Stepwise on-Surface Synthesis and Transformations of Two-Dimensional Covalent Organic Frameworks by Controlled Thermal Stimuli
The on-surface synthesis of atomically precise carbon nanomaterials has emerged as a powerful strategy to overcome the limitations of solution-based chemistry, enabling the fabrication of low-dimensional polymers and networks with unprecedented structural control. In particular, the development of single-layer two-dimensional (2D) covalent organic frameworks (COFs) remains a central challenge due to their promising applications in a variety of fields, as sensing, catalysis, and (opto)electronics. [1,2] Here, we report a stepwise on-surface synthetic approach that exploits steric hindrance and sequential thermal activation to engineer and transform two COFs with atomic precision by scanning probe techniques. An ex-professo designed molecular precursor, functionalized with gem-dibromoalkene groups and a phenanthroline moiety, is employed to guide controlled coupling reactions under ultra-high vacuum conditions. Initial debromination leads to the formation of sterically controlled one-dimensional covalent chains, which self-assemble into a two-dimensional supramolecular network. Subsequent annealing yields a 2D organometallic network. Further thermal activation drives carbon–carbon coupling reactions between wires to form a 2D-COF composed of linear chains connected through ethynylene bridges. Remarkably, continued annealing triggers an unprecedented COF-to-COF transformation, in which the ethynylene linkages convert into antiaromatic pentalene moieties, demonstrating in situ chemical transformations of 2D-COFs in a stepwise manner. This work establishes a general strategy for the bottom-up synthesis and thermal transformation of 2D-COFs at interfaces, validating the combination of steric control and sequential reactions as a viable alternative to symmetric precursor design, and opens new avenues for tailoring the structure and electronic properties of atomically precise 2D-COFs by thermal stimuli. [1] R.-R. Liang et al. "Two-Dimensional Covalent Organic Frameworks With Hierarchical Porosity" Chem. Soc. Rev. 2020, 49, 3920 [2] C. Wang et al. " 2D Covalent Organic Frameworks: From Synthetic Strategies to Advanced Optical-Electrical-Magnetic Functionalities" Adv. Mater. 2022, 34, 2102290