Mechanism of Singlet Oxygen Generation Catalyzed by CoN ₄ -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 _<i>xy</i> ²d _<i>yz</i> ²d _<i>xz</i> ²d_z² ¹d_x²‑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.