang, J.; Wang, G.; Song, Y.; Jiang, L. Macromolecules 2006, 39, 1300–1303. (b) Ichimura, K.; Oh, S.; Nakagawa, M. Science 2000, 288, 1624–1626. (2) (a) Okano, T.; Yamada, N.; Okuhara, M.; Sakai, H.; Sakurai, Y. Biomaterials 1995, 16, 297–303. (b) Kongtong, S.; Ferguson, G. S J. Am. Chem. Soc. 2002, 124, 7254–7255. (3) Klein, J.; Kumacheva, E.; Mahalu, D.; Perahia, D.; Fetters, L. J. Nature 1994, 370, 634–636. (4) Galaev, I. Y.; Warrol, C.; Mattiasson, B. J. Chromatogr. A 1994, 684, 37–43. (5) (a) Yerushalmi, R.; Scherz, A.; van der Boom, M. E.; Kraatz, H-B. J. Mater. Chem 2005, 15, 4480–4487. (b) Katsonis, N.; Lubomska, M.; Pollard, M. M.; Feringa, B. L.; Rudolf, P. Prog. Surf. Sci. 2007, 82, 407–434. (6) Willner, I.; Basnar, B.; Willner, B. AdV. Funct. Mater. 2007, 17, 702–717. (7) Alexander, C.; Shakesheff, K. M. AdV. Mater. 2006, 18, 3321–3328. (8) (a) Sato, O. Acc. Chem. Res. 2003, 36, 692–700. (b) Irie, M. Chem. ReV. 2000, 100, 1683–1684. (c) Yamase, T. Chem. ReV. 1998, 98, 307–326. (d) Wigglesworth, T. J.; Myles, A. J.; Branda, N. R. Eur. J. Org. Chem. 2005, 7, 1233–1238.
Introducing responsiveness to a surface requires that a functional unit, e.g., an electro- and photoresponsive dithienylethene unit as in the present report, is modified with a moiety that enables immobilization onto a surface. Critical to the successful operation of the responsive unit on a surface is the disconnection of the responsive unit functionality from the functionality of the moiety employed in the immobilization, i.e., that the two components (the photochromic dithienylethene and the electropolymerizable methoxystyryl unit) retain their individual properties when connected covalently through a phenyl spacer.10,11 A number of polymer-based systems12 incorporating photochromic diarylethenes and spiropyrans have been reported. The routes employed to make these functional polymers include coupling of dithienylethenes with aromatic disubstituted Horner and Wittig reagents,13 Friedlander condensation,14 radical polymerization of covalently attached styrenes,15 the use of metal ions to form coordination polymers,16 and palladium catalyzed cross-coupling reactions.17 Although in some of these systems (9) (a) Leclerc, M. AdV. Mater. 1999, 11, 1491–1498. (b) Liu, Y.; Mu, L.; Liu, B. H.; Kong, J. L. Chem.—Eur. J. 2005, 11, 2622–2631. (10) (a) Kudernac, T.; van der Molen, S. J.; van Wees, B. J.; Feringa, B. L. Chem. Commun. 2006, 3597–3599. (b) Browne, W. R.; Kudernac, T.; Katsonis, N.; Areephong, J.; Hjelm, J.; Feringa, B. L. J. Phys. Chem. C 2008, 112, 1183– 1190. (11) Areephong, J.; Browne, W. R.; Katsonis, N.; Feringa, B. L. Chem. Commun. 2006, 3930–3932. (12) Wang, J. Y.; Feng, C. G.; Hu, W. X. Prog. Chem. 2006, 18, 298–307. (13) Bertarelli, C.; Bianco, A.; Boffa, V.; Mirenda, M.; Gallazzi, M. C.; Zerbi, G. AdV. Funct. Mater. 2004, 14, 1129–1133. (14) Choi, H.; Lee, H.; Kang, Y.; Kim, E.; Kang, S. O.; Ko, J. J. Org. Chem. 2005, 70, 8291–8297. (15) Kobatake, S.; Kuratani, H. Chem. Lett. 2006, 35, 628–629. (16) (a) Matsuda, K.; Takayama, K.; Irie, M. Chem. Commun. 2001, 363–364. (b) Munakata, M.; Han, J.; Maekawa, M.; Suenaga, Y.; Kuroda-Sowa, T.; Nabei, A.; Ebisu, H. Inorg. Chim. Acta 2007, 360, 2792–2796. (17) Kawai, T.; Nakashima, Y.; Irie, M. AdV. Mater. 2005, 17, 309–314.
10.1021/la703999x CCC: $40.75 2008 American Chemical Society Published on Web 05/16/2008
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retention of electrochemical/photochemical properties (i.e., switching behavior29). In the present contribution, we report a one-step method for fabricating a photoresponsive polymer thin film via electropolymerization. The methoxystyryl-substituted diphenyl-dithienylcyclopentene 2o employed in the present study is based on the dithienylcyclopentene switches reported earlier, i.e., 1o (Figure 1).30 The two methoxystyryl groups were electropolymerized31 on several substrates, including gold, platinum, glassy carbon, and ITO, to allow for investigation of both physical characteristics of the polymer films formed and to facilitate spectroscopic studies of the polymer films. We demonstrate that the dithienylcyclopentene unit in 2o retains its electrochemical and photochemical switching properties after immobilization.
