Comparing Ullmann Coupling on Noble Metal Surfaces: On‐Surface Polymerization of 1,3,6,8‐Tetrabromopyrene on Cu(111) and Au(111)

Abstract

The on-surface polymerization of 1,3,6,8-tetrabromopyrene (Br4 Py) on Cu(111) and Au(111) surfaces under ultrahigh vacuum conditions was investigated by a combination of scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT) calculations. Deposition of Br4 Py on Cu(111) held at 300 K resulted in a spontaneous debromination reaction, generating the formation of a branched coordination polymer network stabilized by C-Cu-C bonds. After annealing at 473 K, the C-Cu-C bonds were converted to covalent C-C bonds, leading to the formation of a covalently linked molecular network of short oligomers. In contrast, highly ordered self-assembled two-dimensional (2D) patterns stabilized by both Br-Br halogen and Br-H hydrogen bonds were observed upon deposition of Br4 Py on Au(111) held at 300 K. Subsequent annealing of the sample at 473 K led to a dissociation of the C-Br bonds and the formation of disordered metal-coordinated molecular networks. Further annealing at 573 K resulted in the formation of covalently linked disordered networks. Importantly, we found that the chosen substrate not only plays an important role as catalyst for the Ullmann reaction, but also influences the formation of different types of intermolecular bonds and thus, determines the final polymer network morphology. DFT calculations further support our experimental findings obtained by STM and XPS and add complementary information on the reaction pathway of Br4 Py on the different substrates.

Formation of Organometallic Intermediate States in On‐Surface Ullmann Couplings

“…This work is organized as follows: First, we present experimental STM results confirming and extending the observations made in reference of surface vacancies in Ullmann couplings of 1,4‐dihexyl‐2,5‐diiodobenzene on metal surfaces for reactions on Au(111), Ag(111), and Cu(111). In the next section, we show results of ab initio molecular dynamics (AIMD) simulations, which question the role of thermodynamically driven adatom formation as an origin of such defects/intermediates.…”

“…[9] Pham et al recently compared the Ullmann coupling of tetrabromopyrene on Cu(111)a nd Au(111). [10] They found spontaneously formed CÀCuÀCb ridges in the former case (at 300 K), while halogen-mediatedi nteractions stabilized an on-covalently bound2 Dn etwork in the latter case. Vacancies in the copper surfacew ere observed, which the authors attributed to the fact that Cu surface atoms were incorporated into the molecular network.…”

“…Pham et al. recently compared the Ullmann coupling of tetrabromopyrene on Cu(111) and Au(111) . They found spontaneously formed C−Cu−C bridges in the former case (at 300 K), while halogen‐mediated interactions stabilized a non‐covalently bound 2D network in the latter case.…”

On‐Surface Assembly of Hydrogen‐ and Halogen‐Bonded Supramolecular Graphyne‐Like Networks

“…Theintramolecular Br-Br distance of d = 0.98 AE 0.05 nm (compared to 1.01 nm in DFT) confirms that the molecules are intact, which is consistent with the unperturbed herringbone reconstruction of the Au substrate. [21] This finding suggests that the removal of the N substituent does not destabilize the Br À C(sp) bond significantly.M oreover,t he hexameric X 6 -synthon leads to an organizational chirality with observed homochiral left-and right-handed domains (see Figure S5).…”

Recent Progress in the Fabrication of Low Dimensional Nanostructures via Surface-Assisted Transforming and Coupling

“…In addition, the metal substrate not only serves as a support of low dimensional nanostructures, but also decreases the activation energy of dehydrogenation, leading to more participants in the process of coupling after C-H bond being separated [19][20][21][22]. On one hand, metal substrates decrease the relatively large bond dissociation enthalpy to let the action be easily done.…”