The role of halogens in on-surface Ullmann polymerization

Abstract

Ullmann coupling is the most common approach to form surface-confined one- and two-dimensional conjugated structures from haloaryl derivatives. The dimensions of the formed nanostructures can be controlled by the number and location of halogens within the molecular precursors. Our study illustrates that the type of halogen plays an essential role in the design, orientation, and extent of the surface-confined organometallic and polymeric nanostructures. We performed a comparative analysis of five 1,4-dihalobenzene molecules containing chlorine, bromine, and iodine on Cu(110) using scanning tunneling microscopy, fast-X-ray photoelectron and near edge X-ray absorption fine structure spectroscopies. Our experimental data identify different molecular structures, reaction temperatures and kinetics depending on the halogen type. Climbing image nudged elastic band simulations further clarify these observations by providing distinct diffusion paths for each halogen species. We show that in addition to the structure of the building blocks, the halogen type has a direct influence on the morphology of surface-confined polymeric structures based on Ullmann coupling.

Dechlorinated Ullmann Coupling Reaction of Aryl Chlorides on Ag(111): A Combined STM and XPS Study

“…Some oligomers are covalently bonded, and the ratio increases with the annealing temperature. Note that the endings of the oligomers point to the bright C−Ag−C protrusions, we therefore propose that the organometallic intermediates 4 are Cl‐terminated due to incomplete dechlorination. Corresponding molecular models are overlaid in Figure c and d. There might exist an attraction between the undissociated C−Cl and Ag in the organometallic intermediates, which acts as the driving force for disorderly aggregated oligomers.…”

“…To do the XPS measurements, DCTP was first evaporated onto the Ag(111) surface at nearly saturation coverage. The Cl 2p spectrum measured at room temperature (bottom curve in Figure a) is dominated by a Cl 2p doublet feature (Cl 2p 1/2 and Cl 2p 3/2 , with an area ratio 1 : 2 and a splitting energy Δ=1.6 eV) with Cl 2p 3/2 centered at a binding energy (BE) of 200.8 eV, corresponding to the C−Cl bonds in the DCTP molecules (blue line). The Cl 2p doublet feature slightly shifts towards a lower BE by nearly 0.1 eV after the sample is annealed to 373 K, with a marginal decrease in intensity, which is probably caused by gradual desorption of DCTP and the relating increase in surface work function .…”

Overcoming Steric Hindrance in Aryl‐Aryl Homocoupling via On‐Surface Copolymerization

“…One of the most used chemical reactions to grow 1D and 2D polymers on metal surfaces is Ullmann coupling . Many studies have given insights into Ullmann‐like reactions on various metal substrates, and the ability of synthetic chemists to create all sorts of building blocks has led to the realization of a wide range of graphene‐related materials …”

Ullmann Coupling Reactions on Ag(111) and Ag(110); Substrate Influence on the Formation of Covalently Coupled Products and Intermediate Metal-Organic Structures

“…However the mechanistic process by which the intermediate state is formed and converted to the covalent product is still debated. Several studies have attempted to address the problem, investigating the effects of both the bulk chemistry 26 and topology of the surface 27 , as well as other factors such as the influence of halogens 28 , 29 and reaction kinetics 30 , on structure formation. The relationship between the alignment of the intermediate and final structures with the substrate is an additional influential factor to be explored and is investigated here.…”