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

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-FeN4 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 dx 2-y 2 and dxy 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.