Acid Strength and Substituent Effects in Morphological Engineering of 2D Single-Crystal-Like Covalent Organic Frameworks

The synthesis of a single-crystal two-dimensional covalent organic framework (2D COF) has persisted with formidable challenges, particularly in achieving orderly morphological development and dimensional scalability. Herein, we proposed a gradient acid catalysis strategy utilizing aniline as a modulator to precisely regulate Schiff-base reaction kinetics, thereby facilitating the synthesis of high-quality 2D COFs with single-crystal-like morphology. Employing modulation of the ortho-substituent configurations in aldehyde monomers, we successfully synthesized three COFs (NEU-305, NEU-309, and NEU-317) with single-crystal-like morphology and demonstrated unique morphological evolution patterns. The crystallization mechanism and morphological development of COFs were systematically investigated through the integration of DFT calculations, elucidating the crucial role of the acid effect in regulating the Schiff-base reaction barrier. The modulating effect of substituent electronic effects on amine reactivity was analyzed, revealing a direct correlation between anisotropic binding differences and crystal growth kinetics. Furthermore, NEU-309 exhibited an exceptional Au adsorption affinity, enabling efficient Au recovery from actual samples. This work established a general strategy for designing COFs with programmable morphologies and structure-property relationships.