Formation and Electrochemical Property of Pyrrole-Terminated SAMs and the Effect of the SAMs on the Physicochemical Properties of Polypyrrole Films Electrochemically Deposited over Them

ω-(N-Pyrrolyl) alkanethiols with various alkyl chain lengths were prepared to study the formation of self-assembled monolayers (SAMs) bearing a bulky headgroup and the electrochemical deposition of polypyrrole over SAM-modified gold electrodes. Polarized IR reflection−absorption, ellipsometry, and quartz crystal microbalance data showed that an alkanethiol with a longer alkyl chain is better with regard to coverage and ordering of the SAMs. The monolayer coverages are 62%, 74%, 78%, and 78% for ω-(N-pyrrolyl) propanethiol, ω-(N-pyrrolyl) hexanethiol, ω-(N-pyrrolyl) nonanethiol, and ω-(N-pyrrolyl) dodecanethiol, respectively. Due to oxidative desorption of the monolayer, the cyclic voltammograms (CVs) of the SAMs of ω-(N-pyrrolyl) alkanethiols on Au in 0.1 M KCl/H<INF>2</INF>O display an irreversible oxidation wave at ca. 1.2 eV versus a Ag/AgCl electrode. No redox reaction of the SAM was observed when the CV measurements were carried out in acetonitrile, using Et<INF>4</INF>NBF<INF>4</INF> as an electrolyte. There is also no voltammetric evidence for polymerization of the surface-confined pyrrole units at the potential cyclically scanned from 0 to +1.4 V. The electrochemical deposition of polypyrrole potentiodynamically over monolayers of ω-(N-pyrrolyl) alkanethiols shows that the polymerization potential of pyrrole over long alkyl chain ω-(N-pyrrolyl) alkanethiol modified gold electrodes was higher than that over short alkyl chain modified electrodes in KCl/H<INF>2</INF>O. The polymerization of pyrrole apparently occurred without monolayer oxidation. In Et<INF>4</INF>NBF<INF>4</INF>/CH<INF>3</INF>CN, SAMs of ω-(N-pyrrolyl) alkanethiols were stable under a potential scan. During the growth of polypyrrole, the pyrrole molecules in ω-(N-pyrrolyl) alkanethiols may serve as nucleation sites for the growth of polypyrrole. Therefore, polypyrrole films electrochemically deposited on ω-(N-pyrrolyl) alkanethiol monolayers have a denser morphological packing and good adhesion. The conductivity of these films is enhanced as a consequence of the change in morphology.