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

Abstract

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.