Unusual Deprotonated Alkynyl Hydrogen Bonding in Metal-Supported Hydrocarbon Assembly

Abstract

We demonstrate that terminal alkynyl moieties represent powerful functional groups for driving thermally stable, on-surface supramolecular structure formation on a reactive substrate. Through a combination of scanning tunneling microscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption-fine-structure spectroscopy and density functional theory calculations, we investigate the molecule–surface interaction and self-assembly of two prototypical hydrocarbon species on Cu(111). For 1,3,5-tris(4-ethynylphenyl)benzene (Ext-TEB) adsorption at low temperature (200 K) results in nonassembling, conformationally adapted intact species. Deprotonation of the terminal alkyne moieties, taking place at temperatures ranging from 300 to 350 K, triggers the formation of room-temperature stable, close-packed supramolecular islands. Through DFT calculations, the stabilizing interaction is identified as a trifurcated ionic C–H···π–δ hydrogen bonding between the π-system of the ionic alkynyl groups and methine moieties of nearby benzene rings, providing an energy gain of 0.26 eV/molecule upon network formation. Robust assemblies result from the combination of this weak directional attraction with the strong surface anchoring also provided by the alkynyl groups. The generality of this novel ionic hydrogen-bonding type is demonstrated by the observation of low-dimensional assemblies of 9,10-diethynyl-anthracene on the same surface, consistently explained with the same type of interaction.