The realization of molecular electronics requires comprehension of single-molecule I-V characteristics. Aside from the electron-transport properties of the molecular framework, the molecule-electrode binding contributes significantly to the contact resistance, R n)0 , and thus to the values of single-molecule resistance. Isothiocyanate (-NCS), a versatile ligand for organometallics, can bind to a metal substrate to complete a metal-moleculemetal configuration for external measurements. Isothiocyanate has the advantage of being a π-conjugated moiety that presumably exhibits a relatively smaller impedance than the commonly used methylene thiol headgroup (-CH 2 SH) in many molecular wires. For example, this study shows that the single-molecule conductance of n-butanediisothiocyanate is an order of magnitude better than that of n-octanedithiol even though they both contain 10 atoms counted from sulfur to sulfur. For a homologous series of molecules, R n)0 can be extrapolated from the intercept of the resistance obtained by the repeated formation of molecular junctions using scanning tunneling microscopy. To isolate the contact effect of the -NCS-Au electrode from other factors, alkanediisothiocyanates were studied because the large HOMO-LUMO gap of alkyl chains is not sensitive to the number of methylene units. The results show two sets of R n)0 values, with the smaller set being 128 kΩ, about 1/12 the other value. A detailed examination of the results suggests that the preferential adsorption site for isothiocyanate on gold is the atop site rather than the 3-fold-hollow sites of thiol on gold.

The use of single-molecule junctions for various functions constitutes a central goal of molecular electronics. The functional features and the efficiency of electron transport are dictated by the degree of energy-level alignment (ELA), that is, the offset potential between the electrode Fermi level and the frontier molecular orbitals. Examples manifesting ELA are rare owing to experimental challenges and the large energy barriers of typical model compounds. In this work, single-molecule junctions of organometallic compounds with five metal centers joined in a collinear fashion were analyzed. The single-molecule i-V scans could be conducted in a reliable manner, and the EFMO levels were electrochemically accessible. When the electrode Fermi level (EF ) is close to the frontier orbitals (EFMO ) of the bridging molecule, larger conductance was observed. The smaller |EF -EFMO | gap was also derived quantitatively, unambiguously confirming the ELA. The mechanism is described in terms of a two-level model involving co-tunneling and sequential tunneling processes.

The influence of a pi-acid cyanide axial ligand on the metal-metal interactions of [Ru(3)(mu(3)-dpa)(4)(X)(2)](0/+) (X = NCS(-), CN(-)) is manifested by the measurements of single-molecule conductance coupled with in situ electrochemical control.

EMACs (extended metal-atom chains) offer a unique platform for the exploration of metal-metal interactions. There has been significant advances on the synthesis of EMACs, such as lengthening the chains up to 11 metal atoms thus far, integrating naphthyridine moieties for tuning the charge carried at metal centers, and manipulation of metal-metal interactions. However, the metal centers in EMACs hitherto are limited to first row transition metals which are more labile than those relatively inert ones with electrons filled in the 4d and 5d shells. In this Communication, the synthesis, crystallographic, magnetic, and electrical conducting studies of [Ru5(mu5-tpda)4Cl2] and [Ru5(mu5-tpda)4(NCS)2], the first pentanuclear EMACs of second-row transition metal, are reported.