Isomeric donor-π-acceptor-π-donor COFs via bisected donor-monomer strategy and directionally π bridges for photocatalytic organic conversion

Covalent organic frameworks (COFs) have emerged as promising semiconductor-like photocatalysts, yet their efficiency is often hampered by insufficient active sites and rapid recombination of photogenerated charge carriers. A key challenge lies in the precise regulation of the electronic structure between donor and acceptor units to promote directional charge migration. To address this, we report a donor-monomer bisected molecular engineering strategy for constructing two novel isomeric 1D-CBTDA-COF and 2D-CBTDA-COF with distinct dimensionality. Both frameworks feature a donor–π–acceptor–π–donor (D–π–A–π–D) unit, wherein: (1) the carbazole donor units enable electron delocalization across the π-conjugated plane via lone-pair electrons on nitrogen, facilitating electron migration; (2) the thiadiazole acceptor units, with their electron-deficient nitrogen sites, capture and stabilize photogenerated electrons; (3) the Schiff-base C N linkages serve as electron-withdrawing π-bridges, which align with the desired electron transfer pathway from donor to acceptor, thereby accelerating charge separation. Compared to the 1D analogue, 2D-CBTDA-COF exhibits markedly enhanced photocatalytic activity in the thiocyanation of indole derivatives, which were attributed to its more excellent photoelectrical properties. It is explained that more extended and robuster π–π stacking network of 2D-CBTDA-COF can further suppresses charge recombination and promote interlayer carrier transport. This study demonstrates the critical role of electronic and structural dimensionality in COF photocatalysis and provides a rational design principle for advanced organic transformation catalysts.