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 Delta E 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 (approximate to 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 essentially barrierless, whereas the former faces a barrier of about 0.9 eV for inverted Cl4TPP on Cu(111). Our STM measurements show that C-Cu-C organometallic species are the main final products in the presence of Cu adatoms, which is explained by our DFT reaction profile when heat dissipation to the substrate is taken into account. This work not only highlights the relevance of surface adatoms in selecting the reaction pathway but also opens the possibility of precisely tailoring 2D molecular assemblies by controlling the supply of Cu adatoms.