Experimental Section
Figure 1. (a) Photo and electrochromic dithienylethene (1o) and (b) a bifunctional molecule incorporating photo/electrochromic dithienylethene and electropolymerizable methoxystyryl units (2o). (c) A dithienylethene based write/read/erase system based on a self-assembled monolayer on ITO, see ref 11. The suffix “o” or “c” after the compound number refers the ring open (e.g., 1o) or ring closed state (e.g., 1c) of the dithienylethene component.
the photochromism is reduced significantly compared with monomeric model compounds, in certain cases improved photochromic properties (i.e., conversion from the open to the closed state in dithienylethenes) have been reported.18 Several approaches to grafting polymers onto surfaces, including activated substrates19 and functionalized polymers,20 photoinitiated grafting of functionalized polymers,21 and plasmainduced grafting, are available.22 The in situ formation of polymers on electrode materials by electrochemical techniques23 is a wellestablished approach also, and has focused primarily on the fabrication of conducting polymer films,24 providing electroactive polymer film layers for, for example, corrosion protection25 and for retaining biomolecules on surfaces.26 Electropolymerization is a simple, yet robust, method that can produce polymer films, the thickness of which can be controlled by variation in the electrolysis time. The polymerization process can be monitored conveniently by cyclic voltammetry and has been shown to be suitable for immobilization of some photochromic systems.27,28 In our previous reports, a dithienylcyclopentene photochromic switching unit was immobilized as a self-assembled monolayer on indium tin oxide (ITO)11 and on gold electrodes10b with (18) Stellacci, F.; Bertarelli, C.; Toscano, F.; Gallazzi, M. C.; Zotti, G.; Zerbi, G. AdV. Mater. 1999, 11, 292–295. (19) Tomlinson, M. R.; Genzer, J. Macromolecules 2003, 36, 3449–3451. (20) Ionov, L.; Sidorenko, A.; Stamm, M.; Minko, S.; Zdyrko, B.; Klep, V.; Luzinov, I. Macromolecules 2004, 37, 7421–7423. (21) Fuhrmann, I.; Karger-Kocsis, J. J. Appl. Polym. Sci. 2003, 89, 1622– 1630. (22) Kai, T.; Ueno, W.; Yamaguchi, T.; Nakao, S. J. Polym. Sci. A: Polym. Chem. 2005, 43, 2068–2074. (23) Lyons, M. E. G. In ElectroactiVe Polymer Electrochemistry, Part 1: Fundamentals; Lyons, M. E. G., Ed.; Plenum Press; New York, 1994; p 1. (24) Biallozor, S.; Kupniewska, A. Synth. Met. 2005, 155, 443–449. (25) Tallman, D. E.; Spinks, G.; Dominis, A.; Wallace, G. G. J. Solid State Electrochem. 2002, 6, 73–84. (26) Cosnier, S. Anal. Bioanal. Chem. 2003, 377, 507–520. (27) Lee, J.; Kwon, T.; Kim, E. Tetrahedron Lett. 2007, 48, 249–254. (28) Yasser, A. E.; Moustrou, C.; Youssoufi, H. K.; Samat, A.; Guglielmetti, R.; Garnier, F. J. Chem. Soc., Chem. Commun. 1995, 471–472.
Materials. Chemicals were purchased from Acros, Aldrich, Fluka or Merck. Solvents for extractions and chromatography were technical grade. All solvents used in reactions were distilled freshly from appropriate drying agents before use. Flash chromatography was carried out using Merck silica gel 60 (230-400 mesh ASTM). 2,2′(Dichlorodithienylethene)-cyclopentene (1) was prepared as described previously.30 [Ru(II)(4,7-diphenyl-1,10-phenanthroline)3](PF6)2 was received as a gift from Gas Sensors Solutions (GSS Irl.). All solvents used for spectroscopic and electrochemical measurements were of UVASOL grade or better unless stated otherwise. Synthesis of 2,2′-(Di(4′′-carboxy-phenyl)dithienylethene)cyclopentene (4o). 3o (100 mg, 0.30 mmol) was dissolved in anhydrous tetrahydrofuran (THF, 4 mL) under a dinitrogen atmosphere, and nBuLi (0.47 mL, 1.6 M in hexane, 0.76 mmol) was added by syringe. This solution was stirred for 30 min at room temperature, and B(OBu)3 (0.21 mL, 0.76 mmol) was added. The resulting solution was stirred for 40 min at room temperature to produce the corresponding boronic ester. A separate flask was charged with p-bromobenzaldehyde (112 mg, 0.60 mmol), [Pd(PPh3)4] (52 mg, 0.05 mmol), THF (2 mL), aqueous Na2CO3 (1.6 mL, 2 M), and ethylene glycol (4 drops). The mixture was heated to 80 °C, and the preformed boronic ester was added slowly by syringe. The reaction mixture was heated at reflux for 3 h, after which it was diluted with diethyl ether (40 mL) and washed with brine (40 mL). The brine solution was washed with additional diethyl ether (40 mL), and the combined organic phases were dried over Na2SO4 and concentrated in vacuo. Chromatography over silica (pentane/ethyl acetate 5/1) yielded 4o (106 mg, 0.23 mmol, 75%) as a green solid. 