Harnessing Reduced-Symmetry Molecules to Guide Surface Assemblies into Covalent 2D Organic Materials

On-surface synthesis has emerged as a pivotal approach for assembling covalent 2D organic materials from molecular precursors, with long range order and tailored electronic structures. Yet, while molecular self-assembly on surfaces is often highly ordered, subsequent intra-/intermolecular C–C bond formation between molecular precursors leads to disordered on-surface polymerization. This unpredictability stems from a lack of control over molecular site reactivity and from an accompanying symmetry loss during dehydrogenation and dehydrohalogenation processes.

Here, we introduce a controlled pathway for these processes, achieved by the targeted functionalization of the pyrrole ring of Zinc Tetraphenylporphyrins. This results in a suite of molecules with consistent 2-fold symmetry. We show that selected functional groups and symmetry confinement of dehydrogenation and dehydrohalogenation reactions lead, under certain conditions to a uniform packing exhibiting long-range order, post on-surface intra-molecular reactions, as observed under STM. Comprehensive XPS/UPS characterizations elucidate the electronic structures of these molecular layers, and by-products of C–C bond formation are tracked using TPD.