title: Growth of Thin, Anisotropic, π-Conjugated Molecular Films by Stepwise "Click" Assembly of Molecular Building Blocks: Characterizing Reaction Yield, Surface Coverage, and Film Thickness versus Addition Step Number.
abstract: We report the systematic characterization of anisotropic, π-conjugated oligophenyleneimine (OPI) films synthesized using stepwise imine condensation, or "click" chemistry. Film synthesis began with a self-assembled monolayer (SAM) of 4-formylthiophenol or 4-aminothiophenol on Au, followed by repetitive, alternate addition of terephthalaldehyde (benzene-1,4-dicarbaldehyde) or 1,4-benzenediamine to form π-conjugated films ranging from 0.6-5.5 nm in thickness. By systematically capping the OPI films with a redox or halogen label, we were able to measure the relative surface coverage after each monomer addition via Rutherford backscattering spectrometry, X-ray photoelectron spectroscopy, spectroscopic ellipsometry, reflection-absorption infrared spectroscopy, and cyclic voltammetry. Nuclear reaction analysis was also employed for the first time on a SAM to calculate the surface coverage of carbon atoms after each stepwise addition. These six different analysis methods indicate that the average extent of reaction is 99% for each addition step. The high yield and molecular surface coverage confirm the efficacy of Schiff base chemistry, at least with the terephthalaldehyde and 1,4-benzenediamine monomers, for preparing high-quality molecular films with π conjugation normal to the substrate.
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States.
authors.forename: Abel T
authors.initials: AT
authors.lastname: Demissie
authors.affiliation: ‡Characterization Facility, University of Minnesota,100 Union Street SE, Minneapolis, Minnesota 55455, United States.
authors.forename: Greg
authors.initials: G
authors.lastname: Haugstad
authors.affiliation: †Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Avenue SE, Minneapolis, Minnesota 55455, United States.
authors.forename: C Daniel
authors.initials: CD
authors.lastname: Frisbie