Letters Surface Plasmon Resonance Spectroscopy as a Probe of In-Plane Polymerization in Monolayer Organic Conducting Films R. Georgiadis,* K. A. Peterlinz, J. R. Rahn, A. W. Peterson, and J. H. Grassi Department of Chemistry, Boston University, Boston, Massachusetts 02215 Received December 31, 1999. In Final Form: June 21, 2000 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. Introduction Chemical reactions in supported molecular assemblies are important in research ranging from biology to materi- als science. Surface-immobilized monomers offer unique opportunities for study, optimization, and exploitation of the effect of the surface environment on the process of interest. Much use has been made of self-assembled monolayer films of alkanethiols in research as a means to promote adhesion at interfaces, to resist corrosion, and to fabricate tailored surfaces of chemical and biological sensors and molecular electronics. 1 Monomolecular con- stituents of the film, when functionalized with polymer- izable moieties, can be chemically or electrochemically induced to polymerize. One class of films that have attracted much attention due to potential applications are monomolecular al- kanethiol films functionalized with a terminal pyrrole group. Pyrrole can polymerize to form polypyrrole, an organic conducting polymer which can be switched between conducting and insulating states by reversible electrochemical doping and dedoping. 2 As usually grown, polypyrrole films do not adhere well to substrates, are rough and irregular, and have conductivities which are too small for most practical applications. For bulk poly- pyrrole, it has been shown that the conductivity can be improved when the material is grown in a more ordered, constrained geometry. 3 Several groups have shown that alkanethiol-modified pyrroles can be tethered to a gold surface, 4-18 but there (1) Whitesides, G. M.; Ferguson, G. S.; Allara, D.; Scherson, D.; Speaker, L.; Ulman, A. Crit. Rev. Surf. Chem. 1993, 3, 49-65. (2) Patil, A. O.; Heeger, A. J.; Wudl, F. Chem. Rev. 1988, 88, 183- 200. (3) Menon, V. P.; Lei, J. T.; Martin, C. R. Chem. Mater. 1996, 8, 2382-90. (4) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823-10824. (5) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302-306. (6) Willicut, R. J.; McCarley, R. L. Adv. Mater. 1995, 7, 759-762. (7) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296-301. © Copyright 2000 American Chemical Society AUGUST 22, 2000 VOLUME 16, NUMBER 17 10.1021/la9917076 CCC: $19.00 © 2000 American Chemical Society Published on Web 07/29/2000