Mechanism of Singlet Oxygen Generation Catalyzed by CoN4‑Graphene: Insights from Embedded Multi-Configurational Theory.

Efficient generation of singlet oxygen (1O2) underpins critical applications in photodynamic therapy, selective oxidation, and environmental remediation. Single-atom catalysts (SACs), particularly transition-metal-nitrogen-carbon single-atom catalysts (TM-N-C SACs) featuring TMN4 motifs embedded in carbon matrices, have shown promise in catalyzing this spin-forbidden transformation of ground-state triplet oxygen (3O2) to 1O2. However, the underlying catalytic mechanisms and governing electronic factors remain elusive due to challenges in both experimental characterization and computational approaches. In this work, we integrate density functional embedding theory (DFET) with second-order N-electron valence state perturbation theory (NEVPT2) to resolve the electronic structure and reactivity of CoN4-graphene (Gr). Embedded NEVPT2 accurately identifies the Co center's electronic configuration as (d xy 2d yz 2d xz 2dz2 1dx2‑y2 0) and predicts a weak, polarized Co-O2 interaction with an adsorption energy of -0.37 eV, consistent with experimental trends. Furthermore, we reveal a stepwise, photoinduced electron-transfer mechanism for 1O2 generation on CoN4-Gr, fundamentally distinct from the energy-transfer pathway operative in photosensitized systems. This work demonstrates the effectiveness and efficiency of combining DFET with high-level wave function methods for accurately describing complex correlated catalytic systems. The insights obtained not only deepen our understanding of SAC-mediated 1O2 generation but also establish a transferable theoretical framework for the rational design of efficient, photosensitizer-free catalysts for reactive oxygen species production.