Imaging the Predicted Isomerism of Oligo(aniline)s: A Scanning Tunneling Microscopy Study

As model systems of poly(aniline), oligo(aniline)s have been previously predicted to exist in a number of isomeric forms, with positional, geometric and conformational isomerism possible. Here, monolayers of phenyl and alkyl-chain end-capped tetra(aniline)s in their half-oxidised emeraldine base (EB) state are thermally evaporated in ultra-high vacuum (UHV) conditions. Scanning tunneling microscopy (STM) is then used to directly visualize individual cis/trans isomers. The influence the observed isomerism has on the two-dimensional self-assembly of these conjugated systems is discussed. Gas-phase density functional theory (DFT) calculations are used to corroborate the experimental data, providing optimized structural data that corresponds well to the isomers observed by STM. The proof of the presence of these isomers provides the basis for understanding and gaining insight into the lack of well-defined self-assembled thin films and single crystals from aniline-based materials in their half-oxidised EB state. The assembly behavior and properties of molecular thin films are thus clearly and intractably linked, highlighting the importance of isomerism and material choice for organic thin film applications.