Engineering Charge Transport by Tunneling in Supramolecular Assemblies through Precise Control of Metal-Ligand Interactions.

Coordination-driven supramolecular assemblies are promising for nanometer-sized electronic devices due to the potential to manipulate metal-ligand interactions and thereby control charge transport via tunneling through these assemblies. Cross-plane charge tunneling is investigated in assemblies of metalloporphyrins and pillar molecules, specifically palladium(II) and zinc(II) octaethylporphyrin (PdOEP and ZnOEP) monolayers and bilayers with bidentate (DABCO) and monodentate (ABCO) pillar ligands on highly oriented pyrolytic graphite (HOPG). Junction measurements and quantum-chemical calculations reveal that metal-ligand interactions significantly influence charge transport via tunneling and thermoelectric effects. Weak interactions in PdOEP assemblies create isolated molecular orbitals on interior pillar ligands, compressing the HOMO-LUMO gap and enhancing tunneling currents with unusual, inverted attenuation behavior and high thermopower. Conversely, strong interactions in ZnOEP assemblies induce localized orbitals on the porphyrin, leading to conventional tunneling decay behavior and low thermopower. The study highlights the potential of metal-ligand interactions as a strategy to engineer molecular orbital distribution, enhancing quantum transport efficiency in molecular-scale devices.