Influence of Low-Symmetry Distortions on Electron Transport through Metal Atom Chains: When Is a Molecular Wire Really “Broken”?

Abstract

In the field of molecular electronics, an intimate link between the delocalization of molecular orbitals and their ability to support current flow is often assumed. Delocalization, in turn, is generally regarded as being synonymous with structural symmetry, for example, in the lengths of the bonds along a molecular wire. In this work, we use density functional theory in combination with nonequilibrium Green's functions to show that precisely the opposite is true in the extended metal atom chain Cr(3)(dpa)(4)(NCS)(2) where the delocalized π framework has previously been proposed to be the dominant conduction pathway. Low-symmetry distortions of the Cr(3) core do indeed reduce the effectiveness of these π channels, but this is largely irrelevant to electron transport at low bias simply because they lie far below the Fermi level. Instead, the dominant pathway is through higher-lying orbitals of σ symmetry, which remain essentially unperturbed by even quite substantial distortions. In fact, the conductance is actually increased marginally because the σ(nb) channel is displaced upward toward the Fermi level. These calculations indicate a subtle and counterintuitive relationship between structure and function in these metal chains that has important implications for the interpretation of data emerging from scanning tunnelling and atomic force microscopy experiments.

From Homonuclear Metal String Complexes to Heteronuclear Metal String Complexes

“…[62] McGrady et al addressed the relationship between the structure and transport phenomena of metal string complexes by analysis of their conduction channels. [27,44,48,[63][64][65][66] With the development of HMSCs, theoretical studies provide directions for the design of molecular rectifiers. Density functional theory calculations suggested that an asymmetric current-voltage response depends strongly on the symmetry of the dominant conduction channel: π channels are more localized and, hence, afford higher rectification ratios.…”

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

“…45,[50][51][52][53] However, as discussed by McGrady in the context of pseudo-1D metal string complexes, efficient through-molecule conductance can be achieved through frontier orbitals which are distributed near, and energetically aligned with, the electrodes and need not be evenly distributed along the entire molecular backbone. 54,55 Recently, the molecular conductances of Pt(II) complexes trans- Through numerous studies of molecular junctions, the nature of the molecule-electrode contact has proven important, 57 and recognition of the role that anchor groups play in the electrical performance of the junction has led to the development of a wide range of contacting groups and electrode materials. [58][59][60][61][62][63] However, even well-known contacting groups such as thiolates bind to a range of surface sites, including terraces or near to adatoms as well as idealised pristine atomically flat terraces, giving rise to a range of molecular conductance signatures from a given molecular backbone.…”

Synthesis and structure of solution-stable one-dimensional palladium wires

“…But few examples of 1-D metal chain polymers supported by metal-metal bonds have been reported, which has limited the knowledge of their conductive properties. Even for well-studied oligomeric metal chains, which can be considered as model systems for extended 1-D metal wires, the factors that determine conductivity are a subject of current debate 21 . Unlike conductive organic polymers, for which the principles of controlling conductivity are well established 22 , examples of polymers based on 1-D metal wires that allow for rational control over conductivity are scarce 23 .…”