title: How surface bonding and repulsive interactions cause phase transformations: ordering of a prototype macrocyclic compound on Ag(111).
abstract: We investigated the surface bonding and ordering of free-base porphine (2H-P), the parent compound of all porphyrins, on a smooth noble metal support. Our multitechnique investigation reveals a surprisingly rich and complex behavior, including intramolecular proton switching, repulsive intermolecular interactions, and density-driven phase transformations. For small concentrations, molecular-level observations using low-temperature scanning tunneling microscopy clearly show the operation of repulsive interactions between 2H-P molecules in direct contact with the employed Ag(111) surface, preventing the formation of islands. An increase of the molecular coverage results in a continuous decrease of the average intermolecular distance, correlated with multiple phase transformations: the system evolves from an isotropic, gas-like configuration via a fluid-like phase to a crystalline structure, which finally gives way to a disordered layer. Herein, considerable site-specific molecule-substrate interactions, favoring an exclusive adsorption on bridge positions of the Ag(111) lattice, play an important role. Accordingly, the 2D assembly of 2H-P/Ag(111) layers is dictated by the balance between adsorption energy maximization while retaining a single adsorption site counteracted by the repulsive molecule-molecule interactions. The long-range repulsion is associated with a charge redistribution at the 2H-P/Ag(111) interface comprising a partial filling of the lowest unoccupied molecular orbital, resulting in long-range electrostatic interactions between the adsorbates. Indeed, 2H-P molecules in the second layer that are electronically only weakly coupled to the Ag substrate show no repulsive behavior, but form dense-packed islands.
authors.affiliation: Physik Department E20, Technische Universität München, James Franck Straße 1, D-85748 Garching, Germany.
authors.forename: Felix
authors.initials: F
authors.lastname: Bischoff
authors.forename: Knud
authors.initials: K
authors.lastname: Seufert
authors.forename: Willi
authors.initials: W
authors.lastname: Auwärter
authors.forename: Sushobhan
authors.initials: S
authors.lastname: Joshi
authors.forename: Saranyan
authors.initials: S
authors.lastname: Vijayaraghavan
authors.forename: David
authors.initials: D
authors.lastname: Écija
authors.forename: Katharina
authors.initials: K
authors.lastname: Diller
authors.forename: Anthoula C
authors.initials: AC
authors.lastname: Papageorgiou
authors.forename: Sybille
authors.initials: S
authors.lastname: Fischer
authors.forename: Francesco
authors.initials: F
authors.lastname: Allegretti
authors.forename: David A
authors.initials: DA
authors.lastname: Duncan
authors.forename: Florian
authors.initials: F
authors.lastname: Klappenberger
authors.forename: Florian
authors.initials: F
authors.lastname: Blobner
authors.forename: Runyuan
authors.initials: R
authors.lastname: Han
authors.forename: Johannes V
authors.initials: JV
authors.lastname: Barth
mesh_terms: D001665:Binding Sites; D047028:Macrocyclic Compounds / Q000737:chemistry*; D008422:Materials Testing; D053768:Metal Nanoparticles / Q000737:chemistry* / Q000648:ultrastructure*; D010316:Particle Size; D044367:Phase Transition; D012834:Silver / Q000737:chemistry*; D055672:Static Electricity; D013499:Surface Properties
chemical_list: D047028:Macrocyclic Compounds; D012834:Silver