Site Density Governs Switching Reactivity Regimes During Aerobic C─H Oxidation in Mn Single‐Atom Photocatalysts

ABSTRACT Understanding how the density of isolated metal sites affects the structure and function of single‐atom catalysts remains a central challenge, particularly for photocatalysts based on polymeric semiconductors. In this study, we show that Mn incorporation in carbon nitride‐supported single‐atom photocatalysts (Mn 1 @CN x ) triggers a regime transition without detectable Mn aggregation. At low‐to‐intermediate loadings, isolated Mn–N x sites are accommodated within a partially delaminated CN x framework, promoting productive carrier trapping, oxygen activation, and selective aerobic benzylic C–H oxidation. Above a critical loading, however, Mn incorporation reorganizes the support into a distorted and densified architecture, modifies the Mn coordination environment, and shifts carrier dynamics toward unproductive recombination. Structural, spectroscopic, and computational analyses connect this transition to framework stacking, electronic disorder, and Mn‐support coupling. During selective aerobic benzylic C–H oxidation, we show that this regime transition translates into a nonmonotonic activity response, in which additional isolated Mn sites become detrimental once support reorganization overrides the benefit of increased site density. Mechanistic experiments and theory support a photoinduced oxygen activation, with the productive regime enabled by the balance between isolated Mn sites and a favorable CN x trap‐state landscape. These findings establish single‐atom site density as a structural and photophysical design parameter for polymer‐supported photocatalysts.