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

Abstract 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.