Spatially Selective Substitution for Switching the Mechanism Pathway of Oxygen Reduction Reaction on Linear Conjugated Polymers

Precise control over oxygen reduction reaction (ORR) pathways in metal-free electrocatalysts remains challenging due to the coupled nature of multielectron transfer, molecular aggregation, and oxygen adsorption configurations. Here, we demonstrate that spatially selective substitution in linear conjugated polymers (LCPs) provides an effective molecular handle to regulate the ORR pathways. By systematically increasing the substituent steric bulk, we induce a distinct J- to H-type aggregation transition in fluorene-based LCPs. In situ UV–vis spectroscopy, combined with electrochemical analysis and density functional theory calculations, reveals that steric-driven aggregation enhances intermolecular charge transfer, thereby stabilizing superoxide intermediates and lowering the O–O bond cleavage barrier. As a result, the ORR pathway is selectively tuned from a dominant two-electron process to a highly efficient four-electron pathway. This work highlights steric engineering as a useful physical chemistry strategy for controlling the multielectron reaction pathway in organic materials.