Mechanistic Picture and Kinetic Analysis of Surface-Confined Ullmann Polymerization

Abstract

Surface-confined polymerization via Ullmann coupling is a promising route to create one- and two-dimensional covalent π-conjugated structures, including the bottom-up growth of graphene nanoribbons. Understanding the mechanism of the Ullmann reaction is necessary to provide a platform for rationally controlling the formation of these materials. We use fast X-ray photoelectron spectroscopy (XPS) in kinetic measurements of epitaxial surface polymerization of 1,4-dibromobenzene on Cu(110) and devise a kinetic model based on mean field rate equations, involving a transient state. This state is observed in the energy landscapes calculated by nudged elastic band (NEB) within density functional theory (DFT), which assumes as initial and final geometries of the organometallic and polymeric structures those observed by scanning tunneling microscopy (STM). The kinetic model accounts for all the salient features observed in the experimental curves extracted from the fast-XPS measurements and enables an enhanced understanding of the polymerization process, which is found to follow a nucleation-and-growth behavior preceded by the formation of a transient state.

The Macrocycle versus Chain Competition in On‐Surface Polymerization: Insights from Reactions of 1,3‐Dibromoazulene on Cu(111)

“…The resulting debrominated azulene units form organometallic oligomers with C‐Cu‐C bonds, as shown in Figure . According to previous work, the C−Cu bond is stable over the duration of the experiment at 300 K . Therefore, we have a non‐equilibrium situation of kinetic reaction control, which means that the initially formed product (i.e., the species that is produced with the fastest rate) prevails.…”

Micrometre-long covalent organic fibres by photoinitiated chain-growth radical polymerization on an alkali-halide surface

“…Key factors for this success are first the confinement to 2D and the adsorption on a well-chosen surface that can favour both thermodynamic and kinetic progression of the polymerization process. 24 Second, in UHV there is a much broader range of annealing temperatures possible. In fact, most of the reactions described so far are initiated by thermal annealing which however often creates defects in the formed structures due to the increased diffusion of the precursors on the surface.…”

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 …”