1H NMR (300 MHz, CDCl3): δ ) 2.03 (s, 6H), 2.11 (t, J ) 7.2 Hz, 2H), 2.86 (t, J ) 7.2 Hz, 4H), 7.19 (s, 2H), 7.62 (d, J ) 8.4 Hz, 4H), 7.83 (d, J ) 8.4 Hz, 4H), 9.96 (s, 2H); 13C NMR (100.6 MHz, CDCl3): δ ) 14.9, 23.3, 38.7, 125.6, 126.2, 126.3, 134.9, 135.0, 137.3, 137.4, 138.4, 140.3, 191.6; MS (EI): 468 [M+]; HRMS calcd for C29H24O2S2 468.1218, found 468.1211. Synthesis of 2o. Sodium hydride (79 mg, 60% in oil, 1.98 mmol) was suspended in anhydrous THF (15 mL) under a dinitrogen atmosphere, and p-MeO(C6H4)CH2P(O)(OEt)2 (0.56 mL, 3.24 mmol) was added slowly by syringe. The mixture was heated at 55 °C for 1.5 h, and a solution of 4o (133 mg, 0.28 mmol) in anhydrous THF (5 mL) was added slowly via a syringe. The reaction mixture was heated at reflux for 20 h, after which it was quenched with water (10 mL) and extracted with dichloromethane (3 × 10 mL). The combined organic phases were dried over Na2SO4 and concentrated in vacuo. Chromatography over silica (dichloromethane) yielded 2o (29) Tian, H.; Yang, S. J. Chem. Soc. ReV. 2004, 33, 85–97. (30) (a) de Jong, J. J. D.; Lucas, L. N.; Kellogg, R. M.; Feringa, B. L.; van Esch, J. H. Eur. J. Org. Chem. 2003, 1887–1893. (b) Lucas, L. N.; de Jong, J. J. D.; Kellogg, R. M.; van Esch, J. H.; Feringa, B. L. Eur. J. Org. Chem. 2003, 155–166. (31) (a) Aranyos, V.; Hjelm, J.; Hagfeldt, A.; Grennberg, H. J. Chem. Soc., Dalton Trans. 2001, 1319–1325. (b) Fabre, B.; Michelet, K.; Simonet, N.; Simonet, J. J. Electroanal. Chem. 1997, 425, 67–75. (c) Beer, P. D.; Kocian, O.; Mortimer, R. J.; Ridgway, C.; Stradiotto, N. R. J. Electroanal. Chem. 1996, 408, 61–66.
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Figure 2. Synthesis of dithienylhexahydrocyclopentene monomer 2o via 4o. (a) nBuLi, THF, 298 K; (b) B(OBu)3, THF, 298 K; (c) p-Br(C6H4)CHO, Pd(PPh3)4, Na2CO3; (a-c 75% yield) (d) p-MeO(C6H4)CH2P(O)(OEt)2, NaH, THF, ∆, (68% yield).
Figure 3. (a) UV/vis absorption spectra in CH2Cl2 solution of 2 in the open (2o) and closed (2c) state. 2c was generated by irradiation of 2o at λexc 365 nm. (b) Cyclic voltammetry of 2o in CH2Cl2 (0.1 M TBAPF6) showing formation of the closed form 2c2+ (and ultimately 2c) upon oxidation of the 2o.
(129 mg, 0.18 mmol, 68%) as a yellow solid. 1H NMR (300 MHz, CDCl3): δ ) 2.02 (s, 6H), 2.10 (t, J ) 7.2 Hz, 2H), 2.86 (t, J ) 7.2 Hz, 4H), 3.84 (s, 6H), 6.86-7.10 (m, 10H), 7.26 (s, 2H), 7.43-7.50 (m, 10H); MS (EI): 676 [M+]; 13C NMR (100.6 MHz, CDCl3): δ ) 14.7, 23.3, 38.7, 55.6, 114.4, 124.1, 125.7, 126.3, 126.9, 127.1, 127.9, 128.2, 130.4, 133.6, 134.8, 136.6, 137.0, 139.7, 159.6; HRMS calcd for C45H40O2S2 676.2470, found 676.2436. Physical Methods. 1H NMR spectra were recorded on a Varian VXR-300 spectrometer (at 300 MHz) at ambient temperature. 13C NMR spectra were recorded on a Varian VXR-400 spectrometer (at 100 MHz). Chemical shifts are denoted in δ (ppm) with respect to tetramethylsilane (TMS) and referenced to the residual CHCl3 peaks. Coupling constants J, are denoted in hertz. Mass spectra were recorded with an MS-Jeol mass spectrometer with ionization according to the EI+ procedure. UV/vis spectra were recorded (in solution) using an HP 8453A diode array spectrophotometer or (for polymer-modified ITO slides) using a JASCO 670 UV/Vis NIR spectrophotometer. Fluorescence spectra were recorded using a JASCO F7200 fluorimeter. Spectra are corrected for lamp and detector response. Contact angles were measured on a Kru¨ss Drop Shape Analysis System DSA 10 Mk2. Quantum yields were determined by standard methods.32 Electrochemical measurements were performed using a Model 630B or 760C Electrochemical Workstation (CH Instruments). Analyte concentrations were typically 0.5 to 1 mM in anhydrous dichloromethane containing 0.1 M tetra-butylammonium perchlorate (TBAClO4). A Teflon shrouded glassy carbon, gold or platinum macro electrode, or a gold or platinum microelectrode was employed as a working electrode (CH Instruments), a Pt wire auxiliary electrode (32) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991–1024.
