Inter- and Intramolecular On-Surface Synthesis of Porphyrin-Based Nanostructures on Au(111) and Cu(111)

Surface-confined synthesis provides alternative reaction pathways to those utilized within solution-phase chemistry and offers a route to extended molecular architectures with nanoscale dimensions and fascinating magnetic, electronic, and catalytic properties. However, these reaction pathways can be complex multistep processes, containing multiple reactive intermediates. Optimizing the selectivity and efficiency of such synthetic routes should be underpinned by detailed mechanistic insight, which requires submolecular spatial resolution in combination with details of chemical evolution throughout the reaction process. A key challenge is the application of an experimental methodology that allows in-depth study of multistep reactions. Here, we combine the spatial resolution of scanning tunneling microscopy with temperature-programmed photoelectron spectroscopies and present a comprehensive characterization of a multistep on-surface reaction utilizing a brominated porphyrin species. The porphyrin species employed is a highly functionalizable “molecular building block” from which nanostructured materials can be built, and within this work we identify key differences between the reaction on Cu(111) and Au(111). Intermolecular Ullmann-type coupling as well as intramolecular ring-closing and self-metalation are observed: specifically, on Au(111) we characterize self-metalation within covalently coupled assemblies of ring-closed TPP. Our results highlight the differing reactivity of Au(111) and Cu(111) and the strong influence of the substrate upon the reaction pathway and preferred products, and we provide spectroscopic and topographical characterization for all reaction steps.