Electrochemistry of Conductive Polymers. XXIV. Polypyrrole Films Grown at Electrodes Modified with β-Cyclodextrin Molecular Templates

Abstract

Organic conducting polymers have been studied extensively due to fundamental interests in reaction mechanisms as well as for their possible applications to practical devices including energy storage devices, electrochemical/chemical sensors, electrochromic devices, and others. 1-3 Most applications require repeated injection and removal of charges from bulk polymers via doping and dedoping processes. Some applications including sensors or electronic devices may eventually require nanosized polymer wires or dots. For this reason, nanosized conducting polymer wires have been prepared electrochemically using hollowed templates made from polycarbonates with neutron beams 4,5 or chemically using the zeolites. 6,7 The polymer wires were then obtained by dissolving polycarbonates or zeolites. Conducting polymer wires thus prepared generally had enhanced conductivity compared to that of bulk polymers. 5,7,8 Conducting polymers were generally grown on metal electrodes by potentiodynamic, potentiostatic, or galvanostatic methods. 3 Depending on the state of electrode surfaces, the overpotential for oxidation of monomer molecules is determined, which affects their film quality; an example includes oxidation of aniline for the preparation of polyaniline. Aniline requires a large overpotential at metal electrodes but once the polymer is formed on the electrode surface, the organic film usually reduces the overpotential for oxidation of monomer molecules, leading to what is called "autocatalytic mechanism." 3,9 When organic molecules lowering the aniline overpotential are adsorbed on the metal electrodes, the quality of polyaniline formed on the electrodes is generally improved to a great deal. 10 In rare cases, however, modified electrodes have been used to prepare conducting polymers.Recently, we reported use of cyclodextrin (CD) covered electrodes for performing molecular-size selective electrochemistry of a few quinone compounds. 11 The cavities created by thiolated CDs adsorbed on gold electrodes act as either an array of ultramicroelectrodes or anchoring sites depending on properties of molecules examined. It occurred to us that the CD-modified electrodes could be used as those offering anchoring sites for monomer molecules, which could lead to a possibility of growing polymers to our specification.We carried out our current study to (i) see how the polymerization site modification would affect the polymer growth, (ii) find whether or not the cavity-containing compounds such as CDs can be used as molecular templates for electrochemical polymerization/growth of conducting polymers, and (iii) explore the possibility of preparing nanosized single-stranded polymer wires using electrodes with appropriately modified surfaces. In this work we used thiolated ␤-CD molecules to modify the gold surfaces. ␤-CD has a cavity diameter of about 7.8 Å, which is large enough to serve as an anchoring site for monomer molecules such as pyrrole. 12 Experimental Pyrrole (Aldrich, 98%) was used after distillation over zinc powder and stored in...

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