Strategies for Improving the Structural Quality of Covalent Networks in On-Surface Ullmann Coupling

Abstract

On-surface Ullmann coupling is an established route for the bottom-up synthesis of two-dimensional (2D) organic nanostructures. Covalent organic networks combine chemical and mechanical stability of covalent bonds with versatility and tunability of organic materials, which is highly interesting for application in (opto)electronic devices. Via Ullmann coupling, halogenated monomers can be readily polymerized into covalently cross-linked networks on solid surfaces. However, the formation of irreversible covalent bonds inhibits reorganization and correction of structural defects. To overcome the limitations in structural quality, the polymerization reaction has to be controlled from the initial to the final reaction step. This article introduces general strategies to steer the coupling reaction to improve long-range order, compactness, and domain size of 2D networks. In this context, careful selection of substrate materials, optimization of reaction conditions, and multistep reaction pathways are discussed.