XPS Study of Self-Assembly of Ruthenium Dimers [((acac)₂Ru)₂bptz]⁰,⁺ on Hydrophobic and Hydrophilic SAMs

Both neutral and monocationic ruthenium complexes form self-limiting monolayers on top of organothiol/gold SAMs that bear hydrophilic terminating groups. The SAMs are soaked in solutions of [((acac) 2 Ru) 2 bptz] 0 or [((acac) 2 Ru) 2 bptz] + (acac = acetylacetonate, bptz = 3,6-bis-pyridyl-1,2,4,5-tetrazine). Saturation coverage is reached in 3 h of soaking time. X-ray photoelectron spectroscopy (XPS) of powder samples and of monolayer samples shows that the complexes remain intact on binding to surfaces and that the underlying SAMs are not damaged by exposure to the metal complexes. The saturation coverage of the neutral Ru dimers on SAMs that terminate in either −OH or −PO 3 H 2 is estimated to be 145 ± 10 Å 2 per molecule. The monocationic dimer occupies 170 ± 10 Å 2 per molecule on the same surfaces, probably due to the inclusion of the PF 6 - counterion (which is observed in the XPS spectrum). These surface coverages and the observed ellipsometric increase in thickness of 7 ± 1 Å are consistent with monolayer coverage and a flat orientation of the molecules. Neither complex binds to methyl-terminated SAMs. In the monolayers of the monocationic mixed-valence species [((acac) 2 Ru) 2 bptz] +, only one set of XPS binding energies is observed for ruthenium 3d and 3p orbitals, implying that both Ru atoms in the dimer are in the same oxidation state on a very short time scale. This is consistent with the strong delocalization of charge in this compound previously inferred from spectroscopic and electrochemical measurements.