Roles of Precursor Conformation and Adatoms in Ullmann Coupling: An Inverted Porphyrin on Cu(111)

Surface diffusion, molecular conformation, and on-surface coupling reactions are key processes for building tailored molecular nanostructures such as graphene nanoribbons, polycyclic aromatic hydrocarbons, and one-dimensional/two-dimensional (2D) polymers. Here, we study the surface diffusion and coupling in situ of a chlorinated porphyrin, namely 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (Cl4TPP), using a combined scanning tunneling microscopy (STM), density functional theory (DFT), and X-ray photoelectron spectroscopy approach. Using STM, we obtain surface migration and rotation barriers ΔE of 0.77 ± 0.09 and 0.93 ± 0.28 eV, respectively, indicative of covalent binding to the surface. In fact, we find that the precursors as well as all the reaction species exclusively (≈100%) adopt a peculiar “inverted” conformation covalently bonded to Cu(111). Using DFT, we have mapped two coupling reaction pathways: direct dechlorination and Cu adatom-mediated Ullmann coupling. We find that the latter is essential...