Synthesis of Well-Ordered COF Monolayers: Surface Growth of Nanocrystalline Precursors versus Direct On-Surface Polycondensation

Abstract

Two different straightforward synthetic approaches are presented to fabricate long-range-ordered monolayers of a covalent organic framework (COF) on an inert, catalytically inactive graphite surface. Boronic acid condensation (dehydration) is employed as the polymerization reaction. In the first approach, the monomer is prepolymerized by a mere thermal treatment into nanocrystalline precursor COFs. The precursors are then deposited by drop-casting onto a graphite substrate and characterized by scanning tunneling microscopy (STM). While in the precursors monomers are already covalently interlinked into the final COF structure, the resulting domain size is still rather small. We show that a thermal treatment under reversible reaction conditions facilitates on-surface ripening associated with a striking increase of the domain size. Although this first approach allows studying different stages of the polymerization, the direct polymerization, that is, without the necessity of preceding reaction steps, is desirable. We demonstrate that even for a comparatively small diboronic acid monomer a direct thermally activated polymerization into extended COF monolayers is achievable.

Selective binding in different adsorption sites of a 2D covalent organic framework

“…13,21 In the present experiments, 1. and left for 60 min. After the thermal treatment, the reactor was taken out of the oven and allowed to cool down for at least 20 min before the samples were removed.…”

Principles of molecular assemblies leading to molecular nanostructures

“…The ability of a surface to act as an additional ligand for porphyrins was reported earlier by Elemans and co-workers [65] for alkylated Mn-porphyrin monolayers adsorbed at the tetradecane/Au(111) surface. Recent years have witnessed a surge in reports describing surface-supported synthesis and STM of two-dimensional covalent networks [66][67][68][69][70]. Selective intermolecular interactions have been widely exploited to couple non-covalently assembled molecular components in a more 'permanent' covalent fashion.…”

Toward printing molecular nanostructures from microstructured samples in ultrahigh vacuum

“…6,7 This technique can be used to build a priori insoluble nanostructures that might have a strong impact in the field of nanomaterials and in molecular electronics. 8,9 However, although many examples of on-surface polymerization have been reported in the last years on metallic surfaces, [10][11][12][13][14][15][16][17] there are very few successful examples on more technologically relevant nonmetallic substrates, for instance, semiconductor or oxide surfaces, 18,19 which are advantageous to electronically decouple the molecules from the surface. A successful polymerization process requires a full control of the balance between the dehalogenation step and the diffusion of molecular species on the surface, because these two properties determine the efficiency of the chemical reaction and consequently the outcome of the entire process.…”