Role of Electrostatics in Metal–Organic Junctions on Surfaces: para-Hexaphenyl-dicarbonitrile on Au(111)

Abstract

Nitrile (C≡N) terminated molecules have proven to be versatile molecular building blocks for engineering complex metal–organic junctions with tailored properties and functions. These junctions involve a rich variety of intermolecular interactions, where the role of electrostatics is not always clearly addressed. To gain deeper insight, we present a detailed combined experimental and computational study of the nature of the interaction between nitrile N and Au atoms. We have performed scanning tunneling microscopy (STM), noncontact atomic force microscopy (nc-AFM) measurements, and slab DFT calculations of para-hexaphenyl-dicarbonitrile (Ph₆(CN)₂) on the Au(111) surface, which self-assembles into three and 4-fold metal–organic junctions. We utilized van’t Hoff plots derived from our experimental data to determine the reaction enthalpies of 123 ± 9 and 100 ± 9 meV for the 3 and 4-fold metal–organic junctions, respectively. To better understand the intrinsic nature of the CN···Au interaction, we performed gas-phase calculations of [Ph₂(CN)₂···Au]^Q clusters for various charges Q to establish the most likely oxidation state of the Au atom. To this end, we carried out a charge population and quantum theory of atoms in molecules (QTAIM) topological analysis of the CN···Au interaction at the bond critical point. We conclude that the nature of the interaction is mostly driven by electrostatics and that the monocation cluster is the most favorable charge state for the metal–organic assembly.