A STM perspective on covalent intermolecular coupling reactions on surfaces

'Covalent self-assembly', i.e. the on-surface synthesis of covalent organic aggregates and networks, has received considerable attention. This review covers recent scanning tunnelling microscopy (STM) based studies on intermolecular reactions carried out on solid substrates that resulted in surface-confined covalently interlinked organic nanostructures. Experiments showed that their defect density crucially depends on the targeted dimensionality: while zero-dimensional aggregates and one-dimensional chains and ribbons can be synthesized on surfaces with utmost structural perfection, i.e. without any topological defects, realization of long-range ordered two-dimensional (2D) covalently interlinked organic networks has revealed itself as a paramount challenge for on-surface chemists. Different types of reactions, foremost condensation and addition reactions have been proven suitable as polymerization reactions for 2D cross-linked covalent networks. Yet, the emergence of topological defects during the polymerization is difficult to avoid. However, the combined experience and creativity of chemists and surface scientists has yielded encouraging first results which may open up ways for realization of extended, long-range ordered 2D polymers. This review summarizes and compares different approaches, i.e. reaction types, monomers, environments and conditions, for the on-surface synthesis of covalent organic nanostructures. The focus on STM as an analytical tool appears justified, since its unique capabilities render the STM an ideal instrument to study and even control covalent coupling reactions of organic molecules on surfaces.