Abstract
Scanning tunnelling microscopy (STM) is commonly used to identify on-surface molecular self-assembled structures. However, its limited ability to reveal only the overall shape of molecules and their relative positions is not always enough to fully solve a supramolecular structure. Here, we analyse the assembly of a brominated polycyclic aromatic molecule on Au(111) and demonstrate that standard STM measurements cannot conclusively establish the nature of the intermolecular interactions. By performing high-resolution STM with a CO-functionalised tip, we clearly identify the location of rings and halogen atoms, determining that halogen bonding governs the assemblies. This is supported by density functional theory calculations that predict a stronger interaction energy for halogen rather than hydrogen bonding and by an electron density topology analysis that identifies characteristic features of halogen bonding. A similar approach should be able to solve many complex 2D supramolecular structures, and we predict its increasing use in molecular nanoscience at surfaces.
Highlights
Scanning tunnelling microscopy (STM) is commonly used to identify on-surface molecular self-assembled structures
While conventional STM measurements cannot determine whether 3,9-Br2PXX assembles via hydrogen bonded (HB) or XB, only by means of high-resolution STM (HR-STM) are we able to identify XB interactions as the driving force leading to the formation of supramolecular assemblies of 3,9-Br2PXX on Au(111)
The Br and O atoms in 3,9-Br2PXX offer the possibility of both HB and XB intermolecular interactions when arranged into supramolecular arrays on a surface
Summary
Scanning tunnelling microscopy (STM) is commonly used to identify on-surface molecular self-assembled structures. The majority of the 3,9-Br2PXX molecules self-assemble into kagome-type structures (phase 1) that develop in the face-centred cubic (fcc) regions and elbow sites of the Au (111) herringbone reconstruction (Fig. 2a).
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