Reversible Sliding Motion by Hole-Injection in Ammonium-Linked Ferrocene, Electronically Decoupled from Noble Metal Substrate by Crown-Ether Template Layer

Abstract

Artificial molecular machines, especially when based on wheel-and-axle complexes, can generate mechanical motions in response to external stimuli. Ferrocene (Fc) is a key component, but it decomposes at 300 K on metal surfaces. Here, a novel method is presented to construct and control the molecular complex composed of ammonium-linked ferrocene (Fc-amm) and tetrabrominated crown ether (BrCR) on a Cu(111) surface. Fc-amm molecules are periodically arranged on a BrCR monolayer film and imaged using scanning tunneling microscopy and spectroscopy. A lateral motion of the Fc groups by ≈0.1 nm is observed for pairs of "edge-on" Fc-amm molecules upon hole injection. This sliding motion is reversible and controlled by the applied voltage. Theoretical analysis indicates that the motion is caused by increased Coulomb repulsion of the hole-doped Fc-amm+ ions and accompanied by a weakening of CH-π interactions. These findings open new avenues for developing nanomolecular devices using on-surface bottom-up processes.

Keywords: Ferrocene; Supramolecule; crown ether; molecular motion; scanning tunneling microscopy.