Molecular Packing Governed by a Single Carbon Atom: Odd–Even Effects in 4-n-Alkyloxybenzoic Acids on Au(111)

Two-dimensional (2D) self-assembly monolayers of 4-n-alkyloxybenzoic acids (4-OBA) on Au(111) were investigated by scanning tunneling microscopy (STM) and semiempirical modeling. The study reveals that the supramolecular organization of these molecules is governed by the interplay between directional intermolecular hydrogen and dipole bonding and nondirectional van der Waals (vdW) interactions with the metallic substrate. We observe a clear odd/even effect: the parity (even or odd) of the carbon number in the alkyloxy chain leads to distinct packing geometries. This behavior is attributed to a compromise between optimal intermolecular bonding, maximization of molecular packing, and substrate-adsorbate interaction. Both assemblies exhibit weak interactions with the substrate, as indicated by the preservation of the Au(111) herringbone reconstruction and molecular mobility. By integrating STM experiments with semiempirical modeling, we correlate real-space imaging with molecular-level conformational analysis, thereby offering a comprehensive picture of the structural determinants of chain-parity effects. These findings highlight the subtle balance of intermolecular and molecule-substrate forces in directing 2D crystal formation and offer a platform for engineering tunable supramolecular monolayers via chain-length control.