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Polymer Films on Electrodes Part 29. Electropolymerized www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 498 (2001) 67–74 Polymer films on electrodes Part 29. Electropolymerized poly(7,14-diphenylacenaphtho[1,2-k]fluoranthene): electrochemistry and conductance of a novel electrochromic hydrocarbon ladder polymer film Maurizio Quinto, Allen J. Bard * Department of Chemistry and Biochemistry, The Uni6ersity of Texas at Austin, Austin, TX 78712, USA Received 13 April 2498; received in revised form 13 June 2000; accepted 22 June 2000 Dedicated to Fred C. Anson for his distinguished contributions to electrochemistry Abstract The electropolymerization of 7,14-diphenylacenaphtho[1,2-k]fluoranthene on a platinum or indium tin oxide (ITO) electrode produces an electroactive film which displays fluorescence and electrochromism. We report a study of the electrochemical properties and other characteristics of this film, such as the resistance dependence by the potential applied to the electrode. In particular, the film showed dramatic changes in the resistance and color as a function of the potential applied to the film because of changes in the electronic state of the polymer. Atomic force microscopy, scanning electrochemical microscopy, and electrochemical quartz crystal microbalance studies were also carried out to characterize the polymer film. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Polymer films; Electrochromic films; Conductance 1. Introduction In a previous preliminary study, a new hydrocarbon ladder polymer (PANF), obtained by electropolymer- In addition to fundamental interest in the properties ization of 7,14-diphenylacenaphtho[1,2-k]fluoranthene of polymer films on electrode surfaces, practical appli- (ANF), showing interesting electrochromic behavior cations, such as electrochromic behavior, have been and high stability of the surface layers was described suggested. These have possible use in displays, smart [1]. A ladder polymer is one in which the monomeric windows, and other applications where the nature and units are coupled to one another with two bonds rather intensity of the color can be controlled electrically. In than the single bond coupling that characterizes poly- general, for practical applications these films should mers like poly(vinylferrocene) and polypyrrole. To un- possess high stability under long-term cycling, rapid derstand better the characteristics of films of PANF optical changes with applied electrical signal required to and to correlate its redox properties and spectroscopic achieve optical tunability, satisfactory color contrast, behavior with molecular structure, a detailed electro- durability, the processability of the chromogenic mate- chemical study was carried out. We also determined the rials into suitable forms (conformational coatings, mul- resistance dependence on the applied electrode poten- tilayers), and a variety of necessary properties beyond tial, surface topography, and film thickness by atomic optical tunability (e.g. mechanical strength). force microscopy (AFM) and scanning electrochemical microscopy (SECM), and mass changes during electro- * Corresponding author. Tel.: +1-512-471-3761; fax: +1-512-471- chemical cycling by electrochemical quartz crystal mi- 8696. crobalance (EQCM) measurements. Measurement of E-mail address: [email protected] (A.J. Bard). the resistance for a film deposited onto an electrode 0022-0728/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S0022-0728(00)00266-7 68 M. Quinto, A.J. Bard / Journal of Electroanalytical Chemistry 498 (2001) 67–74 surface represents an interesting field for a number of ments of the polymer-covered substrate were carried purposes, ranging from the anodic dissolution of metals out with two different mediators depending on the to the study of electrochromic films of indium oxide [2]. potentials applied to the substrate, that is, tetrathiaful- In the past, several techniques were used for such valene (TTF) and 7,7%,8,8%-tetracyanoquinodimethane resistance measurements, based on discontinuous deter- (TCNQ) were used to get approach curves with the minations [3–5], or continuous determinations substrate at −1.15 and 1.1 V, respectively. Preliminary (impedance measurements via a 90° quadrature signal cyclic voltammetry with these compounds suggested the from a lock-in amplifier [6], SECM inside the film [7], potentials that should be applied to the tip for the impedance spectroscopy [8], scanning tunneling mi- approach curve experiments for the electrode reactions croscopy [9], and indirect electrochemical measure- to be diffusion controlled: −0.60 V for TCNQ reduc- ments [10,11]). In this paper a new and simple method tion and 0.05 V for TTF oxidation. The tip was posi- to measure the film resistance as a function of potential tioned close to the polymeric film in the conductive was developed. It employed a four microband-electrode state (E =1.1 V). The electrolyte was MeCN with 0.1 array covered by the electrodeposited film. The resis- sub M TBAPF , and the concentration of the mediator was tance measurement was performed with a d.c. voltmeter 