Surface Plasmon Resonance Spectroscopy as a Probe of In-Plane Polymerization in Monolayer Organic Conducting Films

Several groups have shown that alkanethiol-modified pyrroles can be tethered to a gold surface, but there is often little evidence that, once oxidized, the resulting monolayer film is an organic conducting polymer. Using surface plasma resonance (SPR) spectroscopy, we demonstrate for the first time that, upon electrochemical oxidation, self-assembled alkanethiol−pyrrole films on gold show behavior characteristic of organic conducting polymers: we observe reversible changes in the optical constants of the organic film upon doping/dedoping. Since the optical constants are related to film conductivity, we show that the effective isotropic dielectric constant of the film obtained in the standard SPR data analysis can be interpreted in terms of in-plane and out-of-plane contributions to film conductivity. We find that the in-plane conductivity of oxidized 3-(ω-mercaptoundecyl)pyrrole is smaller, but of the same order of magnitude, than that found for thick films of polypyrrole. Most importantly, we observe reversible changes in the optical constants of the polymerized film, which are consistent with electrochemical switching of an organic conducting polymer whose conductivity is largest for the doped state and decreases for the dedoped state.