Abstract
The effect of molecular structure on single molecular conductance is studied by STM (scanning tunneling microscopy), c-AFM (conductive atomic force microscopy), and the repeated formation of single molecular junctions (STM break junction). The target molecules are rigid metal strings (Figure 2-1) and somewhat flexible furan oligoaryls (Figure 3-1). For the former, those metal strings have linear metal-atom chains which are helically wrapped by four all-syn oligo-□-pyridylamine ligands (L) to form strings of [MnL4(NCS)2] (Mn = Cr3, Co3, Ni3, Cr5, Co5, Ni5, and Cr7). The metal-metal bond orders for strings of nickel, cobalt, and chromium cores are, respectively, 0, 0.5, and 1.5 which correlate well qualitatively and quantitatively with their relative conductance. Penta- and heptachromium strings exhibit two types of conductance, ascribed to conformations of symmetric and alternating Cr-Cr bond lengths. We manifest the effect of conformation on single-molecule conductance by relatively flexible furan oligoaryls with 6~18 conjugated double bonds. Also demonstrated is that the resistance at the molecule-electrode contact, instead of being a constant, is affected by the hybridization degree of the benzenedimethanethiol headgroup with the □-conjugated backbone.