Role of site-specific coordination for the structural and electronic properties of a Co-HATCN network on silver

Overview & Synopsis

Two-dimensional metal-organic porous networks (2DMOPNs) on metal surfaces provide a versatile platform for exploring emergent electronic and magnetic phenomena in designed surface nanostructures. While the porous architecture is governed by coordination between molecular building blocks and metal centers, most 2DMOPNs utilize only one type of coordination site, leaving the impact of multiple coordination sites largely unexplored. We address this question by investigating the structural formation of a Co-coordinated 2DMOPN on Ag(111) with the linker molecule hexaazatriphenylene-hexacarbonitrile (HATCN), which has two chemically distinct nitrogen coordination sites at the peripheral nitrile groups and pyrazine-type core nitrogen atoms. Scanning tunneling microscopy (STM) reveals the transformation from a simple honeycomb HATCN structure to a Co-coordinated porous network with three structurally and electronically inequivalent molecules in a 6:3:1 ratio. The existence of two distinct coordination sites leads to selective Co coordination geometries for all three molecules, accompanied by site-specific charge transfer from Co into the former lowest unoccupied molecular orbital (LUMO), as revealed by momentum-resolved photoemission and density functional theory. Our results demonstrate that site-selective Co coordination not only controls the overall architecture of the porous network structure but also strongly influences the local electronic properties through site-specific metal-molecule interaction, providing a route to tailor the site-specific electronic structure of 2DMOPNs at the molecular level.