and a Ag/AgCl ion quasi-reference or a saturated calomel electrode (SCE) were employed (calibrated externally using 0.1 mM solutions of ferrocene, all potentials reported are relative to SCE). Cyclic voltammograms were obtained at sweep rates between 10 mV/s and 5000 mV/s. For reversible processes, the half-wave potential values are reported. Redox potentials are +/-10 mV. Irradiation of samples at 365 nm was carried out using the emission from a Spectroline ENB280C lamp, and at >400 nm using a standard 8 W fluorescent bulb together with a 400 nm cut off filter. The photostationary state (PSS) achieved at 365 nm irradiation was >98% (by NMR and fluorescence spectroscopy, Figure S12, Supporting Information), and full reversion to the open state is achieved with >450 nm irradiation. For atomic force microscopy (AFM), the morphology and rootmean-square (rms) roughness of poly-2c films on ITO were obtained from phase and height images collected using an atomic force microscope (PicoLE, Scientec). The images were collected in intermittent contact mode with ultrasharp silicon cantilevers. Data visualization was achieved using WsXm software.33 For the X-ray photoelectron spectroscopy (XPS) measurements, the polymer films were prepared by electropolymerization onto gold films supported on mica. The open form of the polymer film (poly2o) was prepared by irradiation of poly-2c with visible (>400 nm) light. XPS spectra of 2o/2c were obtained by depositing a drop of a dilute dichloromethane solution of 2o/2c onto evaporated gold films followed by drying under an argon gas stream. The samples were introduced through a load lock system into an SSX-100 (Surface (33) Horcas, I.; Ferna´ndez, R.; Go´mez-Rodrı´guez, J. M.; Colchero, J.; Go´mezHerrero, J.; Baro, A. M. ReV. Sci. Instrum. 2007, 013705.
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Table 1. Spectroscopic and Redox Data 2o 2c poly-2o poly-2c 1oc 1cc
λmax Abs /nma
Ep,ab
362 358, 568 N/A 540 278 360, 527
0.98 1.12 1.10
Ep,cb
E1/2b -1.69, 0.34, 0.59
-1.73
0.38, 0.67
-1.74
0.34, 0.68
(10-4 M) in dichloromethane (DCM). b (10-4 M) in CH2Cl2 (0.1 M TBAPF6) in V vs SCE. c in CH3CN (0.1 M TBAPF6), from ref 35. a
Science Instruments) photoemission spectrometer with a monochromatic Al KR X-ray source (hv ) 1486.6 eV). The base pressure in the spectrometer during the measurements was 10-10 mbar. The energy resolution was set to 1.3 eV to minimize data acquisition time, and the photoelectron takeoff angle was 37°. XPS binding energies were referenced to the Au 4f7/2 core level at 84.0 eV.41 Spectral analysis included a linear background subtraction and peak deconvolution using Gaussian and Lorentzian functions in a least-squares curve-fitting program (WinSpec) developed at the LISE, University of Namur, Belgium. Peak fitting of sulfur bands was performed using a doublet with a 2:1 intensity ratio and splitting of 1.18 eV.
Figure 4. Electrochemical deposition of poly-2o on a glassy carbon electrode by cyclic voltammetry (scanning speed of 0.5 V s-1 during repetitive cycles between 0-1.6 V vs SCE). The decrease in the intensity of the oxidation wave of 2o at 0.98 V is due to the depletion of 2o from the diffusion layer (as it is converted to 2c) and the passivation of the electrode as the polymer film grows.