6 5 mM in both cases. The solutions were bubbled with connected with the two central microbands while the potential applied to the external microbands was Ar before the SECM experiments, and during the mea- varied. surements, the cell was maintained under an Ar atmo- sphere. A model 660 electrochemical workstation (CH Instruments) was employed for the tip characterization 2. Experimental in the absence of the polymeric substrate. Imaging AFM experiments were performed with a Tetra-n-butylammonium hexafluorophosphate Nanoscope III (Digital instruments, Santa Barbara, (TBAPF6) (SACHEM, Austin, TX) was recrystallized CA) in air, at room temperature and atmospheric pres- from EtOH+H2O (4:1) twice and dried at 100°C be- sure. The images were not digitally filtered. The sample fore use. Benzene (Aldrich, anhydrous) and MeCN was manually positioned below the tip and maintained (Burdick and Jackson, UV grade) were used as received perpendicular to the tip and parallel to the tip scan after being transported unopened into an inert atmo- plane through manipulation of the microscope adjust- sphere drybox (Vacuum Atmospheres). All the other ment screws. For a depth profile study and to deter- chemicals were used as received. The reference electrode mine the film thickness with AFM measurements, part was calibrated versus an SCE by the addition of fer- of the film was removed from the ITO surface using a rocene (Fc) as an internal standard taking the formal razor blade. + potential of Fc/Fc as 0.424 vs. SCE. All the potentials The EQCM measurements were performed with a quoted are referred to the SCE reference electrode. home-built instrument described elsewhere [14]. A sim- Different working electrodes were used during the elec- ple battery driven TTL oscillator with an optoisolator trochemical experiments: Pt disk electrodes (3 mm di- was used to isolate the ground of the QCM circuit from ameter unless otherwise specified), indium-tin-oxide- the potentiostat (PAR 173 and 175). A Philips model coated glass electrodes (ITO, R 40 V square, Delta s / PM6681 frequency counter monitored the output of the Technologies, MN), and an array of four Pt microelec- optoisolator. trodes (IMT, Neuchaˆtel, Switzerland). The microelec- trodes assembled in this array were 1 mm long, 25 mm 2.1. 7,14-Diphenylacenaphtho[1,2-k]fluoranthene wide, and 25 mm apart, and they were fabricated on a Si N passivated Si wafer [12]. The completed structures 3 4 This compound was prepared by a route similar to were mounted on a printed circuit board support and that previously reported [1]. The cyclopentadienone wire bonded and all except the array were covered with epoxy encapsulation. intermediate was prepared by refluxing 1,3-diphenylace- Cyclic voltammograms and electropolymerizations tone (3.72 g, 17.7 mmol) and acenaphthenequinone were carried out with a model 660 electrochemical (3.23 g, 17.7 mmol) in ethanolic KOH for 6 h. The workstation (CH Instruments, Austin, TX). The resis- solvent was removed under vacuum, and the residue tance measurements were carried out by coupling a was dissolved in CH2Cl2 and filtered (the cyclopenta- PAR model 175 universal programmer and a model 173 dienone can be isolated at this step by addition of potentiostat (Princeton Applied Research) with a d.c. pentane to the filtered solution to precipitate the com- voltmeter connected to a computer via a LabView pound; however, isolation is not required for the fol- interface. lowing preparation). The solvent was removed and The experimental setup for SECM measurements has acenaphthylene (2.70 g, 17.8 mmol) and xylene (40 ml) been described previously [13]. Potentiostatic experi- were added and the resulting solution was refluxed for M. Quinto, A.J. Bard / Journal of Electroanalytical Chemistry 498 (2001) 67–74 69 2 days. The solvent was removed by distillation and the the central 7 and 14 positions with alkyl or aryl groups residue was placed under vacuum to remove unreacted prevents reactions at these carbons and directs reactiv- acenaphthylene. Then DDQ (4.0 g, 18 mmol) and suffi- ity of the radical cation toward the ends of the cient CH2Cl2 to dissolve the residue was added, and the molecule, promoting the linear polymerization via an solution was stirred at 45°C for 1 h. The solution was oxidative coupling route. As shown in a previous paper filtered through silica (3 cm) and the solution volume [1], the cyclic voltammogram of the monomer (Fig. 2) was reduced under vacuum. The product precipitated shows two reversible reduction processes (at −1.61 as yellow crystals, which were isolated by filtration. The and −2.10 V), and an oxidation wave (Epa =1.52 V) supernatant was reduced further and cooled to induce that is chemically irreversible, indicating that the radi- the product to crystallize (5.15 g total, 61%). The cal cation is not stable and quickly undergoes a follow- compound was recrystallized from toluene before use in ing chemical reaction. The separation between the first the electropolymerization experiments. and second reduction is about 0.5 V; this value is typical for polyaromatic hydrocarbons [15–17].
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