On-Surface Synthesis of 1D and 2D Porphyrin Metal-Organic Networks on Ag(100) Tuned by Substrate Temperature

The production of single-layer polymer ribbons and sheets composed of fused porphyrin rings, named porphene, an analog to graphene, has been aimed over the years as a promising material for applications. Several attempts to polymerize porphyrin rings have used the Ullmann-coupling reaction among the different synthesis routes. In this paper, we report the on-surface synthesis of organometallic molecular chains (1D) and porous networks (2D) from the free-base 5,10,15,20-tetrakis(4-bromophenyl)-porphyrin (H₂TBrPP) on Ag(100). We investigated the adsorption of H₂TBrPP using a comprehensive multitechnique approach combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS), with density functional theory (DFT) based calculations. We find that distinct thermal stimuli applied to the substrate during the deposition lead to different final nanostructures (1D and 2D). This shows an evident thermal selectivity related to porphyrin conformation and substrate interaction, revealing different activation energies. Free-based porphyrins are shown to adopt the so-called “inverted” conformation on Ag(100), which was previously reported exclusively on the Cu(111) surface. Here, we explain the coexistence of these two porphyrins’ molecular conformations (“inverted” versus “saddle”). Moreover, based on DFT calculations, we propose a mechanism to overcome the kinetically trapped intermediate organometallic state to allow complete C-C coupling during the Ullmann reaction of porphyrin ring derivatives.