Controlling on-surface synthesis of 2D covalent organic networks using hydrogen

On-surface synthesis using dehalogenative Ullmann coupling on noble metals has been extensively used for creating metal-supported 2D covalent organic networks in ultrahigh vacuum. In principle, rational design of the precursor molecules determines the network and its electronic properties. In this work, we propose the introduction of atomic hydrogen into the reaction chamber as an additional route to engineer the on-surface reactions. Recent reports have demonstrated that atomic hydrogen assists in debromination and subsequent hydrogenation of adsorbed molecules[1]. We extend this technique to control the degree of hydrogenation of tribrominated heterotriangulene precursors[2] during the coupling process, which enables the creation of self-assembled islands of hydrogenated monomers, covalently-linked dimers, and macrocycle hexamers. We study these assemblies using STM and assess their structure using mass spectrometry. The sequence of n-mer assemblies enables a comparative study of electronic properties from single monomer to continuous 2D covalent network.

[1] Zuzak et al. ACS Nano, 14(10) 13316 (2020).
[2] Bieri et al., Chem. Commun. 47 (37) 10239 (2011).