Protected electronic structures of unoccupied molecular orbitals in metal-organic contacts

Molecular electronic properties are prone to be modified upon bonding with metal atoms, which, to a large extent, hinders the design and engineering of molecular electronic devices. Here, we report on protected electronic structures of unoccupied molecular orbitals in metal–organic contacts, investigated by using low temperature scanning tunneling microscopy/spectroscopy (STM/STS). On Au(111), Dicyanovinyl-hexathiophene (DCV6T) molecules self-assemble into diverse nanostructures including Au-atom-coordinated chains, where orbital realignment and redistribution are induced by Au-ligand hybridization. When, instead, cobalt atoms are deposited on Au(111) previous to DCV6T deposition, Co-atom-coordinated chains are formed. The hybridization with Co atoms results in band gap states at the ligands, presumably caused by the mixing of the cobalt 3d states and the occupied molecular orbitals. Whereas, STS measurements resolve that the lowest unoccupied molecular orbital (LUMO) and LUMO + 1 show the same features in DCV6T bonding with Co atoms as those in the uncoordinated molecules, in terms of orbital spatial distribution and energy alignment. Our study demonstrates that the protection of desired orbital structures in metal–organic contacts could be achieved by tuning the metal/ligand combination.