Decoupling Chemically Active 2D Molecular Overlayers from the Substrate: Chlorophenyl Porphyrins on Graphene/Ir(111)

The synthesis of atomically precise chemically active 2D molecular overlayers may be hindered by chemical interactions with the underlying substrate, especially when based on chlorophenyl porphyrins. At the same time, the chlorination of graphene, i.e., the covalent bonding of chlorine atoms with sp² carbon atoms, is known to have a significant influence on the electronic properties of pristine graphene. In this study, we deposit a chlorinated porphyrin molecule, namely 5,10,15,20-tetrakis(4-chlorophenyl)porphyrin (Cl₄TPP), on graphene/Ir(111). Employing a combined experimental and theoretical approach, we demonstrate that the porphyrin layer physisorbed on graphene self-assembles into a periodic square-like arrangement. This carpet-like growth is unperturbed by the step edges of the substrate, neither in its periodicity nor in its orientation. In addition, the molecular overlayer is thermally stable and does not alter the electronic properties of graphene. Remarkably, we show that Cl₄TPP does not experience a dechlorination reaction with the underlying substrate, even after postdeposition annealing temperatures as high as 550 K. Moreover, postdeposition annealing at 700 K suggests the Cl₄TPP molecules desorb intact without affecting graphene’s electronic properties. In so doing, we demonstrate the effectiveness of graphene physisorbed on Ir(111) to both promote the formation and preserve the properties of chemically reactive 2D overlayers based on chlorophenyl porphyrins. These results show physisorbed graphene’s potential as a general templating material for the formation of highly reactive self-assembled 2D overlayers.