The achievement of atomic control over the organic–inorganic interface is key to engineering electronic and spintronic properties of molecular devices. We leverage insights from inorganic chemistry to create hard–soft acid–base (HSAB) theory-derived design principles for incorporation of single molecules onto metal electrodes. A single molecule circuit is assembled via a bond between an organic backbone and an under-coordinated metal atom of the electrode surface, typically Au. Here, we study molecular composition factors affecting the junction assembly of coordination complexes containing transition metals atoms on Au electrodes. We employ hetero- and homobimetallic lantern complexes and systematically change the coordination environment to vary the character of the intramolecular bonds relative to the electrode-molecule interaction. We observe that trends in the robustness and chemical selectivity of single molecule junctions formed with a range of linkers correlate with HSAB principles, which have traditionally been used to guide atomic arrangements in the synthesis of coordination complexes. We find that this similarity between the intermolecular electrode-molecule bonding in a molecular circuit and the intramolecular bonds within a coordination complex has implications for the design of metal-containing complexes compatible with electrical measurements on metal electrodes. Our results here show that HSAB principles determine which intramolecular interactions can be compromised by inter molecule-electrode coordination; in particular on Au electrodes, soft–soft metal–ligand bonding is vulnerable to competition from soft–soft Au-linker bonding in the junction. Neutral donor–acceptor intramolecular bonds can be tuned by the Lewis acidity of the transition metal ion, suggesting future synthetic routes toward incorporation of transition metal atoms into molecular junctions for increased functionality of single molecule devices.

Metallophilic interactions, weak interactions between closed-shell metal atoms, have been recently utilized to create unique nanostructures with anisotropy of electronic properties along the direction of the interaction. The strength of the metallophilic interaction is an important factor for the design of these nanostructures. Recently, Doerrer and co-workers presented a general metathesis route to create extended chains of metallophilic double salts with two modular opposite-charge ions with Au­(I) and Pt­(II) centers without bridging ligands. Here, we apply theoretical and experimental angle-resolved Raman spectroscopy to identify the vibrational signature associated with the Au­(I)–Pt­(II) interaction in the double salt wire, [Pt­(terpy)­Cl]­[Au­(CN)2)]. Our study reveals six Raman-active low-energy phonon modes below 75 cm–1 that are anisotropic, as shown by their polarization dependence. By analysis of the low-energy Raman spectrum and the nature of the associated phonon modes, we identify one mode to be associated with the intrachain Pt–Au interactions with a frequency of 57 cm–1. We show that the polarization dependence of the Raman spectrum is the key to elucidating directional metallophilic modes.

Understanding and manipulating quantum interference (QI) effects in single molecule junction conductance can enable the design of molecular-scale devices. Here we demonstrate QI between σ and π molecular orbitals in an ∼4 Å molecule, pyrazine, bridging source and drain electrodes. Using single molecule conductance measurements, first-principles analysis, and electronic transport calculations, we show that this phenomenon leads to distinct patterns of electron transport in nanoscale junctions, such as destructive interference through the para position of a six-membered ring. These QI effects can be tuned to allow conductance switching using environmental pH control. Our work lays out a conceptual framework for engineering QI features in short molecular systems through synthetic and external manipulation that tunes the energies and symmetries of the σ and π channels.