On-surface Hydrogen-induced Covalent Coupling of Polycyclic Aromatic Hydrocarbons via a Super-hydrogenated Intermediate.

The activation, hydrogenation, and covalent coupling of polycyclic aromatic hydrocarbons (PAHs) are processes of great importance in fields like chemistry, energy, biology, or health, among others. So far, they are based on the use of catalysts which drive and increase the efficiency of the thermally- or light-induced reaction. Here, we report on the catalyst-free covalent coupling of non-functionalized PAHs adsorbed on a relatively inert surface in presence of atomic hydrogen. The underlying mechanism has been characterized by high-resolution scanning tunnelling microscopy and rationalized by density functional theory calculations. It is based on the formation of intermediate radical-like species upon hydrogen-induced molecular super-hydrogenation which favors the covalent binding of PAHs in a thermally-activated process resulting in large coupled molecular nanostructures. The mechanism proposed in this work opens a door toward the direct formation of covalent, PAH-based, bottom-up synthetized nano-architectures on technologically relevant inert surfaces.