Rational Synthesis of Photocatalytic Acridinium-Based Covalent Organic Frameworks via Single-Atom Skeletal Editing

The rational synthesis of covalent organic frameworks (COFs) with customized functionalities and enhanced stability requires innovative bond-forming strategies beyond conventional dynamic covalent chemistry. Herein, we report a novel single-atom skeletal editing approach to construct acridinium-based crystalline COFs through an irreversible Katritzky-type reaction. This strategy enables precise transformation of 9,9'-(2,3,5,6-tetramethyl-1,4-phenylene)bis(3,6-di-tert-butylxanthylium) with either 1,3,5-tris(4-aminophenyl)benzene (TAPB) or 1,2,4,5-tetra(4-aminophenyl)benzene (TADB) into acridinium-linked COFs, denoted as Acr-TAPB and Acr-TADB, respectively. The transformation proceeds via a unique single-atom oxygen-to-nitrogen replacement, converting the xanthylium precursor into acridinium frameworks. The resulting COFs exhibit high crystallinity, robust stability, strong redox ability, and efficient charge separation. Notably, Acr-TADB functions as a highly effective metal-free photocatalyst for oxidative nucleophilic substitution and [3 + 2] cycloaddition reactions, supporting gram-scale synthesis in both batch and flow reactors with excellent stability. This work establishes skeletal editing as a powerful strategy for engineering functionalized COF architectures for advanced applications.