Tunable interface of ruthenium porphyrins and silver

The adsorption and monolayer self-assembly of functional metal-organic blocks on solid surfaces are critical for the physicochemical properties of these low-dimensional materials. Although modern microscopy tools are very sensitive to the lateral arrangement of such blocks, they are still unable to offer directly the complete structural analysis especially for nonplanar molecules containing different atoms. Here, we apply a combinatorial approach for the characterization of such interfaces, which enables unexpected insights. An archetypal metalloporphyrin on a catalytically active surface as a function of its molecular coverage and substituent arrangement is characterized by low-energy electron diffraction, scanning probe microscopy, X-ray photoelectron spectroscopy, normal-incidence X-ray standing waves, and density functional theory. We look into Ru tetraphenyl porphyrin (Ru-TPP) on Ag(111), which is also converted into its planarized derivates via surface-assisted cyclodehydrogenation reactions. Depending on the arrangement of the phenyl substituents, the Ru atoms have distinct electronic structures and the porphyrin macrocycles adapt differently to the surface: saddle shape (pristine Ru-TPP) or bowl shape (planarized Ru-TPP derivates). In all cases, the Ru atom resides close to the surface (2.59/2.45 Å), preferably located at hollow sites and in the interface between the plane of the porphyrin macrocycle and the Ag surface. For the more flexible pristine Ru-TPP, we identify an additional self-assembled structure, allowing the molecular density of the self-assembled monolayer to be tuned within ∼20%. This precise analysis is central to harnessing the potential of metalloporphyrin/metal interfaces in functional systems.