Self-assembly networks of azobenzene with hydrogen and halogen bonds on Au(111)

The self-assembly of azobenzene, a prototypical photo-switchable molecule, has been extensively studied through hydrogen-bond-driven motifs. However, systems incorporating halogen bonds, which often compete with hydrogen bonds, remain rare. Here, we investigate the self-assembled structures of azobenzene derivatives terminated with hydroxyl and bromo groups on the Au(111) surface using scanning tunneling microscopy. Three distinct supramolecular motifs-triangular, hexagonal, and striped-were observed. Density functional theory calculations support the proposed molecular models. The triangular assemblies are stabilized by hydrogen bonds (O-H⋯O and Br⋯H), while the hexagonal and striped motifs involve both hydrogen bonds and halogen bonds (Br⋯Br). Our results show that the striped motif exhibits the highest stability among the three, with its preference driven by the collective thermodynamic stability of the dense layer rather than by the adsorption strength of individual molecules. Our findings demonstrate the cooperative and competitive roles of hydrogen and halogen bonds in directing the assembly of azobenzene-based nanostructures.