Results Synthesis. The dimethoxystyryl-substituted dithienyl-hexahydrocyclopentene monomer 2o was obtained from dialdehyde 4o by a double Horner-Wadsworth-Emmons reaction (Figure 2) as a mixture of --cis, cis-trans, and trans-trans isomers. The geometrical isomers were not separated. Electronic Spectroscopy and Electrochemistry of 2o in Solution. The UV/vis spectra and cyclic voltammetry of 2 in both the open (2o, colorless) and closed (2c, colored) state are shown in Figure 3. The open form 2o shows a main absorption at 362 nm, which decreases in intensity upon irradiation at 365 nm with a concomitant increase in absorption at 568 nm due to the formation of the closed state 2c (see Supporting Information, Figure S1). The open form is regenerated upon irradiation with visible light. Isosbestic points are maintained over several cycles, indicating that the cis/trans isomerization of the styryl double bonds is quenched effectively by the dithienylcyclopentene unit (Figure S1). The red shift of the absorption of both the open and, to a lesser extent, the closed forms of 2 in comparison to 1 (Figure 1 and Table 1), is expected due to the electronic absorption of the methoxystyryl unit and the electron donating effect of the same unit on the chromophoric unit of the dithienylethene moiety, respectively.34 Compound 2o is moderately fluorescent (Φfl ) 0.04, Figure S2) while 2c is nonfluorescent. The electrochemical properties of 2 are as expected for a dithienylcyclopentene-based photochromic switch.35 Cyclic voltammetry in the range -0.5 and 1.35 V (vs SCE) shows features comparable with to that of the model compound 1o.35 (34) (a) Hara, M.; Tojo, S.; Majima, T. J. Photochem. Photobiol., A: Chem. 2004, 162, 121–128. (b) Beer, P. D.; Kocian, O.; Mortimer, R. J.; Ridgeway, C. J. Chem. Soc., Dalton Trans. 1993, 2629–2638. (35) For redox properties of related diarylcyclopentenes in solution, see. (a) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.; Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.—Eur. J. 2005, 11, 6414–6429. (b) Browne, W. R.; de Jong, J. J. D.; Kudernac, T.; Walko, M.; Lucas, L. N.; Uchida, K.; van Esch, J. H.; Feringa, B. L. Chem.—Eur. J. 2005, 11, 6430–6441. (c) Koshido, T.; Kawai, T.; Yoshino, K. J. Phys. Chem. 1995, 99, 6110–6114. (d) Fraysse, S.; Coudret, C.; Launey, J.-P. Eur. J. Inorg. Chem. 2000, 125, 1581– 1590. (e) Guirado, G.; Coudret, C.; Hliwa, M.; Launay, J. P. J. Phys. Chem. B. 2005, 109, 17445–17459. (f) Peters, A.; Branda, N. R. J. Am. Chem. Soc. 2003, 125, 3404–3405. (g) Peters, A.; Branda, N. R. Chem. Commun. 2003, 954–955. (h) Gorodetsky, B.; Samachetty, H. D.; Donkers, R. L.; Workentin, M. S.; Branda., N. R Angew. Chem., Int. Ed. 2004, 43, 2812–2815. (i) Zhou, X.-H.; Zhang, F. S.; Yuan, P.; Sun, F.; Pu, S.-Z.; Zhao, F.-Q.; Tung, C.-H. Chem. Lett. 2004, 33, 1006–1067.
Figure 5. Clean (dotted line) and poly-2c modified (solid line) glassy carbon electrode in an electrolyte solution (CH2Cl2/TBAClO4) containing ∼1 mM [Ru(II)(4,7-diphenyl-1,10-phenanthroline)3](PF6)2. (scan rate 0.5 V s-1)
The lower potential for the oxidation of 2o compared with 1o is expected on the basis of the electron donating effect of the methoxystyryl group. For these dithienylhexahydrocyclopentene photochromic switches, oxidation of the open form leads to immediate conversion to the closed form (in the dicationic state, see Figure 3), i.e., the irreversible anodic oxidation of the open form 2o at Ep,a ) 0.98 V (vs SCE) leads to ring closure to 2c2+, which can then be reduced, first to 2c1+ at 0.62 V and finally to 2c at 0.40 V (Figure 3b). On the return wave, 2c is reoxidized to 2c1+ at 0.40 V and to 2c2+ at 0.62 V. The reduction of 2c is quasi-reversible especially at higher scan rates (>0.5 V s-1) in contrast to the irreversibility of the reduction of 1c. The increase in reversibility is in agreement with previous studies35b of analogues of 1c para-substituted with electron donating groups. Oxidative Electropolymerization. Repeated cyclic voltammetry of 2o between 0.0 and 1.6 V (vs SCE, Figure 4) results in a steady increase in the current response at ca. 0.4 and 0.6 V assignable to formation of 2c (Figure 3b). The increase has two contributions. The first minor contribution is due to the formation of 2c2+ from 2o at the working electrode (see Figure 3b). The second and most significant contribution arises from the further oxidation of 2c2+, specifically the methoxystyryl groups thereof,
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Figure 6. Left: Cyclic voltammetry of a poly-2c-modified glassy carbon electrode in monomer (2o)-free CH2Cl2 (0.1 M TBAPF6) solution at 0.20, 0.50, 0.75, and 1.00 V s-1 (with iR compensation, glassy carbon working electrode, A ) 0.071 cm2). Right: linear dependence of current on scan rate between 0.1 and 1 V s-1. µ ) 7.5 × 10-10 mol cm-2.
