Molecular and Electronic Structure of Self-Assembled Monolayers Containing Ruthenium(II) Complexes on Gold Surfaces

Ru(II) bipyridyl complexes were covalently bonded to self-assembled monolayers (SAM) on Au surfaces. Their molecular and electronic structure was studied by means of polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS), photoelectron spectroscopies, scanning tunneling microscopy (STM) and density functional theory (DFT) calculations. We found that attaching the Ru complex to the SAM does not cause great modifications to its molecular structure, which retains the alkyl chain 30 deg tilted with respect to the surface normal. Furthermore, the Ru center is located 20 A away from the metal surface, i.e., at a sufficient distance to prevent direct electronic interaction with the substrate. Indeed the electronic structure of the Ru complex is similar to that of the free molecule with a HOMO molecular orbital mainly based on the Ru center located 2.1 eV below the Fermi edge and the LUMO molecular orbital based on the bipyridine groups located 1 eV above the Fermi level.