<p>Although polymeric carbon nitride holds promise for solar-driven hydrogen production, its scalability is constrained by reliance on costly vacuum conditions and scarce freshwater supplies. Herein, ultrathin carbon nitride nanosheets are covalently linked to electron-donating pyrene units via π-bridges. The resulting donor–π–acceptor frameworks, featuring a biphenyl π-bridge, exhibit reduced exciton binding energy and long-lived charge-separated states. In situ spectroscopic and electrochemical analyses collectively demonstrate efficient intramolecular electron transfer and a strengthened built-in internal electric field. Theoretical calculations suggest that electron accumulation on heptazine units may enhance the adsorption of Na⁺/Mg²⁺–triethanolamine complexes, accelerating hole consumption in seawater. As a result, the optimal donor–π–acceptor catalyst shows efficient photocatalytic seawater splitting under ambient pressure and natural sunlight, achieving a hydrogen evolution rate of 134 mmol h⁻¹ g⁻¹. Here, we show a molecular design strategy that advances photocatalytic ambient-pressure seawater splitting and promotes the commercialization of green hydrogen production.</p>