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/twodimensional (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 (Cl 4 TPP), 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 essentially barrierless, whereas the former faces a barrier of about 0.9 eV for inverted Cl 4 TPP 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.

Metal adatoms play a key role in surface diffusion, adsorption conformation, and self-assembly of porphyrin molecules on metal surfaces. Herein, we study the specific influence of coadsorption of Fe, Co, and Pd atoms on the behavior of 2H-tetrakis­(p-cyano)­phenylporphyrin (2H-TCNPP) on Cu(111) using scanning tunneling microscopy. Upon co-deposition of Fe and Co, the molecules form one-dimensional (1D) linear chains after mild annealing on Cu(111) driven by the interaction of its cyano groups with metal adatoms. A similar behavior has been observed previously on Cu(111), mediated by Cu adatoms, where the functional CN groups were also found to lower the reaction rate of the so-called porphyrin self-metalation reaction with Cu atoms significantly, in comparison to the non-cyano-functionalized porphyrin. Upon co-deposition of Pd and mild annealing, we find a remarkably different behavior, that is, a massive reorganization from 1D molecular chains to a peculiar rectangular 2D (two-dimensional) network. The molecular appearance changes to a clover shape, which is attributed to a Pd-induced dehydrogenation and subsequent ring closure reaction of the phenyl and pyrrole groups.

Monolayer hexagonal boron–nitrogen–carbon (h-BNC) is considered a prominent candidate for the next generation of semiconductor electronic devices. Nevertheless, experimental evidence of h-BNC formation is limited, including a detailed study of its morphological and electronic properties. Here, successful growth of h-BNC from an unexplored single molecular precursor (hexamethyl borazine, C6H18B3N3) using a conventional CVD approach on Ir(111) is reported. The conformation structure of the monolayer and its correlation with the local electronic properties are discussed based on scanning tunneling microscopy/spectroscopy (STM/STS) and X-ray photoelectron spectroscopy (XPS) results. The results show an h-BNC structure that can be described as BN-doped graphene since the moiré lattice parameter is preserved along with the alloy. This BN-doped cluster, renamed as h-BN “nanodonuts” according to the electronic density exhibited in STM images, have a tendency to place specific positions within the moiré superstructure, and it is constituted by at least (BN)8 units arranged in a 6-fold BN rings conformation, as evidenced by simulation of STM images based on density functional theory (DFT). For a BN concentration of about 17%, a band gap between 1.4 and 1.6 eV was determined. The versatility of the novel molecular precursor is proven by the growth of a high-quality h-BN monolayer on Rh(111).

On-surface coupling reactions and molecular conformation are essential processes for building tailored functional molecular nanostructures. Here, we study the thermal debromination and reactivity of free-base tetra(4-bromophenyl)porphyrin (H 2 TBrPP) on Cu(111) as a function of the substrate temperature. It has been previously reported in the literature that C−Br bonds remain intact at room temperature (RT) and that the Br•••Cu(111) interaction induces a drastic surface reconstruction around the molecule periphery and a distortion in the adsorbate itself. However, based on a combination of STM and XPS experiments, supported by density functional theory (DFT) calculations, we instead demonstrate that debromination readily occurs at RT, leading to a new interpretation of both the adsorption behavior and the molecular coupling of H 2 TBrPP on Cu(111). For the molecules deposited on the metallic substrate held above RT, our STM measurements show the growth of ordered 2D metal−organic frameworks (MOFs).

ResumoNeste projeto de IC estudamos a coordenação molecularformação de nanofios moleculares de 2H-TCNPP na superfície de Cu(111) mediados por átomos de Fe ou Pd, além de analisar a sua variação com a temperatura. O estudo envolveu a preparação deste particular substrato, evaporação das moléculas, aquecimento da amostra e sua investigação pelas técnicas de microscopia de tunelamento eletrônico (STM) e espectroscopia de elétrons excitados por raios X (XPS). No trabalho procuramos desvendar o papel do Fe ou Pd na formação dos nanoestruturas moleculares.