Axial Coordination Orchestrates Antibonding‐State Fermi‐Level Proximity and Oxygen Affinity in Single‐Atom Iron for Selective Chemiluminescence

ABSTRACT Selective chemiluminescence sensing systems face challenges due to poor specificity of radical pathways in aqueous environments. To address this issue, axially coordinated Fe single‐atom catalysts (Fe‐SACs) with asymmetric high‐spin N−FeN 4 sites were synthesized via one‐step calcination, enabling selective reactive iron species (RFeS) generation under neutral conditions. In situ x‐ray absorption fine structure and the density functional theory analyses revealed that the axial N‐coordination stabilizes high‐spin states, reduces orbital splitting energy, and enhances d – p hybridization with peroxymonosulfate (PMS), thereby lowering the formation energy of RFeS by 42%. Concurrently, it modulates d x 2 ‐ y 2 and d xy orbitals to weaken oxygen binding and promote electron delocalization. This dual modulation achieves > 99% RFeS selectivity, suppressing non‐selective radicals. The Fe‐SACs/PMS/luminol system exhibits 826‐fold signal amplification and < 5.8% interferent‐induced signal variation, enabling ultrasensitive evaluation of total phenolics (using phenol as a model) in complex water with < 8.3% deviation from high‐performance liquid chromatography. This work establishes axial‐coordination‐directed electronic modulation as a universal design principle for selective catalysis.