Stepwise On-Surface Synthesis and Transformations of Two-Dimensional Covalent Organic Frameworks by Controlled Thermal Stimuli.

The development of covalent organic frameworks (COFs) is currently a primary objective in materials science, taking into account the envisioned applications in a variety of fields, including gas and energy storage, sensing, catalysis, and optoelectronics. Recently, the advent of on-surface covalent synthesis has allowed the design of one-atom-thick COFs, although the in situ transformations of such materials at interfaces have remained elusive. In this work, advantage is taken of an ex-professo synthesized molecular precursor endowed with gem-dibromide functional groups and a phenanthroline moiety to exploit steric hindrance as a synthetic controlling concept and, by subsequent chemical coupling reactions through thermal activation, afford COF transformations at interfaces in a controlled stepwise manner. In a first step, 1D covalent molecular chains are formed and self-assembled in a 2D supramolecular network, which, upon annealing, gives rise to a 2D porous organo-metallic network. Further annealing at higher temperatures affords the formation of a 2D-COF comprising linear chains based on ethynylene bridges at the cores of the monomers and carbon-carbon couplings at their peripheries. Such ethynylene linkages are transformed into antiaromatic pentalene moieties upon subsequent annealing, thus exemplifying the conversion of 2D-COFs at interfaces. These results provide new avenues toward the engineering and in situ chemical transformations of 2D-COFs in a stepwise manner, anticipating the tailoring of the structure and electronic properties of monolayer 2D-COFs by thermal stimuli.