Striding to extend the length of metal-atom strings, oligo-α-pyridylamino ligands are modulated with naphthyridyl moieties leading to the undeca-nickel mixed-valence complexes [Ni(bnatpya)Cl] (1) and [Ni(bnatpya)Cl] (2). The first single-molecule conductance measurements of a linear undeca-nickel chain were performed.

Metal string complexes contain a linear metal‐atom chain in which the metal centers are coordinated by four equatorial and two axial ligands. With a variety of transition‐metal elements and ligands, the structural framework drives the flourishing of molecular design and properties. The one‐dimensional configuration makes the compounds suitable for the studies of quantum transport across molecular junctions. In this study, we report the conductance measurements and transmission spectra of three trinickel metal strings, [Ni3(dpa)4(NCS)2] (1), [Ni3(dzp)4(NCS)2] (2), and [Ni3(dpa)4(CN)2] (3) (Hdpa = dipyridylamine, Hdzp, diazaphenoxazine) in which 1 is a prototypical compound, dzp of 2 represents an equatorial ligand more rigid than dpa of 1, and ─CN is an axial ligand with a ligand‐field effect stronger than ─NCS of 1. Measurement results of molecular junctions for 1, 2, and 3 are 2.69, 3.24, and 17.4 MΩ, respectively. The highest occupied molecular orbital and lowest unoccupied molecular orbital (HOMO–LUMO) gaps calculated by density functional theory in the gas phase for 1, 2, and 3 are about 2.65, 2.34, and 3.85 eV, respectively. Zero‐bias transmission spectra of 1–3 show that transmission peaks lie just above EFermi (the Fermi energy of the gold electrode), suggesting LUMO‐dominant transport pathways. The transmission peaks at EFermi are associated with LUMO+2 found in the gas phase. LUMOs in the free space are located at nearly 1 eV below EFermi. The shift of molecular orbitals from their isolated form and the alignment of LUMO+2 with the electrode Fermi level manifest the importance and significant of the electrodes' self‐energy on electron transport.

Charge transport across molecular junctions can be described by G = G contact exp(−βL), envisioned as sequential propagation through electrode-molecule contacts (G contact ) and the molecular backbone (exp(−βL)). How G contact and exp(−βL) are modulated by the chemical potentials of the electrodes (E F ), although essential, remains relatively unexplored because E F is typically driven by the applied V bias and hence limited to a small range in that a large V bias introduces complicated transport pathways. Herein, the interrelated E F and V bias are electrochemically disentangled by fixing V bias at a small value while potentiostatically positioning the electrode E F in a 1.5 V range. The results show that E F affects G contact more pronouncedly than the molecular backbone. For the covalently anchored acetylene-electrode (CC−Au) junctions, the energy level of the frontier molecular orbital (E FMO ) is found to shift nonlinearly as E F changes; |E FMO − E F | is independent of E F in the range of −0.25 to 0.00 V (vs E Ag/AgCl ) and is narrowed by ∼32% at 0.00−0.75 V. These findings are elucidated by the refined Simmons model, Newns-Anderson model, and single-level Breit−Wigner formula and quantitatively shed light on the influence of electrodes on the molecular orbitals (viz., the self-energy, Σ).

Energy-level alignment (ELA) between Fermi levels of electrodes and frontier molecular orbitals (FMOs) dictates single-molecule I–V bias characteristics. Proposed herein to better achieve ELA is the drive of E FMO toward E Fermi via interactions between the anchoring group and the undercoordinated gold atom at the electrode apex, where the interactions and the shift of E FMO resemble the ligand-field-modulated orbital splitting. This concept is demonstrated by −CC–electrode and −CC–CC–electrode junctions. The junction current of the latter, with an additional ethynyl moiety, is slightly larger than that of the former for molecules with the same backbone. This finding is contradictory to the length-dependent exponential decrease in current. Simulations show that the orbital splitting creates an FMO composed of 5d xz orbitals of gold, with contributions not only from the ligated metal atom but also, intriguingly, from neighboring atoms or atoms in the underneath second layer, indicative of a distinct pinning effect. When the other terminal group is a −CN group, asymmetric I–V bias curves are observed. The degree of electric rectification is associated with the electrode configuration (e.g., pyramidal, stepped, or planar) and thus the strength of ethynyl–electrode ligand–metal orbital mixing. The scanning tunneling microscopy-based break junction technique shows a more pronounced rectification for junctions of the −CC–CC–electrode than for junctions of the −CC–electrode.

The ligand, 2,7-bis(α-5-phenylpyrazinamino)-1,8naphthyridine (H 2 bphpzany), was synthesized by the reaction of 2,7-dichloro-1,8-naphthyridine with 2-amino-5-phenylpyrazine in the presence of potassium tert-butoxide under palladium(0)-catalyzed conditions. Linear defective penta-cobalt metal-string complex [Co 5 (bphpzany) 4 (NCS) 2 ] (1) and hexa-cobalt Co 6 11+ complex [Co 6 (bphpzany) 4 (NCS) 2 ](PF 6 ) (2) each containing four bphpzany 2ligands were synthesized, and their structure was determined using single-crystal X-ray diffraction. The structure of complex 1 consists of two lantern-type dinuclear Co 2 fragments at the terminal positions and one rare octacoordinated cobalt at the center forming a linear