Regulation of Excitonic Effect in Donor–Acceptor Carbon Nitride for H 2 O 2 Photosynthesis and Water Treatment

ABSTRACT Constructing a donor–acceptor (D–A) structure in polymeric carbon nitride (PCN) extends the π ‐conjugation for enhanced H 2 O 2 photosynthesis, yet its impact on the excitonic effect remains underexplored. Herein, the exciton behavior and charge dynamics in D–A pairs were systematically regulated via rational donor design. Benzene derivatives (from benzene to perylene) with varied binding sites were employed to construct nine D–A models. A clear donor‐size‐activity relationship is established, revealing that the anthracene donor achieves an optimal balance between π‐extension and exciton dissociation. Guided by the density functional theory calculations, seven photocatalysts are synthesized. Therein,2‐position anthracene‐modified PCN (2‐AnCN) exhibited exceptional H 2 O 2 production efficiency, achieving 3.3 times higher than that of the pristine PCN without any sacrificial agent. This enhancement originates from a fundamental mechanism transition from a 4e − ‐dominated oxygen reduction reaction (ORR) process in PCN to a 2e − ‐dominated pathway in 2‐AnCN, driven by the extended π ‐delocalization and suppressed exciton binding. The system also demonstrates practical efficacy in photocatalytic bacterial inactivation and organic degradation. This work demonstrates that donor size is the key parameter governing exciton behavior, charge separation, and H 2 O 2 production, providing a molecular‐level strategy to balance π ‐extension, binding site, and exciton management for rational photocatalyst design.