Ligand-Field-Modulated Molecular Junctions: On Covalently Bonded Ethynyl–Electrode Interfaces

Energy-level alignment (ELA) between Fermi levels of electrodes and frontier molecular orbitals (FMOs) dictates single-molecule I – V bias characteristics. Proposed herein to better achieve ELA is the drive of E FMO toward E Fermi via interactions between the anchoring group and the undercoordinated gold atom at the electrode apex, where the interactions and the shift of E FMO resemble the ligand-field-modulated orbital splitting. This concept is demonstrated by −C≡C–electrode and −C≡C–C≡C–electrode junctions. The junction current of the latter, with an additional ethynyl moiety, is slightly larger than that of the former for molecules with the same backbone. This finding is contradictory to the length-dependent exponential decrease in current. Simulations show that the orbital splitting creates an FMO composed of 5d xz orbitals of gold, with contributions not only from the ligated metal atom but also, intriguingly, from neighboring atoms or atoms in the underneath second layer, indicative of a distinct pinning effect. When the other terminal group is a −CN group, asymmetric I – V bias curves are observed. The degree of electric rectification is associated with the electrode configuration ( e.g ., pyramidal, stepped, or planar) and thus the strength of ethynyl–electrode ligand–metal orbital mixing. The scanning tunneling microscopy-based break junction technique shows a more pronounced rectification for junctions of the −C≡C–C≡C–electrode than for junctions of the −C≡C–electrode.