Figure 7. Visible absorption spectrum of poly-2c on an ITO electrode.
to the cationic radical species (Figure 4). The polymerization occurs upon oxidation of the methoxystyryl groups, which form cation radicals and undergo radical-radical coupling31 to form dimers and oligomers. This results in a loss of vinylic character of the methoxystyryl group, which is manifested in a blue shift31 in the electronic absorption of the polymer (Figure 7, Table 1). The extent of polymerization achievable is, however, limited (Figure S3). Indeed, even with potentiostatic deposition at 1.6 V, the thickness of the polymer film formed reaches an effective limit (Vide infra) and does not progress thereafter, regardless of the length of time at which the oxidizing potential is held (Figure S4). The onset potential at which electropolymerization occurs was examined. Below 1.35 V, although electrochemical switching35 of 2o to 2c was observed to take place at the electrode (Figure 3b), formation of polymer films did not occur. At higher switching potentials, polymer film formation was observed, and, above 1.50 V, there was no significant dependence of film formation on the electrochemical switching potential employed. Electropolymerization at monomer concentrations below 0.25 mM over a fixed number of cycles resulted in thinner films than those above a [2o] of 0.5 mM; however, at monomer concentrations between 0.25 mM and 10 mM, although the polymer film formed within fewer cycles, no significant difference in the final polymer
films formed was observed, i.e. the maximum film thickness of poly-2c achievable is constant (Vide infra).36 The scan rate dependence for the formation of electropolymerized films was examined between 0.1 and 10 V s-1. The polymer films formed between 0.5 and 1 V s-1 were essentially identical, while, at scan rates faster than 1 V s-1, the degree of electropolymerization was found to decrease significantly. This is not unexpected, as at higher scan rates the cell resistance becomes significant and serves to decrease the effective oxidative switching potential (Vide supra). At 0.1 V s-1 the degree of electropolymerization was found to be decreased also, and frequently electropolymerization was not observed to take place at all, or mechanically unstable films were formed. This is most probably due to a inhibition of deposition of the polymer on the electrode due the instability of poly-2c in its oxidized state (see Figure S10, Vide infra). As the first layers of polymer are deposited at this scan rate the film degrades rapidly to an insulating state and prevents further deposition. Indeed pretreatment of a glassy carbon electrode, by oxidative cycling between 0.0 and 1.4 V in 0.1 M H2SO4, results in less stable polymer films, whereas sonication of the treated electrode in toluene followed by sonication in dichloromethane or roughening by mechanical abrasion allowed for much more stable films to be formed. For 2c, essentially the same behavior and film thicknesses were obtained (see Figure S11). Reductive Electrochemistry. For 2o and 2c, reductive electrochemistry is not observed at potentials more positive than -1.5 V (vs SCE). At more negative potentials, the reduction of the closed form of the switch is observed at approximately -1.6 V for 2c and poly-2c (Vide infra, Figure 5 and Figure S6) similar to that of 1c.35 For poly-2c, the reduction appears irreversible; however, a prewave to the poly-2c f poly-2c+ is observed at ∼0 V, which can be associated with the reduction at -1.6 V (i.e., the prewave is not observed when the switching potential is more positive than the -1.5 V). The occurrence of such a prewave is consistent with the presence of trapped charges in the polymer film after the reductive cycle,37 which are released only at potentials where significant conductivity in the film is generated (i.e., the first oxidation of poly-2c) by injection of holes. Although this reduction cycle does not lead to immediate destruction of the polymer film, repeated cycling shows that, on the third and (36) The film thickness was less than the limit of detection by of a dektak profilometer (i.e. 420 nm) of a glassy carbon electrode on which the switch was polymerized showed a decrease in the current intensity of the redox process at 0.34 V (Figure 9), indicating the formation of the open form (poly-2o). Indeed, the poly-2omodified electrode can be converted quantitatively to poly-2c by cycling between 0.00 and 1.35 V (Figure 10), through oxidative ring closure via poly-2c2+ (Vide supra). (41) Moulder, F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Physical Electronics, Inc.: Chanhassen, MN, 1995.
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Figure 9. Cyclic voltammetry of poly-2c on a glassy carbon electrode at 0.75 V s-1 in DCM (0.1 M TBAClO4) before (a) and after (b) photochemical ring opening by visible irradiation (>420 nm) of poly-2c in monomer-free solution, and after (c) oxidative ring closure (see Figure 10).
Photochemical closing of the polymer film after ring opening to the poly-2o form is possible by irradiation with UV light (365 nm). However, although ring closure is observed within a few seconds of exposure to UV light, rapid irreversible destruction of the polymer film is observed (Figure 11). The photochemical ring opening and subsequent electrochemical ring closure of the polymer film deposited on ITO was followed by UV/vis spectroscopy (Figure S9). The absorption of the closed state in the visible region is seen to decrease upon photochemical ring opening and increase again after electrochemical ring closure (Vide supra). These data confirm that the ability of 2 to switch from open to closed forms, and vice versa, both by electrochemical and photochemical techniques, respectively, is retained in the electropolymerized state. X-ray Photoemission Spectroscopy (XPS). A recent study of the electronic structure and chemical nature of dithienylethene switch multilayer films on Au(111) has demonstrated XPS to be a useful technique, which can distinguish between the “open” and “closed” forms of these photochromic switches and detect the presence of species that contain
Figure 10. Cyclic voltammetry of poly-2o on a glassy carbon electrode prepared by visible irradiation (>420 nm) of poly-2c in monomer-free solution at 0.1 V s-1 in DCM (0.1 M TBAClO4). The first cycle is shown as a thick line.
