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 use of single-molecule junctions for various functions constitutes ac entral 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 owingt oe xperimental challenges and the large energy barriers of typical model compounds.I nt his work, singlemolecule junctions of organometallic compounds with five metal centers joined in acollinear fashion were analyzed. The single-molecule i-V scans could be conducted in ar eliable manner,and the E FMO levels were electrochemically accessible. When the electrode Fermi level (E F )i sc lose to the frontier orbitals (E FMO )o ft he bridging molecule,l arger conductance was observed. The smaller j E F ÀE FMO j gap was also derived quantitatively,u nambiguously confirming the ELA. The mechanism is described in terms of atwo-level model involving co-tunneling and sequential tunneling processes.Electron transport is an essential theme across major disciplines and is of utmost importance to the success of molecule-based electronic devices.F or further improving such systems,a nu nderstanding of the interfacial transport processes,which can be analyzed in terms of the energy-level alignment (ELA), is crucial.[1] As mall barrier height (f B ), that is,the difference between the electrode Fermi level (E F ) and the frontier molecular orbitals (E FMO ), is expected to lead to facile transport. Thedegree of ELA for thin-film devices is typically unveiled by ultraviolet photoelectron spectroscopy (UPS).[1] However,f or ultimate miniaturization down to the single-molecule level, the relatively large beam size of UPS renders it unsuitable for probing local structures.M oreover, each single-molecule junction confers only one f B .The lack of systematic changes in f B makes the correlation with electrontransport efficiencydifficult. To this end, E FMO can be shifted with ag ate electrode by the field effect with single-molecule transistors (SMTs).[2] Unfortunately,S MT studies are very limited owing to fabrication difficulties.S canning tunneling microscopy (STM) based break junctions have been shown to be ac onvenient method for creating single-molecule junctions, [3] and the tuning of E F and f B is achievable through electrochemical control (hereafter referred to as EC-STM BJ) by driving the potential of the working electrode (E wk ) against that of the reference electrode. [4][5][6][7][8] Thus far, the effect of f B on the single-molecule conductance has only been proposed sporadically.[6-9] Most EC-STM BJ studies were performed at fixed E wk values and unable to monitor the system upon E F approaching E FMO .EC studies of single-molecule conductance focus mostly on organic redox moieties. [5,[7][8][9][10] Examples of organometallic compounds are rare and limited to those with one [11][12][13] or two [14,15] metal centers,s uch as ferrocenedicarbo...

The synthesis, crystal structure, magnetic properties, and single-molecule conductance of two new trinuclear metal string complexes, [Ni(3)(dzp)(4)(NCS)(2)] (2) and [Co(3)(dzp)(4)(NCS)(2)] (3), containing the rigid Hdzp ligand (1, 1,9-diazaphenoxazine) are reported. X-ray structural analyses show that compounds 2 and 3 exhibit smaller torsion angles and longer metal-metal distances than those exhibited by the corresponding dpa(-) analogues (dpa(-) = dipyridylamido anion) due to the rigidity of Hdzp ligands. The longer metal-metal distance observed for 2 and 3 results in variations in their magnetic properties. The exchange interaction (J = -160 cm(-1)) between two high spin (HS) Ni(II) ions in 2 decreases slightly in comparison with those of trinickel dpa(-) analogues. The doublet-quartet gap of 3 is smaller than that of [Co(3)(dpa)(4)(NCS)(2)] (4), which causes compound 3 to show spin-crossover behavior even at low temperature.