Self-assembly and reactivity of double-decker 4,5,15,16-tetrabromo[2.2]paracyclophane on Au(111)

We investigated the self-assembly and the reactivity of double-decker 4,5,15,16-tetrabromo[2.2]paracyclophane molecules on Au(111) using low-temperature scanning tunnelling microscopy and ab initio calculations. When sublimated in vacuum on the substrate kept at room temperature, the molecules self-assemble into ordered islands with hexagonal symmetry extending over thousands of squared nanometres. The molecules adsorb with the top and bottom benzene rings nearly parallel to the Au(111) surface. The interaction between the adsorbed molecules and the substrate is weak and characterized by a charge transfer of 0.21 e-/molecule towards the substrate. Upon annealing in vacuum or direct molecular sublimation onto a hot Au(111) surface, dimerization occurs via an Ullmann-like coupling reaction when the unsaturated C atoms in two 4,5,15,16-tetrabromo[2.2]paracyclophane monomers bind via a [2 + 2] cycloaddition. This study is thus one of the still rare reports of on-surface synthesis of layered molecular structures.