Dual-Quenching Charge Transfer Unlocks Record Nitrate-to-Ammonia Photocatalytic Conversion in Redox-Active Eosin Y Polymers.

Efficient photocatalytic nitrate reduction reaction (NO3RR) is vital for mitigating nitrogen pollution and producing green ammonia (NH3). Although organic polymer photocatalysts show great potential for NO3RR, they frequently suffer from low charge separation efficiency. This limitation largely comes from the lack of suitable redox-active moieties incorporated in the polymer photocatalysts toward NO3RR. Herein, we embed the redox-active Eosin Y (EY) into a conjugated polymer backbone to synthesize a series of EY-X polymers (where X = benzene, biphenyl, or fluorene), in which the extended π-conjugation can promote dual dynamic and static quenching for directional electron transfer. Upon visible-light excitation, EY forms a long-lived radical anion (EY•-) that stores and relays electrons to nitrate, while ground-state complexation between polymer and nitrate preorganizes the substrate for photoinduced directional electron transfer. This dual-pathway mechanism extends charge-separated lifetimes, inhibits recombination, and enhances electron delivery. Consequently, under cocatalyst-free conditions, the EY-BE polymer achieves a record high NH3 production of 215 µmol g-1 h-1. Experimental and computational investigations support the reversible EY/EY•- cycle and the nitrate-binding ground-state complex as the origin of activity and selectivity. This work demonstrates a rational strategy leveraging reversible redox-active chromophores to integrate dual quenching for designing high-performance NO3RR photocatalysts.