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

• Published in: Journal of the American Chemical Society Vol. 137; no. 27; pp. 8819 - 8828
• Main Authors: Demissie, Abel T, Haugstad, Greg, Frisbie, C. Daniel
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 15-07-2015
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/jacs.5b04512

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.