Selective Supramolecular Fullerene–Porphyrin Interactions and Switching in Surface-Confined C₆₀–Ce(TPP)₂ Dyads

The control of organic molecules, supramolecular complexes and donor–acceptor systems at interfaces is a key issue in the development of novel hybrid architectures for regulation of charge-carrier transport pathways in nanoelectronics or organic photovoltaics. However, at present little is known regarding the intricate features of stacked molecular nanostructures stabilized by noncovalent interactions. Here we explore at the single molecule level the geometry and electronic properties of model donor–acceptor dyads stabilized by van der Waals interactions on a single crystal Ag(111) support. Our combined scanning tunneling microscopy/spectroscopy (STM/STS) and first-principles computational modeling study reveals site-selective positioning of C₆₀ molecules on Ce­(TPP)₂ porphyrin double-decker arrays with the fullerene centered on the π-system of the top bowl-shaped tetrapyrrole macrocycle. Three specific orientations of the C₆₀ cage in the van der Waals complex are identified that can be reversibly switched by STM manipulation protocols. Each configuration presents a distinct conductivity, which accounts for a tristable molecular switch and the tunability of the intradyad coupling. In addition, STS data evidence electronic decoupling of the hovering C₆₀ units from the metal substrate, a prerequisite for photophysical applications

How Surface Bonding and Repulsive Interactions Cause Phase Transformations: Ordering of a Prototype Macrocyclic Compound on Ag(111)

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 <i>via</i> 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

Self-Terminating Protocol for an Interfacial Complexation Reaction <i>in Vacuo</i> by Metal–Organic Chemical Vapor Deposition

The fabrication and control of coordination compounds or architectures at well-defined interfaces is a thriving research domain with promise for various research areas, including single-site catalysis, molecular magnetism, light-harvesting, and molecular rotors and machines. To date, such systems have been realized either by grafting or depositing prefabricated metal–organic complexes or by protocols combining molecular linkers and single metal atoms at the interface. Here we report a different pathway employing metal–organic chemical vapor deposition, as exemplified by the reaction of <i>meso</i>-tetraphenylporphyrin derivatives on atomistically clean Ag(111) with a metal carbonyl precursor (Ru₃(CO)₁₂) under vacuum conditions. Scanning tunneling microscopy and X-ray spectroscopy reveal the formation of a <i>meso</i>-tetraphenylporphyrin cyclodehydrogenation product that readily undergoes metalation after exposure to the Ru-carbonyl precursor vapor and thermal treatment. The self-terminating porphyrin metalation protocol proceeds without additional surface-bound byproducts, yielding a single and thermally robust layer of Ru metalloporphyrins. The introduced fabrication scheme presents a new approach toward the realization of complex metal–organic interfaces incorporating metal centers in unique coordination environments