High-Conductive Organometallic Molecular Wires with Delocalized Electron Systems Strongly Coupled to Metal Electrodes

Abstract: Besides active, functional molecular building blocks such as diodes or switches, passive components as, e.g., molecular wires, are required to realize molecular-scale electronics. Incorporating metal centers in the molecular backbone enables the molecular energy levels to be tuned in respect to the Fermi energy of the electrodes. Furthermore, by using more than one metal center and sp-bridging ligands, a strongly delocalized electron system is formed between these metallic "dopants", facilitating transport along the molecular backbone. Here, we study the influence of molecule--metal coupling on charge transport of dinuclear X(PP)$_2$FeC$_4$Fe(PP)$_2$X molecular wires (PP = Et$_2$PCH$_2$CH$_2$PEt$_2$); X = CN (1), NCS (2), NCSe (3), C$_4$SnMe$_3$ (4) and C$_2$SnMe$_3$ (5)) under ultra-high vacuum and variable temperature conditions. In contrast to 1 which showed unstable junctions at very low conductance ($8.1\cdot10<sup>{-7}$ G$_0$), 4 formed a Au-C$_4$FeC$_4$FeC$_4$-Au junction 4' after SnMe$_3$ extrusion which revealed a conductance of $8.9\cdot10<sup>{-3}$ G$_0$, three orders of magnitude higher than for 2 ($7.9\cdot10<sup>{-6}$ G$_0$) and two orders of magnitude higher than for 3 ($3.8\cdot10<sup>{-4}$ G$_0$). Density functional theory (DFT) confirmed the experimental trend in the conductance for the various anchoring motifs. The strong hybridization of molecular and metal states found in the C--Au coupling case enables the delocalized electronic system of the organometallic Fe$_2$ backbone to be extended over the molecule-metal interfaces to the metal electrodes to establish high-conductive molecular wires.