Wesenhagen et al.
Figure 11. Cyclic voltammetry of poly-2c on a glassy carbon electrode at 0.75 V s-1 in DCM (0.1 M TBAClO4) (a) before and (b) after photochemical ring opening by visible irradiation (>420 nm) of poly-2c in monomer-free solution, and (c) after photochemical ring closure by irradiation with UV light.
oxidized sulfur atoms (see Table 2).42 In Figure 12, the XPS signals of the sulfur 2p core levels are shown for both poly-2c and poly-2o on Au(111) together with least-squares best fit analysis of the peaks. In the S 2p spectrum of poly-2c, only two peaks were observed, one with the S 2p3/2 component at a binding energy of 163.4 eV, assigned to the closed form of the polymerized dithienylethene switch, and a less intense peak with the S 2p3/2 component at 164.2 eV assigned to the open form of the dithienylethene component of the polymer. The absence of a component at 168 eV indicates that the sulfur atoms were not oxidized (to/from, e.g., -SO, -SO2, etc.) in the electropolymerized film.42 In the S 2p spectrum of poly-2o, four sulfur species can be identified: the component with a S 2p3/2 line at 163.5 eV corresponds to the closed form of the polymerized dithienylethene switch. The S 2p3/2 line at 164.6 eV is assigned to the open form of the polymerized dithienylethene switch, and at 168 eV, is assigned to the S 2p3/2 line of oxidized sulfur (8%).40 XPS shows the S 2p3/2 core level binding energy dependence on the isomeric chemical structure, by going from closed form (163.5 eV) to open form (164.6 eV) in the polymer switch. This is evidence of the presence of both structures in the closed and open forms. The line at 162 eV binding energy is more difficult to assigned and may be due to sulfur chemisorbed on gold;41 however, the intensity of this peak would suggest that half of the sulfur atoms in the film are interacting with the gold surface, which is not consistent with a multilayer film. A further difficulty in observing the open and closed forms arises as a result of partial charging of the film, which will result in the formation of cationic species in the film giving rise to peaks of higher binding energy. Furthermore, this species will catalyze both ring opening and closing of the dithienylethene unit.35 Indeed, for 2c deposited onto gold slides by evaporation from CH2Cl2 solution, and 2o prepared in situ by visible irradiation (Figure 13), the XPS data of the S 2p core level region both open (42) (a) Mendoza, S. M.; Lubomska, M.; Walko, M.; Feringa, B. L.; Rudolf, P. J. Phys. Chem. C. 2007, 111, 16533–16537. (b) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799–6806. (c) Noh, J.; Ito, E.; Araki, T.; Hara, M. Surf. Sci. 2003, 532, 1116–1120. (d) Sako, E. O.; Kondoh, H.; Nakai, I.; Nambu, A.; Nakamura, T.; Ohta, T. Chem. Phys. Lett. 2005, 413, 267–271.
Thin PhotoactiVe Diarylethene Redox Polymer Films
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Table 2. X-ray Photoelectron Spectroscopy polymer poly-2o
poly-2c
monomer
S 2p3/2 binding energy (eV)
peak assignment
162 (54%) 163.5 (24%) 164.6 (14%) 168 (8%) 163.4 (70%) 164.2 (30%)
S-Au closed form open form oxidation closed form open form
Figure 12. XPS spectra of the S 2p core level region of (a) poly-2o (obtained by visible irradiation of poly-2c at >420 nm) and of (b) poly2c.
and closed forms can be identified in each sample in approximately similar ratios. In the spectrum of 2o (Figure 12a), three peaks can be identified: the first one, with a S 2p3/2 binding energy of 163.6 eV, corresponds to the closed form, a second peak, with the S 2p3/2 component appearing at 164.1 eV, can be assigned to the open form, and the last peak, where the S 2p3/2 signal is at 168 eV. The latter corresponds to sulfur oxidation (4%) due to the irradiation to obtain 2o, as observed in the case of the polymer (see Figure 12). In the S 2p photoemission spectrum of 2c (Figure 13b), two sulfur species can be identified: the first, with S 2p3/2 peaked at 163.6 eV, corresponds to the closed form, and the second, with S 2p3/2 peaked at 164.2 eV, can be assigned to the open form. This suggests that, in the case of this particular dithienylethene, the propensity for electrochemically driven ring opening and closing makes it difficult to address each state independently by XPS.
