Revisit of trinickel metal string complexes [Ni3L4X2] (L = dipyridylamido, diazaphenoxazine; X = NCS, CN) for quantum transport

Abstract

AbstractMetal 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.