Theoretical understanding of the inversion of rectification direction in ferrocenyl-embedded tridecanethiolate single-molecule rectifiers

The rectifying properties of single-molecule diodes, which consist of one ferrocenyl-embedded tridecanethiolate (SCnFc13−n, n = 0-13) sandwiched between two symmetric Ag(111) electrodes, have been theoretically studied by first-principles calculations. The numerical results show that rectification direction as well as the rectification ratio of the single-molecule rectifiers can be well controlled by judiciously designing the site of ferrocenyl (Fc) group within the tridecanethiolate backbone. Further analysis reveals that frontier molecular orbitals (FMOs) of the molecules are spatially localized on the Fc group. Asymmetrically positioning of the Fc group within the tridecanethiolate backbone causes asymmetric evolutions of the FMOs under forward and backward bias voltages, which is the underlying rectification mechanism for the single-molecule diodes. Furthermore, varying the location of Fc group within the tridecanethiolate backbone modifies the evolving feature of the FMOs under bias voltages and hence changes the rectification performance and rectification direction of the molecular diodes. In addition, a simple model is proposed here to understand the asymmetric evolutions of the spatially localized FMOs, which are ascribed to the electric potential energy effect of electronic wavefunctions in external electric field. Our first-principles and modeling results give a deeper insight into the recent experimental observation [Nat. Commun.6, 6324 (2015)] and are of great help to future rational design of molecular rectifiers.