S 2p3/2 binding energy (eV)
peak assignment
2o
163.6 (64%) 164.1 (32%) 168 (4%)
closed form open form oxidation
2c
163.6 (86%) 164.2 (14%)
closed form open form
diphenyl-1,10-phenanthroline)3](PF6)2 that the inhibition of growth of the polymer film is due to the formation of an evenly distributed thin nonconducting polymer matrix, in which the dithienylethene switch units are embedded. For the cationic complex [Ru(II)(4,7-diphenyl-1,10-phenanthroline)3](PF6)2, under the electrochemical conditions employed (i.e., at scan rates of >0.1 V s-1) electron transfer from the electrode to the complex is effectively blocked. The redox potential of the complex is >100 mV from the oxidation and reduction potentials of the dithienylethene, indicating the polymer film behaves as a redox polymer. In contrast, the redox response of decamethylferrocene shows only a minor reduction in the rate of electron transfer, despite its redox potential being equally far removed from that of the polymer’s dithienylethene unit. However, for the decamethylferrocene, the polymer is in a neutral state at low potentials, and hence the diffusion into the matrix may be significant. In the case of the Ru(II)/Ru(III) process at 1.28 V (vs SCE), the cationic complex is less likely to diffuse into the polymer matrix itself, and at potentials at which oxidation of the complex occurs, the polymer is cationic itself, and hence electron transfer across the polymer film is required. This confirms that the polymer film is an effective barrier to electron transfer at potentials distant from the dithienylethene units’ first and second redox processes (Table 1). For polymerization to take place, the monomer must first be oxidized to the dicationic state, and, subsequently, further oxidation at higher potential produces methoxystyryl cationic radicals (Figure 14). As the polymer film grows in thickness the
Discussion The covalent connection of a photo/electrochromic unit (a dithienylethene) with a electropolymerizable methoxystyryl unit allows for the formation of electropolymerized films in which the photo/electrochromic unit retains its physical properties. The electroactive polymer is formed as layers of limited thickness, regardless of the concentration of monomer present in solution or the deposition potential or time. It is apparent from AFM and the effect of the polymer film on electron transfer to decamethylferrocene and [Ru(II)(4,7-
Figure 13. XPS spectra of the S 2p core level region of the monomer, deposited by evaporation of hexane solutions of 2o and 2c, as thin films on gold showing the open form (a) and the closed form (b).
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to that of the electropolymerizable methoxystyryl group, and hence electron transfer to the monomer can be mediated by the redox polymer formed. Hence the primary limit to film growth in the present system can be ascribed to saturation of the polymer backbone and a concomitant loss of conductivity at potentials required (i.e., > 1.35 V, Figure 8) for polymerization to continue. The stability of the polymer films with respect to adhesion was found to be best on rough (e.g., ITO) surfaces and on glassy carbon electrodes. In the case of the latter electrode type, the formation of covalent bonds between the surface and the electrode may help adhesion. The mechanical instability of the polymer on gold and platinum electrodes supports this. Furthermore, whereas previously11 we demonstrated that immobilization of the dithienylethene as a self-assembled monolayer allows for read-write-erase functionality (Figure 1), the instability of the film at low scan rates and under the UV irradiation precludes similar functionality in the present system.
Conclusions
Figure 14. Sequential oxidation and ring closure steps that precede polymerization: 2o is oxidized and forms 2c2+. At higher potentials, the methoxystyryl units of 2c2+ are oxidized to form cationic radicals, which can undergo polymerization.
electron transfer from the electrode to species in solution becomes increasingly retarded, and at a limiting thickness, electron transfer effectively ceases. The fact that this is the case is supported by the simple series resistor/capacitor character of the electrochemical impedance spectrum at potentials removed from the redox couples of the dithienylethene unit. In related cationic monomers,31a with net positive charges as high as +4 and +5, efficient electropolymerization is observed to take place. However, for these systems, in place of a dithienylethene unit, a [Ru(2,2-bipyridine)3]2+ type unit is employed, the oxidation potential of which is very close
The incorporation of two distinct functional units in the same molecule, i.e., a photochemically and electrochemically switchable dithienylcyclopentene unit and an electropolymerizable methoxystyryl unit separated by a phenyl spacer, allows for retention of the functions of each functional unit in the monomer (2o). Electropolymerization to form thin films is achieved readily on a range of substrates, and the retention of both the photo- and electrochemical properties of the dithienylethene unit were demonstrated by UV/vis and XPS spectroscopy and by electrochemistry. The origin of the limited film thickness is proposed to be due to the lack of conductivity of the polymer at potentials required to achieve electropolymerization and the low redox potential of the dithienylethene component, which is insufficient to mediate electron transfer to the solution from the electrode. In future studies, the approach taken here will be extended to the analogues of 1o, e.g., the dithienylhexafluorocyclopentene based polymers, which are oxidized at redox potentials closer to that at which electropolymerization takes places in order to allow for variation in the thicknesses of the polymer films formed and to enable tuning of character of the polymer film-coated surfaces formed (e.g., increased hydrophobicity, color, etc.). Acknowledgment. This work received financial support from the MSCplus program of the Zernike Institute for Advanced Materials, the Dutch Foundation for Fundamental Research on Matter (FOM), the Breedtestrategie program of the University of Groningen, The Netherlands Organization for Scientific Research (NWO-CW), the National Research School Combination Catalysis (NRSC-C), NanoNed, NWO-CW Vidi grant (W.R.B.), and Ubbo Emmius (J.A.). Supporting Information Available: Electrochemical, spectroscopic and AFM data. This material is available free of charge via the Internet at http://pubs.acs.org. LA703999X
