Cs ₃ Bi ₂ Br ₉ Quantum Dots‐Driven Photogenerated Bromine Radical Relay Enables Efficient Cleavage of C α /C β ─O Bonds for Lignin Depolymerization

With the remarkable activation efficiency, hydrogen atom transfer (HAT) strategy has emerged as a promising approach for promoting lignin depolymerization; yet, it faces substantial challenges in using stoichiometric HAT reagents or acidic/basic additives. Herein, a catalytic system comprising Cs 3 Bi 2 Br 9 quantum dots (CBB QDs) and catalytic amounts of bromine sources that achieves 99% conversion in depolymerizing α ‐O‐4 and β ‐O‐4 model compounds to benzaldehyde (∼87%) and phenol (∼92%) through a bromine radical (•Br) relay mechanism. Also, this system accomplishes the first effective photocatalytic depolymerization of an advanced ( α ‐O‐4)‐( β ‐O‐4) dilinkage model compound, helping to clarify the substantial reactivity discrepancies between monomeric models and native lignin. Catalytic reactions of birch lignin demonstrate cleavage of C α /C β ─O bonds with > 99% selectivity, yielding high‐value aromatic aldehydes and phenolic products. Notably, this system overcomes the inherent cycling instability of QDs photocatalysts, maintaining 95% of the initial conversion after seven recycles. Mechanistic studies establish that the exceptional performance originates from the unique halogen exchange capability of CBB QDs, coupled with the optimal band alignment, which enables continuous •Br generation and sustains persistent HAT cycles.