Empowering non-covalent hydrogen, halogen, and [S-N]₂ bonds in synergistic molecular assemblies on Au(111)

Non-covalent bonds are fundamental for designing self-assembled organic structures with potentially high responsiveness to mechanical, light, and thermal stimuli. The weak intermolecular interaction allows triggering charge-transport, energy-conversion, enzymatic, and catalytic activity, to name a few. Here, we discuss the synergistic action that multiple highly-directional and purely electrostatic bonds have in assembling one molecular specie, namely 4,7-dibromobenzo[c]-1,2,5-thiadiazole (2Br-BTD), in two different patterns on the Au(111) surface. We find, using scanning tunneling microscopy (STM) and density functional theory (DFT), that multiple secondary-interactions strengthen the electrostatic attraction between the pnicogen and chalcogen atoms forming [S-N]₂ heterocycles, the building block of the two networks. Among these interactions, there are halogen-halogen bonds that form characteristic supra-molecular synthons of 3, 4, or 6 molecules. However, not all these nodal structures contribute to the cohesion of the system. In such cases, other secondary bonds involving hydrogen or nitrogen compensate for the eventual deficiency.