Insight into Acceptor Engineering with Excitonic Effect in Donor–Acceptor Covalent Organic Framework

Donor–acceptor (D–A) covalent organic frameworks (COFs) show great potential as photocatalysts due to their tunable energy levels. However, strong excitonic effects severely limit free charge carrier generation in photocatalytic processes, and their mitigation through molecular engineering remains challenging. Herein, we systematically investigate how elongating electron-withdrawing benzothiadiazole (BT) acceptor units in four isomorphic D–A type COFs (TAPB-BT0-COF to TAPB-BT3-COF) affect excitonic effects and photocatalytic activity. Combined experimental and theoretical analyses demonstrate that elongating electron-deficient BT acceptor progressively decreased the exciton binding energy (Eb). Nevertheless, photocatalytic sulfide oxidation yields followed a volcano-shaped trend, peaking at TAPB-BT2-COF due to its optimal charge transfer characteristics. Crucially, while elongating acceptor fragments in COFs reduces Eb, maximizing photocatalytic performance requires balancing this reduction with enhanced charge delocalization and transfer, not merely extending the unit. These findings provide new insights for designing BT-based COFs via acceptor engineering to regulate excitonic effects and achieving high photocatalysis.